Cell Membranes

Question 1

Why is the membrane known as fluid

Answer 1

Because phospholipids can move laterally in the membrane

Question 2

4 main types of phospholipids mainly found in eukaryote plasma membranes

Answer 2

Phosphatidylcholine
Phosphatidylethanolamine
Phosphatidylserine
Sphingomyelin

Question 3

What does amphipathic mean

Answer 3

Means phospholipids have a polar hydrophilic head and non polar hydrophobic fatty acid tails that are insoluble
Regions of polar and non polar

Question 4

How many fatty acids does each phospholipid have
How long are they
What are the two different types of chain

Answer 4

Each phospholipid has 2 fatty acid chains
They can be 12-20 carbons long
They can be saturated or unsaturated

Question 5

Structure of phosphatidylcholine

Answer 5

Glycerol backbone with a phosphate attached and then a choline group attached to the phosphate
2 fatty acid chains attached to the glycerol

Question 6

What are the charges of the 4 phospholipids

Answer 6

All neutrally charged expect for phosphatidylserine

Question 7

Structure of the 4 phospholipids

Answer 7

All have a glycerol backbone expert for sphingomyelin which has a sphingersine backbone and is a sphingersine molecule with a fatty acid chain, a phosphate and then attached to the phosphate a choline group

Question 8

What does the amphipathic nature of phospholipids cause them to do

Answer 8

Causes them to form
Lipid micelle- the fatty acid tails face inwards and the phosphate heads face outwards to form a sphere
Lipid bilayer- the fatty acids are inside and the phosphate heads are on the outside this then spontaneously close to form sealed compartments as it is energetically unfavourable for the sides of the bilyaer- fatty acid tails- to be exposed to the aq environment to seal to form energetically favourable compartments

Question 9

Why is fluidity important in the membrane

Answer 9

• allows lipids and proteins to diffuse in the lateral plane and interact with each other
• allows membranes to fuse with other membranes
• ensures cells are shared equally between daughter cells after splitting
• allows for cell mobility

Question 10

How do bacteria and yeast regulate the fluidity of their membrane

Answer 10

In colder environments they synthesise shorter fatty acid chains that are unsaturated this decreases the interactions between fatty acids keeping the membrane more fluid at lower temperatures

Question 11

Role of cholesterol in membranes

Answer 11

Modulates the properties of lipid bilayer

Inserts between fatty acid tails as it is amphipathic so tightens the bilayer decreasing permeability of small molecules

Question 12

Where are lipid bilayers formed

Answer 12

They are formed in the endoplasmic reticulum on the outer cytosolic leaflet

Question 13

What are the names of the two layers of the membrane of the endoplasmic reticulum

Answer 13

The endoplasmic reticulum membrane has the outer cytosolic leaflet and the inner luminal leaflet

Question 14

Synthesis of phosphatidylcholine

Answer 14

Fancy acid binding protein transports the fatty acid to the outer cytosolic leaflet of the endoplasmic reticulum- this is so it doesn’t form a micelle while free in the cytosol

The fatty acids are embedded into the membrane and in a succession of steps the glycerol, phosphate and choline are added

The endoplasmic reticulum enzyme scrambles catalysed flip flop which is transmembrane movement so the phospholipids distribute equally between the outer and inner leaflet

Question 15

What happens once phospholipids have been synthesised at the ER

Answer 15

The newly synthesised membrane is transported to the plasma membrane and other organelles
During this process the inner luminal leaflet of the ER becomes the extra cellular leaflet of the plasma membrane
Plasma membrane flippase flips phosatidylethanolamine and phosphatidylserine from the extra cellular leaflet of plasma membrane to the inner cytosolic leaflet. e plasma membrane is therefore asymmetric

Question 16

What are glycolipids structure and location

Answer 16

They are located on the extra cellular leaflet of the plasma membrane
They are based off sphingosine and they contain a sugar

