Cell Membranes, Transport, and Cellular Metabolism: Study Notes for General Biology

Structure and Function of Biological Membranes

Overview of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds all cells, controlling the movement of substances in and out. It is composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol, providing both structure and function.

• Phospholipid Bilayer: Forms the fundamental structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Embedded or attached to the membrane, serving various roles such as transport, signaling, and structural support.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), important for cell recognition and signaling.

• Cholesterol: Interspersed within the bilayer, modulating membrane fluidity and stability.

• Extracellular Matrix (ECM): Network of fibers outside the cell, providing structural support and mediating cell interactions.

• Cytoskeleton: Microfilaments attached to the membrane on the cytoplasmic side, maintaining cell shape and facilitating movement.

Example: The fluid mosaic model describes the dynamic arrangement of lipids and proteins in the membrane.

Membrane Proteins

Membrane proteins are classified based on their association with the lipid bilayer and their function.

• Peripheral Proteins: Bound to the membrane surface, often attached to integral proteins or phospholipid heads. They play roles in signaling and maintaining cell shape.

• Integral Proteins: Penetrate the hydrophobic core of the bilayer. Those that span the membrane are called transmembrane proteins.

• Transmembrane Proteins: Have hydrophobic regions (often alpha helices of nonpolar amino acids) that interact with the membrane interior, and hydrophilic regions exposed to aqueous environments.

Example: Channel proteins and carrier proteins are types of transmembrane proteins involved in transport.

Membrane Transport

Permeability of the Lipid Bilayer

The plasma membrane's selective permeability allows some substances to cross more easily than others.

• Hydrophobic (nonpolar) molecules: Such as hydrocarbons, can dissolve in the lipid bilayer and pass through rapidly.

• Polar molecules: Such as sugars (e.g., glucose) and ions (e.g., Na+, K+, Mg2+, Ca2+) do not cross the membrane easily.

Transport Proteins

Transport proteins facilitate the movement of hydrophilic substances across the membrane.

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions to pass through (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

• Each transport protein is specific for the substance it moves.

Types of Membrane Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input. Includes simple diffusion and facilitated diffusion.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually from ATP).

Comparison of Passive and Active Transport

Feature	Passive Transport	Active Transport
Energy Requirement	No	Yes (ATP)
Direction	Down gradient	Against gradient
Examples	Diffusion, facilitated diffusion	Sodium-potassium pump

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

• Hypotonic Solution: Lower solute concentration outside; water enters the cell, which may lyse (burst).

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves the cell, which may shrivel.

Cellular Metabolism

Metabolic Pathways

Metabolism is the totality of an organism's chemical reactions, organized into metabolic pathways.

• Catabolic Pathways: Release energy by breaking down complex molecules (e.g., cellular respiration).

• Anabolic Pathways: Consume energy to build complex molecules (e.g., protein synthesis).

Forms of Energy

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy due to position or structure.

• Chemical Energy: Potential energy available for release in a chemical reaction.

Energy can be converted from one form to another.

Free Energy and Spontaneity

The change in free energy ($\Delta G$) determines whether a process is spontaneous.

• Spontaneous processes: Only occur when $\Delta G$ is negative.

• Exergonic Reactions: Release free energy ($\Delta G < 0$), are spontaneous.

• Endergonic Reactions: Absorb free energy ($\Delta G > 0$), are nonspontaneous.

Comparison of Exergonic and Endergonic Reactions

Feature	Exergonic	Endergonic
Free Energy Change ($\Delta G$)	Negative	Positive
Spontaneity	Spontaneous	Nonspontaneous
Energy Flow	Release	Require input
Product Energy	Lower than reactants	Higher than reactants

Enzymes and Metabolic Regulation

Enzyme Function

Enzymes are biological catalysts that speed up reactions by lowering the activation energy ($E_a$) barrier, without affecting $\Delta G$.

• Active Site: Region on the enzyme where the substrate binds.

• Induced Fit: Enzyme changes shape to better fit the substrate.

Factors Affecting Enzyme Activity

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

• pH: Each enzyme has an optimal pH.

Enzyme Inhibition

• Competitive Inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive Inhibitors: Bind elsewhere, causing the enzyme to change shape and reducing activity.

• Examples: Toxins, poisons, pesticides, antibiotics.

Feedback Inhibition

In feedback inhibition, the end product of a metabolic pathway inhibits an earlier step, preventing overproduction and conserving resources.

Cellular Respiration and Redox Reactions

Redox Reactions

Redox (oxidation-reduction) reactions involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons (reduces positive charge).

• Example: $\text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^-$ (Na is oxidized, Cl is reduced)

Stages of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose in the presence of oxygen. It occurs in three main stages:

1. Glycolysis: Breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, generating ATP, NADH, and FADH2.

3. Oxidative Phosphorylation: Accounts for most ATP synthesis via the electron transport chain and chemiosmosis.

Summary of Glycolysis

• Glucose $\rightarrow$ 2 pyruvate + 2 ATP (net) + 2 NADH

• ATP formed: 4 total, 2 used, net 2 ATP

Citric Acid Cycle

• Occurs in the mitochondrial matrix.

• Per turn: 1 ATP, 3 NADH, 1 FADH2 generated.

Electron Transport Chain and Chemiosmosis

• Electrons from NADH and FADH2 are transferred to the electron transport chain (ETC).

• ETC consists of a series of redox reactions, passing electrons to oxygen (final electron acceptor).

• Energy released is used to pump protons (H+) across the mitochondrial membrane, creating a proton gradient.

• ATP synthase uses the flow of protons back into the matrix to drive ATP synthesis (chemiosmosis).

Photosynthesis

Overview of Photosynthesis

Photosynthesis converts solar energy into chemical energy in chloroplasts. It consists of light reactions and the Calvin cycle.

• Light Reactions: Occur in the thylakoid membranes; split water, release O2, produce ATP and NADPH.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to convert CO2 into sugar.

Photosystems

• Each photosystem consists of a reaction-center complex and light-harvesting complexes.

• Light-harvesting complexes capture photons and transfer energy to the reaction center.

ATP and NADPH Production

• ATP and NADPH are produced on the stroma side, where the Calvin cycle occurs.

• Light reactions increase the potential energy of electrons by moving them from H2O to NADPH.

Comparison: Chemiosmosis in Mitochondria and Chloroplasts

Feature	Mitochondria	Chloroplasts
Proton Gradient Location	Intermembrane space	Thylakoid space
ATP Synthesis Location	Matrix	Stroma
Energy Source	Food molecules	Light energy

Additional info: The notes above are expanded and clarified for academic completeness, with logical groupings and context added for fragmented content. All tables are reconstructed for clarity and comparison.