To stay alive we must provide our cells with the fuel needed to do cell work. This fuel (or food) must be organic molecules, and preferably, glucose. Cells must do a series of chemical reactions, called cellular respiration, to obtain energy for the synthesis of ATP from their fuel molecules. You may recall that all cell work is actually accomplished when molecules of a single substance, ATP, are broken, releasing energy to do the cell work. We will see how all of this comes together in this section of the course.

We have learned that we obtain our glucose from the process of photosynthesis, which occurs in the chloroplasts of plant cells. Photosynthesis transforms light energy into chemical energy, and uses that energy to produce the carbohydrate, glucose, using water and carbon dioxide molecules for the "ingredients". The process of photosynthesis also produces oxygen, which can be used by the cells of all living organisms during cell respiration.

In this section, we are going to discuss these two processes, which are so critical for each and every living organism.


Photosynthetic Requirements
Photosynthesis occurs in all parts of plants which contain the green pigment, chlorophyll. In most plants, however, photosynthesis occurs mostly in leaves, where chloroplasts are concentrated. Let's first discuss the photosynthetic requirements.

The typical photosynthesis reaction produces glucose. In order to produce glucose you need:
Chlorophyll
6CO2 + 12H20 + 686 kcal -----> C6H12O6 + 6H20+ 6O2

Chlorophyll
(Carbon dioxide + water + light energy ------> glucose + water + oxygen)

Before looking at the details of how photosynthesis works, let's look at what we need for photosynthesis: The "raw" materials.

1. Chloroplasts
The chloroplast has a double membrane with a series of internal stacked membranes. Light energy is captured by the pigments found on special membranes in the chloroplast called thylakoids, which are folded into disk-shaped stacks called grana.

the reactions needed to produce carbohydrates occur in the stroma region of the chloroplast. Enzymes are located here.

2. Light Absorbing Pigments
Chlorophyll is the primary pigment which absorbs light energy in photosynthesis. In plants, there are two forms of chlorophyll (a and b) as well as important accessory pigments, such as the carotenes. Each pigment absorbs certain wavelengths, and collects and concentrates light energy for the photosynthetic process.

Pigment molecules do not work alone. They are arranged on the grana in precise clusters of several hundred molecules, called photosystems. Each photosystem contains a specific pair of chlorophyll a molecules in a reaction center.

There are two types of photosystems (Photosystem I and Photosystem II). Each has a reaction center activated by a slightly different wavelength of light.

3. Light Waves
Not all wavelengths of light are equally useful in photosynthesis. The light absorbing pigments used in photosynthesis can not absorb all wavelengths of light. Some light energy can not be absorbed (is reflected instead) and some is transmitted. Only those wave lengths which can be absorbed by the photosystem pigments can be used in the process of photosynthesis. The light waves most useful to photosynthesis are reds and blues. Not surprisingly, green light is almost useless for photosynthesis...

4. Water
Water serves as one of the inorganic raw materials for photosynthesis. Specifically, water is the hydrogen donor for the process of photosynthesis.

Light energy is used to split water molecules, forming 2H+ , 2e- , and Oxygen .

As we have learned, water is obtained from the environment, absorbed by roots and conducted throughout the plant by the xylem of the vascular system.

5. Carbon Dioxide
Carbon dioxide provides the carbon source for manufacturing the carbohydrates in photosynthesis.

We have learned that carbon dioxide diffuses from the atmosphere, through the stomata . The rate of diffusion of carbon dioxide and availability of carbon dioxide usually limit the rate and amount of photosynthesis which occurs in a plant.

6. Oxidation/Reduction Molecules or redox
In any chemical reaction, electrons from the atoms and molecules involved are transferred or shared. This process often changes the energy level of those electrons.

• A chemical reaction where an electron is lost is an oxidation.
  
• A chemical reaction where an electron is gained is a reduction.
  
  

The light-dependent reactions require chlorophyll and occur in the thylakoid membranes of the grana of the chloroplast.

