The binomial system

There are millions of species on our planet. Although species can be very different from each other, many have similar features that allow us to put them into groups. This is called classification.

Kingdoms

The first big division of living things in the classification system is to put them into one of five kingdoms. These are based on what an organism's cells are like.

The five kingdoms are:

1. animals (all multicellular animals)
2. plants (all green plants)
3. fungi (moulds, mushrooms, yeast)
4. prokaryotes (bacteria, blue-green algae)
5. protoctists (Amoeba, Chlorella and Plasmodium)

Further divisions

Living things can then be ranked according to:

• phylum
• class
• order
• family
• genus
• species

The binomial system of naming species uses Latin words. Each name has two parts, the genus and the species. For example, human beings belong to the genus Homo, and our species is sapiens - so the scientific name is Homo sapiens.

Vertebrates

Vertebrates are animals with backbones. They can be classified according to their features, and include bony fish, amphibians, reptiles, birds and mammals. 

The vertebrates are animals with a backbone. Scientists separate this group into smaller groups because of their features:

• how the animal takes in oxygen – lungs, gills or through the skin
• thermoregulation – maintains own temperature (homeotherms) or temperature varies with surroundings (poikilotherms)
• reproduction – internal or external fertilisation, lay eggs (oviparous) or give birth to live young (viviparous).

What is a species?

Organisms of a species:

have more characteristics in common than they do with organisms of a different species

• can interbreed to produce fertile offspring.

Similar species tend to live in similar habitats, and are closely related in evolutionary terms. . Closely related species living in different types of habitat may have different characteristics. You can use keys to identify organisms according to their features.

 organisms do not always reproduce sexually, and some hybrids are fertile.

Sometimes classification can be complicated by:

• variation within a species
• hybridisation (closely related species breed to produce offspring that have characteristics of both – the hybrids are often infertile)
• ring species – neighbouring populations of species may have slightly different characteristics but can still interbreed as part of a chain but the two ends of the chain can’t interbreed.

It can also be difficult to classify a certain organism. For example, the single-celled organism called Euglena has some confusing characteristics. It has:

• chloroplasts, like a plant
• no cell wall, like an animal
• a flagellum to swim with, like some bacteria.

A fifth kingdom, called the protoctista, was made for organisms like Euglena.

Viruses

Scientists do not classify a virus as a living thing. This is because:

• it does not show all seven processes for life
• when it enters a cell it changes the way a cell works so it can make copies of the virus.

Animalia

Multicellular, no cell wall or chlorophyll, heterotrophic feeders. Examples: all multicellular animals, including: jellyfish, worms, arthropods, molluscs, echinoderms, fish, amphibia, reptiles, birds and mammals

Plantae

Multicellular, have cell walls and chlorophyll, autotrophic feeders. Examples: all green plants, including: algae, ferns and mosses (plants that do not produce seeds), conifers and flowering plants (plants that do produce seeds)

Fungi

Multicellular, have cell walls, do not have chlorophyll, saprophytic feeders. Examples: moulds, mushrooms, yeast

Protoctista

Usually unicellular, with a nucleus eg amoeba. Examples: amoeba and paramecium

Prokaryotes

Unicellular, with no nucleus eg bacteria. Examples: bacteria and blue-green algae

phylum chordates -animals with a supporting rod running the length of the body eg backbone c

lass mammals animals that are warm-blooded, have lungs and body hair, produce milk and give birth to live young

order primates ape-like animals

family hominids human-like animals

genus homo humans

species sapiens modern humans

Variation

Organisms are adapted to survive in different conditions. Over many generations, these adaptations have come about through variation. Variation involves small changes between organisms which may allow that organism to compete better for survival. Variation can have environmental or genetic causes.

Adaptation

Darwin's theory of evolution explains how species of living things have changed over geological time. The theory is supported by evidence from fossils and by the rapid changes that can be seen to occur in microorganisms such as antibiotic-resistant bacteria. Many species have become extinct in the past, and the extinction of species continues to happen.

Every organism has certain features or characteristics that allow it to live successfully in its habitat. These features are called adaptations, and we say that the organism is adapted to its habitat.Organisms living in different habitats need different adaptations.

