CIE AS/A Level Biology -12.1 Energy- Study Notes- New Syllabus
CIE AS/A Level Biology -12.1 Energy- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -12.1 Energy- Study Notes- New Syllabus
Key Concepts:
- outline the need for energy in living organisms, as illustrated by active transport, movement and anabolic reactions, such as those occurring in DNA replication and protein synthesis
- describe the features of ATP that make it suitable as the universal energy currency
- state that ATP is synthesised by:
• transfer of phosphate in substrate-linked reactions
• chemiosmosis in membranes of mitochondria and chloroplasts - explain the relative energy values of carbohydrates, lipids and proteins as respiratory substrates
- state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration
- calculate RQ values of different respiratory substrates from equations for respiration
- describe and carry out investigations, using simple respirometers, to determine the RQ of germinating seeds or small invertebrates (e.g. blowfly larvae)
Energy in Living Organisms
🌱 Why Living Organisms Need Energy?
Living organisms require a continuous supply of energy to carry out essential processes that maintain life. The main source of energy is ATP (adenosine triphosphate), produced during cellular respiration.
🔋 Examples of Energy Use
1. Active Transport
- Movement of substances against a concentration gradient (low → high).
- Requires ATP to power carrier proteins or pumps in membranes.
- Examples:
- Uptake of mineral ions by root hair cells.
- Sodium–potassium pump in neurons.
2. Movement
- Energy needed for muscle contraction.
- ATP provides energy for actin and myosin filament sliding in muscle fibres.
- Also needed for:
- Beating of cilia and flagella.
- Cytoplasmic streaming inside plant cells.
3. Anabolic Reactions (Building Up Molecules)
- Energy required for biosynthesis of large molecules from smaller units.
- DNA Replication:
- ATP provides energy for joining nucleotides by DNA polymerase.
- Needed for unwinding DNA strands (helicase action).
- Protein Synthesis:
- ATP required for activation of amino acids and their attachment to tRNA.
- Energy also needed for peptide bond formation during translation.
- Other anabolic reactions: synthesis of polysaccharides (e.g., starch, glycogen) and lipids.
📌 Key Point
Energy is essential for maintaining order and function in cells. Without ATP, active processes stop, leading to cell death.
🧠 Summary Box:
Energy use in organisms:
– Active transport → movement of molecules across membranes.
– Movement → muscle contraction, cilia, flagella.
– Anabolic reactions → DNA replication, protein synthesis, macromolecule formation.
ATP is the universal energy currency that powers these processes.
Features of ATP as the Universal Energy Currency
🌱 What is ATP?
ATP (Adenosine Triphosphate) is a small, soluble nucleotide molecule. It carries immediately usable energy for cellular processes and is produced mainly in the mitochondria during respiration.
⭐ Features Making ATP Suitable
- Immediate energy source: Hydrolysis of ATP → ADP + Pi releases small, controlled amounts of energy. Prevents energy wastage compared to releasing large amounts at once.
- Rapidly regenerated: ATP is quickly resynthesised from ADP + Pi during respiration, ensuring a continuous energy supply.
- Soluble and easily transported: Diffuses readily within cells, supplying energy to all parts of the cell.
- Universal energy carrier: Found in all living organisms; powers processes like active transport, movement, and biosynthesis.
- Releases energy in a single reaction: Only one bond (terminal phosphate) is broken to release energy, making it efficient.
- Coupling of reactions: ATP hydrolysis can be directly linked to energy-requiring processes, ensuring effective energy use.
- Small but sufficient energy yield: About 30.5 kJ mol⁻¹ released per ATP hydrolysed – enough to drive cellular reactions without excessive energy lost as heat.
ATP is small, soluble, rapidly regenerated, and universal. It provides controlled, immediate, and usable energy for all cells. This makes ATP the universal energy currency of life.
Synthesis of ATP in Cells
🌱 ATP Formation Pathways
ATP can be synthesised by two main mechanisms in cells:
Substrate-Level Phosphorylation
- Definition: Direct transfer of a phosphate group from a phosphorylated substrate to ADP → ATP.
- Occurs in:
- Glycolysis (cytoplasm)
- Krebs cycle (mitochondrial matrix)
- Example: In glycolysis, phosphoenolpyruvate (PEP) donates a phosphate to ADP → ATP.
- Key feature: Does not require oxygen or an electron transport chain.
Chemiosmosis
- Definition: The synthesis of ATP using the energy of a proton (H⁺) gradient across a membrane.
- Mechanism:
- Electrons pass along an electron transport chain (ETC) in mitochondria or chloroplasts.
- This drives pumping of protons across a membrane → creates a proton gradient.
- Protons flow back through ATP synthase by facilitated diffusion.
- The energy released phosphorylates ADP + Pi → ATP.
- Occurs in:
- Mitochondria (inner membrane – oxidative phosphorylation).
- Chloroplasts (thylakoid membranes – photophosphorylation).
- Key feature: Oxygen acts as the final electron acceptor in mitochondria, forming water.
🧠 Summary Box:
ATP is synthesised in two ways:
• Substrate-level phosphorylation – direct phosphate transfer.
• Chemiosmosis – proton gradient and ATP synthase in membranes.
Together, these pathways supply the cell with its universal energy currency.
