A-LevelBiologyBiological molecules9700/23 · May/June 2021 · Q47 Marks

Phloem sap is transported within phloem sieve tubes.

(a)Monosaccharides and disaccharides are transported as part of phloem sap, but polysaccharides are not components of phloem sap. Define the terms disaccharide and polysaccharide.[2]
(b)(i)Sieve tubes are composed of phloem sieve tube elements. Fig. 4.1 is a diagram of one sieve tube element and its companion cell. Some of the structural features of the sieve tube element have not been included in the diagram. [Figure 4.1] Complete Fig. 4.1 by drawing and labelling the structural features of the sieve tube element that have not been included in the diagram.[3]
(b)(ii)A pressure gradient in a sieve tube causes the mass flow of phloem sap from the source to the sink. At the source, a decrease in water potential in the phloem sap and an increase in the hydrostatic pressure of the phloem sap can be measured. Describe the events that cause each of these changes in the phloem sap. decrease in water potential increase in hydrostatic pressure[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[1] disaccharides - two, monosaccharides / sugar monomers / sugar units / simple sugars ; R sugar molecule
[1] polysaccharides - more than two / three or more / many / large number of, monosaccharides / sugar monomers / sugar units / simple sugars; A sugar molecule / carbohydrate monomer
[1] if 'monomers' is the only term given for both allow one mark (unless contradicted e.g. amino acids are monomers)
[1] correct ref. to glycosidic bond(s);
[1] max 2 if large vacuole or nucleus drawn
[1] 1 sieve plate drawn and labelled; A sieve pore label must be to a gap doesn't need to show pores I plasmodesmata
[1] any two from
[1] 2 cytoplasm labelled and drawn as, thin layer against wall / patches; if drawing not clear, allow if label indicates peripheral cytoplasm
[1] 3 mitochondrion / mitochondria, drawn within cytoplasm / at edge, and labelled; internal details not required
[1] 4 endoplasmic reticulum drawn within cytoplasm / at edge, and labelled ; AER RRER/SER if no indication of peripheral cytoplasm, or structures located within peripheral cytoplasm/at edge, allow one mark for drawing and labels of two of mp2,3,4
[1] 5 cell surface membrane drawn in (close to cell wall) and labelled; A plasma membrane
[1] 6 AVP; e.g. p-proteins drawn as, strands / AW, and labelled plasmodesmata shown as strands of cytoplasm entering sieve tube cell from companion cell and labelled
[1] influx / entry / AW, of sucrose / organic compounds / assimilates / photosynthates / named ; A active loading for entry R active transport / cotransport/facilitated diffusion R incorrect cell type named as donor cell e.g. xylem / mesophyll must have idea of entry – stating that they are present in the phloem sieve tube is not enough
[1] water enters (from xylem), by osmosis / down water potential gradient / from high(er) to low(er) water potential / from less negative to more negative water potential; A Ψ for water potential A water enters, increasing volume