PHOTOSYNTHESIS
Photosynthesis is the process by which green plants, some bacteria and
some Protoctists their own food using water, carbon dioxide and energy
from the sun.
The process of photosynthesis mostly takes place in the green part of the
plants, mostly in the leaf
Photolysis is the process whereby water is broken down into hydrogen
and oxygen ions.
The Leaf’s Adaptation for Photosynthesis.
The external structure of a leaf can be viewed using either using hand lens
or unaided eyes.
❖ The petiole
The petiole or leaf stalk attaches the leaf to the branch or stem. It
keeps the lamina in a position that will enable it to get maximum
amount of sunlight.
❖ The lamina is the expanded portion or blade of a leaf. It has a
large surface area. This maximizes the absorption of sunlight
energy and carbon dioxide. The lamina is also thin so that carbon
dioxide gas can diffuse and sunlight energy can penetrate over a
short distance to reach cells.
❖ The mid-rib which gives rise to veins and veins contain xylem
and phloem. Xylem transports water and mineral salts and Phloem
transports manufactured food from the leaf to other parts of the
plant.
Figure; 8.1: External structure of leaf
❖ The cuticle: This is the outermost transparent and waxy layer of
the leaf.
✓ It allows light to penetrate into the photosynthetic cells.
✓ It also protects the leaf from injury, pests, and excessive
loss of moisture. There is no cuticle on the stomata in
order to allow gaseous exchange
❖ The epidermis – allows sunlight to penetrate the leaf easily
❖ Stomata (singular is stoma) are small pores in the epidermis.
They allow oxygen and carbon dioxide to diffuse in and out of
the leaf. Stomata are surrounded by guard cells that close and
open the pores. The guard cells also have chloroplasts to allow
photosynthesis.
Note; Broad leaves, such as those of bean plants, have more
stomata on the lower epidermis than on the upper epidermis
❖ Vascular bundles. These are xylem and phloem tissues which
serve as conducting tissues. Xylem transports water and mineral
salts. Phloem transports manufactured food from the leaf to
other parts of the plant.
Importance of photosynthesis
❖ Photosynthesis produces food for the plants and animals that
feed on plants
❖ Photosynthesis produces oxygen that helps to replace the
oxygen that has been used up during burning, respiration and
other processes
❖ The process of photosynthesis uses carbon dioxide from the
atmosphere which is harmful to the environment
❖ Photosynthesis is the source of fuel. The remains of dead plants
and animals form fossil fuels such as coal, oil and natural gas.
❖ Photosynthesis is important for the lives of plants as well as
animal.
Conditions necessary for photosynthesis
❖ Chlorophyll that is responsible for trapping sunlight energy
that is used during photosynthesis.
❖ Sunlight(Light energy) causes photolysis, a chemical
process whereby water molecules (H
2
O) are split into
hydrogen ions (H
+
) and hydroxyl ions (OH
-
).
Raw material of photosynthesis
5. Place the leaf in the watch glass or white tile or petri dish and add
few drops of iodine solution and observe the colour change. If the
leaf changes into blue-black this means starch is present in the leaf.
Note if you are given variegated leaf, blue-black colour will be
observed in the green part of the leaf only, Why, due to presence of
chlorophyll which is the one of the condition for
photosynthesis(photosynthesis takes place only on the green part of
the leaf)
Note
If you are provided with variegated leaf, only green part of the variegated
leaf blue-black colour will be observed and the aim of the experiment an
experiment to determine whether chlorophyll is necessary for
photosynthesis
Example
2. You are provided with specimen P.
Carry out an experiment as instructed below:
1.1. Make a drawing to show the colour pattern of specimen P.
1.2. Dip specimen P in hot water for about one minute.
1.3. Boil specimen P in alcohol using a hot water bath.
1.4. Dip the boiled specimen P in hot water.
1.5. Spread specimen P on a white tile and add 2 - 3 drops of iodine
solution on the specimen.
Questions
(a) (i) Identify specimen P
(ii) What observation did you make after applying iodine solution to
specimen P?
(b) Why was specimen P
(i) Dipped in hot water at the beginning of the experiment
(ii) Boiled in alcohol
(iii) Dipped in hot water after boiling in alcohol?
(c) Why was the alcohol boiled using a hot water bath?
(d) (i) What is the aim of the experiment?
(ii) Name the physiological process which was being investigated.
(iii) What conclusion can you make from this experiment?
Note
The specimen was variegated leaf
PRACTICAL ACTIVITY
1.
You are provided with specimen M, hibiscus leaf, methylated spirit,
iodine solution, source of heat, test tube, 250 ml beaker, dropper,
forceps, water and white tile
Procedure
(i)
Take a leaf from a plant that has been in the sunlight at least
6 hours.
(ii)
Put the leaf in boiling water for 2-3 minutes
(iii)
Remove the leaf rom the methylated spirit and rinse it in warm
water. The water softens the leaf.
(iv)
Put the leaf on a White tile. Add 2-3 drops of iodine solution on
the leaf, what do you observe?
Questions
(a) (i) Identify specimen M by its common name
(ii)What observation did you make after applying iodine solution to
specimen M?
(b) Why was specimen M
(i) Dipped in hot water at the beginning of the experiment
(ii) Boiled in alcohol
(iii) Dipped in hot water after boiling in alcohol?
(c) Why was the alcohol boiled using a hot water bath?
(d) (i) What is the aim of the experiment?
(ii) Name the physiological process which was being investigated.
(iii) What conclus1on can you make from this experiment?
Answers
(a) (i) specimen M was hibiscus leaf
(ii) The blue-black colour was observed on the leaf
(b) (i) In order stops any chemical from taking place(photosynthesis)
(ii) This decolourise the leaf (removal of chlorophyll)
(iii) To soften the leaf
(c) Because it is highly flammable.
(d) (i) Testing a plant leaf for starch
(ii)Photosynthesis
(iii)The blue-black colour indicates the presence of starch
