Question 114 marks
Question
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. cytoplasm X …… Z …… Y …… Fig. 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] * 0000800000003 * DFD 3 (d) 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. (i) 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] (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] [Total: 14] * 0000800000004 * DFD 4
Mark scheme
1(a) X = (large / permanent) vacuole ; 3 Y = tonoplast ; A vacuolar membrane Z = nucleus ; 1(b) if correct named cells are given, allow as a tissue if plural, otherwise ignore e.g. 2 I cortex cell A cortex cells I root hair cell at start and, xylem / vascular bundle / stele, at end I cell structures epidermal (tissue) / epidermis cortical (tissue) / cortex A parenchyma endodermal (tissue) / endodermis pericycle any three in correct sequence ;; if incorrect tissue(s) included with a correct full sequence of three, max 1 mark only two tissues stated but are in correct sequence allow one mark three tissues stated but not in correct sequence, and no incorrect tissues, allow one mark 1(c) any three from: 3 cellulose, (molecules / fibres), hydrophilic / allow adhesion of water (molecules) ; idea that arrangement of cellulose (fibres) gives spaces (for water flow) ; AW e.g. many gaps 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 ref. to hydrogen bonding ; in correct context e.g. cellulose molecules form hydrogen bonds with water molecules I hydrogen bonds between cellulose molecules hemicellulose / pectin, hydrophilic ; 1(d)(i) rough ER site of, protein / enzyme, synthesis and smooth ER site of, lipid / sorgoleone, synthesis ; 2 A protein modification A lipid storage (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(d)(ii) student X to max 3 4 (mechanism could be) exocytosis ; R diffusion / facilitated diffusion see ecf below plus max two details from: for, bulk transport / larger quantities ; idea that may be too large to exit cell (by other mechanisms) ; A are large molecules presence of, (small) Golgi bodies ; A presence of Golgi body ref. to vesicles as, Golgi / secretory, vesicles ; A description e.g. packaging into vesicles vesicles, move to / fuse with, cell (surface) membrane ; ecf if, diffusion / facilitated diffusion, stated as the mechanism: move down a. concentration / diffusion, gradient ; A move from high to low concentration student Y to max 3 check one of three different options (A enzymes for acid phosphatases and vice versa) option 1 enzymes, facilitated diffusion and sorgoleone, (simple / passive) diffusion ; plus two marks for all information in table correct ;; enzymes sorgoleone hydrophilic / are not hydrophobic / polar / AW hydrophobic / not hydrophilic / non-polar / AW pass through, transport / membrane / carrier, proteins cross hydrophobic core / between fatty acid AW tails / phospholipid bilayer AW A channel protein if two marks not gained, one mark for a correct row or correct column 1(d)(ii) option 2 sorgoleone exocytosis and enzyme facilitated diffusion ; plus one additional point about exocytosis ; detail from enzyme column (shaded dark grey) ; option 3 enzymes exocytosis and sorgoleone diffusion ; plus one additional point about exocytosis ; sorgoleone column (shaded light grey) ;