IGCSE Biology - Organisms and their Environment

sunlight

main source of energy for biological systems

food chain

shows the transfer of energy from one organism to another

food webs

show interconnecting food chains

producers

plants that make their own organic nutrients from simple raw materials (eg carbon dioxide & water) and provide energy in the form of carbon compounds (proteins, carbohydrates & fats) for consumers and decomposers

consumers

obtain their energy by eating other organisms, either plants of animals or both

decomposers

fungi and bacteria that gain their energy from waste organic material

trophic level

position of organisms in a food chain or food web

primary consumers

herbivores that eat the producers

secondary consumers

carnivores that eat the herbivores

tertiary consumers

aka top carnivores carnivores that eat other carnivores

pyramids of numbers

show how many individuals there are at each trophic level but give no indication of their size

pyramids of biomass

indicate the mass of living material at each trophic level, but give no indication of the rate of growth

vegetarian diet

can support far more people than one that includes meat products

energy losses

occur between trophic levels as a result of respiration, and in waste materials produced

carbon cycle

photosynthesis, respiration, feeding, decomposition, fossilisation, combustion

water cycle

evaporation, condensation, precipitation (and transpiration)

carbon recycling

decomposers like bacteria & fungi are important in the recycling of nutrients

nitrogen

available in the atmosphere as a gas, but not available to most organisms as it is not reactive

nitrogen-fixing bacteria

live inside root nodules of legumes & convert nitrogen to ammonia

ammonia

result of bacteria decomposing protein and urea.

Nitrifying bacteria convert it to nitrate ions

Denitrifying bacteria

convert nitrate ions to nitrogen gas

nitrate ions

absorbed by plants and converted to amino acids which they use to make proteins

population

a group of individuals of the same species living in the same habitat at the same time

population growth rate

affected by food supply, predation & disease

sigmoid population growth curve

curve on a graph showing four main phases in the growth of a population of bacteria

lag phase

first phase in the growth of a population when doubling of the numbers has little effect as the numbers are so small

exponential (log) phase

second phase in the growth of a population, when the population is increasing rapidly by doubling, with no limiting factors (eg food/ water)

stationary phase

third phase in the growth of a population, when cells are dying at the same rate as they are being produced

death phase

fourth and final phase in the growth of a population, when more cells are dying than are being produced, because of oxygen shortage, lack of food or build-up of toxic waste products

human population growth

has increased exponentially over the past 300 years due to improved agriculture, nutrition, public health & medical care