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Showing posts with label Videos. Show all posts
Showing posts with label Videos. Show all posts
Thursday, 16 April 2015
TYPES OF PLANTS KINGDOMS OR CLASSIFICATION
Biology: Scientists Reclassify Plants
into Three Separate Kingdoms
into Three Separate Kingdoms
Plants should
actually be classified as belonging to three distinct kingdoms rather than to
just one, according to the conclusions of a five-year project that involved 200
scientists from 12 countries.
actually be classified as belonging to three distinct kingdoms rather than to
just one, according to the conclusions of a five-year project that involved 200
scientists from 12 countries.
The conclusions were
presented on August 4, 1999, to the XVI International Botanical Congress in St.
Louis, Missouri. The analysis is widely considered the most comprehensive
outline of how plant life evolved. But scientists involved in constructing the
new outline noted that their findings were still preliminary and that the new
picture of plant evolution was likely to change with the collection of fresh
data.
presented on August 4, 1999, to the XVI International Botanical Congress in St.
Louis, Missouri. The analysis is widely considered the most comprehensive
outline of how plant life evolved. But scientists involved in constructing the
new outline noted that their findings were still preliminary and that the new
picture of plant evolution was likely to change with the collection of fresh
data.
According to the
research team, there are at least five major kingdoms, or branches, of
multicellular life: animals, fungi, green plants, red plants, and brown plants.
In the past, all known varieties of plants were typically grouped within a
single plant kingdom. Team leaders said their work also confirmed earlier
research suggesting that fungi—including mushrooms and yeast—belonged in a
separate kingdom.
research team, there are at least five major kingdoms, or branches, of
multicellular life: animals, fungi, green plants, red plants, and brown plants.
In the past, all known varieties of plants were typically grouped within a
single plant kingdom. Team leaders said their work also confirmed earlier
research suggesting that fungi—including mushrooms and yeast—belonged in a
separate kingdom.
Among the team's
major findings was that plants now growing on the land evolved from freshwater
plants. This challenges the long-accepted view that land plants originally
evolved from plants living in the sea. The team also traced all living land
plants to a common ancestor, a primitive type of green plant. In addition, the
researchers said they have identified the closest living relative of the
world's first flowering plant—a single species found only on the island of New
Caledonia in the South Pacific Ocean.
major findings was that plants now growing on the land evolved from freshwater
plants. This challenges the long-accepted view that land plants originally
evolved from plants living in the sea. The team also traced all living land
plants to a common ancestor, a primitive type of green plant. In addition, the
researchers said they have identified the closest living relative of the
world's first flowering plant—a single species found only on the island of New
Caledonia in the South Pacific Ocean.
The new analysis is
based largely on a classification approach known as cladistics. Unlike the
classical approach, in which organisms are grouped according to shared physical
characteristics, cladistics classifies organisms according to evolutionary
characteristics shared with a common ancestor. Using the fossil record, genetic
analysis, and other tools, cladists (scientists who specialize in
cladistics) create a family tree, or cladogram, to indicate when and where
different branches split off from the common ancestral line. Cladistics has
grown increasingly sophisticated in recent years, as advances in genomics,
a field that identifies evolutionary changes in the structure of genetic
material, have allowed researchers to quickly identify relationships among
living things.
based largely on a classification approach known as cladistics. Unlike the
classical approach, in which organisms are grouped according to shared physical
characteristics, cladistics classifies organisms according to evolutionary
characteristics shared with a common ancestor. Using the fossil record, genetic
analysis, and other tools, cladists (scientists who specialize in
cladistics) create a family tree, or cladogram, to indicate when and where
different branches split off from the common ancestral line. Cladistics has
grown increasingly sophisticated in recent years, as advances in genomics,
a field that identifies evolutionary changes in the structure of genetic
material, have allowed researchers to quickly identify relationships among
living things.
The researchers'
analysis showed that green plants, red plants, and brown plants evolved from
three different varieties of one-celled, plantlike organisms and should
therefore be grouped into separate kingdoms. Green plants comprise the largest
of the plant kingdoms and include shrubs, trees, grasses, ferns, mosses, and
flowering plants—about 500,000 species in all. Brown and red plants have
survived mostly as species of seaweed and microscopic algae known as diatoms.
analysis showed that green plants, red plants, and brown plants evolved from
three different varieties of one-celled, plantlike organisms and should
therefore be grouped into separate kingdoms. Green plants comprise the largest
of the plant kingdoms and include shrubs, trees, grasses, ferns, mosses, and
flowering plants—about 500,000 species in all. Brown and red plants have
survived mostly as species of seaweed and microscopic algae known as diatoms.
The team's work
points to a new understanding of how plant life emerged from an aquatic
environment and then exploded over the Earth's lands. Rather than invade the
land directly from the sea, where all plantlike organisms first evolved,
single-celled green plants first migrated to fresh water, where they developed
into multicellular organisms. To survive on land, green plants had to adapt,
developing new ways to remain moist and to reproduce. The researchers said
their evidence suggested these adaptations occurred more than 450 million years
ago.
points to a new understanding of how plant life emerged from an aquatic
environment and then exploded over the Earth's lands. Rather than invade the
land directly from the sea, where all plantlike organisms first evolved,
single-celled green plants first migrated to fresh water, where they developed
into multicellular organisms. To survive on land, green plants had to adapt,
developing new ways to remain moist and to reproduce. The researchers said
their evidence suggested these adaptations occurred more than 450 million years
ago.
Brent Mishler, a
biologist from the University of California at Berkeley and one of the team
leaders, noted that green plants had likely invaded the land at many different
times. However, the team's analysis suggested that only one plant lineage
survived and eventually diversified into all known land plants. This finding
challenges a common view among biologists that land plants evolved from several
distinct ancestors, including for example one for mosses and another for
flowering plants. The new work “indicates there's an Eve, a common ancestor, in
the primordial soup of green plants,” Mishler said. The team's data indicated
that the common ancestor was probably closely related to tiny green plants
known as coleochaetes, which still live in some of the world's pristine fresh
waters.
biologist from the University of California at Berkeley and one of the team
leaders, noted that green plants had likely invaded the land at many different
times. However, the team's analysis suggested that only one plant lineage
survived and eventually diversified into all known land plants. This finding
challenges a common view among biologists that land plants evolved from several
distinct ancestors, including for example one for mosses and another for
flowering plants. The new work “indicates there's an Eve, a common ancestor, in
the primordial soup of green plants,” Mishler said. The team's data indicated
that the common ancestor was probably closely related to tiny green plants
known as coleochaetes, which still live in some of the world's pristine fresh
waters.
