Session 5: Cell Membranes

What are the functions of the biological membrane?

- Form a continuous & highly selective permeability barrier

- Allow control of enclosed chemical environment

- Allow communication between cell & environment

- Generate signals in response to electrical & chemical stimuli

What does 'amphipathic' molecules mean?

Molecules that contain both a hydrophilic and hydrophobic group

What are phospholipids composed of?

glycerol, 2 fatty acids, a phosphate group, and a polar 'head group'

What are some examples of phospholipid polar head groups?

Choline, amines, amino acids, sugars

What is the most prevalent length of fatty acid chains?

C16 and C18 length

The ___ double bond introduces a kink into the structure of the fatty acid chains

The cis double bond introduces a kink into the structure of the fatty acid chains

What is the range of length for fatty acid chains?

C14 to C24

What is the predominant class of lipids in cell membranes?

Phospholipids

What is Sphingomyelin?

ceramide + phosphate + choline

It is an example of a glycolipid

Glycolipids

Sphingomyelin, Cerebrosides, Gangliosides

What is the head group of cerebrosides?

Sugar monomer head group

What is the head group of gangliosides?

Oligosaccharide head group

Approximately how much of the membrane lipid does cholesterol comprise?

~45%

What is the role of cholesterol in the membrane?

Help modulate membrane fluidity under physiological conditions

What two structures do amphipathic molecules produce in aqueous solution?

Micelles and bilayer (preference to form bilayer)

What is an 'unsaturated' fatty acid?

Fatty acid that contains a double bond in the cis conformation

How does the presence of a double bond affect membrane fluidity in phospholipids?

Reduces phospholipid packing and therefore affects membrane fluidity

What structure do amphipathic molecules such as glycolipids and phospholipids prefer to produce in aqueous solution?

Bilayers

What force drives the formation of lipid bilayers?

Van der Waals forces between hydrophobic tails

The cooperative structure of membranes is stabilised by non-covalent forces, electrostatic attraction and ___ bonding between hydrophilic moieties and interactions of hydrophilic groups and water

hydrogen bonding

What are the four modes of phospholipid motion in membranes?

- Intra-chain motion = kink formation in the fatty acyl chains

- Fast axial rotation

- Fast lateral diffusion

- Flip-flop = movement of lipid molecules from one half of bilayer to the other half on a one for one exchange basis

What are other factors affect the fluidity of the lipid bilayer?

- Unsaturated cis double bonds produce ‘kinks’ = reduce phospholipid packing and increase fluidity

- Cholesterol = stabilises the membrane fluidity

What is the function of membrane proteins?

Transporters - control movement across membrane

Anchors - attachment points to the membrane

Recognition - markers for cell-cell interaction

Glue - junctions connecting cells

Enzymes - localising metabolic pathways

Transduction - receptors carry signals into the cell

What are the two types of membrane protein?

integral and peripheral

What are the three modes of protein motion?

1) Conformational change

2) Rotational

3) Lateral movement

What motion found in lipid motion is absent in protein motion in the membrane?

Membrane proteins DON'T flip-flop

What reasons are there for restrictions in protein mobility in the bilayer?

1) Lipid-mediated effects = proteins tend to separate out into the fluid phase or cholesterol poor regions

2) Associations between membrane proteins

3) Associations with extra-membranous proteins (peripheral proteins) including those of the cytoskeleton

What is the name of the scientist who discovered the fluid mosaic model?

Singer-Nicholson model (1972)

What does the Singer-Nicholson (fluid-mosaic) model state?

This model states that membrane proteins may be deeply embedded in the bilayer (‘integral’) or associated with the surface (‘peripheral’)

What are peripheral proteins?

- Bound to the surface of the membrane

- Electrostatic & hydrogen bonding

- Removed by: changes in pH or ionic strength

What are integral proteins?

- Penetrate the hydrophobic interior of the lipid bilayer

- Cannot be removed by manipulating pH/ionic strength

- Removed by: agents that compete for non-polar interactions e.g., detergents & organic solvents

Osmolysis of RBC membranes create ___ membranes

ghost membranes

Analysis of ghost membranes by gel electrophoresis techniques reveals >___ major proteins

Analysis of ghost membranes by gel electrophoresis techniques reveals >10 major proteins

Most of these major proteins in RBC are released when ghost membranes are treated with high ___ strength medium, or by changing the ___

high ionic strength medium, pH

This indicates that they are PERIPHERAL proteins

Other proteins in the ghost membranes of the RBC can only be removed by adding ___ and so are considered integral proteins

detergents

The cytoskeletal protein in RBCs = ___ interacts with membrane proteins via adaptor proteins (Ankyrin & Band 4.1) and so = restrict membrane mobility

The cytoskeletal protein in RBCs = SPECTRIN interacts with membrane proteins via adaptor proteins (Ankyrin & Band 4.1) and so = restrict membrane mobility

What membrane protein in RBCs is important for antigenic sites for blood groups?

