PHOTOSYNTHESIS – HOW PLANTS MAKE THEIR FOOD

Did you know that plants can make their own food? Yes – plants and even algae, cyanobacteria, kelp and phytoplankton can do so by a process called photosynthesis. Photo – light (Greek); Synthesis – putting together (Greek).

This takes place in an organelle called chloroplast of green leaves where glucose and oxygen are produced with the help of sunlight (visible spectrum), carbon dioxide and water. Plants use glucose for themselves and give out oxygen which we humans breathe!

Let us now understand how this happens:-

OVERALL REACTION –

6 CO₂ + 6 H₂ O —-light—-> C₆ H₁₂ O₆ (Glucose) + 6O₂

SITE OF PHOTOSYNTHESIS

This occurs in chloroplast within the mesophyll cells of green leaves.

PIGMENTS INVOLVED

There are four pigments in leaf –

• Chlorophyll a (blue green)
• Chlorophyll b (yellow green)
• Xanthophylls (yellow)
• Carotenoids (yellow-orange)

Out of these, chlorophyll a is the main pigment responsible for photosynthesis.

It has been observed that photosynthesis mostly occurs in the wavelengths of blue and red light of visible spectrum. These happen to be the same wavelengths which are maximally absorbed by chlorophyll a.

The other three pigments called accessory pigments also help in photosynthesis by absorbing light and transferring energy to chlorophyll a.

The pigments are organized into photosystems, namely – PS1 and PS2.

PS1 is also known as P700 as it has peak absorption at 700 nm while PS2 is called P680 as its absorption peak is at 680 nm.

LIGHT REACTION

These reactions are directly driven by light. This is also called the photochemical phase.

These result in formation of ATP,NADPH and oxygen.

SITE OF LIGHT REACTION – Thylakoid membrane of chloroplast

Let us discuss the steps:-

ELECTRON TRANSPORT

When sunlight is absorbed by PS2, the electrons within it become excited. Electrons are then taken up by an electron acceptor and enters an electron transport system. Then they are passed on to PS1.

At the same time, the electrons in PS 1 are also stimulated as PS1 absorbs light. Again electrons are picked up by electron acceptor and then are carried to NADP, which is reduced to NADPH + H⁺ .

SPLITTING OF WATER

On the inner side of thylakoid membrane, water is broken down into H⁺ , [O] and electrons. This happens within PS2. The electrons thus generated replenish the lost electrons from PS2.

NON CYCLIC PHOTO PHOSPHORYLATION

When both the photosystems take part in succession, first PS2 then PS1, it results in production of ATP as well as NADPH + H⁺ by non cyclic photo phosphorylation.

CYCLIC PHOTO PHOSPHORYLATION

When only PS1 is involved, there is production of only ATP which is known as cyclic photo phosphorylation. It occurs in stroma lamellae.

CHEMIOSMOTIC HYPOTHESIS

ATP synthesis occurs by the process of chemiosmosis within chloroplast.

Protons move from lumen to stroma via a proton gradient. As a result, ATP synthase undergoes conformational change resulting in production of ATP.

TO SUMMMARIZE –

In light reaction, light absorption by PS2 stimulates electrons which initiates an electron transport chain that carries electrons to PS1 then to NADP reducing it to NADPH + H⁺ .

Splitting of water into H⁺ and [O] generates electrons to replace lost electrons of PS2.

ATP is produced.

Thus, light reaction yields – NADPH , O₂ and ATP.

DARK REACTION

This is also known as CALVIN CYCLE.

It is the biosynthetic phase where ATP and NADPH produced in light reaction are used to form glucose.

It takes place in stroma.

It is not directly driven by light but that does not necessarily mean that it occurs in dark.

Let us now see the steps involved in dark reaction:-

CARBOXYLATION

CO₂ is fixed into a stable intermediate organic molecule.

The 5 carbon compound RuBP (Ribulose 1,5 – bisphosphate) is the primary acceptor of CO₂ .

RuBP is carboxylated by CO₂ to form two molecules of 3- PGA (Phosphoglyceric acid).

RuBP carboxylase (RuBisCO) acts as enzyme for this step.

REDUCTION

A series of reactions occur to form glucose from the 3 carbon PGA.

2 molecules of ATP and 2 molecules of NADPH are utilized.

The cycle has to be repeated 6 times to form a single glucose molecule (C₆ H₁₂ O₆ ).

REGENERATION

RuBP is regenerated by utilizing 1 ATP so that the cycle continues.

Thus we see that –

3 molecules of ATP and 2 molecules of NADPH are required for 1 CO₂ entering the Calvin cycle.

Therefore, 6 Calvin cycles are needed to synthesize 1 molecule of glucose.

That means 6CO₂ , 18 ATP and 12 NADPH are used per molecule of glucose.

Now that we have understood photosynthesis, let us discuss something about C4 Cycle.

THE C4 PATHWAY

This occurs in plants of hot and dry environments as an adaptation to decrease water loss and wasteful process of photorespiration.

The first carboxylation product in C4 cycle is a 4 carbon molecule – Oxaloacetic acid.

But the main biosynthetic pathway here too is Calvin cycle.

C4 plants have bundle sheath cells which have large number of chloroplasts, no intercellular spaces and thick walls impermeable to gas. Examples are – maize, corn, sugarcaine.

HATCH AND SLACK PATHWAY –

In the mesophyll cell –

PEP (Phosphoenolpyruvate) – a 3 carbon molecule is carboxylated to form Oxaloacetic acid.

Then other 4 carbon molecules form in mesophyll cells like malic acid and aspartic acid.

The 4 carbon molecule then enters bundle sheath cell and is decarboxylated.

The CO₂ thus released enters Calvin cycle and 3 carbon molecule is returned back to mesophyll cell where it is regenerated.