The Nucleus controls the activities of the cell.
The cytoplasm is where most of the chemical reactions take place.
a cell membrane controls the passage of substances into and out of the cell.
The mitochondria is where most energy is released in respiration.
The ribosomes is where protein synthesis occurs.
Plant and algal cells have a cell wall made of cellulose, which strengthens the cell.
chloroplasts absorb light energy to make food.
A vacuole is filled with cell sap.
A bacterial cell consists of cytoplasm and a membrane surrounded by a cell wall; the genes are not in a distinct nucleus.
Yeast is a single-celled organism. Yeast cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall.
Cells may be specialised to carry out a particular function. E.g. A sperm cell or a Red blood cell is specialised.
Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration to a region with a lower concentration. The greater the difference in concentration, the faster the rate of diffusion
Dissolved substances can move into and out of cells by diffusion.
Oxygen required for respiration passes through cell membranes by diffusion.
Large multicellular organisms develop systems for exchanging materials. During the development of a multicellular organism, cells differentiate so that they can perform different functions.
A tissue is a group of cells with similar structure and function. Examples of tissues include:
■ muscular tissue, which can contract to bring about movement
■ glandular tissue, which can produce substances such as enzymes and hormones
■ epithelial tissue, which covers some parts of the body.
Organs are made of tissues. One organ may contain several tissues. The stomach is an organ that contains:
■ muscular tissue, to churn the contents
■ glandular tissue, to produce digestive juices
■ epithelial tissue, to cover the outside and the inside of the stomach.
Organ systems are groups of organs that perform a particular function. The digestive system is one example of a system in which humans and other mammals exchange substances with the environment.
The digestive system includes:
■ glands, such as the pancreas and salivary glands, which produce digestive juices
■ the stomach and small intestine, where digestion occurs
■ the liver, which produces bile
■ the small intestine, where the absorption of soluble food occurs
■ the large intestine, where water is absorbed from the undigested food, producing faeces.
Plant organs include stems, roots and leaves. Examples of plant tissues include:
■ epidermal tissues, which cover the plant
■ mesophyll, which carries out photosynthesis
■ xylem and phloem, which transport substances around the plant.
Photosynthesis is summarised by the equation:
carbon dioxide + water ---> glucose + oxygen
■ light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and algae
■ this energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugar (glucose)
■ oxygen is released as a by-product.
The rate of photosynthesis may be limited by:
■ shortage of light
■ low temperature
■ shortage of carbon dioxide.
Light, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits photosynthesis.
The glucose produced in photosynthesis may be converted into insoluble starch for storage. Plant cells use some of the glucose produced during photosynthesis for respiration.
Some glucose in plants and algae is used:
■ to produce fat or oil for storage
■ to produce cellulose, which strengthens the cell wall
■ to produce proteins.
To produce proteins, plants also use nitrate ions that are absorbed from the soil.
The shape of an enzyme is vital for the enzyme’s function. High temperatures change the shape.
Different enzymes work best at different pH values.
Some enzymes work outside the body cells. The digestive enzymes are produced by specialised cells in glands and in the lining of the gut. The enzymes then pass out of the cells into the gut where they come into contact with food molecules. They catalyse the breakdown of large molecules into smaller molecules.
The enzyme amylase is produced in the salivary glands, the pancreas and the small intestine. This enzyme catalyses the breakdown of starch into sugars in the mouth and small intestine.
Protease enzymes are produced by the stomach, the pancreas and the small intestine. These enzymes catalyse the breakdown of proteins into amino acids in the stomach and the small intestine.
Lipase enzymes are produced by the pancreas and small intestine. These enzymes catalyse the breakdown of lipids (fats and oils) into fatty acids and glycerol in the small intestine.
The stomach also produces hydrochloric acid. The enzymes in the stomach work most effectively in these acid conditions.
Some microorganisms produce enzymes that pass out of the cells. These enzymes have many uses in the home and in industry. In the home:
■ biological detergents may contain protein-digesting and fat-digesting enzymes (proteases and lipases)
■ biological detergents are more effective at low temperatures than other types of detergents. In industry:
■ proteases are used to ‘pre-digest’ the protein in some baby foods
■ carbohydrases are used to convert starch into sugar syrup
■ isomerase is used to convert glucose syrup into fructose syrup, which is much sweeter and therefore can be used in smaller quantities in slimming foods.
In industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require expensive, energy-demanding equipment. However, most enzymes are denatured at high temperatures and many are costly to produce.
Aerobic respiration takes place continuously in both plants and animals.
Most of the reactions in aerobic respiration take place inside mitochondria.
Aerobic respiration is summarised by the equation:
glucose + oxygen ➞ carbon dioxide + water (+ energy)
Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid. As the breakdown of glucose is incomplete, much less energy is released than during aerobic respiration. Anaerobic respiration results in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water.
Body cells divide by mitosis. In body cells the chromosomes are normally found in pairs.
When a body cell divides by mitosis:
■ copies of the genetic material are made
■ then the cell divides once to form two genetically identical body cells.
Mitosis occurs during growth or to produce replacement cells. Body cells have two sets of chromosomes; sex cells (gametes) have only one set.
When a cell divides to form gametes:
■ copies of the genetic information are made
■ then the cell divides twice to form four gametes, each with a single set of chromosomes.
An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.
An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.
Extinction may be caused by:
■ changes to the environment over geological time
■ new predators
■ new diseases
■ new, more successful, competitors
■ a single catastrophic event, eg massive volcanic eruptions or collisions with asteroids
■ through the cyclical nature of speciation.
New species arise as a result of:
■ isolation – two populations of a species become separated, eg geographically
■ genetic variation – each population has a wide range of alleles that control their characteristics
■ natural selection – in each population, the alleles that control the characteristics which help the organism to survive are selected
■ speciation – the populations become so different that successful interbreeding is no longer possible.