Ch 2 · 12.2 Respiration · AS & A Level Biology 9700

Topic 2 of 2 in Energy and respiration — AS & A Level Biology 9700 — 78 lessons.

Introduction

Respiration is the controlled release of energy from organic molecules to generate ATP. In eukaryotes this happens in four linked stages: glycolysis in the cytoplasm, then the link reaction, the Krebs cycle and oxidative phosphorylation, all associated with the mitochondrion. Each stage strips hydrogen from the substrate and ultimately delivers it to an electron transport chain, where most of the cell's ATP is made. (Syllabus section 12.2.)

Key ideas — The mitochondrion

The mitochondrion is the site of the later stages of aerobic respiration, and its structure reflects this role.

🧑‍🏫 Teacher's explanation

Think of the mitochondrion as a highly specialized power plant within a cell. A power plant needs a large floor space to house massive turbines and a controlled environment to manage energy flow. In the cell, the 'floor space' is the inner membrane (cristae), which is folded to provide maximum surface area for the machinery that produces ATP.

📖 Definitions

🔍 Worked example — Easy

A student observes a mitochondrion under an electron microscope. They note that the inner membrane is highly folded. Explain how this structural feature relates to the function of the mitochondrion.

🔍 Worked example — Standard

Calculate the percentage increase in surface area if the inner membrane folds from a flat surface of 20 mum^2 to a total cristae surface area of 150 mum^2.

🔍 Worked example — Exam-style

A respirometer is used to measure the rate of oxygen consumption in yeast. If the volume of oxygen consumed increases from 0.5 cm^3 to 1.2 cm^3 in 10 minutes, calculate the rate of respiration in cm^3 min^-1.

⚠️ Common errors

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🎯 Exam question

Key ideas — Glycolysis

Glycolysis takes place in the cytoplasm and does not require oxygen.

🧑‍🏫 Teacher's explanation

Think of glycolysis as the 'splitting' stage of a fuel source. Imagine you have a large, stable log (glucose) that is hard to burn directly. First, you have to chop it into smaller, more reactive pieces (phosphorylation) to make it unstable enough to break apart. Once it splits into two smaller pieces (triose phosphate), it is much easier to burn completely.

📖 Definitions

🔍 Worked example — Easy

Calculate the net yield of ATP produced from the glycolysis of one molecule of glucose.

🔍 Worked example — Standard

A student is investigating the rate of respiration in yeast using DCPIP. If the color change from blue to colorless is observed faster at 40^°C than at 20^°C, what does this indicate about the rate of glycolysis?

🔍 Worked example — Exam-style

Complete the following sequence for the oxidation of glucose: Glucose (6C) arrow Fructose 1,6-bisphosphate (6C) arrow [X] (3C) arrow [Y] (3C).

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🎯 Exam question

Key ideas — The link reaction and the Krebs cycle

When oxygen is available, pyruvate is actively transported into the mitochondrial matrix. Both stages occur there.

🧑‍🏫 Teacher's explanation

Think of aerobic respiration as a highly efficient factory assembly line. Glycolysis is the initial breakdown in the factory floor (cytoplasm), but to get the maximum energy out, the products must be sent into a specialized high-energy workshop: the mitochondria.

📖 Definitions

🔍 Worked example — Easy

Calculate the total number of carbon atoms released as CO_2 during the link reaction and the Krebs cycle for one single molecule of glucose.

🔍 Worked example — Standard

A student measures the rate of respiration in yeast using a respirometer. The volume of oxygen consumed in 5 minutes is 0.4 cm^3. Calculate the rate of oxygen consumption per minute.

🔍 Worked example — Exam-style

Compare the fate of pyruvate in a muscle cell during vigorous exercise (anaerobic) versus a muscle cell at rest (aerobic).

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🎯 Exam question

Key ideas — Oxidative phosphorylation

Oxidative phosphorylation occurs across the inner mitochondrial membrane and generates the majority of ATP in aerobic respiration.

🧑‍🏫 Teacher's explanation

Think of oxidative phosphorylation as a hydroelectric dam. The electron transport chain acts like a pump, using the energy from flowing electrons to push protons (water) into a high-altitude reservoir (the intermembrane space). This creates a massive amount of potential energy due to the concentration gradient. When these protons flow back down through the 'turbines' (ATP synthase), they drive the production of ATP.

📖 Definitions

🔍 Worked example — Easy

State the location of oxidative phosphorylation and the final electron acceptor in a eukaryotic cell.

