Review Questions
Plasma Membrane
1. What is the function of the plasma membrane?
The plasma membrane forms the outer boundary of all cells. Described as
semi-permeable, the membrane regulates the passage of atoms and
molecules in and out of the cell. All membrane-bound organelles are also
built of plasma membrane.
2. Draw a phospholipid bilayer and label the polar heads and the non-polar
tails.
The plasma membrane is made of a phospholipid bilayer. Phospholipids have
a glycerol backbone attached to two fatty acid chains (one is unsaturated)
and one phosphate group. The phosphate has a negative charge. The rest of
the lipid is nonpolar. Water is attracted to the phosphate group and repelled
by the rest of the molecule, so a phospholipid has a dual nature: a hydrophilic
region (“head”) and a hydrophobic region (“tails”). When placed in water, the
phospholipids form a bilayer. The heads face outward and the tails stay
inside. The bilayer is semi-permeable barrier. Some substances can freely
cross the membrane whereas others are stopped.
3. Why is the plasma membrane described as a “fluid mosaic”?
The plasma membrane is a fluid because the phospholipid molecule is
unsaturated. At room temperature, this causes the bilayer to have the
consistency of salad oil. The term “mosaic” refers to the variety of embedded
transmembrane proteins scattered throughout the membrane.
4. Why is cholesterol an important component of animal cell membranes?
Cholesterol is a component of the cell membranes of animals. Cholesterol
makes the membrane less fluid and therefore more impermeable to biological
molecules. In a sense, cholesterol creates a more solid membrane and a
more restrictive membrane. At low temperatures, however, cholesterol
prevents the close packing of the phospholipids and keeps the membrane
fluid.
5.
Name 7 specialized proteins that are embedded in the phospholipid bilayer.
Describe the function of each of the 7 proteins.
Transmembrane
Protein
Glycoprotein
Structure
Function
Small carbohydrate chain projects
out from the cell
Channel protein
Protein has a small tunnel through
the center
Carrier protein
Protein is shaped to grasp
substance and shuttle it across
Receptor protein
Protein has a small binding site that
fits the shape of a specific atom or
molecule (e.g. glycoprotein, tastant,
neurotransmitter, hormone, etc.)
Enzymes with active sites
embedded in membrane;
organelles with a lot of membrane
contain loads of enzymatic proteins
(e.g. cristae of mitochondria,
thylakoids of chloroplasts)
Attach membrane to cytoskeleton
Membrane proteins of adjacent
cells hook together
Cell recognition (a chemical
nametag) identify self vs.
nonself. Successful organ
transplants require
matching glycoproteins
Allows ions passage
through cell membrane;
highly specific
Transports molecules
across membrane (e.g.
glucose, amino acids, etc)
sensory input,
communication, initiates
metabolic change in target
cell (signal transduction)
Metabolism (build and
degrade molecules)
Enzymatic protein
Anchoring proteins
Intercellular Joining
Supports cytoskeleton
Holds cells to each other
6. What kinds of substances can pass easily through a plasma membrane (be
specific)? What kinds of substance cannot pass through a plasma
membrane (be specific)?
Can pass through
1. Small and relatively uncharged
Examples:
O2, CO2, H2O
Cannot pass through
1. Large
Examples:
monosaccharides, amino
acids, nucleotides, biological
monomers and polymers
2. Lipid soluble
Examples:
Steroids, Hydrocarbons,
Alcohol, Anesthetics
2. Charged
Examples:
Ions (Na+, K+, H+, Ca2+, Cl-,
PO4-, HCO3-, OH-, etc.)
7. What is diffusion? Give an example of diffusion in the human body.
Diffusion is the net movement of solutes (or solvents) to regions of lower
concentration as a result of random spontaneous motions. Diffusion
produces a uniform distribution of particles.
The atoms of any piece of matter above absolute zero are in motion. In
liquids and gases, atoms are colliding and bouncing off each other randomly.
A glass of water may look still but inside those water molecules are in
constant motion. Add dye molecules and the dye will spread out until it is
equally dispersed. The random motion of the water molecules and the dye
mixed them into a homogenous solution.
