Respiration: WJEC A level Biology revision notes

Lesson 1 of 13

Why do all living organisms need to respire?

These notes cover topic 3.3, releases chemical energy in biological processes, in WJEC A level Biology Unit 3: Energy, and the Environment. Each lesson follows the WJEC specification, and the exam tips are based on BioWelsh analysis of WJEC past papers.

Every living cell needs a constant supply of energy, day and night, for as long as it is alive. is the process that releases the chemical energy stored in organic molecules, such as glucose, so that cells can use it. It happens in every living cell: in bacteria, fungi, plants and animals. Plants respire all the time too, not only when it is dark.

Cells cannot use the energy in glucose directly. Instead, uses that energy to make from and (). Wherever energy is needed in the cell, is back to and , releasing a small, manageable amount of energy (30.6 kJ per mole) in a single step. (If is new to you, read the ATP lesson on why cells need ATP first.)

If glucose were burned in one go, all of its energy would be released at once as heat, which a cell could not use and which would damage it. releases the energy gradually, in a long series of small steps. Each step is catalysed by its own , and the product of one reaction becomes the for the next. A sequence like this is called a . is a pathway, because it breaks large molecules down into smaller ones.

What organisms use the ATP from respiration for
Use of ATP	Why energy is needed	Example
	Moving ions and molecules against their	Sodium-potassium pumps in ; root hair cells taking up mineral ions
Synthesis of large molecules	Forming bonds to build polymers (anabolic reactions)	Protein synthesis at , DNA replication, making polysaccharides
Cell division	Replicating DNA and separating chromosomes	Mitosis in growing tissues and in single-celled organisms
Movement	Changing the shape of proteins	Muscle contraction in animals; beating cilia and flagella
Keeping warm	Energy not transferred to is released as heat	Mammals and birds maintaining their body temperature

Cells that use a lot of contain a lot of , the where most is made. Examples are the cells lining the of the kidney, which actively reabsorb sodium ions and glucose, and , which pump sodium and potassium ions. If stops, these processes stop too. For example, roots in waterlogged soil cannot get enough oxygen for , so they cannot actively absorb mineral ions.

The three ways cells make ATP
Type of phosphorylation	Where it happens	Where the energy comes from
	(cytoplasm) and the (mitochondrial )	A phosphate group is transferred directly to from another molecule, using energy released by a reaction
	Inner mitochondrial membrane	Energy from electrons passing along the ; needs oxygen
	membranes of	Energy from light; this happens in , not

Organisms grouped by their need for oxygen
Group	Respiration	Examples
	Must have oxygen; respire aerobically	Most animals and plants
	Respire aerobically when oxygen is present, and anaerobically when it is not	Yeast and many bacteria
	Respire without oxygen and cannot grow when oxygen is present	Some bacteria and

Lesson 2 of 13

What are the stages of aerobic respiration and where do they happen?

uses oxygen and breaks glucose down completely to carbon dioxide and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. As with , the equation only shows the start and the end. In between are four stages, each happening in a particular part of the cell.

Three of the four stages happen inside , so it helps to know their structure first. The labelled illustration below shows a .

Labelled cut-away illustration of a sausage-shaped mitochondrion. Labels show the outer membrane, the intermembrane space, the folded inner membrane forming cristae, the matrix with granules, ribosomes and a loop of mitochondrial DNA, and small stalked ATP synthase particles on the cristae. A circular inset shows pores called porins in the outer membrane. — The structure of a mitochondrion, with a cut-away view showing the cristae and matrix. Photo: Kelvinsong, CC0.

The is surrounded by two membranes: a smooth outer membrane and a folded inner membrane.
The folds of the inner membrane are the . They give a large surface area for the and (also called ), shown as small stalked particles.
The narrow gap between the two membranes is the . are pumped into it.
The fluid inside the inner membrane is the . It holds the of the and the , and also mitochondrial DNA and .

The transmission electron micrograph below shows two real from a mammal's lung cell, cut in section.

Black and white electron micrograph of two round, dark-edged organelles in a grainy background. Each is surrounded by a double boundary and contains several dark lines running across the inside. A white scale bar labelled 200 nm is in the bottom right corner. — Transmission electron micrograph of two mitochondria in a mammalian lung cell, with a 200 nm scale bar. Photo: Louisa Howard, Public domain.

Each round shape is one , surrounded by a dark boundary made of the two membranes.
The dark lines running across the inside are , folds of the inner membrane.
The grainy material between the is the .
The scale bar is 200 nm, so each is about 500 nm, or 0.5 µm, across.

The diagram below summarises the four stages, where each one happens and what each makes from one molecule of glucose.

Diagram of a cell with a large mitochondrion inside. Box 1, glycolysis in the cytoplasm, takes in glucose and gives 2 pyruvate, net 2 ATP and 2 reduced NAD. An arrow labelled pyruvate goes to box 2, the link reaction in the matrix, giving 2 acetyl CoA, 2 carbon dioxide and 2 reduced NAD. An arrow goes to box 3, the Krebs cycle in the matrix, giving 4 carbon dioxide, 6 reduced NAD, 2 reduced FAD and 2 ATP. Dashed blue arrows from all three boxes go to box 4, the electron transport chain on the inner membrane, which takes in reduced NAD, reduced FAD and oxygen and gives up to 34 ATP, water, NAD and FAD. — The four stages of aerobic respiration, where they happen and what they make from one glucose molecule. Original BioWelsh diagram.

happens in the cytoplasm. Glucose (6C) is split into two (3C), with a net gain of 2 and 2 .
is transported into the mitochondrial . In the each becomes (2C), releasing carbon dioxide and making .
enters the , also in the . The cycle releases carbon dioxide and makes , and a little .
The dashed blue arrows show and carrying hydrogen to the on the inner membrane.
The uses oxygen and makes most of the , forming water.

