Membrane Structure and FunctionChapter 5

Membrane Models•Lipid-soluble molecules enter cells more

rapidly than water soluble molecules.▫Lipids are a component of the plasma

membrane.

Fluid-Mosaic Model•Fluid-mosaic model—model for the

plasma membrane based on the changing location and pattern of protein molecules in a fluid phospholipid bilayer.

Plasma Membrane Structure and Function•Plasma membrane separates the internal

environment of the cell from the external environment.▫Regulates the entrance and exit of

molecules into the cell.▫Helps the organism maintain a steady

internal environment.

Plasma Membrane Structure and Function•Plasma membrane is made up of a

phospholipid bilayer with embedded proteins.▫Proteins form a mosaic pattern.

•Hydrophilic (water loving) polar heads of the phospholipid molecules face the outside and inside of the cell where water is found.

Plasma Membrane Structure and Function•Hydrophobic (water-fearing) nonpolar tails

face each other.

•Cholesterol—one of the major lipids found in animal plasma membranes▫Makes the membrane impermeable to many

molecules.▫Stiffens and strengthens the membrane by

helping regulate its fluidity.

Plasma Membrane Structure and Function•Glycoproteins—protein in the plasma

membranes that bears a carbohydrate chain.▫Phospholipids and proteins attached to

carbohydrate chains.

Plasma Membrane Structure and Function•Carbohydrate Chains

▫Carbohydrate chains of proteins give the cell a “sugar coat”, called the glycocalyx. Glycocalyx protects the cell, facilitates

adhesion between cells, reception of signal molecules, and cell-to-cell recognition.

▫In humans, carbohydrate chains are the basis for the A, B, and O blood groups.

Functions of Proteins within the Plasma Membrane•Channel proteins

▫Involved in the passage of molecules through the membrane.

•Carrier proteins▫Transports sodium and potassium ions

across a nerve cell membrane. Carrier protein is essential in nerve

conduction.

Functions of Proteins within the Plasma Membrane•Cell recognition proteins

▫Glycoproteins Help the body recognize when it is being invaded

by pathogens so that an immune reaction can occur.

•Receptor proteins▫Have a shape that allows a specific molecule to

bind to it.▫The binding of this molecule causes the protein

to change its shape, and resulting in a cellular response.

Functions of Proteins within the Plasma Membrane•Enzymatic proteins

▫Carry out metabolic reactions directly.▫Without enzymatic proteins degradation

and synthetic reaction would not occur.

Permeability of the Plasma Membrane

Permeability of the Plasma Membrane•Differentially (selectively) permeable—

ability of plasma membranes to regulate the passage of substances into and out of the cell.▫Allowing some to pass through and

preventing the passage of others.

Permeability of the Plasma Membrane•Passive transport

▫Involves diffusion or facilitated transport does require a carrier protein. Does not require chemical energy.

▫Diffusion occurs without benefit of a carrier protein, whereas facilitated transport does require a carrier protein.

Permeability of the Plasma Membrane•Active Transport

▫Requires a carrier protein and chemical energy.

▫Vesicle formation can take a molecule out of a cell (called exocytosis) or into a cell (called endocytosis).

Permeability of the Plasma Membrane•Concentration gradient—gradual change

in chemical concentration from one point to another.▫Concentration of a substance moves from a

high concentration area to an area where their concentration is low.

Permeability of the Plasma Membrane• Diffusion—movement of molecules from a higher

to a lower concentration until equilibrium is achieved.▫Spontaneous ▫Requires no chemical energy

• Solution—contains both a solute and a solvent.▫Solute—substance being dissolved within the

solvent.▫Solvent—substance in which the solute is

dissolved within.

Permeability of the Plasma Membrane•Gas exchange in lungs.

▫Oxygen diffuses into the capillaries of the lungs because there is a higher concentration of oxygen in the alveoli (air sacs) than in the capillaries.

After inhalation, the concentration of oxygen in the alveoli is higher than that in the blood; oxygen diffuses into the blood.