Question 17

How are the glycolipids and glycoproteins distributed in the plasma membrane

Answer 17

Glycolysation occurs in the lumen of the endoplasmic reticulum or Golgi therefore the glycolipid is in the inner lumen as the enzyme that adds it in is in the inner lumen of the endoplasmic reticulum budding then occurs and the glycolipid is transported in a vesicle to fuse with the plasma membrane and end on the extra cellular leaflet

Question 18

What are the three types of integral protein

Answer 18

Single pass transmembrane protein- alpha helix
Multi pass transmembrane protein- multiple alpha helix
Multi pass transmembrane protein- beta pleated sheet

Question 19

How are soluble membrane proteins passed through the membrane from the ribosomes to the lumen of the ER

Answer 19

Proteins are synthesised by ribosomes in the cytoplasm
Soluble proteins for export from the cell contain 15-20 amino acid hydrophobic peptide signal at the N-terminal

The signal peptide directs the growing polypeptide chain as it emerges from the ribosome to a translocator in the ER and the growing polypeptide is threaded through the membrane

The signal peptide is cleaved and the soluble protein is free in the ER lumen

Question 20

How are single pass transmembrane proteins inserted into the membrane

Answer 20

Some proteins contain a signal start transfer peptide and a stop peptide
The signal peptide directs the growing polypeptide chain to the translocator and the growing polypeptide is threaded through the membrane
When the translocator interacts with the stop transfer sequence translocations stops and the translocator discharges the protein laterally into the bilyaer

Question 21

Integration of a double pass transmembrane protein into the membrane

Answer 21

They have an internal sequence (start transfer sequence) and a stop transfer sequence so the start sequence is not at the N terminal instead internal in the protein so is directed to the translocator and is threaded through and when the translocator gets to the stop transfer sequence translocation stops and the protein is discharged laterally into the membrane and the signal peptide is not cleaved off

Question 22

Why are glycoproteins important

Answer 22

Cell recognition, protection, inflammatory response

Question 23

What is the glycocalyx

Answer 23

Carbohydrate rich layer surrounding cells consisting of glycoproteins and glycolipids
Protects cells against chemical and mechanical damage

Question 24

Nucleus double membrane

Answer 24

The nuclear envelope consists of an inner and outer membrane which is continuous with the ER
The environment is penetrated by nuclear pore complexes allows bidirectional exchange between nucleoplasmic and cytoplasmic compartments

Question 25

Nuclear pore complex

Answer 25

Largest protein complex in the cell More than 400 individual protein molecules 30 distinct nucleoporins Incorporates internal and peripheral membrane proteins

Question 26

Why is it important for ions and small molecules to be able to cross the membrane

Answer 26

Regulation of intercellular ion concentration Uptake of nutrients Excretion of metabolic waste

