1

Membrane Structure and Function

Transport of Substances through the cell membrane

Chapter 2 (cell membrane parts) and 4 of Guyton and Hall, 11th ed.

2

Lecture outline

I. Membrane Function and StructureA. PhospholipidsB. ProteinsC. CarbohydratesD. Cholesterol

II. Transport across the membraneA. Passive

i. Osmosis and osmotic pressure

ii. Simple a. Factors that

influence b. Examples

iii. Facilitated a. characteristics

iv. Rates of simple vs. facilitatedB. Active

i. Primaryii. Secondary

• In the cell membrane are phospholipids, proteins, sugars, etc., that separate intra and extra cellular fluid, and limit what can travel through it. Proteins create channels or pores. They can be perceived as antigens. There are some proteins that are only on the inside of the cell membrane, which turn on activities in the cell.

3

• Phospholipids are antipathic (water loving and water hating). One side can attach to other water loving molecules on the phosphate portion. The two fatty acid (FA) tails are made of long chains of hydrocarbons. The FAs dislike water, but can bind with hydrophobic molecules. If we only had one layer of phospholipids, the membrane would orient toward the water loving fluid. But FAs don’t like the water, so with a bi-layer, the FA can be happy, and the phosphate can be happy. Substances that love lipids can get to the middle of the membrane, but water loving has a hard time crossing.

4

5

Membrane Function• Organizes chemical activities

of cell– separates cells from outside

environment– controls passage of molecules

across membranes– partitions organelle function in

eukaryotes– provides reaction surfaces and

organizes enzymes and their substrates

– Proteins embedded provide function, too!

• Channels• Carriers• Receptors• Cell adhesion• Enzymes• Identification markers, etc.

6

Membrane Structure– phospholipids have polar “head”

(hydrophilic) and nonpolar “tail” (hydrophobic)

– form stable bilayer in water with heads out and tails in

– hydrophobic interior from fatty acid tails forms barrier to hydrophilic molecules

– Chemistry: glycerol + two fatty acids and phosphate head

• Proteins can be integral (throughout the membrane) or peripheral (one side or the other).

• Integral protein can create a pore, or channel with a gate that can open and close. Proteins on the surface of the membrane can bind a chemical. A peripheral protein on the inside of the membrane can instigate a series of enzymatic reactions within the cell. Some proteins can bind substances on the outside of the membrane and transport them into cell (facilitative diffusion)

7

8

•Provide function to a membrane•Can move laterally•membrane also shows “sidedness”

•interior - attachment to cytoskeleton•exterior - carbohydrates, extracellular matrix (next slide)

• defined by mode of association with the lipid bilayer– integral: channels, pores, carriers, enzymes, etc.– peripheral: enzymes, intracellular signal mediators, etc.

K+

Proteins:

• Sugars outside of the cell can attach to the phosphate heads or to the proteins (that will now be a glycoprotein). If there are many glucose molecules on the outside of the cell, it will make the outside of the membrane negatively charged. Every cell is set up like a battery, with a separation of charges across the cell membrane. This is called potential; one area is more negative than another area. There is storage of electricity, like a battery. The inside of the cell should be more negative than the outside of the cell. But if there is a glycocalyx (sugar bundle) on the outside of the cell, they make it a negative charge.

9

10

•glycoproteins (majority of integral proteins)• proteoglycans•glycolipids (approx. 10%)• involved in cell-cell attachments/interactions• play a role in immune reactions

GLYCOCALYX

(-) (-) (-) (-)(-)

(-)(-)

Carbohydrates

• Cholesterol maintains the fluidity of cell membrane so the lipids are not frozen in place, but not so much that there are gaps in the cell membrane. There needs to be a balance of flexibility and stability. Cholesterol is a lipid, so it’s located in the middle of the membrane. If you try to apply a lipid to a phospholipid membrane without proteins in it, and you will see that hydrophobic molecules get through it easier than hydrophilic. Gases like CO2, O2, and small molecules like ethanol could get through. If you try to add water loving molecules (charged molecules like glucose and positive ions), they can pass. Water can also get through. Water loving substances get across the lipid center by active and passive transport.

11

12

• present in membranes in varying amounts• increases membrane FLEXIBILITY and STABILITY during different temperatures• helps to increase hydrophobicity of membrane

(-) (-) (-) (-)(-)

(-)(-)

Cholesterol:

13

Transport across a membrane: Understand this!

• barrier to water and water-soluble substances•Allow lipid soluble substances to cross through membrane

hydrophilic“head”

hydrophobicFA “tail”

ions H2O

CO2

LIPIDS by themselves are a:

O2

N2

glucose

14

Movement across the cell Membrane

ionsglucose

H2O

… but, in a living cell, hydrophilic molecules still get across! How?

