organisms and their environment

Question 1

Food chain

Answer 1

The transfer of energy from one organism to the next, beginning with the producer

Question 2

Food web

Answer 2

A network of interconnected food chains

Question 3

Producer

Answer 3

An organism that makes its organic nutrients, usually using energy from sunlight through photosynthesis

Question 4

Consumer

Answer 4

An organism that gets its energy by feeding on other organisms

Question 5

Herbivore

Answer 5

An animal that gets its energy by eating plants

Question 6

Carnivore

Answer 6

An animal that gets its energy by eating other animals

Question 7

Decomposer

Answer 7

An organism that gets its energy from dead or waste material

Question 8

Population

Answer 8

A group of organisms of one species, living in the same area, at the same time

Question 9

Trophic level

Answer 9

The position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass

Question 10

Community

Answer 10

All of the populations of different species in an ecosystem

Question 11

Ecosystem

Answer 11

A unit containing a community of organisms sms

Question 12

Energy flow

Answer 12

The Sun is the principal source of energy input to biological systems. The Earth receives two main types of energy from the Sun: light (solar) and heat. Photosynthetic plants and some bacteria can trap light energy and convert it into chemical energy.

Question 13

Heterotrophic organisms obtain their energy

Answer 13

by eating plants or animals that have eaten plants. Thus, all organisms, directly or indirectly, get their energy from the Sun. The chemical energy produced is passed from one organism to another in a food chain but, unlike water and elements such as carbon and nitrogen, energy does not return in a cycle. The energy given out by organisms is lost to the environment.

Question 14

Energy is transferred between

Answer 14

organisms in a food chain by ingestion. Food chains are lists of organisms that show the feeding relationship between them, as in the example below.

Question 15

A food chain usually starts with

Answer 15

a producer (photosynthetic plant), which gains its energy from the Sun. The arrows used to link each organism to the next represent the transfer of energy. They always point towards the ‘eater’ and away from the plant. The feeding level is known as the trophic level.

• Plants are producers (they make, or produce, food for other organisms).
• Animals that eat plants are primary consumers (a consumer is an ‘eater’). They are also called herbivores. • Animals that eat other animals are secondary, or possibly tertiary, consumers, depending on their position in the chain. They are also called carnivores.

Question 16

number pyramid

Answer 16

Question 17

Energy is lost at each level in the food chain, as it is transferred between trophic levels. The following examples show how the energy is lost:

Answer 17

• Energy lost through the process of respiration (as heat).
• Energy used up for movement (to search for food, find a mate, escape from predators, etc.).
• Warm-blooded animals (birds and mammals) maintain a constant body temperature – they lose heat to the environment.
• Warm-blooded animals lose heat energy in faeces and urine.
• Some of the material in the organism being eaten is not used by the consumer; for example, a locust does not eat the roots of maize, and some of the parts eaten are not digestible.

Question 18

Even plants do not make use of all the light energy available to them. This is because some light:

Answer 18

• is reflected off shiny leaves;
• is the wrong wavelength for chlorophyll to trap;
• passes through the leaves without passing through any chloroplasts;
• does not fall on the leaves.

Question 19

This means that the transfer of energy

Answer 19

between trophic levels is inefficient – a lot is lost. On average, about 90% of the energy is lost at each level in a food chain. This means that, in long food chains, very little of the energy entering the chain through the producer is available to the top carnivore. Thus, there tend to be small numbers of top carnivores, and food chains usually have fewer than five trophic levels.

Question 20

energy units

Answer 20

Question 21

bio mass pyramid inverted or uppright

Answer 21

Question 22

CARBON CYCLE

Answer 22

• Carbon moves into and out of the atmosphere mainly in the form of carbon dioxide.
• Plants take carbon dioxide out of the air by photosynthesis.
• Plants convert carbon dioxide into organic materials (carbohydrates, fats and proteins).
• Herbivores obtain carbon compounds by feeding on plants. Carnivores gain carbon compounds by feeding on other animals.
• Animals and plants release carbon dioxide back into the air through respiration.
• When organisms die, they usually rot (the process of decomposition). Decomposers break down the organic molecules through the process of respiration to release energy. This also releases carbon dioxide into the air.
• If a dead organism does not decompose, the carbon compounds are trapped in its body. Over a long period, this can form fossil fuels such as coal, oil or gas (fossilisation).
• Combustion of fossil fuels releases carbon dioxide back into the air.

Question 23

Plants do not start respiring when they stop

Answer 23

photosynthesising (at night) – they respire all the time, but during the day there is usually a net intake of carbon dioxide and output of oxygen. If there is an increase in the combustion of fossil fuels or if more trees are cut down and not replaced, carbon dioxide levels in the atmosphere will increase. This is thought to contribute to global warming. Carbon dioxide forms a layer in the atmosphere, which traps heat radiation from the Sun. This causes a gradual increase in the atmospheric temperature which can: • melt polar ice caps, causing flooding of low-lying land; • change weather conditions in some countries, increasing flooding or reducing rainfall and changing arable (farm) land to desert; • cause the extinction of some species that cannot survive at higher temperatures.

Question 24

water cycle

Answer 24

• Plants release water vapour into the air through transpiration.
• Water evaporates from seas, lakes, rivers and soil.
• Water vapour condenses in the air, forming clouds.
• Water returns to the land as rain (precipitation), draining into streams, rivers, lakes and seas.
• Plant roots take up water by osmosis. In addition, animals lose water to the environment through exhaling and sweating, and in urine and faeces.

