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How thick is the cell membrane?

as thick as a protein


What must the membrane be able to do?

- selectively permeable
- impermeable to macromolecules
- permeable to nutrients and waste products


What are droplets of lipid in water called?



What is the most abundant class of phospholipid? Its structure and charge

phophatidylcholines - has two fatty acids, a glycerol, phosphate and at the top a choline
Overall neutral charge


What are some other examples of phospholipids and their overall charges?

- phosphatidylserine (negative)
- phosphatidylethanolamine (neutral)
- sphingomyelin (neutral)


Why is the cell membrane much more fluid than calculations predict?

- Phosphoipids can flip-flop between layers of the membrane
- Phospholipids can diffuse between each other in the same layer i.e. with their neighbours, leading to a dynamic membrane


What are membranes impermeable to?

- Cations (K+, Na+, Ca2+ ) but some do leak through down the concentration gradient
- Anions (Cl, HCO3-)
- Small hydrophilic molecules like glucose
- Macromolecules like proteins and RNA


What are the functions of membrane proteins?

- Transport (Na+-Glucose transporter)
- Receptor- for hormones and growth factors
- Cell recognition and adhesion
- Electron carrier (cellular respiration and photosynthesis in mitochondria and chloroplasts)


What is the electrochemical gradient?

The electrochemical gradient is a combination of the concentration and electrostatic gradients
generated by ions
- The chemical gradient tends to move particles down the gradient, spontaneously
- The electrostatic gradient tends to move particles according to their charge

Equilibrium can be reached so that the actual ratio of the intra- and extracellular concentration ultimately depends on the existing membrane potential


What prevents the dissipation of gradients and why?

sodium-potassium pumps
In order to main the osmotic balance and stabilises the cell volume
Also it is exploited by certain processes to drive the transport of the sugars and amino acids, and also to generate electrical signals by deliberately causing the influx of sodium into the cell, disrupting the membrane potential


What can the membrane potential be calculated from?

Nernst equation
- R=gas constant
- Z=charge of ion
- F=faraday's number


What is pinocytosis?

Engulfment by the membrane of extracellular solute and small molecules which end up in small intracellular membrane-bound vesicles


What is exocytosis?

Movement of proteins and other molecules (e.g. hormones, blood clotting factors) from intracellular vesicles towards the extracellular space by fusion with the cell membrane


What is the role of cholesterol?

It increases or decreases the membrane stiffness and permeability depending on the temperature and nature of the membrane. It also changes interactions with the cytoskeleton.


What are symporters and antiporters?

symporter - molecules moved with sodium down its concentration gradient
antiporter- molecules moved in opposite direction to sodium e.g. protons


Sodium potassium pump - structure and how does it work?

Made of two polypeptide chains - alpha and beta
The alpha chain spans the membrane ten times forming a hydrophilic pore through which ions can move

2 K+ in
3 Na+ out
Successive transition of the pump driven by the phosphorylation of the aspartyl residue using ATP, followed by the hydrolysis of the aspartyl phosphate
Causes an ion and charge gradient


Potassium channel

4 subunits with a pore
Scorpion toxin targets this
K+ can leak through



They can also cross membranes - some are lipid soluble e.g. prostaglandins and other ones are impermeable


What are three examples of bulk transport mechanisms?

- phagocytosis
- pinocytosis
- exocytosis


Cell membrane proteins - picket fence model

- Membrane proteins diffuse freely within small compartments and can hop between the compartments
- The proteins appear to be constrained by fences
- Some are less mobile, and appear to be attached to the fence (pickets)
- The fences are made up by a membrane skeleton, which form compartments
- The fences function by steric hindrance i.e. they physically prevent the movement of molecules beyond their boundary
- They seem to be made up of components of the actin cytoskeleton
- The cell membrane proteins are moved in the membrane along the actin filaments by myosin motors i.e. cadherin
- Cell membranes and organelle membranes contain proteins
- These proteins increase the cell fluidity and they confer new properties to the membranes
- Many membrane proteins are involved in transport and in the transmission of signals


K+ concentration and equilibrium

[K+] inside is high, so there is a tendency for K+ to move out of the cell, but this is counterbalanced by the electric potential which opposes the movement of positive charges out of the cell, as this would accentuate the voltage difference across the cell.
An equilibrium is reached when the rate of inward movement of K+ ions down the electrochemical gradient equals the rate of outward movement down the concentration gradient. Thus the electrical imbalance caused by the sodium pump will not quite be
compensated by K+ movement. The end result is a membrane potential, with a voltage difference across the membrane (inside negative). This is important for signalling in nerves, muscles, etc


Protein and lipid composition

The proportion of protein to lipids varies from cell type to cell type. The protein composition is different in the inner and outer leaflets of the lipid bilayer. The protein composition also depends on the organelles.