Cells 3- Cell Membranes

Question 1

Why must cells be separated from their environment

Answer 1

to maintain complex order in a chaotic physical world

they need to control their internal environment- compartmentalisation.

Question 2

How does the plasma membrane appear on a transmission electron micrograph?

Answer 2

membrane is 2 dark bands separated by an electron lucent interior

Question 3

What is the typical thickness of a plasma membrane

Answer 3

3-7nm- varies from cell to cell.

Question 4

What is the role of the plasma membrane

Answer 4

forms the limit of the cell (= plasma membrane or cell membrane)
surround intracellular compartments (organelles)
Selective permeability:
impermeable to macromolecules, biochemical intermediates- need to keep these inside the cell.
permeable to nutrients, waste products

transfer of information (= signal transduction)- cell signalling- to detect and respond to the environment

Question 5

Describe the formation of the phospholipid bilayer in an aqueous environment.

Answer 5

Phospholipids have a hydrophilic head (polar) and hydrophobic tail. To avoid water, the tails pack together.
Suspended in water they form micelles or droplets. They can also arrange themselves into bilayers (a layer two molecules thick, two opposing layers of lipid), called “liposomes”.

Question 6

What does amphiphilic mean

Answer 6

contain both hydrophobic and hydrophilic groups. Hydrophilic groups contain anionic and cationic groups, and can be net anionic or neutral.

Question 7

Describe the structure of a phospholipid

Answer 7

Glycerol links hydrophilic head with hydrophobic tail. Phosphate linked to 3 carbon of glycerol. If fatty acid is saturated- straight. Unsaturated- kink.

Question 8

What is the overall charge of Phosphatidylcholine

Answer 8

Choline has a positive charge, phosphate has a negative charge- hence neutral overall.

Question 9

How does the presence of unsaturated or saturated fatty acids effect the permeability of the membrane.

Answer 9

Kink causes sparsely packed membrane- don’t pack close enough- this created different regions of membrane for different functions- proteins can fit there.
Saturated- More densely packed
- “lipid rafts”

Question 10

How does SM differ

Answer 10

Long chain of acyl based ceramide- not glycerol

Question 11

What is the role of cholesterol in the plasma membrane

Answer 11

decreases permeability

modulates membrane stiffness

• affects interactions with cytoskeleton
  . Packs into gaps between phospholipids- especially in unsaturated region- making it less permeable- as it packs the membrane.

Question 12

What are the properties of cholesterol

Answer 12

Steroid based Rings and tail are hydrophobic- hydroxyl head makes it hydrophilic. Cholesterol is a steroid.

Question 13

Where are the glycolipids

Answer 13

Glycolipids (with sugar headgroups,) on the extracellular side (leaflet) of the membrane

Question 14

How are phospholipids arranged in the bilayer

Answer 14

Differential distribution of phospholipids across membrane- cytoplasmic region has negative charge. Extracellular side is neutral.

Question 15

Can phospholipids switch sides in the bilayer?

Answer 15

Hard- have to move hydrophilic head groupd through hydrophobic core- requires a lot of energy.

Question 16

Can lipids move (diffuse) laterally within a monolayer?

Answer 16

Yes, no restrictions.

Question 17

What was the issue with the artificial membranes testing the early model of the plasma membrane

Answer 17

Cell membranes are more flexible than artificial lipid bilayers
Cell membranes let water through more than lipid bilayers

Question 18

What evidence lead to the fluid mosaic model.

Answer 18

In the 1970s it was found that membrane flexibility as observed in movements such as ‘ruffling’ could not be explained by the current models
More fluid than calculations and models predicted. Fluid as they contain proteins which break up order in the phospholipid bilayer- making it more fluid.

Question 19

Describe freeze-fracture electron microscopy.

Answer 19

Cells frozen and hit with sharp blade- fracturing the membranes down the middle- dots showed proteins
This was also evidence for the fluid mosaic model.

