Everything from the 4 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
The Sun is the principal source of energy input to biological systems. Producers capture light energy in photosynthesis and store it as chemical energy in organic molecules such as glucose, starch and proteins. Chemical energy passes from organism to organism by feeding, but at every stage some is transferred to the environment, mainly as heat from respiration. Energy flows through an ecosystem in one direction; it is not recycled.
Energy is never "used up" or "made". Say it is transferred: from light to chemical energy, then to the environment as heat. And energy is not recycled: nutrients cycle, energy flows through.
Heat lost to the environment cannot be used by plants for photosynthesis. Only light energy can be captured, which is why the flow is one-way.
A food chain shows the transfer of energy from one organism to the next, beginning with a producer; a food web is a network of interconnected food chains. A trophic level is an organism's position in a food chain, food web or ecological pyramid: producers, then primary, secondary, tertiary and quaternary consumers. Pyramids of numbers and biomass compare the levels. Extended: only a small fraction of the energy at one trophic level reaches the next, so food chains usually have fewer than five trophic levels.
Arrows in a food chain show the direction of energy flow, so they point towards the eater: grass → rabbit means the rabbit eats the grass. Drawing them the other way loses the mark.
A pyramid of biomass is better than numbers, but it is still a snapshot at one moment. A fast-growing producer that is eaten quickly (such as plankton) can have a small standing biomass, so even biomass pyramids are occasionally inverted.
Energy is not lost "in" the food chain because animals are wasteful: most of it leaves as heat from respiration, which every organism must carry out to stay alive.
In the carbon cycle, photosynthesis removes carbon dioxide from the air; feeding passes carbon compounds along food chains; respiration by plants, animals and decomposers and combustion of fossil fuels return carbon dioxide to the air; dead material that does not decompose can form fossil fuels over millions of years. Extended: in the nitrogen cycle microorganisms carry out decomposition, nitrification, nitrogen fixation and denitrification, while plants absorb nitrate ions to make amino acids and proteins.
Plants respire all the time, day and night, as well as photosynthesising. Do not draw respiration arrows only from animals; plants and decomposers release carbon dioxide too.
Nitrification and nitrogen fixation are different processes. Fixation starts from nitrogen gas; nitrification starts from ammonium ions already in the soil. Denitrification is the one that returns nitrogen gas to the air.
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, and an ecosystem is a unit containing the community of organisms and their environment, interacting together. The rate of population growth depends on food supply, competition, predation and disease. In an environment with limited resources a population follows a sigmoid curve with lag, exponential (log), stationary and death phases.
In the stationary phase organisms are still being born and still dying. The population stays level because the two rates are equal, not because reproduction has stopped.
Do not say the population levels off "because it is full". Name the limiting factor (food, space, oxygen) and say that it makes the birth rate fall to equal the death rate.