Glycophorin A

Why is the erythrocyte cytoskeleton important?

Maintains deformability needed for RBCs to make their passage through capillaries without lysis/damage

What is the common dominant form of haemolytic anaemia?

Hereditary spherocytosis

- SPECTRIN protein levels in RBCs depleted ~40-50%

- Erythrocytes round up

- Less resistant to lysis during passage through capillary beds

- Removed by spleen

- Shortened in vivo survival and ability of bone marrow to compensate = haemolytic anaemia

What is the other form of haemolytic anaemia?

Hereditary elliptocytosis

- Defect in SPECTRIN protein in erythrocytes

- Unable to form heterotetromers

- Fragile ellipsoid cells

What is the importance of membrane transport systems?

- Maintain intracellular pH

- Maintain ionic composition

- Regulate cell volume

- Concentration of metabolic fuels & building blocks

- Extrusion of waste products of metabolism & toxic substances

- Generation of ionic gradients needed for electrical excitability of nerve & muscle

What is passive diffusion?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

Movement of water across membranes

Osmosis

Example of facilitated diffusion in the human body

In the kidney = mass movement of water facilitated by insertion of water-specific channels called Aquaporins e.g. Aquaporin-2 into the luminal membranes of the epithelial cells that line the collecting ducts

How does facilitated diffusion occur?

Transport proteins - increase permeability enormously for polar substances

Models for facilitated transport include

- Protein pores (channels)

- Carrier molecules (ping pong and flip flop)

Facilitated transport is a ___ process = each carrier can only interact with limited number of ions at any one moment. There is a finite number of transporters present in the membranes.

Facilitated transport is a saturable process = each carrier can only interact with limited number of ions at any one moment. There is a finite number of transporters present in the membranes.

Give examples of gated channels in the membrane

1) Ligand-gated

2) Voltage-gated = open/closes in response to potential differences across the membrane

3) Gap junctions (connexins) = close when cellular calcium rise to >10uM or cell becomes acidic

What is active transport?

Movement of molecules into or out of a cell against a concentration gradient (unfavourable concentration and/or electrical gradient) using energy directly or indirectly from ATP hydrolysis

Give an example of a pump that displays active transport (sodium-potassium pump)

This is a channel protein pump that moves three Na+ ions OUT the cell and two K+ ions INSIDE the cell simultaneously - works against the concentration gradient at the expense of one ATP molecule

What is the function of Na-K pump?

- To keep Na+ concentration high OUTSIDE the cell

- To keep K+ concentration high INSIDE cell

What percentage of ATP in the body is used to power sodium-potassium pumps?

~25% of ATP in the body

In the mitochondria - the gradient of [H+] protons drives ___ synthesis via an ATP-dependent proton transporter

In the mitochondria - the gradient of [H+] protons drives ATP synthesis via an ATP-dependent proton transporter

What is co-transport?

When the transfer of one solute depends on the simultaneous or sequential transfer of of another (same direction = symport; opposite direction - antiport)

What is co-transport also known as?

Secondary active transport

Why is it called 'secondary active transport'

Because the primary source of energy (hydrolysis of ATP) is being used INDIRECTLY to move the second molecule

What is an example of secondary active transport in the human body?

Energy to co-transport glucose and sodium ions from the glomerular filtrate into the PCT of the kidney comes from a gradient created by the active transport of Na+ OUT of the cell and INTO the interstitium

Give examples of some co-transport systems in the human body

1) Na+-glucose co-transport of small intestine and kidney (symport) = entry of sodium provides ATP for entry of glucose

2) Na+-Ca2+ exchange where inward flow of sodium down its concentration gradient drives outward flow of Ca2+ up its conc. gradient (antiport)

3) Na-H+ exchange = inward flow of Na+ down conc. gradient leads to cell alkalisation by ejecting H+ ions (antiport)

Two fatty acid chains linked to a glycerol backbone with a phosphate group and a polar head group

Phospholipid

One fatty acid chain and one sphingosine chain (i.e., a ceramide) linked to a phosphate group and a polar head group

Sphingomyelin

A ceramide linked to a single sugar moiety head group

Cerebroside

A ceramide linked to a sugar polymer head group

Ganglioside

A rare condition resulting in the loss of hexosaminidase A. This leads to accumulation of gangliosides and progressive loss of nerve cell function. Those with the infantile form will die by the age of four

Tay-Sachs disease

A rare disorder caused by deficiency of enzyme glucocerebrosidase. The accumulation of cerebrosides in a range of cells and tissues, particularly leukocytes, leads to a range of symptoms. Life expectancy is mildly impacted in the most common form.