The study also shed
light on the evolution of flowering plants, which first developed about 130
million years ago, team leaders said. Previous research suggested that
magnolias, or possibly water lilies, which have simple unspecialized flowers,
were the most primitive flowering plants still alive. The researchers, however,
identified a species called Amborella, which grows only in New Caledonia
and produces small, cream-colored flowers, as closest to the earliest flowering
plants.
light on the evolution of flowering plants, which first developed about 130
million years ago, team leaders said. Previous research suggested that
magnolias, or possibly water lilies, which have simple unspecialized flowers,
were the most primitive flowering plants still alive. The researchers, however,
identified a species called Amborella, which grows only in New Caledonia
and produces small, cream-colored flowers, as closest to the earliest flowering
plants.
Scientists said the
new classifications were important for understanding how plants evolved and
could also have practical benefits. Researchers searching for plants with
medicinal value, for example, might more easily identify close relatives of
plants now used medicinally, experts said. Other experts noted that knowledge
about where plants fit on the evolutionary tree could help weed-control
specialists devise more effective attacks against invasive plant species by
employing techniques that have proved effective against close relatives of the
pests.
new classifications were important for understanding how plants evolved and
could also have practical benefits. Researchers searching for plants with
medicinal value, for example, might more easily identify close relatives of
plants now used medicinally, experts said. Other experts noted that knowledge
about where plants fit on the evolutionary tree could help weed-control
specialists devise more effective attacks against invasive plant species by
employing techniques that have proved effective against close relatives of the
pests.
Tuesday, 10 March 2015
WHAT IS ABORTION
 |
| Explaining the reality of Abortion |
Abortion, termination of a pregnancy
before birth, resulting in the death of the fetus. Some abortions occur
naturally because a fetus does not develop normally or because the mother has
an injury or disorder that prevents her from carrying the pregnancy to term.
This type of spontaneous abortion is commonly known as a miscarriage. Other
abortions are induced—that is, intentionally brought on—because a pregnancy is
unwanted or presents a risk to a woman’s health, or because the fetus is likely
to have severe physical or mental health problems.
Induced abortion, the focus of this article, is one of
today’s most intense and polarizing ethical and philosophical issues. Modern
medical techniques have made induced abortions simpler and less dangerous. But
in the United States, the debate over abortion has led to legal battles in the
courts, in the Congress of the United States, and state legislatures. The
debate has spilled over into confrontations, which are sometimes violent, at
clinics where abortions are performed.
This article discusses the most common methods used to
induce abortions, the social and ethical issues surrounding abortion, and the
history of the regulation of abortion in the United States.
METHODS OF ABORTION
Induced abortions are
performed using drugs or surgery. The safest and most appropriate method is
determined by the age of the fetus, which is calculated from the beginning of
the pregnant woman’s last menstrual period. Most pregnancies last an average of
39 to 40 weeks. This period is divided into three stages known as trimesters.
The first trimester consists of the first 13 weeks, the second trimester spans
weeks 14 to 28, and the third trimester lasts from the 29th week to birth.
Abortions in the first trimester of pregnancy are easier and safer to perform
while abortions in the second and third trimesters require more complicated procedures
and pose greater risks to a woman’s health. In the United States, a pregnant
woman’s risk of death from a first-term abortion is less than 1 in 100,000. The
risk increases by about 30 percent with each week of pregnancy after 12 weeks.
DRUG – BASE ABORTION METHOD
Drug-based abortion, also
known as medication abortion, typically requires that a woman take two types of
drugs within the first weeks of a confirmed pregnancy. In one method, a
pregnant woman first takes the drug mifepristone, also known as RU-486, which
blocks progesterone, a hormone needed to maintain the pregnancy. About 48 hours
later, she takes another drug called misoprostol. Misoprostol is a
prostaglandin (a hormone-like chemical produced by the body) that causes
contractions of the uterus, the organ in which the fetus develops. These
uterine contractions expel the fetus.
Another type of drug combination
that induces abortion is the use of misoprostol with methotrexate, an anticancer
drug that interferes with cell division. A physician first injects a pregnant
woman with methotrexate. About a week later, the woman takes a pill containing
misoprostol to induce uterine contractions and expel the fetus.
These drug-based abortion methods
effectively end pregnancy in approximately 96 percent of the women who take
them and are most effective when performed very early in a pregnancy. These
methods require no anesthesia. However, the use of drugs to induce abortion has
not been widely adopted by women in the United States for a number of reasons.
These drugs can cause unpleasant side effects—some women experience nausea,
cramping, and bleeding. More serious complications, such as arrhythmia, edema,
and pneumonia, affect the heart and lungs and may cause death. Perhaps the
primary deterrent is that these drug-based abortion methods require at least
two visits to a physician over a period of several days, and these methods are
no cheaper than a surgical abortion.
SURGICAL METHOD
ABORTION
Legal surgical abortion,
when done by a trained provider, is essentially 100 percent effective. A number
of surgical methods can be used to induce abortions. To end a pregnancy before
it reaches eight weeks, a doctor typically performs a preemptive abortion or
an early uterine evacuation. In both procedures a narrow tube called a
cannula is inserted through the cervix (the opening to the uterus) into the
uterus. The cannula is attached to a suction device, such as a syringe, and the
contents of the uterus, including the fetus, are extracted. Preemptive abortion
uses a smaller cannula and is performed in the first four to six weeks of
pregnancy. Early uterine evacuation, which uses a slightly larger cannula, is
performed in the first six to eight weeks of pregnancy. Both types of abortions
typically require no anesthesia and can be performed in a clinic or physician’s
office. The entire procedure lasts for only several minutes. In preemptive
abortions the most common complication is infection. Women who undergo early
uterine evacuation may experience heavy bleeding for the first few days after
the procedure.
Vacuum aspiration is a procedure used for abortions
in the 6th to 14th week of pregnancy. It requires that the cervix be dilated,
or enlarged, so that a cannula can be inserted into the uterus. Progressively
larger, tapered instruments called dilators may be used to dilate the cervix.
During the procedure, the cannula is attached to an electrically powered pump
that removes the contents of the uterus. In some cases, the lining of the
uterus must also be scraped with a spoonlike tool called a curette to loosen
and remove tissue. This procedure is referred to as curettage. Vacuum
aspiration may require local anesthesia and can be performed in a clinic or
physician’s office. Minor bruising or injuries to the cervix may occur when the
cannula is inserted.
Dilation and curettage (D&C), performed during the 6th to 16th week of
pregnancy, involves dilating the cervix and then scraping the uterine lining
with a curette to remove the contents. A D&C often requires general
anesthesia and must be performed in a clinic or hospital. Possible
complications include a reaction to the anesthesia and cervical injuries. Since
the development of vacuum aspiration, the use of D&C has declined.
