Energy — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

Section 1 of 25The need for energy

Anabolic reactions

• Anabolic reactions for example protein synthesis and DNA synthesis from nucleotides.

Active transport

• Active transport such as the sodium-potassium pump, and exocytosis.

Movement

• Movement muscle contraction and movement of chromosomes on the spindle.

Body temperature

• Body temperature maintaining it, in mammals and birds only.

Autotrophs

• Autotrophs plants and algae make organic compounds from CO₂ using light energy in photosynthesis.

Heterotrophs

• Heterotrophs they obtain ready-made organic molecules from food.

Source of energy

• Source of energy almost all energy in organisms comes originally from the Sun and is released by respiration in every cell.

Section 2 of 25ATP as the energy currency

ATP as the energy currency

Hydrolysis

• ATP → ADP + Pi releases about 30.5 kJ per mol.

Regeneration

• ATP is regenerated from ADP + Pi by ATP synthase.

Good currency

• Good currency it is small, soluble, easily hydrolysed in one step, releases energy in small amounts and cannot leave cells.

Continuous synthesis

• Continuous synthesis cells hold only a tiny amount of ATP and it does not cross the surface membrane, so it is resynthesised constantly.
• The synthesis is a condensation reaction (water released) needing energy from respiration or photosynthesis.

Phosphorylated intermediates

• ATP can transfer a phosphate group, for example to glucose in glycolysis.
• The phosphorylated molecule is more reactive, lowering the activation energy for the next step.

Section 3 of 25Two ways of making ATP

1Substrate-linked phosphorylation

• Substrate-linked phosphorylation a phosphate is transferred directly from a substrate to ADP.
• It occurs in the cytoplasm and mitochondrial matrix.
• It yields few ATP per glucose.

2Chemiosmosis

• Chemiosmosis a proton gradient across a membrane drives ATP synthase.
• It occurs in the inner mitochondrial membrane and thylakoid membranes.
• It yields most of the ATP.

Section 4 of 25Glycolysis and the mitochondrion

1Glycolysis

• Glycolysis glucose is phosphorylated using 2 ATP and split into hexose bisphosphate and then two triose phosphates.
• They are oxidised to 2 pyruvate, making 4 ATP (net 2) and 2 reduced NAD.

2Pyruvate

• Pyruvate it is actively transported into the mitochondrial matrix.

3Link reaction

• Link reaction pyruvate is decarboxylated and dehydrogenated and combined with coenzyme A to form acetyl CoA.

4Krebs cycle

• Krebs cycle acetyl (2C) joins oxaloacetate (4C) to make citrate (6C).
• Decarboxylation and dehydrogenation regenerate oxaloacetate.
• Each turn releases 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP.

5Mitochondrion adaptations

• Mitochondrion adaptations folded cristae give a large surface area for the electron transport chain and ATP synthase.
• The matrix contains Krebs enzymes.
• The impermeable inner membrane maintains the gradient.

Section 5 of 25Oxidative phosphorylation

Reduced NAD and FAD release H at the inner membrane.

H splits into H⁺ and electrons.

Electrons pass along carriers in the electron transport chain, releasing energy.

The energy pumps H⁺ from the matrix into the intermembrane space, forming a gradient.

H⁺ diffuse back through ATP synthase (chemiosmosis), making ATP from ADP + Pi.

Oxygen accepts electrons and H⁺, forming water.

Role of oxygen

• Role of oxygen without O₂ the chain stops.

Section 6 of 25Respiratory substrates and RQ

Respiratory substrates and RQ

Energy values

• Energy values lipids have a higher energy value than carbohydrates because they contain more H atoms per unit mass.

Protein

• Protein it is used mainly when other stores are low.

Respiratory quotient (RQ)

Respiratory quotient (RQ) volume CO₂ produced ÷ volume O₂ consumed (no units).

• Typical valuescarbohydrate 1.0, lipid 0.7, protein 0.9.

Interpreting RQ.

• About 0.85 suggests a mixture of carbohydrate and lipid.
• Above 1 suggests carbohydrate is being converted to fat (overfeeding).
• Below 0.7 suggests underfeeding, with unusual substrates in use.

Anaerobic RQ

• Anaerobic RQ in mammalian muscle no O₂ is used and no CO₂ made, so RQ cannot be calculated.
  • (glucose → lactate)
• In yeast, CO₂ is released with no O₂ uptake, so RQ tends to infinity.

Section 7 of 25Respirometer measurements

1

Set up a respirometer with the organisms and soda lime or KOH in one tube, and a control tube with glass beads of equal volume.

2

Equilibrate in a water bath at constant temperature for 10 minutes with taps open.

