DP Biology SL C1.2 Cell Respiration

DP Biology SL C1.2 Cell Respiration - Full notes

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DP Biology SL | C1.2 Cell Respiration

DP BIOLOGY · THEME C: INTERACTION AND INTERDEPENDENCE

C1.2

Cell Respiration

Full notes for SL · IB-style

Contents

1	ATP: the energy currency of the cell
2	What cell respiration is
3	Aerobic respiration
4	Anaerobic respiration in humans and yeast
5	Comparing aerobic and anaerobic respiration
6	Measuring the rate of respiration
7	ATP: the energy currency
8	Respiration versus gas exchange
9	Aerobic respiration overview
10	Anaerobic respiration in humans
11	Anaerobic respiration in yeast
12	Comparing aerobic and anaerobic respiration
13	Respirometers
14	Factors affecting the rate of respiration
15	Respiratory substrates
16	Respiratory quotient (RQ)
17	Glycolysis (overview)
18	Mixed review: worked problems
19	Additional exam-style practice
20	Respiration and exercise
21	Metabolic poisons
22	Common misconceptions and links
23	Exam-style question with markscheme
24	Check your understanding
25	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain the properties and roles of ATP
define cell respiration and explain why it is needed
write word equations for aerobic and anaerobic respiration
explain anaerobic respiration in humans and yeast and its uses
compare aerobic and anaerobic respiration
explain how a respirometer measures the rate of respiration

This booklet covers the SL content of this subtopic. Additional Higher Level material is not included.

Guiding questions

How is energy released from food and made available to cells? Why do cells respire anaerobically when oxygen is short?

How this topic is examined

C1.2 is examined through equations, comparisons of aerobic and anaerobic respiration, and respirometer experiments.

State clearly where each process happens and what goes in and comes out.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 ATP: the energy currency of the cell

ATP carries small, usable packets of energy.

Think about it

Glucose stores a lot of energy, so why don't cells use glucose directly to power muscle contraction or active transport?

Adenosine triphosphate (ATP) is a nucleotide made of adenine, ribose and three phosphate groups. It is the immediate source of energy for almost all processes in cells.

When ATP is hydrolysed to ADP (adenosine diphosphate) and an inorganic phosphate (Pi), energy is released, which can be used to do work. The reaction is reversible: ATP is regenerated from ADP and Pi using energy from respiration (or photosynthesis). A typical cell recycles its whole pool of ATP every minute or so.

ATP is suited to its role because: it is soluble and moves easily within the cell; it releases energy in small, manageable amounts; it is stable at the pH of the cell but easily hydrolysed by enzymes; and it cannot pass through membranes, so each cell keeps its own supply.

Uses of ATP include active transport (B2.1), synthesis of macromolecules (anabolism), movement (muscle contraction, cilia, chromosome movement), and cell division.

Figure 1. The ATP-ADP cycle.

Quick check

1. What are the products of ATP hydrolysis?

2. Give three uses of ATP in cells.

3. Why is ATP better than glucose as an immediate energy source?

Answers: 1. ADP and inorganic phosphate 2. active transport, synthesis, movement 3. releases small usable amounts; easily hydrolysed

2 What cell respiration is

The controlled release of energy from organic compounds to make ATP.

Cell respiration is the controlled release of energy from organic compounds, such as glucose, to produce ATP. It happens in every living cell, all the time. It is not the same as breathing (ventilation), which only moves air into and out of the lungs.

Respiration is a series of enzyme-catalysed reactions that oxidize the organic compound in small steps, so that energy can be captured in ATP rather than lost as heat all at once. Glucose is the main respiratory substrate, but lipids and proteins can also be used.

Common mistake

Confusing respiration with breathing. Breathing is ventilation of the lungs; respiration is a biochemical process in every cell.

3 Aerobic respiration

With oxygen, glucose is fully broken down to CO₂ and water.

Aerobic respiration requires oxygen. Glucose is broken down completely to carbon dioxide and water, releasing a large amount of energy, enough to make about 30 to 32 ATP per glucose molecule. In eukaryotes, most of it happens in the mitochondria.

glucose + oxygen → carbon dioxide + water C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Variables affecting the rate of aerobic respiration include temperature (enzyme activity), availability of substrate and oxygen, and the energy demands of the cell (for example, muscle activity).

4 Anaerobic respiration in humans and yeast

Without oxygen, glucose is only partly broken down.

Anaerobic respiration occurs without oxygen. Glucose is only partially broken down in the cytoplasm, and only 2 ATP are produced per glucose molecule.

