Respiration: WJEC A level Biology cram notes

Lesson 1 of 13

Why do all living organisms need to respire?

Cram notes for topic 3.3, , in WJEC A level Biology Unit 3.

releases chemical energy from organic molecules in many small, -controlled steps and transfers it to . All cells respire all the time.

from is used for , protein and DNA synthesis, and cell division in all organisms.
Cells with high demand, such as kidney tubule cells and , have many .
happens in and the ; happens on the inner mitochondrial membrane.
, such as yeast, respire with or without oxygen; cannot grow in oxygen.

Lesson 2 of 13

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

has four stages: in the cytoplasm, the and the in the mitochondrial , and the on the inner membrane ().

The diagram below summarises the stages for one glucose molecule.

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.

turns glucose into 2 , with net 2 and 2 .
The turns 2 into 2 , releasing 2 CO₂ and making 2 .
The releases 4 CO₂ and makes 6 , 2 and 2 .
and carry hydrogen to the , which uses oxygen and makes most of the .

Lesson 3 of 13

What happens in glycolysis?

Glucose (6C) is phosphorylated by 2 to , which is more reactive. It splits into 2 (3C). Each is to (3C), reducing and making 2 by .

Per glucose: 4 made, 2 used, so net 2 , plus 2 and 2 .

Lesson 4 of 13

What happens in the link reaction?

In the , each is dehydrogenated (reducing ) and (releasing CO₂). The two-carbon acetyl group joins to form .

Lesson 5 of 13

What happens in the Krebs cycle?

(2C) joins a 4C compound to make a 6C compound. The 6C compound is and dehydrogenated to 5C, then to 4C, and the 4C acceptor is regenerated.

The diagram below shows one turn of the cycle.

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.

6C to 5C: one CO₂ released and one .
5C to 4C: one CO₂ released, one and one made.
4C back to the 4C acceptor: one and one .
One turn gives 2 CO₂, 3 , 1 and 1 ; there are two turns per glucose.

Lesson 6 of 13

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

and deliver hydrogen to the chain on the . The hydrogen splits into and electrons. Electrons pass along and their energy is used by to actively transport from the into the .

diffuse back through , which makes from and . This is . (If is unfamiliar, read the ATP lesson on chemiosmosis.)

The diagram below shows the chain 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.

enters at pump 1 and at pump 2.
Electrons (dashed orange) pass along the chain; (blue) are pumped into the .
Oxygen is the and forms water.
flow back through to make .

Lesson 7 of 13

How do respiratory inhibitors affect respiration?

blocks the last , so electrons cannot pass to oxygen and the chain stops.
Oligomycin blocks , so no is made and builds up.
Malonic acid is a of a .
DNP lets leak across the inner membrane, so less is made and more heat is released.

Less and other -requiring processes stop, for example sodium ion pumping in the kidney.

Lesson 8 of 13

What happens in anaerobic respiration?

Without oxygen the stops, is not regenerated, and the and stop. Only can continue, giving net 2 per glucose.

The diagram below compares the two 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.

Animals: is to using hydrogen from .
Yeast and plants: is to and carbon dioxide, and is to .
Both pathways regenerate so can continue.

Lesson 9 of 13

How much ATP does respiration make from one glucose molecule?

One glucose gives 4 by , 10 (× 3 = 30 ) and 2 (× 2 = 4 ): 38 in theory. Real cells make about 30 to 32.

Lesson 10 of 13

How are lipids and proteins used in respiration?

Lipids are to and .
is phosphorylated and dehydrogenated to , which enters .
are split into two-carbon fragments that form , which enters the .
Proteins are to amino acids, which are deaminated in the liver; the amino group becomes and the rest enters as , or a Krebs intermediate.

Fat releases about twice as much energy per gram as carbohydrate, because it has more hydrogen atoms, giving more and . It also needs more oxygen and forms more .

Lesson 11 of 13

How can the rate of respiration in yeast be measured?

