Cells and Energy: A Comprehensive Overview

Energy in Living Things

Energy: The Ability to Do Work (5.1.1)

Energy is defined as the ability to do work. It exists in two states:

  • Kinetic Energy: The energy of motion.
  • Potential Energy: Stored energy, existing in objects not in motion but capable of moving. All work done by living things involves transforming potential energy into kinetic energy.

Energy flows from the sun to Earth, where photosynthetic organisms trap it and store it in carbohydrates as potential energy. This energy is then transferred during chemical reactions.

Laws of Thermodynamics (5.2.1)

The laws of thermodynamics describe energy changes in the universe.

  • First Law of Thermodynamics: Energy cannot be created or destroyed; it can only be changed from one form to another. The total energy in the universe remains constant. Energy exists in different forms like light, electrical, or heat energy.

  • Second Law of Thermodynamics (5.2.2): The conversion of potential energy into random molecular motion (heat) is constantly increasing. This conversion progresses from an ordered, less stable form to a disordered, but stable form.

  • Entropy (5.2.3): Entropy is a measure of disorder in a system and is constantly increasing. Disorder is more likely than order, and energy must be used to maintain order.

Chemical Reactions (5.3.1)

Chemical reactions involve the breaking or formation of covalent bonds.

  • Reactants: The starting molecules in a chemical reaction.
  • Products: The molecules produced by the reaction.
  • Endergonic Reactions: Chemical reactions where the products contain more potential energy than the reactants.
  • Exergonic Reactions: Chemical reactions that release energy and are more likely to occur.

All chemical reactions require an input of energy.

  • Activation Energy (5.3.2): The energy required to start a reaction.

  • (5.3.3) Catalysis: A chemical reaction proceeds faster when its activation energy is lowered.

Enzymes

How Enzymes Work (5.4.1)

Enzymes are macromolecules that lower the activation energy of chemical reactions in cells. They are catalysts. An enzyme binds to the reactant (substrate) at the enzyme's active site. The enzyme is not affected by the reaction and can be reused repeatedly.

Biochemical Pathways (5.4.2)

Sometimes enzymes work in a series of reactions called a biochemical pathway. The product of one reaction becomes the substrate for the next reaction. The enzymes involved are usually located near each other in the cell.

Factors Affecting Enzyme Function (5.4.3)

Factors such as temperature and pH affect enzyme function. Most enzymes have an optimal temperature and pH range. High temperatures can disrupt the bonds that hold the enzyme in its proper shape, reducing its ability to catalyze a reaction. Hydrogen ion concentrations also affect these bonds, so increasing or decreasing the pH can disrupt enzyme function.

How Cells Regulate Enzymes (5.5.1)

Enzymes can be inhibited or activated as a means of regulation by temporarily altering the enzyme's shape.

  • Inhibition: A molecule called a repressor binds to the enzyme, altering the shape of the active site, preventing substrate binding.

  • Activation: A molecule called an activator binds to the enzyme, changing the shape of the active site so it is able to bind the substrate. Enzymes controlled in this way are allosteric enzymes.

  • Competitive Inhibition: A repressor molecule binds to the active site of the enzyme, blocking it directly.

  • Noncompetitive Inhibition: The repressor binds to a different site on the enzyme, altering the shape of the active site, preventing substrate binding.

Enzymes are often regulated by feedback inhibition, where the product of a reaction functions as a repressor, shutting down its own synthesis.

How Cells Use Energy: ATP (5.6.1)

Cells require energy to do work in the form of ATP. ATP contains a sugar, an adenine, and a chain of three phosphates. The three phosphates are held together with high-energy bonds. When the outermost phosphate bond breaks, considerable energy is released. A cell uses this energy to drive reactions by coupling ATP breakdown with other chemical reactions in the cell.