In humans (and other animals), anaerobic respiration produces lactate. It happens in muscles during intense exercise, when oxygen cannot be supplied fast enough. It allows a short burst of extra ATP. Lactate accumulates, and afterwards extra oxygen is needed to break it down (the oxygen debt, or excess post-exercise oxygen consumption).

In yeast (and plants), anaerobic respiration produces ethanol and carbon dioxide. This is fermentation. It is used in baking (CO₂ makes bread rise; ethanol evaporates during baking) and in brewing (ethanol in beer and wine), and in producing bioethanol as a fuel.

Humans: glucose → lactate Yeast: glucose → ethanol + carbon dioxide

Quick check

1. What is produced by anaerobic respiration in muscles?

2. What are the products of anaerobic respiration in yeast?

3. Why is anaerobic respiration useful in exercise?

Answers: 1. lactate 2. ethanol and carbon dioxide 3. rapid ATP when oxygen supply is insufficient

5 Comparing aerobic and anaerobic respiration

Same start, very different yields.

	Aerobic	Anaerobic (humans)	Anaerobic (yeast)
Oxygen	required	not required	not required
Location	cytoplasm and mitochondria	cytoplasm	cytoplasm
Products	CO₂ and water	lactate	ethanol and CO₂
ATP per glucose	about 30 to 32	2	2
Breakdown of glucose	complete	incomplete	incomplete

Key point

Both types begin with glycolysis in the cytoplasm. Only aerobic respiration uses mitochondria and oxygen, which is why it yields far more ATP.

6 Measuring the rate of respiration

A respirometer measures oxygen uptake.

A respirometer measures the rate of oxygen consumption by small organisms, such as germinating seeds or woodlice. The organisms are placed in a sealed tube with soda lime (or potassium hydroxide), which absorbs the carbon dioxide they produce. As they take in oxygen, the volume of gas decreases, so a coloured liquid in a capillary tube moves towards the organisms. The distance moved per minute gives the rate of oxygen uptake.

A control tube with glass beads (same volume) instead of organisms corrects for changes in temperature and pressure. Temperature must be kept constant, for example in a water bath. The respirometer can be used to investigate the effect of temperature on respiration rate.

WORKED EXAMPLE 1 Respirometer calculation

In a respirometer, the dye moved 24 mm in 10 minutes. The capillary tube has a cross-sectional area of 0.8 mm². The seeds had a mass of 4.0 g. Calculate the rate of oxygen uptake in mm³ g⁻¹ min⁻¹.

Solution

Volume = 24 × 0.8 = 19.2 mm³M1

Rate = 19.2 ÷ 10 ÷ 4.0 = 0.48 mm³ g⁻¹ min⁻¹A1

Figure 2. Effect of temperature on respiration rate of germinating seeds.

Quick check

1. Why is soda lime used in a respirometer?

2. What is the purpose of the control tube?

Answers: 1. absorbs CO₂ so volume change reflects O₂ uptake 2. corrects for temperature and pressure changes

7 ATP: the energy currency

Structure and role.

ATP (adenosine triphosphate) consists of adenine, ribose and three phosphate groups. Hydrolysis of the terminal phosphate releases energy (about 30 kJ mol−1) for active transport, muscle contraction, synthesis of macromolecules and movement of chromosomes. ATP is regenerated from ADP + Pi by respiration.

WORKED EXAMPLE 2 Properties (standard)

Explain why ATP is suited to its role as an energy currency.

Solution

Releases a small, manageable amount of energy per moleculeR1

Easily hydrolysed and regenerated; soluble, moves within cellsR1

Quick check

1. List three processes in cells that require ATP.

Answers: 1. active transport; muscle contraction; protein synthesis

8 Respiration versus gas exchange

Avoiding confusion.

Cell respiration is the controlled release of energy from organic compounds to produce ATP, occurring in every living cell. Gas exchange (ventilation) is the exchange of O2 and CO2 with the environment. They are linked: gas exchange supplies O2 for aerobic respiration and removes CO2.

WORKED EXAMPLE 3 Distinguishing (standard)

Distinguish cell respiration from ventilation.

Solution

Respiration: chemical reactions in cells producing ATPR1

Ventilation: movement of air in and out of the lungsR1

Common mistake

Using 'respiration' to mean breathing. In biology, respiration is a chemical process in cells; breathing is ventilation.

9 Aerobic respiration overview

Glucose to carbon dioxide and water.

C6H12O6 + 6O2 → 6CO2 + 6H2O (+ about 30–32 ATP). Glycolysis occurs in the cytoplasm; the link reaction, Krebs cycle and oxidative phosphorylation occur in mitochondria. Oxygen is the final electron acceptor.