After the first 16 weeks
of pregnancy, abortion becomes more difficult. One method that can be used
during this period is dilation and evacuation (D&E), which requires greater
dilation of the cervix than other methods. It also requires the use of suction,
a large curette, and a grasping tool called a forceps to remove the fetus.
D&E is a complicated procedure because of the larger size of the fetus and
the thinner walls of the uterus, which stretch to accommodate a growing fetus.
Bleeding in the uterus often occurs. D&E is often performed under general
anesthesia in a clinic or hospital. It is typically used in the first weeks of
the second trimester but can be performed up to the 24th week of pregnancy.
An induction abortion can
also be performed in the second trimester, usually between the 16th and 24th
week of pregnancy. In this type of abortion a small amount of amniotic
fluid, the fluid that surrounds the fetus, is withdrawn and replaced with
another fluid. About 24 to 48 hours later, the uterus begins to contract and
the fetus is expelled. When this method was first developed, physicians used a
strong saline (salt) solution to abort the fetus; today they may also use
solutions containing prostaglandins or pitocin, a synthetic form of a
chemical produced by the pituitary gland that induces labor. Heavy bleeding,
infection, and injuries to the cervix can occur. This procedure is performed in
the hospital and requires a stay of one or more days.
Abortions performed at the end of
the second trimester and during the third trimester require major surgery. Two
such late-term procedures include hysterotomy and intact dilation and
extraction. In hysterotomy, the uterus is cut open and the fetus is removed
surgically in an operation similar to a cesarean section, but a hysterotomy
requires a smaller incision. Hysterotomy is major abdominal surgery performed
under general anesthesia.
Intact dilation and extraction,
also referred to as a partial birth abortion, consists of partially removing
the fetus from the uterus through the vaginal canal, feet first, and using
suction to remove the brain and spinal fluid from the skull. The skull is then
collapsed to allow complete removal of the fetus from the uterus.
SOCIAL AND ETHICAL
ISSUES
Abortion has become one
of the most widely debated ethical issues of our time. On one side are
pro-choice supporters—individuals who favor a woman’s reproductive rights,
including the right to choose to have an abortion. On the other side are the
pro-life advocates, who may oppose abortion for any reason or who may only
accept abortion in extreme circumstances, as when the mother’s life would be
threatened by carrying a pregnancy to term. At one end of this ethical spectrum
are pro-choice defenders who believe the fetus is only a potential human being
when it becomes viable, that is, able to survive outside its mother’s womb.
Until this time the fetus has no legal rights—the rights belong to the woman
carrying the fetus, who can decide whether or not to bring the pregnancy to
full term. At the other end of the spectrum are pro-life supporters who believe
the fetus is a human being from the time of conception. As such, the fetus has
the legal right to life from the moment the egg and sperm unite. Between these
positions lies a continuum of ethical, religious, and political positions.
A variety of ethical arguments
have been made on both sides of the abortion issue, but no consensus or
compromise has ever been reached because, in the public policy debate, the most
vocal pro-choice and pro-life champions have radically different views about
the status of a fetus. Embryology, the study of fetal development,
offers little insight about the fetus’s status at the time of conception,
further confounding the issue for both sides. In addition, the point during
pregnancy when a fetus becomes viable has changed over the years as medical
advances have made it possible to keep a premature baby alive at an earlier
stage. The current definition of viability is generally accepted at about 24
weeks gestation; a small percentage of babies born at about 22 weeks gestation
have been kept alive with intensive medical care. Despite the most advanced
medical care, however, babies born prematurely are more at risk for long-term
medical and developmental problems.
This combination of medical
ambiguities and emotional political confrontations has led to considerable
hostility in the abortion debate. For many people, however, the lines between
pro-choice and pro-life are blurred and the issue is far less polarized. Many
women who consider themselves pro-life supporters are concerned about possible
threats to reproductive rights and the danger of allowing the government to
decide what medical options are available to them. Similarly, many pro-choice
individuals are deeply saddened by the act of abortion and seek to minimize its
use through better education about birth control, and, in particular, emergency
contraception, birth-control methods that prevent pregnancy after unprotected
sexual intercourse.
REGULATION OF ABORTION
Abortion has been practiced around the world
since ancient times as a crude method of birth control. Although many religions
forbade or restricted the practice, abortion was not considered illegal in most
countries until the 19th century. There were laws prior to this time, however,
that banned abortion after quickening—that is, the time that fetal
movement can first be felt. In 1803 England banned all abortions, and this
policy soon spread to Asia, Africa, and Latin America. Throughout the middle
and late 1800s, many states in the United States enacted similar laws banning abortion.
In the 20th century, however, many nations began to relax their laws against
abortion. The former Union of Soviet Socialist Republics (USSR) legalized
abortion in 1920, followed by Japan in 1948, and several Eastern European
countries in the 1950s. In the 1960s and 1970s, much of Europe and Asia, along
with the United States, legalized abortion.
An estimated 46 million abortions are
performed worldwide each year, of which 20 million are performed in countries
where abortion is restricted or prohibited by law. Illegal abortions are more
likely to be performed by untrained people, in unsanitary conditions, or with
unsafe surgical procedures or drugs. As a result, illegal abortion accounts for
an estimated 78,000 deaths worldwide each year, or about one in seven
pregnancy-related deaths. In some African countries, illegal abortion may
contribute to up to 50 percent of pregnancy-related deaths. In Romania, where
abortion was outlawed from 1966 to 1989, an estimated 86 percent of
pregnancy-related deaths were caused by illegal abortion. In countries where
abortion is legal, less than 1 percent of pregnancy-related deaths are caused
by abortion.
LIGALIZATION OF
ABORTION IN UNITED STATES
An estimated 46 million abortions are
performed worldwide each year, of which 20 million are performed in countries
where abortion is restricted or prohibited by law. Illegal abortions are more
likely to be performed by untrained people, in unsanitary conditions, or with
unsafe surgical procedures or drugs. As a result, illegal abortion accounts for
an estimated 78,000 deaths worldwide each year, or about one in seven
pregnancy-related deaths. In some African countries, illegal abortion may
contribute to up to 50 percent of pregnancy-related deaths. In Romania, where
abortion was outlawed from 1966 to 1989, an estimated 86 percent of
pregnancy-related deaths were caused by illegal abortion. In countries where
abortion is legal, less than 1 percent of pregnancy-related deaths are caused
by abortion.
In 1976 the Supreme Court
recognized the right of pregnant girls under the age of 18, known as mature
minors, to have abortions. Three years later the Court ruled that states may
require the consent of one parent of a minor requesting an abortion. Parental
consent is not necessary if a confidential alternative form of review, such as
a judicial hearing, is made available for young women who choose not to involve
their parents. The Court stated that a judge in a hearing must approve a
minor’s abortion, in place of her parents, if the judge finds that the minor is
mature enough to make the decision on her own. If the judge finds that the
minor is not capable of making this decision on her own, he or she can decide
whether the abortion is in the minor’s best interest.
