Energy, Cellular Metabolism, and Membrane Dynamics: Study Notes

Energy and Cellular Metabolism

Energy: Definition and Types of Work

Energy is the capacity to do work, which is essential for all cellular processes. 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.

Forms of Energy

Energy exists in two primary forms:

• Kinetic Energy: The energy of motion.

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

Example: In cells, potential energy stored in concentration gradients and chemical bonds is converted to kinetic energy to perform chemical, transport, or mechanical work.

Chemical Reactions: Activation Energy and Rate

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

• Low activation energy: Spontaneous reaction.

• High activation energy: Slow or no reaction.

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

Chemical Reactions: Coupling

Cells often couple exergonic (energy-releasing) reactions to endergonic (energy-consuming) reactions. The energy released from exergonic reactions (such as ATP hydrolysis) is used to drive endergonic processes.

Enzymes: Biological Catalysts

Enzymes are proteins that speed up the rate of chemical reactions by lowering the activation energy. They bind to substrates and position them optimally for reaction. Enzymes are not consumed in the reaction and act as biological catalysts.

Metabolism: Catabolic and Anabolic Reactions

Metabolism encompasses all chemical reactions in an organism:

• 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 involves several metabolic pathways:

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

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

ATP Production: Glycolysis

Glycolysis is the first step in glucose metabolism and occurs in the cytosol. It does not require oxygen.

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

ATP Production: Pyruvate Metabolism

• Anaerobic metabolism: If oxygen is lacking, pyruvate is converted to lactate. Net yield: 2 ATP per glucose, no NADH.

• Aerobic metabolism: If oxygen is present, pyruvate is converted to acetyl CoA, producing NADH and CO2.

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

ATP Production: Electron Transport Chain

High-energy electrons from NADH and FADH2 are transferred through the electron transport chain, driving ATP synthesis. Oxygen is the final electron acceptor, forming water.

Metabolism Summary Table

Pathway	ATP Yield	NADH	FADH2
Glycolysis	2	2	0
Citric Acid Cycle	2	6	2
Electron Transport	28	-	-
Total (Aerobic)	32	-	-

Each NADH = 2.5 ATP, Each FADH2 = 1.5 ATP

Equation:

$\text{Total ATP} = 4 + (10 \times 2.5) + (2 \times 1.5) = 32$

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 transcribed into mRNA in the nucleus.

• Translation: mRNA is translated into a polypeptide chain at the ribosome, using tRNA and amino acids.

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

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

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 necessarily 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: Properties and Fick's Law

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

• 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 molecules: Can diffuse through the phospholipid bilayer.

• Lipophobic molecules: Cannot cross the lipid bilayer; require transport proteins.

Membrane Proteins: Functions

• Structural proteins: Provide cell shape and adhesion.

• Enzymes: Catalyze reactions.

• Receptor proteins: Mediate chemical signaling.

• Transporters: Move molecules across membranes.

Types of Membrane Transporters

Channel Proteins	Carrier Proteins
Gated channels (open/closed)	Uniport, Symport, Antiport carriers
Open channels	Bind substrate, change conformation

Gating 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., temperature, tension).

Facilitated Diffusion

Facilitated diffusion allows sugars and amino acids to cross membranes via specific transporters. It requires a concentration gradient, does not use energy, and stops at equilibrium.

Active Transport

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

• 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 small vesicles.

• Exocytosis: Cell expels materials 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).

• Osmolarity: Number of particles in solution.

• Tonicity: Describes the effect of a solution on cell volume.

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

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.