BIOLOGY 2b SUMMARY SHEET Information in italics is only needed at higher tier. B2.5.1 PROTEINS Protein molecules made up of long chains of amino acids. These long chains are folded to produce a specific shape that enables other molecules to fit into the protein (like a lock and key). Proteins act as: Structural components of tissues such as muscles Hormones Antibodies catalysts Catalysts increase the rate of reactions. Biological catalysts are called enzymes. Enzymes are proteins. B2.5.2 ENZYMES The shape is vital for the enzyme to function. High temperature destroys this special shape and stops the enzyme working. 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 ones which can be absorbed through the gut wall. Name of Enzyme Where it is produced What it does Amylase Protease Lipase Salivary glands, pancreas and small intestine Stomach, pancreas and small intestine Pancreas and small intestine Breakdown starch into sugars in the mouth and small intestine Breakdown proteins into amino acids in the stomach and small intestine Breakdown 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 best in acidic conditions. The liver produces bile which is stored in the gall bladder before being released into the small intestine. Bile neutralises the acid that was added in the stomach. This provides alkaline conditions in which enzymes in the small intestine work most effectively.
Some microorganisms produce enzymes which pass out of the cells. These enzymes can be used in the home and in industry. In the home, biological detergents may contain protein digesting and fat digesting enzymes (proteases and lipases). Biological washing powders have to be used at a low temperature to avoid damaging the enzymes. 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 can therefore be used in smaller amounts in slimming foods. Enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise need expensive, energy demanding equipment. However, most enzymes are damaged by high temperatures and can be costly to produce. B2.6.1 AEROBIC RESPIRATION The chemical reactions inside cells are controlled by enzymes. During aerobic respiration (respiration using oxygen) chemical reactions occur which: Use glucose (a sugar) and oxygen Release energy Aerobic respiration takes place continuously in plants and animals. Most of the reactions in aerobic respiration occur inside the mitochondria Aerobic respiration can be summarised by the equation: glucose + oxygen carbon dioxide + water ( + energy) The energy that is released during respiration is used: To build up large molecules using smaller ones In animals, to enable muscles to contract In mammals and birds, to maintain a steady body temperature in colder surroundings In plants to build up sugars, nitrates and other nutrients into amino acids which are then built up into proteins. During exercise a number of changes take place: Heart rate increases Rate and depth of breathing increases These changes increase the rate of blood flow to the muscles and so increase the supply of sugar and oxygen and increase the rate of removal of carbon dioxide. Muscles store glucose as glycogen, which can then be converted back to glucose for use during exercise a sort of reserve store.
B2.6.2 Anaerobic respiration During exercise, if there is not enough oxygen reaching the muscles, they use anaerobic respiration to obtain energy. This 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. It results in an oxygen debt that has to be paid back to oxidise the lactic acid to carbon dioxide and water (complete the respiration). This is why you continue to be out of breath after exercise the body still needs more oxygen. If muscles are involved in long periods of exercise they become fatigued (tired) and so stop contracting efficiently. One cause of this is a build up of lactic acid in the muscles (can cause cramp). Blood flowing through the muscle removes the lactic acid. B2.7.1 Cell division In body cells the chromosomes are normally found in pairs. Body cells divide by mitosis. The chromosomes contain the genetic information. Body cells have two sets of chromosomes; sex cells (gametes) have only one set. For an organism to grow it needs to produce more cells. More cells are also needed to repair and replace old or damaged cells. This happens by existing cells dividing into two new cells. When this happens: copies of the chromosomes are made first (to make two sets of 23 pairs) the cell splits into two with a complete set of chromosomes in each new cell. Chromosomes contain the the genetic information this means that every cell in a body has exactly the same genetic information This type of cell division is called mitosis. The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells. They contain the same pairs of genes (alleles) as the parents. Cells in reproductive organs - testes and ovaries (in humans) - divide to form sex cells (gametes). This type of cell division is called meiosis. Gametes have only one set of chromosomes. 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 (one of each pair). When gametes fuse at fertilisation a single cell with new pairs of chromosomes is formed. (One of each pair of alleles comes from each parent this is one reason why sexual reproduction leads to variation.) A new individual then develops from this cell by repeated mitosis. Most types of animal cells differentiate at an early stage (only some of the genes are switched on so the cell cannot perform a wide range of roles.)
