Fig. 3.1 shows some fruits (grapes) of the grapevine, Vitis vinifera. Sucrose is transported in the phloem of the grapevine to the fruits. In the fruits, sucrose is hydrolysed by the enzyme sucrase, which is found in cell walls. The glucose and fructose produced by the hydrolysis of sucrose pass through membrane proteins, known as hexose transporters, into the cytoplasm of the fruit cells.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

1. [1] polar / water soluble / hydrophilic, substances cannot pass through the, phospholipid bilayer / hydrophobic core (in cell surface membranes); I too large / active transport / facilitated diffusion / (hydrophobic) fatty acid 'tails'
2. [2] rate of uptake, becomes constant / reaches a plateau or at high concentrations / concentrations of > 150 µmol dm⁻³, rate of uptake remains constant; A levels off protein carriers / (hexose) transporters / VvHT1s, are working at their highest rate / are saturated ; A the number of protein carriers is limiting factor / AW A Vmax is reached in context of transporters
3. [2] any two from: I size / quality of grapes / grapevines 1 grapes will, be sweeter / have improved taste ; 2 idea that concentration of sugars in grapes determines uptake of water by osmosis ; 3 idea of more energy for growth of the grapes ; 4 larger yield ; 5 increased, profit / income, for farmers ; 6 AVP; e.g. producing grapes in shorter time
4. [1] (foreign) protein / glycoprotein, that stimulates, an immune response / production of antibodies / activation of lymphocytes ; A polysaccharide / molecule / foreign substance / foreign antigen
5. [4] any four from: accept T-cells for T-lymphocytes 1 (group / clone of) T-lymphocytes have receptors complementary to an antigen ; A specific T-lymphocytes have receptors to an antigen 2 (specific) T-lymphocytes, bind / AW, to antigen (on surface of antigen-presenting cell / macrophage) ; 3 (selected) T-lymphocytes, divide many times (by mitosis) / undergo clonal expansion ; 4 T-helper cells secrete, cytokines / interleukins / interferon ; 5 detail of effect of cytokine ; e.g. activation of B-lymphocytes / increased phagocytosis by macrophages / 'angry' macrophages 6 T-killer cells, bind / attach, to infected cells and destroy them; 7 detail of action of T-killer cells ; e.g. ref. to, perforin / granzyme / hydrogen peroxide / toxins 8 production of memory cells (in context of T-lymphocytes / T-helper cells / T-killer cells);