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L03 - The cell surface

 

Membrane structure and composition

The membrane is composed mainly of phospholipids and transmembrane proteins
 

Role of cholesterol (sterols)

Cholesterol adds rigidity to the membrane (regulated membrane fluidity)
 

Glycolipids

These are located mainly in the plasma membrane, on the noncytosolic half
 

Fluid mosaic model

The cell membrane is described as being a fluid mosaic
 

Importance of fluidity

Fluidity of the membrane is required in order to:
 

Protein association with bi-layer

There are 4 types of associations:
 



 

Transmembrane proteins

These proteins span the membrane and have several functions including:
 

Linking of transmembrane proteins

A polypeptide chain usually cross the bi-layer as an a-helix
 

Cell cortex

The shape of cells and mechanical properties are determined by a meshwork of fibrous proteins (cell cortex)
 

Glycoproteins

Most of the proteins in the plasma membrane have sugars covalently linked to them
 

Function

By forming a physical layer on top of the bi-layer, it prevents mechanical and chemical damage
 

Transport across cell membranes

There are 2 types of transport across cell membranes:
 

Active transport
 

Example: Sodium-potassium pump

The sodium potassium pump couples Na+ out and K+ in, in the following way:
 

This allows for the maintenance of an electrochemical gradient

 

Coupled Active transport (secondary active transport)Active transport may be coupled, i.e. two molecules may be moved together (one with conc grad, other against)
 

 

 

 

 

 

 

 

 

 

 

 

 

Example: Na/Glucose symporter
 

This type of active transport is driven by the electrochemical gradient

 

Example: Na/K+ Antiport



 

 

Ca2+ Pumps

Ca2+ is kept at a low concentration in the cytosol, compared to extracellular fluid
 

 

Passive transport

Passive transport does not require any energy input, there are 2 types:
 

Both involve movement down the concentration and electrical gradient

 

Simple diffusion

Simple diffusion involves the movement of molecules down their concentration gradient
 

Facilitated diffusion

Facilitated diffusion uses 2 types of membrane proteins:
 

Channel protein

Channel protein are pores which discriminate on size and charge
 

Carrier proteins

Carrier proteins are highly selective
 

Electrochemical gradients

Movement across the membrane depend on 2 forces:
 

This is known as the electrochemical gradient. It needs to be maintained to ensure osmotic balance and to drive transport across the membrane

 

Movement across the membrane- No proteins

The membrane allows and blocks certain molecules:
 

Intracellular signal transduction lipids

    • There are 2 opposing sheets of lipids known as the phospholipid bi-layer
    • They prevent the passage of water-soluble molecules
    • Each phospholipid has a hydrophobic tail and a hydrophilic head
    • They are arranged so that that the hydrophobic tails are on the inside
    • Cholesterol is also present
    • This is through making it more difficult for proteins to move through
    • This means that the membrane is less fluid and less permeable
    • This allows proteins to function better as they are held in position rather than dotted about
    • This allows for easier communication
  1. They form part of the carbohydrate layer
  2. They acquire their sugar in the golgi apparatus
  3. They are important for recognition, stability and cell attachment
    • This is due to the flexible nature of the membrane (as the fatty acid tails are unsaturated)
    • Proteins also span the membrane which gives the mosaic appearance
    • These proteins can move via the use of actin filaments
    • Allow for the fusing of other membranes (during endo/exocytosis)
    • Distribute membrane evenly between daughter cells following cell division
    • Allow signaling lipids and membrane proteins to diffuse and interact during cell signaling
  1. Transmembrane
  2. Membrane associated (associated with inner leaflet of bi-layer, protein is in cytosol)
  3. Lipid linked
  4. Protein attached
    • Receptors: By binding ligands
    • Anchors: Link to intracellular matrix and actin to allow cells to move around
    • Transporters: Such as the Na+ pump
    • Signal transduction molecules
    • Enzymes
  1. The transmembrane segment is composed of Non-polar (hydrophobic) amino acids
  2. This is as the side chains cannot form favorable interactions with water molecules (fine in lipid conditions)
  3. The polypeptide backbone (hydrophilic) forms hydrogen bonds (in the a-helix), as water is absent
  4. They are attached to the cytosolic surface of the membrane, to transmembrane proteins
  5. An example of this is spectrin in red blood cells
  6. These are known as glycoproteins (with short chains of sugars, oligosaccharides)
  7. Proteins with one or more long polysaccharide chain attached are proteoglycans
  8. These are located on the noncytosolic side
  9. This forms a sugar coating, known as the carbohydrate layer
  10. The sugars also absorb water, giving it a slimy surface (acts as lubrication)
  11. They also have an important role in cell-cell recognition and adhesion
    • Active
    • Passive
    • This form of transport requires energy from the hydrolysis of ATP
    • This is because substances are moved against the concentration gradient
    • 3 Na+ bind to the catalytic subunit
    • This activates ATPase
    • This causes the hydrolysis of ATP into ADP +pi, the latter is transferred to the pump
    • This is known as phosphorylation
    • 3 Na+ are ejected from the cell as the phosphorylation causes a change in conformation
    • 2 K+ then bind
    • Dephosphorylation occurs from the catalytic subunit
    • This causes a reversion to initial conformation
    • This means that the 2K+ move into the cell
    • This is an ATP driven pump thus known as primary active transport
    • Symport: Both molecules are moved in the same direction
    • Antiport: Molecules moved in the opposite direction
    • High concentration of Na in gut
    • Low concentration of glucose in gut
    • The binding of Na and glucose is co-operative (binding of glucose dependent on binding of Na)
    • Therefore, as Na is more likely to move into the cell, glucose is more likely to bind in the initial state
    • Therefore, both enter the cell more often than they leave it
  1. This is because Ca2+ can bind tightly to molecules in the cell altering their activity
  2. This allows it to act as a signalling molecule (e.g. trigger muscle contraction)
  3. A low Ca2+ concentration is achieved by ATP-driven Ca2+ pumps
    • Simple diffusion
    • Facilitated diffusion
    • There are no membrane proteins involved
    • No energy is required
    • Channel protein
    • Uniporter carrier proteins
    • The pores are hydrophilic, allowing for rapid diffusion
    • Most channel proteins are non directional
    • They have some selectivity
    • They can be controlled via gates
    • The molecule has to bind to the carrier protein thus this method is slower
    • It is needed for small organic molecules such as glucose
    • Chemical (concentration gradient
    • Voltage across membrane
    • Allow: Hydrophobic molecules, small uncharged polar molecules (only some, most blocked)
    • Block: Large, uncharged polar molecules, ions
    • Rapidly generated and destroyed in response to a specific signal
    • Spatially + temporally generated giving a highly specific signal (amplification)
    • Bind specifically to conserved regions
    • Causes conformation change in protein