Cell Membranes & Transport

Question 1

What is the physiological role of membranes?

Answer 1

1) Provide a selectively permeable barrier which protects the interior of the cell and associated organelles, etc. while allowing the movement of certain metabolites in/out of the cell.
2) Allow for transport of various things - waste, O2, nutrients, ions, etc.
3) Give cell shape and structure
4) Compartmentalizations of organelles
5) Offer sites for enzyme and hormone binding
6) Allow for electrochemical gradient to form

Question 2

What is a metabolite?

Answer 2

Small molecules (usually) that are intermediate or end products of metabolism.

Question 3

What is the general composition of a cell membrane?

Answer 3

1) Lipids, proteins and carbohydrates

Question 4

Primary component of a cell membrane

Answer 4

Phospholipid

Question 5

Describe the structure of the membrane

Answer 5

1) Asymmetric bilateral - refers to arrangement of proteins embedded or attached within or to the membrane.
2) Semi-permeable
3) Amphipathic - hydrophilic head and hydrophobic tail (form hydrophobic core, interior)

Question 6

What are the 3 types of membrane lipids?

Answer 6

1) Phospholipids (PL)
2) Glycolipids
3) Cholesterol

Question 7

Phospholipid types

Answer 7

-Type is dependent open the backbone of the lipid

1) Glycerophospholipid (alcohol)
- glycerol backbone, 2 fatty acids and phosphate group
- Ex. Phosphatidylcholine, phosphatidylserine

2) Sphingolipids (SL)
- Sphingosine backbone, 1 fatty acid, phosphorylcholine
- Ex. Sphingomyelin (IMPORTANT!!!)

Question 8

Clinical relevance of sphingomyelin

Answer 8

Niemann-Pick Disease

 - Rare autosomal recessive disorder
 - Results in sphingomyelinase deficiency, preventing breakdown sphingomyelin
 - Build up of lipid leads to hepatoslenomegaly (liver and spleen enlargement) and neurological damage (retardation, seizures) 
 - Victims die before Age 2.

Question 9

Structure of Sphingomyelinase

Answer 9

Phosphorylationcholine + ceramics

Question 10

Describe a glycolipid structure

Answer 10

1) Sphingosine backbone
2) Carbohydrate (oligosaccharide) backbone
- Note: MAKES IT DIFFERENT FROM SPHINGOPHOSPHOLIPID

-Located on outside of lipid membrane (aka outer leaflet)

Question 11

Describe cholesterol structure and location

Answer 11

1) Steroid
2) Hydroxyl group
3) Hydrocarbon side-chain

-Embedded in bilateral, between PLs

Question 12

Why are cholesterol microdomains (area of membrane with distinct structure/function important?

Answer 12

• “Lipid rafts”
• Essential for cell signaling
  - Because

Question 13

Location of membrane lipids

Answer 13

Inside (Inner sheet)

- Glycolipids
- Sphingomyelin
- Phosphatidylcholine

I.e. sphingolipids

Outside (outer sheet)

- Phosphatidylinositol
- Phosphatidylserine
- Phosphatidylethanol

I.e. glycerophospholipids (except phosphatidylcholine)

Question 14

Types of Membrane Proteins

Answer 14

1) Integral - embedded
2) Peripheral - attached
3) Lipid-anchored - tethered

Question 15

Describe an Integral Membrane Protein

Answer 15

1) Protein within (embedded) the bilayer

Ex. Polytopic transmembrane protein

Question 16

What is a polytopic transmembrane protein

Answer 16

An integral membrane protein that goes across the entire width of the membrane and as such, not only interacts with the interior and exterior of the cell but serves and important transport role

- Includes transport proteins, ion channels and receptors
       - Move things in and out of cell and responsible for cell signaling

Question 17

What is a Peripheral protein

Answer 17

• A protein loosely attached to the membrane
• Attached via electrostatic interactions with either lipids or other proteins.

Tip - when you hear peripheral think outside, doesn’t matter so it is loosely held to the cell which wouldn’t mind losing it.

