Structure of Cells
Cell—the structural and functional unit of all tissues
Robert Hooke (1660)—discovered cells while observing a very thin section of cork from a plant
stem.
Cell Theory—from conclusions of Schleiden, Schwann, and Virchow
1.
All organisms are composed of cells
2.
Cells are the basic units of structure and function
in organisms
3.
All cells come from pre-existing cells
Cell types grouped according to organization of nucleus:
1. Prokaryotes—do not have a membrane bound nucleus or other membrane bound
organelles. The word prokaryote means “before nucleus”. The nuclear material may be in
the form of a single circular strand of DNA called a plasmid. Prokaryotic organisms include
bacteria and blue-green bacteria.
2. Eukaryotes—have a membrane bound nucleus and other membrane bound organelles.
Three major parts/areas of a cell (outside to inside):
1. Cell membrane (plasma membrane)—holds the cell together and regulates what enters
and leaves the cell. The fluid-mosaic model of the cell membrane states that it is a biphospholipid layer with proteins embedded in it. The cell membrane is said to be
“selectively permeable” because small pores formed by the proteins only allow
certain substances to pass through the cell membrane.
2. Cytoplasm—everything within the cell except the nucleus. About 70 percent water and
30 percent proteins, fats, carbohydrates, nucleic acids, and ions. Consistency of non
solidified gelatin. Contains numerous organelles (literally “little organs”) that carry out
particular functions within the cell. The cytoplasm is in constant motion called
“cytoplasmic streaming”.
Eukaryotic cell organelles in the cytoplasm include:
A.
Ribosomes—composed of RNA and protein, not a membrane bound organelle.
Sites of protein synthesis. Free ribosomes interspersed throughout the cell
typically make proteins that are to be used inside the cell.
B.
Endoplasmic reticulum (ER)—composed of plasma membranes forming a
network of interconnected, flattened, tube-like structures that end in blind
alleys. One function of ER is transport of proteins throughout the cell. May be:
(1)
Smooth ER (does not have ribosomes attached to it) Within the winding
channels of the smooth endoplasmic reticulum are the enzymes needed
for the construction of molecules such as carbohydrates and lipids.
(2)
Rough ER (has ribosomes attached to it) The endoplasmic reticulum and
its bound ribosomes are particularly dense in cells that produce many
proteins for export, such as the white blood cells of the immune system,
which produce and secrete antibodies.
C.
Golgi bodies—sack-like structures that resemble ER and lie close to it. Are
more disk-like than ER. Used in packaging and modification of cellular
secretions or substances to be used in the cell. Are composed of plasma
membranes.
D.
Vesicles—produced by the golgi bodies. Vesicles store and transport cellular
products and wastes. They also digest food and other particles taken into the
cell. Are composed of plasma membranes.
E.
Mitochondria—aerobic respiration occurs on the inner membranes of these
organelles. The inner membranes on which the reactions occur are called
cristae. They contain their own DNA in the form of a plasmid. Are composed
of plasma membranes.
F.
Plastids—used to store lipids or starches, or to hold pigments. Found in cells of
plants or algae. Examples of plastids include chloroplasts, chromoplasts, and
leucoplasts. Are composed of plasma membranes.
G.
Lysosomes—vesicles from the golgi bodies that contain enzymes to digest food
or foreign bodies. Are composed of plasma membranes.
H.
Vacuoles—act as reservoirs for food, water, and minerals. More prominent in
plant cells. Animal cells contain vacuoles but they are not as large. Are
composed of plasma membranes.
I.
Cytoskeleton—provides the framework for the cell. Composed of
microfilaments and microtubules. Microtubules are thin, hollow proteins
involved in movement.
J.
Centrioles—play an important part in cell reproduction at which time they
produce microtubules that aid in the movement of the chromosomes. Found in
animal cells and some algae and fungi.
K.
Cilia and flagella—involved in movement of the cell or movement of liquids
surrounding the cell. Composed of microtubules. Cilia are small hair-like
structures that may cover the surface of a cell. Flagella are usually less
numerous but much longer.
L.
Cell wall—found in plants and fungi; enables a plant cell to absorb water into
the central vacuole and swell without bursting. The resulting pressure in the
cells provides plants with rigidity and support for stems, leaves, and flowers.
