Gr 12 Detailed Process of Photosythesis

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Photosynthesis occurs in two stages

Photosynthesis produces sugar molecules as a food source for the plant. Sugars, such as glucose, are held together by covalent bonds. It requires energy to create those covalent bonds, and the source of that energy can ultimately be traced back to the Sun.

The first stage of photosynthesis is a set of reactions that ‘trap’ light energy and convert it to the chemical energy of ATP. The second stage of photosynthesis is a set of reactions in which ATP is used to help bond carbon dioxide and water moleculestogether to create a sugar, such as glucose.

Introduction of the Light Dependent and Light Independent (sometimes called Dark) Reactions of Photosynthesis. Each part of Photosythesis will be dicussed in detail further down the page.

Light Dependent

Light Independent

The first stage of photosynthesis is a set of reactions typically referred to as the light dependent reactions.
In this set of reactions, chlorophyll (and other photosynthetic pigments) absorb light energy and convert that energy to a form of chemical energy, specifically ATP.
In addition, light energy is used to accomplish a reaction that is called photolysis of water.
In this reaction, a water molecule is split into its component elements: hydrogen and oxygen.
The oxygen that is split away due to the photolysis of water is typically released from the plant leaf as a waste product.
From the plant’s perspective, the useful products formed during this stage of photosynthesis are ATP and hydrogen.
Light-dependent reactions take place in the intermembrane space of the thylakoids
Reduced NADP and ATP are produced in the light-dependent reactions

How does the NADH work?

The second stage of photosynthesis is a series of reactions collectively referred to as the light-independent reactions.
ATP and hydrogen are used as forms of chemical energy to convert carbon dioxide and water into useful organic molecules for the plant.
Glucose, a typical product of photosynthesis, is an organic molecule.
It requires six inorganic carbon dioxide molecules to form one glucose molecule.
This conversion of an inorganic form of an element to an organic form is known as fixation.
Therefore, photosynthesis can be described as a series of reactions in which carbon dioxide and water are fixed into glucose, and oxygen is produced as a by-product.
The fixation reaction described above requires energy. The energy to create the glucose comes directly from the ATP and hydrogen created in the first stage of photosynthesis.
Ultimately, this energy can be traced back to sunlight. It is also important to note that glucose is only one of the many possible organic molecules that can be formed from photosynthesis.
Light-independent reactions take place in the stroma

Light is absorbed by chlorophyll, which results in the production of ATP (chemical energy)
Light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen
The hydrogen and ATP are used in the light independent reactions, the oxygen is released from stomata as a waste product

ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions
The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.)
The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together

How does the NADH work?

Light Dependent - Photophosphorylation

The light dependent reactions include the splitting of water by photolysis to give hydrogen ions (protons) and the synthesis of ATP in photophosphorylation. The hydrogen ions combine with a carrier molecule NADP to make reduced NADP. ATP and reduced NADP are passed from the light dependent to the light independent reactions. Photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in one or both types of photosystem.

Steps of Light Dependent Reaction

Excitation of photosystems by light energy
Production of ATP via an electron transport chain
Reduction of NADP⁺ and the photolysis of water

Step 1: Excitation of Photosystems by Light Energy

Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane
Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm)
When a photosystem absorbs light energy, delocalised electrons within the pigments become energised or ‘excited'
These excited electrons are transferred to carrier molecules within the thylakoid membrane

Step 2: Production of ATP via an Electron Transport Chain

Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane
As the electrons are passed through the chain they lose energy, which is used to translocate H⁺ ions into the thylakoid
This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force
The H⁺ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis)
ATP synthase uses the passage of H⁺ ions to catalyse the synthesis of ATP (from ADP + Pi)
This process is called photophosphorylation – as light provided the initial energy source for ATP production
The newly de-energised electrons from Photosystem II are taken up by Photosystem I

Step 3: Reduction of NADP+ and the Photolysis of Water

Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP⁺
This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions
The electrons lost from Photosystem I are replaced by de-energised electrons from Photosystem II
The electrons lost from Photosystem II are replaced by electrons released from water via photolysis
Water is split by light energy into H⁺ ions (used in chemiosmosis) and oxygen (released as a by-product)

Summary of PHOTOPHOSPHORYLATION

The electrons lost from Photosystem I are replaced by the de-energised electrons from Photosystem II
The electrons lost from Photosystem II are replaced following the photolysis of water
The products of the light dependent reactions (ATP and NADPH) are used in the light independent reactions

The light dependent reactions occur within the intermembrane space of the thylakoids
Chlorophyll in Photosystems I and II absorb light, which triggers the release of high energy electrons (photo activation)
Excited electrons from Photosystem II are transferred between carrier molecules in an electron transport chain
The electron transport chain translocates H⁺ ions from the stroma to within the thylakoid, creating a proton gradient
The protons are returned to the stroma via ATP synthase, which uses their passage (via chemiosmosis) to synthesise ATP
Excited electrons from Photosystem I are used to reduce NADP⁺ (forming NADPH)

Z Scheme - the energy changes of Photosynthesis can be shown as a Z scheme as follows

First vertical bar: Photosystem II electrons are energised by light (electrons replaced by photolysis of water molecules)
Diagonal bar: Electrons lose energy as they pass through an electron transport chain (synthesising ATP)
Second vertical bar: Photosystem I electrons are energised by light (electrons used to reduce NADP⁺)

2 Types of Photophosphorylation Cyclic and Non-Cyclic

Video and Animation Resources for this topic

Light Independent - Calvin Cycle

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

This animation of the Calvin cycle is designed to show, in 3 stages, the major events in the synthesis of sugar in the stroma of chloroplasts. For the sake of clarity all of the enzymes, except Rubisco, have been omitted and only the carbon skeletons of the intermediates are shown.

http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html

This animation of the light reactions of photosynthesis is designed to show, in 4 stages, the major events in the synthesis of ATP and NADPH in the thylakoids of chloroplasts.

Step 1. Carbon Fixation

Step 2: Reduction of Glycerate-3-Phosphate

Step 3: Regeneration of RuBP

Important Video to show the entire process of Photosynthesis, Light Dependent and Light Independent Reactions, Calvin Cycle and Electron Transport

Calvin Cycle

Super easy to understand!!!

SUMMARY VIDEO!

Watch last to check your understanding

When watching the video, create a table about Light Independednt and Light Dependent Reactions.

Keep track of the reactants, products and what structures are involved.

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