Chapter 6 Review Notes - Cell Membranes

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BIOLOGY 1107

Chapter 6

Learning Outcomes

Sadava 10e

1.

Describe the fluid mosaic model of biological

membranes.

The structure of biological membranes is described by the Fluid Mosaic

Model. In an aqueous environment, phospholipids form a bilayer, which is like a

“lake” in which a variety of proteins “float”.

2.

Describe the composition of the membrane and the

main functions of each component.

-In an aqueous environment, phospholipids form a bilayer, which is

like a “lake” in which a variety of proteins “float”.

-Phospholipids have a polar, hydrophilic “head” and hydrophobic fatty acid “tails.”

Lipids spontaneously maintain the bilayer organization, allowing membranes to

fuse easily. Lipids composition varies; phospholipids vary in fatty acid chain

length, degree of saturation, and phosphate groups; this influences fluidity.

Animal cell membranes may be up to 25% cholesterol, important for membrane

integrity. Cholesterol and long-chain, saturated fatty acids pack tightly,

making a less fluid-membrane.

-As temperature decreases, movement of molecules and cellular

processes slow. Some organisms change the lipid content of the cell membranes when

they get cold.

-Proteins: The numbers of proteins in membranes vary depending on

membrane function. Peripheral membrane proteins lack exposed hydrophobic groups

and do not penetrate the bilayer. Integral membrane proteins have hydrophobic

and hydrophilic regions or domains. Some extend across the lipid bilayer;

others are partially embedded. Transmembrane proteins extend all the way

through the phospholipid bilayer. They have one or more domains; those on the

inner and outer sides can have specific functions. Peripheral membrane proteins

are located on one side of the membrane. Some membrane proteins can move freely

within the bilayer, while others are anchored to a specific region. Membranes

are dynamic and are constantly forming, transforming, fusing, and breaking

down.

-Membranes also have carbohydrates on the outer surface that serve as

recognition sites for other cells and molecules. *Glycolipids= carbohydrate +

lipid

*Glycoprotein= carbohydrate + protein

3.

Name and describe three types of cell junctions.

Cell junctions are specialized structures that hold cells together.

-Tight junctions help ensure directional movement of materials.

-Desmosomes are like “spot welds”.

-Gap junctions allow communications.

4.

Categorize passive vs. active methods of

transport across membranes.

Membranes have selective permeability—some substances can pass through, but not

others.

-Passive transport—no outside energy required

-Active transport—energy required

5.

Explain the meaning of selective permeability as

it relates to membranes.

Membranes have selective permeability—some substances can pass through, but not

others.

6.

Define diffusion, and give examples of substances

that move across membranes by simple diffusion.

Diffusion is the process of random movement toward equilibrium.

Diffusion is the net movement or particles from regions of greater

concentration to regions of lesser concentration. Diffusion rate depends on

diameter of the molecules or ions, temperature of the solution, and

concentration gradient. Diffusion works very well over short distances (e.g.,

within a cell). A membrane is permeable to solutes that move easily across it;

impermeable to those that cannot. Simple diffusion is diffusion of small

molecules through the phospholipid bilayer. Water and lipid-soluble molecules

can diffuse across the membrane. Electrically charged and polar molecules

cannot pass through easily.

7.

Define osmosis and explain the importance of a

cell's osmotic balance.

Osmosis is the diffusion of water across a membrane.

Depends on relation concentrations of water molecules on each side of the

membrane:

-A hypertonic solution has a higher relative solute concentration.

-Isotonic solutions have equal solute concentrations.

-A hypertonic solution has a lower relative solute concentration.

Osmosis can modify the shapes of cells.

8.

Distinguish among hypertonic, isotonic, and hypotonic solutions, and describe how

cans are affected when placed in these solutions.

-A hypertonic solution has a higher relative solute concentration.

-Isotonic solutions have equal solute concentrations.