Light energy is also used to split water (Photolysis of water) into:

H2O -----> 2H+ + 2e- + 1/2 O2

The light reactions remove electrons from excited chlorophyll molecules in both Photosystem I and Photosystem II and pass the higher energy electrons along an electron transport chain, releasing energy to make ATP (from ADP and P), or transferring the electrons to NADP.

The light reactions must occur several times to produce enough ATP and NADPH to "run" the Calvin cycle.

How do we really get this energy transferred and produce molecules of ATP?

The Chemiosmotic Model of ATP Synthesis
Electrons released from molecules (such as chlorophyll) travel down an electron transport system by a series of redox reactions, releasing their energy in controlled bits. This energy can then be used for the synthesis of ATP.

In photosynthesis, the molecules of electron transport system are located in the thylakoid membrane. The energy released from electron transport systems is used to move Hydrogen ions (H+), produced by the photolysis of water, into the inner thylakoid compartments. This concentration of H+ inside the membrane establishes a pH and electrical gradient in the thylakoid compartment which has an inherent energy value.

These two gradients (concentration of H+ and the electrical charge of the positive ions) move through special channels in the thylakoid membranes which are coupled to the ATP synthesis enzyme, ATP synthase. As the H+ ions flow down the gradient in the protein channels, energy is released to make ATP from ADP and P on the other side of the thylakoid membrane in the stroma.

Note: Peter Mitchell won the 1978 Nobel prize in chemistry "for his contribution to the understanding of biological energy transfer through the formulation of the chemiosmotic theory". ATP is synthesized in the mitochondria during cell respiration by a similar mechanism.

Stage II: Calvin Cycle or C-3 Photosynthesis
(Sometimes called the Dark Reactions)

Six molecules of Carbon dioxide each combine with a 6 molecules of a 5-carbon sugar (Ribulose bisphosphate) and undergo a reduction to form 3-carbon molecules (Glyceraldehyde 3 Phosphate or G3P).
Ten of the 12 molecules of G3P are used to regenerate more ribulose bisphosphate to keep the cycle going.
Two of the 12 G3P are converted to the carbohydrate, glucose.

These photosynthetic reactions do not use light energy for the energy source. They use the ATP produced in the light reactions for their energy source, and the energy transfer molecule, NADPH

Carbohydrate molecules are produced in Calvin Cycle in the stroma of the chloroplast.

The requirements for the Calvin-Benson cycle are:

Water

There are 3 parts to the Calvin-Benson cycle:
Reduction
6 CO2 + 6 RuBP (Ribulose bisphosphate) --> 12 G3P

• NADPH provides the Hydrogen for the reduction.
  
• ATP provides the energy.
  

Regeneration
10 G3P--> 6 RuBP (Ribulose bisphosphate)

• This uses more ATP
  

Surplus
2 G3P --> 1 Glucose

Although the details of the process of photosynthesis are one of my favorite subjects, we lack the time and chemistry preparation in this
course to go into them.. They are discussed a bit more in your text and you are encouraged to read this section carefully.

What comes after Photosynthesis?
Although glucose is the typical end product of photosynthesis, and as we shall discuss, the primary fuel molecule for living organisms, plants are capable of synthesizing all of their organic molecules from photosynthetic intermediates, notably G3P and glucose phosphate . Plants are more versatile in their synthetic abilities than are animals. In addition there are whole groups of plant products, called secondary metabolites, synthesized directly or perhaps as by-products of plant activity.

Some of these metabolites are protective in nature, such as toxins; some, such as lignin, are used in plant structure.


An Alternative Photosynthetic Pathway -- Solving some of the complications of photosynthesis in stress environments

The common limit to photosynthesis is the availability of carbon dioxide, which must diffuse from the atmosphere into the leaves through pores in the leaf surface, called stomata. Unfortunately, these pores also permit diffusion of water, from the interior of the leaf, and can be a significant sources of water loss to the plant. (As much as 90% of the water taken in to a plant is lost this way.)

Under very hot and dry conditions, many plants close their stomata to minimize water loss. During these times the ratio of oxygen to carbon dioxide in the leaf increases, and this favors a process called photorespiration .