Yeast releases carbon dioxide in aerobic and , so the volume of carbon dioxide collected per minute measures the rate of . Rate = volume ÷ time.

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 find how temperature affects the rate of in yeast by measuring carbon dioxide release.

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 and glucose in pH 7 buffer
Yeast suspension and glucose in pH 7 Organism and .
Water bath
Water bathControls temperature.
Boiling tube with delivery tube
Boiling tube with delivery tubeCarries gas to the syringe.
Gas syringe
Measures carbon dioxide volume.

Safety

Yeast allergenRiskIrritation of skin or eyes.ControlEye protection; wash hands.

Variables

Independent variableTemperature

Water bath, 20 to 45 °C.

Dependent variableRate of respiration

Carbon dioxide volume per minute.

Controlled variablesVolume and concentration of yeast, Glucose concentration and volume, pH

Volume and concentration of yeast

Same stirred batch, 10 cm³.

Glucose concentration and volume

Same solution, 10 cm³.

pH

pH 7 .

Control experimentBoiled and cooled yeast

No gas released.

Variables in brief

Independent: temperature.
Dependent: carbon dioxide volume per minute.
Controlled: yeast, glucose, pH, times.

Method, step by step

1)
Mix 10 cm³ of stirred yeast suspension with 10 cm³ of glucose in pH 7 .
Why?
Same number of cells and each time.
2)
Equilibrate in a water bath for 5 minutes, then connect a .
Collecting carbon dioxide from respiring yeast in a gas syringe. Original BioWelsh diagram.
Open full size
3)
Record the volume of carbon dioxide after 5 minutes.
4)
Repeat three times with fresh yeast at 20 to 45 °C; rate = volume ÷ 5.

Results and analysis

Example data

rate of carbon dioxide production by yeast at each temperature

Temperature / °C	rate / cm³ min⁻¹
20	0.30
25	0.55
30	0.93
35	1.39
40	1.49
45	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

The rate rose from 0.30 cm³ min⁻¹ at 20 °C to a peak of 1.49 cm³ min⁻¹ at 40 °C, as molecules gained kinetic energy.

At 45 °C the rate fell to 0.64 cm³ min⁻¹ because respiratory began to denature.

Worked calculations

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

Evaluation

Carbon dioxide dissolves in the mixture.

Effect on resultsRate underestimated.

ImprovementTreat as a systematic error.

Readings every 5 °C.

Effect on resultsOptimum not precise.

ImprovementUse 1 °C steps near the peak.

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

Same volumes of stirred yeast and glucose in pH 7 .
Water bath at each temperature with 5 minutes to equilibrate.
to measure carbon dioxide volume in a time.
Three repeats with fresh yeast; rate = volume ÷ time.
At least five temperatures; with boiled and cooled yeast.

What the answer needs to cover

Same yeast and glucose
Equilibration
Gas volume over time
Repeats and
and

Exam practice

Why is the glucose made up in a pH 7 buffer?Show answer

Carbon dioxide dissolves to form an acid, and the keeps the pH constant so that only temperature affects the .

Carbon dioxide lowers pH
keeps pH constant

Lesson 12 of 13

How does a respirometer measure the rate of oxygen uptake?

Sodium hydroxide absorbs the carbon dioxide released, so as the organisms use oxygen the gas volume falls and the coloured liquid moves towards them. Volume of oxygen = πr² × distance moved.

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.

Organisms sit on a wire gauze above sodium hydroxide solution.
The tube is kept in a water bath, with the tap open while it equilibrates.
The distance the coloured liquid moves along the scale is measured over time.

Lesson 13 of 13

How do the ideas in respiration fit together?

in five steps

1) : glucose to 2
2) : to
3) : CO₂, and
4) : pumped
5) : made, oxygen forms water

releases energy, it never produces it.
Net from is 2.
are pumped; electrons are passed along.
and regenerate .
is not .