Cell Surface Wk2

Question 1

Give a basic overview of cell membranes

Answer 1

• Cells have a membrane to protect inside from outside
• Composed of lipids (mainly phospholipids) and proteins formed into bilayers
• There are two opposing sheets of lipids into which proteins are inserted
• Each lips has a hydrophobic tail and a hydrophilic head which defines the lipid bilayer structure

Question 2

What features does the fluid mosaic model membrane have?

Answer 2

• Can deform membranes
• Proteins can move (dynamic structure)
• Can expand and contract
• Can break off and form other organelles
• Forms a barrier (lipid bilayer)
• Decides what goes in and out of the structure (protein molecule)

Question 3

What is the organisation of lipids in phospholipid bilayer?

Answer 3

• Hydrophobic tails face each other (inner core of membrane)
• Hydrophilic heads face out towards fluids
• Fatty acid chains determine fluidity of membrane
• Charged so can interact with aqueous solutions
• Phospholipids are amphipathic (both philic and phobic)

Question 4

What are the four major phospholipids in the mammalian plasma membrane?

Answer 4

• Phophatidylethanolamine
• Phosphatidylserine
• Phosphatidylcholine
• Sphingomyelin

Question 5

What are intracellular signal transduction lipids?

Answer 5

• e.g. phosphatidylinositol, ceramide etc.
• Minor proportion of phospholipid content of intracellular membranes. Derived from lipids residing in the plasma membrane.
• Rapidly generated/destroyed by enzymes in response to a specific signal
• Spatially and temporally generated = highly specific signal
• Bind specifically to conserved regions found within many different proteins and once bound, induce confirmation and/or localisation and activity change within these proteins

Question 6

What does cholesterol do?

Answer 6

• Inserts (intercalates) between membrane phospholipids
• This tightens packing in the bilayer/membrane rigidity (and density) and decreases membrane permeability to small molecules (goes against fluid mosaic model)

Question 7

Why do biological membranes have to be fluid?

Answer 7

• Allow signalling lipids and membrane proteins to rapidly diffuse in the lateral plane and interact with one another e.g. in cell signalling (tyrosine kinase)
• Allows membrane to fuse with other membranes e.g. in exocytosis (transport vesicles)
• Ensures membranes are equally shared between daughter cells following cell division (membranes have to be fluid in order to split)

Question 8

What are membrane protein functions?

Answer 8

• Transport
• Enzymatic activity
• Signal transduction
• Cell-cell recognition
• Intercellular joining
• Attachment to the cytoskeleton and extracellular matrix (ECM)

Question 9

What are integral and peripheral membrane proteins?

Answer 9

• Single pass or multi-pass transmembrane proteins (integral)
• Peripheral membrane protein
• The transmembrane domains of integral membrane proteins are comprised of hydrophobic amino acids, which are organised into alpha-helical structures

Question 10

What is a transporter?

Answer 10

Transport molecules across membrane - needed for polar/charged ions e.g. Na+ pump - move Na+ across the proteins

Question 11

What are anchors?

Answer 11

Link structures to intracellular scaffolds (intern, actin)

Question 12

What are receptors?

Answer 12

Bind ligands and/or generate a signal inside cell

Question 13

What are signal transduction molecules?

Answer 13

Pass on and amplify signals (e.g. from outside cell)

Question 14

What are two passive transport systems?

Answer 14

Simple diffusion:
- No membrane proteins involved
- Driven by concentration gradients
- Down a concentration gradient
- The ability of a solute to cross the membrane by simple diffusion depends on: concentration gradient, hydrophobicity/charge and size
- Membranes are highly impermeable to ions
Facilitated diffusion:
- Membrane proteins involved (have to help it move)
- Driven by concentration gradients
- Involves membrane proteins - 2 classes - Channels (discriminates mainly on size and charge) and uniporter carrier proteins (involves a binding site for solutes). They transport ions/small molecules across the membrane passively along their concentration/electrochemical gradients.
No energy input (ATP) required for either

Question 15

What is a protein channel?

Answer 15

• Membrane proteins that form hydrophilic pores through the plasma membrane
• Most are non-directional ion channels (fundamentally a channel through the membrane)
• Shows some selectivity e.g. big pore = big ions
• Fast - up to 107 ions per second
• Gated channels offer more control than a simple membrane pore

Question 16

What are uniporter carrier proteins/.