Since these decisions, about 40
states have enacted and enforced parental consent or notification laws,
although some laws have been contested in courts for years. In 1990, for
example, in Hodgson v. Minnesota, the Supreme Court upheld a law
requiring that prior notice be provided to both parents of a minor before an abortion
is performed. In a similar case arising in Ohio that same year, the court
upheld a requirement for notice or consent of one parent. In 2000, however, the
New Jersey Supreme Court struck down a law requiring parental notice for
unmarried girls under age 18.
Other state-imposed restrictions
regulate who pays for abortions, where abortions are performed, and what
information is provided to women seeking abortions. For example, in 1977 the
Supreme Court allowed states to limit the use of Medicaid funds (government
assistance for health care) for payment of elective abortions—that is,
those abortions not medically required. A law upheld by the Supreme Court in
1980 restricted the availability of federal Medicaid funding for abortions
deemed medically necessary. After that ruling, abortion payments for poor women
in many states were limited to cases in which pregnancy threatened the woman’s
life. Also in 1977, the Supreme Court allowed the city of St. Louis, Missouri,
to exclude elective abortions from procedures performed in a public hospital.
In 1983 the Court found
it unconstitutional to require that a woman considering an abortion be given
information developed by the state about risks or consequences and wait 24
hours after receiving information before having the abortion. Similarly, in
1986 the Court struck down a comprehensive Pennsylvania law requiring that
state-developed materials about abortion be offered to women undergoing the
procedure.
Since the 1989 Supreme Court
decision in Webster v. Reproductive Health Services, the Court
has permitted several state-imposed restrictions to stand. The Webster
case upheld a Missouri law that prohibits the use of public facilities or
public employees for abortion and requires a physician to determine the viability
of a fetus older than 20 weeks before performing an abortion. In the 1991 case
of Rust v. Sullivan, the Court upheld a federal policy that
prevented health-care providers who received federal funding from engaging in
any activities that encouraged or promoted abortion as a method of family
planning. President Bill Clinton later revoked this policy in 1993.
In 1992 the Supreme Court
decided Planned Parenthood of Southeastern Pennsylvania v. Casey,
a case in which the Court reaffirmed the central ruling of Roe v. Wade—that
no undue burden on access to abortion should exist for a woman over 18 years of
age prior to fetal viability. But the case also permitted states more freedom
in regulating abortion. The Court overturned prior rulings, making it possible
for states to again require that a woman be given state-developed information
about abortion risks and consequences and wait 24 hours before undergoing the
procedure.
In 1996 the Congress of
the United States enacted a bill banning the practice of so-called partial
birth abortions, also known as the intact dilation and extraction procedure.
President Clinton vetoed the law because it failed to permit use of the
procedure when a fetus displays severe abnormalities or when carrying a
pregnancy to term presents a serious threat to a woman’s health or life. Over
30 states passed laws in the 1990s banning use of the procedure.
In June 2000, in Stenberg
v. Carhart, the Supreme Court struck down a Nebraska ban on partial
birth abortion. The Court stated that the ban was an unconstitutional violation
of both Roe v. Wade and Planned Parenthood of Southeastern
Pennsylvania v. Casey. But after Congress passed the Partial Birth
Abortion Ban Act of 2003 and President George W. Bush signed it into law, the
Court revisited the issue in a 2007 ruling in Gonzales v. Planned
Parenthood and Gonzales v. Carhart. This time, with Justice
Samuel A. Alito, Jr. replacing the retired Justice Sandra Day O’Connor, the
Court upheld the ban on the partial birth abortion procedure in a 5 to 4
decision. Under the law, physicians who perform the banned procedure could face
fines and up to two years in prison. The law allows for use of intact dilation
and extraction only in cases where the mother’s life is endangered without the
procedure.
Since the Supreme Court ruling
that legalized abortion in 1973, opponents of abortion have worked continuously
to reverse the decision. They have lobbied state and federal officials to place
restrictions on women seeking abortions or on individuals providing abortions.
They have also held protests directed at clinics that perform abortions and, in
some cases, have accosted and obstructed patients and health-care providers at
such clinics. In May 1994 the Freedom of Access to Clinic Entrances Act was
enacted, which made it a federal crime to use force, threat of force, or
physical obstruction to injure, intimidate, or interfere with reproductive
health-care providers and their patients. That same year, in a case known as Madsen
v. Women’s Health Center, the Supreme Court upheld the basic right
to protest in peaceful, organized demonstrations outside abortion clinics. But
the case upheld a Florida law that created a 36 ft (11 m) buffer zone around a
clinic to ensure that demonstrations do not prevent access to clinics or
disrupt clinic operations. In February 1997 the Court upheld buffer zones
around clinics but struck down certain floating, or moveable, buffer zones
around individuals approaching clinics.
The Supreme Court’s ruling in
2007 upholding the federal Partial Birth Abortion Ban Act of 2003 was expected
to spur further attempts to restrict abortion, if not overturn Roe v. Wade.
Supporters of the right to abortion noted that the Court’s majority opinion in
the 2007 decision represented the first time since the 1973 Roe ruling
that the Court permitted a ban on an abortion procedure, effectively intruding
on the privacy of a decision between a woman and her physician. Supporters of
the ruling countered that the Court’s decision addressed the moral and ethical
concerns put forward by opponents of abortion. They cited Justice Anthony
Kennedy’s argument in the majority opinion that “the government has a
legitimate and substantial interest in preserving and promoting fetal life.”
Monday, 9 March 2015
Cell
 |
| Cells |
What is cell?
Cell (biology), basic unit of life.
Cells are the smallest structures capable of basic life processes, such as
taking in nutrients, expelling waste, and reproducing. All living things are
composed of cells. Some microscopic organisms, such as bacteria and protozoa,
are unicellular, meaning they consist of a single cell. Plants, animals, and
fungi are multicellular; that is, they are composed of a great many cells
working in concert. But whether it makes up an entire bacterium or is just one
of trillions in a human being, the cell is a marvel of design and efficiency.
Cells carry out thousands of biochemical reactions each minute and reproduce
new cells that perpetuate life.
Cells vary considerably in size. The smallest cell, a
type of bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in
diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a
human hair. Among the largest cells are the nerve cells that run down a
giraffe’s neck; these cells can exceed 3 m (9.7 ft) in length. Human cells also
display a variety of sizes, from small red blood cells that measure 0.00076 mm
(0.00003 in) to liver cells that may be ten times larger. About 10,000
average-sized human cells can fit on the head of a pin.
