A-LevelBiologyThe mitotic cell cycle9700/22 · May/June 2023 · Q214 Marks
Fig. 2.1 is a photomicrograph of a longitudinal section (LS) through a root tip. Two different regions are visible: • the root apical meristem • the root cap. Cells in the root cap synthesise a gel-like, sticky secretion known as mucilage, which is important in reducing friction between soil and the growing root. It is composed mainly of polysaccharides and also contains some amino acids and enzymes. [Figure 2.1]
- (a)Describe three differences, visible in Fig. 2.1, between root apical meristem cells and root cap cells.[3]
- (b)Mucilage acts as a glue to bind tiny soil particles together, forming small clumps close to the root. These small clumps help to maintain the soil water around the root tip and prevent the loss of water. With reference to the cohesive and adhesive properties of water, suggest and explain how the formation of small clumps of soil helps to maintain the soil water around the root tip.[3]
- (c)Enzymes present in mucilage catalyse the breakdown of organic compounds in the soil. This increases the presence of mineral ions in the soil. State the term used to describe enzymes that act outside the cells that synthesise them.[1]
- (d)(i)The polysaccharides and amino acids present in the mucilage are a source of nutrients for soil microorganisms that live in the area surrounding the root. Some of these microorganisms can break down soil compounds to release mineral ions. Explain what is meant by a polysaccharide.[2]
- (d)(ii)The soil microorganisms use amino acids to synthesise proteins. All of the twenty different amino acids that are present in proteins have the same general structure. Draw the general structure of an amino acid in the space provided and use this drawing to explain how it is possible to have many different amino acids.[2]
- (d)(iii)Mineral ions are usually present in the soil in very low concentrations. The action of mucilage enzymes and soil microorganisms can help to increase the presence of mineral ions. Root hair cells are specialised for the uptake of these mineral ions and for the absorption of water from the soil. Suggest and explain how the presence of mineral ions in the root hair cell can increase the absorption of water by the root hair cells.[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 2023). The mark or mark type is shown in brackets.
- [1] root apical meristem cells: (some) cells in, mitosis / different stages of cell cycle; OR root cap cells: no cells in mitosis / all cells in interphase ; A no chromosomes visible / only chromatin visible
- [1] root apical meristem cells: (generally) smaller (size); A less cytoplasm OR root cap cells: (generally) larger (size); A more cytoplasm
- [1] root apical meristem cells: larger nucleus in proportion to, size of cell / quantity of cytoplasm ; OR root cap cells: smaller nucleus in proportion to, size of cell / quantity of cytoplasm ;
- [1] root apical meristem cells: nucleoli / nucleolus, (more) visible / prominent / AW ; OR root cap cells: nucleoli / nucleolus, not visible / not prominent / AW ;
- [1] root apical meristem cells: (generally all) cuboidal / same shape ; A rectangular / regular (shape) OR root cap cells: variety of shapes / shape described ; A irregular / circular
- [1] root apical meristem cells: ref. arrangement of cells; e.g. rows / chains / tightly packed/compact / uniform / regular OR root cap cells: ref. arrangement of cells ; e.g. irregular/loosely packed / more intercellular spaces
- [1] AVP; e.g. root apical meristem cells: all / more, cells have nuclei OR root cap cells: some have no nucleus
- [1] AVP; e.g. root apical meristem cells: cells not, sloughing off / detaching OR root cap cells: some cells, sloughing off / detaching
- [1] water molecules form hydrogen bonds ;
- [1] cohesion between water molecules (in spaces) ;
- [1] adhesion of water molecules to, soil, surface / particles / clumps / mucilage / polysaccharides ;
- [1] clumps provide, areas / (small) spaces / place, in which water collects / idea of stable environment e.g. reduced movement of water, in / around clumps / keeps water around the, soil clumps / the root tip ;
- [1] extracellular (enzymes) ;
- [1] more than two / three or more / many / large number of / polymer of, monosaccharides / sugar monomers / sugar units / simple sugars / carbohydrate subunits / AW ;
- [1] joined by / AW, glycosidic bonds ; / idea of repeating, subunits / units / monomers / AW ;
- [1] (drawing shows) all correct (NH2 or COOH groups on left or right) ;
- [1] R-group / side chain, is variable OR different amino acids have different, R-groups / side chains ;
- [1] mineral ions / nutrients, are solutes / dissolve in water ;
- [1] (solutes) decrease / lower, water potential, within cell / in cytosol / in cytoplasm ;
- [1] increases / steep(er)/ presence of/AW, water potential gradient ;
- [1] water follows osmotically / water enters by osmosis ;
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