The environment comprises of plants, animals, and human beings. Animals and human beings take food from the plants. The food we take is the plants and naturally, plants do take food from the sunlight and the process is called as photosynthesis. The process by which the green plants and some other organisms use the sunlight to synthesize the nutrients from carbon dioxide and water is termed as Photosynthesis. Photosynthesis in plants generally comprises the green pigment chlorophyll and generates oxygen as a by-product.

Photosynthesis

The process by which the plants, some bacteria, and some other protistans use the energy from the sunlight and produce glucose from carbon dioxide and water is called as Photosynthesis. The glucose formed can be converted into pyruvate which would releases adenosine triphosphate (ATP) by the cellular respiration. In other words, Photosynthesis is the process in which energy from the sunlight is used to convert the carbon dioxide and water into the molecules needed for growth. These molecules comprise sugars, enzymes, and chlorophyll. The green chemical chlorophyll absorbs light energy, and the energy allows the production of glucose by the reaction between the carbon dioxide and water. Oxygen is also produced as a waste product. The word origin of Photosynthesis has come from the Greek where Photo means “light,” and synthesis means “putting together.”

The word equation may summarize photosynthesis as:

carbon dioxide + water arrow with sunlight and chlorophyll glucose + oxygen

The conversion of usable sunlight energy into the chemical energy is associated with the action of green pigment chlorophyll.

 Photosynthesis

Chlorophyll is a complex molecule. Several modifications of chlorophyll will occur among plants and other photosynthetic organisms. All the photosynthetic organisms have chlorophyll a. The accessory pigments will absorb energy whereas the chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), carotenoids (such as beta-carotene) and xanthophylls. Chlorophyll a will absorbs its energy from the violet-blue and reddish orange-red wavelengths, and few from the intermediate (green-yellow-orange) wavelengths.

Structure of Plant Leaves

Plants were the only photosynthetic organisms to have the leaves (and not all plants have leaf). A leaf may be observed as a solar collector filled with photosynthetic cells. For plants to perform photosynthesis, they need light energy from the sun, water, and carbon dioxide. Water gets absorbed from the soil into the cells of root hairs. Water enters the root and is transported up to the leaves through the specialized plant cells known as the xylem vessels. Land plants must defend against drying out and so have evolved specialized structures known as stomata that allow the gas to enter and exit the leaf.

Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it could enter the leaf through the stoma (singular form of stomata), flanked by two guard cells. Likewise, oxygen produced during the photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gasses are moving between the inside and outside of the leaf, and a large amount of water is also lost.

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The Structure Of Chloroplast & Photosynthetic Membranes

The thylakoid is the structural unit of the photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs or vesicles containing photosynthetic chemicals. Only eukaryotes have the chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks collectively known as grana. The areas in between the grana are referred to as stroma. Whereas the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments.

 Photosynthesis

Structure Of Chloroplast & Photosynthetic Membranes

The typical plant leaf comprises the following

  • Upper and Lower Epidermis: The outer layer of the cells that controls the amount of water that is lost through the transpiration is the upper epidermis.
  • Stomata: The pores or holes in the leaves that are responsible for the exchange of gasses between the plant leaves and the atmosphere is called Stomata. Carbon dioxide is absorbed from the atmosphere and oxygen is released.
  • Mesophyll: The photosynthetic (parenchyma) cells that are located in between the upper and lower epidermis is called the Mesophyll. These cells contain the chloroplasts.
  • Vascular Bundle: These are tissues that form part of the transport system of the plant.  Vascular bundles consist of xylem and phloem vessels which will transport the water, dissolved minerals and the food to and from the leaves.

Process of Photosynthesis

When chlorophyll a absorbs the light energy, an electron gains the energy and is ‘excited.’ Then the excited electron is transferred to another molecule which is called a primary electron acceptor. The chlorophyll molecule is oxidized (loss of an electron) and has a positive charge. Photoactivation of chlorophyll a will results in the splitting of water molecules and the transfer of energy to the ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP).

The chemical reactions involved include:

  • Condensation Reactions is responsible for the water molecules splitting out, including phosphorylation, the addition of a phosphate group to an organic compound.
  • Oxidation or Reduction (redox) reactions involves the electron transfer

Photosynthesis in plants takes place in two stages. These stages were known as the Light Dependent Reactions and the Calvin Cycle.

Light-Dependent Reactions

The first stage in the photosynthesis is the light-dependent reactions. These reactions will take place on the thylakoid membrane inside the chloroplast.  During which, the light energy is converted into ATP (chemical energy) and NADPH (reducing power).

Light is absorbed by the two Photosystems called Photosystem I (PSI) and Photosystem II (PSII).  These protein complexes contain light-harvesting chlorophyll molecules and accessory pigments called antenna complexes. The photosystems are also equipped with the reactions centers (RC). These are complexes of proteins and pigments that are responsible for energy conversion. The chlorophyll molecules of PSI will absorb light with a peak wavelength of 700nm and are called as P700 molecules. The chlorophyll molecules of PSII will absorb light with a peak wavelength of 68Onm and are called as P68O molecules.

