DEMO

Photosynthesis

By The End Of The Subtopic Learners Should Be Able To;
  1. Define photosynthesis.
  2. Investigate conditions necessary for photosynthesis.
  3. Investigate gaseous exchange in plants.
  4. Describe factors that affect the rate of photosynthesis. 
  5. Describe how leaf structure is adapted for photosynthesis.


  • Photosynthesis is a process in which green plants manufacture carbohydrates using carbon dioxide, water and light energy trapped by chlorophyll.
  • Light energy is trapped and converted into chemical energy (glucose).
  • Plants can manufacture their own food using inorganic substances hence they are called autotrophs or producers.
  • This can be summarised by the following equation

Carbon dioxide + Water chlorophylllight energy Glucose + Oxygen

CO2 + H2O chlorophylllight energy C6H12O6 + O2

  • The equation only shows glucose as a food product and oxygen is released as a by-product of this reaction.
  • The glucose produced is stored in the plant cells as starch.

Experiment: Starch test on a leaf

Materials

  • boiling water bath
  • methylated spirit or ethanol
  • boiling tube
  • test tube holder
  • test tube rack
  • petri dish or glass/ceramic tile
  • iodine solution
2.1.1.JPG (28 KB)

Procedure

  1. Bring some water to boiling either in a beaker of water heated by a Bunsen burner or in an electric water bath.
  2. Submerge a leaf in boiling water for one minute. This kills the leaf as it destroys membranes, making it easier to extract chlorophyll.
  3. Place this leaf in a boiling tube with enough alcohol to just cover the leaf and place the boiling tube in the boiling water bath for 10             minutes.
Precaution: When the alcohol begins to boil, turn off the Bunsen. This will prevent the alcohol from boiling too vigorously and shooting out of the mouth of the boiling tube.
 4. Wash the leaf in cold water which removes the ethanol and rehydrates the leaf and makes it easy to spread out.
 5. Spread the leaf flat out on a tile or Petri dish and put some drops of iodine solution on it.

Expected observations

  • If the leaf turns blue-black, starch is present and if it stays red/brown there is no starch.

Requirements for photosynthesis

  • As from the equation of photosynthesis, the requirements of photosynthesis are:

1. Availability of light

  • Light provides the energy that drives photosynthesis.

2. Chlorophyll

  • It is the green pigment found in the chloroplasts of leaf cells.
  • This pigment absorbs light energy.

3. Carbon dioxide and water

  • Carbon dioxide and water are the raw materials used to synthesise glucose.

Experiment: Is light necessary for photosynthesis?

Materials

  • potted plant that has been kept in the dark for 48 - 72 hours
  • aluminium foil or black tape
  • scissors
  • starch test materials

Procedure:

  1. Cut a small piece of aluminium foil or black tape.
  2. Cover a part of one leaf on both sides with a strip of aluminium foil.
  3. Leave the plant in the sun for 3 hours (the exposed parts of the leaf form the control of the experiment).
  4. After 3 hours remove the leaf from the plant and form a starch test.

Expected observations

  • In the experiment because light is the energy needed for photosynthesis, the leaf will appear as in fig.2.1.2 (b).
  • The part in sunlight will show presence of starch.
2. Fig.2.1.2 (a).jpg (133 KB)

Experiment: Is chlorophyll necessary for photosynthesis?

Materials

  • plant with variegated leaves that has been kept in the dark for 48-72hours
  • starch test materials

Procedure

  1. Remove the plant from the dark and put it in the sun for 3 hours.
  2. After 3 hours remove the leaf from the plant and form a starch test.
3. Fig.2.1.3 (a)_1.jpg (77 KB)
3. Fig.2.1.3 (a)_2.jpg (68 KB)
3. Fig.2.1.3 (a)_3.jpg (73 KB)

Expected observations

  • The part that contained chlorophyll (green colour) showed presence of starch concluding that chlorophyll is needed for photosynthesis.

Experiment: Is carbon dioxide necessary for photosynthesis?

