Download The plasma membrane consists of two layers of lipid molecules

1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in
specialized areas of the organism’s cells. As a basis for understanding this concept:
a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their
surroundings.
The plasma membrane consists of two layers of lipid molecules organized with the polar (globular) heads of the
molecules forming the outside of the membrane and the nonpolar (straight) tails forming the interior of the
membrane. Protein molecules embedded within the membrane move about relative to one another in a fluid fashion.
Because of its dynamic nature the membrane is sometimes referred to as the fluid mosaic model of membrane
structure.
Cell membranes have three major ways of taking in or of regulating the passage of materials into and out of the cell:
simple diffusion, carrier-facilitated diffusion, and active transport. Osmosis of water is a form of diffusion. Simple
diffusion and carrier-facilitated diffusion do not require the expenditure of chemical bond energy, and the net
movement of materials reflects a concentration gradient or a voltage gradient or both. Active transport requires free
energy, in the form of either chemical bond energy or a coupled concentration gradient, and permits the net transport
or “pumping” of materials against a concentration gradient.
Notes:
The plasma membrane's main purpose is to maintain a stable internal environment even though the external
environment is constantly changing.
In order to maintain homeostasis the plasma membrane is selective and capable of actively moving materials in or out of
the cell. Each cell has a membrane specially designed to support the special function of that cell.
The selective process may be passive or active.
The plasma membrane is selectively permeable because of its structure. To better understand permeability you need to
know some features of the typical plasma membrane.
Its main component is the phospholipid (
)
Phospholipids are arranged in a bilayer.
Membrane surfaces (upper and lower) are water loving (hydrophilic) phosphate groups which are polar.
The internal portion of the bilayer where the fatty acid tails meet is water repelling (hydrophobic).
Special membrane proteins are embedded in the bilayer. The proteins can move in the bilayer relative to each other.
Each protein has a unique function or role to play.
Cholesterol molecules stabilize plasma membranes by regulating how fluid they are and prevent their degradation.
Phospholipids are related to triglycerides but instead of 3 fatty acids the phospholipid has 2 fatty acids and a phosphate
group.
Because the phosphate is hydrophilic (water loving) and the fatty acids are hydrophobic (water fearing) the phospholipid
bilayer is able to regulate what can pass through it. It can act as a barrier to most water soluble substances because the
oily fatty acid tails of the phospholipids repel polar or ionic molecules.
Fat soluble (nonpolar) materials and very small polar molecules can slip between the fluid phospholipids. Larger polar
molecules and ions can only pass through membranes by entering pores in special transport proteins located in the
membranes.
Cellular Transport
Materials can enter or leave a cell in two ways:
1. passively - that is without any energy needed by the cell
2. actively - in which the cell must use energy to compete the transport process
Passive Processes
Simple Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
Solute particles in a liquid solvent are always on the move. Their movement is unpredictable and often too slow for many
cell processes. Nevertheless diffusion is a crucial mechanism the cell takes advantage of to distribute needed materials.
Diffusion is caused by molecular vibrations due to thermal energy (heat). The warmer it is the faster molecules can
diffuse.
The smaller the molecule the faster it will be distributed by diffusion
Because both water and solute particles move randomly, colliding with each other they tend to scatter until they are
evenly mixed. It is the natural tendency of all things to move apart and become randomly distributed.
In other words, substances tend to go from where they are highly concentrated to areas of lower concentration.
Once a condition is reached where concentrations no longer change by simple diffusion a state of dynamic equilibrium
exists. Diffusion continues but no change is observable.
Osmosis (This is a special variety of diffusion)
Osmosis is the diffusion of water through a membrane such as the plasma membrane. Generally only water, oxygen,
nitrogen, carbon dioxide, and other small uncharged particles can freely diffuse through the plasma membrane. Large
particles are prevented from freely passing through membranes because they are bigger than the pores or openings in the
membrane. The plasma membrane is also selective because its hydrophobic interior stops polar molecules and ions from
getting through.
Two Solutions separated by a membrane can be:
Isotonic. When two solutions are isotonic they are at equilibrium. There is no concentration gradient. The
concentrations of solvent and solute are equal.
Hypotonic. A hypotonic solution has a higher concentration of solute (dissolved particles). When a cell is hypotonic
relative to its surroundings it will attract water which tends to move into the cell because its concentration is lower there.
The cell will expand. The increase of water within the cell creates a type of pressure called osmotic pressure. Animal cells
will burst but plant cells with their cell walls thrive. In plant cells this pressure is called turgor pressure.
Hypertonic. An hypertonic solution is lower in concentration compared to its surroundings. Water tends to leave
hypertonic solutions. A cell placed in a hypertonic solution will lose water which is attracted to the higher salt
concentrations in the surroundings. The cell shrivels up. This shrinking process is called plasmolysis.
Passive Transport
The diffusion of materials across a plasma membrane through the channels created by special carrier or transport
proteins is called facilitated diffusion. This is a type of passive transport because no energy is expended by the cell.
The way transport proteins work is not completely understood, but the current hypothesis is that when the substance
(such as glucose) binds to the carrier protein its shape changes creating a temporary pore for it to enter through.
These transport proteins have many characteristics in common with enzymes.
1. they bind to only specific substances
2. they can become saturated which means they can only transport so many molecules in a given time
3. they can be inhibited if a similar molecule competes for the same active site and thus blocks passage of the normal
molecule
Dynamic equilibrium can be established where a concentration gradient is maintained. This requires the expenditure of
energy and will be discussed next.
Energy Consuming Processes
Active Transport - Transport proteins
If a cell needs to accumulate rare or necessary materials, or build a concentration gradient it can do so by using energy.
Special proteins in the cell membrane can change shape and open a channel or translocate a substance when activated by
an energetic phosphate bond. Activation by the phosphate only occurs when the substance to be transported makes
contact with the transport protein.
Most ions, such as Na+ or K+, are brought into the cell against their concentration gradients by transport proteins called
ion pumps. All nerve cells must constantly pump potassium ions into the cell while pumping sodium ions out of the cell
by a transport protein called the sodium-potassium ion pump.
Active Transport - Membrane action
Entire sections of the plasma membrane can capture and transport materials into or out of the cell. These processes
normally only occur in animal cells
Endocytosis
When large particle of food such as a bacterium presents itself to a hungry cell it can pinch in and enfold the object to
form a vacuole. This is called phagocytosis.
Normally the leading edge of a cell appears to constantly flutter, pinching inward at many locations apparently sampling
its environment creating numerous small vesicles in a process called pinocytosis.
Both of these processes reduce the amount of plasma membrane and the cell would tend to shrink in size if it where not
for the continual manufacture of new membrane by the smooth endoplasmic reticulum.
Exocytosis
Exocytosis is the means a cell uses to export materials made within the cell to its surroundings. Digestive enzymes,
antibodies, hormones and some waste products can all be sent out of the cell when membrane bound vacuoles or
vesicles fuse with the plasma membrane.