The polar bear

Polar bears are well adapted for survival in the Arctic. They have:

• a white appearance, as camouflage from prey on the snow and ice
• thick layers of fat and fur, for insulation against the cold
• a small surface area to volume ratio, to minimise heat loss
• a greasy coat, which sheds water after swimming.

Some of the features of the different organisms in a species show continuous variation, and some features show discontinuous variation.

Continuous variation

Human height is an example of continuous variation. Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values. So it is continuous variation.

For any species a characteristic that changes gradually over a range of values shows continuous variation. Examples of such characteristics are:

• height
• weight
• foot length.

Discontinuous variation

Human blood group is an example of discontinuous variation. There are only four types of blood group. There are no other possibilities and there are no values in between. So this is discontinuous variation.

A characteristic of any species with only a limited number of possible values shows discontinuous variation. Here are some examples of discontinuous variation:

• gender (male or female)
• blood group (A, B, AB or O)
• eye colour.

Causes of variation

Some variation within a species is inherited, and some variation is due to the environment.

Inherited causes of variation

Variation in a characteristic that is a result of genetic inheritance from the parents is called inherited variation.

Children usually look a little like their father, and a little like their mother, but they will not be identical to either of their parents. This is because they get half of their inherited features from each parent.

Each egg cell and each sperm cell contains half of the genetic information needed for an individual. When these join at fertilisation a new cell is formed with all the genetic information needed for an individual.

Here are some examples of inherited variation in humans:

• eye colour
• hair colour
• skin colour
• lobed or lobeless ears.

Gender is inherited variation too, because whether you are male or female is a result of the genes you inherited from your parents.

Environmental causes of variation

Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will will grow taller as it tries to reach more light.

Variation caused by the surroundings is called environmental variation. Here are some other examples of features that show environmental variation:

• language and religion
• flower colour in hydrangeas - these plants produce blue flowers in acidic soil and pink flowers in alkaline soil.

DNA are large and complex molecules.They carry the genetic code that determines the characteristics of a living thing.

A gene is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.

The cell’s nucleus contains chromosomes made from long DNA molecules.

The diagram shows the relationship between the cell, its nucleus, chromosomesin the nucleus, and genes.

Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.

Different forms of the same gene are called alleles (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

Alleles are dominant or recessive:

• the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
• the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair

For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.

In a genetic diagram, you show all of the possible alleles for a particular characteristic. There will be two alleles from one parent and two from the other parent, making four altogether. You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways but some or all of them could be repeated.

In genetic diagrams, the dominant allele (this allele will always have an affect) is shown as a capital letter, while the recessive allele (this allele will not be seen if a dominant is present) is shown as a lower-case letter.

The alleles in the organism are the genotype. What the organism looks like, eg red flower is the phenotype.

Genetic diagrams (Punnett squares)

Genetic diagrams or Punnett squares are used to show the possible outcomes of a particular cross. A dominant allele is shown by a capital letter, and a recessive allele by a lower case letter.

Cystic fibrosis

People with CF produce abnormally thick and sticky mucus in their lungs and airways. As a result, they are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics can fight infection. CF also affects the gut and pancreas, so food is not digested efficiently.

Inheriting copies of the allele

You need to inherit two copies of the faulty allele to be born with CF. If you have just one copy, you are a carrier, but will not experience any symptoms. If two carriers have a child together, there is a one in four chance (or 25 per cent) of it inheriting the disorder.

Sickle cell disease is a recessive condition so the sufferer has two copies of a faulty gene. The red blood cells of sufferers are misshapen and can stick together which can block blood vessels. Sickle cell disease sufferers can become very tired and quickly get out of breath. If the sickle cells block a blood vessel, this can be fatal.

It is important that the body’s internal environment is controlled. For example, the amount of carbon dioxide in the bloodstream must be carefully controlled.

Maintaining a constant internal environment is called homeostasis. The nervous system and hormones are responsible for this.

Blood sugar level

This is controlled to provide cells with a constant supply of energy. The blood sugar level is controlled by the release and storage of glucose, which is in turn controlled by a hormone called insulin.

Body temperature

This is controlled to maintain the temperature at which enzymes work best, which is 37°C. Body temperature is controlled by:

• controlling blood flow to the skin
• sweating
• shivering

The body’s water content

This is controlled to protect cells by stopping too much water from entering or leaving them. The process is called osmoregulation.

