BIOLOGY&

H.S.BLesson

PHOTOSYNTHESIS

Syllabus: Specific Objectives

• At the end of this lesson you should be able to:

• Explain the process of photosynthesis;

• Investigate the effect of light and chlorophyll on the production of starch.

• Explain the ways in which other living organisms depend on plants directly/indirectly for food.

What is Photosynthesis?

Definition:

• The process by which green plants (and some bacteria) use light energy, water and carbon dioxide to make theirown food.

• The light energy is usually obtained from the sunlight and it is absorbed by chlorophyll.

• The food produced is glucoseand Oxygen is a by-product of this reaction.

Word and Chemical Equation used to summarize the process

Chemical Equation:

• 6CO2 + 6H20 C6H12O6 + 6O2

• Word equation: Carbon Dioxide + water = glucose + oxygen

• Raw materials: carbon dioxide + water

• Products: Glucose + oxygen

SUNLIGHT

CHLOROPHYLL

Site of Photosynthesis

• Plant Organ: Leaf

• Plant tissue: Palisade Mesophyll tissue

• Plant Cell: Palisade Mesophyll cell

• Cell Organelle: Chloroplast

Chloroplasts present in Elodea plant leaf cells

Leaf Anatomy

A section of a Leaf under the microscope

Internal structure of A Leaf • The leaf has a waxy cuticle to stop

it losing water.

• The epidermis is a protective layer of cells and contains no chloroplasts.

• The palisade layer contains the most chloroplasts as it is near the top of the leaf. The chloroplasts contain the pigment chlorophyll. It is here that photosynthesis takes place.

• The palisade cells are arranged upright. This means the light has to pass through the cell lengthways and so increases the amount of light absorbed.

Stages of Photosynthesis• There are two main stages of photosynthesis, one

that depends on light and one that can occur in the dark i.e., light independent.

• 1.) Light Reaction:• Light energy splits water molecules (H2O) to

produce hydrogen (H2) and oxygen (O2).

• Dark Reaction:• The Hydrogen (released from water splitting),

combines with Carbon dioxide (CO2) to produce sugar (glucose- C6H12O6).

Fate of the Products of Photosynthesis• Glucose is the main product of photosynthesis.

• It can be used in the following ways:

• To produce energy (in respiration)

• To build starch (for storage)

• To build cellulose (for cell walls)

• To form amino acids (for growth and repair)

• To form lipids (for storage/energy)

Fate of the Products of Photosynthesis (contd)• Oxygen is a by-product of photosynthesis.

• It is needed for respiration inside the plant to make energy.

• Excess is released into the atmosphere to provide other organisms with the oxygen supply needed to respire.

Role of Light and Chlorophyll in

Starch Production

• Light: Required to activate and split water molecules intohydrogen and oxygen.

• Chlorophyll: Green pigment within chloroplasts of plant cells that absorb light energy.

• Starch (complex sugar) is formed as a storage moleculeafter photosynthesis. The more glucose produced, the more starch can be formed

Testing a Leaf For Starch(iodine test)1. Place leaf in a beaker of boiling water. This will break down the

cell membranes and cell walls and allow the iodine to enter the cells.

2. Turn off flame and place leaf in a boiling tube of ethanol inside the beaker of water. The ethanol will extract the chlorophyll for better observation of colour change when iodine is added.

3. Remove leaf and place on white ceramic tile, then add a few drops of iodine.

• Result: A colour change from brown to blue-black occurs if starch is present.

Testing a Leaf For Starch(iodine test)

Boiling tube of ethanol inside a beaker of hot water

Drops of iodine placed on the leaf which is on the white tile.

Step 1 & 2Step 3

To Investigate if light is needed for

photosynthesis

• Take a healthy plant and leave it in a cupboard for a few days to destarch it.

• Test a leaf for starch to make sure it is starch-less.

• Fasten a folded piece of black paper/foil, firmly over both sides of the leaf on your plant with some parts uncovered.

• Expose the plant to normal conditions for a couple days. E.g. sunlight, water, CO2

• Remove cover from the leaf and then test both areas for starch (covered & uncovered).

• What would you expect to happen?

Results And Conclusion of Investigation

• Before Investigation:• Destarching- Darkness prevents

plants from photosynthesizing and thus they use up their starch stores in order to survive.

• After Investigation:• The area of leaf covered up from

light does not turn blue-black after iodine test. No starch present. However, uncovered areas test positive for starch.

• Conclusion: Light is needed for photosynthesis to occur.

Click link for videohttps://www.youtube.com/watch?v=k-HiZdafzt4

Investigation: To see if chlorophyll is needed for photosynthesis

• Take a healthy plant with variegated leaves Variegated leaf (some parts green, some not)

• Test both the green and yellow/white areas of the leaf for starch.

White area- No chlorophyll

Green area –Chlorophyll present

Results/Conclusion

• The green parts of the leaf contain chlorophyll so will have a positive test for starch. (brown to blue-black)

• The yellow/white areas do not contain chlorophyll so will have a negative test for starch. (no colour change)

• Click link for videohttps://www.youtube.com/watch?v=x17XhQJ3PxI

Living Organisms’ Dependence on Plants

• Energy (from the sun) enters the living world through green plants. Hence All animals depend on plants directly or indirectly for food.

• Animals also depend on plants for oxygen

• Plants also provide many materials such as wood and many products such as drugs/ medicines.