WJEC Biology for AS Level Student Book: 2nd Edition

Stretch & challenge Plant cells each have up to about 100 000 plasmodesmata connecting them to other cells, with about 10 per µm 2 . Plants do not have a nervous system, and it may be that this high connectivity through the cytoplasm can serve a similar function. Differences between plant and animal cells Generalised plant and animal cells have a nucleus, cytoplasm and a cell membrane. The table below shows the differences between them. Animal cell Plant cell Cell wall absent present – surrounds cell membrane Chloroplasts absent present – in cells above ground Plasmodesmata absent present Vacuole present – small, temporary; scattered throughout cell present – large, permanent, central; filled with cell sap Centrioles present absent Energy store glycogen starch Organelles are interrelated Organelles have been described here separately but their functions within the cell are often related: ▪ The nucleus contains chromosomes in which the DNA encodes proteins. ▪ Nuclear pores in the nuclear envelope allow mRNA molecules, transcribed off the DNA, to leave the nucleus and attach to ribosomes in the cytoplasm or on the rough ER. ▪ Ribosomes contain rRNA, transcribed from DNA located at the nucleolus. ▪ Protein synthesis occurs on ribosomes, producing proteins in their primary structure. ▪ Polypeptides made on the ribosomes are moved through the RER and are packaged into vesicles. The vesicles bud off the RER and carry the polypeptides to the Golgi body, where they are chemically modified and folded. ▪ The Golgi body produces vesicles containing newly synthesised proteins. These may be lysosomes, containing digestive enzymes, used within the cell. They may be secretory vesicles, which carry the proteins to the cell membrane for exocytosis. ▪ Phospholipids and triglycerides move through the smooth endoplasmic reticulum to various destinations in the cell. Interrelationship of organelles 1. nucleus 2. nuclear pore 3. RER 4. vesicle 5. Golgi body 6. secretory vesicle 7. cell membrane 1 2 3 4 5 6 7 1.2 Cell structure and organisation Knowledge check Complete the paragraph by filling in the gaps: Plant and animal cells have many features in common but, close to their nucleus, animal cells have a pair of ........................ , absent from plant cells. Plant cells also have a large, central .................... surrounded by a membrane called the ........................, a cellulose ................. ................. and photosynthetic cells have ........................ 2.3 Knowledge check Match the structures 1–3 with the descriptions of their functions A–C. 1. Chloroplast 2. Cell wall 3. Vacuole A. Provides mechanical support B. Site of photosynthesis C. Its osmotic activity controls a cell’s turgor 2.4 39