Cell Biology - 1.3 Membrane Structure

Question 1

Davson-Danielli Model (1930) - the Sandwich (theory)

Answer 1

Described as a bilayer of phospholipids sandwiched between a layer of protein on either side:

• Chemical analysis showing that the membrane was composed of phospholipid and protein
• Electron micrograph images (1950) showed membranes as two dark lines separated by a lighter area
• Membranes are semi-permeable… hence proteins could be acting as a barrier to some substances

(Skill analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli Model)

Assumptions:

• all membranes were of a uniform thickness and would have a constant lipid-protein ratio
• all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)

Question 2

Singer-Nicolson Model - the Fluid Mosaic

Answer 2

developed through evidence accumulating that did NOT support the Davson-Danielli model (in the 1950s/60s)

• Electron micrographs showed globular proteins were in the center of the phospholipid layer
• analysis of protein showed some were hydrophobic - ie some extended all the way through the phospholipid layer and some within

(Skill analysis of falsification of the Davson-Danielli Model that led to that Singer-Nicolson Model)

Question 3

Relationship between amino acids and proteins on membranes.

Answer 3

Polar amino acids on the surface of proteins make them water-soluble. Polar amino acids can create channels through which hydrophilic substances can diffuse. Non-polar amino acids allow proteins to remain embedded in membranes.

Question 4

hydrophilic (definition)

Answer 4

“water-loving” ie. attracts water molecules

Question 5

Hydrophobic (definition)

Answer 5

“water-fearing” ie. does NOT attract water molecules

Question 6

Facts about membrane structure (need to know)

Answer 6

• All cells have a membrane
• it is a double layer of phospholipids with proteins and cholesterol embedded in it
• It is semi-permeable - ie. allows some molecules to get through BUT not all (ie. controls what goes in and out of the cells)

Question 7

Phospholipids (must also be able to draw)

Answer 7

• make up the cell membrane (are amphipathic ( having both hydrophilic (POLAR) and hydrophobic parts (NON-POLAR)))

“have two long (NON-POLAR) fatty tails are hydrophobic”

• the head is solid and tail is like a liquid === cell membrane is fluid (allows for it to change shape and to transport materials across)

Question 8

Phospholipids IN WATER

Answer 8

(Understanding = Phospholipids form bilayers in water due to the amphipathic properties of the Phospholipid molecules)

“when phospholipids are mixed with water they naturally become arranged into two layers. They self-organise to keep their heads wet and tails dry - the attraction of the tails to one another makes the membrane very stable but also flexible”

Question 9

Cholesterol (- understanding)

Answer 9

• it is a component of animal cell membranes (making up about 20%) - embedded in the phospholipid. (They have hydrophobic and hydrophilic ends/regions (ie. it is an amphipathic molecule - they reduced the fluidity and the permeability of the membrane to ions)
  (understanding: cholesterol is a component of animal cell membranes - in mammalian membranes they reduce membrane fluidity and permeability to some solutes)
• It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
• It functions to separate phospholipid tails and so prevent crystallisation of the membrane
• It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Question 10

Membrane proteins

Answer 10

(the individual effects of each protein contribute to the semi-permeable nature of the membrane)

1. Integral = (transmembrane) proteins embedded in the bilayer
2. Peripheral (cell adhesion process) = proteins attached to the outer surface involved in cell to cell communications through specialised adhesion proteins in the cell membrane.
3. Channel proteins = act as punts (eg. Potassium channels in nerve cells)
4. Glycoproteins = have sugar units attached to the outer surface of the membrane - important for cell recognition by the immune system and as hormone receptors

Question 11

Functions of membrane proteins (TRACIE)

Answer 11

T = Transport = protein channels (facilitated diffusion) and protein pumps (active transport)
R = Receptors = eg. for hormones
A = Anchorage = Cytoskeleton and cell wall attachments
C = Cell recognition = immune system proteins
I = Intercellular joinings = cells attached to each other (eg. plasmodesmata)
E = Enzymatic activity = the movement of ions back and forth across the membrane (eg. respiration)

Question 12

Phospholipids (production)

Answer 12

Phospholipids are synthesized by the smooth endoplasmic reticulum and transported as vesicles to the cell membrane

Question 13

Integral membrane proteins

Answer 13

Integral membrane proteins are embedded in the phospholipid bilayer.
Most integral proteins span the entire phospholipid bilayer.

The amino acid sequence determines the three-dimensional conformation of a protein Integral proteins have hydrophobic amino acids that interact with hydrophobic fatty acid tails of the membrane phospholipids (1.3.U1), thus anchoring the protein to the membrane

Question 14

Peripheral membrane proteins

Answer 14

Peripheral membrane proteins are indirectly or loosely attached to the surface of the cell membrane, but may dip slightly into the lipid bilayer.

Peripheral proteins attach to the membrane via by reversible electrostatic interactions or hydrogen bonds with phospholipid heads at the membrane surface or with another membrane protein.

Question 15

Carrier proteins

Answer 15

… bind the specific solute to be transported and undergo a series of shape changes to transfer the bound solute across the membrane

Question 16

Channel proteins

Answer 16

… form pores that extend across the lipid bilayer; when these pores are open, they allow specific solutes (usually inorganic ions of appropriate size and charge) to pass through them and thereby cross the membrane

Question 17

Active transport (pump) process

Answer 17

Cells also have carrier proteins that will actively pump certain solutes across the membrane:

• Requires a source of metabolic energy, such as ATP hydrolysis.
• Moves the solute against the concentration gradient (from areas of lower concentration to areas of higher concentration).

Question 18

Receptor proteins

Answer 18

they are proteins that are embedded in the cellular membrane that bind to specific chemical signals from outside the cell.

When the chemical signal binds, the membrane protein triggers a response by the cell.

Question 19

Peripheral (cell adhesion process)

Answer 19

(proteins attached to the outer surface involved in cell to cell communications through specialised adhesion proteins in the cell membrane.)

Adhesion can occur either through: direct contact between the surfaces of two cells attaching to an extracellular matrix, a gel-like structure containing molecules released by cells into spaces between them.

Question 20

Properties of the Phospholipid Bilayer:

Answer 20

• The bilayer is held together by weak hydrophobic interactions between the tails
• This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
• Hydrophilic / hydrophobic layers restrict the passage of many substances
• Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility

Question 21

falsification of the Davson-Danielli Model

Answer 21

Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size

Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane

Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place

Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively - When the two cells were fused, the markers became mixed throughout the membrane of the fused cell - This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)

Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane

These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure

Question 22

Fluid (definition):

Answer 22

the phospholipid bilayer is viscous and individual phospholipids can move position

Question 23

Mosaic (definition):

Answer 23

the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components