Nutrient Cycles and Populations

The Carbon Cycle

Nutrients are recycled in nutrient cycles. In the carbon cycle, carbon moves between the carbon dioxide in the air, living organisms and fossil fuels. The cycle is limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion.

Worked example (reading the carbon cycle): In Fig 19.20, name the process that moves carbon from the carbon dioxide in the air into plants. Step 1. Find the arrow that points from the carbon dioxide in the air to the plants. Step 2. It is labelled photosynthesis. Answer: photosynthesis.

Worked example (reading the carbon cycle): In Fig 19.20, name the process that moves carbon from fossil fuels into the air. Step 1. Find the arrow that leaves the fossil fuels and points to the carbon dioxide in the air. Step 2. It is labelled combustion. Answer: combustion.

Carbon Cycle Processes

Worked example (carbon dioxide into the air): State two processes that return carbon dioxide to the air. Step 1. Respiration by plants, animals and decomposers releases carbon dioxide. Step 2. Combustion of fossil fuels also releases carbon dioxide. Answer: respiration; combustion.

Worked example (carbon in food chains): Describe how carbon in a plant can pass to a rabbit. Step 1. The plant made organic nutrients containing carbon by photosynthesis. Step 2. The rabbit eats the plant, so the carbon is passed on by feeding. Answer: by feeding.

Worked example (decomposition): Explain how decomposers return carbon to the air. Step 1. Decomposers feed on dead organisms and waste, which contain carbon. Step 2. They respire, and respiration releases carbon dioxide into the air. Answer: by respiration.

The Nitrogen Cycle

ExtendedThe nitrogen cycle

The nitrogen cycle describes how nitrogen moves between the air, the soil and organisms. Nitrogen gas makes up most of the air, but plants can only use it after it has been changed into nitrogen compounds, such as nitrate ions.

1. Plants absorb nitrate ions from the soil and use them to make amino acids and proteins.

2. Animals get protein by feeding, and digest it to amino acids.

3. Excess amino acids are broken down in the liver by deamination, forming urea.

4. Decomposers break down dead organisms and waste such as urea, releasing ammonium ions.

Worked example (reading the nitrogen cycle): In Fig 19.21, name the process that turns ammonium ions into nitrate ions. Step 1. Find the arrow that leaves the ammonium ions and points to the nitrate ions. Step 2. It is labelled nitrification. Answer: nitrification.

Worked example (nitrate and protein): Describe how nitrate ions in the soil become protein in a plant. Step 1. The plant absorbs nitrate ions from the soil. Step 2. It uses them to make amino acids, and then proteins. Answer: absorbed, then amino acids and proteins.

Nitrogen Cycle Processes

ExtendedRoles of microorganisms

Decomposition, nitrification, nitrogen fixation and denitrification are all carried out by microorganisms (apart from nitrogen fixation by lightning).

Worked example (roles of microorganisms): State the role of microorganisms in decomposition in the nitrogen cycle. Step 1. Decomposers break down proteins in dead organisms and waste. Step 2. This releases ammonium ions into the soil. Answer: break down protein to ammonium ions.

Worked example (nitrogen fixation): State two ways in which nitrogen gas can be changed into nitrogen compounds. Step 1. Lightning can change nitrogen gas into nitrogen compounds. Step 2. Bacteria also carry out nitrogen fixation. Answer: lightning; bacteria.

Populations, Communities, Ecosystems

A population is a group of organisms of one species, living in the same area, at the same time. A community is all of the populations of different species in an ecosystem. An ecosystem is a unit containing the community of organisms and their environment, interacting together.

Worked example (using the terms): A pond contains frogs, fish, water plants, water and mud. State what is meant by the group of all the frogs in the pond. Step 1. The frogs are all of one species, in the same area, at the same time. Step 2. So they are a population. Answer: a population.

Worked example (community and ecosystem): Explain the difference between the community and the ecosystem of the pond. Step 1. The community is all the populations of different species in the pond: frogs, fish and water plants. Step 2. The ecosystem is the community and its environment (water and mud) interacting together. Answer: ecosystem = community + environment.

Factors Affecting Population Growth

The rate of population growth depends on the factors that change how many organisms are born or die. The factors you need are food supply, competition, predation and disease.

Worked example (food supply): Explain why a rabbit population grows faster in a year with plenty of grass. Step 1. With plenty of grass, more rabbits get enough food, so more survive. Step 2. More rabbits survive to reproduce, so the population grows faster. Answer: more food: more survive and reproduce.

Worked example (predation): Foxes eat rabbits. Describe the effect on the rabbit population of an increase in the number of foxes. Step 1. More foxes eat more rabbits. Step 2. So the rabbit population grows more slowly, or decreases. Answer: rabbit growth slows or falls.

Worked example (disease): A disease spreads through a population of deer. Describe the effect on population growth. Step 1. The disease kills deer or stops them reproducing. Step 2. So the population grows more slowly, or decreases. Answer: growth slows or falls.

The Sigmoid Growth Curve

When a population grows in an environment with limited resources, the graph is an S-shaped, or sigmoid, curve. It has four phases.

Worked example (identifying a phase): Use Fig 19.23 to state the phase of growth between 4 hours and 8 hours. Step 1. The curve is rising steeply between 4 and 8 hours. Step 2. Rapid growth is the exponential (log) phase. Answer: exponential (log) phase.

Worked example (interpreting a graph): Use Fig 19.23 to describe the change in the population between 17 hours and 24 hours. Step 1. The curve falls from about 9 100 cells to about 3 600 cells. Step 2. The population decreases: this is the death phase. Answer: decreases; death phase.

Explaining the Phases

ExtendedWhy each phase happens

ExtendedLimiting factors

The factors from page 8 act as limiting factors: food supply, competition, predation and disease. As the population grows, these limit its growth, so the curve levels off. The growth curve is only an S-shape when the resources are limited.

Worked example (explaining the stationary phase): Explain why the population stops increasing in the stationary phase. Step 1. A limiting factor, such as food supply, has become short. Step 2. So the number of births is equal to the number of deaths and the population stays constant. Answer: limiting factor; births equal deaths.

Worked example (explaining the exponential phase): Explain why the population grows rapidly in the exponential phase. Step 1. Food is plentiful and there is little competition, so many organisms reproduce. Step 2. Few organisms die, so births are greater than deaths. Answer: plentiful resources; births greater than deaths.