Photosynthesis - Dark and Light Reaction stages | Biology

Photosynthesis is a complex process that involves a series of reactions. It can be summarized into two main reactions; Light reaction/Light stage and Dark reaction/Dark stage.

- This is the first stage of photosynthesis.
- It occurs in the presence of light.
- Without light it cannot take place.
- Light stage occurs in the grana of the chloroplasts.
- During light stage, two fundamental processes occur:

1. Photolysis of water
   - This refers to the splitting of water molecules using sunlight energy to give hydrogen ions and oxygen gas.
   - This is aided by the fact that the grana contain chlorophyll molecules that trap sunlight energy for photolysis.
   - The oxygen gas produced can either be released into the atmosphere or be utilized by the plant for respiration.

   Water ---> Hydrogen atoms + Oxygen gas

2. Formation of adenosine triphosphate (ATP)
   - Some of the sun light energy is used to combine Adenosine Diphospate molecule in the plant tissues with a phosphate molecule to form Adenosine Triphosphate (ATP). - ATP is an energy rich molecule that stores energy for use in the dark stage when sunlight energy could be unavailable.
   

   ADP + P ---> ATP

   - The hydrogen ions and ATP formed during light stage are later used in dark stage.

- These reactions are light independent. The energy that propels these reactions are derived from the ATP formed during light stage.

- Also known as carbon (IV) oxide fixation, dark stage involves combination of carbon (IV) oxide molecule with hydrogen ions to form a simple carbohydrate and a water molecule.
- Dark reactions take place in the stroma.

CO₂ + 4H⁺ ----->(CH₂O)_n + H₂O

- Other food materials are then synthesized from the simple sugars through complex synthesis reactions.
- The simple sugar formed in dark stage is quickly converted to starch which is osmotically inactive.
- When a lot of simple sugars accumulate in the chloroplasts, osmotic pressure of the guard cells would increase causing the guard cells to draw a lot of water through osmosis.
- This makes the guard cells to bulge and open the stomata.
- This can result into excessive water loss.
- To prevent, this, the simple sugars are quickly converted to starch.
- To test whether photosynthesis has taken place in a leaf, therefore, a test for presence of starch and not simple sugars is carried out.

Requirements

Iodine solution, Methylated spirit/alcohol, Boiling tube, Droppers, Water, White tile, Fresh leaf, Means of heating, Timer.

The following procedure should be followed.

• Detach a leaf that has been exposed to light for about six hours. This duration ensures that the leaf has photosynthesized.
• Put the leaf in boiling water for 10 minutes. This kills the protoplasm, denatures the enzymes and stops any chemical reactions in the leaf.
• Remove the leaf and put it in a boiling tube containing methylated spirit or alcohol and boil in a water bath. Methylated spirit is highly flammable hence should be boiled indirectly. Boiling with methylated spirit or alcohol decolourises the leaf (removes the chlorophyll). This ensures that the leaf becomes white so that colour changes can be observed easily when iodine is added.
• Remove the leaf and wash off in hot water to remove methylated spirit and to soften the leaf.
• Spread the leaf on a white tile and add drops of iodine solution onto the leaf and observe.

Observations

- If there is formation of blue black patches on the leaf then starch is present If the yellow/brown colour of iodine persists on the leaf then starch is absent in the leaf.

1. Carbon (IV) oxide concentration
   - While the concentration of carbon (IV) oxide in the atmosphere is fairly constant at 0.03%, an increase in carbon (IV) oxide concentration translates into an increase in the rate of photosynthesis up to a certain point when the rate of photosynthesis becomes constant.
   - At this point, other factors such as light intensity, water and temperature become limiting factors.

   

2. Light Intensity
   - The rate at of photosynthesis increases with an increase in light intensity up to a certain level.
   - Beyond the optimum light intensity the rate of photosynthesis becomes constant.
   - To this effect, plants photosynthesize faster on bright and sunny days than on dull cloudy days.
   - Light quality/wavelength also affects the rate of photosynthesis. Most plants require red and blue wavelengths of light for photosynthesis.
   - Light duration also affects photosynthesis rate.

   

3. Temperature
   - Photosynthesis is an enzyme controlled process. At very low temperatures the rate of photosynthesis is slow because the enzymes are inactive. - As temperature increases, the rate of photosynthesis increases because the enzymes become more active. - Rate of photosynthesis is optimum at (35-40)°C.
   - Beyond 40°C the rate of photosynthesis decreases and eventually stops since the enzymes become denatured.

   

4. Water
   - Water is a raw material for photosynthesis. - At extreme level of water shortage, rate of photosynthesis will be severely affected.

Requirements

Water plant e.g. elodea, spirogyra, Nymphea (water lily), glass funnels, beakers, small wooden blocks, test tubes, wooden splints and sodium hydrogen carbonate.

The following procedure should be followed.

1. Set up the apparatus as shown in the figure below.

   

2. Place the set up in the sunlight to allow photosynthesis to take place.
3. Leave the set up in the sun until sufficient gas has collected in the test tube.
4. Test the gas collected with a glowing splint.
5. Record your observations.

NOTE
- In this experiment, sodium hydrogen carbonate is added to the water to boost the amount of carbon (IV) oxide in the water since water has a low concentration of carbon (IV) oxide.
- A water plant is also selected because water plants are adapted to photosynthesis under the low light intensity in water where terrestrial plants cannot easily photosynthesize.
- This experiment can also be used to investigate the factors affecting the rate of photosynthesis:

• Carbon (IV) oxide concentration: Carry out the experiment using different amounts of dissolved sodium hydrogen carbonate e.g 5g, 10g, 15g, 20g and examine the rate at which the gas collects.
• Light intensity: An artificial light source can be used. Illuminate the plant and vary the distance between the set up and the light source while recording the time it takes for the gas jar to fill or counting the number of bubbles per unit time.
• Temperature: carry out the experiment at varying temperatures and record the rate at which the gas collects.

Requirements

Methylated spirit, iodine solution, water, white tile, droppers, beaker, source of heat, boiling tube, light proof material e.g. aluminium foil, potted plant and clips.
The following procedure should be followed.

1. Cover two or more leaves of a potted plant with a light proof material.
2. Place the plant in a dark place for 48 hours (keeping the plant in the dark for 48 hours is to ensure that all the starch in it is used up. This makes the leaves ideal for investigating whether starch would form in the experimental period. This is called destarching).
3. Transfer the potted plant to light for 5 hours.
4. Detach and uncover the leaves and immediately test for starch in one of the covered leaves and one that was not covered.

Requirements

- Sodium hydroxide pellets, flask, jelly
- The following procedure should be followed.

1. De-starch the plant for 48 hours
2. Place a few pellets of sodium hydroxide in the flask
3. Bore a hole in the cork of the same size as the petiole of the leaf being used
4. Cut the cork lengthwise.

Requirements

- For this experiment, a variegated leaf is required. This is a leaf in which some patches lack chlorophyll.
- These patches could be yellow. They lack chlorophyll hence photosynthesis does not take place in them.

- The following procedure should be followed.

1. De-starch or remove variegated leaf that has been exposed to light for at least three hours.
2. Draw a large diagram of the leaf to show the distribution of the chlorophyll.
3. Test the leaf for starch and record observations.