Energy Flow and Photosynthesis in Biological Systems

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Energy in Biological Systems

Definition and Forms of Energy

Energy is a fundamental concept in biology, essential for all life processes. It exists in various forms and can be transformed from one form to another within living systems.

Energy is defined as the capacity to do work. Work involves the movement of an object after the application of a force.
Energy can be taken in from the environment, converted to useful forms, and released as heat or waste.

Kinetic Energy and Potential Energy

Natural systems contain different forms of energy, and living systems continuously convert one form of energy to another.

Kinetic energy is the energy of motion, such as when a child moves down a slide.
Potential energy is stored energy due to an object's location or structure, such as a child poised at the top of a slide. In biological systems, potential energy is often stored in chemical bonds.

Chemical Energy

One form of energy that is particularly important to all biological systems is chemical energy. Chemical energy is stored in the bonds that hold atoms together in molecules.

Living organisms can break these bonds, releasing the stored energy.
The released chemical energy can then be put to work powering life's functions. For example, ATP (adenosine triphosphate) is a common energy carrier in living cells.
Breaking a bond in ATP results in one molecule of ADP and one phosphate group, releasing energy that can be used for other processes.

Conservation of Energy

Energy can be converted from one form to another, but it cannot be created or destroyed (the First Law of Thermodynamics).

Within living systems, no energy conversion is perfectly efficient. Some energy is always lost as heat, a form of kinetic energy stored in the random motions of atoms and molecules.

Heat and Entropy

Heat energy is very chaotic and is therefore hard to recapture in a biological system. As energy is converted, the entropy, or amount of disorder, of the system increases. Increasing entropy (randomness) is a natural consequence of life's processes.

To restore order, considerable energy must be expended.

Energy Flow Through an Ecosystem

Sources of Energy

Nearly every organism on Earth ultimately derives its energy from the sun. Some organisms capture solar energy directly through the process of photosynthesis, while others rely on chemical energy stored in sugars and other organic molecules.

Producers (such as plants, algae, and some bacteria) absorb the sun's energy and convert it to chemical energy stored in sugars and organic molecules.
Consumers are organisms that cannot use sunlight directly and must obtain energy by eating producers or other consumers.
Most ecosystems depend on producers as the base of the food web.

Photosynthesis

Photosynthesis is the process by which producers capture the energy of sunlight and convert it to chemical energy stored in sugars.

The chemical ingredients for photosynthesis are carbon dioxide and water.
In the cells of plants and algae, organelles called chloroplasts use light energy to rearrange the atoms of these ingredients, producing sugars and other organic compounds.
The by-products of photosynthesis are oxygen gas and heat, which are released into the environment.

Consumers

Many organisms—including humans—cannot obtain energy directly from the sun. Instead, they obtain energy by eating producers, which gives them access to the sugars and other molecules made by photosynthesis.

Cellular Respiration

All organisms—both producers and consumers—are able to release the chemical energy stored in sugars through the process of cellular respiration.

Cellular respiration is the process by which cells break down sugars and other organic molecules to produce ATP, the energy currency of the cell.
In eukaryotic cells, enzymes in the cytoplasm and within organelles called mitochondria help break the chemical bonds of food molecules, releasing energy that can be used to produce ATP.
During cellular respiration, oxygen is used and carbon dioxide is released as a by-product.
It is important to remember that only producers perform photosynthesis, but both producers and consumers perform cellular respiration.

Photosynthesis in Chloroplasts

Energy of Sunlight is Used to Produce Sugars

Within the process of photosynthesis, a producer uses water, carbon dioxide, and the energy in sunlight to produce sugars and oxygen gas. Photosynthesis occurs inside organelles called chloroplasts.

Within plant cells, the interior of leaves have the most chloroplasts, where photosynthesis occurs at the greatest rates.
By capturing sunlight, plants use this energy to produce sugar (a building block for the mass of a plant), and most energy is released to the nearby air as heat.

Where and How Photosynthesis Occurs

Most of the chemical reactions of photosynthesis take place within chloroplasts. A stack of internal membranes in chloroplasts is called a granum (plural: grana), and each individual disk is called a thylakoid. To proceed, photosynthesis requires water and carbon dioxide. In most plants, water is absorbed by the roots. Carbon dioxide enters through tiny pores called stomata (singular: stoma), which are found mainly on the undersides of leaves. Oxygen gas is given off as a by-product and exits through these same pores. Heat is also released from the surface of the plant.

Chlorophyll

Chlorophyll is the primary pigment in chloroplasts.

Molecules of chlorophyll located within the thylakoid membranes selectively absorb blue/violet and orange/red light.
Green/yellow light is not absorbed, and because this reflected light is what reaches our eyes, plants appear green to us.

Summary Table: Key Processes in Energy Flow

Process	Organisms	Inputs	Outputs	Main Organelle
Photosynthesis	Producers (plants, algae, some bacteria)	Sunlight, CO2, H2O	Sugars, O2, Heat	Chloroplast
Cellular Respiration	All organisms	Sugars, O2	ATP, CO2, H2O, Heat	Mitochondrion

Key Equations

Photosynthesis:

Cellular Respiration: