Respiration
What you'll learn
- What cellular respiration is, and why every living cell needs it.
- How aerobic and anaerobic respiration compare in animals, plants and fungi.
- Why respiration is an exothermic reaction that supplies ATP.
- How sugars, amino acids, fatty acids and glycerol are used to build and break down biological molecules.
The big idea: cells need usable energy
A cell is not just a tiny bag of chemicals. It is constantly doing work: making new molecules, moving substances across membranes, growing, dividing, and in muscle cells, contracting.
All these activities are part of metabolism, which means all the chemical reactions happening in a cell or organism. Metabolic reactions are controlled by enzymes, which are biological catalysts: they speed up reactions without being used up.
Respiration is one of these metabolic processes. It uses organic molecules — carbon-based molecules found in living things, such as glucose — as fuels.
Cellular respiration
Cellular respiration is a universal chemical process, occurring continuously in living cells, that breaks down organic molecules and supplies ATP for cell activities.
ATP
ATP, or adenosine triphosphate, is a small molecule that transfers energy to cell processes such as active transport, muscle contraction and synthesis of new molecules.
Respiration is not breathing
This is one of the most important distinctions in the topic.
Ventilation means moving air in and out of the lungs. In everyday language, this is “breathing”.
Gas exchange means oxygen and carbon dioxide diffusing between air spaces and the blood, for example in the lungs.
Respiration is a chemical process inside cells. It happens in animals, plants, fungi and microorganisms.
Breathing is not respiration
Do not write that respiration is “breathing”. Breathing helps supply oxygen for aerobic respiration in many animals, but respiration itself is the chemical reaction inside cells.
Respiration is exothermic
An exothermic reaction transfers energy to the surroundings. Cellular respiration is exothermic because energy is transferred from the chemical store of molecules such as glucose.
In cells, not all the energy is lost as heat. Some is transferred to ATP, which is then used for cell processes.
Energy is transferred, not created
Respiration does not make energy from nothing. It transfers energy from organic molecules into ATP and heat.
A simple practical idea is to compare germinating seeds with dead seeds in insulated containers. Germinating seeds are alive and respiring; boiled seeds are dead, so they act as a control.
Interpreting temperature change
A student puts germinating seeds in one insulated flask and boiled dead seeds in another. Both start at 21.0 °C. After one hour, the germinating seeds are at 24.5 °C and the dead seeds are at 21.2 °C.
- Compare the temperature changes: the germinating seeds increase by 3.5 °C, while the dead seeds increase by only 0.2 °C.
- Link the larger increase to living cells: the germinating seeds are respiring continuously, so chemical energy is being transferred.
- Use the control: because the dead seeds show almost no temperature rise, the larger rise is evidence that respiration is exothermic.
Aerobic respiration
Aerobic respiration is respiration using oxygen. The main fuel, or substrate, is usually glucose.
The word equation is:
glucose + oxygen → carbon dioxide + water
Aerobic respiration releases a relatively large amount of ATP from each glucose molecule. In animal and plant cells, much of aerobic respiration happens in the mitochondria.
Anaerobic respiration
Anaerobic respiration is respiration without oxygen. It still supplies ATP, but the ATP yield is much lower than in aerobic respiration because glucose is not broken down as completely.
In animals, such as humans:
glucose → lactic acid
This can happen in muscle cells during vigorous exercise, when oxygen cannot be supplied quickly enough.
In plants and fungi, including yeast:
glucose → ethanol + carbon dioxide
This is often called fermentation. Yeast fermentation is useful in bread-making and brewing because it produces carbon dioxide and ethanol.
Comparing aerobic and anaerobic respiration
| Feature | Aerobic respiration | Anaerobic respiration in animals | Anaerobic respiration in plants and fungi |
|---|---|---|---|
| Oxygen needed? | Yes | No | No |
| Main substrate | Glucose and oxygen | Glucose | Glucose |
| Products | Carbon dioxide and water | Lactic acid | Ethanol and carbon dioxide |
| Relative ATP yield | High | Low | Low |
| Typical example | Most cells when oxygen is available | Muscle cells during intense exercise | Yeast fermenting sugar |
Spotting the pathway
If oxygen is present, think aerobic respiration. If oxygen is absent, think anaerobic respiration — then use the organism to choose the products.