WORKED EXAMPLE 4 Equation (basic)

Write the word and balanced equation for aerobic respiration of glucose.

Solution

glucose + oxygen → carbon dioxide + waterA1

C6H12O6 + 6O2 → 6CO2 + 6H2OA1

10 Anaerobic respiration in humans

Lactate fermentation.

Without enough oxygen, muscle cells convert pyruvate to lactate, regenerating NAD so glycolysis can continue, producing only 2 ATP per glucose. Lactate builds up, lowering pH; it is later converted back in the liver, requiring extra oxygen (oxygen debt / EPOC).

WORKED EXAMPLE 5 Sprinting (exam level)

Explain why sprinters continue to breathe heavily after a race.

Solution

Repay oxygen debtR1

Oxygen needed to oxidize lactate and restore ATP and creatine phosphateR1

11 Anaerobic respiration in yeast

Alcoholic fermentation.

Yeast converts pyruvate to ethanol and CO2 under anaerobic conditions. This is used in brewing (ethanol) and baking (CO2 makes dough rise; ethanol evaporates during baking).

WORKED EXAMPLE 6 Baking (standard)

Explain why dough rises.

Solution

Yeast respires anaerobically producing CO2R1

Gas bubbles trapped in the doughR1

In the real world

Bioethanol fuel is produced by yeast fermentation of sugars from crops such as sugar cane and maize.

12 Comparing aerobic and anaerobic respiration

Yield and products.

Feature	Aerobic	Anaerobic (humans)	Anaerobic (yeast)
oxygen	required	not required	not required
products	CO2, H2O	lactate	ethanol, CO2
ATP per glucose	about 30–32	2	2
location	cytoplasm and mitochondria	cytoplasm	cytoplasm

13 Respirometers

Measuring oxygen uptake.

A respirometer measures the rate of respiration by the uptake of O2: soda lime (or KOH) absorbs CO2, so the volume decrease equals O2 consumed, shown by movement of coloured liquid in a capillary tube. A control tube with glass beads accounts for temperature and pressure changes.

WORKED EXAMPLE 7 Calculation (exam level)

Liquid in a capillary of radius 0.5 mm moves 24 mm in 10 minutes for 2.0 g of germinating peas. Calculate the rate of O2 uptake per gram per minute.

Solution

Volume = π(0.5)2 × 24 = 18.8 mm3M1

Rate = 18.8/(10 × 2.0) = 0.94 mm3 g−1 min−1A1

Quick check

1. Why is soda lime included?

2. Why is a control tube used?

Answers: 1. to absorb CO2 so volume change reflects O2 uptake 2. to correct for temperature and pressure changes

14 Factors affecting the rate of respiration

Temperature and substrate.

Rate of respiration increases with temperature up to an optimum (enzymes), with glucose and oxygen availability, and with activity level. Very high temperatures denature enzymes.

Quick check

1. Predict the effect of cooling germinating seeds on the respirometer reading.

Answers: 1. slower movement: lower rate of respiration

15 Respiratory substrates

Carbohydrates, lipids, proteins.

Glucose is the main substrate, but lipids (fatty acids) and proteins (amino acids) can also be respired. Lipids release more energy per gram because they have more C–H bonds; they require more oxygen per gram.

WORKED EXAMPLE 8 Energy yield (standard)

Why do lipids release more energy per gram than carbohydrates?

Solution

More hydrogen atoms (C–H bonds) per gramR1

More electrons available for the electron transport chainR1

16 Respiratory quotient (RQ)

CO2 produced/O2 used.

RQ = volume of CO2 produced/volume of O2 consumed. Carbohydrates give RQ = 1.0; lipids about 0.7; proteins about 0.9. RQ indicates which substrate is being respired.

WORKED EXAMPLE 9 RQ (standard)

Calculate RQ for the respiration of the fat tripalmitin: 2C51H98O6 + 145O2 → 102CO2 + 98H2O.

Solution

102/145M1

= 0.70A1

17 Glycolysis (overview)

Splitting glucose.

Glycolysis occurs in the cytoplasm: glucose is phosphorylated using 2 ATP, split into two triose phosphates, then oxidized to two pyruvate, producing 4 ATP (net 2 ATP) and 2 reduced NAD. It does not require oxygen.

WORKED EXAMPLE 10 Net yield (standard)

State the net yield of glycolysis per glucose.

Solution

2 ATP, 2 NADH, 2 pyruvateA2

18 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 11 Mixed problem (exam level)

Explain why aerobic respiration yields much more ATP than anaerobic respiration.