Question 25

The nitrogen cycle roles of the three main types:

Answer 25

• Nitrogen-fixing bacteria – convert nitrogen gas into compounds of ammonia.
• Nitrifying bacteria – convert compounds of ammonia into nitrates.
• Denitrifying bacteria – break down nitrites into nitrogen gas.

Question 26

nitrogen cycle

Answer 26

The element nitrogen is a very unreactive gas. Plants are not able to change it into nitrogen compounds, but it is needed to form proteins. Nitrogen compounds become available for plants in the soil in a number of ways, including:

• nitrogen-fixing bacteria (some plants – legumes such as peas, beans and clover – have roots with nodules that contain these bacteria, so the plant receives a direct source of nitrates);
• breakdown of dead plants and animals by decomposers (bacteria, fungi and invertebrates);
• the addition of artificial fertilisers, compost (decaying plant material) and manure (decaying animal waste – urine and faeces);
• lightning – its energy causes nitrogen to react with oxygen. Plants absorb nitrates into their roots by active uptake (see Chapter 3). The nitrates are combined with glucose (from photosynthesis) to form amino acids and proteins. Proteins are passed through the food chain as animals eat the plants. When animals digest proteins, the amino acids released can be reorganised to form different proteins. Some soil bacteria – denitrifying bacteria – break down nitrogen compounds and release nitrogen back into the atmosphere. This is a destructive process, commonly occurring in waterlogged soil. Farmers try to keep soil well drained to prevent this happening – a shortage of nitrates in the soil stunts the growth of crop plants. Nitrates and other ammonium compounds are very soluble, so they are easily leached out of the soil and can cause pollution.

Question 27

Factors affecting the rate of population growth

Answer 27

The rate of growth of a population depends on the following.

Food supply – ample food will enable organisms to breed more successfully to produce more offspring; a shortage of food can result in death or can force emigration, reducing the population.

Predation – if there is heavy predation of a population, the breeding rate may not be sufficient to produce enough organisms to replace those eaten, so the population will drop in numbers. There tends to be a time lag in population size change for predators and their prey. As predator numbers increase, prey numbers drop, and as predator numbers drop, prey numbers rise again (unless there are other limiting factors).

Disease – this is a particular problem in large populations, because disease can spread easily from one individual to another. Epidemics can reduce population sizes very rapidly.

Question 28

Human population growth

Answer 28

Human population growth has been the result of having no limiting factors. In 2014, the human population size was 7.2 billion. Human population size has increased exponentially (Figure 19.12) because of improvements in food supply and the development of medicine to control diseases. Infant mortality has decreased, while life expectancy has increased. Such a rapid increase in population size has social implications. These include increasing demands for basic resources including food, water, space, medical care and fossil fuels. The presence of a larger human population creates greater pressures on the environment (more land needed for housing, growing crops and road building, as well as wood for fuel and housing) and, potentially, more pollution. The presence of a larger population of young people results in greater demands on education, while more old people results in greater demands on health care.

Question 30

comunity population eco system

Answer 30

In a lake, the animal community will include populations of fish, insects, crustaceans, molluscs and protoctists. The plant community will consist of rooted plants with submerged leaves, rooted plants with floating leaves, reed-like plants growing at the lake margin, plants floating freely on the surface and filamentous algae in the surface waters. A lake is an ecosystem, which consists of the plant and animal communities mentioned above, and the non-living part of the environment (mud, water, minerals, dissolved oxygen, soil and sunlight) on which they depend.

Question 31

Effect of a limiting factor on population

Answer 31

growth When a limiting factor influences population growth, a sigmoid (S-shaped) curve is created, as shown in Figure 19.13 for a colony of yeast. You need to be able to identify the lag, exponential (log), stationary and death phases on a graph of population growth.

A limiting factor such as food takes effect as the population becomes too large for supplies to be sufficient. The population growth rate reduces until births and deaths are equal. At this point, there is no increase in numbers – the graph forms a plateau. As food runs out, more organisms die than are born, so the number in the population drops. This is the death phase.

Question 32

Abundant food supplies can lead

Answer 32

to more people becoming obese; this results in greater demands on health care due to increasing numbers of sufferers of heart disease, diabetes, blindness, etc. In the long term, this may reduce average life expectancy, as poor health becomes a limiting factor. You need to be able to explain the factors that lead to the different phases shown in Figure 19.13:

• Lag phase – the new population takes time to settle and mature before breeding begins. When this happens, a doubling of small numbers does not have a big impact on the total population size, so the line of the graph rises only slowly with time. • Log (exponential) phase – there are no limiting factors. Rapid breeding in an increasing population causes a significant increase in numbers. A steady doubling in numbers per unit of time produces a straight line.
• Stationary phase – limiting factors, such as shortages of food, cause the rate of reproduction to slow down and there are more deaths in the population. When the birth rate and death rate are equal, the line of the graph becomes horizontal.
• Death phase – the mortality rate (death rate) is now greater than the reproduction rate, so the population numbers begin to drop. Fewer offspring will live long enough to reproduce. The decline in population numbers can happen because the food supply is insufficient, waste products contaminate the habitat or disease spreads through the population.