Metabolic Pathways: Reversibility and Alternative Routes

Metabolic pathways, controlled by enzymes, can be reversible or use alternative routes.

Example: Anaerobic respiration: glucose → pyruvate → lactate (in the absence of oxygen)

Example: Fermentation in plants/yeast: glucose → pyruvate → ethanol + CO₂

Alternative routes may bypass certain stages in a pathway.

Highlight: The flexibility of metabolic pathways allows cells to adapt to different environmental conditions and energy needs.

Cellular Membranes and Their Functions

Membranes play a crucial role in cellular organization and function.

Definition: The plasma membrane separates the cell from its environment, while other membranes divide eukaryotic cells into specialized compartments.

The phospholipid bilayer structure forms the basis of cellular membranes, with embedded proteins serving various functions:

1. Pores for diffusion (e.g., chlorine ions)
2. Pumps for active transport (requires ATP)
3. Structural support
4. Enzymes (e.g., ATP synthase)

Highlight: The phospholipid bilayer function and structure is essential for maintaining cellular organization and facilitating various biochemical processes.

Specialized Membrane Structures: Mitochondria and Chloroplasts

Mitochondria and chloroplasts have specialized membrane structures:

1. Outer membrane
2. Inner membrane
3. Cristae (folded inner membranes)
4. Matrix

Highlight: The folded inner membranes increase surface area, enhancing reaction rates and substrate concentration.

Activation Energy in Biochemical Reactions

Chemical reactions involve building or breaking down molecules. Activation energy (Ea) is required for a reaction to occur.

Definition: Enzymes lower the activation energy needed for a reaction, allowing biochemical reactions to proceed rapidly at relatively low temperatures.

Highlight: The role of enzymes in metabolic pathways is crucial for maintaining efficient cellular processes at physiological temperatures.

Enzyme Structure and Function

Enzymes play a vital role in metabolic pathways by catalyzing reactions.

Highlight: The active site of an enzyme is flexible, not rigid.

When a substrate binds to the active site, it changes slightly, improving the fit between the enzyme and substrate. This is known as induced fit.

Definition: Affinity refers to the tendency of the substrate to bind with the active site.

• Substrates have high affinity
• Products have low affinity

Highlight: The flexibility and specificity of enzyme active sites contribute to the efficiency of metabolic enzymes.

Enzyme Inhibition

Inhibitors are substances that decrease the rate of enzyme-controlled reactions. There are three main types of inhibition:

1. Competitive inhibition
2. Non-competitive inhibition
3. Feedback inhibition

Competitive Inhibition

Definition: Competitive inhibitors have a similar shape to the substrate molecule and compete for the active site on the enzyme.

This type of inhibition can be overcome by increasing substrate concentration.

Non-competitive Inhibition

Definition: Non-competitive inhibitors bind away from the active site but change the shape of the active site, preventing substrate binding.

This type of inhibition cannot be reversed by increasing substrate concentration.

Highlight: Understanding enzyme inhibition is crucial for comprehending the regulation of metabolic pathways and developing therapeutic interventions.

Feedback Inhibition in Metabolic Pathways

Feedback inhibition is a regulatory mechanism in metabolic pathways.

Definition: As the concentration of the end product builds up, some of it binds to an enzyme earlier in the pathway, slowing down the reaction and preventing further synthesis of the end product.

This mechanism helps maintain homeostasis in cellular metabolism.

Highlight: Feedback inhibition is a key regulatory mechanism in anabolic and catabolic reactions in metabolism.

Cellular Respiration and ATP Production

Cellular respiration is a series of metabolic pathways that release energy from food, producing ATP.

Definition: ATP (Adenosine Triphosphate) is the energy currency of the cell.

When the bond between ADP and phosphate is broken, energy is released.

Phosphorylation

Definition: Phosphorylation is an enzyme-controlled reaction where a phosphate group is added to a molecule.

ATP plays a crucial role in phosphorylation reactions throughout cellular metabolism.

Highlight: Understanding ATP production and utilization is essential for comprehending energy flow in anabolic and catabolic reactions in metabolism.

Glycolysis: The First Stage of Cellular Respiration

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm.

Highlight: Glycolysis does not require oxygen $anaerobic$ and is enzyme-controlled.

Glycolysis consists of two phases:

1. Energy Investment Phase: Requires ATP to phosphorylate glucose intermediates
2. Energy Payoff Phase: 4 ATP molecules are formed

Example: The net gain of ATP in glycolysis is 2 molecules.

Vocabulary: Dehydrogenase enzymes remove hydrogen ions and electrons from molecules, passing them to coenzyme NAD to form NADH.

If oxygen is present, pyruvate progresses to the citric acid cycle.

Highlight: Glycolysis is a fundamental process in both anabolic and catabolic reactions in metabolism, providing energy and intermediates for various cellular processes.

Citric Acid Cycle: The Second Stage of Cellular Respiration

The citric acid cycle, also known as the Krebs cycle, is the second stage of cellular respiration.

Metabolic Pathways: Anabolic and Catabolic Reactions

Metabolism encompasses all reactions within a cell, divided into anabolic $build-up$ and catabolic (breakdown) processes.

Definition: Anabolic reactions build complex molecules from simple building blocks, requiring energy. An example is protein synthesis.

Definition: Catabolic reactions break down complex molecules into simple building blocks, releasing energy. An example is aerobic respiration.

These reactions are linked, as the energy generated by catabolic reactions enables anabolic reactions to occur.

Highlight: Anabolic and catabolic reactions in metabolism are interconnected, forming the basis of cellular energy management.