Photosynthesis: Process, steps and reactions

What is photosynthesis?

Photosynthesis is the process in which most of the green plants use carbon dioxide and water to produce sugar (simple form of carbohydrates) in the presence of sunlight. The term photosynthesis means ‘synthesis using sunlight’. But why photosynthesis is so important? If you think it is useful for the production of oxygen and ultimately for the survival of most of the species on the earth, then you might be partly right. With the help of photosynthesis plants produce sugar which is necessary for most of the herbivorous animals. It also helps to maintain the level of carbon dioxide in the atmosphere. Lastly it produces about 170 million tons of dry matter of which 90% is produced in the ocean.

The overall chemical reaction of photosynthesis is represented as follows:

6CO2 + 12 H2O    ————>    sugar (glucose) + 6O2 + 6H2O

Site of photosynthesis

The process of photosynthesis takes place in the chloroplast. But where is this chloroplast present in plants? They are present in the middle layers (mesophyll cells) of leaf. If you carefully observe the structure of leaf, you will see the upper epidermis and below that there is a palisade mesophyll followed by spongy mesophylls. Now most of the chloroplasts are present in the palisade mesophylls. The chloroplast is surrounded by two membranes. It is filled with the fluid inside it known as stroma. There are flattened fluid filled sacs called as thylakoids or lamellae. When these thylakoids are arranged in a stack like pile of coins they are known as grana. Various grana are joined by intergranal or stroma lamellae. One should note that the process of photosynthesis occurs in two phases or two reactions. First is light reaction that occurs in grana region while second is dark reaction which occurs in the stroma of chloroplast.  

Layers of leaf
Mesophyll layer in leaf

Pigments involved in photosynthesis

Different pigments of photosynthesis are present in the chloroplast. The basic function of these pigments is to absorb different wavelengths of light. They belong to two main groups:

  • Chlorophylls

These are green pigments with two principle types as chlorophyll a (bluish green) and chlorophyll b (olive green). Chlorophylls absorb mainly violet-blue and red region of visible light (VIBGYOR region).

  • Carotenoids

These are orange-yellow pigments with two principle types as carotene (beta carotene) that is orange in color and xanthophyll that is yellow in color.

NOTE: Chlorophyll a mainly absorbs violet and red light

             Chlorophyll b mainly absorbs blue and red light

Raw material for photosynthesis

  • As per the overall reaction of photosynthesis you can observe that water and carbon dioxide act as reactants, hence they can be said as raw material for the process.
  • Since carbon dioxide is available in the atmosphere, plants utilize that during photosynthesis and released it during respiration.
  • Similarly when water is used as raw material it undergoes it gives H+ and OH- ions during photolysis. The obtained hydrogen ions are then used to synthesize energy rich molecules.

Mechanism of photosynthesis

As we discussed earlier that the process of photosynthesis occurs in two phases

  • Light reaction or photochemical phase
  • Dark reaction or biosynthetic phase

NOTE: It is important to understand that in the light phase the plant is making certain chemical or energy rich molecules such as NADPH and ATP which can be later used to synthesize the sugar in the bio-synthetic phase. Hence the goal of photo-chemical phase is to prepare energy rich molecule and in bio-synthetic phase it is important to synthesize sugar i.e. glucose.  

  1. Light phase or photo-chemical phase

As the name suggest it is clear that this phase requires light or specifically presence of photons. It occurs in the grana of thylakoid membrane. It involves several steps such as

  • Activation of chlorophyll

When the chlorophyll molecule receives the light energy or photons (smallest unit of light), it gets excited and emits electron. This emitted electron then travel through ETC (electron transport chain). The reaction can be represented as

Chlorophyll a  +  hv (photons of light) ————->  Chlorophyll a  + e (emitted electron).

Light and dark reaction in photosynthesis
  • Photolysis of water

This step involves splitting of water molecule with the help of light energy. The water molecule splits into it H+ and OH- ions.

