The Biology Web Home page

All organisms require energy for their chemical reactions. These reactions may be involved with reproduction, growth, or other activities. Photosynthetic organisms such as plants use light energy to produce carbohydrate (glucose). Glucose can be used at a later time to supply the energy needs of the cell. Photosynthesis is therefore a process in which the energy in sunlight is stored in the bonds of glucose for later use. 

This experiment was published by Jan Baptisa van Helmont in 1648:

"...I took an earthenware vessel, placed in it 200 pounds of soil dried in an oven, soaked this with rainwater, and planted in it a willow branch weighing 5 pounds. At the end of five years, the tree grown from it weighed 169 pounds and about 3 ounces. Now, the earthenware vessel was always moistened (when necessary) only with rainwater or distilled water, and it was large enough and embedded in the ground, and, lest dust flying be mixed with the soil, an iron plate coated with tin and pierced by many holes covered the rim of the vessel. I did not compute the weight of the fallen leaves of the four autumns. Finally, I dried the soil in the vessel again, and the same 200 pounds were found, less about 2 ounces. Therefore 169 pounds of wood, bark, and root had arisen from water only."

Most of the weight of the tree described in the above experiment came from carbon dioxide and water. The equation for photosynthesis shows that these compounds are used to produce glucose.

6CO₂ + 6H₂O + Energy ® C₆H₁₂O₆ + 6O₂

C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O + Energy

Light behaves as if it were composed of "units" or "packets" of energy that travel in waves. These packets are photons.

The wavelength of light determines its color. For example, The wavelength of red is about 700 nm and the wavelength of blue light is about 470 nm.

Visible light is a part of a larger spectrum of radiation called the electromagnetic spectrum.

Ultraviolet radiation (UV) is dangerous to cells because it breaks chemical bonds.

Pigments are molecules that absorb light. When a photon of light strikes a photosynthetic pigment, an electron in an atom contained within the molecule becomes excited. Energized electrons move further from the nucleus of the atom.

The excited (energized) molecule can pass the energy to another molecule or release it in the form of light or heat.

Chlorophyll A is the main photosynthetic pigment in all organisms except bacteria. Other pigments called accessory pigments absorb slightly different wavelengths of light. The combination of all of the pigments increases the range of colors that plants can use in photosynthesis.

Accessory pigments include chlorophyll b and a group of pigments called carotenoids. They do not participate directly in photosynthetic reactions but are able to pass their energy to chlorophyll a.

The photograph below is an elodea leaf X 400. Individual cells are clearly visible. The tiny green structures within the cells are chloroplasts.

Click the image (above) to enlarge it.

Thylakoids are membranous disk-like structures that are stacked together in larger structures that resemble stacks of coins. Chlorophyll and carotenoid pigments are located in the membranes of the thylakoids. The thylakoid membranes also contain the electron transport system.

The diagram below is a drawing of a chloroplast showing the thylakoids.

The fluid-filled space surrounding the grana is the stroma. Many enzymes needed in photosynthesis are found in the stroma.

The light-dependent reactions require light.

These reactions occur in the thylakoid membrane.

They produce ATP and NADPH, which are needed to produce glucose in the light-independent reactions (below).

Notice how the equation for photosynthesis relates to the reactions shown in the diagram below.
6CO₂ + 6H₂O + Energy ® C₆H₁₂O₆ + 6O₂

Light-independent reactions occur in stroma of the chloroplast in light or dark conditions.

They function to reduce CO₂ to glucose.

The closely packed pigment molecules and the reaction center form a unit referred to as an antenna complex.

Photons of light that are picked up by any of the pigment molecules in the antenna pass their energy to nearby pigment molecules until it is eventually passed to a special molecule of chlorophyll a called the reaction center.

The reaction center molecule becomes ionized and it loses its electron to an electron acceptor. This electron will need to be replaced.

The antenna, the reaction center, and the electron transport molecules make up a photosystem. There are two kinds of photosystems in eucaryotes. The reaction center chlorophyll molecule of photosystem I absorbs 700 nm light best and is therefore called P₇₀₀. The reaction center of photosystem II absorbs 680 nm light best and is called P₆₈₀.