Controlling body temperature

Human enzymes usually work best at 37ºC, which is human body temperature. This can be measured in several places, including the ear, finger, mouth and anus.

There are various ways to measure body temperature, including using a clinical thermometer, heat-sensitive strips, digital probes or thermal imaging cameras.

Extremes of body temperature are dangerous:

• high temperatures can cause dehydration, heat stroke and death if untreated
• low temperatures can cause hypothermia and death if untreated

The body’s temperature is monitored by the brain. If you are too hot or too cold, the brain sends nerve impulses to the skin, which has three ways to either increase or decrease heat loss from the body’s surface:

1. Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.
2. If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal.
3. Blood vessels supplying blood to the skin can swell or dilate - vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. Blood vessels can shrink down again - vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.

Muscles can also receive messages from the brain when you are cold. They respond by shivering, which warms you up.

Too hot

• sweat glands in the skin release more sweat when we get too hot. This evaporates, removing heat energy from the skin.
• vasodilation occurs. Blood vessels leading to the skin capillaries become wider (dilate) allowing more blood to flow through the skin, and more heat to be lost.

Too cold

• muscles contract rapidly and we shiver when we're cold. These contractions need energy from respiration, and some of this is released as heat.
• vasoconstriction occurs - blood vessels leading to the skin capillaries become narrower (constrict) letting less blood flow through the skin and conserving heat in the body.

Hormones are chemicals secreted by glands in the body. Different hormones affect different target organs.

The bloodstream transports hormones from the glands to the target organs. Bodily reactions to hormones are usually slower and longer lasting than nervous reactions.

Move the mouse over the different glands to see what they do. You need to know the locations of the pancreas, ovaries and testes. You should also know which hormones they produce.

Gland           Hormone        Target organs

Ovary          Oestrogen       Ovaries,

Ovary          Progesterone  Uterus

Pancrease   Insulin             Liver

Testes         Testosterone  Male reproductive organs

Glucose is needed by cells for respiration. It is important that the concentration of glucose in the blood is maintained at a constant level. Insulin is a hormone produced by the pancreas that regulates glucose levels in the blood.

Glucose levelEffect on pancreasEffect on liverEffect on glucose level too high insulin secreted into the blood liver converts glucose into glycogen goes down too low insulin not secreted into the blood liver does not convert glucose into glycogen goes up

Blood sugar level meter and insulin shot pen

Diabetes is a disorder in which the blood glucose levels remain too high. It can be treated by injecting insulin. The extra insulin allows the glucose to be taken up by the liver and other tissues, so cells get the glucose they need and blood-sugar levels stay normal. There are two types of diabetes.

Type 1 diabetes

Type 1 diabetes is caused by a lack of insulin. It can be controlled by:

• monitoring the diet
• injecting insulin

People with type 1 diabetes have to monitor their blood sugar levels throughout the day as the level of physical activity and diet affect the amount of insulin required.

Type 2 diabetes

Type 2 diabetes is caused by a person becoming resistant to insulin. It can be controlled by diet and exercise. There is a link between rising levels of obesity (chronic overweight) and increasing levels of type 2 diabetes.

A 'tropism' is a growth in response to a stimulus. Plants grow towards sources of water and light, which they need to survive and grow.Auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weedkillers, rooting powder and to control fruit ripening.The direction of plant growth

Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water.There are different types of tropisms:

• positive tropism – towards the stimulus
• negative tropism – away from the stimulus
• phototropism – growth in response to the direction of light
• geotropism – growth in response to the direction of gravity

response part of plant direction of growth advantage positive phototropism stem tip growth towards light to get maximum light for photosynthesis negative phototropism root tip growth away from light less chance of drying out positive geotropism root tip towards gravity more chance of finding moisture negative geotropism stem tip away from gravity more chance of finding

Auxin is a plant hormone responsible for controlling the direction of growth of root tips and stem tips in response to different stimuli including light and gravity.

Auxin is made at the tips of stems and roots. It's moved in solution to older parts of the stem and root where it changes the elasticity of the cells. More elastic cells absorb more water and grow longer, causing bending in the stem or root. It's thought that light and gravity can interfere with the transport of auxin causing it to be unevenly distributed.

Auxin is produced in the tip of growing shoots.

If the tips are removed from growing shoots they cannot produce auxin, so phototropism cannot occur.

