Double membrane enclosing stacks of green disc like structures called grana. These grana make up what is called the thylakoids. These thylakoids are surrounded by a dense fluid called the stroma.


Nature of Sunlight

The electromagnetic spectrum:










380 nm

450 nm

500 nm

550 nm

600 nm

650 nm

700 nm

Plants use light in the 450 and 700 nm range. Meaning the plants like indigo-blue and orange-red light.

Certain colors in light rays are important for proper plant growth. Leaves reflect and derive little energy from many of the yellow and green rays of the visible spectrum. Yet the red and blue parts of the light spectrum are the most important energy sources for plants, and plants require more rays from the red range than from the blue. Plants growing outdoors, in greenhouses or close to windows are exposed to a natural balance of the blue and red light rays that plants need. Where plants receive little or no natural light, you must provide additional light from artificial sources.

Various plant pigments help use light. Carotenoids, chlorophyll a, b, and c. Chlorophyll a absorbs indigo and red lights, b absorbs blue and orange -red, c absorbs blue and orange in smaller amounts.

Chlorophyll is a molecule containing 2 main parts: a complex ring with a magnesium ion in the center and a nonpolar tail.


The reactions of photosynthesis take place in two main stages:

a). those that capture energy (Light Reactions)

b). those that use energy to make carbohydrates (Calvin Cycle)



These reactions take place in the thylakoid membranes. They involve 2 sets of light -absorbing reactions and 2 sets of electron transport chain reactions.

STEP 1. Light hits Photosystem II (P 680) causing electrons to be boosted to a higher energy level and pass into an electron transport chain. As a result some of the H+ from the stroma are carried through the thylakoid membrane and released into the space inside. ATP is produced here.


STEP 2: at the end of the chain a low energy electron enters Photosystem I (P- 700). Here it gets energized by more sunlight. This energizes the electrons and moves them into the NADPH electron chain. This chain passes electrons to

NADP+ in the stroma. Each NADP+ accepts 2 electrons and reacts with a H+ in the stroma to form NADPH. The result is to move the electrons out of the thylakoid into the stroma. These electrons are replaced by the splitting of water, that also produces H+ and O2. The H+ stays in the thylakoid and becomes part of the H+ reservoir that will power the chemiosmotic synthesis of ATP.

The Calvin Cycle

 ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce carbon dioxide to sugar.

The Calvin cycle is similar to the Krebs cycle in that the starting material is regenerated by the end of the cycle. Carbon enters the Calvin cycle and leaves as sugar. ATP is the energy source, while NADPH is the reducing agent that adds high energy electrons to form sugar.

The Calvin cycle actually produces a 3 carbon sugar glyceraldehyde 3-phosphate.

The Calvin cycle may be divided into 3 steps.

Step 1: Carbon Fixation. This phase begins when a carbon dioxide molecule is attached to a 5 carbon sugar, ribulose biphosphate (RuBP).

This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco) one of the most abundant proteins on earth.

The products of this reaction is an unstable 6 carbon compound that immediately splits into 2 molecules of 3-phosphoglycerate.

For every 3 molecules of carbon dioxide that enter the cycle via rubisco, 3 RuBP molecules are carboxylated forming 6 molecules of 3-phosphoglycerate.

Step 2: Reduction. This endergonic reduction phase is a 2 step process that couples ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde phosphate.

An enzyme phosphorylates (adds a phosphate) 3-phosphoglycerate by transferring a phosphate from the ATP. The product is 1-3-bisphosphoglycerate.

Electrons from the NADPH reduce the carboxyl group of the 1-3-bisphosphoglycerate to the aldehyde group of glyceraldehyde-3-phosphate.

For every three carbon dioxide molecules that enter the Calvin cycle, 6 glyceraldehyde-3-phosphates are produced, only one can be counted as a net gain. The other 5 are used to regenerate 3 molecules of RuBP.

Step 3: Regeneration of RuBP. A complex series of reactions rearranges the carbon skeletons of 5 glyceraldehyde-3-phosphate molecules into 3 RuBP molecules.

These reactions require 3 ATP molecules.

RuBP is thus regenerated to begin the cycle again.

C4 Plants: Many plants begin the Calvin cycle with a 4 carbon compound instead of a 3 carbon compound. These are called the C4 plants. They include the grasses (sugar cane and corn). These plants live in areas that are very hot and semiarid.


Click here for The AP Lab 4: Plant Pigments and Photosynthesis.