Along with their differences in size, cells present
an array of shapes. Some, such as the bacterium Escherichia coli,
resemble rods. The paramecium, a type of protozoan, is slipper shaped; and the
amoeba, another protozoan, has an irregular form that changes shape as it moves
around. Plant cells typically resemble boxes or cubes. In humans, the outermost
layers of skin cells are flat, while muscle cells are long and thin. Some nerve
cells, with their elongated, tentacle-like extensions, suggest an octopus.
In multicellular organisms, shape is typically tailored
to the cell’s job. For example, flat skin cells pack tightly into a layer that
protects the underlying tissues from invasion by bacteria. Long, thin muscle
cells contract readily to move bones. The numerous extensions from a nerve cell
enable it to connect to several other nerve cells in order to send and receive
messages rapidly and efficiently.
By itself, each cell is a model of
independence and self-containment. Like some miniature, walled city in
perpetual rush hour, the cell constantly bustles with traffic, shuttling
essential molecules from place to place to carry out the business of living.
Despite their individuality, however, cells also display a remarkable ability
to join, communicate, and coordinate with other cells. The human body, for
example, consists of an estimated 20 to 30 trillion cells. Dozens of
different kinds of cells are organized into specialized groups called tissues.
Tendons and bones, for example, are composed of connective tissue, whereas skin
and mucous membranes are built from epithelial tissue. Different tissue types
are assembled into organs, which are structures specialized to perform
particular functions. Examples of organs include the heart, stomach, and brain.
Organs, in turn, are organized into systems such as the circulatory, digestive,
or nervous systems. All together, these assembled organ systems form the human
body.
The components of cells are molecules, nonliving
structures formed by the union of atoms. Small molecules serve as building
blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and
lipids, which include fats and oils, are the four major molecules that underlie
cell structure and also participate in cell functions. For example, a tightly
organized arrangement of lipids, proteins, and protein-sugar compounds forms
the plasma membrane, or outer boundary, of certain cells. The organelles,
membrane-bound compartments in cells, are built largely from proteins.
Biochemical reactions in cells are guided by enzymes, specialized proteins that
speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA)
contains the hereditary information for cells, and another nucleic acid,
ribonucleic acid(RNA), works with DNA to build the thousands of proteins the
cell needs.
Photosynthesis
 |
| Photosynthesis |
What is Photosynthesis?
Photosynthesis, process by which green
plants and certain other organisms use the energy of light to convert carbon
dioxide and water into the simple sugar glucose. In so doing, photosynthesis
provides the basic energy source for virtually all organisms. An extremely
important byproduct of photosynthesis is oxygen, on which most organisms
depend.
Photosynthesis occurs
in green plants, seaweeds, algae, and certain bacteria. These organisms are
veritable sugar factories, producing millions of new glucose molecules per
second. Plants use much of this glucose, a carbohydrate, as an energy source to
build leaves, flowers, fruits, and seeds. They also convert glucose to
cellulose, the structural material used in their cell walls. Most plants
produce more glucose than they use, however, and they store it in the form of
starch and other carbohydrates in roots, stems, and leaves. The plants can then
draw on these reserves for extra energy or building materials. Each year,
photosynthesizing organisms produce about 170 billion metric tons of extra
carbohydrates, about 30 metric tons for every person on earth.
Photosynthesis has far-reaching
implications. Like plants, humans and other animals depend on glucose as an
energy source, but they are unable to produce it on their own and must rely
ultimately on the glucose produced by plants. Moreover, the oxygen humans and
other animals breathe is the oxygen released during photosynthesis. Humans are
also dependent on ancient products of photosynthesis, known as fossil fuels,
for supplying most of our modern industrial energy. These fossil fuels,
including natural gas, coal, and petroleum, are composed of a complex mix of
hydrocarbons, the remains of organisms that relied on photosynthesis millions
of years ago. Thus, virtually all life on earth, directly or indirectly,
depends on photosynthesis as a source of food, energy, and oxygen, making it one
of the most important biochemical processes known.
WHERE PHOTOSYNTHESIS OCCUR
Plant photosynthesis occurs
in leaves and green stems within specialized cell structures called
chloroplasts. One plant leaf is composed of tens of thousands of cells, and
each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped
structure, is divided by membranes into numerous disk-shaped compartments.
These disklike compartments, called thylakoids, are arranged vertically in the
chloroplast like a stack of plates or pancakes. A stack of thylakoids is called
a granum (plural, grana); the grana lie suspended in a fluid
known as stroma.
Embedded in the membranes
of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping
pigment required for photosynthesis. Additional light-trapping pigments,
enzymes (organic substances that speed up chemical reactions), and other
molecules needed for photosynthesis are also located within the thylakoid
membranes. The pigments and enzymes are arranged in two types of units,
Photosystem I and Photosystem II. Because a chloroplast may have dozens of
thylakoids, and each thylakoid may contain thousands of photosystems, each
chloroplast will contain millions of pigment molecules.
HOW PHOTOSYNTHESIS
WORKS
Photosynthesis is a very
complex process, and for the sake of convenience and ease of understanding,
plant biologists divide it into two stages. In the first stage, the
light-dependent reaction, the chloroplast traps light energy and converts it
into chemical energy contained in nicotinamide adenine dinucleotide phosphate
(NADPH) and adenosine triphosphate (ATP), two molecules used in the second
stage of photosynthesis. In the second stage, called the light-independent
reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms
that help form glucose, and ATP provides the energy for this and other
reactions used to synthesize glucose. These two stages reflect the literal
meaning of the term photosynthesis, to build with light.
LIGHT DEPENDENT
REACTION
Photosynthesis relies
on flows of energy and electrons initiated by light energy. Electrons are
minute particles that travel in a specific orbit around the nuclei of atoms and
carry a small electrical charge. Light energy causes the electrons in
chlorophyll and other light-trapping pigments to boost up and out of their
orbit; the electrons instantly fall back into place, releasing resonance
energy, or vibrating energy, as they go, all in millionths of a second. Chlorophyll
and the other pigments are clustered next to one another in the photosystems,
and the vibrating energy passes rapidly from one chlorophyll or pigment
molecule to the next, like the transfer of energy in billiard balls.
Light contains many colors,
each with a defined range of wavelengths measured in nanometers, or billionths
of a meter. Certain red and blue wavelengths of light are the most effective in
photosynthesis because they have exactly the right amount of energy to
energize, or excite, chlorophyll electrons and boost them out of their orbits
to a higher energy level. Other pigments, called accessory pigments, enhance
the light-absorption capacity of the leaf by capturing a broader spectrum of
blue and red wavelengths, along with yellow and orange wavelengths. None of the
photosynthetic pigments absorb green light; as a result, green wavelengths are
reflected, which is why plants appear green.