In photorespiration, oxygen bonds to RuBP (It competes for the enzyme that normally bonds carbon dioxide to RuBP), and forms a 2-carbon compound (which is eventually broken down into CO2 and H2O) instead of forming PGA, needed for the Calvin cycle and glucose formation. It is estimated that as much as 50% of the photosynthetic capability of some plants is lost to photorespiration when oxygen levels are high and carbon dioxide levels are low in these hot and dry conditions.

C-4 Plants have alternative methods of trapping and accumulating carbon dioxide, and in addition , have an internal anatomy which separates the oxygen producing light-independent reactions of photosynthesis from the Calvin cycle, minimizing photorespiration. In C-4 plants, these two processes occur in modified chloroplasts which are located in different cells within the leaves.

Modified carbon dioxide fixation system in the C-4 plant.
CO2 combines with RuBP in the first step of the Calvin cycle. In C-4 plants, CO2 is fixed before the Calvin cycle starts. Any CO2 which enters the leaf, at any time, can be combined with a common metabolic intermediate, PEP (phosphoenol pyruvate) in mesophyll cells, forming a 4-carbon acid. This 4-carbon acid is then transferred to the bundle sheath cells where it can be released for re-fixation in the Calvin-Benson cycle.

This is a more efficient trap for carbon dioxide since the 4-carbon acid can accumulate during non-light periods, concentrating carbon dioxide when photosynthesis can not occur, and can be used during periods of low moisture when stomata are closed to prevent water loss. However, regenerating PEP requires ATP, so C-4 photosynthesis may not always be more productive than the C-3 pathway. Unfortunately, the pathway is genetic, so plants can't choose.

Modified anatomy of the C-4 plant leaf (Krantz Anatomy)
Light reactions occur in leaf mesophyll cells, just as in C-3 plants. The chloroplasts of the C-4 plant mesophyll cells have lots of grana. Recall that oxygen is produced only in the light reactions.

The Calvin Cycle, however, occurs in the chloroplasts of special bundle sheath cells*, cells that surround the veins of the leaf. These chloroplasts have almost no grana, so very little, if any, oxygen is produced in these cells. The bundle sheath is the outer layer of the leaf veins, which normally adds strength to the leaf.


CAM - Another Conservation Mechanism
Some C-3 plants close stomata at night to conserve water, and can use the CO2 trap discussed above. In the daytime, this CO2 is released for "normal" C-3 photosynthesis, while the stomata can remain closed to prevent excessive water loss. The CAM is derived from the types of plants in which it was first discovered, Crassulaceans, and for the fact the CO2 trapped forms acids. These plants do not have the Krantz anatomy.

Cell Respiration

We have just concluded a discussion of the process of photosynthesis, the product of which, glucose, is the primary fuel molecule for the cells of living organisms. Now we shall turn our attention to the process that releases the energy of fuel molecules for the formation of ATP needed to do cell work. This process is known as cell respiration . Cell respiration involves a series of oxidations (which removes electrons) of a fuel molecule, usually glucose. These reactions occur, for the most part, in the mitochondria of the cell.

Most organisms require a process of cell respiration which is known as aerobic respiration, because oxygen is required in the cell respiration pathway. There are also organisms which do cell respiration without oxygen (anaerobic), and all organisms do some type of anaerobic respiration during times of oxygen deficit, although it may not be sufficient to sustain the organism's ATP needs.

Cell Respiration - An Overview
As with many metabolic processes, Cell respiration has a number of stages. No matter what molecule ultimately accepts the electrons removed from glucose, the initial stage of cell respiration is a process called glycolysis . In aerobic respiration, the second stage is the Krebs cycle , and the final stage is the electron transport chain with the synthesis of ATP. For each glucose molecule oxidized, the cell gets about 36 ATP for cell work.

Although we will not be studying the process of cell respiration in this course, it would be valuable to read about this process in your text.
Both cellular respiration and photosynthesis are discussed thoroughly in Biology 101 and Biology 201.

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