Energy Values of Respiratory Substrates
🌱 Respiratory Substrates
Respiratory substrates are organic molecules oxidised in respiration to release ATP. The main substrates are:
- Carbohydrates (mainly glucose)
- Lipids (fats)
- Proteins (amino acids, under special conditions)
🔬 Relative Energy Values
| Respiratory Substrate | Energy Value (kJ g⁻¹) | Explanation |
|---|---|---|
| Carbohydrates (e.g., glucose) | ~15.8 | Contain many C–H bonds; easily oxidised. Glucose is the primary substrate because it can directly enter glycolysis. |
| Lipids (e.g., triglycerides) | ~39.4 (about 2× carbohydrates) | Contain more C–H bonds per gram → more reduced → release more ATP per gram. Oxidised via glycerol (→ glycolysis) and fatty acids (→ Krebs cycle via acetyl-CoA). |
| Proteins (amino acids) | ~17.0 | Usually a last-resort substrate (during starvation). Amino acids must be deaminated (removal of –NH₂ group) before entering respiration → less efficient. |
📌 Key Points to Remember
- Lipids provide the highest energy yield per gram (more hydrogen atoms to oxidise → more NADH/FADH₂ → more ATP).
- Carbohydrates are the preferred substrate because glucose can be respired quickly and efficiently.
- Proteins are not usually used unless carbohydrates and lipids are depleted.
• Carbohydrates: 15.8 kJ g⁻¹ → primary, quick energy source.
• Lipids: 39.4 kJ g⁻¹ → most energy-dense substrate.
• Proteins: 17 kJ g⁻¹ → emergency substrate (less efficient due to deamination).
Respiratory Quotient (RQ)
🌱 Definition
The respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide (CO₂) produced to the number of molecules of oxygen (O₂) taken in during respiration.
🔬 Typical RQ Values
| Respiratory Substrate | RQ Value | Reason |
|---|---|---|
| Carbohydrates (e.g., glucose) | 1.0 | Equal amounts of CO₂ produced and O₂ consumed (e.g., C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). |
| Lipids | ~0.7 | Require more O₂ for oxidation compared to CO₂ produced (due to high hydrogen content). |
| Proteins | ~0.8–0.9 | Intermediate value; depends on amino acid type and deamination process. |
📌 Key Notes
- RQ indicates which substrate is being respired:
- 1.0 → mainly carbohydrates
- 0.7 → mainly lipids
- ~0.8–0.9 → mainly proteins
- RQ > 1 → anaerobic respiration occurring (extra CO₂ from lactate buffering).
RQ is the ratio CO₂ produced / O₂ consumed.
• Carbohydrates: 1.0
• Lipids: 0.7
• Proteins: ~0.8–0.9
Useful for identifying which substrate is being respired.
Respiratory Quotient (RQ)
📌 Definition
Respiratory Quotient (RQ) = ratio of CO₂ produced to O₂ consumed during respiration.
📌 Calculation from Equations
1. Carbohydrate (Glucose)
Equation:
CO₂ produced = 6, O₂ consumed = 6
🔹 Interpretation: Carbohydrates have RQ = 1.0.
2. Fat (e.g., Tripalmitin \( C_{51}H_{98}O_6 \))
Equation:
CO₂ produced = 102, O₂ consumed = 145
🔹 Interpretation: Fats have RQ ≈ 0.7 (less CO₂ per O₂).
3. Protein (average amino acid)
General equation (simplified):
Typically, RQ ≈ 0.8–0.9
🔹 Interpretation: Proteins produce less CO₂ relative to O₂ than carbohydrates.
4. Anaerobic Respiration (Yeast)
Equation:
No O₂ used → O₂ consumed = 0, CO₂ produced = 2
🔹 Interpretation: RQ is not meaningful for anaerobic respiration.
📊 Summary Table
| Respiratory Substrate | General RQ Value |
|---|---|
| Carbohydrate (glucose) | 1.0 |
| Fat | ≈ 0.7 |
| Protein | ≈ 0.8–0.9 |
| Anaerobic | Undefined / ∞ |
🧠 Key Points
- RQ > 1.0 → anaerobic respiration taking place (excess CO₂).
- RQ < 1.0 → fat or protein being used as substrate.
- RQ = 1.0 → pure carbohydrate respiration.
Investigating Respiratory Quotient (RQ) Using Simple Respirometers
📌 Principle
A respirometer measures oxygen consumption and/or carbon dioxide production during respiration.
Respiratory Quotient (RQ):
RQ helps determine the respiratory substrate being used.
🔬 Apparatus
- Simple respirometer (with manometer or capillary tube containing coloured fluid)
- Potassium hydroxide (KOH) or soda lime → absorbs CO₂
- Germinating seeds or small invertebrates (e.g. blowfly larvae)
- Control tube (no organisms, to account for pressure/temperature changes)
- Stopwatch
- Water bath (constant temperature)
⚙️ Method
- Place the living organisms (e.g. germinating seeds) in one tube of the respirometer.
- Add KOH/soda lime to absorb any CO₂ produced.
- Connect to a manometer with coloured fluid.
- Set up a control tube with no organisms.
- Keep in a water bath to maintain constant temperature.
- Record movement of the manometer fluid over time → indicates oxygen uptake.
- Repeat without CO₂ absorbent → allows measurement of both O₂ consumed and CO₂ released.
- Use the formula to calculate RQ.
📊 Typical RQ Values
| Substrate | RQ Value | Explanation |
|---|---|---|
| Carbohydrates | 1.0 | Equal O₂ consumed and CO₂ released |
| Lipids | ~0.7 | More O₂ used, less CO₂ released |
| Proteins | ~0.8–0.9 | Intermediate values |
| Anaerobic respiration | >1.0 | Extra CO₂ produced without O₂ uptake |
🧠 Key Notes
- KOH/soda lime removes CO₂ → ensures only O₂ uptake is measured.
- The manometer fluid movement shows change in gas volume.
- By comparing experiments with and without CO₂ absorbent, both O₂ and CO₂ values can be determined.
- RQ indicates whether the organism is using carbohydrates, fats, or proteins, or if anaerobic respiration is occurring.