CO2O2N2

15

Passive Transport Active Transport• occurs down a concn. gradient• no mediator (simple) •or involves a “channel” or “carrier”(facilitated)• no additional energy beyond kinetic energy

• occurs against a concn. gradient• involves a “pump”• requires cellular ENERGY (ATP)

Figure 4-2; Guyton & HallPassive transport

Osmosis &

• Passive transport means no cellular energy required, no ATP used.

• Active transport means ATP is used, either directly or indirectly.

• Passive transport makes substances move from high to low concentration, down their gradient.

• Active transport is when at least one solute is moved against its concentration gradient.

16

• Osmosis is passive, no ATP is used. Water moves from high to low concentration. That is, water moves from low particles to high particles. If you have two sides of a membrane, and the particles can’t move, water will move. How does it get through? There are aqua pores created just for water passage. You have to have a gene to make these pores. The wrong amount of pores causes water imbalances.

17

18

Osmosis:- net flow of water across a semipermeable

membrane (permeable to water but not solute) Osmosis occurs from pure water toward a water/salt solution. Water moves down its concentration gradient.

Figure 4-9; Guyton & Hall

Sel

ectiv

ely

perm

eabl

e m

embr

ane

This movement is affected by the solute concentration (osmotic force) and hydrostatic forces (more on this later in the course

• When you did the PhysioEx osmosis activity, you applied pressure, did not see volume change, just measured the hydrostatic pressure.

• The idea of molarity and osmolarity is expressed in this example:

• Solution A is 100 g of something added to water• Solution B is 1000g added to water.• The g% is different.

19

• To calculate molarity, you have to divide grams by molecular weight. Both of the above solutions have same molarity, one mole per liter. That means they both have the same number of molecules. They are different sizes, but still the same number. If neither side dissociates, same number of particles, but if A dissociates into 3 particles, how many osmoles is it? Three. If A is separated from B by a membrane that only allows water to move, where will water go? It moves from B to A, and the volume in A will climb, unless you apply pressure (3osm) to stop it from rising. Molarity is the number of molecules.

20

21

Major determinant of osmotic pressure- differences in total solute or particle concentration NOT MASS!

Which has the greatest molar concentration?Which has the greatest number of molecules?(6.02 x 1023 Molecules)Which solution has the greatest osmolality? (assume no dissociation)If “A” has a dissociation factor of 3, now which solution has the greatest osmolality?

Solute AMw = 100

Solute BMw = 1000

100 gin 1 L

1000 gin 1L

mOsm (millisomolar) = index of the concn or mOsm/L of particles per liter soln

mM (millimolar) = index of concn of or mM/L molecules per liter soln

• Simple diffusion of a solute is also passive. Rate of diffusion depends on

• How big is the gradient? How steep is the slide? The greater the difference, the faster the rate of diffusion, if the solute is permeable across the gradient.

• Is the solute permeable?

22

• Simple diffusion of small molecules can diffuse without any protein assistance. Water loving larger molecule needs a protein. Some pores are open all the time, and those that can use it, will diffuse when they want. If always open, is a pore. If not always open, it is a channel. Channels are gated. The gate can be open or closed. They open when a special chemical (ligand) binds to it, called ligand operated channels (LOC), like a key. Some open by electrical change, like garage door opener, called VOC voltage operated channel. If it doesn’t have permeability, gate closed, can only get through slowly. If it is open, solute can diffuse. There is no ATP used, not active transport.

23

24

Non-carrier mediated transport

1. Simple Passive Diffusion • is tendency of molecules to

spread out spontaneously from area of high concentration to area of low concentration

• At equilibrium, there is not net gain nor loss of cell fluid.

• It is passive; molecule diffuses down concentration gradient without input of cellular energy

• Need permeability • Need concentration gradient

(chemical/ electrical) Can a molecule move from side B to side A?

Figure 4-8; Guyton & Hall

25

(a) lipid-soluble molecules move readily across the membrane(rate depends on lipid solubility)

(b) water-soluble molecules cross via channels or pores (these are proteins!).

•ungated•Gated channels- Chemical and Electrical gated channels

(c) Different molecules diffuse independently of each other

(a) (b)

Simple Diffusion

Voltage gated channel

• These cause a change in the electrical potential (separation of charges). They are specific, for instance, one may only allow sodium to cross. You would need a different one to allow potassium to pass. The amino acids dictate what things can go through them. Need many different types of proteins, many pores.

• Ligand gated channel

• A chemical binds to open the gate. 26

27

ungated• determined by size, shape, distribution of charge, etc.

Characteristics:

Na+

in

outNa+ and other ions

gated• voltage (e.g. voltage-dependent Na+ channels)• ligand activated (e.g. nicotinic ACh receptor channels)

Ion Channels- allow simple diffusion

How was this discovered?