Answer 16

• Glucose transporter (Glut2) in gut epithelia
• Highly selective - transported molecule bound to carrier
• Relatively slow (<1000 molecules per second)
• Glucose passes from outside to inside via con transporter proteins
• Goes from outward facing state to inward facing state

Question 17

What is an electrochemical gradient?

Answer 17

• Negative membrane inside, positive membrane inside - negative membrane means that +ve ions repulsed by positive membrane inside
• An electrochemical gradient (produced by a change in charge = ‘membrane potential’) combines the concentration gradient and membrane potential
• The force driving. charged solute (e.g. Na+ ions) across a membrane is the concentration gradient and the membrane potential

Question 18

Why do cells maintain electrochemical gradients?

Answer 18

• To drive transport across membranes
• To maintain osmotic balance
• Electrical forces inside and outside of the cell must be balanced (most of the time) (though small localised differences at the plasma membrane are allowed)
• Without active transport to maintain electrochemical gradients, ions would flow down their gradients through channels, disturbing the osmotic balance

Question 19

What is active transport?

Answer 19

• It moves solutes against their electrochemical gradients

- To achieve this movement against the electrochemical gradient, energy is required

Question 20

What are ATP driven pumps?

Answer 20

• Moves solutes against the concentration/electrochemical gradient by expending energy (primary active transport)
• E.g. Na+ and K+ electrochemical gradient
• In the absence of Na+/K+ ATPase ions would flow down their gradients disturbing osmotic balance and preventing ‘secondary’ active transport

Question 21

How is the Na+ electrochemical gradient maintained?

Answer 21

1. Protein hydrolyses ATP and is phosphorylated. 3Na+ ions bind.
2. Na+ dependent phosphorylation causes pump to undergo conformational change. 3Na+ pumped out.
3. 2K+ pumped in and pump is dephosphorylated.
4. K+ dependent dephospho rylation causes the pump to return to its original conformation. K+ is transferred across the membrane and released.
   - 30% of cell’s total energy consumption is used in operating this pump
   - Operated continuously to expel Na+ that enters cell through other carrier proteins and channels
   - Hydrolyses ATP to ADP e.g. is both an enzyme and a carrier protein
   - Couples the export of Na+ to the import of K+ hence the name

Question 22

How do cells carry out active transport?

Answer 22

• ATP driven pumps - couple the transport of a solute against its gradient to the hydrolysis of ATP (all done on same protein) - primary active transport
• Coupled transporters - couple the transport of one solute with the gradient to another against the gradient - secondary active transport

Question 23

What are coupled transporters?

Answer 23

• Move solutes against concentration/electrochemical gradient by coupling transport to Na+ gradient created by Na+/K+ ATPase.
• Do not depend directly on the hydrolysis of ATP (e.g. secondary active transport)

Question 24

What is a symport?

Answer 24

• Na+/glucose symporter

- Both things move in together

Question 25

What is an antiport?

Answer 25

• Na+/Ca2+ antiporter

- Work in opposite to each other

Question 26

Explain the Na+/gluose symporter in gut epithelia

Answer 26

• Na+ electrochemical gradient used to drive the movement of glucose against its gradient
• Binding of Na+ and glucose is co-operative i.e. binding of glucose is dependent on Na+ because Na+ is much higher outside the cell glucose is more likely to bind facing extracellular space than intracellular space
• Protein wants both things to bind together - its doesn’t use ATP itself but Na+/K+ ATPase creates electrochemical gradient
• Overall result is that Na+ and glucose enter the cell more often than leave it - net flow is into the cell

Question 27

What is the Na+/Ca2+ anti porter?

Answer 27

• Important in cardiac muscle: cardiac muscle cell contraction is triggered by a rise in intracellular Ca2+ - Ca2+ therefore reduced inside cell to not increase heart rate so much
• More Na+ outside than inside
• Energy of Na+ moving in down its electrochemical gradient forces Ca2+ outside
• The Na+/Ca2+ anti porter reduces intracellular (Ca2+) and thereby reduces strength of cardiac muscle contraction