The importance of receptors to organisms

Question 1

Humoral immunity

Answer 1

1) Pathogens enters blood stream, phagocytosis occurs.
2) Specific B lymphocytes engulf foreign antigens.
3) Foreign antigen displayed on surface of B lymphocytes.
4) Activated T helper cells bind to presented foreign antigens.
5) Activated T helper cells activate B lymphocyte to divide by mitosis.
6) Clones differentiate to form plasma B and memory B lymphocytes.
7) Plasma B cells produce thousands of specific antibodies.
8) Antibodies released into blood stream and bind with antigens, forming antigen-antibody complexes. PRIMARY RESPONSE.
9) Memory B cells remain are are used for repeated invasions by same pathogen. SECONDARY RESPONSE.

Question 2

Action potentials

Answer 2

1) At resting potential, voltage gated Na+ channels are shut.
2) When a stimulus occurs, a threshold value is reached, causing some of the Na+ channels to open.
3) Na+ diffuse from tissue fluid through the voltage-gated Na+ channels into cytoplasm of axon down electrochemical gradient. More Na+ channels open.
4) Depolarisation.
5) Action potential (+40mV) is established, voltage-gated Na+ channels close, voltage-gated K+ channels open.
6) Electrical gradient is reversed, more open.
7) More K+ diffuse into tissue fluid.
8) Hyperpolarisation
9) Voltage-gated K+ channels close, Na+/K+ pump moves 3 Na+ out and 2 K+ in.
10) Resting potential (-70mV) re-established, axon repolarised.

Question 3

Taxes and Kineses

Answer 3

Kineses.
Random non-directional movement.
Organisms do not move towards or away from stimulus.
More unpleasant the stimulus, the more rapidly the organism moves.

Taxes
Simple directional response.
Direction determined by direction of stimulus.
Move towards stimulus = positive taxis.
Move away from stimulus = negative taxis.

Question 4

Add on to taxes and kineses: Phototropism in the shoot.

Answer 4

1) Cells in shoot tip produce IAA, diffuses down shoot.
2) Light causes movement of IAA from light side to shaded side.
3) Conc. greater in darker side.
4) Attaches to receptors on membrane of cells.
5) Releases hydrogen ions, lowering the pH.
6) Breaks bonds between microfibrils in the cellulose cell wall, making them more easily stretched so they elongate.
7) Shaded side grows faster = shoot bends towards the light.

Question 5

Transcriptional factors

Answer 5

1) Each TF has a DNA binding site that is specific to a particular gene.
2) When a specific protein is required, the gene is stimulated by the specific TF.
3) This initiates synthesis of the protein by binding to the DNA at the specific base sequence - the promoter sequence.
4) Stimulates transcription.
5) Pre-mRNA produced, spliced into mRNA before being translated into a protein.

Question 6

Actions of glucagon

Fall in blood glucose level

Answer 6

Binds to complementary receptor, activates adenylate cyclase, hydrolyses ATP to cAMP, which activates protein kinase. Glycogen hydrolysed into glucose. (second messenger system).

1) Alpha cells from Islets of Langerhans detect blood glucose decrease and secrete glucagon.
2) Travels in blood, reaching target cell, works by 2nd messenger system, causing glycogenolysis.
3) Gluconeogenesis synthesises glucose from amino acids and glycerol.
4) Blood glucose levels increase.