3.1 - Introduction

• Two stages of photosynthesis: the light-dependent reactions, the light-independent reactions (the dark reactions).

• The simplified equation of photosynthesis is:

3.2 - Enzymes

• Proteins that act as catalysts

• Catalysts speed up reactions by lowering the energy (activation energy) needed for the reaction to take place, but are not used up in the reaction.

• Substrates - the substances that enzymes act on.

• Enzymes - selective; interact only with particular substrates.

• Shape of the enzyme provides the specificity.

• The part of the enzyme that interacts with the substrate - the active site.

• The lock and key model - describes a substrate’s interaction with the enzyme’s active site; suggests that the enzyme and the substrate possess specific complementary geometric shapes that fit perfectly.

• The induced-fit model of enzyme-substrate interaction describes the active site of an enzyme as specific for a particular substrate that fits its shape.

• When the enzyme and substrate bind together, the enzyme is induced to alter its shape for a tighter active site–substrate attachment.

• This tight fit places the substrate in a favorable position to react, accelerating the rate of reaction.

• After an enzyme interacts with a substrate, converting it into a product, it is free to find and react with another substrate.

• A small concentration of enzyme can have a major effect on a reaction.

• The effectiveness of the enzyme will suffer and the enzyme could denature if not placed in optimal conditions, causing the protein to unravel and lose its shape.

• Factors that impact the effectiveness of an enzyme:

  • The temperature

  • The pH

  • The concentration of the substrate involved

  • The concentration of the enzyme involved

• Plot showing energy versus time. Height A represents original activation energy; height B represents the lowered activation energy due to the addition of enzymes.

  

• Competitive inhibition - an inhibitor molecule resembling the substrate binds to the active site and physically blocks the substrate from attaching; can sometimes be overcome by adding a high concentration of substrate to outcompete the inhibitor.

• Noncompetitive inhibition an inhibitor molecule binds to a different part of the enzyme, causing a change in the shape of the active site so that it can no longer interact with the substrate.

3.3 - Cellular Energy

• All living organisms rely on a constant input of energy in different forms to survive and thrive.

• This flow of energy follows the laws of thermodynamics that govern all forms of energy - it states that energy cannot be created or destroyed; it can only change form, and it must be obtained through its environment.

• The second law of thermodynamics states that life is in a constant movement toward entropy or a “gradual decline of order” in a system and requires a constant input of energy from its environment that can be used to overcome this decline of order.

• The constant input of energy to overcome energy and the idea that energy cannot be created or destroyed is the foundation upon which trophic or energy dynamics on Earth rest.

• The movement energy comes from - endergonic and exergonic reactions.

• Endergonic reactions - reactions in which energy is absorbed from the surroundings.

• Exergonic reactions - reactions in which free energy is released.

• Energy in chemical reactions. a. In an endergonic reaction, the products of the reaction contain more energy than the reactants, and the extra energy must be supplied for the reaction to proceed. b. In an exergonic reaction, the products contain less energy than the reactants, and the excess energy is released

  

• This constant exchange of free energy - maintained in living systems through controlled and efficient transfer of energy during the metabolic pathway.

• The products of one reaction in a metabolic pathway become the reactants for the next step in the pathway.

• Eg: the production of adenosine triphosphate (ATP) in cells. ATP is constructed from an adenosine diphosphate (ADP) and an inorganic phosphate group (Pi) through phosphorylation (a chemical process in which a phosphate group is added using free energy).

3.4: Aerobic Respiration

Glycolysis

• Occurs in the cytoplasm of cells; is the beginning pathway for both aerobic and anaerobic respiration.

• A glucose molecule is broken down through a series of reactions into two molecules of pyruvate.

• This reaction can occur in oxygen-rich and oxygen-poor environments; but when in an environment lacking oxygen, glycolysis slows because the cells become depleted of NAD+.

• A lack of oxygen prevents oxidative phosphorylation from occurring, causing a buildup of NADH in the cells; this buildup causes a shortage or NAD+.

• Fermentation - the solution to this problem; it takes the excess NADH that builds up and converts it back to NAD+ so that glycolysis can continue.

  Layout of glycolysis

• The first step adds a phosphate to a molecule of glucose with the assistance of an ATP molecule to produce glucose-6-phosphate (G6P).

• The newly formed G6P rearranges to form a molecule named fructose-6-phosphate (F6P). Another molecule of ATP is required for the next step, which adds another phosphate group to produce fructose 1,6-biphosphate.

• F6P - splits into two 3-carbon-long fragments known as PGAL (glyceraldehyde phosphate); with the formation of PGAL, the energy-producing portion of glycolysis begins.

• Each PGAL molecule takes on an inorganic phosphate from the cytoplasm to produce 1,3- diphosphoglycerate. During this reaction, each PGAL gives up two electrons and a hydrogen to molecules of NAD+ to form the all-important NADH molecules.

• The next step - leads to the production of the first ATP molecule in the process of respiration—the 1,3-diphosphoglycerate molecules donate one of their two phosphates to molecules of ADP to produce ATP and 3-phosphoglycerate (3PG).

• Two ATP molecules formed - because before this step, the single molecule of glucose divided into two 3-carbon fragments.

• After 3PG rearranges to form 2-phosphoglycerate, phosphoenolpyruvate (PEP) is formed, which donates a phosphate group to molecules of ADP to form another pair of ATP molecules and pyruvate.

• In total, two molecules each of ATP, NADH, and pyruvate are formed during this process. Glycolysis - produces the same result under anaerobic conditions as it does under aerobic conditions - two ATP molecules.

• If oxygen is present, more ATP is later made by oxidative phosphorylation.