3

Close the taps, start timing and measure movement of the coloured liquid in the capillary over a set time.

4Calculate O₂ uptake

πr² × distance moved (mm³).

5

Repeat with water instead of soda lime to find CO₂ produced − O₂ used.

6

Reading x is gas volume change with soda lime (O₂ used); reading y is change without it (O₂ used − CO₂ made).

7CO₂ produced

CO₂ produced x − y, so RQ = (x − y) ÷ x. If the fluid does not move without soda lime, RQ = 1.

8

Repeat and average.

9Rate of respiration

Rate of respiration volume O₂ used ÷ (time × mass), for example mm³ min^-1 g^-1.

10Redox indicators

• Redox indicators methylene blue, DCPIP and TTC can measure respiration rate in yeast.

Section 8 of 25Anaerobic respiration and rice

Mammals

• Mammals pyruvate is reduced to lactate by lactate dehydrogenase.

Yeast and plants

• Yeast and plants pyruvate is decarboxylated to ethanal, then reduced to ethanol.

Purpose

• Purpose both regenerate NAD so glycolysis continues.

Yield

• Yield only 2 ATP per glucose, because most energy remains in lactate or ethanol.

Lactate

• Lactate it is taken to the liver and converted back to pyruvate or glucose (oxygen debt).

Rice

• Rice it survives flooding.
• Aerenchyma (air spaces) supply O₂ to roots.
• High alcohol dehydrogenase gives ethanol tolerance.
• Fast stem growth lifts leaves above water.

Section 9 of 25More from the specification

More from the specification

Respiratory oxidation of dietary amino acids

• Respiratory oxidation of dietary amino acids deaminating surplus amino acids yields keto acids metabolized through Krebs cycles, providing a means of ATP generation.

Respiratory Quotient Formula

• The respiratory quotient is calculated using the equation RQ = / (volume of oxygen consumed), identifying the respiratory substrate metabolized by living tissue.
  • (volume of carbon dioxide produced)

Respirometer and Manometer Setup

• In a respirometer, an experimental chamber connects to a fluid manometer;
• as organisms consume oxygen and released carbon dioxide is absorbed by potassium hydroxide, the manometer fluid moves.

Proton Concentration Gradient Across Membranes

• During aerobic respiration, electron transport chains pump hydrogen ions across the inner mitochondrial membrane, establishing a steep electrochemical concentration gradient.

ATP Synthesis via Chemiosmosis

• Protons diffuse down their concentration gradient back into the mitochondrial matrix through ATP synthase, driving phosphorylation of ADP into ATP.

Carbon

• Element with four valence electrons that forms the backbone of organic molecules.

Manometer

• U-tube whose fluid moves as gas volume changes.

Concentration gradient

• The difference in concentration of a substance between two regions.

Amino acid

• The unit from which proteins are made; the sequence decides the protein's shape and function.

Section 10 of 25Respiratory substrates

Glucose

• Glucose the main respiratory substrate in most cells.

Other substrates

• Other substrates when glucose is used up, cells respire other carbohydrates, lipids and then proteins.

Amino acids

• Amino acids respired only when other substrates are exhausted because they have essential structural and enzymatic roles.

Section 11 of 25Energy values

Substrate	Energy value / kJ g-1
Carbohydrate	15.8
Lipid	39.4
Protein	17.0

Section 12 of 25Why energy values differ

Hydrogen content

• Hydrogen content substrates with more hydrogen atoms release more energy.

Hydrogen carriers

• NAD and FAD pick up hydrogen atoms and take them to the inner mitochondrial membrane.

Proton gradient

• Proton gradient hydrogen atoms split into protons and electrons, and protons are pumped into the intermembrane space.

ATP synthesis

• ATP synthesis protons flow back through ATP synthase, making ATP by chemiosmosis.

Water

• Water protons are finally oxidised to form water.

Lipids

• Lipids fatty acids are long hydrocarbon chains with a high proportion of hydrogen, so they give the largest proton gradient.

Section 13 of 25The respiratory quotient

Definition

• RQ is the ratio of carbon dioxide molecules produced to oxygen molecules taken in.

What the symbols mean:

RQ = CO₂ produced ÷ O₂ consumed.

What the symbols mean: RQ = respiratory quotient (no unit) · about 1.0 for carbohydrate, 0.7 for lipid, 0.9 for protein

Carbohydrate

• RQ is 1.0, because glucose gives equal volumes of CO₂ and O₂.

Lipid

• RQ is about 0.7, because more oxygen is needed to oxidise the extra hydrogen.

Equal volumes

• Equal volumes the same number of gas molecules occupies the same volume, so mole ratios can be used.