Question 18

What is a lipid-anchored protein

Answer 18

Protein tethered to lipids in the membrane via covalent bonds (“hook”)
-Ex. G proteins

Question 19

Structure and location of carbohydrate in membrane

Answer 19

1) Covalently attached to some membrane lipids and proteins
2) Faces the extracellular environment/space
- Glycocalyx is common shell

Question 20

What is glycocalyx and describe its function

Answer 20

• A carbohydrate shell covering many membranes
• Consists of (1) Glycolipids and (2) glycosylated proteins

Function:

1) Protection/barrier - prevents membrane mechanical breakdown and premature degradation
2) Cell connection/adhesion - allows cell to have better contact with their cells (essential during fertilization and tissue creation/formation)
3) Cellular identification - allows body to differentiate its cells from foreigners (EXAMPLE: glycocalyx on RBC allows for A, B, AB blood type differentiation due to differences in oligosaccharide residues)

Question 21

Why is membrane fluidity important?

Answer 21

1) Proteins and lipid ability to move freely (fluid mosaic) allows for them to go to specific membrane locations to carry out designated functions.

***But, too fluid or too rigid is also not good

Question 22

Factors that impact membrane fluidity

Answer 22

1) Temperature
2) Lipid make up/composition
3) Cholesterol

Question 23

What is Tm

Answer 23

Melting temperature
The temperature at which a membrane goes from a fluid to stiff/rigid state.

too rigid
»>Tm -> too fluid
>Tm -> Juustttt riiighhtt

Question 24

Lipid composition w/ respect to membrane fluidity

Answer 24

Lower membrane fluidity
-Lipids with long, saturated fatty acids
Higher membrane fluidity
-Lipids with short, unsaturated fatty acids (due to kinks in the chain, which prevent dense packing and make MORE FLEXIBLE)

Saturation and fluidity are inversely proportional

Question 25

Cholesterol w/ respect to membrane fluidity

Answer 25

Depending on other factors, membrane fluidity can increase or decrease. ``` Rigid membrane (high saturated fat) + cholesterol = increase in fluidity Fluid membrane (high unsaturated) + cholesterol = decrease in fluidity ``` Buffers a potentially large impact on fluidity from temperature

Question 26

Describe membrane permeability (i.e. types of molecules)

Answer 26

Membrane is permeable to lipophilic molecules (diffusion) while being impermeable to hydrophilic molecules Hydrophilic molecules need transport protein

Question 27

What is a transport protein

Answer 27

A type of transmembrane protein that allows molecules, ions, etc. to move in and out of the cell by either passive or active means

Question 28

What are the important ion gradients for cells? Describe the concentrations of each ion in and out of a typical neuronal cell (In mM).

Answer 28

Na+ / K+ / Ca2+ / Cl- E. 145 / 4 / 2 / 50 ———————————— In. 15. /. 150/. 10. /. 10^-8 (Cl- has 10,000 fold gradient!)

Question 29

Describe Passive Transport

Answer 29

- Does not depend on energy to move molecules across the membrane - Molecules move across membrane by moving from high to low concentration (I.e down their concentration gradient)

Question 30

Describe Active Transport

Answer 30

-Molecules are moving against their concentration gradient (low to high concentration) and thus require transport that NEEDS ENERGY to move them across membrane. ATP hydrolysis provides energy (-12kcal/mol)

Question 31

Describe the types of passive transport

Answer 31

1) Simple Diffusion - No assistance required - Only for small, non polar and polar, uncharged molecules - Greater concentration difference (steeper gradient)=faster diffusion 2) Facilitated Diffusion - Assistance from Transmembrane Proteins required - Function as (a) ion channels or (b) transporters - For large, charged molecules - Really increase speed of transport (similar to enzymes and rxns) - Examples. Aquaporin (for water transfer), Voltage-gated Na+, Glucose transporter

Question 32

Explain the types of facilitated diffusion

Answer 32

1) Ion channels - openings or “gates” in the bilayer that allows charged and polar molecules to pass through (again, down concentration gradient) - very common, extremely present throughout 2) Ligand-gated channel - a gate that undergoes a conformational change when attached by a ligand (neurotransmitter or hormone). The change opens the gate allowing ions to pass through (again, down concentration gradient). - Once ligand is release, gate closes (Ex. Glutamate receptor - antagonist is Mimantine/Namenda) 3) Voltage-gated Ion Channels - open in response to change in membrane electrical potential (large negative charge inside cell)

Question 33

Explain the three conformational states of a Ligand-gated Ion Channel

Answer 33

1) Resting - gate closed 2) Open - ions travel through freely 3) Desensitized - channel doesn’t respond to ligand - remains closed

Question 34

Explain depolarization in regards to Voltage-gated channels

Answer 34

Occurs when a positively charged ions come into the cell, creating a large increase in membrane potential - this is what triggers the opening of the gated channels, allowing SPECIFIC ions to come across membrane. Example. Sodium channel and potassium channel. Common in neurons

Question 35

Stages of Votage-gated ion channel

Answer 35

1) Closed 2) Open 3) Inactivated - brief period following depolarization where new signal does not open channel.

