Question
The image shows a cell in the anther of a lily (Lilium sp.) plant during the first division of meiosis.
(a) (i) Identify the stage of meiosis shown in the image
(ii) State where in the lily flower pollen would be formed.
(b) Distinguish between plant pollination and fertilization.
(c) The diagram shows a cross section through a broad bean (Vicia faba) seed.
(i) On the diagram, label the cotyledon.
(ii) Outline the function of the cotyledon.
▶️Answer/Explanation
(a)
(i) The stage is anaphase I of meiosis, where homologous chromosomes are being pulled to opposite poles.
(ii) Pollen is formed in the anther, which is part of the stamen (the male reproductive organ of the flower).
(b)
- Pollination is the transfer of pollen from the anther (male part) to the stigma (female part) of a flower.
- Fertilization is the fusion of the male gamete (in the pollen) with the female gamete (in the ovule), forming a diploid zygote.
(c)
(i) 
(ii)
- The cotyledon stores food (like starch or protein) for the developing embryo.
- It provides energy and nutrients during germination.
- It helps in the early growth of the seedling, including development of roots and shoots.
Question
Multicellular organisms benefit from cell specialization and division of labour.
(a) Outline the processes occurring during interphase in the cell cycle.[4]
(b) Describe what occurs in a neuron when an action potential is propagated along the axon. [4]
(c) Explain how cells in the bloodstream cause a specific immune response.[7]
Answer/Explanation
(a)
- The cell grows and increases in size.
- Organelles like mitochondria and chloroplasts are duplicated to prepare for cell division.
- DNA replication occurs so each new cell will get a full set of genetic material.
- Proteins and enzymes are synthesized, including mRNA transcription for gene expression.
- ATP is produced through cell respiration to provide energy for all these activities.
(b)
- When a stimulus reaches the neuron, sodium channels open, and Na⁺ ions enter, making the inside of the axon more positive (depolarization).
- Once the threshold is reached, the action potential is triggered and spreads.
- Potassium channels open, and K⁺ ions leave the cell, causing repolarization (membrane returns to negative inside).
- This change in charge creates local currents, triggering the next part of the axon to depolarize, moving the signal along the axon.
(c)
- A specific immune response is triggered when the body detects a particular pathogen.
- Macrophages (phagocytes) engulf the pathogen and present its antigens on their surface.
- These antigens activate helper T cells (a type of lymphocyte).
- The activated T cells then activate specific B cells that can produce antibodies against that antigen.
- Only B cells with the matching antibody are selected and cloned by mitosis to make many identical cells.
- These clones become plasma cells, which secrete large amounts of antibodies into the blood.
- Some B cells become memory cells, which remain in the body and give long-term immunity by responding faster if the same pathogen infects again.
——————————————————————————————————–Markscheme—————————————————————————————————————-
a a. growth/increase in cell size;
b. division of mitochondria/chloroplasts/production of more organelles/number of organelles doubled;
c. replication of DNA/amount of DNA is doubled;
d. transcription of genes/production of mRNA;
e. protein synthesis;
f. cell respiration/production of ATP;
b a. sodium ions/Na+ enter/diffuse in;
b. depolarization/membrane potential/voltage changes from negative to positive;
c. potassium channels open AND potassium ions/K+ exit/diffuse out;
d. repolarization/membrane potential/voltage changes back from positive to negative;
e. local current due to diffusion of sodium ions along the neuron;
f. (local currents) cause next sodium channels to open/next part of axon to depolarize;
g. opening of sodium channels triggered when threshold potential/-50mV reached;
c a. (specific immune response is) production of antibodies in response to a particular pathogen;
b. antibody is specific to/binds to a specific antigen;
c. macrophages/phagocytes engulf/present antigens from pathogens/viruses/bacteria;
d. T lymphocytes activated by antigens/antigen presentation/antigens presented by macrophage;
e. (activated) T lymphocytes activate B lymphocytes;
f. only B lymphocytes that produce antibodies against the antigen/pathogen are activated;
g. (activated) B lymphocytes clone/divide by mitosis to form plasma cells;
h. plasma cells then secrete (large quantity) of an antibody/secrete antibodies of same type;
i. some B lymphocytes/plasma cells form memory cells;
j. memory cells give long lasting immunity/faster response to a disease/pathogen;