AP Biology Notes: Enzymes, Metabolism, and Cell Communication

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Four Things to Know about Enzymes

  1. All enzymes possess an active site, a 3-D pocket within their structures, in which substrate molecules can be held in a certain orientation to facilitate a reaction. The two models of enzyme-substrate interaction are lock-and-key and induced fit.
  2. Enzymes lower the activation energy of a reaction, do not get used up in the reaction, catalyze millions of reactions per second, do not affect the overall free-energy change of the reaction, increase the reaction rate, and do not change the equilibrium of reactions.
  3. Enzymes can be regulated by inhibitors, molecules that bind to the enzyme either at the active site or the allosteric (regulatory) site. Feedback inhibition is when the end product of a biochemical reaction works to block the activity of the original enzyme.
  4. Enzymes can be influenced by reaction conditions such as high temperatures, detergents, or acidic/basic conditions.


Six Things to Know about Metabolism

  1. Photosynthesis is the energy foundation for almost all living systems. In addition, photosynthesis provides almost all of the oxygen present in the Earth’s atmosphere.
  2. All photosynthetic organisms use chloroplasts and mitochondria to perform photosynthesis and cellular respiration. Photosynthesis and cellular respiration are essentially reverse operations of each other.
  3. Photosynthesis has two main parts—the light cycle and the dark cycle (the latter is usually called the Calvin or Calvin-Benson cycle). Light reactions produce energy and dark reactions make sugars. The light reactions occur in the interior of the thylakoid, while the Calvin-Benson cycle occurs in the stroma.
  4. Cellular respiration is an efficient catabolic pathway and yields ATP. Cellular respiration is an aerobic process. The metabolic reactions of respiration occur in the eukaryotic mitochondria and are catalyzed by reaction-specific enzymes.
  5. Cellular respiration can be divided into several stages: glycolysis, pyruvate decarboxylation, the citric acid cycle, and the electron transport chain.
  6. Cellular respiration is a complex process that requires many different products and specialized molecules. Focus on the requirements and overall net production for the major steps.


Four Things to Know About Cell Communication

  1. There are two major ways animal cells communicate: through signaling molecules secreted by cells and through receptor molecules that rest on the cells’ surfaces.
  2. Responses to signal transduction (the conversion of an extracellular signal to a change in an intracellular process) may be stimulatory or inhibitory.
  3. Endocrine signaling refers to the secretion of chemical messengers for widespread distribution throughout the entire organism. Paracrine signaling refers to the process of signaling only nearby cells. Synaptic signaling occurs between a nerve cell and either another nerve cell, a gland cell, or a muscle cell.
  4. The cells of many tissues contain pores known as gap junctions that allow for direct cell-to-cell communication, which allows for incredibly fast, undisrupted signal transmission across large areas. Plasmodesmata are plant cells’ equivalent of gap junctions.


Key Topics–Enzymes and Metabolism


Remember that the AP Biology exam tests you on the depth of your knowledge, not just your ability to recall facts. While we have provided brief definitions here, you will need to know these terms in even more depth for the AP Biology exam.

Enzyme Structure

  • Enzyme: An organic catalyst; typically, a protein
  • Substrate: A substance that is acted upon by an enzyme
  • Cofactor: An ion or molecule that helps to bind an enzyme to a substrate
  • Coenzyme: An organic cofactor required for enzyme activity
  • Vitamin: An organic nutrient required by organisms in small amounts to aid in proper metabolic processes; may be used as an enzymatic cofactor; because it is not synthesized, it must be obtained prefabricated in the diet

Enzyme Regulation

  • pH: A symbol that denotes the relative concentration of hydrogen ions in a solution: the lower the pH, the more acidic a solution; the higher the pH, the more basic a solution; pH is equal to -log [H+]

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Full Enzymes, Metabolism, and Cell Communication Notes

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