Photosystem I evolved very early; photosystem II evolved later.

The diagrams that follow are less magnified views of the chloroplast and thylakoid shown in the diagram above. The antenna shown above is represented by a single green circle below. Notice that there are two photosystems and therefore two antennas. The blue circles represent the electron transport system (discussed later).

During the light reactions, pigment molecules within the P₆₈₀ antenna absorb a photon of light energy. The energy from that molecule is passed to neighboring molecules and eventually makes its way to the reaction center molecule as previously described. When the reaction center molecule becomes excited, it loses its electron to an electron acceptor.

The electron transport system is found embedded within the thylakoid membrane and functions in the production of ATP. The system contains membrane-bound electron carriers that pass electrons from one to another. As a result of gaining an electron (reduction), the first carrier of the electron transport system gains energy. It uses some of the energy to pump H⁺ into the thylakoid.

The carrier then passes the electron to the next carrier. Because it used some energy to pump H⁺, it has less energy (reducing capability) to pass to the next H⁺ pump.

This carrier uses some of the remainder of the energy to pump more H⁺ into the thylakoid.

The electron is passed to the next carrier which also pumps H⁺.

The electron transport system functions to create a concentration gradient of H⁺ inside the thylakoid. The concentration gradient of H⁺ is used to synthesize ATP. 

ATP is produced from ADP and P_i when hydrogen ions pass out of the thylakoid through ATP synthase. This method of synthesizing ATP by using a H+ gradient in the thylakoid is called photophosphorylation.

Photosystem I

At this point, the electron has little reducing capability (little energy is left). It is passed to the P700 antenna.

A pigment molecule in the P700 antenna absorbs a photon of solar energy. 

The energy from that molecule is passed to neighboring molecules within the antenna. The energy is eventually passed to the reaction center of this antenna.

As a result of being energized, the P700 reaction center loses the electron to an electron acceptor.

The acceptor passes it to NADP⁺, which becomes reduced to NADPH. According to the following equation, NADP⁺ has the capacity to carry two electrons. NADP⁺ + 2e^- + H⁺ ® NADPH

The electron transport system and photophosphorylation in the chloroplast is similar to the system found in the mitochondria to produce ATP during cellular respiration.

The diagram below is a summary of the light reactions. High-energy components of the system are shown near the top of the diagram.

Water

The electron that was lost from the antenna complex of photosystem I is replaced by splitting water (see diagram above).

In the light reactions, electrons move one way from water to NADPH and the energy of sunlight is used to produce ATP.

Calvin Cycle (Light-independent Reactions)

The products of the light reactions (ATP and NADPH) are used to reduce CO₂ to carbohydrate in the Calvin cycle.

The words "CO₂ fixation" refer to the attachment of CO₂ to an organic compound: each CO₂ binds to a 5-carbon ribulose biphosphate (RuBP) molecule.

Carbon dioxide fixation is catalyzed by RuBP carboxylase (rubisco).

The diagram below shows that six carbon dioxide molecules are combined with six RuBP molecules to produce six molecules of a 6-carbon compound. Each of the carbon dioxide molecules contains one carbon atom and each RuBP molecule contains 5 for a total of 6 carbons.

Rubisco makes up 20 - 50% of the protein in chloroplasts. It acts very slowly, catalyzing 3 molecules per second. This compares to 1000 per second for typical enzymatic reactions. Large quantities are needed to compensate for its slow speed. It may be the most abundant protein on earth.

Each of the resulting 6-carbon molecules formed by carbon dioxide fixation (above) splits into two 3-carbon molecules (phosphoglycerate [PGA]) for a total of 12 PGA molecules (below).

The two molecules of PGA are reduced to form PGAL (phosphoglyceraldehyde). The PGAL molecules also have three carbon atoms each. This reaction requires energy from ATP and electrons from NADPH.

Two of the PGAL molecules are used to form one glucose molecule (C₆).  Ten PGAL remain.

The remaining 10 PGAL (3 carbons each, total = 30 carbons) can therefore be reassembled into 6 RuBP (5 carbons each, total = 30 carbons). This rearrangement uses 6 ATP.

