Ch 1 · 12.1 Energy · AS & A Level Biology 9700

Topic 1 of 2 in Energy and respiration — AS & A Level Biology 9700 — 68 lessons.

Introduction

Every energy-requiring process in a cell, from pumping ions to building proteins, is powered by one molecule: ATP. Respiration does not release energy directly to these processes; instead it is used to regenerate ATP from ADP and inorganic phosphate, and the hydrolysis of ATP then drives the work of the cell. This lesson establishes why ATP, rather than glucose or fat, is the molecule cells actually spend, and why different respiratory substrates yield different amounts of it, ending with the respiratory quotient and how to measure it. (Syllabus section 12.1.)

Key ideas — ATP as the universal energy currency

ATP (adenosine triphosphate) is a nucleotide made of the base adenine, the pentose sugar ribose, and three phosphate groups joined in a chain. Several features make it ideal as an energy currency.

🧑‍🏫 Teacher's explanation

Imagine you are at a large international airport. You cannot buy a coffee with a gold bar, even though gold is worth a lot of money. The gold is too valuable for such a small transaction, and it is too heavy to carry in your pocket. Instead, you use small, standardized banknotes. ATP is the 'cash' of the cell. While glucose or lipids are like the 'gold bars' (large, stable energy stores), ATP is the small, portable, and instantly spendable currency that every cellular 'shop' accepts.

📖 Definitions

🔍 Worked example — Easy

Identify the three components that make up an ATP molecule.

🔍 Worked example — Standard

Calculate the Respiratory Quotient (RQ) for the aerobic respiration of glucose, given the equation: C_6H_12O_6 + 6O_2 arrow 6CO_2 + 6H_2O.

🔍 Worked example — Exam-style

A cell is performing active transport to pump ions against a concentration gradient. Explain why ATP is used for this process rather than glucose.

⚠️ Common errors

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🎯 Exam question

Key ideas — ATP synthesis as a phosphorylation reaction

ATP is synthesised by adding an inorganic phosphate group to ADP, a condensation reaction called phosphorylation:

🧑‍🏫 Teacher's explanation

Think of ATP as a rechargeable battery. Just as a battery stores electrical energy that can be used to power a device, ATP stores chemical energy that can be used to power cellular work. When the cell needs energy, it 'uses' the battery by breaking off a phosphate group; to 'recharge' it, the cell must add that phosphate group back on using energy from food (glucose) or light.

📖 Definitions

🔍 Worked example — Easy

A cell performs substrate-level phosphorylation. If the reaction ADP + P_i arrow ATP + H_2O occurs, identify the type of reaction and the enzyme required to catalyze the reverse reaction (hydrolysis).

🔍 Worked example — Standard

Compare the two methods of ATP synthesis mentioned in the syllabus.

🔍 Worked example — Exam-style

Calculate the change in the number of phosphate groups in a system where 5 molecules of ADP are converted to 5 molecules of ATP via chemiosmosis.

⚠️ Common errors

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🎯 Exam question

Key ideas — How cells use ATP

All organisms need energy to stay alive. ATP hydrolysis supplies it for three broad categories of work:

🧑‍🏫 Teacher's explanation

Think of ATP as the 'rechargeable battery' of the cell. While a cell might store large amounts of energy in 'power banks' like lipids or carbohydrates, it cannot use those large stores directly to power a tiny motor. Instead, it must convert that energy into ATP, which is small, portable, and ready to be 'spent' instantly.

📖 Definitions

🔍 Worked example — Easy

A cell uses ATP to power the sodium-potassium pump. If the hydrolysis of 1 mole of ATP releases 305 ext kJ of energy, calculate the total energy released when 0.05 ext moles of ATP are hydrolyzed.

🔍 Worked example — Standard

Compare the energy yield of a carbohydrate versus a lipid. If a carbohydrate provides 17 ext kJ/g and a lipid provides 38 ext kJ/g, calculate the percentage difference in energy density between the two.

