Photosynthesis

Review

What is Photosynthesis?

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.

Equation for Photosynthesis

"...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.

$\mathrm{6CO_2 + 6H_2O + Energy \rightarrow C_6H_{12}O_6 + 6O_2}$

Equation for Cellular Respiration

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + Energy}$

Light

Electromagnetic Spectrum

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.

Photosynthetic Pigments

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.

Chloroplast structure

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.

2 Sets of Reactions

Light-Dependent Reactions

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.

$\mathrm{6CO_2 + 6H_2O + Energy \rightarrow C_6H_{12}O_6 + 6O_2}$

Light-Independent Reactions

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

They function to reduce CO2 to glucose.

Photosystems

A photosystem is composed of a reaction center complex surrounded by light-harvesting complexes. Each light-harvesting complex contains pigment molecules (chlorophylls and carotenoids) bound to a protein complex. The reaction center complex contains two chlorophyll a molecules and a primary electron acceptor bound to a protein complex.

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

The chlorophyll a molecule becomes ionized and lose an electron to an electron acceptor. This electron will need to be replaced.

There are two kinds of photosystems in Eukaryotes. 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.

Details of the Light-Dependent Reactions

Photosystem II

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.

Photophosphorylation

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 carrier 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 Pi 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. $\mathrm{NADP^+ + 2e^- + H^+ \rightarrow NADPH}$

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 nearer the top of the diagram. The red arrows show the path of electrons from water to NADP+ to form NADPH.

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 Calvin cycle produces a 3-carbon sugar called glyceraldehyde 3-phosphate (G3P). G3P is be used to synthesize glucose and other organic compounds needed by the cell.

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

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

The diagram below shows that three carbon dioxide molecules are combined with three RuBP molecules to produce three 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 six PGA molecules (below).

PGA is reduced to form G3P (glyceraldehyde 3-phosphate). The G3P molecules also have three carbon atoms each. This reaction requires energy from ATP and electrons from NADPH.

G3P is used to form glucose and other organic compounds. For each G3P used, five G3P remain.

The remaining five G3P (3 carbons each, total = 15 carbons) can therefore be reassembled into 3 RuBP (5 carbons each, total = 15 carbons). This rearrangement requires phosphorylation from 3 ATP.

Two G3P are used to produce one glucose molecule. 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 and CO2 Concentration

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. The resulting reactions do not produce sugar and they consume ATP. In addition, organic compounds that are involved in photosynthesis are broken down.

 During photosynthesis, CO2 is fixed by the enzyme rubisco. During hot, dry conditions, the level of CO2 drops and rubisco adds oxygen to organic molecules.

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.

The Calvin Cycle in C4 Leaves

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.

In C4 plants, CO2 is fixed in the mesophyll cells by PEP carboxylase, an enzyme that is more efficient than rubisco. The resulting organic compound is transported to the mesophyll cells where the CO2 is released for the Calvin Cycle. The movement of CO2 into the bundle sheath cells maintains a higher concentration than would otherwise be possible.

Mechanism of CO2 Fixation in C4 Plants

CO2 fixation occurs when CO2 is bonded 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.

The diagram below summarizes the path of CO2 in C4 plants.

Efficiency of C4 Photosynthesis

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

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

CAM plants keep their stomata closed during the day to conserve water. At night, the stomata open to allow CO2 to enter. CO2 is incorporated into organic acids which are then stored within the mesophyll cells. During the day, CO2 is released from the organic acids to supply the Calvin Cycle.

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

The diagram below summarizes the path of CO2 in CAM plants.

Photosynthesis Review

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).