(a) Munch hypothesis (10 marks) (b) Correlation and Regression (10 marks) (c) Heat Shock Proteins (HSP's) (10 marks) (d)…

(a) Munch hypothesis (10 marks)

The Munch (pressure-flow or mass-flow) hypothesis, proposed by Ernst Münch in 1930, explains translocation in phloem as bulk flow driven by a turgor pressure gradient between source and sink.

Mechanism:

• At the source (mature leaf), sucrose is actively loaded into the sieve tubes through companion cells by an H⁺-sucrose symporter, lowering their water potential.
• Water enters from the xylem by osmosis and turgor becomes high.
• At the sink (root, fruit, storage organ), sucrose is unloaded and used or stored, water leaves for the xylem and turgor falls.
• The pressure difference pushes the sap in bulk through the sieve tubes, at about 30 to 100 cm per hour.

Evidence: sap exudes under positive pressure from aphid stylets, girdling causes accumulation above the ring, and ¹⁴C-sucrose moves from source to sink.

Objections: sieve plates offer resistance and some tubes show two-way flow; yet the hypothesis is widely accepted, with energy needed only for loading and unloading.

(b) Correlation and regression (10 marks)

Correlation measures the degree and direction of a linear relationship between two variables, such as plant height and yield. It is given by the Pearson coefficient, r = Σ(x - x̄)(y - ȳ) / √[Σ(x - x̄)² × Σ(y - ȳ)²], which lies between -1 and +1: +1 is a perfect positive relation, -1 a perfect negative one, and 0 no linear relation. It is unit-free, and r² gives the proportion of variation explained. Significance is tested by a t-test. Correlation does not prove cause and effect.

Regression describes the dependence of one variable (y) on another (x) so that y can be predicted from x, by the line y = a + bx, fitted by least squares. The slope b, the regression coefficient, is Σ(x - x̄)(y - ȳ) / Σ(x - x̄)², equal to r × (sy/sx), and a is the intercept. There are two regression lines, y on x and x on y, which coincide when r = ±1. Regression is used to predict, for example yield from rainfall.

(c) Heat shock proteins (10 marks)

Heat shock proteins (HSPs) are highly conserved proteins, found in bacteria, plants and animals, that are induced when cells are exposed to high temperature. Ritossa discovered them in 1962 as chromosome puffs in Drosophila salivary glands after heat shock, and Tissières identified the proteins in 1974.

Induction: by heat (in plants usually 5 to 10 °C above the optimum) and also by cold, drought, heavy metals and oxidative stress. Heat shock transcription factors (HSF) bind heat shock elements in the promoters and switch on the genes.

Classes, named by size: HSP100, HSP90, HSP70, HSP60 (chaperonins) and small HSPs (15 to 30 kDa), which are abundant in plants.

Functions: they are molecular chaperones that prevent misfolding and aggregation of proteins, refold denatured proteins, help new proteins to fold and cross membranes, and direct damaged proteins for degradation. This gives thermotolerance. HSPs are important in breeding for heat tolerance and are drug targets (HSP90 inhibitors in cancer).

(d) Biogeochemical cycle (10 marks)

A biogeochemical cycle is the cyclic movement of an element or compound, such as carbon, nitrogen, phosphorus, sulphur or water, between living organisms (bio) and the physical environment (geo: atmosphere, hydrosphere and lithosphere). Each has a large, slow reservoir pool and a smaller exchange pool between organisms and their surroundings.

Types:

• Gaseous cycles, with the reservoir in the atmosphere or hydrosphere: carbon, nitrogen, oxygen and hydrogen. They are fast and largely self-regulating.
• Sedimentary cycles, with the reservoir in the earth's crust: phosphorus, calcium, iron, potassium and, partly, sulphur. They are slow, and material is lost in deep sediments.

Examples. In the carbon cycle, plants fix CO₂ by photosynthesis, and respiration, decomposition, combustion and ocean exchange return it. In the phosphorus cycle, weathering releases phosphate that plants absorb, consumers pass on, and decomposers return; there is no gaseous phase, and much is lost to sediments.

The cycles keep nutrients available and ecosystems stable, and are disturbed by fossil fuel burning, fertiliser run-off (eutrophication) and deforestation.

(e) Regulation of nif genes (10 marks)

The nif genes encode nitrogenase and its associated proteins. In Klebsiella pneumoniae, the best studied, about 20 nif genes lie in a 24 kb cluster of seven or eight operons: nifHDKTY (H for the Fe protein, D and K for the MoFe protein), nifENX and nifB (FeMo cofactor), nifUSVWZM, nifF, nifJ, and nifLA (regulators).

Regulation is mainly at transcription:

• The nif promoters need the alternative sigma factor sigma-54 (RpoN) and the activator NifA, which binds upstream and uses ATP to open the promoter.
• nifLA itself is activated by the Ntr system (NtrC) only when fixed nitrogen (ammonium, glutamine) is scarce.
• NifL is an anti-activator: it inhibits NifA when oxygen is present or fixed nitrogen is high, switching nif genes off.
• High temperature (above about 30 °C) and lack of molybdenum or iron also reduce expression.

In some bacteria, nitrogenase is also switched off directly by ADP-ribosylation of the Fe protein. In Rhizobium, nifA is activated under low oxygen by the FixL-FixJ system.