Below: Summary of the Calvin Cycle

For each six CO₂ molecules that enter the cycle one glucose molecule is produced.

About 30% of the energy available in ATP and NADPH is finally present in the glucose produced.

Photorespiration

Stomata (singular stoma) are microscopic openings on the undersurface of leaves that allow gas exchange and water evaporation from inside the leaf. Because dehydration can be a serious problem, the stomata close when the plant is under water stress. When closed, CO₂ needed for the Calvin cycle cannot enter.

A stoma can be seen in the diagram of a leaf below.

CO₂ Fixation in C₃ Plants

Plants that undergo photosynthesis as described above are called C₃ plants because the end result of CO₂ fixation is two 3-carbon molecules (PGA). In C₃ plants, CO₂ is fixed to RuBP to form a 6 carbon compound by the enzyme RUBISCO.

When the concentration of CO₂ is low, oxygen will bind to the active site of RUBISCO. When oxygen is bound to RUBISCO, RuBP is broken down and CO₂ is released.  This wastes energy and is of no use to the plant. It is called photorespiration because oxygen is taken up and CO₂ is released. Normally, photosynthesis reduces CO₂ to carbohydrate.

Leaf Structure of C₃ and C₄ Plants

The leaves of C₄ plants are structured differently than those of C₃ plants. Photosynthesis occurs within the mesophyll cells in C₃ plants, which form a dense layer on the upper surface of the leaf and a spongy layer on the lower surface. Bundle-sheath cells surrounding the veins are not photosynthetic.

Below: A cross-section of a C₃ leaf.

Dense layers of mesophyll cells surround the bundle-sheath cells of C₄ plants. Both the bundle-sheath cells and the rings of mesophyll cells are photosynthetic.

Below: A cross-section of a C₄ leaf.

In addition to the Calvin cycle, C₄ plants have a pathway that allows them to fix CO₂ at lower concentration than C₃ plants.   This gives them an advantage during hot, dry conditions because they can keep the stomata closed for longer periods of time to prevent dehydration before photorespiration occurs.

The Calvin cycle occurs in the mesophyll cells of C₃ plants but only in the bundle-sheath cells of C₄ plants. The CO₂ concentration of the bundle-sheath cells of C₄ plants is kept high because the mesophyll cells that surround them move CO₂ into the bundle sheath. Photorespiration in C₄ plants is reduced when the stomata are closed because CO₂ movement maintains a higher CO₂ concentration in the bundle sheath than would otherwise be possible.

Mechanism of CO₂ Fixation in C₄ Plants

C₄ plants have an enzyme that is capable of adding CO₂ to a 3-carbon compound to produce a 4-carbon compound. This enzyme functions well even at extremely low CO₂ concentrations because it is unaffected by oxygen. This CO₂ fixation occurs within the mesophyll cells that surround the bundle sheath cells. The 4-carbon compound then moves into the bundle sheath and releases the CO₂, thus raising the level of CO₂ and preventing photorespiration.

The enzyme that fixes CO₂ in the mesophyll cells is PEP carboxylase (PEPCO).

This process is called C₄ photosynthesis because the product of carbon fixation is a 4 carbon compound.

This mechanism requires extra ATP but under hot, dry conditions C₄ plants are  two to three times more efficient than C₃ plants.

In moderate weather, C₃ plants are at an advantage.

CAM (crassulacean-acid metabolism) photosynthesis is found in most desert plants, particularly the succulents (plants that store water in thick, fleshy leaves).

The Calvin cycle occurs in mesophyll cells of these plants but the stomata open only at night when it is cool and more humid. CO₂ fixation occurs when the stomata are open at night. CO₂ is stored as a 4-carbon compound and is released to the cells during the day.

CAM plants are 5 to 7 times more efficient than C₄ plants.

Identify each component in the two diagrams below.

How many carbon atoms does each molecule have in the diagram below? The molecules are represented by letters.

Identify each component (except C and F) in the diagram below.

 

Fill in the table below.

	Light Reactions	Light-Independent Reactions
Inputs
Produced

Light-Dependent Reactions - Draw details of the light-dependent reactions.

Light-Independent Reactions - Draw details of the light-independent reactions (Calvin Cycle).