DSE Biology | Respiration & Gas Exchange Lesson-Prep Guide (Tutor Edition)

DSE Biology | Respiration & Gas Exchange Lesson-Prep Guide (Tutor Edition) | Aerobic, Anaerobic and RQ

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can recite “glucose plus oxygen gives carbon dioxide and water”, yet cannot say why they pant after running. This lesson turns the equation into an energy account: how much oxygen goes in, how much carbon dioxide comes out, how much energy is left.

Objectives — Students can: (1) write the aerobic and anaerobic equations and state the energy difference; (2) calculate respiration rate and respiratory quotient from gas volumes; (3) explain that ventilation rises during exercise to supply oxygen and clear carbon dioxide, not to “change the air” for its own sake.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Molar volume	Volume of 1 mol of gas at room conditions	24 dm3
Diffusion	Which way gas moves	High concentration to low
Energy units	How many J in 1 kJ	1000 J

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Recall: the two equations	Aerobic and anaerobic	Write 6CO2 + 6H2O
8-20 min	The energy budget	0.010 mol glucose	Releases 29 kJ
20-32 min	Anaerobic fermentation	Same amount of glucose	0.92 g ethanol + 480 cm3 CO2
32-44 min	Gas exchange	500 cm3 per breath, 12 breaths a minute	6.0 dm3 per minute
44-54 min	Respiratory quotient	CO2 200, O2 250	RQ = 0.80
54-60 min	Exit check	Three questions, marked at once	Independent

03 Teaching script (read it out)

Scene 1: Respiration is an energy account, not a slogan

Complete oxidation of 1 mol of glucose releases about 2870 kJ; 0.010 mol therefore gives 29 kJ. Most of that leaves as heat rather than ATP, which is exactly why exercise makes you hot.

Board note: Respiration is an energy account, not a slogan

Scene 2: Anaerobic: the same glucose, a fraction of the return

0.010 mol of glucose fermented gives 0.020 mol of ethanol (0.92 g) and 0.020 mol of CO2 (480 cm3). Only around 210 kJ per mol is released, so most of the energy stays locked in the ethanol.

Board note: Anaerobic: the same glucose, a fraction of the return

Scene 3: Respiratory quotient: reading the fuel from gas volumes

An animal takes in 250 cm3 of O2 and gives out 200 cm3 of CO2 in 10 minutes. Respiration rate = 250 / 10 = 25.0 cm3/min; RQ = 200 / 250 = 0.80. An RQ near 0.8 means a mixed diet, near 1.0 means mostly carbohydrate.

Board note: Respiratory quotient: reading the fuel from gas volumes

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Respiration rate	Change in volume / time	2.4 cm3/min
Calculation	Respiratory quotient	CO2 out / O2 in	0.80
Calculation	Anaerobic yield	mol glucose x 2	0.92 g ethanol
Explanation	Panting in exercise	Supply O2, clear CO2 and lactate	Ventilation rises

Sanity check (teach this): Volumes must be at the same temperature and pressure; if pressure changes, correct with the gas laws. Volume ratio equals mole ratio only at the same T and P. Anaerobic CO2 is not aerobic CO2: the two equations give 6 mol against 2 mol per glucose.

05 Independent practice (3 questions)

Question	Answer	What to watch
Minute volume for 500 cm3 x 12	6.0 dm3/min	x / 1000
Rate using 24 cm3 of O2 in 10 min	2.4 cm3/min	divide by 10
RQ for CO2 200, O2 250	0.80	volume ratio
Ethanol from 0.010 mol glucose	0.92 g	mol x 2 x 46

06 Common mistakes and what to say

Mistake	Student writes	Say this
Swapping the two equations	Glucose gives lactate plus 6CO2	“Aerobic always gives six CO2; lactate fermentation gives none.”
Treating RQ as efficiency	RQ 0.8 means 80% energy	“RQ only tells you the fuel, not the efficiency.”
Forgetting the volume-to-mole step	Treating 24 cm3 as 24 mol	“24 cm3 is 0.001 mol, not 24.”
Thinking plants stop respiring at night	Saying they only take in CO2 at night	“Light controls photosynthesis; respiration runs all day.”

07 Homework (paper, section and question numbers)

DSE Biology practice paper (respiration and metabolism) — Questions 3-6
DSE Biology practice paper (gas exchange experiments) — Questions 7-8

08 Exit check (last 5 minutes)

Question	Answer	Decision
Rate for 24 cm3 in 10 min	2.4 cm3/min	Wrong: redo the time division
What RQ 0.80 means	Mixed diet	Wrong: revisit fuels
Ethanol from 0.010 mol glucose	0.92 g	Wrong: redo mol x 2

09 Tutor FAQ

Why does anaerobic respiration give CO2 but no water?

The oxygen ends up in ethanol and CO2, leaving no extra oxygen to form water.

Can RQ exceed 1?

Briefly, during fat synthesis, but DSE questions stay between about 0.7 and 1.0.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.