Biology - Topic 9

  • Created by: 13martinl
  • Created on: 05-12-17 17:34



all the conditions that surround a living organism


the place where an organism lives


all the members of a single species that live in a habitat


all the populations of different organisms that live together in a habitat


a community and its habitat

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Describe the different levels of organisation

Individual, Species, Organism:  An individual is any living thing or organism. Individuals do not breed with individuals from other groups. Animals, unlike plants, tend to be very definite with this term because some plants can cross-breed with other fertile plants.

Population: A group of individuals of a given species that live in a specific geographic area at a given time. Note that populations include individuals of the same species, but may have different genetic makeup such as hair/eye/skin colour and size between themselves and other populations.

Community:  This includes all the populations in a specific area at a given time. A community includes populations of organisms of different species. In the diagram above, note how populations of gold fishes, salmons, crabs and herrings coexist in a defined location. A great community usually includes biodiversity.

Ecosystem:  As explained in the pages earlier, ecosystems include more than a community of living organisms (abiotic) interacting with the environment (abiotic). At this level note how they depend on other abiotic factors such as rocks, water, air and temperature.

Biome: A biome, in simple terms, is a set of ecosystems sharing similar characteristics with their abiotic factors adapted to their environments.

Biosphere: When we consider all the different biomes, each blending into the other, with all humans living in many different geographic areas, we form a huge community of humans, animals and plants, and micro-organisms in their defined habitats. A biosphere is the sum of all the ecosystems established on planet Earth. It is the living (and decaying) component of the earth system.

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What are Abiotic factors&what are the effects?

Abiotic factors
Abiotic factors are the physical and chemical conditions of an environment. For example : heat, salinity, pressure, light, wind, pH ...

•       soil (pH, nutrients, nitrogen content, etc.)



•amount of rain and/or wind


Imagine being a particular organism or standing in a particular ecosystem. What do you see and feel that is not living. Any of those observations qualify as abiotic factors.

Cold blooded animals are dependant on the sun. 

Plants are dependant on soil pH and the amount of rain.

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what are biotic factors&what are the effects?

Biotic factors
Biotic factors are all the biological conditions of an environment for a specie/taxa. It can include prey and predator abundance, available food amount, available space, intra and interspecific competition...

Biotic factors results from the activities of living organisms in the environment. Some examples include: predation, competition, disease

Competition -Habitats have limited amounts of the resources needed by living organisms and organisms can only survive if they can get enough of these resources, so they must compete for resources with other organisms. If they are unsuccessful and cannot move to another habitat, they will die.

Animals - The resources that animals compete for include: food – can be affected by disease, both crops and meat, water – can be polluted, can harbour disease, space.

Animals may also compete for mates so that they can reproduce.

Plants- Remember that plants make their own food using photosynthesis, so they do not compete for food. The resources that plants compete for include: water, space, mineral salts.

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What is the importance of interdependence?

Predators are animals that eat other animals. Prey are the animals that get eaten. The size of the predator population and prey population depend on each other.

•The Canadian lynx eats the snowshoe hare. The size of the two populations was estimated each year for 75 years from the number of animals caught by fur traders. There is a rise and fall in the snowshoe hare population with the lynx population following two years afterwards. No other cat is so dependent on a single prey species, which is why there is such a clear pattern of interdependence between the two populations.

•If the prey population grows, predator numbers will respond to the increased food supply by increasing as well, but the growing predator population will eventually reduce the food supply to the point where it can no longer be sustained.

•Ladybirds and aphids - The simulation shows how the population of ladybirds - predators - and the population of aphids - their preychanges over time

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What is parasitism?


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.

Example : A tapeworm

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:

•Head lice - 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.

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What is mutualism?

Mutualism - 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.

•'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.

Nitrogen-fixing bacteria –  Peas, beans and clover are leguminous plants. They have colonies of nitrogen-fixing bacteria in nodules attached to their roots. The plants gain nitrates from the bacteria, and the bacteria gain sugars from the plants. This is another example of mutualism.

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Core Practical: Investigate the relationship betwe

This investigation involves the use of a belt transect along a gradient (e.g. shaded area to an area with no shade). It involves students thinking about how to sample their chosen area, including the identification and observation of plants/organisms.