The four stages of aerobic respiration
Stage	Where it happens	Main products from one glucose
	Cytoplasm	2 , net 2 , 2
	Mitochondrial	2 , 2 CO₂, 2
	Mitochondrial	4 CO₂, 6 , 2 , 2
	Inner mitochondrial membrane ()	Most (up to 34), water

Notice how often and appear. and are that act as hydrogen . In the first three stages, called remove hydrogen atoms from the , which are , and or accepts them, becoming . The then carry the hydrogen to the , where its energy is used to make . (If , and hydrogen are unfamiliar, read the ATP lesson on how hydrogen is carried in cells.)

happens in the cytoplasm because glucose cannot cross the mitochondrial membranes, and the for breaking glucose down are not in . , on the other hand, is transported into the . This is why isolated given glucose release no carbon dioxide, but given do.

Lesson 3 of 13

What happens in glycolysis?

'splitting sugar'. It is the first stage of both aerobic and , it happens in the cytoplasm, and it does not need oxygen. In one glucose molecule, with six carbon atoms, is split into two molecules of , each with three carbon atoms. You need to know the main steps, but the names of most intermediates are not required.

The flow diagram below shows the steps of for one glucose molecule.

Vertical flow diagram. Glucose (6C) is phosphorylated, with 2 ATP becoming 2 ADP, to form hexose bisphosphate (6C). This splits into two arrows to 2 × triose phosphate (3C). A further arrow leads to 2 × pyruvate (3C); beside it, 2 NAD become 2 reduced NAD in dehydrogenation and 4 ADP plus 4 inorganic phosphate become 4 ATP in substrate-level phosphorylation. A box on the right gives the yield per glucose: 4 ATP made, 2 used, net gain 2 ATP, 2 reduced NAD and 2 pyruvate. — Glycolysis: glucose is split into two pyruvate in the cytoplasm. Original BioWelsh diagram.

At the top, glucose (6C) is phosphorylated: two molecules each give a phosphate group, and become . The product is (6C).
splits into two molecules of (3C).
Each is by . The hydrogen is accepted by , so two are made in total.
At the same time, four are phosphorylated to four by , two for each .
The end products are two (3C). The box on the right shows the yield: 4 made minus 2 used gives a net gain of 2 .

The first step costs energy. Phosphorylating glucose with two makes it more reactive, which lowers the needed to split it. The phosphate groups also make the sugar charged, so it cannot diffuse out of the cell.

The energy is paid back later. When each is converted to , hydrogen is removed and passed to , and enough energy is released to make two directly from and phosphate groups. This is : no is involved.

in brief

1) Glucose (6C)
2) Phosphorylated using 2
3) (6C)
4) Split into 2 (3C)
5) : 2 , 4 made
6) 2 (3C)

What happens next to the products of glycolysis
Product	If oxygen is available	If oxygen is not available
	Transported into the mitochondrial for the	Converted to (animals) or (yeast and plants) in the cytoplasm
	Carries hydrogen to the , giving more	Gives its hydrogen to or , regenerating

Lesson 4 of 13

What happens in the link reaction?

The connects to the . If oxygen is available, is transported from the cytoplasm into the mitochondrial . There it is converted into , usually shortened to .

The diagram below shows the for one molecule.

A dashed vertical line separates the cytoplasm on the left from the mitochondrial matrix on the right. Pyruvate (3C) in the cytoplasm has an arrow across the line labelled transported in. Above the arrow, NAD becomes reduced NAD, labelled dehydrogenation. Below it, carbon dioxide is released, labelled decarboxylation. Coenzyme A joins the arrow, labelled joins the 2C acetyl group, and the product is acetyl CoA (2C), which goes to the Krebs cycle. A note says this happens twice per glucose. — The link reaction converts pyruvate into acetyl coenzyme A in the mitochondrial matrix. Original BioWelsh diagram.

On the left, (3C) is in the cytoplasm. The dashed line is the boundary of the , and the is transported across it into the .
Above the arrow, becomes : hydrogen is removed from . This is .
Below the arrow, carbon dioxide is released. Removing a carbon atom as carbon dioxide is .
What remains is a two-carbon acetyl group, which joins to form (2C).
The goes on to the .

The can be summarised as: + + → + + CO₂.

The two types of enzyme you need to name in respiration
Enzyme	Reaction it catalyses	Product removed
	: removing hydrogen atoms	Hydrogen, accepted by or
	: removing a carboxyl group	Carbon dioxide

Lesson 5 of 13

What happens in the Krebs cycle?

The is a series of -controlled reactions in the mitochondrial . It is called a cycle because the molecule that accepts the acetyl group at the start is regenerated at the end, ready to accept the next one. Its job is to release the remaining energy from the acetyl group, mostly as hydrogen carried by and .

