Essay - Membrane

Question 1

Reabsorption of glucose and water by proximal convoluted tubule.

Answer 1

• Sodium ions actively transported using ATP out of epithelial cells lining PCT into blood capillaries where they are taken away.
• Sodium ions diffuse down concentration gradient via carrier proteins via facilitated diffusion.
• The molecules that have been co-transported into cells of the PCT diffuse into blood. All valuable molecules are reabsorbed.
• In descending limb of Loop of Henle, walla are very permeable to water, so it passes out filtrate via osmosis into the interstitial space.
• Sodium ions diffuse out of filtrate and move up ascending limbs before being actively transported out using ATP, filtrate WP increases.
• More sodium ions actively transported out distal convoluted tubule.
• Collecting duct permeable to water, water passes out it via osmosis, lowering filtrate water potential. u r i n e.

Question 2

Cholinergic synapse

Answer 2

• Action potential arrives at presynaptic neurone, calcium channels open.
• Calcium ions enter presynaptic neurone.
• Vesicles fuse to presynaptic membrane, releasing acetylcholine into synaptic cleft.
• Acetylcholine binds to receptor sites on sodium protein channels in postsynaptic neurone, opening sodium channels, sodium ions diffuse in.
• Generates an action potential in postsynaptic neurone.
• Acetylcholinesterase hydrolyses ester bonds in acetylcholine, diffuses back across synoptic cleft into presynaptic membrane, ready to be put back in vesicles for next action potential.
• Enzyme stops acetylcholine from continuously binding to postsynaptic membrane which would prevent transfer of information across synapses.

Question 3

Humoral immunity

Answer 3

• Pathogen enters blood stream, phagocytosis occurs.
• Specific B lymphocytes engulf foreign antigens.
• Foreign antigen displayed on surface of B lymphocytes.
• Activated T helper cells bind to presented foreign antigen, activating B lymphocyte to divide by mitosis.
• Clones differentiate to form plasma B and memory B lymphocytes.
• Plasma B cells produce thousands of specific antibodies.
• Antibodies release into blood stream, bind with antigens, forming antigen-antibody complexes.
• Memory B cells remain and are used for repeated invasions of same pathogens.
• Membranes needed for immunity!