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How do you determine the no.of organisms?

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How is energy passed through food chains?

They are all ways of showing how energy is passed on from one organism to another. Biomass is the food available for the next trophic level (feeding level) in a food chain. Biomass is used as a source of renewable energy to reduce our dependence on fossil fuels.

Food chains -A food chain shows how energy is passed on from one organism to another. The original source of energy is the Sun.   A pyramid of numbers - The number of organisms in a food chain can be represented graphically in a pyramid. Each bar represents the number of individuals at each trophic level (feeding level) in the food chain. The pyramid of numbers usually shows that the number of organisms at each trophic level gets smaller towards the top.  It usually takes a large number of plants to provide sufficient energy for the consumers in the food chain.  Pyramid of biomass - A pyramid of biomass is a more accurate indication of how much energy is passed on at each trophic level. Biomass is the mass of living material in each organism multiplied by the total number of organisms in that trophic level.  Pyramids of biomass usually are a true pyramid shape (each level is smaller than the one below it). This is because biomass is a measure of the amount of food available. When animals eat, only a small proportion of their food is converted into new tissue, which is the food for the next trophic level. Most of the biomass that animals eat is either not digested or used to provide the energy needed for staying alive.

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How do you calculate efficiency of energy transfer

Energy transfer - Energy is transferred along food chains from one trophic level to the next. However, the amount of available energy decreases from one trophic level to the next. 

Energy loss - In a food chain only around 10 per cent of the energy is passed on to the next trophic level. The rest of the energy passes out of the food chain in a number of ways:

•it is used as heat energy 

•it is used for life processes (eg movement) 

•faeces and remains are passed to decomposers  

Less energy is transferred at each level of the food chain so the biomass gets smaller. As a result, there are usually fewer than five trophic levels in food chains. 

Percentage efficiency of energy transfer

The percentage efficiency of energy transfer between trophic levels can be calculated using the equation: energy transferred to next level ÷ total energy in × 100

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What are the advantages&disadvantages of fish farm

It is a process in which either non-native or native species are bred in an enclosed area, usually for conservation or commercial purposes. For example, there are many places in England, in which we farm fish such as salmon.

Pros •Creates income as there are more fish to sell

•Maintains food sustainability as there is more fish to eat

•Can help to save a certain species of fish 

Cons •Fish are kept in confined spaces so water gets dirty very easily

•Therefore, diseases are spread very quickly unless the area is treated often

•The water treatments can be expensive

•It can be expensive to feed all of the fish

•It can be difficult to control the increasing population of fish

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What are the adv&disadv of eutrophication?


When fertilisers are added to farmland in large quantities, plants and animals in nearby rivers and lakes can sometimes be killed, but not by poisoning. The steps in eutrophication are quite complicated.


•Farmers have a higher crop yield due to the fertilisers

•This then increases the food we can eat

•This also increases the money that the farmers/shops make

•This also creates food sustainability because we have enough nutritious food to eat


•Kills sea plants

•Kills fish

•Costs money to correct

•Could go back into the crops via the water cycle and make the fertilisers less effective, therefore the cycle worsens and worsens

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what are the adv&disadv of introducing new species

Non-indigenous means that they are not naturally native to the place they are in now, for example, the pandas in London zoo are non-indigenous animals as they were not born here, but when cases like this happen, we try to emulate their climate, habitat and eating habits, to keep them breeding and to stop the species from going extinct. 

PROS •Creates chances of new species      •Creates more food

•Creates extra link in the food chain            •Species can be biologically studied without the       •Can be saved from extinction                      researchers travelling

CONS •can cause the spreading of native or non-native diseases

•predation/prey, competition

•can disrupt a food chain completely/make a hole in it

•species may die if brought into a place where their habitat isn’t the same- it may not be able to survive in the conditions it I being put into

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What is bioaccumulation?

Bioaccumulation is when toxins build up in a food chain. The animals at the top of the food chain are affected most severely.

This is what happens:

•Small amounts of toxic substances - often from human activity - are taken up by plants.

•These plants are eaten by primary consumers.

