Cystic fibrosis Cystic fibrosis is caused by a recessive allele that controls the way mucus is formed. Mucus produced by people with cystic fibrosis is abnormally thick and sticky. As a result they have congested lungs and are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics are used to fight infection.
Cystic fibrosis also affects the gut and pancreas so that food is not digested as well
Sex-linked diseases The human X-chromosome carries some alleles for which there are no partners on the Y-chromosome. The result of inheriting an allele like this is different for males and females, and so they are called sex-linked characters. A sex-linked allele inherited by a male is expressed even if it is recessive because it is not hidden by another allele. Haemophilia is an example of a sex-linked disease which is caused by a recessive allele. The normal dominant allele causes the production of a blood clotting factor while the recessive allele does not. Males only inherit one allele because the Y-chromosome does not have one. So inheriting one recessive allele means that their blood does not clot and there is a danger of bleeding to death. There is less chance of females inheriting two recessive alleles for haemophilia, but it is possible. In the past there was little chance of people with haemophilia surviving, but now blood clotting factors can be injected regularly to help control the condition. Females who are heterozygous for a sex-linked allele are called carriers. Their sons have a fifty-fifty chance of inheriting this type of disease.Predicting genetic disease People who have a history of a genetic disease in their family often seek advice about what will happen if they have children. This is called genetic counselling and it gives a couple an idea of how likely they are to have children with the particular disease, so they can decide whether to have children or notVariations Inside the nucleus of all plant and animal cells are chromosomes. These chromosomes are like threads and carry the genes that control characteristics such as the colour of a flower, the shape of a leaf, height and hair colour. Inside your normal body cells there are 23 pairs of chromosomes. There are different numbers of chromosomes for different species of plants and animals. The gametes (for example sperm, pollen and egg cells) contain half the number of chromosomes. This halving in the number of chromosomes happens during meiosis (see the Revision Bite on Cell Division). So each new baby carries a unique set of chromosomes, half from the mother and half from the father.
chromosome A thread like structure in the nucleus of a cell. It is made up of thousands of genes.
gamete A male or female sex cell, such as a sperm, pollen or egg cell.
zygote A fertilised egg
What is a mutation?
A mutation is a change in a gene or chromosome. During meiosis, chromosomes are copied and new gametes are formed. If the chromosomes are not copied exactly a mutation occurs. This can occur naturally for no apparent reason. However, it is known that ionising radiation and some chemicals may increase the occurrence of mutations.
In chromosome mutations a major change occurs and may affect part of or a whole chromosome. Down's syndrome is caused by the presence of an extra chromosome in chromosome set 21.
In gene mutations a chemical change occurs in an individual gene. Although this change may be very small, it can give rise to albinism, sickle cell disease or cystic fibrosis. Gene mutations are usually recessive and the effects can be hidden by a dominant allele (see the Revision Bite on dominant and recessive genes in the Inheritance section).
For instance, cystic fibrosis is caused by a recessive allele, so to have the disease a person must have two recessive alleles. Heterozygous people are called 'carriers' because they carry the recessive allele but do not get the disease.
Sometimes a mutation causes new characteristics that help the organism to survive. For example, plants can become resistant to drought or moths are born with a darker colour that helps camouflage them. In these cases the mutated organisms not only survive but natural selection helps them to live more successfull
mutation A change that takes place in a gene or a chromosome during meiosis. natural selection The survival of organisms best fitted to the conditions in which they live.There is more than one type of reproduction. Sexual reproduction requires two parents. Asexual reproduction requires just one parent.
Animal reproduction
Animals can reproduce asexually or sexually.
It is the simpler animals, such as hydra, that reproduce asexually.
Scientists have cloned animals such as tadpoles, and recently a sheep (called 'Dolly').
Humans reproduce sexually. This involves a male sex cell, called a gamete, joining with a female sex cell.
