5.5 Metabolism—Organized, Enzyme-Mediated Reactions
A. Types of Metabolic Pathways
1. Metabolic pathways are sequences of enzyme-mediated reactions by which cells build, rearrange, or tear down substances.
a. Anabolic pathways require a net energy input to build organic compounds from smaller molecules.
b. Catabolic pathways yield a net release of energy—they break-down molecules to smaller products.
2. We categorize all organisms based on how they get the carbon necessary for metabolism.
a. Autotrophs are producers who get carbon directly from carbon dioxide in their environment.
b. Heterotrophs are consumers who obtain their carbon from organic molecules created by autotrophs.
c. Both extract energy from organic compounds, most via aerobic respiration.
d. Many metabolic pathways are linear, a straight line from reactants to products. Others may be branched, with reactants, or some may be intermediates funneled into more than one sequence of reactions. Still other reactions may be cyclical in nature.
B. The Direction of Metabolic Reactions
1. Metabolic reactions do not always run from reactants to products. Most also run in reverse.
2. Reactions tend toward chemical equilibrium, where concentrations of reactants and products no longer change because the rate of the reaction is similar in both directions.
a. This does not mean the presence of reactants and products are present in equal amounts.
b. Many metabolic pathways in humans are mediated by the control of a few steps of reversible pathways.
C. Redox Reactions
1. Oxidation-reduction reactions are simply electron transfers between molecules.
a. The donor molecule loses an electron and is oxidized.
b. The receptor molecule gains an electron and is reduced.
2. In electron transfer chains, molecules accept and give up electrons in an orderly, stepwise fashion to control the release of energy.
a. Electrons are at a higher energy level when they enter the chain than when they leave.
b. It is helpful to think of the electrons as descending a staircase losing a bit of energy at each step.
5.6 Diffusion, Membranes, and Metabolism
A. What Is a Concentration Gradient?
1. Concentration gradient refers to the number of molecules (or ions) per unit volume of a substance between two regions.
2. The thermal energy of the molecules drives the movement of molecules.
a. Molecules constantly collide and tend to move down a concentration gradient (high to low).
b. The net movement of like molecules down a concentration gradient is called diffusion; each substance diffuses independently of other substances present as illustrated by dye molecules in water.
c. Any substance tends to diffuse in a direction set by its own concentration gradient.
B. Diffusion Rates
1. Several factors influence the rate and direction of diffusion: concentration differences, temperature (higher = faster), molecular size (smaller = faster), electric gradients (a difference in charge), and pressure gradients.
a. Small molecules diffuse faster than larger ones.
b. The diffusion rate is higher when there are steeper gradients.
c. More heat energy makes molecules move faster.
d. An electric gradient (difference in electric charge between regions) can affect the rate and direction of diffusion.
e. When gradients no longer exist, there is no net movement (dynamic equilibrium).
C. Diffusion and Membrane Permeability
1. All cell membranes show selective permeability; that is, some substances can cross, others cannot.
2. Membrane barriers and crossings are necessary to maintain the cell's capacity to increase, decrease, and maintain concentrations of molecules necessary for its reactions.
D. How Substances Cross Membranes
1. Selective permeability arises from the structure of the membrane.
a. Certain molecules like gases and small nonpolar molecules cross through gaps in the lipid bilayer.
b. The lipid bilayer is impermeable to ions and large polar molecules.
2. Passive transport, or "facilitated" diffusion, describes the movement of any substance through a membrane, or protein channel in the membrane, from high to low concentration without the expenditure of energy.
3. Bulk movement of substances in vesicles out of the cell is called exocytosis; the reverse action is endocytosis.