cell 20011
TRANSCRIPT
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The Plasma Membrane
is the boundary between the inside of the
cell, termed the cytoplasm, and the
outside of the cell.
Her integrity is necessary to the life of
the cell.
is a phospholipid bilayer with attached orembedded proteins.
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The Plasma Membrane
At body temperature, the phospholipid bilayer is a
liquid; it has the consistency of olive oil, and the
proteins are able to change their positions bymoving laterally.
Our plasma membranes also contain a substantial
number of cholesterol molecules. These molecules
lend stability to the phospholipid bilayer andprevent a drastic decrease in fluidity at low
temperatures.
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The Plasma Membrane
Short chains of sugars are attached to the outer
surfaces of some protein and lipid molecules
(called glycoproteins and glycolipids,respectively).
These carbohydrate chains, specific to each cell,
mark the cell as belonging to a particularindividual and they account for such
characteristics as blood type or why a patient's
system sometimes rejects an organ transplant.
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Importance of the cell membrane: Holds the contents of the cell in place.
Contact inhibition: A cell will not invade another
cell's territory.
Selective permeability: The cell membrane allowsdesirable substances to come into the cell/ but keeps
undesirable substances out of the cell.
Receptor sites for hormones: Important for
hormones and neurotransmitters.
Identification: Surface proteins serve as positive
identification tags since they occur only in the cells
of that individual.
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The nucleus
The nucleus is the control center for the
cell.
It contains deoxyribonucleic acid (DNA)
that is essential to the cell because its genes
contain the information necessary for the
synthesis of the various proteins that the cellmust produce to stay alive.
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The nucleus
These proteins include enzymes that are
used to synthesize other substances,
including carbohydrates and lipids, made bythe cell.
The genes also represent the individual
units of inheritance that transmitinformation from one generation to another.
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The DNA
Each DNA molecule is made up of two extremely long
double-stranded helical chains containing variable
sequences of four nitrogenous bases.
These bases form the genetic code. Within the nucleus, each double-stranded DNA molecule is
periodically coiled about basic proteins called histones,
forming regularly spaced spherical structures called
nucleosomes that are similar to beads on a string. The stringof beads is further wound into filaments that make up the
structure of chromatin. Further coiling produces structures
known as chromosomes, which are visible during cell
division.
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Endomembrane System
The endoplasmic reticulum, the Golgi
apparatus, lysosomes, and peroxisomes
are structurally and functionally relatedmembranous structures that comprise
the endomembrane system. They work
together to produce, transport, store, orsecrete cellular products.
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Importance of RNA
During the process cal1ed transcription the
double stranded DNA unwinds and a special
type of RNA (messenger RNA) isformed.Thus DNA has passed on its pattern
to RNA.
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TYPES OF RNA
1. Messenger RNA (mRNA): Takes the
blueprint from the nucleus to the ribosomes.
2. Transfer RNA (tRNA): Brings individual
amino acids to the ribosomes to complete the
pattern.
3. Ribosomal RNA (rRNA): A subunit
structure where mRNA and tRNA complet their
work.
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The Golgi apparatus
The Golgi apparatus is especially well
developed in cells that secrete (export) a
product
for example, in the pancreatic cellsthat- make digestive enzymes or the bronchial
cells that produce mucus.
When the Golgi apparatus packages a product
for export, the product is enclosed within a
vesicle that moves toward the plasma
membrane, where the vesicle discharges its
contents.
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Peroxisomes
Peroxisomes (pe-roks'i-somz) are membranous sacs
that contain enzymes. Hydrogen peroxide, a normal
product of metabolism, is harmful if allowed to
accumulate. Peroxisomal enzymes break down hydrogen
peroxide to water and molecular oxygen.
In the liver and kidney, peroxisomes detoxify
harmful substances, such as formaldehyde andalcohol.
Peroxisomes are believed to bud directly from the
smooth ER.
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Plasma Membrane Transport The plasma membrane allows only certain
molecules to enter and exit the cytoplasm freely;
therefore, the plasma membrane is said to be
selectively permeable. Transport of molecules across the membrane
can be active or passive.
Active transport requires the use of ATP energy;passive transport does not.
