Energy

1. Overview

Every living cell needs a continuous supply of energy to drive processes that cannot happen on their own, such as active transport, movement and anabolic (building-up) reactions. Cells do not use energy released by respiration directly; instead they channel it through a single intermediate molecule, ATP (adenosine triphosphate), which acts as the cell's universal energy currency. ATP is synthesised from ADP and inorganic phosphate either by substrate-linked reactions or by chemiosmosis across membranes. This topic also looks at how much energy different respiratory substrates (carbohydrates, lipids and proteins) release, and at the respiratory quotient (RQ), which tells us which substrate an organism is using.

Key Definitions

• Energy currency: a short-term, immediately usable store of energy that links energy-releasing processes to energy-requiring processes in a cell.
• ATP (adenosine triphosphate): a nucleotide made of adenine, ribose and three phosphate groups; its hydrolysis releases a small, usable amount of energy.
• Substrate-linked (substrate-level) phosphorylation: the synthesis of ATP by the direct transfer of a phosphate group from a phosphorylated substrate molecule to ADP.
• Chemiosmosis: the synthesis of ATP driven by protons moving down a proton gradient through ATP synthase in the inner mitochondrial membrane or the thylakoid membrane.
• ATP synthase: the membrane enzyme that catalyses the synthesis of ATP from ADP and inorganic phosphate as protons pass through it.
• Respiratory substrate: an organic molecule, such as a carbohydrate, lipid or protein, that is oxidised in respiration to release energy for ATP synthesis.
• Respiratory quotient (RQ): the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in during respiration.
• Respirometer: apparatus used to measure the rate of gas exchange of small organisms in order to investigate respiration and determine RQ.

Content

Why living organisms need energy

Many cellular processes are endergonic — they require an input of energy and will not occur spontaneously. The energy released by respiration is transferred to these processes through ATP. Three important examples illustrate the need for energy:

• Active transport — moving ions and molecules across membranes against a concentration gradient (for example, the sodium-potassium pump). This requires energy from ATP because the movement is from a region of lower to higher concentration.
• Movement — the sliding of protein filaments in muscle contraction, the beating of cilia and flagella, and the movement of vesicles along the cytoskeleton all use energy transferred from ATP.
• Anabolic reactions — the building of large molecules from smaller ones. In DNA replication, nucleotides are joined into new polynucleotide strands, and in protein synthesis amino acids are joined by peptide bonds on the ribosome; both require energy. Note that energy is transferred to drive these reactions, not "produced" or "created".

Why ATP is a good energy currency

ATP is a nucleotide consisting of the base adenine, the pentose sugar ribose, and a chain of three phosphate groups. When the terminal phosphate is removed by hydrolysis, ATP becomes ADP (adenosine diphosphate) and inorganic phosphate ($P_i$), releasing a usable quantity of energy. Several features make it ideally suited as the universal energy currency:

• It releases energy in small, manageable amounts (the hydrolysis of one molecule releases about 30.5 kJ mol⁻¹), so little energy is wasted as heat.
• It can be rapidly hydrolysed and resynthesised in a single reversible step, so it is regenerated quickly as needed.
• It is soluble and moves easily around the cell, but it cannot cross the cell surface membrane, so it stays where it is made.
• It is universal — the same molecule is used by all cells and links many different energy-releasing and energy-requiring reactions, often by phosphorylating an intermediate to make it more reactive.

Because ATP is constantly broken down and remade, it is best described as being synthesised or regenerated from ADP and $P_i$, rather than "made" once and stored in bulk.

How ATP is synthesised

ATP is synthesised by adding a phosphate group to ADP (phosphorylation) in two ways:

• Substrate-linked (substrate-level) phosphorylation — a phosphate group is transferred directly from a phosphorylated substrate molecule onto ADP. This occurs, for example, in glycolysis and in the Krebs cycle.
• Chemiosmosis — the major route. As electrons pass along the electron transport chain in the inner mitochondrial membrane (or the thylakoid membrane in chloroplasts), energy is released and used to pump protons (H⁺) across the membrane, building up a high concentration of protons on one side. This creates a proton gradient. The protons then diffuse back down the proton gradient through the enzyme ATP synthase, and this flow drives the synthesis of ATP from ADP and $P_i$. In mitochondria the protons move from the intermembrane space back into the matrix.