• “Patch Clamp”• Nobel Prize in Physiology & Medicine -1991

• Neher and Sakmann

Na+

in

out

• Facilitated diffusion is still passive, no ATP is used. It is the same end result as simple diffusion. The difference is that it requires a protein to physically bind to it and move it across the cell membrane. Therefore, it can be saturated. The rate at which solute is moved is limited by the number of carriers you have. When drunken people in a bar want to go home when the bar closes, and there is only one taxi, it would take a long time for all the people to get home. To get home faster, need more carriers. If each carrier moves one carrier, rest of molecules has to wait their turn. If there are too many glucose molecules in the nephron, you will reabsorb some of them them in the bloodstream, and some will spill out in the urine. This is because glucose transporters are saturated in the nephron.

29

30

Facilitated Diffusion (also called carrier mediated diffusion)

Figure 4-7; Guyton & Hall

• Specific proteins facilitate diffusion across membranes

– no cellular energy required

– Carrier protein interacts with solute

– Specificity – carrier only acts upon specific substrates.

– Saturation – the rate of transport will reach a maximum based on the number of carriers available

in the membrane. (This is animated on next slides)

Rate of diffusion is limited by • Vmax of the carrier protein • the density of carrier

proteins in the membrane (i.e., number per unit area)

31

1

2

3

4

5

=solute= transporter

Ex. Pass-through rate is 1 each minute

Transport maximum is reached when carriers are saturated, Vmax.

1/min

2/min

3/min

4/min

5/min

Rate of Simple vs. Facilitated Diffusion

• If you increase concentration gradient, rate increases as well.

• Facilitative will reach velocity maximum. When it is saturated, it levels off.

32

33

rate of diffusion

Concn of substance

simple diffusion

Simple vs. Facilitated

Tm

facilitated diffusion

What limits maximum rate of facilitated diffusion?

Vmax

rate of diffusion (Co-Ci)

Primary active transport

• This uses ATP directly. A protein whose name ends in ATPase is one that hydrolyses ATP, creating a concentration gradient. Going skiing, do you climb the mountain? No, you take the lift, using the energy in the chair lift. Ski down a slope with a rope around your waist, that rope will pull up the next person. That provides the stored energy to pull the second person up. This is secondary active transport. The protein does not use the ATP. As one goes down, liberates energy that helps a different molecule to move against its concentration gradient.

34

35

Active Transport

Primary Active Transport– molecules are “pumped”

against a concentration – gradient at the expense

of energy (ATP) – direct use of cellular

energy

Secondary Active Transport– transport is driven by the

energy stored in the concentration gradient of another molecule (Na+)

– One molecule down gradient

– One molecule against gradient

– indirect use of energy

• Most ATPs are used for primary active transport. The most common is sodium-potassium ATPase. It moves two solutes against their gradients. It keeps sodium outside and potassium inside. When a channel is made, the substance that comes first tells you the protein has a preference for that substance. Sodium-potassium ATPase moves 3 sodium ions for every two potassium ions. They still need carrier proteins. This job can only be done at a certain rate.

36

37

Primary Active Transport• Cells expend energy for

active transport– transport protein involved in

moving solute against concentration gradient

– energy from ATP– rate limited by Vmax of the

transporters

• up to 90% of cell energy expended for active transport!– active transport of two

solutes in opposite directions often coupled

Na+/K+ ATPaseplays an important role in regulating osmotic balance by maintaining Na+ and K+ balance requires one to two thirds of cell’s energy!Others exist- calcium ATPase and H+ ATPase

Secondary active transport

• ATP is not directly used by the protein. Three sodium ions are kicked out, 2 potassium ions are pulled in.

• Another integral protein creates a protein, binds to sodium, allowing it to move down the concentration gradient. If it had high levels of glucose, it would pull in glucose against its concentration gradient, and into the cell. This Na-glucose system is a co-transporter. A co-transporter takes two substances in the same direction across the cell membrane. An anti-porter takes two substances in opposite directions.

38

39

Secondary Active Transport

1. Co-transport (co-porters): substance is transported in the same direction as the “driver” ion (Na+)

Examples:

inside

outside

Na+ AA Na+ gluc 2 HCO3-Na+

- co-transport and counter-transport -

40

2. Counter-transport (anti-porters): substance is transported in the opposite direction as the “driver” ion (Na+)

Examples:

Na+

Ca2+

Na+

H+ Cl-/H+

Na+/HCO3-

outside

inside

• Sample test questions: given the following list, answer the questions below.

• Osmosis

• Simple Diffusion

• Facilitative Transport

• Primary active Transport

• Secondary active Transport

• Which has net movement of water? Simple diffusion

• Select all that apply: This type of transport moves solutes down the concentration gradient. Simple, facilitative, secondary,

• Which ones have a solute moved against its gradient: primary and secondary

• Which is moved against its gradient and ATP is directly used: primary

41