30 terms in 3.5
Energy and ecosystems
Plants use carbon dioxide from the air or water to build organic molecules, such as sugars. This process makes plants the entry point for carbon and energy in every ecosystem.
Energy transfers in and between organisms
Energy and ecosystems
Plants produce sugars — primarily glucose — during photosynthesis. These sugars have two possible fates once made. The majority enter respiration, where cells break them down to release ATP (the cell's usable energy currency). This ATP powers every active process in the plant: active transport, cell division, protein synthesis, and more. The energy released during respiration is ultimately lost t
Energy transfers in and between organisms
Energy and ecosystems
Biomass (the total mass of organic material in organisms) can be measured in two main ways: 1. **Dry mass of tissue per unit area** — fresh tissue contains variable amounts of water, which stores no chemical energy and skews comparisons. Drying the sample (usually in an oven at around 80 °C until mass is constant) removes all water, leaving only organic and inorganic solids. Results are expressed
Energy transfers in and between organisms
Energy and ecosystems
Gross primary production (GPP) measures all the chemical energy that plants lock into their biomass through photosynthesis. It covers a specific area or volume of an ecosystem.
Energy transfers in and between organisms
Energy and ecosystems
Plants use some of their captured energy for their own respiration. Net primary production (NPP) is the energy that remains after subtracting those respiratory losses from the total energy fixed.
Energy transfers in and between organisms
Energy and ecosystems
After plants use some energy for respiration, the remaining energy — called net primary production — goes into growing new plant tissue. Herbivores and other organisms can then access that stored energy by eating the plant.
Energy transfers in and between organisms
Energy and ecosystems
Animals cannot use all the energy in their food. Net production is the energy left over after losses in faeces, urine, and respiration — energy available to build new biomass.
Energy transfers in and between organisms
Energy and ecosystems
Productivity measures how fast an organism builds up biomass. Primary productivity applies to plants; secondary productivity applies to animals. Both use units of energy per area per year.
Energy transfers in and between organisms
Energy and ecosystems
Energy efficiency in farming means maximising the proportion of energy from crops or livestock that reaches humans. Two broad strategies achieve this. **Strategy 1 — Simplifying food webs.** In a natural ecosystem, energy leaks into many non-human food chains: insects eat crops, foxes eat chickens, weeds compete with wheat. Farmers reduce these losses by: 1. Using pesticides to kill insects and o
Energy transfers in and between organisms
Energy and ecosystems
Plants capture energy from sunlight and lock it into organic molecules, but not all of that energy is available to the rest of the ecosystem — some is lost through respiration (the process cells use to release energy for life processes). By distinguishing between gross primary production (the total chemical energy fixed by plants) and net primary production (what remains after respiratory losses), you can quantify exactly how much energy is available to pass up through trophic levels — the feeding levels of a food chain. Understanding these energy budgets explains why food chains are short, why farming practices aim to reduce energy losses, and how productivity — the rate at which biomass is built up — can be measured and improved.
Energy transfers in and between organisms
Nutrient cycles
Unlike energy, nutrients such as nitrogen and phosphorus are not lost from ecosystems. Living organisms, dead matter, and the environment continuously pass these elements between each other in repeating cycles.
Energy transfers in and between organisms
Nutrient cycles
Microorganisms break down dead organic matter and convert chemical elements into forms that living organisms can reuse. Without them, essential elements like nitrogen and phosphorus would stay locked in dead material forever.
Energy transfers in and between organisms
Nutrient cycles
Saprobionts are organisms that break down dead organic matter. They release nutrients locked inside dead material back into the environment for other organisms to reuse.
Energy transfers in and between organisms
Nutrient cycles
Mycorrhizae are fungi that grow into and around plant roots. They massively increase the surface area available for absorbing water and mineral ions from the soil.
Energy transfers in and between organisms
Nutrient cycles
Bacteria drive the nitrogen cycle by converting nitrogen compounds between different forms. Each type of bacterium performs a specific job — from breaking down dead matter to fixing atmospheric nitrogen gas into usable compounds.
Energy transfers in and between organisms
Nutrient cycles
You do not need to memorise the scientific names of specific bacteria. Focus on what each type of bacteria does in the nitrogen cycle, not what it is called.
Energy transfers in and between organisms
Nutrient cycles
When farmers harvest crops or remove animals, they take nutrients out of the soil permanently. Fertilisers — either natural or artificial — replace the nitrates and phosphates that would otherwise be gone.
Energy transfers in and between organisms
Nutrient cycles
Fertilisers added to farmland can wash into rivers and lakes. This triggers a chain of events that removes oxygen from the water and kills aquatic life.
