Autotrophic and Heterotrophic Organisms

Many organisms must eat food to get the energy for life processes because they cannot produce their own food molecules from inorganic substances. They are called heterotrophs. This means that they need a ready- made organic substance to provide them with energy and materials for building cytoplasm. They are, therefore, dependent directly or indirectly on the synthetic activities of green plants. In order to obtain food, most animals have to go foraging and therefore must be mobile. This need for mobility puts a limit on the maximum body size that an animal can attain and still function efficiently. Heterotrophic organisms must rely on autotrophs for energy. All life depends on autotrophs because only they are able to convert light and energy into food.

Green plants and many onecelled organisms take in the energy by capturing it directly from the sun, they use inorganic molecules to produce organic food molecules. Such organisms are called autotrophs. This means that they do not depend on an outside source of organic food but are able to synthesize all of their organic requirements from simple inorganic molecules. During photosynthesis, carbon dioxide and water are used to produce carbohydrates. Nitrate absorbed from the immediate environment provides the nitrogen needed for protein synthesis. Since all the necessary chemicals are ‘to hand’, green plants produce their own food “on site” and have no need to be mobile. This sessile existence allows many plants to attain a body size greater than any animal.

Photosynthesis: Using Energy For Food

Plants that contain chlorophyll can trap energy from the sun and store it. This stored energy can then be used to make sugars. The whole process is called photosynthesis. Study the equation for photosynthesis (2.1).

Stored energy from the sun

Carbon Dioxide + Water ========== Sugar + Oxygen

6CO₂ + 6H₂O =========== C₆H₁₂O₆ + 6O₂ (2.1)

Photosynthesis requires light, chlorophyll, and raw materials. Enzymes are needed for the reactions to proceed. The chlorophyll present in plants traps the light of the sun. Carbon dioxide drawn from the air, and water absorbed through the roots are the usual raw materials for photosynthesis. Oxygen and water are byproducts. The equation above only summarizes the overall process of photosynthesis but does not show the many individual reactions.

The rate of photosynthesis depends on factors such as the availability of the raw materials, the intensity of the sunlight, and the temperature. If water becomes scarce, as when a severe drought occurs, photosynthesis may stop altogether. The greater the intensity of the sunlight, in general, the higher is the rate of photosynthesis. A temperature range of between 20^o C and 35^o C is the best. Above 35^o C and below 0^o C, the activity of the enzymes required for chemical reactions is lessened and the rate of photosynthesis decreases.

Photosynthesis involves two major steps. During the first step, chlorophyll traps light energy from the sun. This light energy is then changed into chemical energy and stored. In the second step, the plant uses the stored chemical energy to change carbon dioxide and water into sugar. Energy from the sun is stored in the sugar that the plant produces. This sugar may be stored or used by the plant for energy.

Notice that oxygen is also produced in photosynthesis. Part of this oxygen is used by the plant to carry on life processes. However, most of the oxygen is released into the air through the stomates. This released oxygen is needed for life by most living organisms.

A chloroplast consists of flattened structures enclosed by a double membrane. Photosynthesis begins in the grana, stacks of tiny disklike sacs. These stacks look like dark dots when seen under an electron microscope. A dense fluid, stroma, fills the space between the grana and the outer membranes.

Biochemists divide the process of photosynthesis into two phases. The first phase is called the light reactions, or light phase. As the name implies, this phase uses light energy. Light reactions involve the trapping of light energy and the formation of materials required in the next phase of the process. Light reactions can proceed only in the presence of light.

The dark reactions comprise the second phase of photosynthesis. The dark reactions use the products from the light reactions to form glucose. Thus the light reactions must take place for the dark reactions to proceed. The two phases together form one continuous process. The reactions of the dark phase are not part of the light-trapping process. The dark reactions can occur with or without light; they do not require light.

The light reactions. The light reactions of photosynthesis use some of the trapped energy to convert ADP into ATP, which stores the energy for later use. Some energy is also used to split water molecules into hydrogen and oxygen.

The light reactions can be described as a series of steps. Some of these steps happen simultaneously:

1. The chlorophyll molecules in the grana absorb photons of light;

2. The energy from the photons boosts electrons from the chlorophyll molecules to a higher energy level;

3. The energized electrons move from one molecule to another in a series of reactions called an electron transport chain. Each time a transfer is made, some energy is released;

4. The energy released from the electrons as they move down the electron transport chain is used ultimately to form ATP molecules by uniting ADP molecules and phosphates. Both ADP and phosphates are readily available in the stroma and chloroplast.

5. The electrons lost from the chlorophyll molecule are replaced by electrons from a water molecule. This process splits the water molecule into hydrogen ions and oxygen gas. The hydrogen combines with a hydrogen acceptor molecule, a molecule that readily accepts hydrogen ions. The oxygen escapes into the atmosphere.

The dark reactions. The second phase of photosynthesis uses energy stored in ATP and the hydrogen locked into the hydrogen acceptor to form glucose. This phase also uses carbon dioxide as a source of carbon for the glucose. The dark phase requires several enzymes and forms several byproducts. This second phase of photosynthesis takes place in the stroma of the chloroplasts.

The dark phase is also known as the Calvin cycle. This name recognizes the work of Melvil Calvin, the American scientist who first identified the process in the 1950s. The cycle begins and ends with a five-carbon sugar, RDP (ribulose diphosphate), which is abundant in chloroplasts. There are 4 major steps in the cycle:

1. Carbon dioxide from the atmosphere combines with RDP in a series of reactions. These reactions use ATP as the energy source and form a substance called PGA (phosphoglyceric acid), a molecule containing 3 carbon atoms.

2. PGA reacts with hydrogen from the light reactions to form PGAL (phosphoglyceraldehyde).

3. Most of the PGAL formed during the dark reactions is used to make more RDP. This RDP then unites with more carbon dioxide, beginning another cycle of dark reactions.

4. Some of the PGAL is combined to form glucose. Two three- carbon molecules of PGAL are required to form one molecule of glucose (C ₆H₁₂ O₆).

Glucose formed in photosynthesis is often not immediately used by the plant. Excess glucose is stored by plants in the form of starch, a polysaccharide made of thousands of glucose molecules. When glucose is needed, the cell breaks down starch and releases just as much glucose as is demanded. In this way plants regulate the amount of glucose available to the cells. If large amounts of starch must be stored, the plant often forms a special storage organ.

Plants also combine glucose to form other carbohydrates, including sucrose, the commonly used table sugar. Cellulose, the material that makes up plant cell walls, is also formed from the union of many glucose molecules.