Question 36

Consequences of blocking ion channels?

Answer 36

Pufferfish toxin - blocks sodium pump - can be fatal! | Topical aesthetics - block sodium channel, inhibiting neurotransmission for pain!! Cool!!!

Question 37

What is Active Transport? Types?

Answer 37

Transport requiring energy input to move molecules across against concentration gradient. Directed by Integral Proteins (Polytopic transmembrane protein transporters) 1) Primary - requires direct input of energy 2) Secondary - use of energy secondhand/indirectly (stored in concentration gradient) - thus it is connected to a primary transport system.

Question 38

What are the two types of Primary Active Transporters

Answer 38

1) P-type ATPases - When ATP is hydrolyzed, the protein becomes phosphorylated 2) ABC Transporters - the above phosphorylation does not occur.

Question 39

Explain P-type ATPases

Answer 39

- Uses ATP hydrolysis to transport against gradient - After hydrolysis, covalent bond between the free phosphate is formed with the protein - phosphorylation of aspartame residue. - HUGE conformational changes occur, allowing for transport Examples: Na+/K+-ATPase (Sodium/Potassium pump -> 3Na out, 2K+ in) and Ca2+-ATPase (Ca2+ out)

Question 40

Uniporter vs. Antiporter vs Symporter

Answer 40

Uniporter is a transport protein which moves one ion in one direction. (Mitochondrial calcium transporter) Antiporter - moves molecules in opposite directions. (Sodium calcium exchanger (NCX)) Symporter - moves two molecules in same direction (Sodium-glucose transporter, Lactose permease)

Question 41

Explain ABC Transporters and how it works

Answer 41

- Moves small molecules (large diversity) against concentration gradient, out of the cell. - ATP is energy source Example. Glycoproteins (has carbohydrate group attached) How it works: 1) Substrate binds 2) Transporter undergoes conformational change 3) Increase want (affinity) for ATP 4) ATP binding 5) Substate expelled following ATP binding conformational change 6) Substrate expelled out of cell 7) ATP hydrolysis resets and protein goes back to resting state.

Question 42

Importance of ABC transporters in drug resistance?

Answer 42

Some pathogens increase the expression of ABC transporters, which in turn expel more drugs out of the cell. Therefore, the cells aren’t affected by the drug, resulting in “drug resistance.”

Question 43

Explain Secondary Active Transport

Answer 43

- ATP not used directly - coupled with primary transport mechanism - Rather, molecules going against gradient are transported along with a molecule that is going with gradient due to the difference in concentration created by a primary transporter (I.e. Sodium-glucose transport works by glucose coupling with Na - glucose is going against gradient while sodium is going down gradient due to Na/K-ATPase)

Question 44

How does NCX move molecules?

Answer 44

NCX is a secondary transporter, moving Ca2+ against it’s gradient by utilizing the energy from the Na+ gradient set by NA/K-ATPase. NCX is an antiporter. Movement -> 3Na moves down gradient, 1 Ca2+ move upgradient

Question 45

Explain how Dietary Monosaccharides are moved from intestine to blood stream

Answer 45

Monosaccharides from polysaccharides (starch) and disaccharides (sucrose and lactose) are moved from the lumen to the blood via: 1) Active Transport and 2) Facilitated Diffusion 1) D-Glucose and D-galactose enter intestinal epithelial cells via secondary active transport permitted by sodium-glucose transporter 1 (SGLT1) 2) Glucose Transporter (GLUT2) moves glucose across enterocyte (cell of intestinal lining) via Facilitated Diffusion and into bloodstream Fructose -> facilitated diffusion -> move down gradient using GLUT5 (apical side) and GLUT2 (basal side) transporters into blood stream See page 95 in Panini