Energy transfers in and between organisms
Nutrient cycles
Unlike energy, which flows in one direction through an ecosystem and is lost as heat, chemical elements such as nitrogen and phosphorus are continuously recycled — broken down, transformed, and made available again for living organisms to use. Microorganisms are central to this process: saprobionts (decomposers that feed on dead organic matter) and specialised bacteria convert nutrients between different chemical forms, while mycorrhizae (fungi that form close associations with plant roots) help plants absorb the inorganic ions that result. Understanding these cycles also explains why human activities like applying fertilisers can disrupt ecosystems, leading to problems such as eutrophication — where excess nutrients trigger algal overgrowth that ultimately depletes oxygen in water.
Energy transfers in and between organisms
Photosynthesis
The light-dependent reaction takes place on the thylakoid membranes inside the chloroplast — think of these as stacked, flattened sacs packed with green chlorophyll pigment. 1. Chlorophyll absorbs light energy. This causes photoionisation: electrons in chlorophyll gain enough energy to escape the molecule entirely. 2. These high-energy electrons pass along a series of proteins called the electron
Energy transfers in and between organisms
Photosynthesis
The light-independent reaction takes molecules made during the light-dependent reaction and uses their energy to build a simple sugar. It needs both reduced NADP and ATP to do this.
Energy transfers in and between organisms
Photosynthesis
The Calvin cycle runs continuously in the stroma (the fluid-filled space) of the chloroplast. Think of it as a molecular assembly line that fixes atmospheric carbon dioxide into usable organic molecules. 1. Carbon dioxide (CO₂) from the air combines with a five-carbon acceptor molecule called ribulose bisphosphate (RuBP). The enzyme rubisco (ribulose bisphosphate carboxylase/oxygenase) catalyses
Energy transfers in and between organisms
Photosynthesis
Photosynthesis converts light energy into chemical energy stored in organic molecules, and at A-level you study exactly how this happens across two linked stages inside the chloroplast. The light-dependent reaction uses light to split water and generate ATP (the cell's energy currency) and reduced NADP (an electron carrier), while the light-independent reaction — the Calvin cycle — uses those products to fix carbon dioxide into triose phosphate, a simple sugar that can be built into glucose and other organic compounds. Understanding these mechanisms underpins the whole of energy transfer in ecosystems, and connects directly to how respiration, productivity, and nutrient cycles are driven by the organic molecules plants produce.
Energy transfers in and between organisms
Respiration
Cells need energy to do work. Respiration is the process that releases energy from glucose and uses it to build ATP — the molecule that directly powers everything a cell does.
Energy transfers in and between organisms
Respiration
Glycolysis — from the Greek for 'sugar splitting' — is the universal opening stage of respiration. Every living cell that respires begins here, whether or not oxygen is available. The process takes place in the cytoplasm, the fluid that fills the cell, rather than inside any organelle. Think of glycolysis as the entry gate to respiration: glucose must pass through it before anything else can happ
Energy transfers in and between organisms
Respiration
Glycolysis splits glucose into two smaller molecules called pyruvate. The cell spends a little ATP to start the process, then gains more ATP back — along with an energy-carrying molecule called reduced NAD.
Energy transfers in and between organisms
Respiration
When cells run out of oxygen, they convert pyruvate into either lactate or ethanol. This regenerates NAD so glycolysis can keep making ATP.
Energy transfers in and between organisms
Respiration
After glycolysis, pyruvate moves into the mitochondria to continue aerobic respiration. The cell uses energy to actively pump pyruvate across the mitochondrial membranes.
Energy transfers in and between organisms
Respiration
Aerobic respiration after glycolysis runs through three connected stages inside the mitochondria. 1. **Link reaction (mitochondrial matrix):** Pyruvate loses a carbon dioxide molecule and gets oxidised, forming a two-carbon acetate group. This oxidation reduces NAD to reduced NAD — think of NAD as an empty energy-carrier that picks up hydrogen here. Acetate immediately joins coenzyme A (a carrier
Energy transfers in and between organisms
Respiration
Every energy-demanding process in a living cell depends on ATP — a small molecule that acts as the cell's universal energy currency — and respiration is the metabolic pathway that produces it. Starting with glycolysis in the cytoplasm, glucose is broken down through a sequence of reactions that, in aerobic conditions, continues inside the mitochondria through the link reaction, Krebs cycle, and oxidative phosphorylation — a process in which electrons pass down a chain of proteins, driving the synthesis of large amounts of ATP. Understanding respiration explains not only how organisms release energy from glucose, but also how fats and amino acids feed into the same pathway, linking directly to how energy flows through entire ecosystems.
Energy transfers in and between organisms