Membranes: The Basics

Question 1

Membrane Lipids

Answer 1

Phospholipids, Cholesterol and Glycolipids

Question 2

Membrane Primary Functions

Answer 2

Keeping toxic substances out

Allowing specified transmembrane transport

Separating vital metabolic processes

Question 3

Biomembrane Structure

Answer 3

Primarily Lipids and Proteins:

Lipids = 20-80% of membrane, giving flexibility

Proteins maintain chemical environment by mediating transport:
- Peripheral, Integral and Lipid linked (attach membrane via lipid linker)

Question 4

Role of membrane proteins

Answer 4

Structural Support
Receptors
Transport proteins
Cell-cell communication: Glycoproteins

Question 5

Nucleus’ membrane

Answer 5

2 membranes (inner and outer) separated by perinuclear space
- protect genetic material and control import/export

Question 6

ER’s membrane

Answer 6

Single membrane, largest organelle in cell

• membrane contact sites for inter-organelle communication
• Site of protein folding inside ER lumen

Question 7

Golgi’s membrane

Answer 7

Fused flat enclosed cisternae stacks (40-100 in mammals)

Cis –> Medial –> Trans (Trans Golgi Network, TGN)

Question 8

Vesicles

Answer 8

Liquid enclosed in lipid bilayer (membrane = lamellar phase)

- different environment to cytosol

Question 9

Mitochondria

Answer 9

Outer and inner membrane

• Inner forms cristae containing matrix within for citric acid cycle
• Intermembrane space - oxidative phosphorylation

Question 10

Varying membrane composition across organelles

Answer 10

Differing levels of lipid compositions:

• e.g. more Sphingomyelin in Plasma membrane than Golgi
• e.g. Golgi has more Phosphatidylserine than mitochondria

Question 11

Lipid Bilayer Structure: Phospholipids

Answer 11

2x fatty acids + glycerol + phosphate group

• Long aliphatic carbon chains with terminal polar group
• Either saturated or Unsaturated (Cis double bond)
• 1 of 2 fatty acids has a kink (due to varying saturation) and a double bond giving a 30 degree bend
• Saturated = straight fatty acids, tightly packed, viscous
• Unsaturated = less tight packing, fluid membrane

Question 12

Glycerophospholipids

forming different phospholipid types

Answer 12

Glycerol-3-phosphate - C1 and C2 esterified with fatty acids: Phosphoryl group linked to another polar group:

Phosphatidyl-

• ethanolamine (PE)
• choline (PC)
• serine (PS)
• inositol (PI)

Can be further phosphorylated into PIP, PIP2 or PIP3 each with different signal characteristics

Question 13

Sphingolipids

Answer 13

Not synthesised from glycerol, have an amide bond between a fatty acid and sphingosine
Functions:
- structural protection from harmful environments and signalling through sphingolipid metabolism

E.g. Sphingomyelin (most common sphingolipid)
- Myelin sheath, exoplasmic leaflet of cell membrane

Broken down into ceramide by sphingomyelinase-2

Question 14

Lipid Bilayer Structure: Cholesterol

Answer 14

A type of modified sterol (carbon rings present)

• Important in membrane structural integrity and fluidity
• Steric reason of fluidity: between 2 phospholipids, prevents them coming too close (viscous) at low temperatures)
• It’s hydroxyl group binds phosphate head; it’s non-polar hydrophobic tail attached to bilayer centre
• Structural stability at high temperatures: attracting polar -OH groups together prevents breakdown

Question 15

Membrane Phase Transition Temperature

Answer 15

Above phase transition temperature:
- bilayer becoomes increasingly fluid until it can’t act as a barrier (fluid form)

Below phase transition temperature:

• Bilayer solidifies into a gel-like state - diffusion of lipids decreases
• Loss of functionality

Question 16

Lipid Bilayer Structure: Glycolipids

Answer 16

Lipids, phospholipids or sphingolipids with a sugar group attached to its head group (by glycosidic bond)

• Provides membrane stability and cellular recognition
• Allows membranes to attach to form tissues
• Extracellular recognition of specific chemicals

Question 17

Lipid Rafts

Answer 17

Dynamic, non-uniform areas of bilayer

• rich with cholesterol and sphingolipids that cluster with specific raft proteins
• important in vesicle transport

Question 18

Lateral Diffusion in Membrane

Answer 18

The membrane bilayer is very dynamic and constantly moving

• lateral movement of lipids, phospholipids and proteins
• A typical lipid molecule exchanges place 10^7 times per second
• Observed with Fluorescence Recovery After Photobleaching (FRAP): non-fluorescent spot disappears as photobleached phospholipids quickly exchange
• Observed by fusing oppositely labelled mouse and human cells: half and half merges into one

Question 19

Transverse Movement of Lipids

Answer 19

Moving lipids from extracellular face of bilayer to cytosolic face (and vice versa)

• Flippase proteins catalyse the otherwise slow process of diffusion of phospholipids from one face to the other

Question 20

Roles of cellular lipids

Answer 20

Energy storage

Signalling Molecules

Storing excess lipids

Question 21

Cellular lipids for energy storage

Answer 21

Triglycerides stored in adipose tissue

• adipocytes designed for constant synthesis and breakdown of triglycerides (controlled by activation of hormone-sensitive lipases)
• complete oxidation of fatty acids provides high caloric content

Question 22

Cellular lipids as signalling molecules

Answer 22

Diaglycerol (DAG) and PIPs involved in calcium-mediated protein kinase C activation

• Prostaglandins used in inflammation/immunity: derived from eicosanoid
• Sphingosine-1-phosphate involved in regulating calcium mobilization, cell growth and apoptosis: derived from ceramide

Question 23

Storing excess lipids in lipid droplets

Answer 23

Lipid ester core + surfuce phospholipid monolayer

• formed when lipid levels exceed required amount