Take care with terminology: the enzyme that makes ATP is ATP synthase (not "ATPase", which breaks ATP down). It is also more accurate to say that reduced NAD and reduced FAD release hydrogen atoms which split into protons and electrons, and that energy is released as electrons move along the chain.

Energy values of respiratory substrates

Carbohydrates, lipids and proteins can all be used as respiratory substrates, but they release different amounts of energy per gram when oxidised:

Substrate	Approximate energy value / kJ g⁻¹
Lipids	~39
Proteins	~17
Carbohydrates	~16

Lipids release roughly twice as much energy per gram as carbohydrates. This is because of their chemical structure. Follow the chain of reasoning:

• Lipid molecules contain a far higher proportion of hydrogen atoms (and carbon) relative to oxygen than carbohydrates do.
• More hydrogen means more reduced NAD and reduced FAD are produced as the substrate is oxidised in respiration.
• These reduced coenzymes deliver more electrons to the electron transport chain.
• More electrons passing along the chain means more protons are pumped, building a larger proton gradient.
• This drives the synthesis of more ATP by chemiosmosis per gram of substrate.

By contrast, carbohydrates are already partly oxidised (they contain more oxygen), so they yield less energy per gram. This high energy density makes lipids an efficient long-term energy store.

Respiratory quotient (RQ)

The respiratory quotient compares carbon dioxide given out with oxygen taken in:

$$RQ = \frac{\text{volume (or molecules) of CO}_2 \text{ produced}}{\text{volume (or molecules) of O}_2 \text{ taken in}}$$

Different substrates have characteristic RQ values, so RQ can show which substrate is being respired:

• Carbohydrate: RQ = 1.0
• Lipid: RQ ≈ 0.7
• Protein: RQ ≈ 0.9

An RQ greater than 1.0 suggests that some anaerobic respiration is taking place (producing CO₂ without using O₂), as happens in plant tissues or yeast that are partly fermenting.

Calculating RQ from equations

You can calculate RQ directly from a balanced equation for respiration by reading off the moles of CO₂ produced and O₂ used. For the complete aerobic respiration of glucose:

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$ $$RQ = \frac{6\ CO_2}{6\ O_2} = 1.0$$

For a typical fatty acid such as oleic acid, the equation is:

$$C_{18}H_{34}O_2 + 25.5\,O_2 \rightarrow 18\,CO_2 + 17\,H_2O$$ $$RQ = \frac{18}{25.5} \approx 0.71$$

The low RQ reflects the small amount of oxygen within lipid molecules, so a large amount of oxygen must be taken in to oxidise them fully.

Worked example

Exam-style question: The complete aerobic respiration of the triglyceride tripalmitin can be represented by the equation below. Use it to calculate the RQ of tripalmitin and state what your answer tells you about the substrate. [3]

$$2\,C_{51}H_{98}O_6 + 145\,O_2 \rightarrow 102\,CO_2 + 98\,H_2O$$

Model answer:

• From the equation, molecules of CO₂ produced = 102 and molecules of O₂ taken in = 145. (1)
• $RQ = \dfrac{102}{145} \approx 0.70$ (1)
• An RQ of about 0.7 is characteristic of a lipid being respired aerobically; the low value shows that a large amount of oxygen is needed because the molecule contains very little oxygen of its own. (1)

Investigating RQ with a respirometer

A respirometer can be used to find the RQ of living material such as germinating seeds or small invertebrates (for example blowfly larvae):

1. Place the living material in a sealed tube connected to a manometer (a U-tube containing coloured fluid) with a graduated capillary.
2. First run — include soda lime (or potassium hydroxide) in the tube to absorb all the CO₂ released. The fluid then moves only because of oxygen uptake, giving the volume of O₂ used.
3. Second run — remove the CO₂ absorber. The fluid movement now reflects the net difference between O₂ taken in and CO₂ given out.
4. Combine the two readings to work out the volumes of O₂ used and CO₂ produced, then calculate RQ.

