1. MEMBRANES
TOPIC 2.4

2. Assessment Statements
2.4.1 Draw and label a diagram to show the structure of a membrane
2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes
2.4.3 List the functions of membrane proteins
2.4.4 Define diffusion and osmosis
2.4.5 Explain passive across membranes by simple diffusion and facilitated diffusion
2.4.6 Explain the role of protein pumps and ATP in active transport across membranes

3. 2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane
2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis

4. Membrane structure Fluid-mosaic model
Proteins floating in a phospholipid bilayer

5. Phospholipids Composed of 3-carbon glycerol Hydrophobic area
2 carbons have fatty acid tails attached
1 carbon has a highly polar alcohol attached
Hydrophobic area
Hydrophilic area
Fatty acids not strongly attached, therefore, it is flexible and allows for endocytosis

6. Cholesterol
Determines membrane fluidity, which changes with temperature
Presence of cholesterol allows effective membrane function at a wider range of temperatures

7. Proteins Embedded in fluid matrix of the phospholipid bilayer
Two types
Integral (goes through membrane)
Peripheral (bound to the surface of the membrane)

8. Membrane protein functions
Hormone binding sites – have specific shapes exposed to the exterior that fit the shape of specific hormones; attachment causes change in shape and results in message relay
Enzymatic action – often grouped as metabolic pathway
Cell adhesion – form junctions
Cell-to-cell communication – provide identification of type or species
Channels for passive transport – substances move through from high to low conc.
Pumps for active transport – movement of substance by changing shape; requires energy

9. Passive transport Does not require ATP
Movement of substance from high to low concentration
Movement occurs along a concentration gradient
Types
Diffusion
Substances other than water move between phospholipid molecules or through proteins which possess channels
Facilitated diffusion
Non-channel protein carriers change shaped to allow movement of substances other than water
Osmosis
Only water moves through the membrane using aquaporins which are proteins with specialized channels for water movement

10. Molecule crossing Small and non-polar molecules cross membrane easily
Ex. Oxygen, carbon dioxide, nitrogen
Large and polar molecules cross membranes with difficulty
Ex. Chloride ions, potassium ions, sodium ions, glucose, sucrose
Water and glycerol cross fairly easily

11. Active transport Requires ATP Involves movement against conc. Gradient
Allows cell to maintain interior conc. of molecules that are different from exterior conc.
Sodium-potassium pump
A specific protein binds to three intracellular sodium ions
Binding of sodium ions causes phosphorylation by ATP
Phosphorylation causes the protein to change its shape, thus expelling sodium ions to the exterior
Two extracellular potassium ions bind to different regions of the protein and this causes the release of the phosphate group
Loss of the phosphate group restores the protein’s original shape thus causing the release of the potassium ions into the intracellular space

12. Endocytosis and exocytosis
Allows macromolecules to enter cell
Depends on fluidity of membrane
Occurs when a portion of the plasma membrane is pinched off to enclose macromolecules
Vesicle is formed that then enters the cytoplasm
Ends of membrane reattach b/c of the hydrophobic and hydrophilic properties of the phospholipids and the presence of water
Ex. Phagocytosis, pinocytosis
Exocytosis
Allows macromolecules to leave cell
Depends on fluidity of membrane
Protein produced by the ribosomes of the rough ER enters the lumen of the ER
Protein exits the ER and enters the cis face of the Golgi apparatus
As the protein moves through the Golgi apparatus, it is modified and exits on the trans face inside a vesicle
The vesicle with the modified protein moves to and fuses with the plasma membrane resulting in the secretion of the contents from the cell