Photosynthesis and Respiration, Grade 8
Boiling a leaf in ethanol over a naked Bunsen flame is how school laboratories catch fire.
The starch test is the best practical in Grade 8 Term 1 and it is the one most likely to go wrong, because the step everybody rushes is the step that matters: the ethanol never goes near the flame. It sits in a small beaker, inside a larger beaker of hot water, with the burner switched off.
This page covers the starch test in a leaf properly, the two carbon dioxide tests that finish the topic, and the one number in the textbook that is worth arguing with.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 1 |
| Strand | Life and living |
| Topic | Topic 1, Photosynthesis and respiration |
| Status | Not a formally assessed practical task. The topic revision in the book is 20 marks |
| Marks | None prescribed. Our worksheet is 40 marks |
The two equations, and why one of them is not a chemical equation
Photosynthesis and respiration are the same equation read in opposite directions. Get that across and half the topic teaches itself.
| Photosynthesis | Respiration | |
|---|---|---|
| Where | Green parts of plants, in the chloroplasts | Every living cell, plant and animal |
| When | Only in light | All the time, day and night |
| Takes in | Carbon dioxide and water | Glucose and oxygen |
| Gives out | Glucose and oxygen | Carbon dioxide and water |
| Energy | Stores light energy as chemical energy | Releases that energy for the cell to use |
Plants respire too, and they respire all night. This is the single most common misconception in the topic. A plant is not the opposite of an animal; it does both jobs, and only one of them needs the sun.
A note on wording. Textbooks often print the word version and call it "the chemical equation for respiration". It is a word equation. The chemical equation uses formulae and is balanced:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Grade 8 only needs the word equation. Calling it by the right name now saves an argument in Grade 10.
Practical 1: testing a leaf for starch
Why starch and not glucose: a leaf makes glucose, then converts it to starch for storage almost immediately. Starch stays put and gives a colour change you can see across a room. Glucose moves away and needs a heated water bath to test.
Before the lesson: destarch the plant
Put the plant in a dark cupboard for two full days. Skip this and every leaf tests positive, including your control, and the practical proves nothing.
A geranium (pelargonium) is the standard plant because the leaves are thin, they destarch reliably and they are in almost every South African garden. A variegated geranium, one with white patches, is worth hunting for, because it gives you the control and the test on a single leaf.
What you need
Destarched geranium leaf · Bunsen burner · tripod and gauze · 500 ml beaker · small beaker or boiling tube · forceps · white tile · dropping pipette · iodine solution · ethanol or methylated spirits · water · safety goggles
Method
- Boil the leaf in water for one minute. This kills the cells and softens the leaf so the stain can get in
- TURN THE BURNER OFF. Not down. Off, and move it away from the bench
- Put the leaf in a small beaker of ethanol, and stand that small beaker in the hot water you just boiled. The water bath does the heating
- Wait until the leaf is almost white and the ethanol has gone dark green. Five to ten minutes. The ethanol is now full of the chlorophyll that was hiding the result
- Dip the leaf back into the hot water for a few seconds. Ethanol leaves the leaf brittle, and this softens it enough to unfold without tearing
- Spread it flat on a white tile
- Cover it with iodine solution and wait a minute
Ethanol boils at 78 °C and its vapour catches from a flame across the bench. The burner is off before the ethanol comes out of the cupboard. This is not caution, it is the reason this practical is sometimes banned in schools that have had an incident.
What you should see
| Leaf | Colour with iodine | Means |
|---|---|---|
| Destarched, then in light for a day | Blue-black | Starch present. Photosynthesis happened |
| Destarched, kept in the dark | Orange-brown, the colour iodine already is | No starch. No light, no photosynthesis |
| Variegated leaf, green part | Blue-black | Chlorophyll present |
| Variegated leaf, white part | Orange-brown | No chlorophyll, no starch, on the same leaf, in the same light. This is the whole lesson in one specimen |
Orange-brown is not a failed test. It is a negative result, and a negative result next to a positive one is what makes the experiment an experiment. Learners who write "the test did not work" have lost the point and usually the marks.
Practical 2 and 3: the two carbon dioxide tests
Lime water goes milky when carbon dioxide bubbles through it. That one reaction carries both of these.
