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AQA GCSE biology b2

for gcse revision
by

kristof bolhuis

on 13 May 2014

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Transcript of AQA GCSE biology b2

Biology B2 B2 6
old and new species B2 6.1 the origins of life on earth the earth was formed 4.5 billion years ago and life began 3.5 billion years ago. We can find out when certain organisms existed because we can date the rocks which their fossils are found in. Fossils fossils can be formed from
-the hard parts of animals that do not decay easily such as bones
-from parts of organisms that do not decay because some of the conditions for decay are not present
-when parts of an organism are replaced by minerals
- as preserved traces of organisms such as footprints however many early life forms were soft bodied so few traces were left behind.
many traces have also been destroyed by numerous geological activity. Maths!

4.5 billion years= 4.5x10power9 B2 6.2 exploring the fossil evidence The fossil record is often incomplete but we can learn from animals that are alive today. Extinction is where all members of a species die out. Extinction can be caused by
-a new disease
-geological changes in the environment
-a new predator
-a new competitor
-natural changes in the species over time
-a single catastrophic event For extinction to happen there must be a change. B2 6.3 extinction The biggest influence on the survival of a species are changes in the environment. Climate change is an important cause of extinction. An animal well adapted to hot conditions may die out if there is an ice age. Changes in temperature or precipitation may cause the food source of an organism to be reduced and this could also lead to extinction. The fossil record shows that in the past there have been several mass extinctions; such as the Permian extinction and the extinction of the dinosaurs. A mass extinction can be caused by
-a single catastrophic event such as a volcanic eruption
-dramatic changes in the environment The extinction of the dinosaurs was caused by a giant asteroid crashing into the earth. this caused
-Huge fires
-volcanic eruptions
-earthquakes
-landslides
-tsunamis
-24hour darkness due to ash and dust blocking the sun. this lowered temperatures and meant plants found it difficult to grow. B2 6.4 isolation and evolution of new species New species can evolve from an existing species if a group becomes isolated from the rest. Geographical isolation can occur if an island separates from the mainland or a river separates two areas. This can also be caused by mountains. Example : two groups of the same tortoise are split up because the island splits from the mainland. On this island the tortoises cannot find their usual food of grass. The only food is high up leaves on small trees and bushes. The tortoises that manage to eat this new food survive through natural selection. Over time the island tortoises will be able to easily eat this new food. An example of an adaptation that could help them is long necks because this adaptation is beneficial to their survival. Because of a new selection pressure the tortoise populations are completely different species. This is due to different factors for differentiation in the environment. This is called speciation as the isolated population is a new species. We say it is a separate species when the two populations can no longer interbreed. Adaptations occur due to a change in alleles which control the characteristics. This results in genetic variation. Another example is Darwin's finches where they evolved separately on different islands. They all have different beaks for different foods. One finch has even evolved to feed on blood. B2 5 simple inheritance in plants and animals B2 5.1 cell division and growth Cell division is necessary for the growth of an organism or for the repair of damaged tissue. Mitosis is where 2 identical cells are produced from the original cell. The chromosomes contain the genes that must be passed on to each new cell. A copy of each chromosome is made before the cell divides so each new cell has exactly the same chromosomes as the adult cell. In early development of plants and animal embryos the cells are unspecialised. These are called stem cells. Most animals differentiate early in development and cell division is mainly used for replacement and repair. Plant cells differentiate throughout the life of the plant as it continues to grow. Cells of offspring produced by asexual reproduction are produced by mitosis. They contain the same alleles as the parent cell. B2 5.2 cell division in sexual reproduction Cells in the reproductive organs divide by meiosis to form gametes. In humans these are the sperm and ova. Each gamete has 1 chromosome from the original pair. All of the cells produced by meiosis are slightly different from the parent cell. Sexual reproduction results in variation because the gametes from each parent fuse. This means half the genetic information comes from each parent. When gametes fuse after fertilization a new body cell with new pairs of chromosomes is formed. The offspring will then develop from this cell by mitosis. Meiosis Before division a copy of the chromosomes is made. The cell then divides twice to form 4 gametes. Each gamete has a single set of chromosomes with a different combination of genes to the others. B2 5.3 Stem cells Stem cells are unspecialised. They can be found in human embryos and bone marrow. Stem cells can differentiate into all other types of cell. In an embryo layers of cells differentiate into the cells needed. In adult bone marrow stem cells differentiate into other types of cells such as red blood cells. It is hoped that stem cells can be made to differentiate into specific cells to treat diseases such as paralysis. B2 5.4 mendel and DNA Gregor Mendel was a monk who discovered how characteristics were inherited. Mendel was the first person to suggest the idea of separately inherited factors. It took a long time for Mendel's ideas to be accepted because people did not know about genes and chromosomes at the time. Mendel's "factors" are now called genes. Genes are found on the chromosomes. Chromosomes are made of DNA. A gene is a section of DNA. Every individual apart from identical twins has a unique DNA fingerprint. Every gene codes for a specific combination of amino acids which make a specific protein. B2 5.5 inheritance