-A hypotonic concentrations has a lower relative solute concentrations.

9.

Describe facilitated diffusion, and cite

differences and examples of substances moved with the aid of channel vs.

carrier proteins.

Facilitated diffusion is diffusion facilitated by protein channels or

carriers. This is a passive method of transport. There are two kinds of

proteins that assist:

1. Channel proteins–integral membrane proteins that form a channel.

2. Carrier proteins—membrane proteins that bind some substances and speed their

diffusion through the bilayer.

Ion channels: Channel proteins with hydrophilic pores. Most are

gated—can be closed or open to ion passage. (e.g. Na+, K+).

Gate opens when protein is stimulated to change shape by chemical signal

(ligand) or electrical charge difference (voltage-gated). The potassium channel

allows K+ in the unhydrated state to pass through, but hydrated Na+

is too large to pass.

Carrier proteins transport polar molecules such as glucose across

membranes in both directions. Glucose binds to the protein, causing it to

change shape and release the glucose on the other side. In carrier-mediated

transport, the rate of diffusion is limited by the number of carrier proteins

in the cell membrane. When all carriers are loaded with solute, the diffusion

system is saturated. Cells that need lots of energy (e.g., muscle cells) have

many glucose transporters.

10.

Describe active transport , the three types of membrane

proteins that can allow this type of

transport, and examples of substances

that move across membranes by this method.

Active transport moves substances against a concentration and/or

electrical gradient. Requires energy source – usually ATP. Active transport is

directional. Three kinds of proteins can carry out active transport:

-Uniporter—moves one substance in one direction.

-Symporter—moves two substances in one direction.

-Antiporter—moves two substances in opposite directions.

Primary active transport requires direct hydrolysis of ATP.

Secondary active transport is energy that comes from an ion concentration

gradient that is established by primary active transport. In secondary active

transport, energy can be “regained” by letting ions move across a membrane with

the concentration gradient. Aids in uptake of amino acids and sugars. Uses symporters

and antiporters.

The sodium-potassium (Na+-K+) pump is an example of

primary active transport found in all animal cells. The pump (antiporter) is an

integral membrane glycoprotein.

In active transport, energy is used to move a solute against its concentration

gradient.

11.

Distinguish between endocytosis and exocytosis,

and distinguish among these three subcategories of endocytosis: phagocytosis, pinocytosis, and

receptor-mediated endocytosis.

-Endocytosis is the processes that brings molecules and cells into a eukaryotic cell by formation of a vesicle. The plasma membrane folds in around the material, forming a vesicle. Endocytosis is used by eukaryotic cells to take up and release fluids, large molecules, and particles. Smaller cells, such as invading bacteria, can be taken up by endocytosis.

-Receptor mediated endocytosis is highly specific. Depends on receptor proteins - integral membrane proteins - to bind specific substances. Sites are called coated pits - coated with other proteins such such as clathrin. The receptor proteins in a coated pit bind specific macromolecules, which are then carried into the cell by a coated vesicle. Mammalian cells take in cholesterol by receptor-mediated endocytosis. In the liver, cholesterol is packaged into low-density lipoprotein, or LDL, and secreted to the bloodstream. Cells that need cholesterol have receptors for the LDLs in clathrin-coated pits.

-Exocytosis: material in vesicles is expelled from a cell. Indigestible materials are expelled. Other materials leave cells such as digestive enzymes and neurotransmitters. Exocytosis is used in eukaryotic cells to take up and release fluids, large molecules, and particles.

-Phagocytosis: molecules or entire cells are engulfed. Some protists feed this way. Some white blood cells engulf foreign substances in this way. A food vacuole or phagosome forms, which fuses with a lysosome. Vesicles are much smaller than in phagocytosis.

-Pinocytosis: a vesicle forms to bring small dissolved substances or fluids into a cell. Vesicles are much smaller than in phagocytosis. Pinocytosis is constant in endothelial (capilary) cells.