Photosynthesis begins when light strikes
Photosystem I pigments and excites their electrons. The energy passes rapidly
from molecule to molecule until it reaches a special chlorophyll molecule
called P700, so named because it absorbs light in the red region of the
spectrum at wavelengths of 700 nanometers.
Until this point, only energy
has moved from molecule to molecule; now electrons themselves transfer between
molecules. P700 uses the energy of the excited electrons to boost its own
electrons to an energy level that enables an adjoining electron acceptor
molecule to capture them. The electrons are then passed down a chain of carrier
molecules, called an electron transport chain. The electrons are passed from
one carrier molecule to another in a downhill direction, like individuals in a
bucket brigade passing water from the top of a hill to the bottom. Each
electron carrier is at a lower energy level than the one before it, and the
result is that electrons release energy as they move down the chain. At the end
of the electron transport chain lies the molecule nicotine adenine dinucleotide
(NADP+). Using the energy released by the flow of electrons, two
electrons from the electron transport chain combine with a hydrogen ion and
NADP+ to form NADPH.
When P700 transfers its
electrons to the electron acceptor, it becomes deficient in electrons. Before
it can function again, it must be replenished with new electrons. Photosystem
II accomplishes this task. As in Photosystem I, light energy activates
electrons of the Photosystem II pigments. These pigments transfer the energy of
their excited electrons to a special Photosystem II chlorophyll molecule, P680,
that absorbs light best in the red region at 680 nanometers. Just as in
Photosystem I, energy is transferred among pigment molecules and is then
directed to the P680 chlorophyll, where the energy is used to transfer
electrons from P680 to its adjoining electron acceptor molecule.
From the Photosystem II
electron acceptor, the electrons are passed through a different electron
transport chain. As they pass along the cascade of electron carrier molecules,
the electrons give up some of their energy to fuel the production of ATP,
formed by the addition of one phosphorus atom to adenosine diphosphate (ADP).
Eventually, the electron transport carrier molecules deliver the Photosystem II
electrons to Photosystem I, which uses them to maintain the flow of electrons
to P700, thus restoring its function.
P680 in Photosystem II is now
electron deficient because it has donated electrons to P700 in Photosystem I.
P680 electrons are replenished by the water that has been absorbed by the plant
roots and transported to the chloroplasts in the leaves. The movement of
electrons in Photosystems I and II and the action of an enzyme split the water
into oxygen, hydrogen ions, and electrons. The electrons from water flow to
Photosystem II, replacing the electrons lost by P680. Some of the hydrogen ions
may be used to produce NADPH at the end of the electron transport chain, and
the oxygen from the water diffuses out of the chloroplast and is released into
the atmosphere through pores in the leaf.
The transfer of electrons in a
step-by-step fashion in Photosystems I and II releases energy and heat slowly,
thus protecting the chloroplast and cell from a harmful temperature increase.
It also provides time for the plant to form NADPH and ATP. In the words of
American biochemist and Nobel laureate Albert Szent-Gyorgyi, “What drives life
is thus a little electric current, set up by the sunshine.”
THE LIGHT –
INDEPENDENT REACTION
The chemical energy required
for the light-independent reaction is supplied by the ATP and NADPH molecules
produced in the light-dependent reaction. The light-independent reaction is
cyclic, that is, it begins with a molecule that must be regenerated at the end
of the reaction in order for the process to continue. Termed the Calvin cycle
after the American chemist Melvin Calvin who discovered it, the
light-independent reactions use the electrons and hydrogen ions associated with
NADPH and the phosphorus associated with ATP to produce glucose. These
reactions occur in the stroma, the fluid in the chloroplast surrounding the
thylakoids, and each step is controlled by a different enzyme.
The light-independent reaction requires
the presence of carbon dioxide molecules, which enter the plant through pores
in the leaf, diffuse through the cell to the chloroplast, and disperse in the
stroma. The light-independent reaction begins in the stroma when these carbon
dioxide molecules link to sugar molecules called ribulose bisphosphate (RuBP)
in a process known as carbon fixation.
With the help of an enzyme,
six molecules of carbon dioxide bond to six molecules of RuBP to create six new
molecules. Several intermediate steps, which require ATP, NADPH, and additional
enzymes, rearrange the position of the carbon, hydrogen, and oxygen atoms in
these six molecules, and when the reactions are complete, one new molecule of
glucose has been constructed and five molecules of RuBP have been
reconstructed. This process occurs repeatedly in each chloroplast as long as
carbon dioxide, ATP, and NADPH are available. The thousands of glucose
molecules produced in this reaction are processed by the plant to produce
energy in the process known as aerobic respiration, used as structural
materials, or stored. The regenerated RuBP is used to start the Calvin cycle
all over again.
Friction
 |
| FRICTION |
What is Friction?
Friction, force that opposes the
motion of an object when the object is in contact with another object or
surface. Friction results from two surfaces rubbing against each other or
moving relative to one another. It can hinder the motion of an object or
prevent an object from moving at all. The strength of frictional force depends
on the nature of the surfaces that are in contact and the force pushing them
together. This force is usually related to the weight of the object or objects.
In cases involving fluid friction, the force depends upon the shape and speed
of an object as it moves through air, water, or other fluid.
Friction occurs to some degree in almost all
situations involving physical objects. In many cases, such as in a running
automobile engine, it hinders a process. For example, friction between the
moving parts of an engine resists the engine’s motion and turns energy into
heat, reducing the engine’s efficiency. Friction also makes it difficult to
slide a heavy object, such as a refrigerator or bookcase, along the ground. In
other cases, friction is helpful. Friction between people’s shoes and the
ground allows people to walk by pushing off the ground without slipping. On a
slick surface, such as ice, shoes slip and slide instead of gripping because of
the lack of friction, making walking difficult. Friction allows car tires to
grip and roll along the road without skidding. Friction between nails and beams
prevents the nails from sliding out and keeps buildings standing.
When friction affects a moving object, it turns the
object’s kinetic energy, or energy of motion, into heat. People welcome the
heat caused by friction when rubbing their hands together to stay warm.
Frictional heat is not so welcome when it damages machine parts, such as car
brakes.
CAUSES OF FRICTION
Friction occurs in part
because rough surfaces tend to catch on one another as they slide past each
other. Even surfaces that are apparently smooth can be rough at the microscopic
level. They have many ridges and grooves. The ridges of each surface can get
stuck in the grooves of the other, effectively creating a type of mechanical
bond, or glue, between the surfaces.
Two surfaces in contact
also tend to attract one another at the molecular level, forming chemical bonds.