The diagram below shows one turn of the . The compounds are named only by their number of carbon atoms.

Circular cycle diagram with four boxes clockwise: 6C at top right, 5C at bottom right, 4C at bottom left and 4C at top left. Acetyl CoA (2C) enters at the top between the top-left 4C and the 6C, and coenzyme A is released. Between 6C and 5C, carbon dioxide is released and NAD becomes reduced NAD. Between 5C and 4C, carbon dioxide is released, NAD becomes reduced NAD and ADP plus inorganic phosphate become ATP. Between the two 4C compounds, FAD becomes reduced FAD and NAD becomes reduced NAD. A box says each turn releases 2 carbon dioxide and makes 3 reduced NAD, 1 reduced FAD and 1 ATP, and the cycle turns twice per glucose. — One turn of the Krebs cycle in the mitochondrial matrix. Original BioWelsh diagram.

At the top, brings a two-carbon acetyl group. It combines with a four-carbon compound (4C) to form a six-carbon compound (6C), and is released.
On the right, the 6C compound loses a carbon dioxide molecule and a pair of hydrogen atoms, which reduce . This forms a five-carbon compound (5C).
At the bottom, the 5C compound loses another carbon dioxide and more hydrogen, which reduces again. One is made by . This forms a four-carbon compound.
On the left, the four-carbon compound is changed back into the original 4C acceptor. Hydrogen is removed twice: once to reduce and once to reduce .
The box at the bottom sums up one turn: 2 CO₂, 3 , 1 and 1 .

Types of reaction in one turn of the Krebs cycle
How many times per turn	Enzyme	Product
2		2 CO₂
4		3 and 1
1	(not required)	1

Products of the Krebs cycle
Product	Per turn (per acetyl CoA)	Per glucose (two turns)
Carbon dioxide	2	4
	3	6
	1	2
	1	2

You do not need to learn the names of the intermediates, but exam questions sometimes give them with their formulae so you can place them. Count the carbon atoms. For example, citric acid (C₆H₈O₇) must be the 6C compound, and α-ketoglutaric acid (C₅H₆O₅) must be the 5C compound.

Lesson 6 of 13

How do reduced NAD and FAD make ATP in the electron transport chain?

By the end of the , the glucose molecule has been broken down completely into carbon dioxide, but only 4 have been made. Most of the energy is now held by the hydrogen atoms carried by and . The on the inner mitochondrial membrane uses this energy to make most of the , by . (For the general idea of an , read the ATP lesson on the electron transport chain.)

The diagram below shows the on the inner mitochondrial membrane.

A horizontal membrane separates the intermembrane space, labelled high proton concentration, at the top from the matrix, labelled low proton concentration, at the bottom. Three purple proton pumps span the membrane, with small blue electron carriers between them. Reduced NAD delivers hydrogen to pump 1 and reduced FAD to pump 2, each splitting into NAD or FAD, protons and electrons. Dashed orange arrows show electrons passing from pump to carrier to pump and then to oxygen in the matrix, where 2 protons, 2 electrons and half an oxygen molecule form water. Blue arrows show protons moving up through each pump into the intermembrane space. On the right, a pink ATP synthetase lets protons flow down into the matrix, and ADP plus inorganic phosphate become ATP. — The electron transport chain and ATP synthetase on the inner mitochondrial membrane. Original BioWelsh diagram.

The blue region at the top is the and the orange region at the bottom is the . The band in between is the inner membrane.
, bottom left, delivers its hydrogen to 1. The hydrogen atoms split into (H⁺) and electrons (e⁻), and is regenerated.
delivers its hydrogen further along, at 2.
The dashed orange arrows show electrons passing from pump to to pump along the chain. As they pass, they release energy.
The blue arrows show each pump using that energy to move from the into the .
At the end of the chain, oxygen accepts the electrons and combines with to form water.
On the right, flow back into the through , which uses their energy to join and into .

step by step

1) and deliver hydrogen
2) Hydrogen splits into and electrons
3) Electrons pass along , releasing energy
4) Pumps move into the
5) flow back through
6) + →
7) Oxygen accepts electrons and , forming water

Pumping makes their concentration much higher in the than in the . The inner membrane is impermeable to , so they can only flow back down this through channels in . This flow, coupled to making , is . (If are unfamiliar, read the ATP lesson on proton gradients; for how uses the gradient, read the ATP lesson on chemiosmosis.)

Oxygen is the : 2H⁺ + 2e⁻ + ½O₂ → H₂O. Without oxygen to remove electrons at the end, electrons cannot move along the chain, are no longer pumped and no is made by . and cannot be , so no or is available for the and the , and they stop too.

and do not give the same amount of . delivers its hydrogen at the start of the chain, so its electrons pass all three . delivers its hydrogen at a lower energy level, at the second pump, so its electrons pass only two pumps. Fewer are pumped, so less is made.

ATP from each reduced coenzyme
Reduced coenzyme	Enters the chain at	Proton pumps used	ATP made (theoretical)
	1	3	3
	2	2	2

How mitochondria are adapted for oxidative phosphorylation
Feature	How it helps
Inner membrane folded into	Large surface area for many electron transport chains and molecules
Inner membrane impermeable to	can only return through , so the gradient is kept and used
Narrow	build up quickly, making a steep gradient
close to the	and from the reach the chain quickly

Lesson 7 of 13

How do respiratory inhibitors affect respiration?