Section 14 of 25Calculating RQ from an equation

1

Write the respiration equation for the substrate.

2

Balance the equation for carbon, hydrogen and oxygen.

3

Take the number before CO₂ and the number before O₂.

4

Divide CO₂ by O₂.

Section 15 of 25RQ in anaerobic respiration

Lactate fermentation

• RQ cannot be calculated because no oxygen is used and no CO₂ is made.

Ethanol fermentation

• RQ tends to infinity because CO₂ is made but no oxygen is used.

Equation

• Equation glucose gives 2 ethanol and 2 CO₂ with no oxygen.

Section 16 of 25Interpreting RQ values

Change in RQ

• Change in RQ means the respiratory substrate has changed.

Mixture

• Mixture an RQ of about 0.85 suggests carbohydrate and lipid are used together.

RQ above 1

• RQ above 1 suggests excess carbohydrate or calorie intake.

RQ below 0.7

• RQ below 0.7 suggests underfeeding.

Order of use

• Order of use carbohydrates, then lipids, then proteins.

Section 17 of 25More from the specification

1ATP hydrolysis and energy coupling

• Living organisms utilize adenosine triphosphate (ATP) as a universal energy currency;
  • the enzymatic hydrolysis of its terminal phosphate group releases approximately 30.5 kJ mol^-1 to drive metabolic endergonic reactions.

2Chemiosmosis across inner membranes

• During oxidative phosphorylation, proton pumps establish a steep proton concentration gradient across the inner mitochondrial membrane, driving ATP synthesis via ATP synthase.

3Respirometer and manometer measurements

• A respirometer with a fluid manometer measures oxygen consumption in respiring organisms by absorbing carbon dioxide with potassium hydroxide (KOH),
• causing manometer fluid displacement.

4Aerobic vs anaerobic respiration

• In anaerobic respiration, glucose undergoes glycolysis without oxygen to yield net 2 ATP per molecule, with respiration formula calculations yielding distinctive respiratory quotients (RQ).

5ATP hydrolysis and energy coupling

• Living organisms utilize adenosine triphosphate (ATP) as a universal energy currency;
  • the enzymatic hydrolysis of its terminal phosphate group releases approximately 30.5 kJ mol^-1 to drive metabolic endergonic reactions.

6Chemiosmosis across inner membranes

• During oxidative phosphorylation, proton pumps establish a steep proton concentration gradient across the inner mitochondrial membrane, driving ATP synthesis via ATP synthase.

7Respirometer and manometer measurements

• A respirometer with a fluid manometer measures oxygen consumption in respiring organisms by absorbing carbon dioxide with potassium hydroxide (KOH),
• causing manometer fluid displacement.

8Aerobic vs anaerobic respiration

• In anaerobic respiration, glucose undergoes glycolysis without oxygen to yield net 2 ATP per molecule, with respiration formula calculations yielding distinctive respiratory quotients (RQ).

9Organism

• A living thing, such as a plant or an animal.

10Manometer

• U-tube whose fluid moves as gas volume changes.

11Hydrolysis

• A reaction that splits a molecule by adding water.

12Anaerobic respiration

• Respiration without oxygen, releasing much less energy per glucose molecule.

13Concentration gradient

• The difference in concentration of a substance between two regions.

14Substrate-linked phosphorylation

• Direct transfer of a phosphate from a substrate to ADP.

Section 18 of 25Respirometer principle

Respirometer principle

Closed system

• Closed system a respirometer measures gas volume changes in a sealed apparatus.

Organisms

• Organisms germinating seeds or small invertebrates are used.

Oxygen uptake

• Oxygen uptake lowers the pressure.

Carbon dioxide production

• Carbon dioxide production raises the pressure, so the net change depends on both gases.

Two readings

• Two readings one with CO₂ absorbed and one without absorption are needed for RQ.

Section 19 of 25Method

1

Set up the respirometer with soda lime in both tubes, which absorbs CO₂.

2

Read the manometer on the side closest to the organisms and calculate the volume change x in cm³ min^-1.

3

Reset the apparatus by letting air in through the screw cap and resetting the fluid with the syringe.

4

Remove the soda lime from both tubes and run again.

5

Calculate the second volume change y in cm³ min^-1.

Section 20 of 25Interpreting x and y

x

x the volume of oxygen consumed.

y

y the volume of oxygen consumed minus the volume of CO₂ produced.

x − y

• x − y the volume of CO₂ given off.

RQ = (x − y) ÷ x.

Glucose

• Glucose equal O₂ and CO₂ means the fluid does not move, y is 0 and RQ is 1.

