Chapter 10

Autotrophic organisms are capable of generating all of their organic molecules from inorganic sources. This anabolic process requires energy. Organisms that use light (from the sun) as a source of this energy are photoautotrophs, while some bacteria capable of obtaining energy by oxidizing inorganic molecules are chemoautotrophs. Organisms that require a source of organic molecules to convert into the organic molecules in their cells are called heterotrophic. Animals are heterotrophs and must eat either plants or the animals that eat plants as a source of organic material.

Photoautotrophs are very important for life and they function via the process of photosynthesis (Fig. 10.5). Before studying photosynthesis we should understand certain properties of light energy.

Light represents electromagnetic energy radiated from the sun (Fig. 10.6). This energy occurs at all different wavelengths. We visualize only a narrow range of these wavelengths (380-750 nm). The colors we recognize represent particular wavelengths of light. When light strikes an object some is reflected, some is transmitted through the object and some is absorbed by the object. Only absorbed light energy can be used by that object to perform work. When an object appears a particular color, that means it did not absorb energy of that wavelength.

Photosynthesis is an anabolic pathway where CO₂ is reduced to organic material. In its simplest possible form:

CO₂ + H₂O + Energy à CH₂O + O₂ + H₂O (Fig. 10.4)

In this reaction, the carbon and oxygen in the organic matter comes from the carbon dioxide and the hydrogen comes from breaking down water. The water shown as a product is formed during the process and comes from carbon dioxide (the O) and the initial water (the H). The oxygen gas given off comes from the initial water. When considering photosynthesis we usually consider two separate sets of reactions: the light dependent reactions and the light independent reactions (sometimes called the dark reactions).

The light dependent reactions are where sunlight energy is captured and transformed into chemical energy (Fig. 10.7, 10.9). This takes place because pigment molecules imbedded in the membranes of the thylakoids may have their electrons stimulated by light from the sun. These pigment molecules occur in groups of about 400 within the thylakoid membranes and consist of molecules of chlorophyll a, chlorophyll b, and carotenoids (Fig. 10.10). Each category of these molecules is capable of absorbing different component wavelengths of the visible spectrum. Most of these pigments simply act as antennas and pass the energy to a centrally located molecule of chlorophyll a that is called the reaction center. Electrons in the reaction center when excited can leave their orbit and serve to reduce a nearby carrier molecule. As the electrons are passed along a series of carrier molecules imbedded in the thylakoid membranes via redox reactions H⁺ are pumped from the stroma into the thylakoid compartment creating a free energy gradient with a higher concentration of H⁺ inside the thylakoid than in the stroma. The H⁺ passively leak back into the stroma via channels lined with ATP synthase resulting in the phosphorylation of ADP into ATP. Since the energy required is obtained from light this process is referred to as photophosphorylation. (Fig. 10.4)

Two forms of photophosphorylation (cyclic (Fig. 10.12, 10.15)and noncyclic (Fig. 10.13, 10.14, 10.17) are recognized. These phosphorylation processes represent the energy gathering part of photosynthesis and are referred to as the light dependent processes. In noncyclic photophosphorylation two photosystems are employed each having a different pigment complex and sets of associated carrier molecules. Photosystem I has P700 as its reaction center and Photosystem II has P680. In noncyclic photophosphorylation when electrons are excited from P700 and used to reduce a primary acceptor carrier molecule they are soon passed to an intermediate NADP⁺ (Nicotinamide adenine dinucleotide phosphate). If two electrons are added to NADP⁺ an H⁺ will combine with it forming NADPH a reduced form (energy rich). In this process a need for replacement electrons for P700 exists. These electrons are provided from P680 of Photosystem II via a series of carrier molecules embedded in the thylakoid membranes. Some of these carriers will only accept electrons others will only accept electrons if they are accompanied with hydrogen protons. When the H⁺ are picked up they are picked up from the stroma and when they are given off they are given off into the thylakoid compartment. This generates ATP when the H⁺ passively diffuse through channels lined with ATP synthase. Now, however, P680 will need replacement electrons. These are provided by the process of photolysis where under enzymatic control water is broken down in the thylakoid compartments into 2H⁺ + 2e^- + ½ O₂. The H⁺ add to the chemiosmotic gradient, the e^- are supplied to P680 and the O₂ is given off as a waste product. This is where the O₂ that now constitutes 21% of the gaseous atmosphere of the earth originated.