Permeability of the Plasma Membrane•Osmosis—diffusion of water through a

selectively permeable membrane.

•Osmosis pressure—measure of the tendency of water to move across a selectively permeable membrane.▫Visible as an increase in liquid on the side

of the membrane with higher solute concentration.

Permeability of the Plasma Membrane•Osmosis

▫Isotonic solution—solution that is equal in solute concentration to that of the cytoplasm of a cell. Causes cell to neither lose nor gain water by

osmosis.

Tonicity—osmalarity of a solution compared to that of a cell. If the solution is isotonic to the cell, there is no net

movement of water. If the solution is hypotonic the cell gains water. If the solution is hypertonic the cell loses water.

Permeability of the Plasma Membrane•Osmosis

▫Hypotonic solution—lower solute concentration than the cytoplasm of a cell. Causes cell to gain water by osmosis. Lower concentration of solute.

▫Turgor pressure—pressure of the cell contents against the cell wall. In plant cells, determined by the water

content of the vacuole and provides internal support.

Permeability of the Plasma Membrane•Osmosis

▫Hypertonic solution—higher solute concentration than the cytoplasm of a cell. Causes cell to lose water by osmosis. Higher percentage of solute

Crenation—shriveling of the cell due to water leaving the cell when the environment is hypertonic.

Plasmolysis—contraction of the cell contents due to the loss of water.

Permeability of the Plasma Membrane•Transport by Carrier Proteins

▫Carrier proteins are specific; each can combine with only a certain type of molecule or ion, which is then transported through the membrane.

Carrier proteins are REQUIRED for both facilitated transport and active transport.

Permeability of the Plasma Membrane•Facilitated Transport—passive transfer of

a substance into or out of a cell along a concentration gradient by a process that requires a carrier.

▫During facilitated transport, a carrier protein speeds the rate at which the solute crosses the plasma membrane toward a lower concentration. Carrier protein undergoes a change in shape

as it moves a solute across the membrane.

Permeability of the Plasma Membrane•Active Transport—use of a plasma

membrane carrier protein to move a molecule or ion from a region of lower concentration to one of higher concentration.▫It opposes equilibrium and requires energy.▫Opposite to the process of diffusion.

Permeability of the Plasma Membrane•Active Transport

▫Proteins involved in active transport often are called pumps.

▫One type of pump in animal cells, moves sodium ions to the outside of the cell and potassium ions to the inside of the cell. These two event are linked, and the carrier

protein is called a sodium-potassium pump.

Permeability of the Plasma Membrane•Sodium-potassium pump

▫Steps Carrier has a shape that allows it to take up

3Na ions. ATP is split, and phosphate group attached to

carrier. Change in shape results and causes carrier to

release 3Na ions outside the cell. Carrier protein now has a shape that allows it

to take up 2K ions.

Permeability of the Plasma Membrane•Sodium-potassium pump

▫Steps Phosphate group is released from the carrier.

Phosphate group is donated by ATP when it is broken down by the carrier.

Change in shape results and causes carrier to release 2K ions inside the cell.

▫Sodium-potassium pump results in both a concentration gradient and an electrical gradient for these ions across the plasma membrane.

Permeability of the Plasma Membrane•Membrane-Assisted Transport

▫Macromolecules are too large to be transported by carrier proteins. Vesicles form to transport into or out of the

cell by vesicle formation. Vesicle formation is an energy-requiring

process, which include exocytosis and endocytosis.

Permeability of the Plasma Membrane•Exocytosis—process in which an

intracellular vesicle fuses with the plasma membrane so that the vesicle’s contents are released outside the cell (secretion occurs).▫Often these vesicles have been produced by

the Golgi Apparatus and contain proteins.

Permeability of the Plasma Membrane•Exocytosis

▫The membrane of the exocytosis vesicle becomes part of the plasma membrane.

▫Exocytosis occurs automatically during cell growth.

▫The proteins are then released from the vesicle adhere to the cell surface or become incorporated in an extracellular matrix.