Animal reproduction Animals can reproduce asexually or sexually. It is the simpler animals, such as hydra, that reproduce asexually. Scientists have cloned animals such as tadpoles, and recently a sheep (called 'Dolly'). Humans reproduce sexually. This involves a male sex cell, called a gamete, joining with a female sex cell. from the ovary, this is called ovulation. The egg cell then moves into the oviduct. Sperm are deposited in the vagina, they go through the cervix, into the uterus and along the oviduct. Sperm meet the egg cell in the oviduct and fertilisation takes place here. Plant reproduction Plants can reproduce asexually or sexually. Asexual reproduction Some plants develop new plantlets, such as runners (strawberries) or side branches (busy lizzy). In others, such as potatoes, storage organs develop underground which grow into new plants. This is called vegetative propagation and examples include tubers, bulbs and rhizomes. We can take a few cells from a plant and grow them into a complete specimen. We do this using tissue culture and call it cloning. We can make cuttings or grafts, which can grow into a new plant. As only one parent is involved in asexual reproduction, all the offspring have exactly the same genes as their parent. The offspring are identical and they are called clones. Because of this, genetic problems are always passed on to the new generation. Sexual reproduction Plants can also reproduce by sexual reproduction. The offspring have the genes from two parents - their genes are different, so the offspring will not be identical. There is variety in the species. These are the four stages of sexual reproduction: An insect or the wind carries pollen grains from another flower to this one. The pollen grains land on the stigma and a pollen tube grows down through the style to the ovary. The nucleus of the pollen grain passes down the tube. It fertilises the egg cell inside the ovule. The fertilised egg cell develops into an embryo. The ovary becomes the fruit and the ovule becomes the seed.
Ecology
Adaptation, predation and competition
The survival of living organisms depends on how well they have adapted to their environment. This exercise will remind you of some of the ways they fight to survive by covering the following subjects:
Adaptation
Predation
Competition
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Competition occurs when two organisms need the same resource in the same place at the same time.
- Plants compete for minerals, water and light.
- Animals compete for water, food and breeding sites.
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Predators catch and eat other animals, prey gets eaten.
Adaptation
Surviving in difficult places
Learn these key features of organisms in hot and cold places.
Hot places
Animals are small and have a large surface area to volume ratio for better heat loss. Animals are active only at dawn and dusk. They spend the heat of the day in a burrow. They have little fat for insulation. They have the ability to survive with little or no water.
Cold places
Animals are large and have a small surface area to volume ratio for little heat loss. They have thick layers of insulation (fat or fur). They have a white coat in winter and a grey/brown coat in s
Predation
There is a continuous war between predators and their prey. The predators need to be adapted to catch enough food to survive. The prey need to be adapted so that enough of them can escape to survive. For example, look at eagles and hares:EaglesHaresLight hollow bones allow them to flyLong legs allow them to run quicklyStrong beak and talons for catching preyLarge ears to hear approaching predatorsEyes on the front of their heads to judge distancesEyes on the side of their heads for all-round visionPlants can be adapted to avoid being eaten by having sharp spines or by producing toxins. Disease acts like a predator in controlling population growth.summer for best camouflage.
Competition
A coral reef is a good place to look for examples of competition. Read through this description of coral reefs and the questions and answers which follow. Most tropical seas are surprisingly short of life because they hold fewer minerals than colder waters. In tropical shallow seas, where the water temperatures are above 18°C, coral reefs can develop. The main species there are corals, which can grow in waters of up to 30m deep. Corals are animals but they have algae inside their cells which can photosynthesise. Corals feed by filtering small floating pieces of food and on the food that leaks out from the algae in their cells. Large and complicated food webs are built up around a coral reef, with many animals feeding on the coral itself, and others feeding on these. Yet other animals feed on scraps and particles in the water. Around a coral reef higher levels of minerals can build up. In the 1960s and 1970s many areas of the Great Barrier Reef in Australia were destroyed by crown-of-thorns starfish. These starfish feed on coral and their population numbers had exploded. This was thought to be linked to the collection of triton shellfish or the overfishing of trigger fish and puffer fish. Another suggestion was that the coral was already weakened by pollution.What is carbon? Carbon is an element which forms an important part of all living organisms and their environments. In its pure state (as an element) it can be found in several very different forms. These can include diamond and graphite, but carbon can combine with other atoms to form a wide variety of molecules. Some of these have names which suggest that they contain carbon, for example: carbon dioxide (CO2), or carbohydrates (such as starch and sugar). The names of other molecules, such as proteins and fats, don't give you this clue so you'll need to remember them. It's important to remember that all living things contain a lot of carbon in one form or another because all cells contain proteins, fats and carbohydrates. All the cells in plants have a cell wall around them. This is made of cellulose, a carbohydrate.
