Energy is required for all processes that make life possible. Organisms obtain energy from the environment and use it to carry out chemical reactions. Energy is also required for maintaining homeostasis within an organism and providing regulation throughout the entire environment. According to the Laws of Thermodynamics...

1. Energy is neither created nor destroyed, but can be converted from one form to another.
2. Without a constant input of energy, a system will tend towards entropy.

Molecules and Cells

• Chemical Reactions: Energy Conversions

Chemical reactions are either catabolic reactions, or anabolic reactions. Catabolic reactions release energy, and anabolic reactions require energy.

Oxidation reactions release energy. Reduction reactions require energy.

Often, the reduction of one substance is coupled with the oxidation of another. The electrons are simply exchanged between substances. The reactions of glycolysis illustrate this principle with the formation of ATP coupled with the conversion of glucose.

• Enzymes

Enzymes are protein catalysts important in lowering the amount of energy of activation. Without enzymes, many chemical reactions would not occur at a temperature that can be tolerated by most organisms.

• Macromolecules

The four biological macromolecules, lipids, proteins, carbohydrates, and nucleic acids are all made of smaller units. They are created by assembling monomers into an ordered arrangement. This process is known as condensation, or dehydration synthesis, which releases water. The disassembly of macromolecules is known as hydrolysis because the insertion of water will break the bond between two monomers.

Monomers of the Macromolecules

• Lipids: 3 fatty acids and 1 glycerol
• Proteins: amino acids
• Carbohydrates: polysaccharides: monosaccharides
• Nucleic Acids: nucleotides
• ATP

ATP is a coenzyme in which all cells temporarily store energy. The phosphorylation, or addition of a phosphate, to ADP forms ATP. ATP has potential energy in the bonds that hold the phophates to the adenosine. Should the cell need energy, ATP is dephosphorylated, or hydrolized, and the energy is released for use.

ATP is required for all active transport, whether across a membrane or through the phloem of a vascular plant.

ATP is very important in muscle contraction in vertebrates. A large supply of ATP is necessary for the proper functioning of a muscle cell. Should the cell be ATP deficient, cramping may occur.

• Photosynthesis and Cellular Respiration

Photosynthesis converts solar energy to the chemical bond energy of glucose. Cellular respiration converts the chemical bond energy of glucose to the usable energy of ATP.

• Concentration Gradients

For substances to be transported against a concentration gradient, energy is required.

The excretory system has to make urine concentrated. It works against the concentration gradient that is set up along the membrane of the collecting tubule. Water and wastes would tend towards equilibrium on either side of the membrane, therefore ATP is required to concentrate the wastes on one side.

Energy is required to operate ion pumps. The functioning of a neuron depends on this mechanism.

The movement of the action potential along the axon of a neuron requires ATP to facilitate the depolarization of the membrane. As ATP is hydrolized, the movement of sodium ions (Na⁺) into the cell and potassium ions (K⁺) out of the cell creates a negative charge on the exterior, known as the action potential.

Energy is stored in an ion gradient. This is important in both photosynthesis and cellular respiration. (Chemiosmosis)

Photosynthesis: During the light reactions of photosynthesis, a hydrogen ion (H⁺) or proton, concentration builds up within the thylakoid lumen. The energy released when the protons travel through the ATP synthase and down the gradient is used to phosphorylate ADP and create ATP. This ATP is later dephosphorylated during the Calvin Cycle, and the energy released is used to form glucose.

Cellular Respiration: The NADH and FADH₂ (from the Krebs Cycle) that travel down the electron transport system, generate a high concentration of protons in the inner membrane space as they go. The movement of these protons through the ATP synthase (very similar to the one in chloroplasts) and down the gradient, into the matrix of the mitochondrion releases energy that phosphorylates ADP. Thus the energy in NADH and FADH₂ has been converted into usable ATP energy.

Heredity and Evolution

• Energy Expenditure for Transcription and Translation

A cell must spend energy to transcribe and translate genes. Entropy decreases as monomers are assembled into macromolecules.

Energy is needed to edit transcribed mRNA pieces and discard the introns.

Organisms and Populations

• The Food Chain

All organisms must obtain energy from some source in their environment. Autotrophs may perform either photosynthesis or chemosynthesis, but heterotrophs must eat autotrophs or other heterotrophs. All food chains follow this basic pattern:

• Producers (autotrophs) e.g. grasses
• Primary consumers (herbivorous heterotrophs) e.g. mouse
• Secondary consumers (carnivorous heterotrophs) e.g. snake
• Tertiary consumers (carnivorous heterotophs) e.g. hawk

**There may be more or fewer levels of consumers, but there are seldom fewer than two, or more than five.

Omnivores can eat at any stage of the food chain. Dead organisms from all levels are consumed by decomposers (heterotrophs). They are the final step in the food chain. Food chains are important in understanding energy flow and transmission among organisms. A food web, that displays all possible feeding paths within an ecosystem, is constructed with many food chains.

• Ecological Pyramids

Ecological pyramids graphically represent amounts of energy, biomass, and numbers of organisms at all levels. Each layer of the pyramid represents a trophic level, and each level sits on top of its food source.

Energy pyramids map the amount of energy transferred between trophic levels. The 10% Law states that only 10% of the energy at one level is transferred to the organisms at the next level. Much of the energy is lost as heat, or used used by the trophic level at which it begins. This has interesting implications in terms of how much energy an ecosystem needs, and how much an individual must eat to satisfy its energy needs. Energy pyramids cannot be turned upside-down.

Biomass pyramids diagram the amount of biomass, or dry organic weight per m², present at each level. Generally, biomass pyramids are right side up, but they can be inverted.

Pyramids of numbers represent the number of organisms at each level. They are generally right side up, but they can be inverted.