A-LevelBiologyTransport in plants9700/22 · May/June 2025 · Q114 Marks
Root hair cells are specialised plant cells located in the outer layer of young roots of plants. Root hair cells have an essential role in the uptake of water and dissolved mineral ions from the soil solution. The transport of water across the root to reach the central xylem tissue can occur by the symplast pathway or apoplast pathway.
- (a)Fig. 1.1 is a diagram of a photomicrograph of a root hair cell. [Figure 1.1] On Fig. 1.1, name cell structures X, Y and Z.[3]
- (b)In the symplast pathway, water passes through the cells of the different tissues in the root before entering the xylem vessels of xylem tissue. Name the tissues of the root, in the correct sequence, through which water passes in the symplast pathway.[2]
- (c)In the apoplast pathway, water passes along the cell walls of adjacent plant cells and through the intercellular spaces. This is more efficient than the symplast pathway. Explain the structural features of plant cell walls that make the apoplast pathway an efficient pathway for the transport of water.[3]
- (d)(i)Root hair cells also synthesise and secrete substances into the soil. Electron microscopy of the structure of root hairs has identified endoplasmic reticulum (ER), a number of small Golgi bodies, and numerous vesicles. • Root hair cells of the barley plant secrete enzymes known as acid phosphatases, which catalyse the release of inorganic phosphate ions from organic phosphates in the soil. • Root hair cells of the sorghum plant secrete a hydrophobic, lipid compound known as sorgoleone, which slows down the growth of neighbouring plants. Explain why the proportion of rough ER to smooth ER may be different in the root hair cells of barley plants compared with the root hair cells of sorghum plants.[2]
- (d)(ii)Student X stated that acid phosphatases and sorgoleone could be transported out of root hair cells using the same process. Student Y stated that acid phosphatases and sorgoleone are transported out of root hair cells using different processes. Suggest the reasons given by student X and by student Y to support their statements. student X student Y[4]
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 2025). The mark or mark type is shown in brackets.
- [1] X = (large / permanent) vacuole
- [1] Y = tonoplast ; A vacuolar membrane
- [1] Z = nucleus
- [2] any three from: epidermal (tissue) / epidermis, cortical (tissue) / cortex A parenchyma, endodermal (tissue) / endodermis, pericycle, in correct sequence
- [1] cellulose, (molecules / fibres), hydrophilic / allow adhesion of water (molecules)
- [1] idea that arrangement of cellulose (fibres) gives spaces (for water flow) ; AW e.g. many gaps
- [1] freely permeable / not impermeable, (to water) ; A idea of few obstacles to, water, movement / flow e.g. (cell walls) not selective / allow continuous movement of water / allow water to move at a high rate / allow all water to pass through / (offer) path of least resistance
- [1] ref. to hydrogen bonding; in correct context e.g. cellulose molecules form hydrogen bonds with water molecules I hydrogen bonds between cellulose molecules
- [1] hemicellulose / pectin, hydrophilic
- [1] rough ER site of, protein / enzyme, synthesis and smooth ER site of, lipid / sorgoleone, synthesis ; A protein modification A lipid storage
- [1] (barley plant), higher proportion of rough ER / lower proportion of smooth ER or (sorghum plant), higher proportion of smooth ER / lower proportion of rough ER
- [1] (student X) (mechanism could be) exocytosis ; R diffusion / facilitated diffusion see ecf below
- [1] (student X) for, bulk transport / larger quantities
- [1] (student X) idea that may be too large to exit cell (by other mechanisms) ; A are large molecules
- [1] (student X) presence of, (small) Golgi bodies ; A presence of Golgi body
- [1] (student X) ref. to vesicles as, Golgi / secretory, vesicles ; A description e.g. packaging into vesicles
- [1] (student X) vesicles, move to / fuse with, cell (surface) membrane
- [1] (student X ecf) move down a. concentration / diffusion, gradient ; A move from high to low concentration
- [1] (student Y option 1) enzymes, facilitated diffusion
- [1] (student Y option 1) sorgoleone, (simple / passive) diffusion
- [1] (student Y table details for enzymes) hydrophilic / are not hydrophobic / polar / AW OR pass through, transport / membrane / carrier, proteins AW A channel protein
- [1] (student Y table details for sorgoleone) hydrophobic / not hydrophilic / non-polar / AW OR cross hydrophobic core / between fatty acid tails / phospholipid bilayer AW
- [1] (student Y option 2) sorgoleone exocytosis and enzyme facilitated diffusion
- [1] (student Y option 3) enzymes exocytosis and sorgoleone diffusion
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