Photosynthesis Notes

Photosynthesis is the biochemical, anabolic and reductive process in which green plants absorb light energy and make organic food from simple molecules like CO₂ and H₂O. It is often called carbon assimilation.

Light energy is changed into chemical energy and stored in organic matter (carbohydrates), and O₂ is released as a by-product. The raw materials are CO₂, which enters from the air through stomata, and H₂O, which is absorbed from the soil by roots. It mainly takes place in the mesophyll cells of the leaf, and inside the cell it takes place in the chloroplast. So the chloroplast is called the “kitchen of the cell”.

The oxygen comes from the breakdown of water, which is called photolysis of water.

Types of photosynthesis

Balanced equation

Significance of photosynthesis

Photosynthetic pigments are the coloured substances in the thylakoid membrane of the chloroplast that absorb light for photosynthesis. There are three groups.

Photosynthetic pigments are arranged with proteins in groups called photosystems, in the thylakoid membrane. Each photosystem has a light-harvesting (antenna) complex of 300 to 400 chlorophylls and other pigments, and a reaction centre.

In non-cyclic photophosphorylation both photosystems work. Electrons flow in a straight line from water to NADP and do not come back to P₆₈₀, so it is called non-cyclic. It works along with the photolysis of water.

The Hatch-Slack pathway is the alternative pathway of the dark reaction, proposed by Hatch and Slack. It is also called the C₄ cycle, because the first stable product is a 4-carbon compound, oxaloacetic acid (OAA). The plants that use it are C₄ plants, for example maize, sugarcane, Sorghum and Amaranthus.

Leaf anatomy. A ring of bundle sheath cells surrounds each vascular bundle (Kranz anatomy). Mesophyll chloroplasts are many and small, have grana but no RuBisCO. Bundle sheath chloroplasts are fewer and larger, have no grana but have RuBisCO.

Mechanism

CO₂ is thus fixed twice. The overall equation is 6CO₂ + 6H₂O + 6PEP + 30ATP + 12NADPH₂ → C₆H₁₂O₆ + 6PEP + 30ADP + 12NADP.

The CO₂-fixing enzyme of C₃ plants is RuBisCO, which has less affinity for CO₂. When the CO₂ concentration falls below 100 ppm, RuBisCO behaves as an oxygenase and binds with O₂. This leads to photorespiration.

Photorespiration is a wasteful process that gives neither glucose nor ATP. It uses up ATP, and it takes place in three organelles: chloroplast, peroxisome and mitochondrion. The first product is a 2-carbon compound, phosphoglycolic acid, so it is also called the C₂ cycle.

In C₄ plants, PEP carboxylase fixes CO₂ first. It has a high affinity for CO₂ and does not bind O₂. So photosynthesis goes on even when the CO₂ concentration is very low (0 to 10 ppm), and there is no photorespiration. That is why C₄ plants are photosynthetically more efficient.

Light

Temperature. The dark reaction is controlled by enzymes, so it depends on temperature. The optimum temperature is 15 to 35 °C. When other factors are not limiting, the rate doubles for every 10 °C rise until the optimum is reached. If the temperature is too high, the enzymes are denatured and the rate falls. Photosynthesis occurs in conifers at high altitude at 35 °C, and some algae of hot springs do it even at 75 °C.

Photosynthesis is the biochemical process in which green plants, algae and some bacteria change light energy into chemical energy. They use sunlight to turn water and carbon dioxide into oxygen and energy-rich organic compounds such as glucose. It is an anabolic process.

Chlorophyll. Chlorophyll is the dominant green pigment of the thylakoid membrane of the chloroplast. It absorbs light mainly in the blue and red areas, and the green light is scattered or reflected, so the plants look green. It has a porphyrin ring with magnesium at the centre and a long phytol tail.

Types of chlorophyll in plants

Photosynthesis has two steps: the light-dependent step (light reaction, Hill’s reaction) and the light-independent step (dark reaction, Blackman’s reaction).

Light reaction (photochemical phase). It takes place in the thylakoid membrane of the chloroplast. The radiant energy is absorbed by the photosystems and changed into the chemical energy of ATP and NADPH₂. It has three steps.

Photophosphorylation is of two types, non-cyclic and cyclic, as explained in the earlier questions.

Differences between light reaction and dark reaction

The Calvin-Benson cycle is the common pathway of the dark reaction. It was first studied in Chlorella (a green alga) by Calvin. The first stable product is the 3-carbon phosphoglyceric acid (PGA), so it is also called the C₃ cycle. It takes place in the stroma. About 95 % of plants are C₃ plants, for example rice, wheat, cotton, oat and tobacco.

Overall reaction: 6RuBP + 6CO₂ + 18ATP + 12NADPH₂ → C₆H₁₂O₆ + 6RuBP + 18ADP + 12NADP

Steps of the Calvin cycle

For one glucose molecule six CO₂ must be fixed, so the cycle must turn six times, using 18 ATP and 12 NADPH₂.

Photosynthesis is affected by external factors and internal factors. The light reaction depends on light, water and pigments, and the dark reaction depends on temperature and available CO₂. According to Blackman’s law of limiting factors, when many factors affect a process, its rate is controlled by the factor that is in the least amount.

A. External factors

B. Internal factors