Where does the carbon come from? Animals get their carbon from the foods they eat (which contain carbohydrates, fats and proteins). These nutrients are broken down (digested) into smaller molecules containing carbon, which can be used by the body to: make new cells make materials, such as hair, in mammals make energy. These molecules are burned up in a process called respiration, in which CO2 (carbon dioxide) is produced and exhaled out. feed plants. Green plants get their carbon from the carbon dioxide in the air. This enters the leaves and is used up by the plant during the process of photosynthesis. During this process sugar is produced, which contains carbon. What happens to the carbon in animal droppings and in dead animals and plants? Several things could happen: The dead animals and plants could be eaten by other animals called scavengers, for example, crows. The remains could be digested by decomposers, for example, microbes such as fungi or bacteria. They could become fossilised to form coal, oil or gas. What is the carbon cycle? You may have noticed that there is a sort of 'recycling' going on here. The way in which carbon is 'recycled' in the environment is called the carbon cycle.Make sure you understand what is happening in respiration and photosynthesis, in particular which gas is used up and which gas is released in each process. When answering questions involving a diagram of the carbon cycle, check carefully which way the arrows are pointing
Energy transfer in ecosystems
This exercise will help you to revise the process of energy transfer in ecosystems. For your exam you need to be able to describe the routes that energy can take through an ecosystem. You'll have to identify where energy is lost and apply these ideas when calculating the efficiency of food production. And you should understand the relative merits of eating a vegetarian or meat-based diet.
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Energy is transferred along food chains from one trophic level to the next.
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Energy always decreases from one trophic level to the next.
- Not all the energy at one trophic level is absorbed by the next one.
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Not all the energy taken in by an animal can be digested - some is excreted.
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Some energy is used in respiration for movement, transport and other maintenance activities, ending up as wasted heat.
- The increase in biomass in growth or the production of offspring is available to the next trophic level.
The biggest loss of energy is from the sun's energy to plant tissues in photosynthesis. Food chains rarely have more than four steps because so much energy is lost at each level. The energy lost in waste and faeces is transferred to the decomposer food chain. More food can be produced from a given area of land if plants are fed to people, not to farm animals. Energy losses in farm animals can be reduced by controlling the temperature of their surroundings or by limiting their movement, but there are animal welfare concerns raised by these farming practices.
Food chains
You need to be familiar with the idea of food chains. In its simplest sense, a food chain shows who eats what in a particular habitat, for example a cabbage is eaten by a caterpillar which is eaten by a blue-tit. A food chain is often referred to as a feeding relationship.
Food chains give two other types of useful information:
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They show how materials (biomass) move through an ecosystem, as one organism feeds on another.
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They show how energy flows through an ecosystem.
The food has to come from somewhere in the first place so remember that the first organism in a food chain must be a green plant (a producer) which can make its own food by photosynthesis. The other organisms in the food chain are called consumers, as they eat (consume) other organisms to get materials and energy.
Food webs
In its natural habitat it is unusual for an animal to eat only one particular organism. A more realistic way of showing feeding relationships is to draw interconnecting food chains. This is called a food web.
Look at this example of a food web by the seashore:
Pyramids of number and biomass
Food chains and webs show the flow of materials and energy in habitats but they do not give you any idea of how many organisms there are in the habitat. To show this you need to draw a pyramid of numbers. Look at this example of a food chain.
