Molecules

The chemistry of life

The chemistry of life is the small set of molecules every living thing is built from: water, carbohydrates, proteins, lipids and nucleic acids.

CAPS

Grade 10 · Life Sciences · Molecules

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

Where it is examined
Grade 10 is the chemistry of life, cells, tissues, organs, support and transport, and the biosphere, biodiversity and the history of life.
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 call an enzyme a carbohydrate, or to think Benedict’s tests for starch. Iodine is starch. Benedict’s is reducing sugar.

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 maize porridge meal. The starch in the maize is a carbohydrate. Iodine on a scrap goes blue-black, so starch is there. The milk on the side has protein, so Biuret would go purple. Cooking does not turn starch into protein. An enzyme in saliva, amylase, starts breaking that starch into sugar, but only while it is the right shape. Boiling the saliva first would denature the enzyme and the starch would stay starch.

More worked examples

Bread, egg and cooking oil
Iodine on fresh bread goes blue-black, so the bread is mostly starch. Biuret on a piece of boiled egg white goes purple, so that is protein. Cooking oil shaken with ethanol and water turns milky, so that is fat. Heat the saliva before it touches the bread and the iodine still goes blue-black afterwards: the amylase was denatured and never broke the starch down.
Apple juice and a boiled egg white
Iodine on apple juice stays brown, so the juice is not starch. Benedict’s, heated with the same juice, turns orange, so a reducing sugar is there. Biuret on a boiled egg white goes purple, and Benedict’s on that egg white does not turn orange. Boiling the egg white did not turn the protein into sugar. It only changed the shape. The test names the molecule. The colour is not a decoration.

In ordinary words

Water is the solvent. Carbohydrates store and supply energy. Proteins do the work, including enzymes. Lipids store energy and build membranes. Nucleic acids store the instructions. Enzymes are proteins that speed one reaction. Heat or the wrong pH changes their shape, so they stop working. Food tests name the molecule: iodine turns blue-black with starch, Biuret turns purple with protein, Benedict’s turns orange with a reducing sugar, and an ethanol emulsion shows fat.

In South Africa

Pap, bread, milk and cooking oil on a South African table are the four food tests in real food.

Part 1 / 4

What it is

The chemistry of life is the small set of molecules every living thing is built from: water, carbohydrates, proteins, lipids and nucleic acids.

The chemistry of life / biological-molecules — Diagram

More worked examples

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

Example 1 / 20

A learner adds iodine solution to four foods. Bread and potato turn blue-black; apple juice and egg white stay yellow-brown. Which foods contain starch, and which test should be used next to check the egg white for protein?

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 learner adds iodine solution to four foods. Bread and potato turn blue-black; apple juice and egg white stay yellow-brown. Which foods contain starch, and which test should be used next to check the egg white for protein?

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

If you have never seen this before: Iodine turns blue-black only with starch. Protein is tested with Biuret reagent. 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?

Iodine test results for four foods.

3. Which method — and why this one?

Iodine turns blue-black only with starch. Protein is tested with Biuret reagent.

4. Step-by-step working

01 / 05
Identify the given
Blue-black = starch present: bread and potato. Yellow-brown (no change) = no starch: apple juice and egg white.
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
Apply
Add Biuret reagent (sodium hydroxide + copper sulfate) to egg white. Purpleviolet=protein present.
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
Bread and potato contain starch. Test egg white with Biuret reagent; it should turn purple.
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

Bread and potato contain starch. Test egg white with Biuret reagent; it should turn purple.

Copy onto your answer sheet — in this order

Answer: Bread and potato contain starch. Test egg white with Biuret reagent; it should turn purple.

6. Check it like a marker

Iodine never tests for protein or sugar — match each reagent to its nutrient.

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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