Made of cellulose in plant cells and chitin in fungus cells.
3.
Nucleus—control center of the cell. Composed of chromatin. Chromatin is
composed of individual chromosomes while in their uncoiled form. Chromosomes are
composed of protein and DNA, and the DNA in the nucleus ultimately controls all the
activities of the cell.
The nucleus is surrounded by the nuclear membrane or envelope and contains one or
more nucleoli (spherical bodies made of DNA, RNA, and proteins). Thought to be
sites of ribosome production. The nucleus performs two important functions:
A.
Controls most activities that take place within the cell.
B.
Transmits hereditary information (DNA). DNA and proteins form
chromosomes in the nucleus.
FLUID MOSAIC MODEL OF THE PLASMA MEMBRANE
When viewed through the electron microscope, the plasma membrane appears as two
thin lines. Chemical analysis shows that the membrane consists of a lipid bilayer, made
up of two layers of molecules called phospholipids.
A normal lipid molecule consists of a molecule of glycerol and three fatty acids, but in a
phospholipid, a phosphate group replaces one of the fatty acids. Fatty acids are nonpolar molecules and do not dissolve in water. The phosphate group, however, is polar
and easily dissolves in water.
Because they are soluble in water, the polar phosphate heads of the molecule point
toward the outside of the cell and inside of the cell where water is plentiful. Therefore, the
heads are said to be hydrophilic, or water loving. Conversely, the nonpolar tails point
away from the water and toward the middle of the membrane forming two layers of
phospholipid molecules. Therefore, the tails are said to be hydrophobic, or water
fearing.
As a result of these properties, the two layers of the plasma membrane form
spontaneously. Once formed the bilipid layer can be maintained without energy from the
cell.
Other components of the plasma membrane include:
1.
Cholesterol molecules—rigid molecules imbedded
in the bilayer of many cells to restrict the movement
of the phospholipids.
2.
Proteins—also have polar and non-polar regions
that determine their position as the float in the
plasma membrane. They may be:
a. Channel proteins that extend completely through the membrane
and function in passive transport.
b. Proteins with attached carbohydrates floating on the surface of the membrane act as
tags or markers.
c. Transport or carrier proteins extend completely through the membrane
and function in active transport.
d. Enzymes float on the surface or interior of the membrane and catalyze
reactions necessary for the cell to function properly.
e. Receptors for hormones and neurotransmitters float on the surface of the
membrane.
TRANSPORT IN CELLS
Homeostasis—balanced internal state. One of the ways cells maintain homeostasis by controlling
what enters and leaves them.
General types of processes involved in movement of substances through cell membranes include:
1)
Active (physiological) transport— energy for the movement of a substance across a
membrane comes from chemical reactions occurring in the cells. Occurs against or
up a concentration gradient of that substance.
2)
Passive (physical) transport— energy for the movement of a substance comes from
some other source and not from a living cell’s chemical reactions.
Occurs down a concentration gradient of that substance.
Specific processes involved in movement of substances through cell membranes include:
1)
Diffusion (passive transport) –the movement of solute and solvent particles in all
directions through a solution or in both directions through a membrane. Fat soluble
(non-polar) molecules can diffuse directly through the phospholipids that make up the
cell membrane while polar molecules or ions cannot.
a)
Net diffusion—refers to the movement of more particles of a substance in
one direction than in the opposite direction. Net diffusion always occurs
down a concentration gradient of a solute; that is from an area of greater
concentration of solute particles to an area of lesser concentration of
solute particles.
b)
Equilibrium—occurs when net diffusion of the solute in one direction through
a membrane and of water in the opposite direction makes the solute
concentration of two solutions equal on either side of a membrane.
2)
Facilitated diffusion (passive transport)—diffusion of molecules or ions across a
membrane through transport proteins
3)
Osmosis (passive transport)—refers to the diffusion of water across a selectively
permeable membrane from and area of lesser concentration of solute to an area of
greater concentration of solute. A selectively permeable membrane is one that
maintains at least one concentration gradient across itself.
a)
Net osmosis—occurs down a concentration gradient of water and tends
to equilibrate solutions separated by a selectively permeable
membrane. Net osmosis occurs into the solution with the highest
concentration of solute particles, increasing its volume and decreasing
its concentration.
b)
Osmotic pressure—the pressure that develops in a solution as a result of
c)
net osmosis into that solution. Osmotic pressure develops in the solution
that originally contained the higher concentration of solute.
