Summary C3 ENZYMES 9700 AS LEVEL

Chapter 4: Cell Membrane and Transport
Monday, 25 April 2022 9:30 PM



FLUID MOSAIC MODEL OF MEMBRANE

• Cell surface membranes create an enclosed space separating internal cell environment from external cell environment,
controls the exchange of material across them (partially permeable) and acts as interface for communication
• Cell membranes are formed from phospholipid bilayer (7 nm wide)

PHOSPHOLIPIDS

• Contains 2 regions: polar head and non-polar tails
• Phosphate head is polar (hydrophilic) and soluble in water
• Fatty acid tail is non-polar (hydrophobic) and insoluble in water
• Form phospholipid monolayers on surface of water




• When shaken with water, they form spheres with hydrophilic phosphate heads outside and hydrophobic fatty acid tails
inside. This is called a micelle.




• Two-layered structures may form in sheet - phospholipid bilayer (basic structure of cell membrane)




• Phospholipid bilayers can form compartments – the bilayer forming the cell surface membrane establishing the boundary
each cell

STRUCTURE OF MEMBRANE

• Bilayers making the membrane also contain proteins which can be either intrinsic (integral) or extrinsic (peripheral)
• Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophob
regions
• Extrinsic proteins are found on the outer or inner surface of the membrane
• FLUID - Phospholipids and proteins can move around via diffusion
MOSAIC - Scattered pattern produced by the proteins within the bilayer




AS BIOLOGY NOTES Page 1

, COMPONENTS OF CELL SURFACE MEMBRANE

1. Phospholipid

 Forms a bilayer with hydrophobic tails pointing inwards and hydrophilic head pointing outwards
 Tails form a hydrophobic core, acting as a barrier to water-soluble substances
 Individual phospholipid molecules can move around within their own monolayer by diffusion
 Can be chemically modified to act as signalling molecule

2. Cholesterol

 Hydrophobic tails and hydrophilic head
 Fits between phospholipid molecules; oriented in same way
 Increases fluidity of membrane, by stopping membrane from becoming too rigid at low temperatures
 Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by
stopping the membrane from becoming too fluid
 Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack
more closely together
 They also contribute to the impermeabilty of the membrane to ions
 Increase mechanical strength and stability of membrane
 Absent in prokaryote membrane

3. Glycolipids

 Lipids with carbohydrate chains - enabling them to act as receptor molecules
 Carbohydrate chains project into fluid surrounding the cell
 Some act as antigens for cell-to-cell recognition

4. Glycoproteins

 Proteins with carbohydrate chains - enabling them to act as receptor molecules
 Carbohydrate chains project into fluid surrounding the cell
 Some act as antigens for cell-to-cell recognition

5. Proteins

 Intrinsic proteins- proteins embedded within the membrane
 Transmembrane proteins - proteins that span the entire membrane (eg transport proteins)
 Extrinsic proteins (peripheral proteins) - proteins found on inner or outer surface of membrane
 Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
(including channel protein and carrier protein)
 Each transport protein is specific to a particular ion or molecule
 Transport protein allow the cell to control which substances enter or leave
AS BIOLOGY NOTES Page 2