These bonds can prevent an object from moving, even when it is pushed. If an
object is in motion, these bonds form and release. Making and breaking the
bonds takes energy away from the motion of the object.
Scientists do not yet fully understand the details
of how friction works, but through experiments they have found a way to
describe frictional forces in a wide variety of situations. The force of
friction between an object and a surface is equal to a constant number times
the force the object exerts directly on the surface. The constant number is
called the coefficient of friction for the two materials and is
abbreviated µ. The force the object exerts directly on the surface is called
the normal force and is abbreviated N. Friction depends on this force
because increasing the amount of force increases the amount of contact that the
object has with the surface at the microscopic level. The force of friction
between an object and a surface can be calculated from the following formula:
F = µ × N
In this equation, F is the force of
friction, µ is the coefficient of friction between the object and the
surface, and N is the normal force.
Scientists have measured the coefficient of friction for
many combinations of materials. Coefficients of friction depend on whether the
objects are initially moving or stationary and on the types of material
involved. The coefficient of friction for rubber sliding on concrete is 0.8
(relatively high), while the coefficient for Teflon sliding on steel is 0.04
(relatively low).
The normal force is the force the object
exerts perpendicular to the surface. In the case of a level surface, the normal
force is equal to the weight of the object. If the surface is inclined, only a
fraction of the object’s weight pushes directly into the surface, so the normal
force is less than the object’s weight.
KINDS OF FRICTION
Different kinds of motion give rise to
different types of friction between objects. Static friction occurs between
stationary objects, while sliding friction occurs between objects as they slide
against each other. Other types of friction include rolling friction and fluid
friction. The coefficient of friction for two materials may differ depending on
the type of friction involved.
Static friction prevents
an object from moving against a surface. It is the force that keeps a book from
sliding off a desk, even when the desk is slightly tilted, and that allows you
to pick up an object without the object slipping through your fingers. In order
to move something, you must first overcome the force of static friction between
the object and the surface on which it is resting. This force depends on the
coefficient of static friction (µs) between the object and
the surface and the normal force (N) of the object.
A book sliding off a desk or brakes
slowing down a wheel are both examples of sliding friction, also called kinetic
friction. Sliding friction acts in the direction opposite the direction of
motion. It prevents the book or wheel from moving as fast as it would without
friction. When sliding friction is acting, another force must be present to
keep an object moving. In the case of a book sliding off a desk, this force is
gravity. The force of kinetic friction depends on the coefficient of kinetic
friction between the object and the surface on which it is moving (µk)
and the normal force (N) of the object. For any pair of objects, the
coefficient of kinetic friction is usually less than the coefficient of static
friction. This means that it takes more force to start a book sliding than it
does to keep the book sliding.
Rolling friction hinders the motion
of an object rolling along a surface. Rolling friction slows down a ball
rolling on a basketball court or softball field, and it slows down the motion
of a tire rolling along the ground. Another force must be present to keep an
object rolling. For example, a pedaling bicyclist provides the force necessary
to the keep a bike in motion. Rolling friction depends on the coefficient of
rolling friction between the two materials (µr) and the
normal force (N) of the object. The coefficient of rolling friction is
usually about t that of sliding friction. Wheels and other round objects will
roll along the ground much more easily than they will slide along it.
Objects moving through a fluid
experience fluid friction, or drag. Drag acts between the object and the
fluid and hinders the motion of the object. The force of drag depends upon the
object’s shape, material, and speed, as well as the fluid’s viscosity.
Viscosity is a measure of a fluid’s resistance to flow. It results from the
friction that occurs between the fluid’s molecules, and it differs depending on
the type of fluid. Drag slows down airplanes flying through the air and fish
swimming through water. An airplane’s engines help it overcome drag and travel
forward, while a fish uses its muscles to overcome drag and swim. Calculating
the force of drag is much more complicated than calculating other types of
friction. (see Aerodynamics)
EFFECTS OF FRICTION
Friction helps people
convert one form of motion into another. For example, when people walk,
friction allows them to convert a push backward along the ground into forward
motion. Similarly, when car or bicycle tires push backward along the ground,
friction with the ground makes the tires roll forward. Friction allows us to
push and slide objects along the ground without our shoes slipping along the
ground in the opposite direction.
While friction allows us to convert one form of motion
to another, it also converts some energy into heat, noise, and wear and tear on
material. Losing energy to these effects often reduces the efficiency of a
machine. For example, a cyclist uses friction between shoes and pedals, the
chain and gears, and the bicycle’s tires and the road to make the bicycle move
forward. At the same time, friction between the chain and gears, between the
tires and the road, and between the cyclist and the air all resist the
cyclist’s motion. As the cyclist pedals, friction converts some of the
cyclist’s energy into heat, noise, and wear and tear on the bicycle. This
energy loss reduces the efficiency of the bicycle. In automobiles and
airplanes, friction converts some of the energy in the fuel into heat, noise,
and wear and tear on the engine’s parts. Excess frictional heat can damage an
engine and braking system. The wearing away of material in engines makes it
necessary to periodically replace some parts.
Sometimes the heat that friction produces is
useful. When a person strikes a match against a rough surface, friction
produces a large amount of heat on the head of the match and triggers the
chemical process of burning. Static friction, which prevents motion, does not
create heat.
REDUCING FRICTION
Reducing the amount of
friction in a machine increases the machine’s efficiency. Less friction means
less energy lost to heat, noise, and wearing down of material. People normally
use two methods to reduce friction. The first method involves reducing the
roughness of the surfaces in contact. For example, sanding two pieces of wood
lessens the amount of friction that occurs between them when they slide against
one another. Teflon creates very little friction because it is so smooth.
Applying a lubricant to a surface can also reduce
friction. Common examples of lubricants are oil and grease. They reduce
friction by minimizing the contact between rough surfaces. The lubricant’s
particles slide easily against each other and cause far less friction than
would occur between the surfaces. Lubricants such as machine oil reduce the
amount of energy lost to frictional heating and reduce the wear damage to the
machine surfaces caused by friction.
Digestion
 |
| Digestion in Human |
What do you understand by Digestion?
INTRODUCTION
Digestive System, series of connected
organs whose purpose is to break down, or digest, the food we eat. Food is made
up of large, complex molecules, which the digestive system breaks down into
smaller, simple molecules that can be absorbed into the bloodstream. The simple
molecules travel through the bloodstream to all of the body's cells, which use
them for growth, repair, and energy.
All animals have a digestive
system, a feature that distinguishes them from plants. Plants produce their own
food in a process called photosynthesis, during which they use sunlight to
convert water and carbon dioxide into simple sugars. But animals, including
humans, must take in food in the form of organic matter, such as plants or
other animals.