Because the stages of are , blocking just one step affects the whole pathway. are substances that bind to one or protein and stop it working. Exam questions often describe an inhibitor you have not met and ask you to work out its effects.

Examples of respiratory inhibitors
Inhibitor	What it blocks	Effect on the cell
	Cytochrome oxidase, the last protein in the chain, which passes electrons to oxygen	Electrons stop moving, no , builds up, the stops; only makes , which is not enough, so it is fatal
Antimycin	A in the chain	Fewer are pumped, so the becomes less acidic and less is made
Oligomycin	The channel of	cannot flow back, so no is made; the chain stops and builds up
Malonic acid	A in the , as a	The slows, so less and reach the chain
DNP	Nothing directly: it lets leak across the inner membrane	The is lost, production falls and the energy is released as heat

The knock-on effects of blocking

1) cannot pass through
2) No made by
3) The gradient becomes too steep for pumps to work
4) Electrons stop moving along the chain
5) cannot be and builds up
6) The and stop
7) is to , which accumulates

Malonic acid has a similar shape to the four-carbon of one , succinic acid. It fits into the and forms an -inhibitor complex, so fewer - complexes can form. Because it competes with the , its effect is if more is added.

Less that every process that needs slows or stops. In the kidney, oligomycin stops the of sodium ions out of the cells lining the . Without a sodium ion gradient, glucose cannot be co-transported back into the blood, so it stays in the .

Lesson 8 of 13

What happens in anaerobic respiration?

Without oxygen, there is no , so the stops. can no longer be there, so runs out, and the and stop. Only can continue, and it needs a steady supply of to accept hydrogen. solves this problem by passing the hydrogen from to , or to a molecule made from it, in the cytoplasm.

The diagram below compares the two anaerobic pathways.

Two panels. Top panel, animals: glucose goes by glycolysis, with net 2 ATP, to 2 pyruvate, which is reduced to 2 lactate. Bottom panel, yeast and plant cells: glucose goes by glycolysis to 2 pyruvate, which releases 2 carbon dioxide to form 2 ethanal, which is reduced to 2 ethanol. In each panel a solid blue curved arrow carries 2 reduced NAD with hydrogen from glycolysis to the final reduction step, and a dashed blue curved arrow returns 2 regenerated NAD to glycolysis. — The lactate pathway in animals and the ethanol pathway in yeast and plants. Original BioWelsh diagram.

In both pathways, turns glucose into 2 , with a net gain of 2 .
The solid blue arrow shows 2 carrying hydrogen from to the next reaction.
In animals (top), is to by accepting this hydrogen.
In yeast and plant cells (bottom), first loses carbon dioxide to form , and then is to .
The dashed blue arrows show being regenerated and returning to , so can continue.

In animals, for example in muscle cells during vigorous exercise, accepts hydrogen from and is to . builds up in the muscles and blood. When oxygen is available again, is carried to the liver and converted back to , which can be respired aerobically or used to make glucose. The extra oxygen needed for this is often called the .

In yeast and in plant cells, for example roots in waterlogged soil, is to , releasing carbon dioxide. accepts hydrogen from and is to . This pathway cannot be reversed, and is toxic, so cells can only survive it for a limited time.

Yeast (Saccharomyces cerevisiae) is a single-celled fungus that can respire either way. The micrograph below shows living yeast cells.

Grey micrograph of six smooth oval cells that look raised on a flat background. Two of the larger cells each have a smaller round bud attached to one end. — Living cells of baker's yeast, *Saccharomyces cerevisiae*, seen with a light microscope using differential interference contrast. Photo: Masur, Public domain.

Each oval shape is one yeast cell, about 5 µm across.
Some cells have a smaller bud attached. This is how yeast reproduces asexually.
Yeast cells contain and respire aerobically when oxygen is present, but switch to the pathway when oxygen runs out.

Comparing the two anaerobic pathways
Feature	Lactate pathway	Ethanol pathway
Organisms	Animals, some bacteria	Yeast, plants
Where	Cytoplasm	Cytoplasm
Carbon dioxide released	No	Yes, one CO₂ per
Hydrogen acceptor
Can it be reversed?	Yes, is converted back to	No
Net per glucose	2	2

Both pathways are reactions. (or ) gains hydrogen, so it is . acts as the hydrogen donor, so it loses hydrogen and is back to .

Lesson 9 of 13

How much ATP does respiration make from one glucose molecule?

An adds up the made at each stage. For WJEC, use the theoretical values: each gives 3 and each gives 2 in the .

Theoretical ATP yield from one glucose molecule in aerobic respiration
Source of ATP	Glycolysis	Link reaction	Krebs cycle	Total ATP
(net)	2	0	2	4
× 3	2 × 3 = 6	2 × 3 = 6	6 × 3 = 18	30
× 2	0	0	2 × 2 = 4	4
Total	8	6	24	38

Real cells make fewer, about 30 to 32 per glucose. Some energy is used to transport , and from into the . Some leak back across the inner membrane without passing through . The energy that is not transferred to is released as heat.

yields so little because glucose is only partly broken down. There is no , no and no , and the hydrogen from is used to regenerate rather than to make .