2000 grass plants
25 voles
1 barn owl
The pyramid of numbers would be as shown here.
f the producer (in this example an oak tree) is a large plant, then the number of primary consumers which feed on the producer (caterpillars in this example) will be much larger.
A way to avoid this problem is to draw a pyramid of biomass. This takes into consideration the amount of biomass(biological material) at each level. Pyramids of biomass are always pyramid shaped.
A pyramid of biomass for the oak tree example looks like this.
The nitrogen cycle
79% of the air around us is nitrogen. Living things need nitrogen to make proteins, but they cannot get it directly from the air because nitrogen gas is too stable to react inside an organism to make new compounds. This exercise will remind you how nitrogen is converted into different forms before being used and excreted by plants and animals. Nitrogen must be changed into a more reactive form to allow plants and animals to use it. Plants can take up and use nitrogen when it is in the form of nitrates or ammonium salts. Changing nitrogen into a more reactive substance is called nitrogen fixation. Nitrogen fixation happens in three main ways: The energy in a lightning bolt can split the di-atomic nitrogen molecule in the air allowing each nitrogen atom to react with oxygen to form nitrogen oxides. These oxides are washed to the ground by the rain where they form nitrates. The Haber process is used by industry to produce ammonia from nitrogen. Ammonia is used to make fertiliser for farmers to feed their crops. Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants fix nitrogen into a usable form. Nitrogen compounds are returned to the soil by excretion and egestion from animals or when plants and animals die and decay. The nitrogen compounds returned in this way are changed back to nitrogen gas by denitrifying bacteria which live in the soil.
Plant
Photosynthesis
Green plants need sunlight. They use the light energy to make a sugar called glucose. Glucose can be converted into another type of sugar called sucrose and carried to other parts of the plant in phloem vessels. Glucose can also be converted into starch and stored. Both starch and sucrose can be turned back into glucose and used in respiration.
Photosynthesis happens in the mesophyll cells of leaves. There are two kinds of mesophyll cells - palisade mesophyll and spongy mesophyll. The mesophyll cells contain tiny bodies called chloroplasts which contain a green chemical called chlorophyll. This chemical is used to catch the light energy needed in photosynthesis.
Conditions needed for photosynthesis
Photosynthesis changes carbon dioxide and water into glucose and oxygen using the energy from sunlight.
Photosynthesis needs:
- chlorophyll
- carbon dioxide (from the air)
- water (from the soil)
- sunlight energy (any light will do except green light)
Photosynthesis produces:
- glucose
- oxygen (a waste product)
Chlorophyll and light energy both need to be present for photosynthesis to take place, but they are not actually part of the reaction - they are not used up. So in the word equation for photosynthesis, remember to write them above the arrow.
The equation for photosynthesis
Photosynthesis is a chemical reaction occurring in the leaves of green plants. Using the energy from sunlight, it changes carbon dioxide and water into glucose and oxygen.
Oxygen is a 'waste' product of photosynthesis.
Glucose can be converted to sucrose and carried to other parts of the plant in phloem vessels. Glucose can also be converted into starch and stored (the starch can later be turned back into glucose and used in respiration).
A cross section of a leaf See if you can learn all the parts in the following diagram
Photosynthesis happens in cells in the middle of the leaf. These cells contain tiny bodies called chloroplasts; these contain a green chemical called chlorophyll.
Chlorophyll is used to convert light energy into chemical energy
The factors that limit photosynthesis Three factors limit photosynthesis from going any faster. Light Sometimes light is a limiting factor. A plant may have lots of water and carbon dioxide, but it will not photosynthesise very fast if there is not enough light; increasing the light intensity will make photosynthesis faster.The factors that limit photosynthesis Three factors limit photosynthesis from going any faster. Light Sometimes light is a limiting factor. A plant may have lots of water and carbon dioxide, but it will not photosynthesise very fast if there is not enough light; increasing the light intensity will make photosynthesis faster. Carbon dioxide Sometimes the level of carbon dioxide is limiting. There may be plenty of light but the plant cannot photosynthesise because it has run out of carbon dioxide. Temperature Temperature can be a limiting factor too. The rate of photosynthesis will be limited if it is too cold for the enzymes to work properlyHormones
Have you ever wondered how plants always manage to grow the right way up, even if you sow seeds upside down? Roots always grow downwards, towards moisture and away from light. Shoots always grow upwards and towards light. This is caused by auxins or 'plant hormones' - chemicals which affect the way the plant grows.