Solution

Anaerobic: only glycolysis (2 ATP)R1

Aerobic: pyruvate fully oxidized; electron transport chain produces most ATPR1

19 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 12 Paper 1 question (standard)

Where does glycolysis occur? A. mitochondrial matrix B. cytoplasm C. inner mitochondrial membrane D. nucleus

Solution

BA1

WORKED EXAMPLE 13 Paper 2 question (exam level)

Describe how to use a respirometer to compare respiration rates of germinating and dry seeds.

Solution

Equal masses in separate respirometers with soda lime; same temperatureR1

Record movement of liquid over time; control tubeR1

Calculate volume of O2 per gram per minuteR1

20 Respiration and exercise

Energy systems.

Short bursts of activity use stored ATP and creatine phosphate; longer intense exercise uses anaerobic glycolysis (lactate); endurance exercise relies on aerobic respiration of glucose and fatty acids. Training increases mitochondria and capillaries in muscles.

Quick check

1. Why does endurance training increase the number of mitochondria?

Answers: 1. more aerobic respiration capacity, more ATP without lactate

21 Metabolic poisons

Disrupting respiration.

Cyanide inhibits cytochrome oxidase in the electron transport chain, stopping aerobic respiration. Uncouplers (such as DNP) let protons leak across the membrane, so energy is released as heat instead of ATP.

Quick check

1. Why is cyanide so toxic?

Answers: 1. it stops the electron transport chain, so cells cannot make enough ATP

22 Common misconceptions and links

Students often think…	Actually…
respiration is breathing	respiration is a chemical process in cells
anaerobic respiration produces no ATP	it produces 2 ATP per glucose
muscles produce ethanol	human muscles produce lactate

How C1.2 connects to the rest of the course

Topic	Connection
C1.1 Enzymes	metabolic pathways
B2.2 Organelles	mitochondrion structure
C1.3 Photosynthesis	chemiosmosis and ATP synthase
B3.3 Muscle (HL)	ATP in contraction
B1.1 Lipids	energy storage

23 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 3. Respiration rate of germinating seeds at different temperatures.

(a) State the word equation for aerobic respiration. [1]

(b) Explain the role of soda lime in a respirometer. [2]

(d) Compare anaerobic respiration in humans and yeast. [3]

Markscheme

Part	Answer	Marks
(a)	glucose + oxygen → carbon dioxide + water	A1
(b)	absorbs CO₂	R1
	so volume change reflects O₂ uptake only	R1
(c)	30 × 1.0 = 30 mm³ in 5 min	M1
	6 mm³ min⁻¹	A1
(d)	both in cytoplasm, 2 ATP, no oxygen	A1
	humans: lactate	A1
	yeast: ethanol and CO₂	A1

Examiner's comments. In (d), the best answers gave both similarities and differences.

24 Check your understanding

Section A: multiple choice

1. ATP is hydrolysed to

A. ADP and Pi B. AMP and glucose C. ADP and glucose D. adenine and ribose

2. Anaerobic respiration in human muscle produces

A. ethanol B. carbon dioxide C. lactate D. oxygen

3. Per glucose, anaerobic respiration yields

A. 30 ATP B. 2 ATP C. 0 ATP D. 38 ATP

4. In a respirometer, soda lime absorbs

A. oxygen B. water C. nitrogen D. carbon dioxide

5. Yeast fermentation is used in

A. cheese making only B. baking and brewing C. photosynthesis D. insulin production

6. Aerobic respiration mainly takes place in

A. mitochondria B. chloroplasts C. ribosomes D. nucleus

Section B: structured

7. Outline the roles of ATP. [3]

8. Compare aerobic and anaerobic respiration. [4]

9. Describe how a respirometer measures respiration rate. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Energy for active transport, synthesis, movement; hydrolysed to ADP + Pi; regenerated by respiration.

8. Aerobic: O₂, mitochondria, CO₂ + water, ~30 ATP; anaerobic: no O₂, cytoplasm, lactate or ethanol + CO₂, 2 ATP.

9. Organisms + soda lime; O₂ uptake reduces volume; dye moves; measure distance per time; control.

25 Summary and topic map

Idea	Remember
ATP	hydrolysed to ADP + Pi; energy for work
Respiration	controlled release of energy to make ATP
Aerobic	O₂; CO₂ + water; ~30 ATP
Anaerobic	lactate (humans), ethanol + CO₂ (yeast); 2 ATP
Respirometer	soda lime; measures O₂ uptake

Before the exam, make sure you…

•never confuse respiration with breathing
•state locations of each process
•give products of anaerobic respiration correctly
•calculate respirometer rates with units

Cell Respiration on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 4. Topic map for Cell Respiration.

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