Diffusion is a free service for cells. The cell does not have to spend any
energy (ATP) for diffusion to occur. All it needs is a concentration gradient. In
our lungs, if there is a higher concentration of oxygen in the air sacs than the
blood, oxygen gas will automatically diffuse into the blood. If there is a higher
carbon dioxide level in the blood than the air sacs, then carbon dioxide will
diffuse out of the blood and into the lungs.
The shape of all life is dictated by diffusion. Every cell depends on diffusion
for survival. Diffusion is the process cells use to exchange respiratory gases,
absorb nutrients, and eliminate wastes.
8. Why are cells so small?
Most cells are invisible to the unaided eye. Cells rely on diffusion for their
survival. Since diffusion occurs across cell membranes, a cell’s surface area
has to be large enough to accommodate sufficient exchange to sustain life. A
cell cannot afford a diffusion bottleneck. An interesting geometrical paradox
kicks into gear when a cell increases in volume. As volume increases surface
area also increases but not at the same rate. Let me give you an example.
A box with a volume of 1 cm3 (1 cm length x 1 cm height x 1 cm depth), has a
surface area of 6 cm2 (6 sides x 1 cm length x 1 cm height). The surface
area-to-volume ratio is 6:1.
Let’s double the volume. A box with a volume of 8 cm3 (2 cm length x 2 cm
height x 2 cm depth) has a surface area of 24 cm2 (6 sides x 2 cm length x 2
cm height). The surface area-to-volume ratio is now only 3:1.
Let’s triple the volume. A box with a volume of 27 cm3 (3 cm length x 3 cm
height x 3 cm depth) has a surface area of 54 cm2 (6 sides x 3 cm length x 3
cm height). The surface area-to-volume ratio has dropped to 2:1.
In fact, every time volume triples, surface area only doubles. If a cell is too
large, the amount of surface area for diffusion is not enough to support life.
So cells are limited in size by the dictates of diffusion. It is much better for a
growing organism not to make its cells bigger but simply to make more of
them.
9. Describe osmosis.
If you cut a potato into strips and submerge one strip into a bowl of tap water
and likewise submerge one strip into a bowl of salt water, after an hour, the
tap water strip will be firm, stiff, and crisp. The salt water strip, on the other
hand, will be limp and flexible. What happened?
This is an example of a special kind of diffusion called osmosis. The potato
strip placed in tap water gained water. Water flowed from the bowl into the
cells of the potato making it stiff and crisp. Salt water had the opposite effect
on the potato strip. Water flowed out of the cells and into the salt water. What
made the water flow differently?
Osmosis is the diffusion of water across a semi-permeable membrane. Just
like any liquid or gas, water will diffuse from a region of high concentration to
a region of low concentration until the concentrations are equal. In our
example, each potato cell has a cell membrane that is selectively-permeable.
Water can cross the membrane without any problem, but the solutes inside
the potato cannot. Since water will follow its concentration gradient, we can
predict where it will flow, depending on the solute concentration on either side
of the membrane.
The cells of the potato placed in tap water have a higher concentration of
solutes than what is in the tap water. This means that there is actually a
higher concentration of water outside the cell than inside (the water has to
share space with the solutes inside the cell so there is less). So water flows
into the cell until the concentration of water versus solute is equal on both
sides.
Conversely, the salt water has a higher concentration of solute relative to
water than the cell. Since there is more water inside the cell than out is the
salt water. Water follows the concentration gradient and will flow out of the
cells.
When cells are placed in an aqueous solution that has a lower solute
concentration than the cells, we call this a “hypotonic” solution, water flows
into the cells. Cells placed in an aqueous solution with a higher solute
concentration, known as a “hypertonic” solution, water flows out of the cells. If
cells are placed into a solution with the same solute concentration as the cell,
no flow occurs. We call this solution “isotonic”.
All cells, not just plant cells, react the very same way but with a few subtle
differences.
An animal cell submerged in a hypotonic solution will swell in size. If the
concentration gradient is big enough, the animal cell with burst. Traditionally,
osmosis in animal cells is demonstrated using red blood cells. The bursting of
red blood cells in a hypotonic solution is called “hemolysis”. In other kinds of
animal cells, we just say “lysis”.
An animal cell submerged in a hypertonic solution will shrivel up and get
smaller. “Crenation” is the term we use to describe this in red blood cells. For
all other animal cells, we just called it “shriveling”.