🔍 Worked example — Exam-style

A respirometer measures a decrease in oxygen volume of 0.4 cm^3 and an increase in carbon dioxide volume of 0.3 cm^3 over 10 minutes in a germinating seed sample. Calculate the Respiratory Quotient (RQ).

⚠️ Common errors

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🎯 Exam question

(a) State the role of the potassium hydroxide (KOH) solution placed in the respirometer. [1] (b) Explain why the volume of gas in the respirometer decreases during respiration. [2] [3 marks]

Key ideas — Energy values and respiratory substrates

The energy value of a respiratory substrate is the quantity of energy released per gram when it is fully oxidised. Typical values are approximately 15.8 kJ g⁻¹ for carbohydrate, 39.4 kJ g⁻¹ for lipid, and 17.0 kJ g⁻¹ for protein.

🧑‍🏫 Teacher's explanation

Think of energy in a cell like the battery in your smartphone. You can have different types of fuel (carbohydrates, lipids, proteins), but the cell can only use one specific 'currency' to power its machinery: ATP. If you want to run a heavy app (like protein synthesis), you need a lot of ATP; if you just want to keep the clock running (maintaining ion gradients), you need less.

📖 Definitions

🔍 Worked example — Easy

Calculate the RQ for the aerobic respiration of glucose, given the equation: C_6H_12O_6 + 6O_2 arrow 6CO_2 + 6H_2O.

🔍 Worked example — Standard

A sample of germinating seeds shows a decrease in oxygen uptake of 0.5 cm^3 and an increase in carbon dioxide production of 0.4 cm^3 over a period of time. Calculate the RQ.

🔍 Worked example — Exam-style

A student is comparing the energy yield of a carbohydrate and a lipid. The carbohydrate provides 15.8 kJ g^-1 and the lipid provides 39.4 kJ g^-1. Calculate the percentage increase in energy value when moving from the carbohydrate to the lipid.

⚠️ Common errors

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🎯 Exam question

Key ideas — The respiratory quotient (RQ)

The respiratory quotient is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in during respiration:

🧑‍🏫 Teacher's explanation

Think of the Respiratory Quotient (RQ) as a 'chemical fingerprint' that tells us exactly what kind of fuel a cell is burning. Just as a car engine might sound different depending on whether it is running on petrol or diesel, a cell's gas exchange pattern changes depending on whether it is burning sugars or fats.

📖 Definitions

🔍 Worked example — Easy

A cell is respiring using glucose. Using the equation mathrmC_6H_12O_6 + 6O_2 arrow 6CO_2 + 6H_2O, calculate the RQ.

🔍 Worked example — Standard

A sample of germinating seeds is placed in a respirometer. Over a period of time, the volume of oxygen consumed is 4.0 cm^3 and the volume of carbon dioxide produced is 3.2 cm^3. Calculate the RQ.

🔍 Worked example — Exam-style

A researcher observes an RQ value of 1.2 in a yeast culture. Explain what this value indicates about the metabolic pathway being used.

⚠️ Common errors

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🎯 Exam question

Key ideas — Measuring RQ with a respirometer

A simple respirometer is a sealed tube containing the organisms (germinating seeds or blowfly larvae, on a gauze platform) connected to a capillary tube with a drop of coloured liquid, or a manometer.

🧑‍🏫 Teacher's explanation

Think of a respirometer like a measuring cup for gas. When organisms breathe, they consume oxygen and release carbon dioxide. If you use a chemical like soda lime to 'trap' the carbon dioxide as it is released, the total volume of gas inside the tube decreases because you are losing oxygen but not replacing it with anything else. This drop in volume tells you exactly how much oxygen was used.

📖 Definitions

🔍 Worked example — Easy

A student uses a respirometer to measure the respiration of germinating peas. In the experimental tube containing soda lime, the liquid moves 0.5 mL towards the organism. In the control tube containing water, the liquid moves 0.2 mL towards the organism. Calculate the RQ.

Ch 1 · 12.1 Energy · AS & A Level Biology 9700

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