Bioenergetics — GCSE Biology Revision
Everything you need to revise bioenergetics for GCSE Biology: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Photosynthesis: The Basics
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, stored in glucose. This vital process uses carbon dioxide from the atmosphere and water absorbed by roots. It occurs primarily in the chloroplasts of plant cells, which contain chlorophyll – a green pigment that absorbs light energy. Oxygen is produced as a waste product and released into the atmosphere, essential for most life forms on Earth. Understanding the inputs and outputs is crucial.
The Photosynthesis Equation
The overall word equation for photosynthesis is: Carbon dioxide + Water → Glucose + Oxygen. The balanced symbol equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Light energy is required for this reaction to take place, often written above the arrow. This equation summarises the transformation of inorganic compounds into organic food molecules, powering almost all ecosystems. It's an endothermic reaction, meaning it takes in energy from the surroundings.
Factors Affecting Photosynthesis Rate
The rate of photosynthesis can be limited by several factors. These include light intensity, carbon dioxide concentration, and temperature. If any of these factors are in short supply, they become a 'limiting factor' – increasing them will increase the rate of photosynthesis, up to a point. Other factors like chlorophyll concentration and water availability can also affect the rate. Farmers often manipulate these factors in greenhouses to optimise crop yield.
Respiration: Releasing Energy
Respiration is the process by which living organisms release energy from organic substances like glucose. This energy is then used for metabolic processes, such as muscle contraction, active transport, and maintaining body temperature. There are two main types: aerobic respiration (with oxygen) and anaerobic respiration (without oxygen). All living cells respire continuously to fuel their activities. It's an exothermic reaction, releasing energy.
Key facts
- Photosynthesis converts light energy into chemical energy (glucose).|Chlorophyll absorbs light energy for photosynthesis.|Aerobic respiration requires oxygen and releases a lot of energy.|Anaerobic respiration occurs without oxygen, releasing less energy.|Limiting factors control the rate of photosynthesis.|Respiration is an exothermic process; photosynthesis is endothermic.
Key terms
- Photosynthesis::The process by which plants and other organisms convert light energy into chemical energy, making food.|Chloroplast::An organelle in plant cells where photosynthesis takes place, containing chlorophyll.|Limiting factor::A factor that restricts the rate of a process, like photosynthesis, if it is in short supply.|Aerobic respiration::Respiration that requires oxygen to release a large amount of energy from glucose.|Anaerobic respiration::Respiration that occurs without oxygen, releasing a smaller amount of energy from glucose.|Glucose::A simple sugar (C₆H₁₂O₆) produced during photosynthesis and broken down during respiration.
Common mistakes
- Confusing the products and reactants of photosynthesis and respiration.|Forgetting that anaerobic respiration also produces energy, just less than aerobic.|Thinking that plants only respire at night and only photosynthesise during the day.|Not understanding that light intensity, CO2 concentration, and temperature are interconnected as limiting factors.
Exam tips
- Practise balancing the photosynthesis and respiration equations until they are second nature.|Be able to describe and explain the effect of each limiting factor on photosynthesis rate.|Learn the word and symbol equations for aerobic and anaerobic respiration in humans and yeast.|Clearly explain the uses of energy released during respiration in an organism.
Quick quiz
1. Which organelle is the primary site of photosynthesis in a plant cell?
- Mitochondrion
- Nucleus
- Chloroplast
- Vacuole
Show answer
Chloroplast — Chloroplasts contain chlorophyll, the green pigment that absorbs light energy for photosynthesis.
2. What are the raw materials needed for photosynthesis?
- Glucose and Oxygen
- Carbon Dioxide and Water
- Water and Oxygen
- Glucose and Carbon Dioxide
Show answer
Carbon Dioxide and Water — Photosynthesis uses carbon dioxide from the air and water from the soil to produce glucose and oxygen.
3. Which of the following is a limiting factor for the rate of photosynthesis?
- Oxygen concentration
- Nitrogen concentration
- Light intensity
- Atmospheric pressure
Show answer
Light intensity — Light intensity, carbon dioxide concentration, and temperature are the main limiting factors for photosynthesis.
4. What is the primary purpose of respiration in living organisms?
- To produce oxygen
- To make glucose
- To release energy from food
- To absorb light
Show answer
To release energy from food — Respiration breaks down organic substances like glucose to release energy for cellular activities.
5. What is the word equation for aerobic respiration?
- Glucose + Oxygen → Carbon Dioxide + Water + Energy
- Carbon Dioxide + Water → Glucose + Oxygen + Energy
- Glucose → Lactic Acid + Energy
- Ethanol + Carbon Dioxide → Glucose + Oxygen
Show answer
Glucose + Oxygen → Carbon Dioxide + Water + Energy — Aerobic respiration requires oxygen to completely break down glucose, producing carbon dioxide, water, and a large amount of energy.
Exam-style questions
Describe the overall process of photosynthesis, including the key reactants and products. [3 marks]
Show mark scheme
- Photosynthesis is the process where plants convert light energy into chemical energy (in glucose).
- It uses carbon dioxide and water as reactants.
- It produces glucose and oxygen as products.
Explain how increasing light intensity affects the rate of photosynthesis. Include any limitations. [4 marks]
Show mark scheme
- As light intensity increases, the rate of photosynthesis increases.
- This is because light provides the energy for the reaction.
- However, at very high light intensities, the rate will plateau.
- This is because another factor, such as CO₂ concentration or temperature, becomes limiting.
Compare aerobic and anaerobic respiration in terms of oxygen requirement and energy released. [4 marks]
Show mark scheme
- Aerobic respiration requires oxygen, whereas anaerobic respiration does not.
- Aerobic respiration releases a large amount of energy.
- Anaerobic respiration releases a much smaller amount of energy.
- Both processes break down glucose.
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