Biological Membranes

Phospholipids and Membrane Structure

Biological membranes are primarily composed of phospholipids, which are amphipathic molecules containing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. These membranes also include proteins and cholesterol.

• Phospholipid bilayer: Forms the basic structure of cell membranes, with hydrophilic heads facing outward and hydrophobic tails inward.

• Cell membrane (plasma membrane): Another term for the biological membrane.

Fluid Mosaic Model

The Fluid Mosaic Model describes membranes as fluid structures with a mosaic of embedded proteins.

• Membranes are composed of 20-80% protein by mass, which move laterally within the membrane.

Types of Membrane Proteins

Integral and Peripheral Proteins

Membrane proteins are classified based on their location and function.

• Integral membrane proteins: Embedded within the membrane and span its entire width.

• Peripheral membrane proteins: Attached to the surface of the membrane.

Membrane Protein Functions (R.A.T.T.L.E.)

• Recognition: Markers for identification.

• Anchorage: Attach cell cytoskeleton and extracellular matrix.

• Transduction: Signal molecule receptors.

• Transport: Move substances across the membrane.

• Linkage: Connects two cells via protein linkage.

• Enzymes: Catalyze enzymatic processes.

Concentration Gradients & Diffusion

Concentration Gradient

A concentration gradient is the difference in concentration between two areas.

• Molecules move down their gradient from high to low concentration (no energy required).

Diffusion

Diffusion is the movement of molecules from high to low concentration until equilibrium is reached.

• Molecules diffuse down their concentration gradient.

Membrane Transport

Selective Permeability

Plasma membranes are selectively permeable, allowing some molecules to pass while blocking others.

• Small, uncharged, nonpolar/hydrophobic molecules (e.g., O2, CO2, N2) can diffuse freely.

• Large, charged, or polar/hydrophilic molecules cannot diffuse freely and require transport proteins.

Passive & Active Transport

• Passive transport: No energy required; molecules move from high to low concentration.

• Active transport: Requires energy (often ATP); molecules move from low to high concentration via transport proteins.

Classes of Membrane Transport Proteins

• Uniporter: Transports one molecule at a time in one direction.

• Symporter: Transports two or more molecules at a time in the same direction.

• Antiporter: Transports two or more molecules at a time in different directions.

Osmosis and Tonicity

Osmosis

Osmosis is the passive diffusion of water across a semi-permeable membrane. The direction of water flow depends on tonicity.

Tonicity

• Isotonic: Equal solute concentration.

• Hypotonic: Lower solute concentration.

• Hypertonic: Higher solute concentration.

Directions of Osmosis

• Water moves from hypotonic to hypertonic solutions.

• Water moves from high water concentration (low solute) to low water concentration (high solute).

Environmental Tonicity Effects

• Hypotonic environments: Water enters cells, causing swelling (lysis in animal cells).

• Hypertonic environments: Water exits cells, causing dehydration (crenation in animal cells).

• Isotonic environments: Water enters and exits cells at equal rates.

Simple & Facilitated Diffusion

Types of Passive Transport

• Simple diffusion: Direct movement of molecules across the membrane.

• Facilitated diffusion: Movement of charged or polar molecules via transport proteins.

Transport Proteins

• Porins/Channels: Form membrane-spanning tunnels for water and small molecules.

• Transporters/Carriers: Undergo conformational changes to move molecules across the membrane.

Active Transport

Primary Active Transport

Directly driven by energy source (ATP hydrolysis).

• ATP-driven pumps: Move molecules against their concentration gradient.

• Na+/K+ pump: Moves sodium and potassium ions in opposite directions.

Secondary Active Transport

Driven by another molecule's concentration gradient.

• Uses energy stored in gradients created by primary active transport.

• Example: Glucose transport coupled to Na+ gradient.

Endocytosis & Exocytosis

Endocytosis

Macromolecule engulfment by the cell membrane, allowing entry into the cell via vesicle formation.

• Phagocytosis: Engulfment of large, solid material (cell eating).

• Pinocytosis: Engulfment of small, liquid material (cell drinking).

• Receptor-mediated endocytosis: Specific uptake using receptor proteins.

Exocytosis

Vesicle fusion with the cell membrane, allowing release of substances outside the cell.

• Examples: Hormones, neurotransmitters, digestive enzymes.

Energy in Biology

Definition and Types

Energy is the ability to perform work. In biology, work is a transfer of energy that causes an overall change.

• Potential energy: Stored energy available to do work.

• Kinetic energy: Energy of motion.

Thermodynamics

The study of energy transfer between bodies of matter.

• System: The portion of matter being studied.

• Surroundings: Everything else outside the system.

• Biological system: Transfers both energy and mass with the surroundings.

Laws of Thermodynamics

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

• Second Law: Energy conversions are never 100% efficient; some energy is lost as heat, increasing entropy (disorder/randomness).

Chemical Reactions

Types of Chemical Reactions

• Endergonic reactions: Require an input of energy (energy enters the reaction).

• Exergonic reactions: Release energy (energy exits the reaction).

Reactants and Products

• Reactants: Starting materials in a reaction.

• Products: Ending materials in a reaction.

ATP (Adenosine Triphosphate)

Structure and Function

ATP is a high-energy molecule used to power cellular activities.

• Composed of three phosphate groups, a pentose sugar, and an adenine nitrogenous base.

ATP Hydrolysis

ATP hydrolysis breaks bonds between phosphate groups, releasing energy.

• ATP → ADP + Pi (inorganic phosphate)

• Energy released is used for cellular work.

Phosphorylation

Transfer of a phosphate group from ATP to another molecule to provide energy.

• Activates target molecules or proteins.

Enzymes

Function and Properties

Enzymes are biological catalysts that speed up chemical reactions without being consumed.

• Functions include building proteins, copying DNA, and digesting food.

Factors Affecting Enzyme Activity

• Temperature

• pH

• Concentration

• Denaturation: Loss of enzyme shape and decreased activity.

Enzyme Activation Energy

• Activation energy (EA): Minimum energy required to start a chemical reaction.

• Enzymes lower EA, increasing reaction rates.

Enzyme Binding

• Substrate binds to enzyme at the active site, forming the Enzyme-Substrate Complex (ES).

• After catalysis, products are released and the enzyme remains unchanged.

Enzyme Inhibition

• Competitive inhibitors: Compete with substrate for the active site.

• Noncompetitive inhibitors: Bind elsewhere (allosteric site), changing enzyme shape.

Metabolism

Introduction to Metabolism

Metabolism is all of an organism's chemical reactions.

• Catabolic pathways: Release energy by breaking down molecules.

• Anabolic pathways: Consume energy to build up larger molecules.

Feedback Regulation

• Negative feedback: Final product inhibits an earlier step in the pathway.

• Positive feedback: Final product stimulates an earlier step in the pathway.

Example Table: Types of Membrane Transport

Example Table: Tonicity Effects on Animal Cells