Question 27

What is passive transport and the two different types

Answer 27

``` Doesn’t require energy and is driven by concentration electrochemical gradients -simple diffusion No membrane proteins are involved -facilitated diffusion Membrane proteins are involved ```

Question 28

What is a membrane potential

Answer 28

Difference in charge between the inside and outside of a cell

Question 29

What is an electrochemical gradient

Answer 29

This combines the chemical gradient- move from area of high concentration to an area of low concentration- and the membrane potential- positive ions will move towards negative vice versa

Question 30

What factors determine if simple diffusion can happen

Answer 30

Concentration gradient Electrochemical gradient Hydrophobicity Size Membranes are impermeable to ions

Question 31

What are the two classes of membrane proteins involved in facilitated diffusion

Answer 31

Channel proteins | Transporter proteins

Question 32

Characteristics of ion channels

Answer 32

Channels are required to transport ions across the membrane as they are impermeable to ions Ion channels exhibit ion selectivity Are driven by concentration and electrochemical gradients They are fast and are regulated- open and close in response to a stimulus

Question 33

Potassium ion channels

Answer 33

They are the most common ion channel and are present in all cells Potassium leaky channels are constantly open They transport potassium from inside the cell to outside

Question 34

How are potassium ion channels selective to potassium

Answer 34

Anions are prevented from entering the channel by negatively charged amino acids at the pore entrance The hydrated potassium ions are too big to pass through the selectivity filter and must be dehydrated which requires energy The energy lost from dehydrating potassium ions is precisely regained by the dehydrated potassium ions interacting with carbonyl oxygens of amino acids lining the selectivity filter Sodium ions would not be able to be transported through this channel as it would be energetically unfavourable as 2 packs of energy would be lost as 4 packs would be required to dehydrate the ion and only 2 packs of energy would be regained on binding with the selectivity filter

Question 35

Characteristics of transporters

Answer 35

They exhibit selectivity They are slow They oscillate between two configurations- state a and state b regardless of whether the solute is bound They transport solutes down their concentration and electrochemical gradient They transport down concentration gradient as when oscillating it is more likely that solute on the side of the membrane with high concentration will enter the solute binding site and then is released on the side with less concentration It is less likely solute will enter the solute binding site on the side with less concentration as less solute present so less likely to pick up solute when oscillating

Question 36

What is the name of the channel protein that transports glucose across the membrane

Answer 36

Gluts They are uriporters as they only transport glucose Produced by most cell types 12 pass membrane spanning protein They are transporters so oscillate between two configurations There are many isoforms

Question 37

Transport of glucose into erythrocytes by glut 1

Answer 37

Glucose is higher in the blood than within the erythrocytes Transported into the cell by glut1 down concentration gradient Glut1 works in both directions so essential conc gradient is maintained which is achieved by glucose being converted to glucose-6-phosphate once in the cell which is not recognised by glut1 so one directional transport is maintained

Question 38

What is active transport and what are the two classes of membrane protein that are involved

Answer 38

Transport of inorganic ions and small molecules actively against concentration and electrochemical gradient ATP-driven pumps Coupled transporters

Question 39

Characteristics of ATP-driven pumps

Answer 39

Transport solutes against concentration and electrochemical gradient by expending energy-ATP hydrolysis This is primary active transport

Question 40

Example of ATP driven pump and characteristics of this pump

Answer 40

Na+ K- ATPase Expressed by all cells Hydrolysed ATP to continually expel sodium ions from the cell in exchange for potassium ions It maintains the sodium potassium electrochemical gradient Without them ions would flow down conc gradient which would ruin oncotic balance and preventing secondary active transport

Question 41

Process of Na+ K+ ATPase

Answer 41

The channel has an alpha and a beta subunit -the alpha subunit is the catalytic subunit 3 sodium ions bind and the pump hydrolysed ATP and is phosphorylated to form ADP this causes the pump to undergo a conformational change and sodium is transported across the membrane and released 2 potassium bind and the pump is dephosphorylated which causes the pump to return to its original conformation and potassium is transferred across the membrane and released

Question 42

What are coupled transporters

Answer 42

They move solutes against the concentration electrochemical gradients by coupling their transport to the sodium gradient created by sodium-potassium ATPase They do not depend directly on the hydrolysis of ATP

Question 43

Two types of coupled transporters

Answer 43

Symport- same direction | Antiport- different directions

Question 44

Example of coupled transporter symporter and characteristics of this

Answer 44

Sodium glucose symporter Expressed by epithelial cells lining the GI tract Glucose conc is higher in the epithelial cells that in the lumen Symporter utilise then sodium electrochemical gradient to transport glucose into the cells against its concentration gradient

Question 45

Steps in the transport of glucose using the sodium/glucose symporter

Answer 45

Oscillates between two conformation state a and state b Binding is cooperative- the binding of sodium increases the affinity of the transporter for glucose so glucose is more likely to bind on the extra cellular side where sodium concentration is high which leads to glucose entering the cell more often than leaving the cell

Question 46

Why are glucose symporter required for the adsorption of glucose from the GI tract getting glucose from lumen to the blood supply where it can be used

Answer 46

Because glucose has to pass through two membranes through the epithelial cell and then through the basement membrane and it cant pass through paracellular junction as there are tight junctions which restrict paracellular movement Barrier and fence functions