Breathing out
Two test tubes of clear lime water. Draw air in through one and blow out through the other, using a straw in each, and count the same number of breaths.
The tube you breathed out into goes milky. The one you drew air in through takes far longer, or stays clear. Exhaled air has about a hundred times more carbon dioxide than the air in the room, which is why the difference is so obvious.
Never suck lime water up the straw. Lime water is calcium hydroxide and it is alkaline. One straw for blowing, one for drawing in, and label them before you start.
Germinating seeds
Soak bean seeds overnight, then put them in a sealed jar with a small open container of lime water inside it. A second jar with boiled, dead seeds is the control.
After a day the lime water in the living-seed jar is milky. Seeds are not green and they are in the dark, so this cannot be photosynthesis. It is respiration, and it is the cleanest proof in the topic that plants respire.
Put a thermometer in each jar and the living seeds read a degree or two warmer. Respiration releases energy, and some of it leaves as heat.
If it does not work
| What you see | What caused it |
|---|---|
| Every leaf goes blue-black, including the dark control | The plant was not destarched. Two days in the dark, not two hours |
| The leaf stays green after the ethanol | Not long enough, or the water bath went cold. Reheat the water with the leaf out of the way |
| The leaf tears to pieces | You skipped the dip back into hot water after the ethanol. Brittle leaf, torn leaf |
| Iodine sits in beads and does not spread | The leaf is still wet. Blot it, then flood it |
| The colour is hard to judge | White tile, good light, and always run a known positive beside it. Judging one colour alone is guesswork |
| Lime water will not go milky at all | Old lime water absorbs carbon dioxide from the air and goes off. Make it fresh, or filter it clear before use |
| The lime water goes milky then clears again | You kept blowing. Excess carbon dioxide redissolves the precipitate. Stop when it turns |
| Both seed jars go milky | The "dead" seeds were not boiled long enough, or the jar was opened. Ten minutes of boiling, then cool, then seal |
Safety
- Ethanol and methylated spirits are flammable. The burner is off, and away, before the ethanol appears. Heat only in a hot water bath
- Iodine stains skin, clothes and benches permanently. One dropper, one tile, goggles on
- Lime water is alkaline. Blow only, never draw in. Rinse any splash off skin at once
- Hot beakers look exactly like cold ones. Tongs, and a heatproof mat
- Tie back long hair before any Bunsen work
What the textbook gets wrong here
Three things worth knowing before you teach this.
- The word equation is printed as "the chemical equation". It is not one. Use the word equation and call it that
- Some editions call the leaf-rolling of erica plants a functional adaptation. A rolled leaf edge is a physical feature, so it is a structural adaptation. Learners get marked on this distinction
- A hand lens will not show you a micro-organism, whatever the introduction to Topic 3 says. At 10x a bacterium is still invisible. That is what the microscope is for
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is the starch test with the variegated leaf, Part B the two carbon dioxide tests, Part C the two word equations and the day-and-night question
- Marking memorandum, with the full answers and the four answers that look right and score nothing
Related practicals
- Food chains and food webs, Grade 8. Where the glucose made here goes next
- Micro-organisms and the growth of yeast, Grade 8. Practical task 1, and respiration again, in a fungus
- Plant and animal cells, Grade 9. The chloroplasts this page depends on, seen at 400x
What you need to run it
Most of this is standard heating apparatus that a school already owns.
The heating set is a Bunsen burner at R600, a stainless steel tripod at R205 and ceramic-centred wire gauze at R26 each. Buy gauze by the ten: it is the piece that warps and gets thrown out.
You need two sizes of glass beaker, a large one for the water bath and a small one to stand inside it, plus borosilicate test tubes from R5,96 for the lime water. Blunt forceps at R20 move a hot leaf without shredding it, and a glass dropping pipette at R10 applies the iodine.
A laboratory thermometer turns the germinating-seed jar into a second measurement, and safety goggles at R30 are not optional near ethanol.
Two chemicals on this page are not in our catalogue yet: iodine solution and lime water. We are sourcing both in school pack sizes. Until then, a pharmacy sells iodine, and lime water is made by shaking builder's lime in water and filtering it clear. Buy the equipment from us and the two chemicals locally, rather than waiting for us.
For the wider range, see laboratory glassware and heating apparatus.