A-LevelBiologyTransport in plants9700/23 · May/June 2019 · Q49 Marks

Phloem sieve tube elements and xylem vessel elements are cells that are involved with the transport of substances in plants.

(a)Describe two differences between the structure of a phloem sieve tube element and a xylem vessel element.[2]
(b)Sucrose is transported in phloem sieve tubes. A student carried out a test to identify the presence of sucrose in a sample of sap taken from inside a phloem sieve tube. One of the steps in the procedure instructed the student to heat the phloem sap with hydrochloric acid. Explain why it is necessary to carry out this step in the procedure.[2]
(c)(i)The process of loading sucrose into a phloem sieve tube element involves a companion cell. Fig. 4.1 shows a stage in the process of loading sucrose into the phloem. [Figure 4.1] Companion cells contain large numbers of mitochondria and ribosomes. Explain the roles of mitochondria and ribosomes for the stage shown in Fig. 4.1.[2]
(c)(ii)Describe and explain how sucrose is transported from the cell wall of the companion cell into the cytoplasm.[3]

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 2019). The mark or mark type is shown in brackets.

[2] any two from:
note that some mark points must have both xylem and phloem mentioned ('v') and some mark points only need xylem or phloem ('or' – below the line)
xylem: no cytoplasm / hollow / no contents (v) phloem: (peripheral / little / some) cytoplasm I protoplasm ; R full of cytoplasm / AW
xylem: has (a few) organelles
A examples of organelles
mitochondria or ER
I has SER / RER / ribosomes
R has nucleus
xylem: lignified (v) phloem: no lignin / (only) cellulose ; R idea of lignin within element
xylem: no end wall(s) / no sieve plate(s) (or) phloem: sieve plate(s) / perforated end wall (s) ; A end walls have broken down A (end walls) have sieve pores
xylem: (side walls) contain pits (or) phloem: no pits ; R piths
xylem: no plasmodesmata (or) phloem: plasmodesmata (to companion cells) ;
xylem: thick(er), cell wall / walled (or) phloem: thin(ner), cell wall / walled ;
thicker lignified wall = thicker wall mark only (for lignin mark need to state what phloem has)
[2] any two from:
to hydrolyse / break, glycosidic bonds ;
to produce, reducing sugars / fructose and glucose ;
A to make reactive groups (to Benedicts) available
to be able to test with Benedict's solution to obtain a positive result ;
A description of positive result
sucrose, is a non-reducing sugar / has no reactive groups when tested with Benedict's solution ;
[2] (large number of mitochondria) provide ATP, for active transport / for protein synthesis ;
(large number of ribosomes) to synthesise the, proton pumps / carrier proteins / AW ;
[3] any three from:
H+, pumped / actively transported (out of companion cell into cell wall)
or
ref. to higher concentration of H+ (in cell wall) / H+ gradient builds up (between cell wall and cytoplasm of companion cell) ;
H+ move back (into companion cell) by facilitated diffusion; I sucrose
through, co-transporter protein / cotransporter; A symporter
H+ cotransports sucrose / sucrose moves (into cell) with H+ ;
sucrose is transported against the concentration gradient ;
A sucrose transported by secondary active transport