Biology - General Certificate of Secondary Education, 2003

AQA

Life Processes and Living Things

Cell Activity

10.1 Plant and Animal Cells	KS3	All animals and plants are made up of cells. Cells may be specialised to carry out a particular function. A group of cells with similar structure and a particular function is called a tissue. Organs are made of tissues. Different organs are combined to form organ systems. Each system in the body carries out a particular function or range of functions.
	FT and HT	Most cells have the following parts: a nucleus which controls the activities of the cell; cytoplasm in which most of the chemical reactions take place; a cell membrane which controls the passage of substances in and out of the cell. Plants also have: a cell wall which strengthens the cell; and often have: chloroplasts which absorb light energy to make food; a permanent vacuole filled with cell sap. Candidates should be able, when provided with appropriate information, to relate the structure of different types of cells to their function in a tissue, an organ, or the whole organism.
	HT	The chemical reactions inside cells are controlled by enzymes. Cytoplasm contains structures called mitochondria, which is where most energy is released in respiration.
10.2 Transport Across Boundaries	FT and HT	Diffusion is the spreading of the particles ot a gas, or of any substance in solution, resulting in a net movement from a region where they are at a higher concentration to a region where thev are at a lower concentration. The greater the difference in concentration, the faster the rate of diffusion. Oxygen required for respiration passes through cell membranes and through gas exchange surfaces, such as alveoli in the lungs, by diffusion. Carbon dioxide enters leaves and leaf cells by diffusion. Osmosis is the diffusion of water trom a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules but not solute molecules. Other substances such as sugar and ions can also pass through cell membranes. Many organ systems are specialised for exchanging materials: in humans the surface area of the lungs is increased by the alveoli, and that of the small intestine by villi; in plants the surface area of the roots is increased by root hairs, and the surface area of leaves by the flattened shape and internal air spaces
	HT	Substances are sometimes absorbed against a concentration gradient. This requires the use of energy from respiration. The process is called active transport. It enables plants to absorb ions from very dilute solutions. Similarly sugar may be absorbed from low concentrations in the intestine and from low concentrations in the kidney tubules. Candidates should be able, when provided with appropriate information, to explain how other gas and solute exchange surfaces in humans and other organisms are adapted to maximise effectiveness.
10.3 Cell Division	FT and HT	The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body. Each chromosome carries a large number of genes. Many genes have different forms called alleles, which may produce different characteristics. In body cells the chromosomes are normally found in pairs. Body cells divide to produce additional cells during growth or to produce replacement cells.
	HT	Before each cell division, a copy of each chromosome is made so that each body cell has exactly the same genetic information. This type of cell division is called mitosis. Cells in reproductive organs — testes and ovaries in humans — divide to form sex cells (gametes). When a cell divides to form gametes: copies of the chromosomes are made; then the cell divides twice to form four gametes, each with a single set of chromosomes. This type of cell division is called meiosis. When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual then develops by this cell repeatedly dividing by mitosis.