WJEC Biology for AS Level Student Book: 2nd Edition

Study point Animal cells contain vacuoles but, unlike in plant cells, these are small, temporary vesicles and may occur in large numbers. Centrioles Centrioles occur in all animal cells and most protoctistans but not in the cells of higher plants. They are located just outside the nucleus. Centrioles are two rings of microtubules, making hollow cylinders positioned at right angles to one another. Together, they are sometimes called the centrosome . During cell division, centrioles organise the microtubules that make the spindle. Electron micrograph of a pair of centrioles Vacuole Most plant cells have a large permanent vacuole which consists of a fluid-filled sac bounded by a single membrane, the tonoplast. Vacuoles contain cell sap, a solution which stores chemicals such as glucose, amino acids and minerals, and may store vitamins and pigments, as in oranges. Vacuoles have a major role in supporting soft plant tissues. Cell wall The cell wall of a plant cell consists largely of cellulose. Cellulose molecules are held together in microfibrils, which are aggregated into fibres, embedded in a polysaccharide matrix called pectin. The cell wall has the following functions: ▪ Transport – the gaps between the cellulose fibres make the cell wall fully permeable to water and dissolved molecules and ions. This space outside the cells, through which solution moves, is called the apoplast. The apoplast pathway is the main way that water crosses the plant root. ▪ Mechanical strength – the structure of cellulose microfibrils and their laminated arrangement make the cell wall very strong. When the vacuole is full of solution, the cell contents push against the cell wall, which resists expansion and the cell becomes turgid, supporting the plant. ▪ Communication between cells – cell walls have pores, called pits, through which strands of cytoplasm, called plasmodesmata (singular – plasmodesma), pass. The plasmodesma occurs where there is no cellulose thickening between two cells. The strand of cytoplasm runs from one cell to the next. The network of cytoplasm in connected cells is called the symplast. The symplast pathway is important in water transport through a plant. Structure of a plasmodesma Link The role of centrioles in cell division is described in Chapter 1.6. Link See p61 for a description of osmosis in plant cells. Link The structure of cellulose is described on p20. The symplast pathway is discussed further on p212. Key term Plasmodesma (plural = plasmodesmata): Fine strands of cytoplasm that extend through pores in plant cell walls, connecting the cytoplasm of one cell with that of another. cell wall cytoplasm endoplasmic reticulum plasmodesma continuous through pit cell membrane 2.2 Knowledge check Match the structures 1–4 with the descriptions of their functions A–D. 1. Ribosome 2. Nucleus 3. Mitochondria 4. Golgi body A. Contains the genetic material B. Site of protein synthesis C. Modifies proteins after their production D. Site of aerobic respiration WJEC Biology for AS Level 38