Types of solutions related to osmosis include:

Isotonic solutions—those solutions whose volumes and pressures will
stay the same if the two solutions are separated by a membrane. They
have the same amount of solute and thus the same potential osmotic
pressure.

Hypertonic solution—one that has a higher solute concentration and
thus a higher potential osmotic pressure than another. One into which
net osmosis occurs when a selectively permeable membrane separates
it from another solution.

Hypotonic solution—has characteristics opposite from those of a
hypertonic solution.
4)
Biological pumps (active transport)—a carrier mediated process (performed by carrier
or transport proteins) that requires energy from the cell in the form of a molecule called
ATP. An example of a biological pump is the “sodium-potassium pump” operates in all of
our cells. It actively transports sodium and potassium ions in opposite directions through
a cell membrane (sodium ions out-potassium ions in), maintaining a higher concentration
of sodium outside the cell membrane and a higher concentration of potassium ions inside
the cell membrane. The carrier molecule is thought to be an enzyme called sodiumpotassium ATPase. This type of pump is very important to the functioning of an
organism’s nervous system.
Other active transport mechanisms active in cells include phagocytosis (cell eating),
5)
which is a type of endocytosis, and exocytosis.
Cell Reproduction
Chromosomes—composed of chromatin that has become tightly wrapped around globules of
proteins called histones. Each chromosome consists of two individual parts called chromatids.
The point at which each chromatid is attached is called a centromere.
In all sexually reproducing organisms, chromosomes occur in homologous pairs. Each
chromosome of a pair has the same size and shape as its homologue. A cell that contains both
chromosomes of a homologous pair is said to be diploid. A cell that contains only one
chromosome from each homologous pair is said to be haploid.
Cell reproduction—occurs when the parent cell divides producing two new daughter cells. Types
of cell reproduction include:
1.
Binary fission—prokaryotic fission, which is binary fission, is a form of asexual
reproduction and cell division used by all prokaryotes (bacteria and archaebacteria)
and some organelles within eukaryotic organisms (e.g., mitochondria).
2.
Budding—one larger cell and one smaller cell produced from the splitting of the
original cell into two cells of different sizes. A unicellular fungus called yeast
reproduces in this manner.
Cell cycle—includes the non-reproducing stage in which cells spend most of their life, called
interphase, and the reproducing stage in which new cells are formed called mitosis. During
interphase, cells are using energy to carry out the basic cell functions. New cell parts such as
mitochondria and ribosomes are being produced early in interphase. About midway through
interphase, each chromosome and the DNA it contains replicates.
Mitosis—division of the nucleus during the reproductive portion of the cell cycle. Occurs in
somatic cells throughout the body.
Mitotic phases:
1.
Prophase
a.
b.
c.
d.
nucleoli disappear
nuclear membrane disintegrates
coiling and shortening of chromosomes give nucleus a grainy appearance
centrioles break apart and begin to migrate toward opposite poles, and the spindle
begins to form
e. during late prophase, chromosomes appear as two joined sister
chromatids joined at the centromere
2.
Metaphase
a. microtubules of the spindle capture the chromosomes by the
centromere
b. chromosomes move to the equator of the spindle and become aligned
3.
Anaphase
a. centromeres split and sister chromatids begin to move apart
b. chromosomes migrate to opposite poles of the cell
4.
Telophase
a. cell cleavage furrow appears in animal cells as plasma membrane is
pulled together by microtubules
b. cell plate begins to appear in plants telophase ends with the reverse of
the events that began prophase
Cytokinesis—division of the cytoplasm of a cell
Control of the cell cycle:
1.
2.
3.
An estimated 25 million cell divisions occur per second in the human body
Some cells divide at a faster rate than others, some never reproduce once they mature
(nerve cells)
Cell-to-cell contact may be one of the mechanisms that control the rate of cell
reproduction