Introduction to the Plasma Membrane

The plasma membrane is a fundamental component of all cells, responsible for regulating the movement of substances into and out of the cell. Its structure and function are essential for maintaining cellular homeostasis and enabling communication with the environment.

Major Ways the Plasma Membrane Regulates Traffic

Overview of Membrane Transport

• Passive Transport: Movement of small molecules across the membrane without energy input, either by diffusion or with the help of transport proteins.

• Active Transport: Movement of ions and molecules against their concentration gradient, requiring both energy (usually ATP) and a transport protein.

• Bulk Transport (Exocytosis and Endocytosis): Movement of large molecules (such as proteins and polysaccharides) into or out of the cell via vesicles.

Structure of Cellular Membranes

Amphipathic Nature of Membrane Components

• Amphipathic molecules have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• Phospholipids are the main amphipathic molecules in membranes, forming a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.

The Fluid Mosaic Model

• The fluid mosaic model describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Membrane proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

Membrane Fluidity

• Membranes are held together mainly by weak hydrophobic interactions.

• Most lipids and some proteins can move sideways; rarely, a lipid may flip-flop across the bilayer.

• Temperature Effects:

  • At lower temperatures, membranes become less fluid and may solidify.

  • Membranes rich in unsaturated fatty acids remain more fluid at lower temperatures.

  • Cholesterol acts as a fluidity buffer in animal cell membranes: at moderate temperatures, it reduces phospholipid movement, decreasing fluidity; at low temperatures, it prevents tight packing, maintaining fluidity.

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral proteins: Bound to the membrane surface.

• Integral proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

Functions of Membrane Proteins

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix

Role of Membrane Carbohydrates

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids).

• They function as markers for cell recognition and play a role in immune response.

Selective Permeability of the Membrane

Permeability of the Lipid Bilayer

• Small, hydrophobic (nonpolar) molecules (e.g., O2, CO2) pass through easily.

• Hydrophilic (polar) molecules and ions (e.g., glucose, Na+) pass through slowly or not at all without assistance.

Transport Proteins

• Channel proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

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

• Transport proteins are specific for the substances they move.

Diffusion and Osmosis

Diffusion

• Movement of particles from an area of higher concentration to lower concentration (down their concentration gradient).

• No energy input is required.

• At dynamic equilibrium, molecules continue to move but there is no net change in concentration.

Osmosis

• Diffusion of water across a selectively permeable membrane.

• Water moves toward the area of higher solute concentration.

Effects of Tonicity on Cells

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic solution: Higher solute concentration outside the cell; cell loses water and shrivels.

• Hypotonic solution: Lower solute concentration outside the cell; cell gains water and may burst (animal cells) or become turgid (plant cells).

Table: Effects of Tonicity on Animal and Plant Cells

Facilitated Diffusion

• Transport proteins (channels and carriers) speed up passive movement of molecules across the membrane.

• Channel proteins can be gated (open or close in response to stimuli).

• Carrier proteins undergo shape changes to move substances down their concentration gradient.

Active Transport

Mechanism and Importance

• Active transport moves substances against their concentration gradient, requiring energy (usually ATP).

• Maintains differences in solute concentrations essential for cell function.

The Sodium-Potassium Pump (Na+/K+ Pump)

• Maintains high K+ and low Na+ inside animal cells.

• Uses ATP to pump 3 Na+ out and 2 K+ in per cycle.

Membrane Potential and Electrochemical Gradients

• Membrane potential: Voltage across a membrane due to differences in ion distribution.

• Electrochemical gradient combines the chemical gradient (concentration) and electrical gradient (charge).

• Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants) generate membrane potential.

Co-Transport

• Active transport of one solute indirectly drives transport of another.

• Example: Proton pumps create an H+ gradient used to drive uptake of sucrose in plants.

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

• Vesicles fuse with the plasma membrane to release large molecules outside the cell.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

• Cell takes in macromolecules by forming vesicles from the plasma membrane.

• Three types:

  • Phagocytosis: "Cell eating"; cell engulfs large particles.

  • Pinocytosis: "Cell drinking"; cell takes in extracellular fluid and dissolved solutes.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors. Example: Uptake of cholesterol via LDL particles.

Key Terms and Definitions

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions.

• Phospholipid bilayer: Double layer of phospholipids forming the core of cell membranes.

Summary Table: Types of Membrane Transport

Example Application: If a cell is placed in a hypertonic solution, water will leave the cell, causing it to shrink. In a hypotonic solution, water enters the cell, which may cause it to burst (animal cell) or become turgid (plant cell).

Chapter 8: An Introduction to Metabolism

Overview of Metabolism

• Metabolism: The totality of an organism’s chemical reactions.

• Metabolic pathway: A series of chemical reactions that transform a starting molecule into a product, each step catalyzed by a specific enzyme.

• Catabolic pathways: “Downhill” reactions that break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: “Uphill” reactions that build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy in Biological Systems

• Thermal energy: Kinetic energy associated with random movement of atoms or molecules; transferred as heat.

• Light energy: Energy in the form of light, used in photosynthesis.

• Potential energy: Stored energy due to location or structure.

• Chemical energy: Potential energy available for release in a chemical reaction (e.g., energy stored in glucose).

Example: The chemical energy in food molecules is converted to kinetic energy for movement and thermal energy (heat).

Thermodynamics and Biological Systems

The First Law of Thermodynamics

• Law: Energy can be transferred and transformed, but cannot be created or destroyed.

• Also called: The principle of conservation of energy.

• Application: Plants convert light energy to chemical energy; animals convert chemical energy to kinetic and heat energy.

The Second Law of Thermodynamics

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

• Implication: Some energy is always lost as heat, making it unavailable to do work.

• Biological relevance: Living organisms increase the disorder of their surroundings through metabolism.

Example: When a plant uses sunlight to make organic molecules, some energy is lost as heat, increasing entropy.

Free Energy and Spontaneity of Reactions

Free energy (Gibbs free energy) determines whether a reaction can occur spontaneously.

• Change in free energy (ΔG): Indicates the spontaneity of a process.

Equation:

• = change in free energy

• = change in enthalpy (total energy)

• = change in entropy

• = temperature in Kelvin

• Spontaneous process: (negative ΔG): can occur without energy input.

• Nonspontaneous process: requires energy input.

Example: Cellular respiration is a spontaneous, exergonic process.

Exergonic and Endergonic Reactions

• Exergonic reaction: Proceeds with a net release of free energy; is negative; spontaneous.

• Endergonic reaction: Absorbs free energy from surroundings; is positive; nonspontaneous.

Example: Synthesis of glucose (photosynthesis) is endergonic; breakdown of glucose (cellular respiration) is exergonic.

ATP and Energy Coupling

• ATP structure: Adenine, ribose, and three phosphate groups.

• ATP hydrolysis: Releases energy by breaking the terminal phosphate bond.

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making it more reactive.

• ATP cycle: ATP is regenerated by adding a phosphate to ADP, using energy from catabolism.

Example: ATP powers muscle contraction, active transport, and biosynthesis.

Enzymes and Activation Energy

• Enzyme: A macromolecule (usually a protein) that catalyzes a specific reaction.

• Activation energy (Ea): The initial energy needed to start a reaction.

• Substrate: The reactant an enzyme acts on.

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

• Induced fit: The enzyme changes shape slightly to fit the substrate more snugly.

How enzymes lower Ea:

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Directly participating in the reaction

Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; activity decreases above or below this point.

• pH: Each enzyme has an optimal pH; extreme pH can denature the enzyme.

• Cofactors: Nonprotein helpers (inorganic ions or organic coenzymes) required for enzyme function.