Energy, Cellular Metabolism, and Membrane Dynamics: Study Notes

Chapter 4: Energy and Cellular Metabolism

Energy in Biological Systems

Energy is essential for all cellular processes and is defined as the capacity to do work. In biological systems, work can be classified into three main types:

• Chemical work: Involves the making and breaking of chemical bonds, such as in metabolic reactions.

• Transport work: Refers to the movement of ions, molecules, and larger particles across cell membranes, creating concentration gradients.

• Mechanical work: Used for movement, including the transport of organelles, cilia and flagella beating, and muscle contraction.

Energy exists in two forms:

• Kinetic energy: Energy in motion.

• Potential energy: Stored energy, such as in chemical bonds or concentration gradients.

Example: In cells, potential energy stored in gradients or bonds is converted to kinetic energy to perform work.

Chemical Reactions and Activation Energy

Chemical reactions in cells require an initial input of energy, known as activation energy, to proceed. The rate of a reaction depends on the activation energy:

• Low activation energy: Spontaneous reactions.

• High activation energy: Slow or no reaction.

• Reaction rate: The speed at which reactants are converted to products.

Chemical Reaction Coupling

Cells often couple exergonic reactions (energy-releasing) with endergonic reactions (energy-consuming) to drive necessary processes. Energy released from exergonic reactions (e.g., breakdown of ATP) is used to fuel endergonic reactions.

Enzymes: Biological Catalysts

Enzymes are proteins that speed up chemical reactions by lowering the activation energy. They bind to substrates, positioning them optimally for reaction, but are not consumed in the process.

• Enzymes are biological catalysts.

• Most enzymes are proteins.

Metabolism: Catabolic and Anabolic Reactions

Metabolism encompasses all chemical reactions in an organism. It is divided into:

• Catabolic reactions: Break down biomolecules to release energy.

• Anabolic reactions: Use energy to synthesize large biomolecules.

ATP Production: Overview

ATP is the primary energy currency of the cell. Its production occurs via aerobic and anaerobic pathways:

• Aerobic pathways: Require oxygen, yield the most ATP.

• Carbohydrates enter as glucose, lipids as fatty acids, proteins as amino acids.

ATP Production: Glycolysis

Glycolysis is the first step in glucose metabolism, occurring in the cytosol and not requiring oxygen.

• Glucose → 2 Pyruvate + 2 ATP + 2 NADH + 2 H2O

$\text{Glucose} \rightarrow 2\ \text{Pyruvate} + 2\ \text{ATP} + 2\ \text{NADH} + 2\ \text{H}_2\text{O}$

ATP Production: Pyruvate Metabolism

• Anaerobic metabolism: If oxygen is lacking, pyruvate is converted to lactate.

• Aerobic metabolism: If oxygen is present, pyruvate is converted to acetyl CoA, entering the citric acid cycle.

$2\ \text{Pyruvate} + 2\ \text{NADH} \rightarrow 2\ \text{Lactate}$ (anaerobic)

$2\ \text{Pyruvate} + 2\ \text{O}_2 \rightarrow 2\ \text{Acetyl CoA} + 2\ \text{NADH} + 2\ \text{CO}_2$ (aerobic)

ATP Production: Citric Acid Cycle

The citric acid cycle (Krebs cycle) occurs in the mitochondria and further oxidizes acetyl CoA.

• 2 Acetyl CoA + 4 O2 + 2 H2O → 6 NADH + 2 FADH2 + 2 ATP + 4 CO2

$2\ \text{Acetyl CoA} + 4\ \text{O}_2 + 2\ \text{H}_2\text{O} \rightarrow 6\ \text{NADH} + 2\ \text{FADH}_2 + 2\ \text{ATP} + 4\ \text{CO}_2$

ATP Production: Electron Transport Chain

High-energy electrons from NADH and FADH2 are transferred through the electron transport chain, generating ATP.

• Oxygen is the final electron acceptor.

• ATP synthase produces ATP from ADP and inorganic phosphate.