The energy released as heat is useful to mammals and birds, which keep a constant body temperature. Newborn babies have brown adipose tissue, whose cells contain many and a good blood supply. It respires quickly and releases a lot of heat, which keeps the baby warm.

Lesson 10 of 13

How are lipids and proteins used in respiration?

Glucose is not the only respiratory . Most cells use carbohydrates first, then lipids when carbohydrate is running low, and protein only when both are used up, for example during starvation. All three can be respired because their breakdown products feed into the same pathway. and the act as a hub that links the metabolism of carbohydrates, lipids and proteins.

The diagram below shows where each type of enters .

Flow diagram. In the centre, polysaccharides are hydrolysed to glucose, which goes by glycolysis to triose phosphate, pyruvate, acetyl CoA and the Krebs cycle. On the left, lipids are hydrolysed to fatty acids and glycerol; glycerol is phosphorylated and dehydrogenated to triose phosphate, and fatty acids are split into two-carbon fragments that join acetyl CoA. On the right, proteins are hydrolysed to amino acids; deamination in the liver sends the amino group to urea, and the remaining carbon skeletons enter at pyruvate, acetyl CoA and the Krebs cycle. The Krebs cycle releases carbon dioxide and sends reduced NAD and FAD to the electron transport chain. — How carbohydrates, lipids and proteins enter respiration. Original BioWelsh diagram.

In the centre, polysaccharides such as are to glucose, which goes through to , and the .
On the left, lipids () are to and .
is phosphorylated and dehydrogenated to , so it joins part way through.
are split into two-carbon fragments, which enter as .
On the right, proteins are to amino acids. After , their remaining carbon skeletons can enter as , or intermediates. The nitrogen is excreted as .

How enters

1)
2) Phosphorylated using
3) Dehydrogenated: is
4)
5) , through the rest of

How enter

1) Long chain
2) Split into two-carbon fragments
3) Hydrogen removed, reducing and
4) Each fragment forms
5) enters the

A gram of lipid releases about twice as much energy as a gram of carbohydrate. have long chains with many carbon and hydrogen atoms and very little oxygen. More hydrogen atoms more and , so more is made in the . Respiring fat also produces more carbon dioxide, uses more oxygen and forms more . This is why desert animals such as camels store fat: respiring it releases water as well as energy.

Energy released by different respiratory substrates
Substrate	Energy released / kJ g⁻¹ (approximate)	When it is used
Carbohydrate	16	First choice for most cells
Lipid	39	When carbohydrate stores are low
Protein	17	Only when carbohydrate and lipid stores are used up

Even though fat releases more energy, muscles store for quick use. Respiring fat needs much more oxygen and produces more carbon dioxide, and a muscle's blood supply can only deliver and remove so much. can also be respired anaerobically, which fat cannot.

Proteins are into amino acids. Amino acids cannot be stored, so any not needed for making proteins are deaminated in the liver: the amino group (-NH₂) is removed and converted into , which is excreted by the kidneys. The rest of the molecule is converted into , or a intermediate, and respired.

How proteins enter

1) Protein
2) to amino acids
3) Deaminated in the liver
4) Amino group becomes and is excreted
5) Remainder enters as , or a intermediate

The hub works in reverse too. If a person takes in more food than they need, excess is used to make . These combine with , made from , in condensation reactions to form , which are stored as fat.

Lesson 11 of 13

How can the rate of respiration in yeast be measured?

Yeast is ideal for investigating : it is safe, it grows quickly, and it releases carbon dioxide whether it respires aerobically or anaerobically. The rate of carbon dioxide production is therefore a good measure of the rate of .

Ways to measure the rate of respiration
What is measured	How	Notes
Volume of carbon dioxide released	Collect gas in a or an upturned measuring cylinder for a time	More accurate than counting bubbles
Number of bubbles released	Count bubbles per minute from a delivery tube in water	Simple, but bubbles vary in size and may be too fast to count
Volume of oxygen taken up	Use a containing sodium hydroxide	Covered in the next lesson
Rate of activity	Time how long an , such as , takes to change colour	The specified practical for the topic

Temperature affects the rate of because every stage is controlled by . As temperature rises, molecules have more kinetic energy, so - complexes form more often and the rate increases. Above the optimum temperature, the begin to denature, their change shape and the rate falls.

Specified practical

Investigating the effect of temperature on the rate of respiration in yeast

Unit 3, section 3, specified practical 1 (BioWelsh entry number)

Aim To investigate how temperature affects the rate of in yeast, by measuring the volume of carbon dioxide released in a set time.

Five panels in order: a beaker of yeast suspension to mix with glucose in pH 7 buffer; a water bath to equilibrate for 5 minutes; a boiling tube with a delivery tube to connect to a gas syringe; a gas syringe to read the volume of carbon dioxide after 5 minutes; a thermometer to change the temperature from 20 to 45 °C with three repeats. — Measuring the rate of respiration in yeast at a glance. Original BioWelsh diagram.

