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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.