•The primary consumers are eaten by secondary consumers, and the secondary consumers are eaten by higher level consumers.

•At each stage (trophic level) of the food chain, harmless substances are excreted but the toxins remain in the tissues of the organisms - so the concentration of toxin becomes most concentrated in the body tissues of the animals at the top of the food chain.

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How do humans affect ecosystems?

Humans rely on natural ecosystems to provide many ‘ecosystem services’- such as pollination of crops, and cleaning air and water. Humans also rely on ecosystems to provide them with fertile soil, mineral nutrients, fish and game. However, human activities often affect ecosystems in negative ways because human systems are not closed loop systems.

Human waste enters other natural ecosystems, where it can cause them to become unbalanced. Sometimes this waste can build up to harmful levels. Examples of human waste that can affect ecosystems are: household waste, industrial waste,agricultural waste, gases produced when fossil fuels are burned

Humans sometimes harvest plants and animals from natural ecosystems and this can also unbalance them. For example, harvesting timber can lead to soil erosion and loss of habitat, while harvesting fish from the oceans can cause species to become threatened and unbalance food webs. In some parts of the world, natural vegetation has been removed and replaced with crops for food or the production of fuels (called ‘biofuels’), or by grazing animals.As well as destroying the natural habitat and reducing biodiversity, soil erosion can cause rivers to become silted up, plus the lack of shade and the lack of moisture in the soil can cause desertification (when fertile land turns into relatively lifeless desert).

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Explain the effect of sustainability on ecosystems

When humans remove substances or organisms from an ecosystem, this is only sustainable if the rate at which we remove them is less than or equal to the rate at which they can be replaced by natural processes, such as reproduction. For example, the way in which crude oil is used is not sustainable because it takes millions of years to form from the decay of dead organisms.

We are removing crude oil from underground reservoirs in the crust very quickly. When it is burned, the energy that is released was captured by photosynthesis millions of years ago when the organisms were alive.  Energy that is trapped in this way can be called ‘fossil sunlight energy’.

In a perfect closed loop system, the inputs and outputs of the organisms are balanced - but the system that formed the crude oil no longer actually exists, so the input when crude oil is burned unbalances the present system (it is an ‘extra’ input).

Sustainable development is more likely to be possible if sunlight is used as a primary energy source, for example by using biofuels. A biofuel is a fuel produced from renewable resources - such as plant biomass - where the carbon dioxide emitted by burning them is balanced by the carbon dioxide absorbed by the plants as they grow.

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What are the benefits of maintaining local biodiv?

Benefits of local biodiversity

•creates food sustainability

•creates a functioning food chain

•helps local businesses survive

        •helps to save habitats

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what are the benefits of maintaining global biodiv

Benefits of global biodiversity

•creates food sustainability

•creates a functioning food chain

•helps businesses survive

•helps to save habitats

•helps to save native and non-native species

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Benefits of reforestation?


It is when you replant trees where they were once cut down


•creates shelter, food and water supplies for many different species

•creates a habitat to survive in

•protects habitats from weathers

•the trees provide oxygen

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Benefits of animal conservation?

Animal conservation 

It is when a group of people come together to protect an endangered species of animals.


•saves species, encourages reproduction 

•maintains food webs/chains

•pest control

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What biological factors affect human population?

Food security is the ability of human populations to access food of sufficient quality and quantity. The rapidly increasing human population around the world has resulted in rising demand for food worldwide. This has led to many concerns about food security.

Food security can be an issue for individuals, households or entire countries. It includes being able to access food that is of an adequate quality and quantity but also being able to afford it.

Some people suggest that current levels of food production will not meet the projected demands in the coming years. Food production must be sustainable and not have a negative impact on the ecosystem and natural resources.

We want to produce more food in the same area of land, therefore increased plant productivity and manipulation of genetic diversity will be required in order to maintain a sustainable food supply.

