2.3 Photosynthesis and the Nutrition of Green Plants

Photosynthesis

This is the process by which green plants build up carbohydrates from carbon dioxide and water.  The energy for this synthesis is obtained from sunlight which is absorbed by chlorophyll. Oxygen is given off as a by product.  in land plants the water is absorbed from the soil by the root system and the carbon dioxide from the air through the stomata.  

Photosynthesis goes on principally in the leaves, though any green part of the plant can photosynthesize.   

 

Organisms obtain nutrition in two main ways:

• Autotrophic Nutrition - this involves the build up of organic molecules, such as glucose, from inorganic molecules, such as carbon dioxide and water. Plants, algae and some bacteria are autotrophs. 

• Heterotrophic Nutrition-   involves feeding on other organisms to obtain organic molecules which are then broken down into simple soluble molecules. Animals, fungi,and some bacteria are heterotrophs.

Autotrophic Nutrition requires the use of energy to manufacture its own organic molecules. Plants are photoautotrophs, i.e., organisms which use light as their source of energy to drive the process of photosynthesis. 

Photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2


While the equation above looks like a simple one step reaction, there are actually quite a few steps between the reactants and products. This complex reaction can be broken down into the following two reaction systems:
Light reactions: H2O O2 + ATP + NADPH2
This system depends on sunlight for activation energy.
Light is absorbed by chlorophyll a which "excites" the electrons in the chlorophyll molecule.
Electrons are passed through a series of carriers and adenosine triphosphate or ATP (energy) is produced.
Water is split, giving off oxygen.

Dark reactions: ATP + NADPH2 + CO2 C6H12O6
While this system depends on the products from the light reactions, it does not directly require light energy.
Energy from the light reaction is used to convert carbon dioxide into a series of carbon sugars.
The ultimate product is glucose.

Chlorophyll: the chemical that makes it all possible.
Chlorophyll is a very large molecule with a chemical formula of C55H70MgN4O6. This molecule is called a pigment because it absorbs certain wavelengths of light. Its color represents the colors of light that it reflects, not absorbs. Therefore, green light is not useful to chlorophyll. Red and blue wavelengths of light are absorbed and provide the energy for photosynthesis.

Types of chlorophyll:

• Chlorophyll a - a bright green pigment that is indispensable to photosynthesis.
• Chlorophyll b - an olive-green pigment that contributes to photosynthesis.
• Chlorophyll c - a yellow-green pigment that is only an accessory pigment.

 
Chloroplasts: the sites of photosynthesis.
Chlorophyll is only found in chloroplasts, cell organells, never in cell cytoplasm. The structure of chloroplasts is quite complex, but these are the major structures:

• The organelle is surrounded by a double membrane.
• Inside the inner membrane is a complex mix of enzymes and water called stroma.
• Embedded in the stroma is a complex network of stacked sacs. Each stack is called a granum.
• Each of the flattened sacs which make up the granum is called a thylakoid.

Reaction centers:
Only 1 in 250 chlorophyll molecules actually converts quanta, units of light energy, into usable energy. These molecules are called reaction-center chlorophyll. The other molecules absorb light energy and deliver it to the reaction-center molecule. These bulk chlorophyll molecules are known as antenna pigments because they collect and channel energy. A unit of several hundred antenna pigment molecules plus a reaction center is called a photosynthetic unit.
The large number of antenna pigment molecules in each photosynthetic unit enables its reaction center to be constantly supplied with quanta of energy.

Factors determining the rate of photosynthesis:

• Light intensity:
  • light-limited - At low light intensities photosynthesis is starved for energy. The system uses most of the quanta the pigments capture and is therefore maximally efficient, but because there are few quanta, the rate is low. Under these conditions the rate may only slightly exceede the respiration rate, so the net photosynthetic production by the cells is actually very poor.
  • light saturation - As the light intensity is raised, the rate of photosynthetic production increases. However, a plateau is reached at about one-fourth the intensity of full sunlight. Light saturation does not result from any limitation in the capacity of chlorophyll to absorb light. It represents the maximum rate at which the dark reactions of photosynthesis can use energy from chlorophyll. A further increase in the energy supply becomes excess energy and it converted to heat and wasted.
• Temperature:
  • The light reactions of photosynthesis are not temperature dependent.
  • The dark reactions of photosynthesis are temperature dependent enzymatic processes.
    • These reactions do have an optimum temperature. Photosynthesis by most plants increases only up to about 25^o C, 77^o F. The rate levels out and then actually declines as the temperature approaches or exceeds human body temperature. This seems odd because we normally think of human body temperature as a physiological optimum temperature.
• Other factors:
  • length of day
  • amount of carbon dioxide available
  • level of air pollution 
  
  
  

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For more information on  Photosynthesis and the Nutrition of Green Plants please follow the links below: 

https://www.xtremepapers.com/revision/gcse/biology/plant_nutrition.php

http://www.smccd.edu/accounts/digennaroc/biology110_labs/6.%20Autotrophic_Nutrition43-48.pdf

http://greatneck.k12.ny.us/GNPS/SHS/dept/science/krauz/regents_bio_lecture_notes/notes%20as%20webpage/notes_on_heterotrophic_nutrition.htm