Materials

  • de-starched potted plant
  • conical flask or polythene bag
  • lime water
  • cotton wool
  • sodium hydroxide or potassium hydroxide
  • starch test materials
4. Fig.2.1.4.jpg (197 KB)

Procedure

  1. Remove the plant from the dark.
  2. Encase a leaf of the plant in a polythene bag or conical flask filled with potassium hydroxide as in fig.2.1.4.
  3. Place the plant in the sun for 3 hours.
  4. Take the leaf enclosed and any other leaf on the plant, which is the control and perform a starch test.

Expected observations

  • After the starch test, the enclosed leaf showed lack of starch showing carbon dioxide is needed for photosynthesis.

Rate of photosynthesis

Limiting factors

  • Limiting factors are the factors that directly affect the rate at which photosynthesis can take place.
  • The main factors affecting rate of photosynthesis are light intensity, carbon dioxide concentration and temperature.

1. Light intensity

  • As light intensity increases the rate of photosynthesis chemical reactions steadily increases in a linear manner as in fig.2.1.5.
  • Eventually, the rate of photosynthesis levels off (becomes constant) due to effect of other factors such as the carbon dioxide concentration or the temperature.
5. Fig.2.1.5.jpg (112 KB)
Experiment: Effect of light intensity on the rate of photosynthesis

Materials

  • pond weed
  • 500cm3 Beaker
  • desk lamp or light source
  • 1 metre ruler
  • stopwatch
  • water at room temperature
  • knife or scissors
  • baking soda
  • test tube
  • thermometer
6. Fig.2.1.6.jpg (120 KB)

Procedure

  1. Cut a segment of the pond weed plant approximately 8cm with scissors and crush the end of the stem at the incision gently.
  2. Submerge the plant into a test tube filled with 40ml room temperature water and 1g baking soda.
  3. Place the test tube in the beaker of water and note the temperature where the beaker acts as a heat shield.
  4. Set the apparatus as in fig.2.1.6.
  5. Darken the laboratory by turning off as many lights as possible.
  6. Turn on the light source and allow the plant to equilibrate or adjust to the light intensity for 2-3 minutes.
  7. Count the number of bubbles given off in one minute.
  8. Move the light 10 cm further back.
  9. Leave for 2-3 minutes for the pondweed to adjust again.
  10. Count the number of bubbles given off in one minute.
  11. Repeat by moving the lamp away by 10 cm intervals until 50 cm is reached.

Expected observations

  • As the light source was continuously being moved further back the number of bubbles given off per minute starts to decrease with increase in distance from the light source.

2. Carbon dioxide concentration

  • An increase in the concentration of carbon dioxide increases the rate at which carbon is incorporated into sugars, thus increase in the rate of photosynthesis.
  • Normally carbon dioxide is present in low concentrations in the atmosphere (0.04%).
  • Increasing the concentration causes a rapid rise in the rate of photosynthesis which eventually becomes constant as shown by the graph in fig.2.1.7.
7. Fig.2.1.7.jpg (100 KB)

Experiment: Effect of carbon dioxide concentration on the rate of photosynthesis

Materials

  • pond weed
  • test tube
  • thermometer
  • sodium bicarbonate solution of varying concentrations
  • 5 boiling tubes
  • beaker
2.18.JPG (33 KB)

Procedure:

  1. Fill each boiling tube with different concentration of sodium bicarbonate solution, label and place in a water bath to warm to 25°C.
  2. Cut the stem of the pond weed at an angle and remove several leaves from around the cut end of the stem.
  3. With the cut end upwards,   immerse the pond weed in the boiling tube with the lowest concentration and place in a beaker as in fig 2.8 above.
  4. Place the water bath with the boiling tube at a measured distance from a light source and allow the plant to adapt for 5 minutes.
  5. Count and record the number of bubbles released per minute and repeat these procedure using different concentrations of sodium bicarbonate.
  6. A graph should be drawn of the rate of bubble production against sodium bicarbonate concentration.

NB: During this experiment only one factor (carbon dioxide concentration) should be varied, whilst the other are kept constant (temperature 25°C and light intensity).