If the tips are covered, light cannot break down the auxin, so phototropism cannot then occur either.

Weedkillers

Selective weedkillers kill some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat plants. The selective weedkiller contains growth hormone that causes the weeds to grow too quickly. The weedkiller is absorbed in larger quantities by the weeds than the beneficial plants.

Rooting powder

Rooting powder makes stem cuttings quickly develop roots. Rooting powder contains growth hormones.

Controlling fruit ripening

Some hormones slow the ripening of fruits and others speed it up. These hormones and their inhibitors are useful for delaying ripening during transport or when fruit is displayed in shops.

The nervous system allows the body to respond, through effectors, to changes in the environment detected by receptors. The process involves neurones and is usually coordinated by the brain. A reflex action is an extra-rapid response to a stimulus: this process also involves the nervous system but it bypasses the brain.

Receptors

Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.

The central nervous system (CNS) in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response

Effectors

An effector is any part of the body that produces the response. Here are some examples of effectors:

• a muscle contracting to move the arm
• a muscle squeezing saliva from the salivary gland
• a gland releasing a hormone into the blood.

Neurones

Neurones are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:

1. sensory neurones carry signals from receptors to the spinal cord and brain.
2. relay neurones carry messages from one part of the CNS to another.
3. motor neurones carry signals from the CNS to effectors.

The diagram below shows a typical neurone: in this case, a motor neurone. It has tiny branches at each end (the dendron) and a long fibre carries the signals (the axon).

The axon is surrounded by a fatty layer known as the myelin sheath. This helps to protect the neurone and allow impulses to travel faster.

Synapses

Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals released by a neurone. The chemical diffuses across the gap makes the next neurone transmit an electrical signal.

1. An electrical impulse travels along an axon.
2. This triggers the nerve-ending of a neuron to release chemical messengers called neurotransmitters.
3. These chemicals diffuse across the synapse (the gap) and bind with receptor molecules on the membrane of the next neuron.
4. The receptor molecules on the second neuron bind only to the specific chemicals released from the first neuron. This stimulates the second neuron to transmit the electrical impulse

The animation below shows a synapse between two neurons:

When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

This is what happens:

1. receptor detects a stimulus - change in the environment
2. sensory neurone sends signal to relay neurone
3. motor neurone sends signal to effector
4. effector produces a respon

The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.

In bright light:

• Radial muscles of the iris relax.
• Circular muscles of the iris contract.
• Less light enters the eye through the contracted pupil.

In dim light:

• Radial muscles of the iris contract.
• Circular muscles of the iris relax.
• More light enters the eye through the dilated pupil.

Drugs are chemicals that cause changes in the body. They can be divided into legal and illegal drugs. Drugs can also be medical (drugs taken to cure illness) or recreational (drugs taken because they have pleasing effects). Some drugs can be addictive – more and more is needed to achieve the same effect. Drugs can be separated into categories – solvents, painkillers, depressants and stimulants.

Stimulants

Stimulants include caffeine - found in fizzy drinks, tea and coffee, cannabis and amphetamines such as speed. They increase the transmission of signals from one nerve cell to the next, which then increases alertness, heart rate and breathing rate. However, in the long term, stimulants can produce 'highs' and then extreme 'lows' or even depression. They can be addictive because the body needs a constant 'top-up' to maintain the effect.

Sedatives

Sedatives or depressants include alcohol and barbiturates (such as the prescribed drug amytal and the illegal GHB). Sedatives are also drugs prescribed by a doctor to help people sleep or to relieve the symptoms of stress. They slow down the nervous system and reactions.

Painkillers or analgesics include paracetamol, aspirin, heroin and morphine. They block nerve impulses from the painful part of the body, or block nerve impulses travelling to the part of the brain responsible for perceiving pain.

Paracetamol is an effective painkiller but an overdose is very dangerous. An overdose damages the liver and can cause death.

Hallucinogens

Hallucinogens change the way our brains work, distorting our senses. This changes our response to what we see, feel and hear. LSD is an example of a hallucinogen.

Nicotine

Nicotine is the addictive substance in tobacco smoke. It reaches the brain within 20 seconds and creates a dependency so that smokers become addicted.

Carbon monoxide

Carbon monoxide combines with the haemoglobin in red blood cells and so reduces the ability of the blood to carry oxygen. This puts extra strain on the circulatory system, and can cause an increased risk of heart disease and strokes.

