Grade 9 Integrated Science Notes: Nutrition in Plants (Photosynthesis, Leaf Structure, Experiments)

Grade 9 Integrated Science · Strand 2.0 Living Things and their Environment · Sub-strand 2.1 Nutrition in Plants

These are free study notes for Grade 9 learners, written to the KICD curriculum design and taken from our full Grade 9 Integrated Science notes. Photosynthesis experiments (testing a leaf for starch, variegated leaves, plants kept in the dark) are favourite KJSEA questions, so read the experiment sections twice.

Green plants make their own food using sunlight, a process called photosynthesis. All the food we eat comes from it, directly or indirectly. In this topic we study the external and internal parts of a leaf, how the leaf is adapted for photosynthesis, the process itself and the experiments that show which conditions it needs.

The leaf

Adaptations of the leaf

Guard cells open the stomata in light and close them in darkness or when the plant is short of water.

Chloroplasts and chlorophyll

A chloroplast has a double membrane. Inside are stacks of discs called grana (one stack is a granum) that hold the green pigment chlorophyll, sitting in a jelly-like fluid called the stroma. Light is trapped in the grana; sugar is made in the stroma.

Chlorophyll absorbs mostly red and blue light and reflects green light, which is why leaves look green. A plant kept in darkness cannot make chlorophyll, turns pale yellow and cannot photosynthesise well.

The process of photosynthesis

Light energy trapped by chlorophyll splits water; the hydrogen combines with carbon dioxide to make glucose and oxygen is released. Glucose is used for respiration, stored as starch (in leaves, cassava, potatoes, maize and beans) or used to make cellulose for cell walls.

Conditions for photosynthesis

Testing a leaf for starch

1. Boil the leaf in water for about a minute to kill and soften it.
2. Put out all flames, then place it in methylated spirit in hot water to remove the chlorophyll.
3. Rinse it in warm water to soften it and spread it on a white tile.
4. Add iodine solution: blue-black means starch present; brown means no starch.

Photosynthesis gives all our food, releases the oxygen we breathe and removes carbon dioxide, and provides wood, charcoal, cotton and paper.

Test yourself: multiple choice

1. Which layer of the leaf carries out most photosynthesis?

   1. Cuticle
   2. Spongy mesophyll
   3. Palisade mesophyll
   4. Lower epidermis
   Show the answer

   C: It is packed with chloroplasts.

2. Gases enter and leave a leaf through the

   1. stomata.
   2. midrib.
   3. petiole.
   4. xylem.
   Show the answer

   A: Stomata are the pores for gas exchange.

3. What colour shows that starch is present when iodine is added?

   1. Brown
   2. Green
   3. Red
   4. Blue-black
   Show the answer

   D: Iodine turns blue-black with starch.

4. Why is a plant kept in darkness for 48 hours before a light experiment?

   1. To make chlorophyll
   2. To remove existing starch
   3. To kill it
   4. To add water
   Show the answer

   B: Destarching makes the test fair.

5. Which gas relights a glowing splint?

   1. Carbon dioxide
   2. Nitrogen
   3. Oxygen
   4. Hydrogen
   Show the answer

   C: Oxygen relights a glowing splint.

Practice questions

1. Write the word equation for photosynthesis. (2 marks)

   Show the answer

   carbon dioxide + water → glucose + oxygen (light and chlorophyll).

2. State three adaptations of the leaf for photosynthesis. (3 marks)

   Show the answer

   Any three: broad lamina; thin; many chloroplasts near the top; stomata and air spaces; network of veins.

3. Describe how to show that light is necessary for photosynthesis. (4 marks)

   Show the answer

   Destarch a potted plant in darkness for 48 hours; cover part of a leaf with foil; leave in light for a day; test for starch: the covered part stays brown and the uncovered part turns blue-black.

4. Why is the leaf boiled in water and then placed in methylated spirit during the starch test? (2 marks)

   Show the answer

   Boiling kills and softens it; methylated spirit removes the chlorophyll so the colour change is seen.