🔍 Worked example — Standard

Explain why the production of ATP decreases if oxygen is absent.

🔍 Worked example — Exam-style

Calculate the percentage decrease in ATP yield if a cell shifts from aerobic respiration (yielding 36 ATP per glucose) to anaerobic respiration (yielding 2 ATP per glucose).

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🎯 Exam question

Key ideas — Anaerobic respiration and energy yield

Without oxygen, the electron transport chain stops, reduced NAD cannot be reoxidised, and the link reaction and Krebs cycle stop. Cells can still make a little ATP by glycolysis if they regenerate NAD another way:

🧑‍🏫 Teacher's explanation

Think of aerobic respiration like a high-efficiency furnace that burns fuel (glucose) completely to extract every bit of heat (ATP) possible. When oxygen is present, the furnace runs perfectly, producing lots of energy and just a bit of waste (CO_2 and H_2O).

📖 Definitions

🔍 Worked example — Easy

Calculate the percentage decrease in ATP yield if a cell switches from aerobic respiration (yielding 38 ATP per glucose) to anaerobic respiration (yielding 2 ATP per glucose).

🔍 Worked example — Standard

A student investigates the rate of respiration in yeast using DCPIP. If the color change from blue to colorless represents the rate of respiration, explain why the rate of color change increases as the temperature increases from 20^°C to 40^°C.

🔍 Worked example — Exam-style

Compare the end products of anaerobic respiration in yeast and mammalian muscle cells, specifying the number of carbon atoms in each product.

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🎯 Exam question

Key ideas — Rice: growing with roots under water

Paddy fields are flooded, so rice roots are surrounded by water, which holds little oxygen. Rice is adapted in three ways:

🧑‍🏫 Teacher's explanation

Imagine you are in a room that is slowly filling with water. If you want to survive, you need two things: a way to get air from the surface down to your feet, and a way to survive the toxic buildup of gases if you can't get enough air. Rice plants face this exact crisis in flooded paddy fields. Because oxygen diffuses very slowly through water compared to air, the roots are essentially 'suffering' from hypoxia (low oxygen).

📖 Definitions

🔍 Worked example — Easy

A rice plant root is submerged in water. Explain why the presence of aerenchyma is essential for the survival of the root cells.

🔍 Worked example — Standard

Compare the metabolic pathways used by rice roots when oxygen is abundant versus when oxygen is depleted.

🔍 Worked example — Exam-style

A researcher observes that rice plants in flooded conditions show significantly increased stem elongation compared to plants in well-drained soil. Explain the evolutionary advantage of this trait.

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🎯 Exam question

(a) State two structural or physiological adaptations of rice that allow it to survive in these conditions. [2] (b) Explain how aerenchyma facilitates the survival of the roots. [2] [4 marks]

Key ideas — Investigating the rate of respiration

Redox indicators take up hydrogen in place of NAD and change colour when reduced: DCPIP goes from blue to colourless and methylene blue from blue to colourless. Mix a yeast suspension with glucose solution in a water bath, add a few drops of indicator, and time how long it takes to lose its colour. Rate = 1 ÷ time.

🧑‍🏫 Teacher's explanation

Imagine you are monitoring the speed of a factory assembly line. To see how fast it is working, you can either count how many finished products come out the other end, or you can measure how much raw material is being consumed. In biology, respiration is like that assembly line, and we use 'indicators' or 'respirometers' to measure its speed.

📖 Definitions

🔍 Worked example — Easy

A yeast suspension is placed in a glucose solution with methylene blue. The indicator turns from blue to colourless in 120 seconds. Calculate the rate of respiration in s^-1.

🔍 Worked example — Standard

In a respirometer experiment, the liquid level in the tube moves 4.0 mm in 5 minutes. Calculate the rate of oxygen uptake in mm · min^-1.

🔍 Worked example — Exam-style

An experiment measures the time taken for DCPIP to decolour in yeast suspensions at different temperatures. At 20°C, time = 300 s. At 40°C, time = 150 s. Calculate the percentage increase in the rate of respiration when the temperature increases from 20\ ext°C to 40°C.

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🎯 Exam question

Worked example

Question. A student added methylene blue to tubes of yeast suspension and glucose solution at different temperatures, and timed how long the blue colour took to disappear. At 30 °C it took 250 s; at 40 °C it took 125 s. (a) Explain why the methylene blue lost its colour. [2] (b) Calculate the rate of respiration at each temperature, as 1000 ÷ time, and explain the difference. [3]

Ch 2 · 12.2 Respiration · AS & A Level Biology 9700

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