Expected observations
  • An increase in the concentration of sodium bicarbonate (carbon dioxide concentration) cause an increase in the number of bubbles produced per minute.

3. Temperature

  • As temperature increases, the rate of photosynthesis also increases until the optimum temperature is reached, any further increase results in decrease in the rate of photosynthesis.
  • The process of photosynthesis is catalysed by enzymes hence is affected by temperature changes.
  • As the enzymes approach their optimum temperature the overall rate of photosynthesis increases as shown in fig.2.1.9.
  • At higher temperatures molecules have more kinetic energy and collide more frequently thus are more likely to react.
  • At low temperatures the enzymes are inactivated and at very high temperatures the enzymes are denatured.
9. Fig.2.1.9.jpg (167 KB)

Experiment: Investigating effect of temperature on the rate of photosynthesis

Materials

  • pond weed
  • test tube
  • thermometer
  • sodium bicarbonate solution
  • boiling tube
  • beaker
  • hot plate or burner

Procedure:

  1. Cool the water bath to 5°C with ice packs.
  2. Fill in the boiling tube with an excess of sodium bicarbonate.
  3. Cut the stem of the pond weed at an angle and remove several leaves from around the cut end of the stem.
  4. With the cut end upwards, immerse the pond weed in the boiling tube and put it in the water bath at 5°C and maintain the temperature.
  5. Place the water bath with the boiling tube at a measured distance from a light source and allow the plant to adjust for 5 minutes as in fig.2.1.10 above as well as maintaining the temperature at 5°C.
  6. Count and record the number of bubbles released per minute.
  7. Repeat the procedure using the temperatures of the water bath at 10°C, 20°C, 30°C, 35°C, 40°C and 50°C.
  8. A graph should be drawn of the rate of bubble production against temperature.
2.18.JPG (33 KB)

Expected observations

  • An increase in the temperature caused an increase in the number of bubbles produced per minute from 5°C to 40°C.
  • At 50°C there was a decrease in the number of bubbles caused by the high temperature which can denature the enzymes involved in photosynthesis.

Structure of the leaf

The vascular bundles (xylem and phloem) form the midrib and veins of the leaf.

A dicotyledonous leaf has a branching network of veins that get smaller as they branch away from the midrib.

11. Fig.2.1.11.jpg (178 KB)



Adaptation Function
Large surface area They have a broad shape with a high surface area to volume ratio to increase the absorption of light
Thin shape Allow light to reach all the cells and create a short distance for carbon dioxide to diffuse in and oxygen to diffuse out.
Leaf stalk (petiole) It holds the leaf in the best position to receive light
Chlorophyll It gives the leaves their green colour and converts light energy to chemical energy.
Veins The network of veins is very fine and water has to pass from a vein through only a few cells to reach other cells.
It supports the structure of the leaf and transport substances to and from the leaf.




Internal structure of a dicotyledonous leaf

  • Leaf structure is a compromise between maximising photosynthesis and minimising water loss.
  • For efficient photosynthesis a leaf needs:
    • water delivery to the leaf
    • removal of the products of photosynthesis (glucose) to storage organs of the plant
    • an efficient means of absorbing light energy
    • a method of gaseous exchange between the leaf and its surrounding
12. Fig 2.1.12.jpg (274 KB)

Features of the leaf and adaptation to photosynthesis

1. Cuticle

  • This is a transparent surface layer which covers the leaf and allows lights to travel to the mesophyll layer.
  • It is usually found on the upper surface of the leaf as it is most exposed to the sun.
  • It is waxy to reduce water loss.

2. Epidermal layer

  • It is usually one cell thick (to reduce distance travelled by light to reach mesophyll cells) and the cells are closely fitting with no air spaces (to reduce evaporation).
  • The cells do not have chloroplasts and are transparent so light can pass through to photosynthesising cells.
  • The upper epidermis secretes a waxy substance which forms the cuticle.
  • The lower epidermis has pores called stomata.
  • Stomata are openings made up of two guard cells which contain chloroplasts.
  • The epidermal layer maintains the shape of the leaf and protects inner cells from bacteria, fungi and mechanical damage and reduces evaporation.