Tar

Carcinogens are substances that cause cancer. Tobacco smoke contains many carcinogens, including tar. Smoking increases the risk of lung cancer, mouth cancer and throat cancer.

The alcohol in alcoholic drinks - such as wines, beers and spirits - is called ethanol. It is a depressant, which means it slows down signals in the nerves and brain.

There are legal limits to the level of alcohol that drivers and pilots can have in the body. This is because alcohol impairs the ability of people to control their vehicles properly.

Short-term effects

Alcohol has short-term effects such as sleepiness and impaired judgment, balance and muscle control. This leads to blurred vision and slurred speech. Vasodilation occurs - blood vessels in the skin carry more blood - leading to heat loss.

Long-term effects

The long-term effects of alcohol include damage to the liver and brain. The liver removes alcohol from the bloodstream because it's a toxic chemical. Over time, alcohol consumption can lead to liver damage (cirrhosis).

If an organ in the body has been damaged then it can be replaced by a healthy organ from a donor – someone who had healthy organs but very recently died from other causes.

A successful transplant has to have:

• similar tissues from donor to patient
• similar ages of donor and patient
• similar locations as organs deteriorate quickly

Organ donation can be an ethical issue especially as the supply of organs is limited. An ethical issue is one that has rights and wrongs. In an exam you will be expected to discuss the ethical issues involved in:

• liver transplants for alcoholics
• heart transplants for the clinically obese

Pathogens are microorganisms that cause infectious disease. Pathogens are mostly bacteria but some are viruses, fungi and protoctists.

Bacteria:

Bacteria come in many shapes and sizes, but even the largest are only 10 micrometres long (10 millionths of a metre).

Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body they release poisons or toxins that make us feel ill.

Viruses

Viruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.

Direct contact

Direct contact means that the disease-causing microbe is passed from one person to another when their bodies touch in some way.

Vertical transmission happens when microorganisms pass from a mother to her unborn baby through the placenta. German measles and HIV can be passed on this way.

Horizontal transmission happens when microorganisms pass from one person to another by touching, kissing or sexual intercourse.

Examples of horizontal transmission

Type of contactBacterial diseaseViral disease touching bacterial gastroenteritis chickenpox kissing bacterial meningitis glandular fever, cold sores sexual intercourse gonorrhoea, syphilis HIV, hepatitis B

Indirect contact happens when microorganisms are carried to a person in some way, instead of by actual body to body contact.

Vehicle-borne transmission involves an object carrying the disease-causing microorganism.

Examples of vehicle-borne transmission

VehicleBacterial diseaseViral disease droplets in the air tuberculosis (TB) colds, flu water cholera polio sharp objects tetanus HIV food Salmonella food poisoning hepatitis A

Vector-borne transmission involves an animal such as an insect. For example, malaria is transmitted by mosquitoes, dysentery by houseflies and plague by fleas.

Most pathogens have to get inside our body to spread infection. Once they are inside, the body provides ideal living conditions, including plenty of food, water and warmth. Standing in their way is our body's immune system - the body's co-ordinated response to the invading pathogens.

The first line of defence is the body's natural barriers. These include:

• skin
• nasal hairs, mucus and cilia
• tears
• stomach acid - it destroys the protein structure of the bacteria’s enzymes. This results in the bacteria being unable to carry out its bodily processes

Antibiotics and antifungals

Antibiotics are substances that kill bacteria or stop their growth. They do not work against viruses: it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

Antifungal agents kill fungi. An example of an antifungal is nyastatin which treats the fungus candida albicans.

How some common antibiotics work

AntibioticHow it works penicillin breaks down cell walls erythromycin stops protein synthesis neomycin stops protein synthesis vancomycin stops protein synthesis ciprofloxacin stops DNA replication

Chemicals that kill microorganisms outside the body are known as antiseptics. Antiseptics can be used to clean an open wound as well as surfaces on objects such as toilets.

One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in hospitals. However, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed.

Development of resistance

The main steps in the development of resistance are:

1. Random changes or mutations occur in the genes of individual bacterial cells.
2. Some mutations protect the bacterial cell from the effects of the antibiotic.
3. Bacteria without the mutation die or cannot reproduce with the antibiotic present.
4. The resistant bacteria are able to reproduce with less competition from normal bacterial strains.

