BIOL 2402 Chapter 3
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Transcript of BIOL 2402 Chapter 3
BIOL 2402 Chapter 3
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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