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How does an ncrease in animal farming and the incr

        •helpful to developing countries, economically 

•more food 

•multipurpose – skins, fibre, fertiliser and fuel

•socially and culturally, lower-income families are closely linked to livestock for farming, economic and food stability

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How does the impact of pests and pathogens affect

•can spread disease within both crops and species

•this decreases the amount of consumable products

•holes in food chain

•less food to sell 

•possibility of pathogens or treatments of pathogens to go into water cycle and potentially bacteria and other pathogens could become immune to the treatment

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describe how different materials cycle through the

Part of what sustains an ecosystem is the constant interchange of matter between its living (biotic) and non-living or 'physical' (abiotic) components. Sustainable ecosystems are dependent upon physical factors such as precipitation, humidity or the lack of, sunlight, temperature, rocks and sedimentary deposits, soil, and overall temperature. These abiotic components play a critical role in shaping the biotic factors which are also present within ecosystems such as plants and animals each of which are adapted to the specific climate and geography of the region in question.

It should also be noted that biotic elements within ecosystems also help shape the abiotic factors as well. For example, the biochemical reactions of nitrogen-fixing bacteria found on the roots of some plants assist in the abiotic nitrogen cycle which is so critical to both the environment and other organisms living within the environment. It is this 'interplay' of abiotic and biotic factors which keeps ecosystems going.

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What is the carbon cycle?

        •Carbon enters the atmosphere as carbon dioxide from respiration and combustion.

•Carbon dioxide is absorbed by producers to make carbohydrates in photosynthesis.

•Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide formed during respiration. The animals and plants eventually die.

•The dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.

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Explain the carbon cycle in the sea?

In the sea, marine animals may convert some of the carbon in their diet to calcium carbonate which is used to make their shells.

Over time the shells of dead organisms collect on the seabed and form limestone.

Due to Earth movements this limestone may eventually become exposed to the air where it's weathered and the carbon is released back into the atmosphere as carbon dioxide.

Volcanic action may also release carbon dioxide. 

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What is the water cycle?

The water cycle works because of heat from the Sun. Water evaporates from the surface of the sea as warm air moves across it. Secondary sources (less important) of water vapour in the air are volcanoes and animal respiration. Water vapour in the air condenses into small droplets, and these droplets form clouds. Water falls from the clouds onto land, as rain or snow. This is called precipitation. Water on the land collects in streams, rivers and lakes, and flows back to the sea, completing the cycle.

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How do we supply safe water to areas of drought?

Water is essential for life - it acts as the solvent in our cells for chemical reactions to take place. 

Water covers about two-thirds of our planet, but the vast majority of it cannot be drunk directly. This is because humans need drinking water with low levels of dissolved salts and microbes. To produce water of a sufficient quality, we must:

•choose an appropriate source of water

•filter the water

•chlorinate the water

Sources of water- Sources of water for drinking should be reliable, and they should also be fresh and free of toxic chemicals (such as heavy metals). In the UK, water resources include lakes, rivers, aquifers and reservoirs. An aquifer is an underground layer of permeable rock, gravel or sand that is soaked with water, while a reservoir is usually an artificial lake, made by building a dam to accumulate and save river water in the valley behind.  In countries where water is scarce, boreholes are drilled to reach water underground.

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How do you purify the water?

Solids in the water, such as leaves and soil, must be removed. The water is sprayed onto specially-prepared layers of sand and gravel called filter beds.

Different-sized insoluble solids are removed as the water trickles through the filter beds. These are cleaned every so often by pumping clean water backwards through the filter.

The water is then passed into a sedimentation tank. Aluminium sulfate is added to clump tiny particles together to make larger particles, which settle out more easily. The water is then passed through a fine filter, such as carbon granules, to remove very small particles.

Water is purified by filtration, sedimentation and the addition of chlorine

Chlorine is added to drinking water to sterilise it. The chlorine kills microbes - including microbes that cause potentially-fatal diseases such as typhoid, cholera and dysentery.

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What is the nitrogen cycle?

•Nitrogen gas is converted to nitrate compounds by nitrogen-fixing bacteria in soil or root nodules. Lightning also converts nitrogen gas to nitrate compounds. The Haber process converts nitrogen gas into ammonia used in fertilizers. Ammonia is converted to nitrates by nitrifying bacteria in the soil.

•Plants absorb nitrates from the soil and use these to build up proteins. The plant may be eaten by an animal, and its biomass used to produce animal protein.

•Urea and egested material is broken down by decomposers. This results in nitrogen being returned to the soil as ammonia. 