  • Formation of NADPH

The hydrogen ions released by splitting of water are used to reduce the NADP molecule to NADPH since it is an energy rich molecule. This energy rich molecule is used to reduce carbon dioxide to produce sugars (glucose).

  • Photophosphorylation

This step involves addition of a phosphate group. When ADP is phosphorylated (addition of phosphate/ IP) and convert into ATP (another energy rich molecule) in the presence of sunlight is called as photophosphorylation.

The overall light reaction can be summarized as,

2H2O  +  2NADP  +  3ADP +  3IP  —————>    2NADPH   +    3ATP

  1. Dark reaction or biosynthetic phase

This phase does not require light and occurs in the stroma of chloroplast. Previously synthesized NADPH and ATP molecules are now utilized in this phase to make glucose.

These molecules helps to convert the carbon dioxide (raw material) into carbohydrates (sugar). This process is called as carbon fixation.

This process was discovered by Melvin Calvin and Andy Benson, it is called as Calvin cycle or Calvin Benson cycle.

This process takes place by series of reactions where RuBP (ribulose biphosphate) act as an acceptor for CO2. The hydrogen that is released by NADPH is combined with CO2 by using energy from ATP molecule glucose is formed. It is important to note that Calvin cycle should repeat 6 times to form 1 glucose molecule.  Refer the overall light reaction and calculate the number of NADPH and ATP as multiple of 6.

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Plant hormones: structure & function- The Biology Bug

Key terms: Plant hormones, chemical nature of hormones, Biological function of hormones, Auxins, Gibberlins, Ethylene, Cytokinin, Abscisic acid

Most of the physiological activities in plants are controlled by certain chemical substances which are called as plant hormones or phytohormones or growth hormones. They are the signalling molecules which act either as growth promoters or growth inhibitors. Plant hormones are simple molecules but have diverse chemical composition for growth and development of plant. Before going into details of phytohormones let us first understand how animal hormones differ from plant hormones.

  • Animal hormones are made by specific glands in endocrine system but in plants the phytohormones are not made by any specialized tissue.
  • Plant hormones have very short life and gets quickly vanished after their immediate action.
  • Unlike animal hormones, plant hormones are made in a small quantity.

There are several types of plant hormones such as auxins, gibberlins, cytokinins, ethylene, abscisic acid, flowering hormones, synthetic growth retardants etc. However, we will discuss first five basic hormones.

Auxins (Cell elongation)

The meaning of auxin is ‘to increase’ or ‘to grow’. The auxin was discovered while studying the bending of coleoptile of canary grass towards light. They observed that when unilateral source of light was provided to the growing coleoptile, it bended in the direction of light. This was because of the presence of auxins.

Chemical nature of Auxin

Indole-3-acetaldehyde, Indole-3-acetonitrile, Indole-3-ethanol, 4-chloro-IAA are found in plants. Certain substances identical to the properties of auxins are IBA (Indole butyric acid), 2,4-dichloro-phenoxy-acetic acid, Indole propionic acid (IPA) etc.

Biological functions in plant

  1. Cell elongation: The primary function of auxins is to stimulate the elongation of cells in shoot. They promote elongation and growth of stem and roots. Similarly the dormancy of buds can be broken for certain plants in temperate region with the help of gibberlin.
  2. Cell division: They help in cell division and promote the growth of certain tissue. For example, when auxin is added during the tissue culture, callus (mass of undifferentiated cells) is formed.
  3. Apical dominance: In most of the vascular plants it is observed that especially the plants which are tall and sparsely branched, if the terminal bud shows higher growth whereas lateral buds remained suppressed. Removal of the apical bud results in rapid growth of lateral buds. This phenomenon is caused by auxins and is known as ‘apical dominance’.
  4. Root initiation: Increase in auxin can promote the lateral branch roots i.e. higher the concentration of auxin, initiates more lateral branch roots. For example, application of IAA in lanolin paste to the cut end of young stem results in an early rooting.

