A-LevelBiologyTransport in plants9700/22 · May/June 2020 · Q515 Marks

In most plants, sucrose is the main sugar that is transported from sources to sinks. Fig. 5.1 shows part of one pathway that is used in plant cells to synthesise sucrose. The enzyme sucrose synthase catalyses the transfer of glucose from UDPG (uridine diphosphate glucose) to fructose. The structure of UDPG is shown in Fig. 5.2.

(a)In the source, sucrose is transferred from a mesophyll cell to a phloem sieve tube through a companion cell. Describe and explain how the transfer of sucrose into a phloem sieve tube from a companion cell can lead to the transport of the sugar to a sink.[4]
(b)(i)Name the type of bond formed when sucrose is synthesised.[1]
(b)(ii)Explain why UDP can be described as a phosphorylated nucleotide.[2]
(b)(iii)Sucrose synthase acts by using an induced fit mechanism rather than a lock and key mechanism. With reference to sucrose synthase and the synthesis of sucrose, outline the difference between the induced fit mechanism and lock and key mechanism of enzyme action.[4]
(c)UDPG is used in some algae (photosynthetic protoctists) to synthesise a storage compound known as floridean starch. The molecular structure of floridean starch has been described as an intermediate between amylopectin and glycogen, with little or no amylose. Describe the molecular structure of floridean starch by completing the passage. Floridean starch is a polysaccharide composed of __________________________________ monomers. The monomers are joined by ____________________________ and ____________________________ linkages, to give a branching structure that is less highly branched than ____________________________________________________________________________.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[4] presence of sucrose (in sieve tube) lowers water potential (of phloem sap); A makes water potential more negative water enters (sieve tube), by osmosis / down water potential gradient; increases volume (in sieve tube); increases hydrostatic pressure (in sieve tube at source) ; A turgor pressure ref. to lower hydrostatic pressure (in sieve tube at sink); allow ecf for no ref. to, hydrostatic/turgor detail; e.g. sucrose removed at sink water follows sucrose that exits sink movement of, (phloem) sap / sucrose, down pressure gradient/from high to low hydrostatic pressure ; mass flow;
[1] glycosidic;
[2] nucleotide components are base, pentose sugar, phosphate; UDP has, uracil and ribose and two phosphates / one extra phosphate;
[4] max three if describing breakdown of sucrose allow points from diagrams any four from induced fit shape of, substrates / UDP and glucose, not (fully) complementary to shape of active site (of sucrose synthase); lock and key shape of, substrates / UDP and glucose, complementary to shape of active site (of sucrose synthase); allow ecf if no ref. to shape induced fit active site flexible / moulds around, substrates / UDP and glucose; AW lock and key active site does not change shape / is not flexible / AW ; induced fit (active site moulds around so) idea of provides better fit / fully complementary ; lock and key substrate fits into active site ;
[4] α-glucose; A glucose R ẞ-glucose α, 1-4 ; α, 1-6; bond types either way round glycogen;

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