•Decomposers also break down the bodies of dead organisms resulting in nitrogen being returned to the soil as ammonia.

•Higher only: In some conditions denitrifying bacteria in the soil break down nitrates and return nitrogen to the air. This is usually in waterlogged soil. Improving drainage reduces this effect, making the soil more fertile.


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what is the importance of decomposers in the Nitro

These aerobic bacteria live in the soil.  

When an organism dies decomposers digest the proteins and DNA that are found in the cells of the organism and produce ammonium ions (NH4+) as a waste product.  

Decomposers will also break down the molecules in animal faeces and will decompose the urea in animal urine into ammonium ions.

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what is the importance of nitrifying bacteria in t

These are also aerobic bacteria that live in the soil.  

They get their energy by converting ammonium ions into nitrates (via an intermediate ion called a nitrite).  Nitrifying bacteria are essential for the Nitrogen cycle because the nitrates they form are the ions that plants will absorb through their roots.

 Nitrates will be used by the plant to make amino acids, proteins and DNA and these can pass up food chains.

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What is the importance of denitrifying bacteria in

These are anaerobic bacteria that thrive in conditions where there is little oxygen in the soil.  This often happens when the soil becomes water-logged so all the air spaces are flooded.  

Denitrifying bacteria are “bad news” for Nitrogen cycle as they get their energy by taking nitrates from the soil and converting them into nitrogen gas.  

This obviously reduces the nitrate available to the plants with roots in the soil and this is one of the reasons farmers like to keep soil well aerated for their crops.

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indicator species shows water pollution

Water pollution is caused by the discharge of harmful substances into rivers, lakes and seas. Many aquatic invertebrate animals cannot survive in polluted water, so their presence or absence indicates the extent to which a body of water is polluted.

if the water is:

  • clean, you will see mayfly larva
  • low level of pollution, you will see freshwater shrimp
  • high level of pollution, you will see water louse
  • very hugh level of pollution, you will see rat-tailed maggots or sludgeworm


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indicator species- the level of air pollution

The most common source of air pollution is the combustion of fossil fuels. This usually happens in vehicle engines and power stations. Sulfur dioxide is released if the fuel contains sulfur compounds. This gas contributes to acid rain. Lichens can be used as air pollution indicators, especially of the concentration of sulfur dioxide in the atmosphere.

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. 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.

•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 is often a sign that the air is heavily polluted with sulfur dioxide.

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effect of temperature on the rate of decomposition


When temperatures are not properly controlled, food can spoil. For example, for every -7°C rise in temperature within the moderate temperature range where most food is handled (10°C to 40°C), the rate of chemical reaction is approximately doubled. As a result, excessive heat will increase the rate of natural food enzyme reactions and the reactions of other food constituents. As a result, protein will breakdown or denature; emulsions will break; some vitamins will be destroyed; moisture will be lost and foods will dry out; and the colour, flavour and odour of some products may be affected Exposing foods to uncontrolled cold temperatures will also cause physical spoilage. Fruits and vegetables that accidentally freeze and thaw have their texture and appearance affected. Skins and surfaces of these products will often crack, leaving them more susceptible to microbial contamination. Some foods that become frozen may also be adversely affected. For example, if mayonnaise freezes, the emulsion will break and the components will separate.

Spoilage is caused in many foods by temperatures that are not extreme. Cold damage of several fruits and vegetables can occur at common refrigerator temperatures (1-6°C). Defects in produce exposed to cold temperatures include the development of off-colours, surface pitting and a variety of decays. Uncut, fresh fruits and vegetables such as bananas, lemons, squash and tomatoes are products that should be held at temperatures no colder than 10°C for best quality.

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effect of water availability on the rate of decomp

Water or moisture greatly affects the keeping qualities of food. Excessive moisture pickup can result in product spoilage and spoilage by:

•Microorganisms – micro-organisms need water to dissolve the food they use. Water allows the food to get into bacterial, yeast and mould cells where it is used for energy and growth. Water also allows waste products to escape from the cells. 

•Chemical Reactions -- the moisture in food also functions to allow chemical reactions to occur between components in the product. 

