PROCESS OF PHOTOSYNTHESIS

What is photosynthesis? Photosynthesis is the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

A general equation for photosynthesis is:

Carbon (IV)Oxide+Water light energy—Glucose+Oxygen chlorophyll

6CO₂+6H₂O light C₆H₁₂O+6O₂ chlorophyll

• The reaction occurs in two main phases or stages
• The initial state requires light and it is called the light dependent stage or simply light stage
• It takes place on the lamellae surfaces
• Its products are used in the dark stage
• The dark stage does not require light although it occurs in the light and is called light independent stage

 

Light-Stage

• Two reactions take place that produce raw materials for the dark stage:
• Light energy splits the water molecules into hydrogen and oxygen
• This process is called photolysis
• The hydrogen is taken up by a hydrogen acceptor called Nicotinamide adenine dinucleotide phosphate (NADP) while oxygen is released as a by-product 2H₂O(l) light energy4H+O₂photolysis
• Light energy strikes the chlorophyll molecules and sets in motion a series of reactions resulting in the production of a high energy molecule called adenosine triphophate (ATP)

 

Dark Stage

• This stage involves the fixation of carbon i.e

the reduction of carbon (IV) oxide by addition of hydrogen to form carbohydrate

• It uses the products formed during the light stage
• ATP

Carbon (IV) oxide + Hydrogen — Carbohydrates

• The synthesis of carbohydrates does not take place in a simple straight line reaction as shown in the equation above
• It involves a series of steps that constitute what is known as the Calvin cycle
• Carbon (IV) oxide is taken up by a compound described as a carbon (IV) oxide acceptor
• This is a 5-carbon compound known as ribulose biphosphate and a six carbon compound is formed which is unstable and splits into two three-carbon compounds
• Hydrogen from the light reaction is added to the three carbon compound using energy (ATP) from the light reaction
• The result is a three carbon (triose) sugar, (phosphoglycerate or PGA)
• This is the first product of photosynthesis
• Glucose, other sugars as well as starch are made from condensation of the triose sugar molecules
• The first product is a 3-carbon sugar which condenses to form glucose (6-C sugar)
• From glucose, sucrose and eventually starch is made
• Sucrose is the form in which carbohydrate is transported from the leaves to other parts of the plant
• Starch is the storage product
• Other substances like oils and proteins are made from sugars
• This involves incorporation of other elements e.g. nitrogen, phosphorus and sulphur

 

Factors Influencing Photosynthesis

• Certain factors must be provided for before photosynthesis can take place
• The rate or amount of photosynthesis is also influenced by the quantity or quality of these same factors

 

Carbon(IV) Oxide Concentration

• Carbon (IV) oxide is one of the raw materials for photosynthesis
• No starch is formed when leaves are enclosed in an atmosphere without carbon (IV) oxide
• The concentration of carbon (IV) oxide in the atmosphere remains fairly constant at about 0.03% by volume
• However, it is possible to vary the carbon (IV) oxide concentration under experimental conditions
• Increasing the carbon (IV) oxide concentration up to 0.1 % increases the rate of photosynthesis
• Further increase reduces the rate

 

Light Intensity

• Light supplies the energy for photosynthesis
• Plants kept in the dark do not form starch
• Generally, increase in light intensity up to a certain optimum, increases the rate of photosynthesis
• The optimum depends on the habitat of the plant
• Plants that grow in shady places have a lower optimum than those that grow in sunny places

 

Water

• Water is necessary as a raw material for photosynthesis
• The amount of water available greatly affects the rate of photosynthesis
• The more water available, the more the photosynthetic rate, hence amount of food made
• Effect of water on photosynthesis can only be inferred from the yield of crops
• It is the main determinant of yield (limiting factor in the tropics)

 

Temperature

• The reactions involved in photosynthesis are catalysed by a series of enzymes
• A suitable temperature is therefore necessary
• The optimum temperature for photosynthesis in most plants is around 30″C
• This depends on the natural habitat of the plant
• Some plants in temperate regions have 20°C as their optimum while others in the tropics have 45°C as their optimum temperature
• The rate of photosynthesis decreases with a decrease in temperature below the optimum
• In most plants, photosynthesis stops when temperatures approach O°C although some arctic plant species can photosynthesise at -2°C or even -3°C
• Likewise, increase in temperature above the optimum decreases the rate and finally the reactions stop at temperatures above 40°c due to enzyme denaturation
• However, certain algae that live in hot springs e.g. Oscilatoria can photosynthesise at 75°C

 

Chlorophyll

• Chlorophyll traps or harnesses the energy from light
• Leaves without chlorophyll do not form starch

 

Also See:

NUTRITION IN PLANTS

BIOLOGY PRACTICAL

ACTIVE TRANSPORT

WATER RELATIONS IN PLANT AND ANIMAL CELLS

OSMOSIS