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Response

Responding to the environment: plants

Plants respond to light, gravity and water by growing toward or away from the stimulus. The growth is controlled by hormones, especially auxin.

CAPS

Grade 12 · Life Sciences · Response

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

  • Where it is examined

    Paper 1 is reproduction, responding to the environment, and homeostasis. Paper 2 is DNA, meiosis, genetics and evolution. Each paper is 150 marks.

  • 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 the plant “moves like an animal” using muscles, or that auxin is only found on the lit side.

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

light

Diagram

Worked theory

Worked theory. A mielie seedling on a windowsill, pot turned so the leaves face the room. Over days the shoot bends back toward the glass. The shaded side elongated. Turn the pot every morning and the bend is less, because no side stays shaded. Lay the same seedling on its side in the dark, so light is not the cause. The shoot still curves up and the root curves down. That second bend is gravity. A plant that has fallen in a storm and then grown a new upright tip is the same theory in a field.

More worked examples

  • The coleoptile experiments

    A grass shoot with its tip covered does not bend toward a lamp. A shoot with the tip cut off does not bend either. Put the tip back, off to one side, in the dark, and the shoot bends away from the side the tip sits on. The tip made a chemical, auxin, and the side that received more of it grew longer. The lamp was not pulling the plant. The shaded side grew.

  • A pot laid on its side

    A seedling in a pot is laid on its side in the dark, so light is not the cause. Overnight the shoot bends up and the root bends down. Auxin on the lower side of the shoot makes that side grow longer, so the shoot curves up. In the root, the extra auxin on the lower side slows growth, so the root curves down. The same chemical, two results. A lamp was not pulling the plant. Gravity was the information.

In ordinary words

A tropism is a growth response. Phototropism is growth toward light. Geotropism is growth in answer to gravity: shoots up, roots down. Auxin gathers on the shaded side of a shoot and makes those cells longer, so the shoot bends toward the light. Roots respond to the same hormone differently, which is why a root on its side curves down. Plants have no nerves and no muscles. The response is growth, so it is slow.

In South Africa

A seedling on a classroom windowsill, bent toward the glass, is the whole of phototropism.

Part 1 / 4

What it is

Plants respond to light, gravity and water by growing toward or away from the stimulus. The growth is controlled by hormones, especially auxin.

More worked examples

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

Example 1 / 20

A seedling is lit from one side and bends towards the light. Explain using auxins.

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 seedling is lit from one side and bends towards the light. Explain using auxins.

Before any working — hunt first

Given

Numbers and units already in the sentence: no numbers yet — only a name, a diagram, or a worded unknown. 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 using auxins. Circle that word — that is the missing variable.

Find

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

Calculate

What we need to calculate: Auxin moves to the shaded side. Then substitute the given, one line at a time, to get Explain using auxins.

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

If you have never seen this before: Auxin moves to the shaded side. 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?

Phototropism.

3. Which method — and why this one?

Auxin moves to the shaded side.

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: no numbers yet — only a name, a diagram, or a worded unknown. 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 using auxins. Circle that word — that is the missing variable. Find: We are required to find: Explain using auxins. Write that as a symbol on the left, with a question mark, before you touch a formula. Calculate: What we need to calculate: Auxin moves to the shaded side. Then substitute the given, one line at a time, to get Explain using auxins.

    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

    Phototropism.

    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

    Auxin moves to the shaded side.

    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

    Auxin accumulates on the shaded side and causes those cells to elongate more, so the stem bends towards light.

    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

    Unequal auxin causes uneven growth towards light.

    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

Unequal auxin causes uneven growth towards light.

Copy onto your answer sheet — in this order

Answer: Unequal auxin causes uneven growth towards light.

6. Check it like a marker

Positive phototropism.

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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Evolution by natural selection

Life Sciences

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