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Energy

Photosynthesis

Photosynthesis is how green plants store light energy as glucose, using carbon dioxide and water, and releasing oxygen.

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 say photosynthesis happens in mitochondria, or that plants do not respire.

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

chloroplast

Diagram

Worked theory

Worked theory. A spinach leaf in sun on a Pretoria windowsill. Light hits the palisade cells. Water has come up the xylem. Carbon dioxide enters through open stomata. Glucose is made and can be stored as starch. At night the light is gone, so photosynthesis stops, even though the leaf is still alive. The same leaf will still respire at night. Cover the leaf with foil and test later with iodine: the covered part stays brown, the lit part goes blue-black. No light, no new starch.

More worked examples

  • Pondweed bubbles

    A sprig of pondweed under a funnel in bright light gives off a stream of bubbles, about 30 in five minutes. Move the lamp away and the count falls to about 8. Switch the lamp off and the count falls to almost none. The gas that relights a glowing splint is oxygen. The same sprig in the dark is still alive. It has just stopped photosynthesising. Add a pinch of bicarbonate, which supplies carbon dioxide, and the bubble count in the light rises again.

  • A leaf with a white edge

    A variegated leaf is left in the dark for two days so stored starch is used up, then put in the light. Iodine later turns the green part blue-black and leaves the white edge brown. The white edge had no chlorophyll, so it made no starch even though it had the same light, the same air and the same leaf stalk. Light alone did not make the starch. Chlorophyll had to be there to catch it.

In ordinary words

It happens in chloroplasts. Chlorophyll absorbs the light. In the light-dependent part, light splits water and oxygen is given off. In the light-independent part, carbon dioxide is built into glucose. The word equation is carbon dioxide + water, with light and chlorophyll, gives glucose + oxygen. A leaf is thin, broad and full of stomata so that light and carbon dioxide can reach the chloroplasts.

In South Africa

A foil-and-iodine test on a leaf from the school garden is the worked case.

Part 1 / 4

What it is

Photosynthesis is how green plants store light energy as glucose, using carbon dioxide and water, and releasing oxygen.

Diagram
Diagram

More worked examples

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

Example 1 / 20

A water plant gives off 12 bubbles per minute with a lamp 10 cm away and 3 bubbles per minute at 20 cm. Explain the result.

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: A water plant gives off 12 bubbles per minute with a lamp 10 cm away and 3 bubbles per minute at 20 cm. Explain the result.

Before any working — hunt first

Given

Numbers and units already in the sentence: 12 bubbles · 10 cm · 3 bubbles · 20 cm. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: Explain the result. Circle that word — that is the missing variable.

Find

We are required to find: Explain the result. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

What we need to calculate: Light intensity is a limiting factor. Then substitute the given, one line at a time, to get Explain the result.

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

If you have never seen this before: Light intensity is a limiting factor. 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?

Bubbles: 12/min at 10 cm, 3min at 20 cm.

3. Which method — and why this one?

Light intensity is a limiting factor.

4. Step-by-step working

  1. 01 / 08

    Step A — Read the question twice

    Slow down. Circle the word that names the unknown. Underline every number and its unit. Do not grab a formula yet — first decide what the paper is actually asking.

    Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

  2. 02 / 08

    Step A2 — Hunt: Given, Missing, Find, Calculate

    Given: Numbers and units already in the sentence: 12 bubbles · 10 cm · 3 bubbles · 20 cm. Copy each one into a column. A name without a number is not given. Missing: After you copy the given, the letter that still has no number is missing. The paper is pointing at: Explain the result. Circle that word — that is the missing variable. Find: We are required to find: Explain the result. Write that as a symbol on the left, with a question mark, before you touch a formula. Calculate: What we need to calculate: Light intensity is a limiting factor. Then substitute the given, one line at a time, to get Explain the result.

    Why this step: Do not name the letters yet. First hunt: what already has a number (Given), what has no number (Missing), the word the paper wants (Find), and which formula still contains the missing letter (Calculate).

  3. 03 / 08

    Step B — Write the given in a column

    Bubbles: 12/min at 10 cm, 3min at 20 cm.

    Why this step: A column with units stops you mixing cm with m, or minutes with seconds. Markers can also see you started correctly.

  4. 04 / 08

    Step C — Name the method in words

    Light intensity is a limiting factor.

    Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

  5. 05 / 08

    Explain

    At 20 cm, light intensity is lower (about ¼, inverse square law), so less light energy drives photosynthesis and less oxygen is produced.

    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.

  6. 06 / 08

    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.

  7. 07 / 08

    Step F — Box the answer with its unit

    Closer lamp → brighter light → faster photosynthesis → more O₂ bubbles.

    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.

  8. 08 / 08

    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

Closer lamp → brighter light → faster photosynthesis → more O₂ bubbles.

Copy onto your answer sheet — in this order

Answer: Closer lamp → brighter light → faster photosynthesis → more O₂ bubbles.

6. Check it like a marker

Keep temperature constant (use a heat shield).

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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