Energy is transferred along food chains from one stage to the next. But not all of the energy available to organisms at one stage can be absorbed by organisms at the next one. The amount of available energy decreases from one stage to the next. Some of the available energy goes into growth and the production of offspring. This energy becomes available to the next stage, but most of the available energy is used up in other ways: energy released by respiration is used for movement and other life processes, and is eventually lost as heat to the surroundings energy is lost in waste materials, such as faeces All of the energy used in these ways returns to the environment, and is not available to the next stage. The animation shows how the level of available energy goes down as it is transferred through a temperate forest food chain.

Parasites are organisms that live on or in a host organism. The parasite benefits from this arrangement, but the host suffers as a result. Fleas are examples of parasites. They live on the skin of other animals and **** their blood: this feeds the flea but weakens the host.

A tapeworm lives inside another animal, attaching itself to the host’s gut and absorbing its food. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting. Parasites do not usually kill the host because this would cut off their food supply.

Other examples of parasites are:

• headlice - they bite other animals such as humans in order to feed off their blood
• mistletoe - the roots of mistletoe grow into the veins of the host tree to absorb nutrients and minerals.

Some organisms rely on the presence of organisms of a different species. For example, oxpecker birds eat ticks and larvae infesting the skin of buffalo and other large animals. For this reason oxpeckers are called a cleaner species. This is an example of mutualism - both species benefit from the arrangement.

Lichens are another example of mutualism. They are formed by algae and fungi living together. Algae can photosynthesise and make food, which is shared by the fungus. The fungus in turn shelters the algae from a harsh climate.

Other examples of mutualism include:

• 'cleaner' fish - these feed off the dead skin and parasites of larger fish such as sharks This provides the cleaner fish with food and keeps the larger fish clean
• chemosynthetic bacteria in deep sea vents - these use chemicals from tubeworms in order to get substances to make food. In return the tubeworms feed off substances made by the bacteria.

pollutanttypical effect smoke deposits soot on buildings and trees, causing them damage. Permeates the air, making it difficult for living creatures to breathe. carbon monoxide poisonous gas carbon dioxide greenhouse gas that contributes to global warming sulfur dioxide

contributes to acid rain

Lichens are plants that grow in exposed places such as rocks or tree bark. They need to be very good at absorbing water and nutrients to grow there, and rainwater contains just enough nutrients to keep them alive. Air pollutants dissolved in rainwater, especially sulfur dioxide, can damage lichens, and prevent them from growing. This makes lichens natural indicators of air pollution. For example:

• bushy lichens need really clean air
• leafy lichens can survive a small amount of air pollution
• crusty lichens can survive in more polluted air.

In places where no lichens are growing, it's often a sign that the air is heavily polluted with sulfur dioxide.

Another indicator of air quality is the blackspot fungus on roses. Blackspot fungus grows well on roses in unpolluted areas because it is killed by the presence of sulfur dioxide that would be found in polluted air.

Increasing human population has led to an increase in pollution. Some of this is due to:

• more fossil fuels being burnt for heat and power
• more food being grown
• land taken over for industry and housing.

As a result there has been an increase in levels of water pollution.

• Nitrate fertilisers are very soluble in water and are easily washed off fields by the rain and then into rivers and reservoirs. Because nitrates are all soluble they cannot easily be removed from the water.
• Pesticides used by farmers to kill weeds or insects may be washed or blown into streams and rivers.
• Sulfur dioxide in the air can dissolve in water to form an acidic solution.

A major problem with the use of fertilisers occurs when they're washed off the land by rainwater into rivers and lakes. The resulting increase of nitrate or phosphate in the water encourages algae growth, which forms a bloom over the water surface. This prevents sunlight reaching other water plants, which then die. Bacteria break down the dead plants and use up the oxygen in the water so the lake may be left completely lifeless.

Removing carbon dioxide from the atmosphere

Green plants remove carbon dioxide from the atmosphere by photosynthesis. The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants.

Returning carbon dioxide to the atmosphere

Organisms return carbon dioxide to the atmosphere by respiration. It is not just animals that respire. Plants and microorganisms do, too.

Passing carbon from one organism to the next

When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal. Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these microorganisms and detritus feeders.