Tropisms
Plants can respond to gravity, moisture and light. These responses are called tropisms - growth movements caused by auxins or 'plant hormones'. Auxins make some parts of a plant grow faster than others; the result may be that the plant bends towards or away from the stimulus.
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Tropisms can be towards a stimulus (positive tropism)
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Tropisms can be away from a stimulus (negative tropism)
Etiolation
When a plant is left in the dark it puts all its energy into growing up to reach the light and makes very little chlorophyll. It will become etiolated. Eventually it should reach the light and then it can start to turn green by making chlorophyll for photosynthesis. Etiolation is the result of atropism
These two plants were grown from cuttings so they are genetically identical. They are both the same age, but one has been left in a dark place.
You may notice that your houseplant turns all its leaves towards the window. It does this because light coming from one side destroys the auxin in a leaf on that side. On the other side auxin makes the leaf grow faster. The result is that the leaves are turned towards the light for photosynthesis. We call this a positive phototropism.
Roots grow down into the ground by a positive geotropism - towards gravity. The shoot from a seed underground can grow upwards, away from gravity, by a negative geotropism.
Auxins are plant hormones. are also known as 'chemical messengers'. affect how fast parts of a plant grow. produce responses called tropisms. You should know that we can use auxins commercially to: make roots grow on cuttings. control the production of fruit. control and kill weeds. As an example of how auxins or plant hormones can be used commercially you could buy geranium plants from a garden shop. These may cost two or three pounds each. It is much cheaper to buy one and take cuttings from it. When your plant is growing nicely, you can cut a piece off it, dip it in rooting powder and then put it in some potting compost. The rooting powder will make it grow strong healthy roots. This is how the garden shop might produce their plants to sell to you.
Transpiration How and why do plants transpire?
Water enters plants through the roots. The roots are covered in millions of tiny root hair cells. These root hair cells have a large surface area so that the plant can absorb enough water from the soil. Water is absorbed into the roots by osmosis.
Water travels from the roots, up the plant, to the leaves. The water is carried in tubes called xylem vessels. Xylem vessels have very narrow diameters - they are microscopic capillary tubes. This helps water to travel up the plant by capillarity (capillary action).
The transpiration streamWater enters plants through the roots. The roots are covered in millions of tiny root hair cells. These root hair cells have a large surface area so that the plant can absorb enough water from the soil. Water is absorbed into the roots by osmosis. Water travels from the roots, up the plant, to the leaves. The water is carried in tubes called xylem vessels. Xylem vessels have very narrow diameters - they are microscopic capillary tubes. This helps water to travel up the plant by capillarity (capillary action). On the underside of the leaves are tiny holes called stomata (singular: stoma) which allow the plant to breathe. When the water reaches the leaves it evaporates and escapes through the stomata. This is called transpiration. As the water escapes, more water is sucked up the xylem. In other words, the leaves create suction pressure to suck water up the plant. Mineral salts dissolved in the water, which include nitrates and phosphates, are needed for growth.
Controlling water loss Plants need to control the water loss from transpiration. They do this by opening and closing their stomata. Two guard cells surrounding each stoma can open and close it. The holes are opened to allow gaseous exchange (breathing) and are closed to reduce loss of water. Transpiration increases when: the leaf stomata are open the air is dry it is warm it is windy the leaves have a large surface area Mitosis occurs wherever an increase in number of cells is needed. It is important in the population growth of uni-cells, and in the growth and repair of multi-cellular organisms. During mitosis a cell produces two copies of itself. Each is identical to the other and to the cell from which they formed. Before a cell divides, its chromosomes are copied exactly. This process is called replication. The DNA of each chromosome is copied to form two chromatids.Pairs of chromatids migrate to the equator of the cell. Contractile spindle fibres are formed, stretching from each pole to the equator of the cell. Each chromatid attaches to a spindle fibre. When the fibres contract the pairs of chromatids are separated and dragged to opposite poles. A complete set of chromosomes is therefore found at each pole. These are then surrounded by a nuclear membrane. In plant cells, the daughter nuclei are separated from each other by the formation of a cell wall. In animal cells, the cytoplasm invaginates to form two daughter cells.