Gaucher’s disease

Loss of function of sphingomyelinase leads to the pathological accumulation of sphingomyelin in the spleen, liver, lungs, bone marrow, and brain. In type A disease, death can occur by 18 months of age.

Niemann-Pick disease

Which one of the following best describes the role of CHOLESTEROL in the cell membrane?

A) Cell adhesion

B) Pathological consequence of high levels of circulating lDLs

C) Facilitates cell-cell recognition

D) Allows tethering to the cytoskeleton

E) Regulates membrane fluidity

E) Regulates membrane fluidity

What impact does the presence of a cis double bond in one of the fatty acid chains of a phospholipid have on the membrane?

It reduces packing efficiency

What RBC membrane component is this…

A heavily glycosylated membrane protein that prevents circulating RBCs from sticking to other cells and vessel walls

Glycophorin

What RBC membrane component is this…

A protein that links transmembrane proteins with the cytoskeleton

Ankyrin (think anchor)

What RBC membrane component is this…

A cytoskeletal protein that stabilises RBC membranes by interacting with membrane proteins

Spectrin (think stabilises)

What RBC membrane component is this…

An anion exchanger important in CO2 transport

Band3

What can you infer about a membrane protein if it is released by a change in pH?

It is a peripheral protein

What can you infer about a membrane protein if it is dissociated by the action of detergents?

It is an integral protein

What properties do amino acid residues of transmembrane protein domains tend to share?

They are hydrophobic

Which mode of membrane protein movement is VERY energetically UNFAVOURABLE?

Flip-flop

Any type of anaemia in which RBCs are LARGER than normal. Can be caused by vitamin B12 and folate deficiency

Macrocytic anaemia

Defects in spectrin leads to formation of elliptical cells

Hereditary elliptocytosis

RBC round up due to loss of cytoskeletal-associated proteins such as spectrin

Hereditary spherocytosis (think round up - spherical)

A type of anaemia in which RBCs are smaller than normal. Often caused by iron deficiency

Microcytic anaemia

Anaemia resulting from loss of globin chains through mutation

Thalassemia

A type of anaemia in which RBC membranes become fragile due to cycles of polymerisation of haemoglobin

Sickle cell anaemia

A type of anaemia in which RBCs are of normal size

Normocytic anaemia

Rank the membrane permeability coefficients of the following from MOST permeable (1) to LEAST permeable (5)

O2, H2O, Glucose, Bicarbonate, Urea

1) O2 (most permeable)

2) H2O

3) Urea

4) Glucose

5) Bicarbonate (least permeable)

Glucose is co-transported with sodium from the lumen of the proximal convoluted tubule following filtration in the kidney. This exploits the sodium concentration gradient established by the action of the Na+/K+ ATPase.

Which one of the following most accurately describes this process?

Select one:

A) Active transport

B) Co-transport

C) Secondary active transport

D) Facilitated diffusion

E) Simple diffusion

C) Secondary active transport

A channel that opens when a chemical binds to a receptor

Ligand-gated

A channel that transports one molecule across a membrane

Uniporter

A channel that transports two different molecules in OPPOSITE directions

Antiporter

A pore between cells that allows for electrical and chemical continuity

Gap junction

A channel that transports two different molecules across a membrane in the SAME direction

Symporter

A channel that opens when the potential difference across a membrane changes

Voltage-gated

Movement of a molecule through a channel AGAINST a concentration gradient

Active transport

Movement of a molecule across a membrane down a concentration gradient through a CHANNEL

Facilitated diffusion

Movement of a molecule across a membrane down a concentration gradient

Simple diffusion

The Na+/K+ ___ pump transports 3 sodium ions OUT the cell, accompanied by the movement of ___ potassium ions back INTO the cell.

The Na+/K+ ATPase pump transports 3 sodium ions OUT the cell, accompanied by the movement of 2 potassium ions back INTO the cell.

Equilibrium potentials for any ion can be calculated using the Nernst equation.

For example: the equilibrium potential for potassium at 370C (EK) can be calculated as follows:

EK=(61/Z)*log10([K+]o/[K+]i) where o=outside, i=inside and Z=charge.

If [K+]o = 4.5 mM and [K+]i = 160mM, what is the equilibrium potential for this ion at 370C?

-95mV