Equipment

Yeast suspension
Yeast suspension10 cm³ per testThe respiring organism; stir before use so cells do not settle.
Glucose solution in pH 7 buffer
Glucose solution in pH 7 10 cm³ per testRespiratory ; the keeps pH constant.
Syringes
Syringes10 cm³Measure the volumes of yeast suspension and glucose solution.
Thermostatically controlled water bath
Thermostatically controlled water bathKeeps the tube at each chosen temperature.
Thermometer
ThermometerChecks the temperature of the water bath.
Boiling tube with bung and delivery tube
Boiling tube with bung and delivery tubeHolds the yeast mixture and carries gas to the syringe.
Gas syringe
20 cm³Measures the volume of carbon dioxide released.
Stop clock
Stop clockTimes the equilibration and the collection period.
Eye protection
Eye protectionProtects eyes from splashes of yeast suspension.

Safety

Yeast can cause an allergic reactionRiskIrritation if it contacts skin or eyes.ControlWear eye protection and wash hands after use.
Mains-powered water bathRiskElectric shock if handled with wet hands.ControlDry hands before touching switches or plugs.
Glass syringe or delivery tubeRiskCuts if glass breaks when fitting the bung.ControlPush bungs in gently, holding the tube near the top.

Variables

Independent variableTemperature

Water bath set to 20, 25, 30, 35, 40 and 45 °C, checked with a thermometer.

Temperature changes the kinetic energy of molecules and the activity of respiratory .

Dependent variableRate of respiration

Volume of carbon dioxide collected in 5 minutes, divided by 5, in cm³ min⁻¹.

Carbon dioxide is released in the and , and in the pathway.

Controlled variablesConcentration of yeast suspension, Volume of yeast suspension, Concentration and volume of glucose solution, pH, Equilibration and collection time

Concentration of yeast suspension

Use the same well-stirred batch throughout.

More yeast cells would release more carbon dioxide.

Volume of yeast suspension

10 cm³ measured with a syringe.

More suspension contains more cells.

Concentration and volume of glucose solution

10 cm³ of the same glucose solution.

concentration affects the rate of .

Glucose solution made up in pH 7 .

Carbon dioxide dissolves to form an acid, and pH affects .

Equilibration and collection time

5 minutes each at every temperature.

The mixture needs time to reach each temperature.

Control experimentBoiled and cooled yeast

Repeat with yeast that has been boiled and cooled, at each temperature.

No gas is released, showing the carbon dioxide comes from of living yeast.

Control experiment or controlled variable?

pH, glucose concentration and the volume of yeast are : they are kept the same at every temperature. Boiled and cooled yeast is a : it is a separate set-up showing that no gas is released without living, respiring yeast.

Method, step by step

1)
Set the water bath to 20 °C and check the temperature with a thermometer.
Why?
Temperature is the , so it must be accurate.
2)
Stir the yeast suspension, then use a syringe to put 10 cm³ into a boiling tube.
Why?
Yeast cells settle, so stirring gives the same number of cells in every test.
3)
Add 10 cm³ of glucose solution made up in pH 7 .
Why?
Glucose is the . The stops the pH falling as carbon dioxide dissolves, because pH also affects .
4)
Place the tube in the water bath and fit the bung, delivery tube and .
Why?
The whole mixture is kept at the chosen temperature and gas can only escape into the syringe.
Collecting carbon dioxide from respiring yeast in a gas syringe. Original BioWelsh diagram.
Open full size
5)
Leave the tube for 5 minutes, then reset the to zero.
Why?
The mixture reaches the temperature of the water bath, and gas stops expanding, before readings begin.
6)
Start the stop clock and read the volume of gas in the syringe after 5 minutes.
Why?
Measuring gas volume is more accurate than counting bubbles, which vary in size.
7)
Repeat twice more at 20 °C, using fresh yeast suspension and glucose from the same batch each time.
Why?
Glucose is used up and yeast cells die, so reusing a mixture would lower the rate. Repeats allow a and show anomalies.
8)
Repeat the procedure at 25, 30, 35, 40 and 45 °C.
Why?
A of at least five temperatures shows the trend and the optimum.
9)
Calculate the volume at each temperature, and the rate as volume ÷ 5 minutes.
Why?
Dividing by time gives a rate that can be compared and plotted.

Results and analysis

Example data

Volume of carbon dioxide collected from yeast in 5 minutes at each temperature

Temperature / °C	Volume 1 / cm³	Volume 2 / cm³	Volume 3 / cm³	volume / cm³	rate / cm³ min⁻¹
20	1.4	1.6	1.5	1.50	0.30
25	2.6	2.9	2.8	2.77	0.55
30	4.4	4.9	4.6	4.63	0.93
35	6.8	7.1	6.9	6.93	1.39
40	7.6	7.2	7.5	7.43	1.49
45	3.3	2.9	3.4	3.20	0.64

Graph of mean rate of carbon dioxide production in cubic centimetres per minute against temperature from 15 to 50 °C. Six crosses joined by straight lines rise from 0.30 at 20 °C to 0.55 at 25 °C, 0.93 at 30 °C, 1.39 at 35 °C and a peak of 1.49 at 40 °C, then fall to 0.64 at 45 °C. — Mean rate of carbon dioxide production by yeast at different temperatures. Original BioWelsh graph.