Many plant cells retain the ability to differentiate throughout life (they are able to adapt to changing circumstances.) e.g. If you rip off someones arm the cells at the shoulder cannot grow a new arm but if you snap off a bit of a plant the broken end will develop roots and it will grow (a cutting). In mature animals, cell division is mainly restricted to repair and replacement. Cells from human embryos and from adult bone marrow (stem cells) can be made to develop into a wide range of cells e.g. nerve cells. Stem cells have the ability to develop into any type of cell. Treatment with these cells may help conditions such as paralysis. You should be able to: Make informed judgements about social and ethical issues concerning the use of stem cells from embryos in medical research and treatments. B2.7.2 Genetic Variation. Young plants and animals produced by sexual reproduction resemble their parents (have similar characteristics) because of information passed on to them in the sex cells (gametes) from which they developed. They have a mixture of features because one of each pair of alleles comes from each parent. The information about the features of organisms is carried by genes. Different genes control the development of different features. Genes are carried on chromosomes. Each chromosome carries a large number of genes. The gene for a particular feature is always in the same position on the chromosome. Chromosomes are found in the nucleus of every cell. There are 46 chromosomes in human cells (except the sex cells) which are arranged as 23 pairs. Both chromosomes in each pair carry the genes for the same features, e.g. type of hair, eye colour etc. This means that there are pairs of genes controlling each feature. This does not mean that both forms of the gene are identical. Many genes have different forms called alleles, which may produce different characteristics. e.g. the gene for tongue rolling has two alleles, able to roll or unable to roll. An allele which 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. In human body cells, one of the pairs of chromosomes carries the genes which determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY)
Chromosomes are made of long molecules of a substance called DNA. (deoxyribo nucleic acid) which has a double helix structure Each person (apart from identical twins) has unique DNA. This can be used to identify individuals a process known as DNA (genetic) fingerprinting. A Gene is a small section of DNA. Each gene codes for a particular combination of amino acids, these then make a specific protein.. Mendel He was the father of Genetics He was the first to suggest the idea of pairs of factors affecting a feature. He came up with the idea of letters (i.e. Bb) to represent the factors. Previous to Mendel people had looked at many variations at the same time Mendel stuck to one feature e.g. tallness or pod colour in Pea plants He also chose features that were clearly different. E.g. peas are either tall or short, their pods are green or yellow It was these two approaches that helped Mendel come up with the ideas of inheritance. Since his death microscopes and staining has enabled us to find and see chromosomes, making his ideas widely accepted. You should be familiar with principles used by Mendel in investigating monohybrid inheritance in peas. You should understand that Mendel s work preceded the work by other scientists which linked Mendel s inherited factors with chromosomes. You should be able, when provided with appropriate information, to explain: why Mendel proposed the idea of separately inherited factors that we now call genes why the importance of Mendel s discovery was not recognised until after his death. Genetic diagrams Characteristics are passed on from one generation to the next in both plants and animals. Simple genetic diagrams can be used to show this. You should be able to interpret genetic diagrams of sex inheritance You should be able to interpret genetic diagrams, including family trees.
Dominant alleles are represented by capital letters (e.g. can roll tongue T) Recessive alleles are represented by lower case letters (e.g. can't roll tongue t) It is easy to predict the outcome of breeding if the original genetics of the parents is known. There are only three combinations of alleles. both dominant TT both recessive tt one dominant, one recessive Tt (the dominant feature is observed) Each of the parents must be one of these types. The possible outcomes of these crosses can be shown by genetic diagrams - (there are various ways of drawing them) Example 1 parent 1 parent 2 homozygous tongue roller Homozygous can't roll TT tt all offspring Tt - can roll t t T Tt Tt T Tt Tt Example 2 parent 1 tt parent 2 (heterozygous) Tt half offspring Tt - can roll T t t Tt tt t Tt tt half tt - can't roll The same principal can be applied to any cross (including the disease examples given later), and sex determination. female XX XY male half the children will be (female) half will be XY (male) X Y X XX XY X XX XY XX
You should be able to predict, explain the outcomes of crosses between individuals for each possible combination of recessive and dominant alleles of the same gene. To do this you will need to be able to construct genetic diagrams and use the terms homozygous, heterozygous, phenotype and genotype. B2.7.3 Genetic disorders There are ethical considerations in treating genetic disorders. Some disorders are inherited. Polydactyly having extra fingers or toes is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder. Cystic fibrosis - a disorder of cell membranes - must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder. They are known as carriers. Embryos can be screened for the alleles that cause these and other genetic disorders. This does raise ethical issues around What you do when you know that information! You should be able to Make informed judgements about the economic, social and ethical issues concerning embryo screening (e.g. genetic disorders) you have studied or from information presented to you. B2.8.1 Old and new species Evidence for early forms of life comes from fossils. Fossils are the remains of organisms from manyyears ago, which are found in rocks. Fossils may be formed in various ways: from the hard parts of animals that do not decay easily from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent when parts of the organism are replaced by other materials as they decay as preserved traces of organisms, eg footprints, burrows and rootlet traces. Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. We can learn from fossils how much or how little different organisms have changed as life developed on Earth.
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.