Biology: Cell Biology, Physiology, Genetics and Ecology
Assam CEE Biology draws from Botany and Zoology: cell biology, plant physiology, genetics, human physiology, reproduction and ecology. The paper favours precise NCERT-line statements and process reasoning rather than diagram labelling.
Core rules
- Cell biology: prokaryotes lack membrane-bound organelles. Mitochondria and chloroplasts contain their own DNA. The plasma membrane follows the fluid mosaic model.
- Photosynthesis: light reactions occur in thylakoids and the Calvin cycle in the stroma. RuBisCO fixes CO₂ in C3 plants; C4 plants use PEP carboxylase and show Kranz anatomy.
- Respiration: glycolysis occurs in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner membrane.
- Genetics: Mendel's law of segregation separates alleles during gamete formation. AB blood group shows codominance; colour blindness is X-linked recessive.
- Physiology: the loop of Henle creates a concentration gradient by countercurrent flow; the SA node is the heart's pacemaker; ADH increases water reabsorption in the collecting duct.
- Ecology: energy transfer follows the 10% law; nutrient cycles may be gaseous or sedimentary.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt.
Each parent produces T and t gametes in equal proportion. A Punnett square gives TT : Tt : tt = 1 : 2 : 1, so the phenotype ratio is 3 tall : 1 dwarf. The reasoning is segregation: each gamete carries one allele, and fertilisation restores the pair. A test cross of Tt × tt would instead give 1 tall : 1 dwarf.
Traps that cost marks
- Calling the cytoplasm the site of glycolysis is correct, but assigning glycolysis to mitochondria is not.
- Confusing thylakoid reactions with the stromal Calvin cycle.
- Mixing ADH with aldosterone when explaining water and sodium balance.
- Saying humans have 46 chromosomes without noting 23 homologous pairs.
- Treating lichen as one organism rather than an alga–fungus association.
- Forgetting that the placenta secretes hCG, progesterone and oestrogen.