Section 21 of 25Oxygen volume

Capillary tube

• Capillary tube volume = π r² h, with radius r in cm and fluid movement h in cm.

Rate

• Rate divide by time in minutes to give cm³ min^-1.

Section 22 of 25Controlling the investigation

Temperature

• Temperature use a controlled water bath.

Control tube

• Control tube use inert material of equal volume to the organisms to compensate for atmospheric pressure changes.

Repeats

• Repeats repeat several times and calculate a mean.

Factors tested

• Factors tested temperature can be varied using a series of water baths and the RQ followed over time.

Section 23 of 25More from the specification

Cellular respiration in living cells

• Cellular respiration in living cells respirometer chambers measure oxygen consumption during glucose oxidation by respiring germinating seeds or yeast cells.

ATP as the universal energy currency

• Adenosine triphosphate (ATP) stores readily accessible biochemical free energy within high-energy phosphoanhydride bonds, hydrolyzed to ADP and inorganic phosphate.

Thermodynamics of metabolic reactions

• Exergonic catabolic reactions with negative delta G release free energy coupled to endergonic anabolic biosynthesis in living organisms.

Substrate-level phosphorylation

• Direct enzymatic transfer of a high-energy phosphate group from an organic metabolic intermediate onto ADP synthesizes ATP independently of electron transport chains.

ATP molecule structure and function

• ATP molecules store chemical energy within high-energy phosphate bonds, releasing transferable energy during cellular metabolic reactions.

Chemiosmotic ATP synthesis

• Chemiosmotic ATP synthesis hydrogen protons flowing down an electrochemical gradient across the inner mitochondrial membrane drive ATP synthase enzyme synthesis.

Respiratory quotients of substrates

• Respiratory quotients of substrates different respiratory substrates consume varying oxygen volumes per carbon dioxide produced, altering respirometer manometer measurements.

Lipids

• Concentrated energy, insulation, cell membranes.

Hydrolysis

• A reaction that splits a molecule by adding water.

Anaerobic respiration

• Respiration without oxygen, releasing much less energy per glucose molecule.

Substrate-linked phosphorylation

• Direct transfer of a phosphate from a substrate to ADP.

Intermembrane space

• The gap between the two membranes where protons accumulate.

Substrate

• The molecule that an enzyme acts on and which binds to the active site.

Gradient

• A difference in concentration.

Chemiosmosis

• ATP synthesis driven by a proton gradient across a membrane.

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Section 24 of 25Key terms

Term	Meaning
ATP	Adenosine triphosphate, the universal energy currency.
Chemiosmosis	ATP synthesis driven by a proton gradient across a membrane.
Substrate-linked phosphorylation	Direct transfer of a phosphate from a substrate to ADP.
Respiratory quotient	CO_{2} produced divided by O_{2} consumed.
Glycolysis	Splitting of glucose to pyruvate in the cytoplasm.
Aerenchyma	Air spaces in plant tissue that carry oxygen to roots.

Term	Meaning
Respiratory substrate	Molecule broken down in respiration to release energy.
Energy value	Energy released per gram of substrate.
Respiratory quotient	Ratio of CO2 produced to O2 consumed.
Hydrogen content	Number of hydrogen atoms available to build a proton gradient.
Chemiosmotic gradient	Difference in proton concentration across the inner mitochondrial membrane.

Term	Meaning
Respirometer	Apparatus measuring gas volume changes from respiring organisms.
Soda lime	Chemical that absorbs carbon dioxide.
Manometer	U-tube whose fluid shows pressure change.
Capillary tube	Narrow tube in which fluid movement is measured.
Control tube	Tube with inert material that corrects for pressure changes.

Section 25 of 25Exam technique

1

Saying respiration produces energy — it produces ATP, which stores and carries energy.

2

Saying oxygen is used in glycolysis — its role is as the final electron acceptor.

3

Saying anaerobic respiration makes as much ATP as aerobic — it makes only 2 ATP per glucose.

4

Saying a cell stores large amounts of ATP — it makes it as needed.

1

Saying protein has the highest energy value — lipid has the highest at 39.4 kJ per gram.

2

Giving RQ as O₂ over CO₂ — RQ is CO₂ produced divided by O₂ consumed.

3

Calculating RQ from an unbalanced equation — balance the equation first.

4

Saying lipids give more energy because they are bigger — they have more hydrogen per gram.

1

Reading the manometer on the wrong arm — read the side closest to the organisms.

2

Forgetting to reset the fluid between runs — air must be admitted and the fluid reset.

3

Thinking the RQ equals x minus y — it is (x − y) divided by x.

4

Omitting the inert control — it corrects for pressure changes.