Cells with cell walls react slightly differently in hypotonic solutions. The cell
walls prevent the cell from bursting. The cell swells and firms up. We say the
cell is turgid. Turgor pressure is the force of the water inside the cell pushing
outward on the wall. In plants, this is a good thing. Since terrestrial plants
have to fight against gravity to stay upright, turgor pressure keeps the cells
rigid. If you stop watering a plant, it will soon wilt. There is not enough water
inside the cell to sustain the necessary turgor pressure. An easy way to crisp
up limp vegetables is to place them in a hypotonic solution.
If the cell walls are flexible, a plant cell will shrivel in a hypertonic solution. We
call this wilting. But an odd thing happens if the cell walls are rigid. The
volume of the cell stays the same, but the cell membrane detaches from the
cell wall and the contents of the cell shrivel into a little ball. We call this
“plasmolysis”. Like wilting, this is reversible by placing the cell into a
hypotonic solution.
10. List some examples of osmosis.
Hypotonic solutions
The automatic sprayers at the grocery store produce section apply a
hypotonic solution to keep the vegetables crisp.
The tiny dung fungus Pilobolus launches its spore-filled capsule dozens
of feet by first filling its stalk with solutes. Water (from rain or dew) will
diffuse into the stalk from the outside and cause it to swell. The pressure
pops the capsule off the stalk and sends it flying.
Freshwater bony fish have a problem with too much water entering their
tissues. Since they live in a hypotonic environment, they maintain their
water balance by producing copious amounts of dilute urine (like a bilge
pump).
Hypertonic solutions
You can kill a plant by spraying it with a hypertonic solution.
You can reduce inflammation by applying a hypertonic solution.
Long before refrigerators, many foods were preserved using hypertonic
solutions: canned fruit in heavy syrup; honey, salt-cured meats.
Drinking sea water dehydrates you.
Marine bony fish live in a hypertonic solution. They have a problem
conserving water. They respond by voiding small amounts of
concentrated urine. They drink constantly and thanks to specialized gills
they can remove most of the excess solute.
Isotonic solutions
Saline solutions for I.V.’s and contact lenses have the same solute
concentration as body fluids.
Your liver makes a protein called serum albumin that circulates in the
bloodstream. A solute, serum albumin, prevents too much water from
leaving the blood and filling the tissues.
Sharks and marine invertebrates don’t have the problems of marine bony
fish. Their cells are isotonic to the sea water.
11. What is facilitated transport (diffusion)?
All solutes in liquids and gases will diffuse. Large molecules and ions,
required for life, cannot cross the phospholipid bilayer without the aid of
specialized transport proteins. Following the concentration gradient, these
substances diffuse through the transport proteins (channel or carrier) into the
cell. Diffusion of substances through transport proteins is facilitated transport.
Most nutrients, signals, and wastes enter and/or exit cells in this way.
12. Describe active transport.
Occasionally, solutes need to be transported across a membrane against the
concentration gradient (moving from a region of lower concentration to a
region of higher concentration). Active transport requires the cell to expend
energy (ATP) and employ transport proteins to assist in this process. Ion
pumps are a great example of active transport. Kidneys used pumps to
actively reabsorb electrolytes. Nerve cells repolarize their cell membranes by
actively pumping Na+ and K+. Mitochondria and chloroplasts pump H+ to
establish a gradient for making ATP.
Other forms of active transport, but at a much larger scale, are endocytosis
and exocytosis. During endocytosis, the cell membrane invaginates,
surrounds, and engulfs a large particle or liquid into a vacuole (or vesicle),
bringing the substance into the cell.
Kinds of endocytosis:
Receptor-mediated endocytosis: When a receptor binds to a specific
solute, the cell membrane responds by enclosing the solute into a vesicle.
For example, iron bound to a transferrin protein binds to special receptors
on the cell membrane and is enclosed inside a vesicle.
Pinocytosis: Cells can sample extracellular solutes through the formation
of membrane vesicles. Pinocytosis (“cell drinking”) is important in cells
that absorb nutrients.
Phagocytosis: Literally “cell eating”, phagocytosis is the engulfing of large
particles within enormous vesicles and bringing them into the cell.
Macrophages, specialized immune system cells, devour bacteria and kill
them using phagocytosis.
Secretion or exocytosis is the process in which material inside the cell, which
is packaged into vesicles, is excreted into the extracellular environment.