BIOL 2402 Chapter 3

Online Tutorial

Topics:

I. Review of membrane structure

II. Membrane transportA. PermeabilityB. Diffusion

I. Review of membrane structure

A. the main structure of the plasma membrane is a fluid lipid bilayer:• made of phospholipids and cholesterol• forms the foundation of the membrane• allows the membrane to be flexible• prevents passage of most polar

molecules

1. characteristics of phospholipids

• polar head is hydrophilic and lines up with water in ICF and ECF

• nonpolar tails are hydrophobic and face the inside of the membrane

• these factors cause a bilayer to form automatically whenever phospholipids come into contact with water

• the molecules are not bonded to each other, so they can move around laterally in the membrane

2. characteristics of cholesterol

• individual molecules are positioned between the tails of the phospholipid molecules

• they prevent the fatty acids from crystallizing

• they stabilize the membrane and promote flexibility

B. proteins are embedded in the lipid bilayer

1. location of proteins• proteins “float” in the lipid bilayer• they are not bonded to the

phospholipids• they may extend through the

membrane• or they may be anchored to the inner or

outer surface of the membrane

2. each protein performs a specific function:• channel• carrier• receptor• enzyme• structural• adhesion• self-recognition

C. carbohydrates are located on the outer surface (see previous slide)

1. carbohydrates are attached to lipids and proteins in the main membrane layer

2. functions of carbohydrates:a. responsible for recognition of selfb. allow cells to recognize cells of the same

typec. help control tissue growth

D. cell to cell adhesions

1. cells are held together bya. cell adhesion moleculesb. the extracellular matrixc. cell junctions

2. there are three types of cell junctionsa. desmosomes hold adjacent cells

together at discrete spotsb. tight junctions form impermeable bonds between adjacent cells and prevent the passage of moleculesc. gap junctions form “tunnels” between adjacent cells and allow the passage of ions from the cytosol of on cell to the cytosol of adjacent cells

II. Membrane Transport

A. whether or not a molecule can pass through a membrane depends on:

• the permeability of the membrane• the penetration of the molecule• the presence of energy (kinetic or ATP)

1. membrane permeabilitybiological membranes are selectively or

semipermeablefactors affecting permeability are:a. the presence of a hydrophobic lipid

bilayer prevents the passage of molecules that are not lipid soluble

b. the presence of channels allows specific ions to go through the membrane

c. the presence of carrier molecules allows specific molecules to be transported through the membrane

2. molecule penetrationmolecules that can go through a membrane

are called “penetrating” factors affecting a molecule’s penetration are:a. solubility in lipids: high solubility increases

penetrationb. electrical charge: charge decreases

penetrationc. polarity: polarity decreases penetrationd. size: large size decreases penetration

self-test: which molecule is most likely to pass through a cell membrane?

molecular weight

polarity

X 56 nonpolar

Y 42 polar

Z 187 nonpolar

B. Diffusion 1. the physical basis for diffusion:• molecules have kinetic energy that

causes them to move constantly and in random directions

• as the molecules move around they collide and bounce off of each other

• given enough time they reach an even distribution

• the NET result is that molecules move from an area of higher to an area of lower concentration (down their concentration gradient)

self-test:

will net diffusion occur if there is no concentration gradient?

2. factors that influence diffusion

rate

a. higher temperature increases diffusion rateb. higher concentation gradient increases

diffusion ratec. higher membrane permeability increases

diffusion rated. higher surface area of membrane increases

diffusion ratee. larger molecule decreases diffusion ratef. longer distance molecule has to diffuse

decreases diffusion rate

3. electrical gradients affect diffusion of ions

like charges repel each otheropposite charges attract each other

the combination of the concentration gradient and electrical gradient is called the “electrochemical” gradient

4. osmosis is the diffusion of water

through a membrane

• water moves down its concentration gradient

• water moves away from the higher water concentration

• water moves towards the higher solute concentration

• think of the solute as “pulling” water to its side of the membrane

a. penetrating vs nonpenetrating

solutes

• if a solute can move through a membrane it will eventually become evenly distributed, osmosis will stop, the volumes on both sides of the membrane will be equal

• if a solute cannot move through a membrane it will continue to “pull” water towards it until water movement is stopped by hydrostatic pressure, and the volume on one side will be larger than the other side

b. osmotic pressure

think of osmotic pressure as a force that “pulls” water to the side of the membrane with the highest solute concentration

the side of the membrane with the higher solute concentration has the higher osmotic pressure

300 mOsm

c. tonicitydescribes the effect of the extracellular fluid

(ECF) on the volume of a cellin the situation diagrammed here the

intracellular fluid (ICF) and extracellular fluid (ECF) both have solute concentrations of 300 milli osmoles

ICF ECF

300 mOsm

a. isotonic solutions

• have the same concentration of solutes as the ICF

• do not cause osmosis• do not cause changes in cell volume

300 mOsm 300 mOsm

300 mOsm

b. hypotonic solutions

have lower solute concentrations than the ICFcause osmosis into the cellcause the cell volume to increase

200 mOsm

water

c. hypertonic solutions

have higher solute concentrations than the ICFcause osmosis out of the cellcause the cell volume to decrease

300 mOsm

400 mOsm

water