Question 20

Describe the fluid mosaic model.

Answer 20

In the ‘fluid mosaic model’, proteins float in a “sea” of lipids in either leaflet of the bilayer, or span both leaflets of lipids that form the bilayer. Could be transmembrane proteins- intrinsic proteins or peripheral proteins.

Question 21

Describe the features of transmembrane proteins

Answer 21

The protein regions in the core of the lipid bilayer have a predominantly –helical conformation – hydrophobic amino acid side chains facing hydrophobic core of the membrane.
Proteins disrupt lipid packing and contribute to fluidity.

Question 22

Describe the role of intracellular “fences” consisting of the actin cytoskeleton in the plasma membrane.

Answer 22

Constrains the lateral movement of membrane proteins- sub-compartmentalising the membrane- creating different domains for different functions.

Question 23

What are lipid bilayers permeable to

Answer 23

water molecules and a few other small, uncharged, molecules likeoxygen (O2) and carbon dioxide (CO2) which diffuse freely in and out of the cell (also many drugs).

Question 24

Describe facilitated diffusion

Answer 24

Facilitated diffusion is movement of hydrophilic (e.g. charged) molecules down their concentration gradient through protein pores that hide the ionic charges from the hydrophobic core of the lipid bilayer. Proteins (or protein assemblies) provide a water-filled channel. The channel can be ‘gated’.

Question 25

What are lipid bilayers impermeable to

Answer 25

Cations - K+, Na+, Ca2+ (but some do leak through, down the concentration gradient) Anions - Cl-, HCO3- small hydrophilic molecules like glucose macromolecules like proteins and RNA

Question 26

How does the protein and lipid composition of the plasma membrane differ between cell types and why is this important.

Answer 26

The proportion of protein to lipids varies between cell types. The protein composition is different in the inner and outer leaflets of the lipid bilayer. The protein composition also depends on the organelles. Myelin Sheath- more lipid as it is a better electrical insulator- don’t want the ions to be dissipated. Mitochondrial Inner Membrane- More protein- protein complexes for electron transport chain.

Question 27

Describe the role of symporters and antiporters

Answer 27

Symporters - sugars and amino acids can be dragged into the cell with Na+, as it moves down its concentration gradient Antiporters - other molecules can move in the opposite direction to Na+ (e.g. H+; Na+-H+ exchanger for intracellular pH regulation)

Question 28

Describe the typical chemical gradients and charge gradients of a plasma membrane.

Answer 28

Outside cell: High Na+. low K+, High Cl-. Inside the cell there is an excess of negative charge- due to the presence of negatively charged proteins and lipids which cannot pass through the phospholipid bilayer.Chloride ions (Cl-) tend to move inward down the concentration gradient through chloride channels. However, Excess negative charge inside the cell (from non-diffusible proteins and lipids) tends to push chloride ions back out of the cell.- opposes chemical gradient

Question 29

What is the 'steady state' and why is it important that it is not reached.

Answer 29

Steady state (not really equilibrium) can be reached so that the actual ratio of intracellular and extracellular concentrations depends on the existing membrane potential- if let long enough equilibrium would be reached- but cells needs gradients for functions. The Na/K pumps prevents the dissipation of ion gradients.

Question 30

Describe the importance of the Na+/K+ pump

Answer 30

The high concentration of fixed anions inside cells (proteins and negatively charged lipids) and their accompanying cations means that water is drawn into the cells by the resulting osmotic gradient. The high concentration of ions (Na+ and Cl-) in the extracellular space means that there is an opposing osmotic gradient. Na+ will tend to move down its concentration gradient into the cell. The Na+-K+ ATPase maintains the osmotic balance and stabilises the cell volume by exporting Na+. The Na+ gradient is thus maintained- and osmotic balance- cell does not swell or shrink. The Na+ gradient is also used to drive the transport of sugars and amino acids (i.e. Symport).

