Energy

Cellular respiration

Cellular respiration is how cells release energy from glucose. It is not the same thing as breathing.

CAPS

Grade 11 · Life Sciences · Energy

Life Sciences is two papers. Use the strand to know which one.

Where it is examined
Grade 11 works through energy transformations, gas exchange, excretion, population ecology and human impact. The same two-paper pattern is used.
How to write it
Name the structure, then say what it does. A labelled diagram only scores if the labels are the biological names. “It helps the body” is not a function.
How a marker reads it
A common mix-up is to write photosynthesis in reverse and call that the whole of respiration, or to place respiration in chloroplasts.

A 2-mark “explain” wants two facts, or one fact and its reason. Extra sentences that contradict the first one can cancel a mark.

Diagram

Worked theory

Worked theory. A learner sprints, then walks. During the sprint the muscle mitochondria cannot get oxygen fast enough, so some glucose is broken down anaerobically and lactic acid builds. That is the burn. In the walk, oxygen catches up and the lactic acid is dealt with. The same learner’s cells were already doing aerobic respiration before the sprint. Plants do this too, in their mitochondria, including at night when they cannot photosynthesise. A breathing rate going up is the supply line, not the reaction itself.

More worked examples

Bread dough and a 100 m
Yeast in dough has little oxygen. It breaks glucose into ethanol and carbon dioxide, and the dough rises from the gas. That is anaerobic, and it pays the yeast very little ATP. A learner in a 100 m race does the human version: muscles make lactic acid, not ethanol, and the ATP is still a fraction of the aerobic yield. Ten minutes later, with oxygen, the same muscles are back to aerobic respiration in the mitochondria. The dough never grows mitochondria for that. The runner already had them.
Thirty-eight and two
A learner sitting and reading is using aerobic respiration. Glucose is broken down fully, in the mitochondria, and the school table gives that yield as about 38 ATP. The same learner in a hard sprint cannot supply oxygen fast enough. Muscles then use anaerobic respiration and the table gives 2 ATP, plus lactic acid. The glucose was available both times. The oxygen, and the mitochondria’s full path, were not. Yeast’s anaerobic product is ethanol. The runner’s is lactic acid. The small yield is the shared point.

In ordinary words

Aerobic respiration needs oxygen and happens mostly in mitochondria: glucose + oxygen gives carbon dioxide + water + a usable packet of energy, ATP. Anaerobic respiration in human muscle has no oxygen and ends in lactic acid, with far less ATP. Yeast’s anaerobic path makes ethanol and carbon dioxide. Breathing only brings the oxygen in and the carbon dioxide out. The energy release is inside the cell.

In South Africa

The burn in a 100 m on sports day, then the heavy breathing after, is anaerobic then aerobic.

Part 1 / 4

What it is

Cellular respiration is how cells release energy from glucose. It is not the same thing as breathing.

Photosynthesis and cellular respiration — Diagram

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

Write the equation for aerobic respiration and name where it mainly occurs.

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Write the equation for aerobic respiration and name where it mainly occurs.

In other words — as if you have never seen this before

If you have never seen this before: Glucose + oxygen. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

Aerobic respiration.

3. Which method — and why this one?

Glucose + oxygen.

4. Step-by-step working

01 / 05
Write the formula
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP).
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Identify the given
Glycolysis in the cytoplasm; Krebs cycle and electron transport in the mitochondria.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
04 / 05
Step F — Box the answer with its unit
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP, mostly in mitochondria.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP, mostly in mitochondria.

Copy onto your answer sheet — in this order

Answer: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP, mostly in mitochondria.

6. Check it like a marker

Aerobic gives far more ATP than anaerobic.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

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Gaseous exchange

Life Sciences

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