Permeability of the Plasma Membrane•Endocytosis—process where cells take in

substances by vesicle formation.

•Endocytosis can occur in 3 different ways:▫Phagocytosis▫Pinocytosis▫Receptor-mediated endocytosis

Permeability of the Plasma Membrane•Endocytosis

▫Phagocytosis—when the material or large substances are engulfed, forming an intracellular vacuole. Common in unicellular organisms.

Example: amoebas

Permeability of the Plasma Membrane•Endocytosis

▫Pinocytosis—vesicles form around a liquid or around very small particles and brings them into the cell. Involves a significant amount of the plasma

membrane because it occurs continuously. The loss of the membrane due to pinocytosis is

balanced by the occurrence of exocytosis.

Permeability of the Plasma Membrane•Endocytosis

▫Receptor-Mediated Endocytosis—selective uptake of molecules into a cell by vacuole formation after they bind to specific receptor proteins in the plasma membrane. Type of pinocytosis but is more efficient.

The receptors for these substances are found at one location in the plasma membrane. This location is called a coated pit because it is a

layer of protein on the cytoplasmic side of the pit.

Modification of Cell Surfaces

Modification of Cell Surfaces•Cell Surfaces in Animals

▫2 different types of animal cell surface features: Junctions between cells The extracellular matrix

Modification of Cell Surfaces•Junction between cells

▫3 types of junctions between cells Adhesion junctions Tight junctions Gap junctions

Modification of Cell Surfaces•Junctions between cells

▫Adhesion junctions—adjacent plasma membranes do not touch but are held together by intercellular filaments attached to buttonlike thickenings.

Modification of Cell Surfaces•Junctions between cells

▫Tight junctions—adjacent plasma membrane proteins join to form an impermeable barrier. Produces a zipperlike fastening.

Examples: intestines and kidney tubules

Modification of Cell Surfaces•Junctions between cells

▫Gap junctions—formed by the joining of two adjacent plasma membranes. It lends strength and allows ions, sugars, and

small molecules to pass between cells. It allows cells to communicate.

Important in the heart muscle and smooth muscle because they permit a flow of ions that is required for the cells to contract as a unit.

Modification of Cell Surfaces•Extracellular Matrix—meshwork of

polysaccharides and proteins in close association with the cell that produced them.▫Collagen is a structural proteins that gives

the matrix strength.▫Elastin fibers are structural proteins that

gives the matrix resilience.

Modification of Cell Surfaces•Extracellular matrix

▫Fibronectins and laminins are adhesive proteins which influence the behavior of the cells. They form “highways” that direct the

migration of cells during development. Permit communication between the

extracellular matrix and cytoplasm of the cell.

Question 11. A phospholipid molecule has a head and

two tails. The tails are founda. At the surfaces of the membrane.b. In the interior of the membrane.c. Spanning the membrane.d. Where the environment is hydrophilic.e. Both a and b are correct.

Question 22. During diffusion,

a. A solvents move from the area of higher to lower concentration, but solutes do not.

b. There is a net movement of molecules from the area of higher to lower concentration.

c. A cell must be present for any movement of molecules to occur.

d. Molecules move against their concentration gradient if they are small and charged.

e. All of these are correct.

Question 33. When a cell is placed in a hypotonic

solution,a. solute exits the cell to equalize the

concentration on both sides of the membrane.

b. Water exits the cell toward the area of lower solute concentration.

c. Water enters the cell toward the area of higher solute concentration.

d. Solute exits and water enters the cell.e. Both c and d are correct.

Question 44. When a cell is placed in a hypertonic

solution,a. solute exits the cell to equalize the

concentration on both sides of the membrane.

b. Water exits the cell toward the area of lower solute concentration.

c. Water enters the cell toward the area of higher solute concentration.

d. Solute exits and water enters the cell.e. Both c and d are correct.

Question 55. Active transport

a. Requires a carrier protein.b. Moves a molecule against its

concentration gradient.c. Requires a supply of chemical energy.d. Does not occur during facilitated

transport.e. All of these are correct.