Question 31

Describe the structure of the Na/K pump

Answer 31

The sodium-potassium pump is found in the plasma membrane of all cells and consists of two polypeptide chains, alpha and beta, with 1000 and 300 amino acids, respectively. The alpha chain spans the membrane 10 times, forming a hydrophilic pore through which the cations (X+) can move. The beta chain is a controller- regulatory

Question 32

Explain how the Na/K pump functions

Answer 32

Transport of 2K+ from left (extracellular) to right (intracellular) in exchange for 3Na+. Therefore it is “electrogenic”, i.e. creates a negative intracellular potential- charge separation created- 3 Na+ bind- ATP binds to aspartyl residue- providing the energy to change the conformational shape of the channel which transports the ions through the membrane. one phosphate group stays bound- this has high affinity for K+ ions- binding causes conformational change, releasing phosphate group- original state.

Question 33

What are the consequences of the actions of the Na/K pump

Answer 33

Ionic gradients are created: less Na+ and more K+ inside the cell than outside. A charge gradient is created, as more positive charges are pushed out than are coming in. This results in the inside of the cell being at a more negative potential than the outside.

Question 34

Describe the formation of the membrane potential

Answer 34

Due to a difference in electric charge on the two sides of a membrane. Can result from activity of electrogenic pumps, e.g. Na+-K+ pump. Also main contributor to potential across mitochondrial inner membrane (electrogenic H+ pump, leading to ATP synthesis). Can result from passive ionic diffusion. A major contributor across plasma membrane of animal cells. K+ inside the cells is high, to balance the fixed anions, and is pumped into the cell by the Na+-K+ ATPase (K+ also can travel through K+ leak channels). [K+]i = 166 mM; [K+]o = 5 mM Near equilibrium for K+: attracted into the cells by fixed anions, and moving out of the cells down the concentration gradient. This imbalance gives rise to a membrane potential: of ~ -70mV

Question 35

Describe the characteristics of the k+ channel

Answer 35

The potassium channel consists of four subunits, with a pore in the middle. Membrane channels are targets for plant and animal toxins (scorpion toxin for K+-channel). It is highly specific - the toxin in the channel environment mimics the K+ and its surrounding water molecules .

Question 36

Describe glucose reabsorption in the PCT

Answer 36

Glucose is membrane-impermeant- requires a channel in the kidney, glucose is reabsorbed from the filtrate in the early proximal tubule lumen (where it is at a low concentration) into the cell where it is at a higher concentration, i.e. against its concentration gradient, and subsequently into the bloodstream on the opposite side of the cell where the concentration is lower Glucose binds to a specific glucose transporter which functions by a flip-flop mechanism – symport The transport is ‘facilitated’, glucose is cotransported with Na+ Na+/K+ pump pumps Na+ back out of PCT to maintain conc gradient. Glucose dissolves down conc gradient into blood.

Question 37

Describe pinocytosis

Answer 37

Pinocytosis: engulfment by the membrane of extracellular solute and small molecules which end up in small intracellular membrane-bound vesicles.

Question 38

describe phagocytosis

Answer 38

Phagocytosis: engulfment by the membrane of extracellular objects such as bacteria, cell debris, other cells, specifically bound to the cell membrane by receptors. Signalling triggers actin cytoskeleton rearrangement. Again these end up in intracellular membrane-bound vesicles.

Question 39

describe exocytosis

Answer 39

Exocytosis: Movement of proteins (e.g. hormones, blood clotting factors) and other molecules from intracellular vesicles into the extracellular space by fusion with the cell membrane.

Question 40

Describe cell signalling

Answer 40

Non-polar-steroid based hormones/prostaglandins- can pass through bilayer- bind to complementary receptor in cytoplasm or on chromosome. Non-steroid based hormones- cannot pass through plasma membrane- bind to complementary trans-membrane receptors- causing conformational change of receptor triggering a secondary messenger.