Control variables carefully: keep the apparatus at a constant temperature in a water bath, allow time for equilibration before taking readings, and include a control tube with non-living material (such as glass beads or boiled seeds) of the same volume to allow for changes in temperature and pressure.

Worked example

Exam-style question: A student used a simple respirometer to investigate respiration in germinating peas at 20 °C. With carbon dioxide absorbed, the coloured fluid showed that 1.8 cm³ of oxygen was taken in over 10 minutes. In a second run without the absorber, the net change in gas volume over the same time was 0.0 cm³. Calculate the RQ and state which respiratory substrate the peas were using. [3]

Model answer:

• In the first run, O₂ taken in = 1.8 cm³.
• With no absorber the net change was 0, so the volume of CO₂ produced equals the volume of O₂ taken in: CO₂ produced = 1.8 cm³. (1)
• $RQ = \dfrac{1.8}{1.8} = 1.0$ (1)
• An RQ of 1.0 indicates the peas were respiring carbohydrate aerobically. (1)

Key Equations

Respiratory quotient: $$RQ = \frac{\text{molecules (or volume) of CO}_2 \text{ produced}}{\text{molecules (or volume) of O}_2 \text{ taken in}}$$

Aerobic respiration of glucose (RQ = 1.0): $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$

Common Mistakes to Avoid

• Saying energy is "produced" or "created". Energy is never made; it is transferred between forms. ATP is synthesised (or regenerated) from ADP and inorganic phosphate, and energy is released when it is hydrolysed.
• Calling the enzyme "ATPase". The enzyme that synthesises ATP during chemiosmosis is ATP synthase; "ATPase" refers to enzymes that hydrolyse ATP. Use the right name.
• Forgetting the proton gradient in chemiosmosis. Always state that protons move down a proton gradient through ATP synthase (from the intermembrane space into the matrix in mitochondria), and link a smaller gradient to fewer protons flowing through and so less ATP made.
• Confusing active transport with facilitated diffusion. Only active transport requires energy from ATP. Facilitated diffusion is passive and relies on the kinetic energy of the molecules and a concentration gradient — it does not use ATP.
• Saying plants stop respiring in the light. Plants (and all living cells) respire continuously, in both the light and the dark; photosynthesis does not replace respiration.
• Loose language about electrons and hydrogen. Energy is released as electrons move along the electron transport chain; reduced NAD and reduced FAD release hydrogen atoms, which then split into protons and electrons — do not say "H⁺ ions are released" from the carriers.
• Getting RQ upside-down. RQ is CO₂ out ÷ O₂ in, not the other way round, and the answer has no units. Quote it to a sensible number of decimal places.

Exam Tips

• When asked why ATP is a good energy currency, give several distinct features (small amounts of energy, rapidly resynthesised, soluble, universal) rather than repeating one idea.
• For chemiosmosis answers, always include the three key terms: proton gradient, diffusion of protons, and ATP synthase — these are the key marking points.
• In RQ calculations, show your working: write the balanced equation (or both respirometer readings), substitute the numbers, then give the final ratio. Method marks are available even if the final value is slightly off.
• Learn the standard RQ values (carbohydrate 1.0, lipid ~0.7, protein ~0.9) and remember that an RQ above 1.0 implies anaerobic respiration.
• For respirometer questions, name the CO₂ absorber (soda lime or potassium hydroxide), explain the need for a control tube and a constant-temperature water bath, and describe how two runs (with and without the absorber) let you find both O₂ uptake and CO₂ output.
• Use precise vocabulary throughout: anabolic, active transport, substrate-linked phosphorylation and chemiosmosis earn marks that vague descriptions do not.