What the results show

Between 20 and 40 °C, the rate rose from 0.30 to 1.49 cm³ min⁻¹. As temperature increased, and molecules had more kinetic energy, so more - complexes formed each second.

The highest rate was at 40 °C, so the optimum temperature for in this yeast is close to 40 °C. Because readings were taken only every 5 °C, the true optimum could lie anywhere between 35 and 45 °C.

At 45 °C the rate fell to 0.64 cm³ min⁻¹. Hydrogen bonds in the respiratory began to break, so their changed shape and fewer - complexes formed.

The repeats at each temperature were close together, so the are reliable enough to show the trend.

Worked calculations

volume at 35 °C = (6.8 + 7.1 + 6.9) ÷ 3 = 6.93 cm³

rate at 35 °C = 6.93 ÷ 5 = 1.39 cm³ min⁻¹

Evaluation

Some carbon dioxide dissolves in the mixture instead of reaching the syringe.

Effect on resultsThe measured rate is lower than the true rate.

ImprovementAccept it as a systematic error, or saturate the mixture with carbon dioxide before starting.

Yeast cells settle during the collection time.

Effect on resultsFewer cells are in contact with glucose, so the rate may vary between repeats.

ImprovementSwirl the tube gently at the same times in every test.

Readings were only every 5 °C.

Effect on resultsThe optimum temperature cannot be found precisely.

ImprovementRepeat at 1 °C intervals between 35 and 45 °C.

The plunger of a gas syringe can stick.

Effect on resultsThe volume reading may jump or be too low.

ImprovementCheck the plunger moves freely, or collect gas in an upturned measuring cylinder over water.

Going further

Other ways to carry out this practical

concentration: keep the temperature at 30 °C and use glucose solutions of 0, 0.2, 0.4, 0.6 and 0.8 mol dm⁻³.
Different sugars: compare glucose, fructose, sucrose and lactose; yeast respires sucrose more slowly at first because it must be , and cannot use lactose.
pH: make up the glucose solution in buffers of different pH at a temperature.
Counting bubbles: invert a syringe of yeast and glucose in a water bath and count the bubbles released per minute; simpler, but bubbles vary in size.
Oxygen uptake: use a with sodium hydroxide solution to absorb carbon dioxide.

Model answer: describe the method

Describe how you would investigate the effect of temperature on the rate of in yeast.

Original practice response · 6 indicative marks

Mix a set volume of stirred yeast suspension with a set volume of glucose solution made up in pH 7 , in a boiling tube.
Place the tube in a thermostatically controlled water bath at the first temperature and leave it for 5 minutes to equilibrate.
Connect the tube to a and measure the volume of carbon dioxide released in a time, such as 5 minutes.
Repeat three times at each temperature using fresh yeast and glucose from the same batch, and calculate the .
Use at least five temperatures, for example 20 to 45 °C, and calculate the rate as volume ÷ time.
Set up a with boiled and cooled yeast to show that the gas comes from .

What the answer needs to cover

Same volume and concentration of yeast and glucose
to keep pH constant
Equilibration time at each temperature
Volume of carbon dioxide measured over a time
Fresh yeast for each repeat, three repeats and a
of temperatures and a with dead yeast

Exam practice

Explain why the number of bubbles may decrease if the same yeast suspension is reused for each repeat.Show answer

The glucose in the suspension is used up in , and some yeast cells may die or be affected by the build-up of , so less carbon dioxide is released.

Glucose used up
Yeast cells die or toxic products build up

State and explain the effect on the pH of the yeast suspension if no buffer is used.Show answer

The pH decreases and the suspension becomes more acidic, because the carbon dioxide released dissolves in the water to form carbonic acid.

pH decreases
Carbon dioxide dissolves to form an acid

Explain why the rate of respiration was lower at 45 °C than at 40 °C.Show answer

At 45 °C, hydrogen bonds in the respiratory break, so the tertiary structure and the shape of the change. Fewer molecules fit, so fewer - complexes form and the rate falls.

Hydrogen bonds break
changes shape
Fewer - complexes

Lesson 12 of 13

How does a respirometer measure the rate of oxygen uptake?

A measures how fast small organisms, such as seeds or woodlice, take in oxygen. The organisms are sealed in a tube with sodium hydroxide solution, which absorbs carbon dioxide. As they respire aerobically, they use oxygen, and the carbon dioxide they release is absorbed. The volume of gas in the tube therefore falls by the volume of oxygen used.

The diagram below shows a simple .

A boiling tube stands in a water bath at constant temperature. Germinating seeds rest on a wire gauze platform above sodium hydroxide solution, which absorbs carbon dioxide. The bung has a side tube with a tap, labelled open while equilibrating then closed, and a capillary tube that runs horizontally over a scale. A red drop of coloured liquid is in the capillary tube, with an arrow showing it moves towards the tube as oxygen is used. — A simple respirometer for measuring oxygen uptake by small organisms. Original BioWelsh diagram.

The organisms sit on a wire gauze platform, so they do not touch the corrosive sodium hydroxide solution below.
The tube stands in a water bath, because temperature affects both the rate of and the volume of a gas.
The tap on the left is left open while the apparatus reaches the temperature of the water bath, then closed before readings begin.
A capillary tube with a drop of coloured liquid runs over a scale. As oxygen is used, the liquid moves towards the tube, and the distance it moves is read from the scale.

