Fig. 5.1 is a transmission electron micrograph showing parts of two plant cells. The function of the middle lamella is cell-to-cell adhesion. The middle lamella is composed of a polysaccharide known as pectin. Pectin interacts with the polysaccharides cellulose and hemicellulose in the cell walls of the plant cells so that the cell walls are held close together, as shown in Fig. 5.1. Golgi body middle lamella X Fig. 5.1

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [2] plasmodesma ; plus any one from R if stated as apoplast 1 allows/AW, symplast, pathway / movement ; 2 allows transport of / AW, water / sucrose / sugars / amino acids / organic substances / ions / mineral salts (A minerals) / lipids / assimilates / hormones / cell signalling molecules; in context of between cells I substance unqualified I osmosis of water if in a longer list where other substances are named correctly R osmosis of water if this is the only item noted 3 cell to cell / AW, communication / signalling; A cell communication
2. [1] any one from (only) polypeptides / proteins, synthesised at ribosomes ; carbohydrates / polysaccharides, not synthesised at ribosomes ; enzymes, for synthesis / required, not located at ribosomes ;
3. [1] exocytosis;
4. [3] accept substrate for cellulose and enzyme for cellulase allow use of extracted data to support a response to confirm awarding a mark 1 correct description; overall trend or more detailed break down e.g. as, cellulose / substrate, concentration increases, rate of reaction increases or as, cellulose / substrate, concentration increases, steep increase in rate of reaction then, less steep increase / (begins to) plateau I 'rate of reaction is fast and then slows down' needs ref. to rate of reaction, describing 'gradient of curve' is not enough plus any two from 2 increasing cellulose concentration increases number of, collisions between enzyme and substrate / enzyme-substrate complexes / ESCs ; A collisions between active site and substrate A idea of increased chances of cellulose binding to active site 3 at low cellulose concentrations, (many) active sites, available / not all used / not saturated ; 4 at higher cellulose concentrations, active sites becoming saturated / most active sites occupied / most active sites not available / AW ; A actives sites are, saturated / occupied 5 named limiting factor(s), qualified; e.g. at low(er) cellulose concentrations, cellulose concentration limiting / enzyme concentration not limiting A idea of, not enough cellulose / enzyme in excess as cellulose concentration increases, change from cellulose concentration limiting to enzyme concentration as limiting factor at higher cellulose concentrations, enzyme concentration limiting / cellulose concentration not limiting A idea of, cellulose in excess / not enough enzyme / enzyme working at its fastest rate / active sites not emptying quickly enough for substrate available
5. [3] mark to max 2 if microorganisms are considered to be enzyme inhibitors lower Km for pectinase, any two from ultrasound increases affinity of enzyme for substrate ; makes shape of active site more complementary ; makes (position of) active site more accessible (to substrate); A makes it easier for substrate to enter (active site) breaks up pectin, aggregates / AW, to expose more substrate for binding; higher Vmax for xylanase, any two from ultrasound increases rate of, collision between enzyme and substrate / more enzyme-substrate complexes per unit time ; I more ESC form needs idea of increased rate increases rate of catalysis after binding; may change substrate for easier hydrolysis ; may lower, more than normal, activation energy required for reaction; A described, e.g. increased ability to, break / form, bonds
6. [1] yes (as all have) increased, catalytic efficiency / productivity / Vmax/Km; A manipulated data for increased catalytic efficiency any two from pectin 24.5/25% cellulase 17.25/17% xylanase 16.8/17%