Digestion generally involves two phases: a mechanical
phase and a chemical phase. In the mechanical phase, teeth or other structures
physically break down large pieces of food into smaller pieces. In the chemical
phase, digestive chemicals called enzymes break apart individual molecules of
food to yield molecules that can be absorbed and distributed throughout the
body. These enzymes are secreted (produced and released) by glands in
the body.
The digestive system of most animals consists
mainly of a long, continuous tube called the alimentary canal, or digestive
tract. This canal has a mouth at one end, through which food is taken in, and
an anus at the other end, through which digestive wastes are excreted. Muscles
in the walls of the alimentary canal move the food along. Most digestive organs
are part of the alimentary canal. However, two accessory digestive organs, the
liver and pancreas, are located outside the alimentary canal. These organs
contribute to chemical digestion by releasing digestive juices into the canal
through tubes called ducts.
DIGESTION IN HUMAN
If a human adult’s digestive
tract were stretched out, it would be 6 to 9 m (20 to 30 ft) long. In humans,
digestion begins in the mouth, where both mechanical and chemical digestion
occur. The mouth quickly converts food into a soft, moist mass. The muscular
tongue pushes the food against the teeth, which cut, chop, and grind the food.
Glands in the cheek linings secrete mucus, which lubricates the food, making it
easier to chew and swallow. Three pairs of glands empty saliva into the mouth
through ducts to moisten the food. Saliva contains the enzyme ptyalin, which
begins to hydrolyze (break down) starch—a carbohydrate manufactured by
green plants.
Once food has been reduced
to a soft mass, it is ready to be swallowed. The tongue pushes this mass—called
a bolus—to the back of the mouth and into the pharynx. This cavity between the
mouth and windpipe serves as a passageway both for food on its way down the
alimentary canal and for air passing into the windpipe. The epiglottis, a flap
of cartilage, covers the trachea (windpipe) when a person swallows. This action
of the epiglottis prevents choking by directing food from the windpipe and
toward the stomach.
THE ESOPHAGUS
The presence of food in
the pharynx stimulates swallowing, which squeezes the food into the esophagus.
The esophagus, a muscular tube about 25 cm (10 in) long, passes behind the
trachea and heart and penetrates the diaphragm (muscular wall between the chest
and abdomen) before reaching the stomach. Food advances through the alimentary
canal by means of rhythmic muscle contractions (tightenings) known as
peristalsis. The process begins when circular muscles in the esophagus wall
contract and relax (widen) one after the other, squeezing food downward toward
the stomach. Food travels the length of the esophagus in two to three seconds.
A circular muscle called the
esophageal sphincter separates the esophagus and the stomach. As food is
swallowed, this muscle relaxes, forming an opening through which the food can
pass into the stomach. Then the muscle contracts, closing the opening to
prevent food from moving back into the esophagus. The esophageal sphincter is
the first of several such muscles along the alimentary canal. These muscles act
as valves to regulate the passage of food and keep it from moving backward.
THE STOMACH
The stomach, located in
the upper abdomen just below the diaphragm, is a saclike structure with strong,
muscular walls. The stomach can expand significantly to store all the food from
a meal for both mechanical and chemical processing. The stomach contracts about
three times per minute, churning the food and mixing it with gastric juice. This
fluid, secreted by thousands of gastric glands in the lining of the stomach,
consists of water, hydrochloric acid, an enzyme called pepsin, and mucin
(the main component of mucus). Hydrochloric acid creates the acidic environment
that pepsin needs to begin breaking down proteins. It also kills microorganisms
that may have been ingested in the food. Mucin coats the stomach, protecting it
from the effects of the acid and pepsin. About four hours or less after a meal,
food processed by the stomach, called chyme, begins passing a little at a time
through the pyloric sphincter into the duodenum, the first portion of the small
intestine.
THE SMALL INTESTINE
Most digestion, as well as
absorption of digested food, occurs in the small intestine. This narrow,
twisting tube, about 2.5 cm (1 in) in diameter, fills most of the lower
abdomen, extending about 6 m (20 ft) in length. Over a period of three to six
hours, peristalsis moves chyme through the duodenum into the next portion of
the small intestine, the jejunum, and finally into the ileum, the last section
of the small intestine. During this time, the liver secretes bile into the
small intestine through the bile duct. Bile breaks large fat globules into
small droplets, which enzymes in the small intestine can act upon. Pancreatic
juice, secreted by the pancreas, enters the small intestine through the
pancreatic duct. Pancreatic juice contains enzymes that break down sugars and
starches into simple sugars, fats into fatty acids and glycerol, and proteins
into amino acids. Glands in the intestinal walls secrete additional enzymes
that break down starches and complex sugars into nutrients that the intestine
absorbs. Structures called Brunner’s glands secrete mucus to protect the
intestinal walls from the acid effects of digestive juices.The small
intestine’s capacity for absorption is increased by millions of fingerlike
projections called villi, which line the inner walls of the small intestine.
Each villus is about 0.5 to 1.5 mm (0.02 to 0.06 in) long and covered with a
single layer of cells. Even tinier fingerlike projections called microvilli
cover the cell surfaces. This combination of villi and microvilli increases the
surface area of the small intestine’s lining by about 150 times, multiplying
its capacity for absorption. Beneath the villi’s single layer of cells are
capillaries (tiny vessels) of the bloodstream and the lymphatic system. These
capillaries allow nutrients produced by digestion to travel to the cells of the
body. Simple sugars and amino acids pass through the capillaries to enter the
bloodstream. Fatty acids and glycerol pass through to the lymphatic system.
THE LARGE INTESTINE
A watery residue of indigestible
food and digestive juices remains unabsorbed. This residue leaves the ileum of
the small intestine and moves by peristalsis into the large intestine, where it
spends 12 to 24 hours. The large intestine forms an inverted U over the coils
of the small intestine. It starts on the lower right-hand side of the body and
ends on the lower left-hand side. The large intestine is 1.5 to 1.8 m (5 to 6
ft) long and about 6 cm (2.5 in) in diameter.The large intestine serves several
important functions. It absorbs water—about 6 liters (1.6 gallons) daily—as
well as dissolved salts from the residue passed on by the small intestine. In
addition, bacteria in the large intestine promote the breakdown of undigested
materials and make several vitamins, notably vitamin K, which the body needs
for blood clotting. The large intestine moves its remaining contents toward the
rectum, which makes up the final 15 to 20 cm (6 to 8 in) of the alimentary
canal. The rectum stores the feces—waste material that consists largely of
undigested food, digestive juices, bacteria, and mucus—until elimination. Then,
muscle contractions in the walls of the rectum push the feces toward the anus.
When sphincters between the rectum and anus relax, the feces pass out of the
body.
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