Meiosis
Meiosis meaning sperms or eggs - contain half this number of chromosomes. A human sperm nucleus has 23 single chromosomes, one from each pair in a testis cell. Similarly, an egg contains one chromosome from each pair in an ovarian cell. Fertilisation is a key feature of sexual reproduction. Here, the nuclei of the sperm and the egg join to form a new nucleus, called the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg. Before a cell divides, the chromosomes are copied exactly. The DNA of each chromosome is replicated to form two chromatids. The chromosomes migrate to the equator of the cell where they pair to form bivalents. Each bivalent is arranged symmetrically, above and below the equator with their centromeres attached to spindle fibres. At this time maternal and paternal chromatids can exchange bits of DNA and become recombinants. This produces new combinations of genetic material and variation in the offspring. The basic genetic information is the same but the details will differ. Spindle fibre contractions drag each bivalent apart. The chromatids of each chromosome stay together at this stage. For each of the 23 pairs there is a 50-50 chance as to which pole the paternal or maternal pair of chromatids go. With over 8 million possibilities there are many opportunities for variation. Two daughter nuclei form with what appears to be single chromosomes, though each chromosome is a pair of chromatids in reality. A second division then takes place, like mitosis, to separate the pairs of chromatids. The end result is four cells each with a complete but single set of 23 chromosomes.
Cells
Unicells
Unicells are one-cell living things which have microscopic structures or organelles inside them. These parts include the chloroplast and nucleus. Different organelles do different jobs in the cell. For example, proteins are made in an organelle called a ribosome. Energy is released by organelles called mitochondria. Organelles allow unicells to perform all the life processes.
Multicellular organisms Multicellular organisms, such as ourselves, are made from specialised cells. These cells too have organelles, but different cells also do different jobs.Groups of cells of the same type are called tissues. Groups of tissues work together as organs to perform particular functions. Moreover, organs in the digestive system then work together. The mouth, gullet, stomach, small intestine - each organ in this organ system does its special job in getting food digested.
Diffusion
Particles in liquids and gases have kinetic energy. They move about, at speed, in all directions. The particles move about randomly. In an area of high concentration, some of the particles collide with each other, lose energy and slow down. Others will escape from the concentrated area to places where there are fewer or none. Very few particles leave an area of low concentration to go to an area where the concentration is higher. This creates a diffusion gradient - the result is that particles diffuse from an area of high concentration to an area of low concentration.
Osmosis
Some membranes in plant and animal cells allow certain particles to pass through them and not others. They are partially (or selectively) permeable.
The diffusion of water through a partially permeable membrane is called osmosis. The rate at which osmosis takes place is affected by the concentration of water in the two solutions on each side of the membrane.
If a partially permeable membrane separates the two solutions, water moves through it in both directions at the same time. However, more water leaves a dilute solution (high water concentration) and passes into a more concentrated solution (low water concentration) than enters it. Although the water appears to move across the membrane in one direction, it is in fact moving in both directions but more one way than the other.
When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions. At this point, the net exchange of water is zero and the system is in equilibrium.
PlaceParticles moveFromToGutDigested food productsGut cavityBlood in capillary of villusLeafOxygenChloroplastAir spaces in mesophyllLungsOxygenAlveolar air spaceBlood circulating around the lungsParticles continue to move from high to low concentration for as long as there is a concentration gradient.
For example, the blood constantly removes oxygen from alveolar air spaces in the lungs, provided that there is more oxygen in the alveolar air spaces than in the blood.
The circulation takes the oxygen-rich blood away and replaces it with blood low in oxygen.