Alevel Bio

Define antigen from an A-Level perspective

A foreign or non-self protein or glycoprotein (or molecule) that stimulates an immune response and the production of specific antibodies.

What is an Antigen-Presenting Cell (APC)? Give two examples

A cell that displays foreign antigens on its own cell-surface membrane. Examples: A phagocyte that has engulfed and hydrolyzed a pathogen, or a virus-infected host body cell.

Where do T lymphocytes and B lymphocytes mature

T cells mature in the thymus gland. B cells mature in the bone marrow.

Describe the mechanism of T-cell activation (Cell-Mediated)

1. Phagocyte engulfs pathogen and presents antigens on its membrane. 2. Specific Helper T cell (Th) binds complementarily to the antigen (clonal selection). 3. Th cell divides rapidly by mitosis (clonal expansion).

What are the four types of cells that cloned Helper T cells differentiate into

1. Memory cells 2. Stimulators (secrete cytokines to increase phagocytosis) 3. B-cell activators (stimulate B cells to divide) 4. Cytotoxic T cells (Tc cells).

How do Cytotoxic T cells (Tc) destroy infected cells

They bind to the foreign antigen on the infected cell and secrete the protein perforin. Perforin makes holes in the cell-surface membrane, making it freely permeable, causing cell lysis and death.

Describe the mechanism of B-cell activation (Humoral Immunity)

1. Specific B cell antibody receptors bind complementarily to an antigen. 2. B cell takes in antigen by endocytosis and presents it. 3. Activated Helper T cell binds to it, stimulating the B cell to divide rapidly by mitosis into plasma and memory cells.

Compare Plasma cells and Memory cells

Plasma Cells: Live a few days

Why is the secondary immune response much faster and stronger

Memory cells are already present. Upon re-exposure, they undergo immediate clonal selection and expansion, producing a higher concentration of antibodies at a much faster rate to eliminate the pathogen before symptoms show.

What is meant by positive cooperativity in haemoglobin binding

The binding of the first oxygen molecule alters the quaternary structure of the haemoglobin. This conformational change uncovers the remaining binding sites, making it much easier for the second and third oxygen molecules to bind.

Explain the shape of the oxygen dissociation curve

Shallow start: Difficult for the 1st O2 to bind due to Hb shape. Steep middle: Positive cooperativity makes subsequent binding rapid/easy. Flattens at top: After 3 sites are filled, probability of the 4th O2 colliding with the last empty site is very low.

What is the Bohr Effect and what causes it at respiring tissues

A decrease in haemoglobin affinity for oxygen in high CO2 environments. High CO2 lowers blood pH, altering the tertiary/quaternary structure of Hb, causing it to unload oxygen more readily.

In terms of oxygen dissociation curves, what does a shift to the right mean

It means haemoglobin has a lower affinity for oxygen. It will release/unload oxygen more easily to respiring tissues at any given partial pressure (pO2).

In terms of oxygen dissociation curves, what does a shift to the left mean

It means haemoglobin has a higher affinity for oxygen. It can load/become fully saturated with oxygen at much lower partial pressures (pO2).

Why does an animal at high altitudes (llama) or in low-oxygen mud (lugworm) have a curve shifted to the left

Because environmental oxygen levels are low. Having haemoglobin with a higher affinity allows the organism to successfully load and transport oxygen from its environment even under low atmospheric oxygen availability.