3. Palisade mesophyll layer

  • These are tall thin cells arranged in columns and separated by very narrow air spaces.
  • The cells contain numerous chloroplasts in the cytoplasm lining the walls.
  • The cell wall and cell membrane are easily permeable to carbon dioxide and water.
  • The chloroplasts are arranged along the side walls close to carbon dioxide in the air spaces and can move up and down the cell depending on the light intensity.
  • The dense packing of these cells allows the absorption of the maximum amount of light.
  • The cells contain a thin layer of water on cell surface that can dissolve carbon dioxide.

4. Spongy mesophyll layer

  • These cells are rounded, rather loosely packed, and are covered with a thin layer of water.
  • The air spaces between them aid the diffusion of gases and water through the leaf to the palisade layer.
  • The cells can photosynthesise but contain less chloroplasts than those of the palisade layer.

5. Vascular bundle

  • This is the transport system in and out of the leaf.
  • The xylem is located towards the upper epidermis and the phloem towards the lower epidermis.
  • The xylem delivers water and mineral salts and the phloem sieve tubes carry away photosynthetic products such as glucose to other parts of the plant and storage organs such as roots.

6. Stomata

  • Usually found on the lower side of the leaf.
  • The guard cells on the stomata can increase or reduce the size of the stoma or close it completely depending on their internal pressure or turgor (movement of water in and out of cells).
  • The stomata tend to close in the absence or in excessive amount of light.
  • This opening and closing of the stomata helps to control gaseous exchange of carbon and oxygen and transpiration rate.
13. Fig 2.1.13.jpg (180 KB)

Photosynthesis and respiration

  • Photosynthesis removes carbon dioxide from the environment and at the same time releases oxygen.
  • Carbon dioxide and oxygen diffuse in and out of leaves through the stomata.
  • Respiration is the opposite of photosynthesis as shown by the equation:

Glucose + Oxygen  Carbon dioxide + water     
           

  • The two processes (photosynthesis and respiration) both occur in green plants.
  • During the day photosynthesis exceeds respiration thus there is a net removal of carbon dioxide and addition of oxygen into the atmosphere.
  • In the dark, photosynthesis is less than respiration and there is a net removal of oxygen and addition of carbon dioxide into the atmosphere.
  • Compensation point is the point where the rates of respiration and photosynthesis exactly balance such that there is no net uptake or loss of carbon dioxide or oxygen.
  • At compensation point glucose consumed by respiration equals glucose produced by photosynthesis.
  • Carbon dioxide produces a weak acid in water, carbonic acid, which has a low pH than water.
  • Overall change in atmospheric carbon dioxide can be demonstrated using bicarbonate indicator or named hydrogen carbonate indicator.
  • The indicator is sensitive to changes in pH caused by the carbonic acid.
  • Bicarbonate indicator changes colour depending on the pH of gases dissolved in it, as shown below.
14. Fig.2.1.14.jpg (157 KB)

  • The amount of glucose used up in respiration is nearly constant whereas amount of glucose produced in photosynthesis varies.

Experiment: Gaseous exchange in plants using bicarbonate indicator

Materials

  • 4 test tubes with corks
  • bicarbonate indicator solution
  • test tube rack
  • 4 equal sizes of fresh leaves
  • beaker
  • strings
  • tissue paper
  • aluminium foil

Procedure:

  1. Tie the leaf stalk with a string long enough to hang it in the test tube midway.
  2. Completely cover two test tubes with aluminium foil and the other with tissue paper.
  3. Boil one of the fresh leaves for 3 minutes.
  4. Add 2ml of bicarbonate indicator to all the 4 test tubes.
  5. Hang the leaves in the test tube and close with a cork and place on a test tube rack.
  6. Place the test tube rack in sunlight for 6 hours.

Expected observations

  • After 6 hours the indicator solution will appear as in fig.2.1.15.
2.16.JPG (25 KB)