Cellular Energetics: Enzymes, Energy, and Glycolysis

Cellular Energetics

Overview

Cellular energetics is the study of how cells obtain, convert, and use energy to sustain life. This topic covers the fundamental processes of respiration and photosynthesis, focusing on how energy is transferred and transformed within living systems. Key processes include aerobic and anaerobic respiration, ATP production, and the light-dependent and light-independent reactions of photosynthesis.

• Aerobic respiration: Glycolysis → Krebs cycle → oxidative phosphorylation (yields up to 36 ATP per glucose molecule).

• Anaerobic respiration: Glycolysis → fermentation (yields much less ATP).

• Photosynthesis: Converts light energy into chemical energy stored in glucose; involves light-dependent and light-independent (Calvin cycle) reactions.

Key equation for photosynthesis:

$\mathrm{6\ CO_2 + 6\ H_2O + light \rightarrow C_6H_{12}O_6 + 6\ O_2}$

Enzymes

Definition and Function

Enzymes are biological catalysts, typically proteins, that speed up chemical reactions in living organisms by lowering the activation energy required for the reaction to proceed. They are essential for regulating metabolic pathways and ensuring efficient energy transfer.

• Catalysts: Substances that increase the rate of a chemical reaction without being consumed.

• Active site: The region on the enzyme where the substrate binds and the reaction occurs.

• Substrate: The specific reactant that an enzyme acts upon.

• Induced fit model: The enzyme changes shape slightly to fit the substrate more snugly, enhancing catalysis.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH range.

• Substrate concentration: Increased substrate concentration increases reaction rate up to a point (saturation).

• Enzyme concentration: More enzyme generally increases reaction rate, assuming substrate is not limiting.

Enzyme Inhibition

• Competitive inhibition: Inhibitor resembles the substrate and competes for binding at the active site. Can be overcome by increasing substrate concentration.

• Noncompetitive inhibition: Inhibitor binds to a site other than the active site, causing a conformational change that reduces enzyme activity. Cannot be overcome by increasing substrate concentration.

Illustrative Figures

• Figure 7.1: Shows how enzymes lower activation energy, making reactions proceed faster.

• Figure 7.2: Depicts competitive inhibition.

• Figure 7.3: Depicts noncompetitive inhibition.

Energy in Biological Systems

Thermodynamics and Energy Transfer

All living organisms require a constant input of energy to maintain order and drive cellular processes. The laws of thermodynamics govern energy transformations:

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

Types of Reactions

• Exergonic reactions: Release energy; products have less free energy than reactants (e.g., cellular respiration).

• Endergonic reactions: Require energy input; products have more free energy than reactants (e.g., photosynthesis).

Reaction Type	Energy Change	Example
Exergonic	Releases energy (ΔG < 0)	Cellular respiration
Endergonic	Requires energy input (ΔG > 0)	Photosynthesis

Glycolysis

Overview and Steps

Glycolysis is the first step in both aerobic and anaerobic respiration, occurring in the cytoplasm. It breaks down one glucose molecule (6 carbons) into two molecules of pyruvate (3 carbons each), producing a net gain of ATP and NADH.

• Location: Cytoplasm

• Inputs: 1 glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 pyruvate, 2 NADH, 2 ATP (net)

Key Steps in Glycolysis

1. Glucose is phosphorylated to glucose-6-phosphate (uses 1 ATP).

2. Glucose-6-phosphate is converted to fructose-6-phosphate.

3. Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate (uses 1 ATP).

4. Fructose-1,6-bisphosphate splits into two 3-carbon molecules (PGAL).

5. Each PGAL is oxidized, reducing NAD+ to NADH and generating ATP by substrate-level phosphorylation.

6. Final products: 2 pyruvate, 2 NADH, 2 ATP (net gain).

Summary Equation for Glycolysis

$\mathrm{Glucose + 2\ NAD^+ + 2\ ADP + 2\ P_i \rightarrow 2\ Pyruvate + 2\ NADH + 2\ ATP + 2\ H_2O}$

Diagram of Glycolysis (Figure 7.5)

• Shows the stepwise conversion of glucose to pyruvate, with ATP and NADH production.

• Key point: One glucose molecule yields 2 pyruvate, 2 ATP (net), and 2 NADH.

Applications and Importance

• Glycolysis provides quick energy and intermediates for other metabolic pathways.

• It is the only energy-yielding pathway in anaerobic conditions.

Additional info: The ATP produced in glycolysis is generated by substrate-level phosphorylation, not oxidative phosphorylation. NADH produced can be used in the electron transport chain (aerobic) or in fermentation (anaerobic).