Photosynthesis Notes

Photosynthesis

Photosynthesis equation: $6CO2 + 6H2O + \text{light} + \text{chlorophyll} \rightarrow C6H{12}O6 + 6O2$
- Carbon dioxide + water + light + chlorophyll $\rightarrow$ Glucose + Oxygen
Key Terminology:
- Radiant energy: Energy from the sun
- Chloroplast: Organelle in plants; site for photosynthesis
- Chlorophyll: Green pigment needed for photosynthesis
- Thylakoids: Part of chloroplast that contains chlorophyll
- Grana: Stacks of thylakoids; site for light-dependent phase
- Stroma: Liquid part of chloroplast; site for light-independent phase
Green plants have chlorophyll in chloroplasts which traps radiant energy from the sun to produce carbohydrates by photosynthesis.
Green plants are producers.
Photosynthesis is an anabolic reaction (builds up).
Photosynthesis has two stages: Light-dependent phase and Light-independent phase (Calvin Cycle).

Role of ATP (Adenosine Triphosphate)

ATP is a nucleotide that performs essential roles in the cell:
- It is the major energy carrier of the cell, providing energy for activities within the cell.
- It is one of the monomers used in the synthesis of RNA + DNA.
- It regulates many biochemical pathways.
During photosynthesis, ATP is synthesized in the chloroplast.
It is used during the formation of polysaccharides, e.g., starch.

Importance of Photosynthesis

To balance the levels of $CO_2$ and oxygen in the atmosphere.
The process uses carbon dioxide ( $CO2$ ) and releases oxygen ( $O2$ ).
To use radiant energy to produce chemical potential energy in the form of glucose, which serves as food for organisms.
Proteins and lipids are made using stored starch.

Environmental Factors Affecting Photosynthesis

Photosynthesis can take place at different rates (speeds).
Factors that affect the rate:
- Intensity of light
- Concentration of $CO_2$
- Temperature

Comparison of Phases

Light-Dependent Phase:
- Occurs in the grana/granum.
- Light is required.
- Light is absorbed by chlorophyll.
- Water is absorbed into the grana of the chloroplast.
- Light causes water to split (photolysis), releasing energy-rich $H^+$ , electrons, and oxygen.
- ATP and NADPH are created; both store energy which is used in the Calvin Cycle (phosphorylation).
Light-Independent Phase (Calvin Cycle):
- Occurs in the stroma.
- Light is not required.
- $CO_2$ is absorbed from the atmosphere.
- $CO_2$ and energy-rich Hydrogen ( $H^+$ ) atoms from the light-dependent phase are combined using ATP to form carbohydrates (glucose).
- Excess glucose is stored as starch granules.

Factors Affecting Rate of Photosynthesis

Intensity of Light:
- At low light intensity, the rate of photosynthesis is low.
- As light intensity increases, the rate of photosynthesis also increases up to a certain point.
- When light intensity is at the optimum amount, photosynthesis will occur most rapidly.
- If light intensity increases past the optimum, the rate of photosynthesis will remain constant.
The concentration of Carbon Dioxide:
- At low carbon dioxide concentration, the rate of photosynthesis is low.
- As the carbon dioxide concentration level increases, the rate of photosynthesis also increases up to a certain point.
- When the optimum amount of carbon dioxide is present, photosynthesis will occur most rapidly.
- If the carbon dioxide concentration is higher than the optimum amount, then photosynthesis will remain constant.
Temperature:
- When the temperature is low, the rate of photosynthesis is low.
- As the temperature increases, the rate of photosynthesis will reach a maximum.
- When the temperature is at the optimum amount, the rate of photosynthesis will reach a maximum.
- If the temperature is higher than the optimum amount, then photosynthesis will decrease in rate because the enzymes used in the process will denature at high temperatures and will no longer function.

Greenhouses

A glass or plastic structure that traps heat and allows light to enter, used to grow plants.
Greenhouse effect: Phenomenon where heat from the sun is trapped on Earth by $CO_2$ in the atmosphere.
Greenhouses can be used to maintain optimum levels of factors affecting photosynthesis.
- Artificial lights can be used to allow the plants to photosynthesize for longer.
- More $CO_2$ can be pumped into the greenhouse or produced by burning gas lamps.
- The temperature can be kept at the optimum level by using heating and cooling devices.