Identifying yeast fermentation
A sealed tube contains yeast and glucose solution. Bubbles are produced and limewater turns milky. No oxygen is supplied.
- Use the condition first: no oxygen means the process is anaerobic respiration, not aerobic respiration.
- Use the organism: yeast is a fungus, so its anaerobic products are ethanol and carbon dioxide.
- Use the evidence: the bubbles and milky limewater show carbon dioxide, which matches fermentation in yeast.
Plants respire too
Plants respire continuously, in light and dark. In the light, they may also photosynthesise, but photosynthesis does not replace respiration.
Why “breakdown” matters
In respiration, larger or energy-rich molecules are broken down chemically. This does not mean they are just physically snapped into pieces like breaking a biscuit. It means their atoms are rearranged into different substances.
Breakdown, not breakup
Use “breakdown” for chemical reactions that change molecules into smaller or different molecules. “Break up” sounds like a physical action and is usually too vague for biology answers.
Biological molecules: building and breaking
Cells need to both synthesise molecules and break down molecules.
Synthesis means making a larger or more complex molecule from smaller units.
A monomer is a small molecule that can join with similar molecules. A polymer is a large molecule made from many repeating monomers.
Sugars and carbohydrates
Simple sugars such as glucose are important because they can be used directly in respiration.
Sugars can also be joined together to make carbohydrate polymers. For example, plants store glucose as starch, and animals store glucose as glycogen.
When carbohydrates are broken down, they release sugars that can be used again in respiration or in synthesis reactions.
Amino acids and proteins
Proteins are polymers made from amino acid monomers. Different proteins are made by joining amino acids in different sequences.
Proteins are vital because they include enzymes, structural proteins and transport proteins. Since enzymes control metabolic reactions, amino acids are needed to build the proteins that keep the cell’s chemistry working.
When proteins are broken down, amino acids are released and can be reused to make new proteins.
Fatty acids, glycerol and lipids
Lipids are made from fatty acids and glycerol. Unlike carbohydrates and proteins, lipids are not true polymers because they are not made from long chains of repeating monomers.
Lipids are important energy stores and are also used in cell membranes. When lipids are broken down, fatty acids and glycerol are released.
Choosing building blocks for synthesis
A cell is making a digestive enzyme and storing extra glucose as starch. Which building blocks are needed?
- Identify the enzyme as a protein, so it must be synthesised from amino acid monomers.
- Identify starch as a carbohydrate polymer, so it is made by joining sugar monomers such as glucose.
- Exclude lipid building blocks: fatty acids and glycerol are used to make lipids, not enzymes or starch.
How this links back to respiration
Respiration and biological molecules are closely connected.
Glucose is a key substrate for respiration. Carbohydrate breakdown can supply glucose when the cell needs ATP. Synthesis reactions, such as making proteins or storage carbohydrates, need ATP supplied by respiration.
So respiration is not a separate “energy topic” floating on its own. It supports the rest of cell metabolism.
In the exam
- If oxygen is present, choose aerobic respiration; if oxygen is absent, choose anaerobic respiration.
- Match anaerobic products to the organism: animals make lactic acid, while plants and fungi make ethanol and carbon dioxide.
- Say respiration “supplies ATP” or “transfers energy” — avoid saying cells “make energy”.
- Remember the building blocks: carbohydrates from sugars, proteins from amino acids, and lipids from fatty acids plus glycerol.
Check yourself
- Why is cellular respiration described as universal and continuous?
- What are the products of anaerobic respiration in animals compared with yeast?
- Which smaller molecules are used to synthesise carbohydrates, proteins and lipids?