Water is controlled in foods by:

•Drying (dehydration), concentration and evaporation -- by removing water to a certain level, deteriorative reactions can be reduced or prevented. Examples of products preserved by these drying techniques include dry milk, potato flakes, drink mixes, evaporated milk, and orange juice concentrate. 

•Freezing - the freezing of foods changes water from liquid to solid form and renders it unavailable to microorganisms and chemical reactions. 

Food Additives - salt and sugar are used in many products to bind water and thereby making it less available for microbial growth and biochemical reactions. Jams, jellies, and cured hams are examples. 

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What is the effect of oxygen availability on the r

Air and Oxygen - One important cause of food spoilage and spoilage is air and oxygen. Because air is colourless, odourless, and tasteless Air consists of 78% nitrogen, 21% oxygen, and a 1% mixture of other gases. While oxygen is essential for life, it can have deteriorative effects on fats, food colours, vitamins, flavours, and other food constituents. It can provide conditions that will enhance the growth of microorganisms; it can cause damage to foods with the help of enzymes; and it can cause oxidation.

Microorganisms. Oxygen can provide conditions that enhance the growth of microorganisms. Some bacteria require oxygen for growth (aerobes) while others can grow only in the absence of oxygen (anaerobes). Moulds and most yeast that cause food to spoil require oxygen to grow. They can often be found growing on the surface of foods when air is present.

Enzymes. Certain enzymes that are naturally present in food are known as oxidizing enzymes. These enzymes catalyse chemical reactions between oxygen and food components, and this leads to food spoilage.  Oxygen. Oxygen can also cause spoilage of foods spontaneously, by itself. Oxidative spoilage is the chief cause of quality loss in fats and fatty portions of foods. When lipids oxidize, short chain carbon compounds are formed; these compounds have very strong odours and flavours and are very undesirable and unacceptable. 

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effect of oxygen availability on decomposition

Oxygen Level

Like all living organisms, the creatures in your soil need oxygen to live. Oxygen comes from the air above the soil, so there must be a means for air to penetrate into the soil. Soil with a loose structure allows for ample spaces between soil particles for oxygen to collect. In such soils, organic matter will decompose faster. Compacted or “tight” soils -- such as soils with a high clay content -- do not provide adequate space for air to collect, causing less biological activity and a slower organic matter break-down.

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effect of water content on the rate of decompositi

Moisture Level

The amount of water in the soil, both indirectly and directly, affects the decomposition rate of organic matter. Indirectly, a wet soil results in a slower break down because water fills the air spaces in the soil, depriving the microbes of oxygen. But all living organisms also require water, so a soil that is too dry directly decreases organic matter decomposition, as the microorganisms found in soil cannot survive without water. Soils that undergo period wet and dry periods -- like a garden that you water weekly -- instead of remaining constantly wet or dry, will cause organic matter to decompose more quickly.

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effect of temperature on the rate of decomposition


At temperatures of 5 degrees C and above, soil bacteria experience some activity. But once temperatures reach 22 C, soil microbes really get going, up to about 38 C. Microbial populations double in the soil with every additional -12 C. Because of this, soil organic matter breaks down faster in the summer, and areas with year-round warm weather will decompose more organic matter annually than areas that experience cold winters.

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What factors affect decay?

The factors that affect the rate of decay include:



•amount of available oxygen.

Microorganisms are more active and digest materials faster when they are in moist, warm and aerobic conditions. They will digest materials more slowly in dry, cold and anaerobic conditions.

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How is decay part of a great cycle?

Some of the substances released during decay are needed by plants for healthy growth. In a stable community of living things, processes that return substances to the environment (such as decay) are balanced by the processes that remove and use substances. In this way, the substances are continuously recycled. 

Practicals you may have encountered worth revising:

•Investigating tropic responses.

•Investigating how indicator species can be used to assess levels of pollution in water or the atmosphere.Investigating animal behaviour using choice chambers.

Decay is an essential life process that digests food or waste matter and recycles materials. Materials from living things decay because they are digested (broken down) by microorganisms. These microorganisms cause decay by releasing enzymes that break down compounds to be absorbed by their cells.

Bacteria and fungi are the main groups ofdecomposer.

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