The light independent processes are where the synthesis of organic material takes place. These reactions take place in the stroma of the chloroplasts and use the ATP and NADPH produced by the light dependent reactions as an energy source for these anabolic processes. These reactions are primarily represented by the Calvin cycle. (Fig 10.18) When carbon dioxide is first incorporated into an organic molecule we refer to that as the fixation of carbon dioxide. This often occurs in the Calvin cycle. It is easier to understand the Calvin cycle if we talk about 3 turns of it simultaneously. Let 3 CO2 molecules combine with 3 5-carbon molecules (ribulose bisphosphate) to form 3 unstable 6-carbon molecules that instantaneously break down into 6 3-carbon molecules. Each 3-carbon molecule is phosphorylated to make it more reactive using an ATP for each. These 3-carbon molecules are then reduced using for each an NADPH as the reducing agent. This produces 6 molecules of Glyceraldehyde Phosphate, the same 3-carbon molecule that is of importance in Glycolysis. One out of six Glyceraldehyde Phosphates can be removed from the cycle and used for other things. The remaining five 3-carbon molecules are converted to three 5-carbon molecules of ribulose phosphate so the cycle is complete. This latter conversion requires 3ATP molecules be dephosphorylated as an energy source. Plants may use the Glyceraldehyde Phosphate in three ways: it can enter the Glycolysis pathway, it can be used to generate ribulose bisphosphate, or it can be combined via condensation reactions to synthesize more complex polysaccharides.

The combination of carbon dioxide with ribulose bisphosphate takes place under control of the enzyme ribulose bisphosphate carboxylase (Rubisco). Rubisco is important because CO₂ and O₂ both compete for its active site. When oxygen binds in the active site, the 5-carbon ribulose bisphosphate is broken down into a 3-carbon and a 2-carbon molecule that wastes energy. This process is called photorespiration. We must understand the anatomy of a leaf to appreciate how this problem is magnified in hot, dry climates.

The leaves of most plants have a defined structure. This type of plant is called a C3 plant because CO₂ is fixed as a 3-carbon molecule as described above. The mesophyll cells contain chloroplasts and are the site of photosynthesis. They are organized as a upper layer of tightly packed cells called the palisades mesophyll and a more loosely packed, less organized grouping called the spongy mesophyll. On the upper and lower edge of the mesophyll layers are layers of epidermal cells. Within the cell are veins that, among other things, are where water necessary for photosynthesis is supplied to the leaf. Imbedded in the lower epidermis are pairs of "hot dog" shaped guard cells. When these guard cells are turgid (filled with water) their shape creates an opening between them called stoma (plural stomata). It is through these stomata that gases such as CO₂ and O₂ diffuse. When the guard cells become flaccid (lose water) the stomata tend to become smaller cutting off the diffusion of gases. This is adaptive because plants must conserve their water when it is hot and dry. When the stomata are closed noncyclic photophosphorylation still is generating oxygen and the Calvin cycle is still reducing the concentration of CO₂ in the leaf, therefore the relative concentration of O₂ to CO₂ increases inside the leaf and competitive inhibition (hence photorespiration) becomes more of a problem. This may lead to as much as 50% of the CO₂ that is fixed by C3 plants being wasted in hot, dry climates.

Natural selection has led to adaptations by some plants to avoid the problem of photorespiration. Some plants have anatomically differently structured leaves than C3 plants, and because these plants first fix CO₂ into a 4-carbon organic molecule they are called C4 plants. The anatomy of a C4 leaf is shown in figure 10.19. In C4 leaves the vein is surrounded by a layer of photosynthetic cells called the bundle-sheath cells, and the bundle-sheath cells are surrounded by mesophyll cells. In C4 plants carbon dioxide fixation occurs in the mesophyll cells under the influence of an enzyme (phosphoenolpyruvate carboxylase) PEP carboxylase. This enzyme has a stronger affinity for CO₂ than Rubisco and O₂ does not act as a competitive inhibitor. The first organic molecule CO₂ is fixed in is Oxaloacetate which is converted to another 4-carbon molecule Malate that diffuses through plasmodesmata into a bundle-sheath cell. There Malate is broken down into the 3-carbon pyruvate and CO₂. This serves to dramatically increase the concentration of CO₂ in the bundle-sheath cells thereby minimizing the competitive inhibition by O₂. Within the bundle-sheath cells the Calvin cycle uptakes CO₂ as in C3 cells. The pyruvate is then transported across the plasmodesmata back into the mesophyll cell, converted to PEP, and the cycle continues. Note that the Calvin cycle does not take place in mesophyll cells. The fixation of CO₂ and the Calvin cycle are separated spatially.

Another adaptation for minimizing the affect of photorespiration occurs in the plant family Crassulaceae (which includes cacti) and is referred to as Crassulacean Acid Metabolism (CAM). CAM plants keep their stomata closed during the heat of the day, but then open them at night. During the night, CO₂ is fixed into organic acids that are stored in the plant vacuoles. During the day these acids are broken down releasing CO₂ that then enters the Calvin cycle. Therefore, CAM plants separate CO₂ fixation from the running of the Calvin cycle in time.