Gibberlins (Cell differentiation)

They are essential for various processes such as seed germination, stem elongation, leaf expansion, pollen maturation etc. Gibberlins were discovered in Japan when rice plants were found to suffer from a disease, bakane (foolish seedling) disease. The diseased rice plants were found to be longer than the healthier ones. This was caused by Gibberella fujikori  and later the substance was isolated and named as ‘gibberlins’.

Chemical nature of Gibberlins

Gibberlins are weakly acidic growth hormones and contain a gibbane ring structure. The structural feature that all gibberlins have in common and defines them as a family of molecules, is that they are derived from ent-kaurene ring.

Biological functions of Gibberlins

  1. Seed germination: Germination starts vigorously if certain seeds are exposed to light or red light. This requirement of light has overcome with the help of gibberlic acid. For example, light sensitive seeds such as tobacco shows poor germination in dark. Therefore treatment of gibberlic acid helps in seed germination.
  2. Bolting effect: In many herbaceous plants the early period of growth shows rosette- habit with a short stem and cauline leaves (arrangement of leaves on stem). Gibberlin triggers the growth of sub-apical meristem which causes the rosette plant to grow faster. In these plants the internodal length is short. Gibberlin promotes the internodal elongation causing rise in the stem height. Such type of growth in stem of rosette plant is known as ‘bolting’.
  3. Leaf expansion: Gibberlins help in making the leaves broader as well as elongate them. This increase in the surface of area provides sufficient space for photosynthesis.

Cytokinins (Cell division)

Cytokinins mainly induce cell division. It either works individually or with auxins. It was discovered by Miller and Skoog, the first hormone they found was named as ‘kinetin‘.

Chemical nature of Cytokinins

Zeatin is the most abundant naturally occurring free cytokinin, but dihydrozeatin (DZ) and isopentyl adenine also commonly found in higher plants and bacteria.

Biological functions of Cytokinins

  1. Cell division: It induces cell division in non meristematic cells. Cytokinins along with the auxins induces root development, bud & shoot development.
  2. Cell enlargement: Cytokinins( specifically kinetin) induces cell enlargement. They promote the expansion of cells in the leaf disks.
  3. Apical dominance: They are antagonistic to auxins with respect to apical dominance which means cytokinins trigger the growth of lateral buds.
  4. Delay of senescence: The aging of leaves caused due to loss of chlorophyll and rapid breakdown of proteins. This process is known as senescence. Cytokinins delay the senescence by controlling protein synthesis and mobilization of resources.

Ethylene (Fruit ripening)

It is the only hormone in plants which exists in the gaseous state.

Chemical nature of Ethylene

It is the simplest olefinic gas which is highly volatile in nature, It is also flammable and undergoes oxidation to produce ethylene oxide.

Biological functions of Ethylene

  1. Fruit ripening: It is known as fruit ripening hormone. Ethylene lamps are used for ripening of certain fleshy fruits. There are different types of fruits that react differently with the exogenous application of ethylne. Hence it is more used in the food industry.
  2. Epinasty: When upper surface of leaf grows faster than the lower surface, the leaf curves downward and this phenomenon is called as ‘epinasty’. Ethylene causes epinasty in dicot plants. NOTE: Monocots do not exibit this response.
  3. Inhibition of geotropism(growth towards gravity): Ethylene can nullify the geotropic effect. Roots become negatively geotrpic while stem turns positively geotrpic.

Abscisic acid /ABA (Abscission)

It is also known as stress hormone because production of this hormone production of this hormone is stimulated by environmental conditions such as drought, water logging etc.

Biological function of Abscisic acid

  1. Abscission: It promotes the abscission of flowers and fruits.
  2. Stomatal closing: ABA controls closure of stomata when the plant undergoes water stress. It is observed that application of ABA to leaves of normal plants causes closing of stomata. It travels from mesophyll cells of chloroplast to the guard cells of stomata during the period of water stress.
  3. It is also known to inhibit the seed germination and causes seed dormancy.

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