Structural Sustainability of Leaf for Photosynthesis

Flat laminae (leaf blade) presenting a large surface area through which sunlight can be absorbed.
Leaves are thin allowing sunlight to penetrate and reach all the cells.
Cuticle and epidermal layers of cells are both transparent to allow light through to the chloroplast.
Containing stomata to enable carbon dioxide to diffuse in and oxygen to diffuse out.
Palisade mesophyll layer of cells are well adapted:
- Vertically arranged, allowing many cells to be exposed to sunlight.
- In the upper part of mesophyll which is nearer to the sun.
- Densely packed with chloroplasts.
- In chloroplast, the chlorophyll molecules are arranged on flat membranes to expose as many as possible to sunlight.

Life Sciences Experiments: Photosynthesis

To determine if a factor is required for photosynthesis
- One plant (the experiment) is given all requirements except for the factor being tested. Another plant is given all of the requirements and is called the control.
- A starch test is performed at the end to prove that photosynthesis took place.
Destarching a plant:
- Plant is placed in a dark cupboard for 48h.
- The plant uses the stored starch for this period.
- This is to ensure that the starch present at the end of the investigation is due to photosynthesis occurring during the investigation.

Investigation 1: Starch Test

During photosynthesis, glucose is produced, which is converted to starch.
Method:
1. Place leaf in boiling water to soften cell wall and kills cells to stop metabolism.
2. Place leaf in beaker/test tube containing ethanol (alcohol).
3. Allow it to stand in boiling water for 10 min to decolourise it.
4. Carefully remove leaf from alcohol and rinse it in water.
5. Spread leaf on petri dish and add iodine solution to it.
Results:
- Iodine turning leaf blue-black proves that starch was produced by photosynthesis.

Investigation 2: Light Necessary for Photosynthesis

Aim: To determine whether light is required for photosynthesis.
Method:
- Destarch a potted plant by placing it in a dark cupboard for 48 h.
- Cover a part of a leaf (still attached to plant) with aluminium foil.
- Place the plant in a sunny area for 48h.
- Pick the leaf and remove the foil.
- Do starch test.
Results:
- Experiment (leaf in foil): the iodine solution remains light brown.
- Control (leaf uncovered): the iodine solution turns blue-black.
Conclusion:
- The parts that turn blue-black in colour contain starch.
- The part which remains reddish-brown does not contain starch.
- Light is required for photosynthesis to take place.

Investigation 3: Carbon Dioxide is Required for Photosynthesis

Aim: To determine whether $CO_2$ is required for photosynthesis.
Sodium hydroxide, potassium hydroxide or sodolime : Remove $CO_2$
Sodium bicarbonate or potassium bicarbonate: add $CO_2$
Method:
- Destarch 2 potted plants.
- Set up an apparatus and water plants well. Sodium hydroxide is used to absorb $CO2$ in jar 1. Sodium bicarbonate releases $CO2$ into jar 2.
- Place sealed bell jars into a sunny area for 48h.
- Pick a leaf from each plant and test for starch.
Results:
- Bell Jar 1 leaf: iodine solution remains reddish brown.
- Bell Jar 2 leaf: iodine solution turns from brown to blue-black.
Conclusion:
- Jar 1: No starch was produced; therefore, photosynthesis can't take place in absence of $CO_2$ .
- Jar 2: Starch is produced; photosynthesis takes place in the presence of $CO_2$ .

Investigation 4: Chlorophyll is Required for Photosynthesis

A variegated leaf is used (contains green parts with chlorophyll and white parts without chlorophyll).
Does not require destarching.
Aim: To determine whether chlorophyll is required for photosynthesis.
Method:
- Place a potted plant with variegated leaf in sunny place (few h).
- Remove a leaf from plant.
- Test for starch.
Results:
- Experiment (white part): iodine solution remains reddish brown.
- Control (green part): iodine solution turns from brown to blue-black.
Conclusion:
- Experiment: no starch, no photosynthesis can occur without chlorophyll.
- Control: contains starch, photosynthesis takes place using chlorophyll.
- Chlorophyll is essential for photosynthesis.

Investigation 5: Photosynthesis Produces Oxygen

A glowing splint is used to show that oxygen is produced during photosynthesis.
A glowing splint re-ignites in presence of oxygen.
Aim: To determine whether oxygen is produced during photosynthesis.
Method:
- Set up the apparatus.
- Place apparatus in sunny area for few hours.
- A small amount of sodium bicarbonate will start to form these bubbles.
- Once enough gas has been trapped in the test tube, remove test tube submerged under the water.
- Seal the test tube using a rubber stopper while still under water.
- Once it has been sealed, remove the test tube from the water.
- Insert a glowing wooden splint into the test tube.
Results:
- The glowing splint re-ignites or burns more brightly.
- Oxygen is present in the test tube.
Conclusion:
- Oxygen is produced during photosynthesis.