MEMBRANES TOPIC 2.4

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

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

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

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

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

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

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

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

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

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

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