Humans have 23 pairs of chromosomes; one pair are the sex chromosomes. Females have sex chromosomes xx while males have sex chromosomes xy. Genes controlling the same characteristic are called alleles. A dominant allele will mask the effect of another. A recessive allele will be masked and the effect blocked by a dominant allele. Genetic diagrams can illustrate how alleles and characteristics are inherited. A genetic diagram is a biological model constructed to predict the inheritance of characteristics. A punnet square can show the odds of getting a particular allele. Phenotype - physical appearance of a characteristic. Genotype - the genetic make up: which alleles were inherited. Homozygous - both alleles are the same Heterozygous- one allele is recessive while one is dominant. What Mendel did; example of inherited characteristics. parents: 1 green pea, 1 yellow pea 1st generation offspring: all green 2nd generation offspring: 3/4 green 1/4 yellow The green allele was dominant so all of the 1st generation offspring were green. But there was 1 in 4 chance for there to be 2 recessive alleles in the 2nd generation hence 1/4 yellow. B2 5.6 inherited conditions in humans There are many different genetic disorders. If an allele is dominant the person only has to inherit one allele to have the genetic disorder. An example of this is polydactyly where people are born with extra digits. Cystic fibrosis can be passed on if both parents have the allele. Even if they do not have the disorder they may be carriers of the allele. If an allele is recessive the person must of have 2 of that allele to get the genetic condition. Cystic fibrosis is caused by a recessive allele. This disease effects cell membranes and causes the production of thick sticky mucus. The mucus can affect major organs such as the lungs and pancreas. A genetic diagram can be used to show how a genetic disorder is inherited and predict if future offspring will have the disease. If a parent is heterozygous for polydactyly each child has a 50% chance of getting the disease. If both parents are heterozygous for cystic fibrosis the child has a 25% chance of getting the disease. A punnet square can show the outcome of a genetic cross. This is a punnet square for polydactyly if one parent is heterozygous for polydactyly. A-dominant allele (polydactyly) a-recessive allele Offspring has 50% of getting polydactyly Punnet square for cystic fibrosis. B2 5.7 science and ethics involving stem cells and embryos. Adult stem cells from bone marrow can be used to treat diseases such as leukemia. Embryonic stem cells have many more potential uses because they can differentiate into any other type of cell. Embryonic stem cells are taken from umbilical cords or spare IVF embryos. Embryonic stem cells can be used to grow new tissues and organs. Some people are opposed to embryonic stem cells because... Research is experimental and this treatment is not widely available yet. IVF embryos have the potential to become babies and a life is lost when stem cells are extracted. IVF embryos cannot give permission for this to happen and they should have rights. research is expensive Embryo screening involves testing the embryo for genetic disorders before it is born. Some parents terminate their child if it has a genetic disorder and some people claim that this is unethical. In IVF only healthy embryos are implanted and the rest are terminated. Some people also claim that this is unethical. B2 4 energy from respiration B2 4.1 aerobic respiration Aerobic respiration takes place continually in plants and animals. The chemical reactions for aerobic respiration take place in mitochondria and are controlled by enzymes the equation for aerobic respiration is... Glucose + oxygen Carbon dioxide + water + energy The energy released can be used for
-building larger molecules from smaller ones
-enable muscle contraction in animals
-maintain a constant body temperature
-build sugars nitrates and other nutrients into amino acids and proteins. Tests for aerobic respiration usually measure the carbon dioxide released. Limewater can test for carbon dioxide becasue it will turn cloudy with particles of limestone if carbon dioxide is present. C6H6O6 + 02 CO2 + H20 + energy B2 4.2 the effect of excercise on the body When you excercise your muscles need more energy in order to contract more. When excercising you need to increase the rate of transport of glucose and oxygen to your muscles and the rate of removal of carbon dioxide. During excercise the heart rate will increase and blood vessels will dilate in order to supply more glucose and oxygen to the muscles. The breathing rate will also increase in order to get rid of more carbon dioxide will uptaking a greater amount of oxygen into the body. Muscles can store glucose as glycogen. This is converted back to glucose during excercise. B2 4.3 anearobic respiration After long periods of excercise your muscles will become tired and stop contracting efficiently. When your muscles cannot get enough oxygen then aerobic respiration cannot take place. You then start respiring anaerobically. In anaerobic respiration the glucose is not completely broken down and lactic acid is prodcued. Anaerobic respiration produces much less energy. The build up of lactic acid can cause muscle fatigue. Blood flow through the muscles can get rid of lactic acid. After excercise the lactic acid has to be broken down and oxygen is needed to do this. The amount of oxygen needed is known as the oxygen debt. The oxygen oxidises the lactic acid into water and carbon dioxide. B2 3 enzymes B2 3.1 proteins, catalysts and enzymes Protein molecules are made up of long chains of amino acids. The shape of a protein depends on its specific function. Proteins can be
-structural components of tissue such as muscle
-hormones
-anitbodies
-catalysts Chemical reactions in the body are controlled by proteins called enzymes. Enzymes act as biological catalysts and speed up reaction. Enzymes have a shape called an active site where other molecules fit, for digestion for example the enzyme breaks down large molecules into smaller ones. The substrate is held in the active site and either broken down or connected to another molecule. Enzymes can
-break down large molecules into smaller ones
-build large molecules from smaller ones
-change one molecule into another.
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