The light dependent reactions begin in PSII

  • A P680 chlorophyll molecule absorbs a photon of light in the light harvesting complex of the PSII. The energy which is generated from the light is then passed from one P680 chlorophyll molecule to another til it reaches the RC of PSII.
  • At the RC is a pair of P680 chlorophyll molecules and an electron in the chlorophyll molecules becomes excited as a result of the higher level of energy.  The excited electron becomes unstable and is released. Another electron gets released followed by the capture of one more photon of light by the light harvesting complex and the transfer of the energy to the RC.
  • The electrons are transported in a chain of protein complexes, and the mobile carriers called as the electron transport chain (ETC). Plastoquinone is the mobile carrier that carries the electrons from the RC of PSII to the Cytochrome b6f Complex.
  • The electrons lost from the PSII are replaced by splitting water with the light in a process called as Photolysis.  Water is utilized as the electron donor in the oxygenic photosynthesis and is divided into electrons (e-), hydrogen ions (H+, protons) and the oxygen (O2).

                           i.e. 2H2O —–> 4H+ + O2 + 4e (photolysis)

  • The hydrogen ions and oxygen gets released into the thylakoid lumen. After which, oxygen gets released into the atmosphere as a by-product of the photosynthesis.
  • While the electrons pass through ETC via Plastoquinone,  hydrogen ions (or protons) from the stroma are also transferred and released into the thylakoid lumen. It results in a higher concentration of the hydrogen ions or the proton gradient in the lumen.
  • As a consequence of the proton gradient in the lumen, hydrogen ions are carried to ATP synthase and thus provide the energy needed for combining ADP and Pi to produce the ATP. Cytochrome b6f transfers the electrons to the Plastocyanin and then transports them to the Photosystem I.  

 Photosynthesis

The electrons have now arrived at PSI

  • They will again receive energy, but this time from the light absorbed by P700 chlorophyll molecules. The electrons are transferred to the mobile carrier, ferredoxin.
  • They are then transported to ferredoxin-NADP reductase (FNR), which is the final electron acceptor.  At this stage, the electrons and a hydrogen ion are combined with the NADP+ to produce NADPH.

                      i.e. NADP+ + 2e + 2H+ —–> NADPH + H+

  • The lost electrons from the PSI are replaced by the electrons from PSII via the electron transport chain.

The energy changes that are accompanying the two sets of changes make a Z shape when drawn out. And this is why the electron transferring process is sometimes called the Z scheme. The key to the scheme is that sufficient energy gets released during the electron transfer to enable ATP to be made from the ADP and phosphate.

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Summary of Light-Dependent Reactions

Flow of the Electrons

      Photosystem II (PSII) —–> b6-f complex —–> Photosystem I (PSI) —-> NADP reductase

The Calvin Cycle

The second stage of photosynthesis is the Calvin Cycle.  These reactions will occur in the stroma of the chloroplast.  Energy from the ATP and the electrons from NADPH are used in converting the carbon dioxide into glucose and other products.

  • One molecule of carbon dioxide (CO2) is combined with one molecule of Ribulose Bisphosphate (RuBP).  It is important to note that RuBP is a 5-carbon molecule and when it is combined with the CO2, the reaction will produce an unstable 6-carbon intermediate.
  • The unstable 6-carbon intermediate will quickly break down to form two 3-carbon molecules and is known as 3-phosphoglycerate (PGA).
  • The two 3-phosphoglycerate molecules will receive energy from ATP and will produce two molecules of 1,3-bisphosphoglycerate (BPGA).
  • An electron from the NADPH is combined with each of the 1,3-bisphosphoglycerate molecules and thus produce two molecules of the Glyceraldehyde 3-phosphate (G3P).

Two G3P molecules are required to make one molecule of glucose.

 Photosynthesis

The next important step in this cycle is to regenerate the RuBP.  The problem here is there is not enough G3P, and we have only run the cycle once with only one molecule of CO2 and one molecule of RuBP. Only two molecules of G3P were produced.  Still, there is a need for an additional ten more molecules of G3P for the cycle to continue.

Take another look at the photosynthesis equation, and you would notice that of about six molecules of carbon dioxide (6CO2) are required for the processing of photosynthesis.

These six molecules of CO2 must be used to produce twelve G3Ps. This means that the steps above need to be repeated five more times to produce ten more molecules of G3P.

Two molecules of G3P would be used to produce glucose and the other ten will be used for the regeneration of RuBP.

Formula & Chemical Equations of Photosynthesis

Thus the balanced equation for the photosynthesis is

            6CO2 + 6H2O + sunlight energy = C6H12O6 + 6O2

The equation can be written out in words as,

With the presence of sunlight, six moles of the carbon dioxide and six moles of water react and form one molecule of glucose and six moles of oxygen.

Photosynthesis can be represented using a chemical equation as,

          Carbon dioxide + water + light energy —–> carbohydrate + oxygen.