The liquid moves because oxygen is used and the carbon dioxide released is absorbed by the sodium hydroxide, so the volume and pressure of gas in the tube decrease. If water replaces the sodium hydroxide, the liquid hardly moves when glucose is respired, because the volume of carbon dioxide released equals the volume of oxygen used.

Steps in using a respirometer and the reasons for them
Step	Reason
Put the organisms on a wire gauze or in a cage	They do not touch the corrosive sodium hydroxide
Leave the apparatus in the water bath with the tap open for about 10 minutes	The gas reaches a constant temperature, so it stops expanding or contracting, and the organisms acclimatise
Close the tap and read the position of the liquid at regular intervals	Gives the distance moved over a known time
Refill the tube with air between runs	The organisms do not run out of oxygen for
Set up a with glass beads or boiled seeds of the same volume	Shows that any movement is caused by , not by changes in temperature or pressure
Use a set mass or volume of organisms, not a set number	Seeds vary in size, so rates can be compared fairly; rates are often given per gram

Lesson 13 of 13

How do the ideas in respiration fit together?

is one long pathway that releases energy from organic molecules in small steps and transfers it to . The tree below shows how the ideas in this topic connect.

How the ideas in respiration connect

Zoom 100%

Zoom in to read each branch. Swipe or scroll sideways to explore.

releases energy to make
- (with oxygen)
  - in the cytoplasm: net 2 , 2
  - in the : , CO₂,
  - in the : CO₂, and ,
  - : , oxygen forms water
- (without oxygen)
  - in animals, and CO₂ in yeast and plants
  - Regenerates so continues
- - 38 in theory, about 40% efficient; anaerobic about 2%
- Other
  - to ; to
  - Amino acids deaminated, then enter as , or Krebs intermediates
- Measuring
  - Carbon dioxide from yeast; oxygen uptake in a

of one glucose molecule

1) Glucose
2) : 2
3) : 2
4) : CO₂ and and
5) : most of the and water

releases energy; it never produces energy.
is in the cytoplasm; the and are in the ; the is on the .
makes 4 but the net gain is 2.
One turn of the gives 2 CO₂, 3 , 1 and 1 ; there are two turns per glucose.
are pumped into the ; electrons pass along the .
Oxygen is the and forms water.
gives 3 , gives 2, because enters the chain at the second pump.
and are made to regenerate , not to make .
Fat releases more energy per gram because it has more hydrogen atoms.
A is not the same as a .

Related learning

ATP
, hydrogen , the , and explain how makes .
Cell structure & organisation
You need the structure of .
Biological molecules
Carbohydrates, and amino acids are the respiratory .
Enzymes
Temperature, pH and inhibitors affect respiratory like all .
Photosynthesis
makes the organic molecules that breaks down, and both use .
Homeostasis & the kidney
makes , and kidney tubule cells need for active reabsorption.
Experimental skills
Rates, , πr²h calculations, controls and evaluation are practical skills tested in Unit 5.

Specification & sources

https://www.wjec.co.uk/media/mxbh1zaf/wjec-gce-biology-spec-from-2015-e-101025.pdf
https://openstax.org/books/biology-2e/pages/7-1-energy-in-living-systems
https://openstax.org/books/biology-2e/pages/7-2-glycolysis
https://openstax.org/books/biology-2e/pages/7-3-oxidation-of-pyruvate-and-the-citric-acid-cycle
https://openstax.org/books/biology-2e/pages/7-4-oxidative-phosphorylation
https://openstax.org/books/biology-2e/pages/7-5-metabolism-without-oxygen
https://openstax.org/books/biology-2e/pages/7-6-connections-of-carbohydrate-protein-and-lipid-metabolic-pathways
WJEC GCE A Level Biology Unit 3 mark schemes, Summer 2017 (Q5 and Q6), 2018 (Q6 and Q7), 2019 (Q1 and Q4), 2022 (Q6), 2023 (Q1, Q3 and Q5), 2024 (Q1) and 2025 (Q1), with examiner reports: depth and exam wording
WJEC GCE A Level Biology Unit 5 mark scheme, Summer 2023 (variant 1, Q1): respirometer technique
Drawings and graphs are original BioWelsh images and are not copied or adapted from any published resource. The other images are used under the licences below. Captions already include credits. Show this list at the bottom of the topic page.
u3.respiration__photo-mito-diagram.jpg · Kelvinsong · CC0 · https://commons.wikimedia.org/wiki/File:Mitochondrion_(standalone_version)-en.svg
u3.respiration__photo-mito-tem.jpg · Louisa Howard · Public domain · https://commons.wikimedia.org/wiki/File:Mitochondria,_mammalian_lung_-_TEM_(2).jpg
u3.respiration__photo-yeast.jpg · Masur · Public domain · https://commons.wikimedia.org/wiki/File:S_cerevisiae_under_DIC_microscopy.jpg
All other images · BioWelsh (original illustrations and graphs) · Owned by BioWelsh · Not applicable
The mitochondrion diagram was rendered from SVG to JPG, and all three images were resized for the web; no other changes were made.
Licence links: CC0 https://creativecommons.org/publicdomain/zero/1.0/ , Public Domain Mark https://creativecommons.org/publicdomain/mark/1.0/