2.4 - Main Organ for Photosynthesis

The Necessity of Photosynthesis in Plants

Plants are autotrophic organisms because of their ability to produce own food from source of raw materials and energy. The process of photosynthesis produces glucose which will be used by the plants for energy generation and other activities such as growth and reproduction.

History of discovery of photosynthesis

• In 1640s, Jan-Baptista van Helmont tested the idea that plants obtained food from the soil. His experiment showed that plants only utilize water that was watered, and not the water from the soil.
• In the year 1772, Joseph Priestly successfully showed that plants released oxygen to the atmosphere.

The Adaptation of the Internal Structure of a Leaf to Photosynthesis

The process of photosynthesis requires chlorophyll pigments which can be found in the chloroplasts. Chlorophyll can absorb energy from sunlight. Besides, plants also need carbon dioxide gas from the outside environment and water from the soil. The products of photosynthesis are glucose and oxygen. Other than the leaves, young stem and other green parts of the plants can also perform photosynthesis process.

Internal structure and Relationship with photosynthesis

• Upper epidermis and Lower epidermis

  • Waxy transparent cuticle enables sunlight to penetrate to the palisade mesophyll
  • The presence of stoma allows the exchange of gases of photosynthesis

• Palisade mesophyll

  • High density of chloroplasts to absorb maximum sunlight
  • Chloroplasts contain chlorophyll which can absorb light energy for photosynthesis

• Spongy mesophyll

  • Density of chloroplasts is lower compared to palisade mesophyll
  • Contains air spaces that allow exchange of gases for efficient photosynthesis

• Vascular bundle

  • Xylem transports water and mineral salts from the roots to the leaves
  • Phloem transports sucrose from the leaves to all parts of the plants

Chloroplast Structure

Chloroplast contains chlorophyll that are capable of absorbing light energy from the sun dan converting it to chemical energy during photosynthesis. The word chlorophyll comes from the Greek word chloros and phyllos where chloros means green and phyllos means leaf. There are a few important components which can be found in the chloroplast.

Granum

• Consist of stack of thylakoids
• Arrangement granum provides a large surface area for optimum photosynthesis

Thylakoid

• Disc-shaped
• Contains chlorophyll
• Contains photosynthetic pigments in the thylakoid membrane

Stroma

• Fluid surrounding granum
• Site for light-independent reaction

Figure 10 - Components in the chlorophyll pigments can be separated using paper chromatography.

Light-Dependent and Ligh-Independent Reactions in Photosynthesis

The process of photosynthesis can be divided into 2 stages. Photolysis is a process where water molecules are broken down into hydrogen ions (H+) and hydroxide ions (OH-) in the presence of light energy and chlorophyll

Light-dependent reactions

1. This process occurs in the thylakoids.
2. Pigments on the surface of the thylakoids absorb light energy from the sun.
3. The light energy absorbed will be used to excite electrons to a higher energy level.
4. The excited electron will undergo a series of electron carriers. The energy released will be used to generate energy in the form of ATP.
5. The electron will be accepted by NADP+ eventually which acts as the final electron acceptor.
6. NADP+ will then combine with H+ ions formed from the photolysis process to form NADPH which is a reducing agent.
7. The chlorophyll pigment will attract electrons from water through photolysis in order to achieve stability.
8. OH- ions will lose electrons and form oxygen and water.

Light-independent reactions

1. This process takes place in the stroma.
2. Carbon dioxide absorbed will be fixed to 5-carbon organic compounds to form 6-carbon organic compounds.
3. NADPH and ATP molecules from light-dependent reaction will reduce the organic compounds to glucose monomers.
4. Glucose monomers will form starch molecules through condensation process. Starch molecules will then be stored in the stroma of chloroplasts.

Photosynthesis reaction: 12H2O (water) + 6CO2 (carbon dioxide) --------> C6H12O6 (glucose) + 6O2 (oxygen) + 6H2O (water)

Comparison between Light-Dependent and Light-Independent Reactions in Photosynthesis

• Similarities

  • Kedua-dua proses dimangkin oleh enzim
  • Kedua-duanya berlaku di dalam kloroplas

• Differences

  • Light-dependent reactions

    • Site of reaction: Thylakoids
    • Process involved: Photolysis of water
    • Reaction substance: Water
    • Reaction products: Oxygen and water molecules
    • ATP molecules: Produced

  • Light-independent reactions

    • Site of reaction: Stroma
    • Process involved: Reduction of carbon dioxide
    • Reaction substance: Carbon dioxide
    • Reaction products: Glucose
    • ATP molecules: Used

Environmental Factors that Affect the Rate of Photosynthesis

1. Carbon dioxide concentration

Figure 11 - In general, the rate of photosynthesis will increase when the concentration of carbon dioxide increases, provided that there are no limiting factors such as temperature and light intensity. At point X, the rate of photosynthesis becomes constant. This means that the rate of photosynthesis will not increase any further despite any increase in the carbon dioxide concentration. This is because light intensity has become the limiting factor, in this case.

2. Light intensity

Figure 12 - Light energy is needed for light-dependent reactions. The rate of photosynthesis will increase and reach a maximum rate if the concentration of carbon dioxide and surrounding temperature are kept constant. Graph A above shows that the rate of photosynthesis reaches its maximum at point X. Any further increase in the light intensity will not increase the rate of photosynthesis. This is because other factors such as carbon dioxide concentration or temperature have limited the photosynthesis process. When carbon dioxide concentration is increased to 0.13% as shown in Graph B, the rate of photosynthesis also increases.

3. Temperature

Figure 13 - The process of photosynthesis is catalysed by enzymes. Therefore, any changes in the temperature will have an effect on the activities of the enzymes and subsequently on the rate of photosynthesis. Can you recall how temperature affects the activity of enzyme from Form 4 Chapter 5? Higher temperature will cause denaturation of enzymes and affect the rate of photosynthesis. In different types of plants, the optimum temperature for photosynthesis process is also different. In general, the optimum temperature lies between 25°C and 30°C.

The Effect of Different Light Intensities and Light Colors on the Rate of Photosynthesis

Other than light intensity, the rate of photosynthesis is also affected by the colors of the light. Different light colors have different wavelengths. The light spectrum consists of 7 colors in a certain sequence (violet, indigo, blue, green, yellow, orange and red). This is also the color of a rainbow. The rate of photosynthesis is the highest in red and blue light. This is because chlorophyll can absorb all red light whereas the carotenoid pigment can absorb the blue light before being transferred to the chlorophyll. Both of these lights have enough energy to excite the electrons in the light-dependent reaction.

Figure 14 - A graph of light absorption against wavelength. Both chlorophyll a and b do not absorb green light. Therefore, chlorophyll gives a green color.