1、1Revision checklistChapter 2All organisms are made of cells.Animal and plant cells have a cell surface membrane, cytoplasm and (usually) a nucleus. Plant cells also have a cellulose cell wall and (often) chloroplasts and a large vacuole containing cell sap.The cell surface membrane controls what ent
2、ers and leaves the cell. Metabolic reactions take place in the cytoplasm. The nucleus contains DNA in the form of chromosomes, which control the activities of the cell.The cell wall of a plant cell helps to maintain the shape of the cell and prevents it from bursting when it fills up with water. The
3、 sap in the vacuole is a solution of sugars and other substances. Chloroplasts contain chlorophyll, which absorbs energy from sunlight. Photosynthesis takes place inside them.A group of similar cells working together to carry out a particular function is called a tissue. Different tissues may be gro
4、uped together to form an organ, and different organs to form an organ system.KeyNot confident Needs more revisionConfident Supplement2There are many different types of animal cell and plant cell, each of which has a structure that is adapted for its functions. Ciliated cells help to sweep mucus up t
5、he trachea and keep the lungs clean. Root hair cells have a large surface area for the absorption of water (by osmosis) and mineral ions (by diffusion and active transport). Xylem vessels conduct water from the roots to other parts of the plant, and provide support. Muscle cells contract to cause mo
6、vement. Red blood cells contain haemoglobin and transport oxygen.Diffusion is the net movement of particles down their concentration gradient. Oxygen and carbon dioxide diffuse into and out of the blood in the lungs.Osmosis is a special kind of diffusion, in which water molecules diffuse down their
7、concentration gradient (water potential gradient) through a partially permeable membrane.Animal and plant cells take up water by osmosis if the solution around them is more dilute than their cytoplasm. Animal cells burst. Plant cells become turgid.Animal and plant cells lose water by osmosis if the
8、solution around them is more concentrated than their cytoplasm. Animal cells shrink. Plant cells become flaccid and then plasmolysed.Cells can take up, or remove, substances against their concentration gradient. This is done by active transport, which uses energy that the cell provides through respiration.