Metabolism Summary Table

Pathway	NADH Produced	FADH2 Produced	ATP Produced
Glycolysis	2	0	2
Pyruvate Metabolism	2	0	0
Citric Acid Cycle	6	2	2
Total (Aerobic)	10	2	4

Each NADH = 2.5 ATP, Each FADH2 = 1.5 ATP

Total ATP from aerobic metabolism: $4\ \text{ATP} + (10\ \text{NADH} \times 2.5\ \text{ATP}) + (2\ \text{FADH}_2 \times 1.5\ \text{ATP}) = 32\ \text{ATP}$

Synthesis: Lipids

Most lipids are synthesized in the smooth endoplasmic reticulum (ER) and cytosol. Glycerol and fatty acids are combined to form triglycerides.

Synthesis: Proteins

Protein synthesis involves transcription and translation:

• Transcription: DNA is converted to mRNA in the nucleus.

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

• Post-translational modifications include folding, cross-linking, cleavage, and addition of other groups.

Nucleotide pairing: DNA: A=T, G≡C; RNA: A=U, G≡C

Chapter 5: Membrane Dynamics

Mass Balance and Homeostasis

The law of mass balance states that to maintain a constant amount of a substance, any gain must be offset by an equal loss. Homeostasis is the maintenance of a stable internal environment, which does not always mean equilibrium.

• Osmotic equilibrium: Equal total solute per volume on both sides of the membrane.

• Chemical disequilibrium: Unequal distribution of specific solutes (e.g., K+ high inside, Na+ high outside).

• Electrical disequilibrium: Difference in charge across the membrane.

Diffusion: Principles and Properties

Diffusion is the passive movement of molecules from high to low concentration. Key properties include:

• Passive process (no ATP required)

• Moves down concentration gradient

• Net movement until equilibrium

• Rapid over short distances, slower over long distances

• Rate increases with temperature

• Rate decreases with increasing molecular size

• Can occur across open systems or barriers

Fick's Law of Diffusion:

$\text{Rate of diffusion} = \text{surface area} \times \text{concentration gradient} \times \text{membrane permeability}$

Membrane Permeability

• Lipophilic (hydrophobic) molecules: Can diffuse through the phospholipid bilayer.

• Lipophobic (hydrophilic) molecules: Cannot diffuse through the lipid bilayer.

Membrane Proteins and Transporters

Membrane proteins serve various functions:

• Structural proteins: Provide cell shape and adhesion.

• Enzymes: Catalyze reactions.

• Receptor proteins: Mediate chemical signaling.

• Transporters: Move molecules across membranes.

Transporters include:

• Channel proteins: Create water-filled passageways.

• Carrier proteins: Bind and transport substrates by changing conformation.

Types of Channel Proteins

• Chemically gated: Controlled by messenger molecules or ligands.

• Voltage-gated: Controlled by electrical state of the cell.

• Mechanically gated: Controlled by physical changes (e.g., tension, temperature).

Facilitated Diffusion

Facilitated diffusion allows molecules like sugars and amino acids to cross membranes via specific transporters, requiring no energy and stopping at equilibrium.

Active Transport

• Primary active transport: Uses ATP directly (e.g., Na+/K+ pump).

• Secondary active transport: Uses energy from one molecule moving down its gradient to move another against its gradient.

Vesicular Transport

• Phagocytosis: Cell engulfs large particles.

• Endocytosis: Cell takes in substances via vesicles.

• Exocytosis: Cell expels substances via vesicles.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane in response to solute concentration gradients. Water moves from areas of low solute concentration (hyposmotic) to high solute concentration (hyperosmotic).

• Tonicity: Describes the effect of a solution on cell volume (isotonic, hypertonic, hypotonic).

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the cell membrane, typically around -70 mV. It is maintained by ion gradients and the activity of the Na+/K+ pump.

Summary Table: Types of Membrane Transport

Transport Type	Energy Requirement	Example
Simple Diffusion	None	O2, CO2
Facilitated Diffusion	None	Glucose, amino acids
Primary Active Transport	ATP	Na+/K+ pump
Secondary Active Transport	Indirect (gradient)	SGLT transporter
Vesicular Transport	ATP	Endocytosis, exocytosis

Additional info: These notes expand on the original slides by providing definitions, examples, and equations for key concepts in energy metabolism and membrane dynamics, suitable for Anatomy & Physiology students.