Passive transport mechanisms include diffusion,
osmosis, and filtration.
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Diffusion
Diffusion is the passive movement of molecules
from an area of higher concentration to an area
of lower concentration. In the body, oxygen enters the blood from the
alveoli (air sacs) of the lungs by diffusion.
Cells do not expend any energy whensubstances can simply diffuse across the plasma
membrane.
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Osmosis Osmosis (oz-mo'sis) is the diffusion of water
across a plasma membrane. It occurs whenever
the concentrations of water on either side of a
selectively permeable membrane are unequal. Normally, body fluids are isotonic (i"so-ton'ik)
to cells; that is, the concentrations of substances
(solutes) and water (solvent) on either side of the
plasma membrane are equal, and cells,
therefore, maintain their usual size and shape,
For this reason, most intravenous solutions are
also isotonic to cells.
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Filtration
Filtration of water and substances in the
region of capillaries is largely responsible
for the formation of tissue fluid, the fluidthat surrounds the cells.
Filtration is also at work in the kidneys
when water and small molecules move fromthe blood to the inside of the kidney tubules.
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Active Transport In active transport, substances accumulate either inside or outside
the cell in the region of higher concentration.
For example:
iodine collects in the cells of the thyroid gland;
sugar is completely absorbed from the gut by the cells lining the digestive
tract;
sodium is sometimes almost completely withdrawn from urine by cells lining
the kidney tubules.
Carrier proteins and an expenditure of energy are both needed totransport substances from an area of lower concentration to an area
of higher concentration. A carrier is a plasma membrane protein that
specializes in combining with and transporting substances across
the plasma membrane.
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Endocytosis
At times, substances are taken into cells by vesicle
formation. This is called endocytosis (en"do-si-to'sis).
When the material taken in is quite large, the process iscalled phagocytosis (fag"o-si-to'sis) (cell eating).
Phagocytosis is common to amoeboid-type cells, such as
macrophages, called the body's scavengers because they
engulf worn-out red blood cells and other types of debris. When cells take in material that is small enough to be
dissolved or suspended in water, the process is called
pinocytosis (pi"no-si-to'sis) (cell drinking).
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Exocytosis
Vesicles within the cytoplasm of the cell can
fuse with the plasma membrane and release
their contents lo the outside of the cell. This iscalled exocytosis (ex^o-si-to'sis).
Some cells of the nervous system release
substances involved in the transfer of nerveimpulses between adjacent cells via exocytosis.
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Cell Division
The two types of cell division are termed
mitosis and meiosis.
Mitosis occurs during growth and repair.
Meiosis occurs during gametogenesis, the
production of gametesthat is, the sperm
and eggs.
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Mitosis
During ordinary cell division, called
mitosis, a mother cell divides, producing
two daughter cells. In humans, the mother cell has forty-six
chromosomes, and the two daughter cells
that result also have forty-six chromosomes.
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Mitosis
The chromosomes are at first single, containing a single
DNA helix.
Before cell division takes place, DNA replication occursas the chromosomes duplicate.
A duplicated chromosome contains two identical parts
(called chromatids).
During mitosis, these portions separate, becoming
daughter chromosomes.
A daughter chromosome contains one DNA helix again
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Mitosis
Mitosis is the type of cell division required for growthand repair of somatic (body) cells.
The process of mitosis assures that each cell in the
body has the same number and kinds of chromosomesand, therefore, the same genes (since the genes are on
the chromosomes).
Ordinarily, a cell only divides about fifty times; with
maturity, a cell stops dividing.
Cancer cells never mature and, instead, continue to
divideindefinitely.
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Meiosis In females, meiosis occurs during oogcnesis (o"o-jen'e-sis)
(egg production) in the ovaries.
In males, meiosis occurs during spermatogenesis(sperm
production) in the testes. Following sexual intercourse, a sperm fertilizes an egg, and a
new individual begins development.
Because a sperm carries twenty-three chromosomes and an
egg carries twenty-three chromosomes, the new individual hasforty-six chromosomes, which is the normal number for
human beings. In this way, both parents contribute twenty-
three chromosomes to the new individual.