Membrane Structure, Function, and Metabolism: General Biology Study Notes

Membrane Structure and Function

Plasma Membrane

The plasma membrane is a fundamental structure that separates the living cell from its external environment. It regulates the movement of substances into and out of the cell and facilitates communication and interaction with the environment.

• Separation: Isolates the cell from the nonliving environment.

• Selective Transport: Controls the entry and exit of molecules.

• Communication: Senses and interacts with external signals.

• Cell Identity: Involved with cell recognition and interactions with other cells.

Intracellular Membranes

Intracellular membranes maintain the structural integrity of organelles and facilitate compartmentalization within the cell.

• Structural Support: Maintain organelle structure and function.

• Compartmentalization: Separate chemical reactions and environments.

• Attachment Sites: Provide surfaces for enzyme and cellular structure attachment.

• Transport: Facilitate movement of substances via vesicles.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of diverse protein molecules embedded in a fluid bilayer of phospholipids.

• Phospholipids: Main structural component; form bilayers with hydrophilic heads and hydrophobic tails.

• Glycolipids: Lipids with carbohydrate chains; contribute to cell recognition.

• Sterols: Cholesterol and phytosterols; maintain membrane fluidity.

• Proteins: Integral and peripheral proteins; involved in transport, signaling, and recognition.

Lipid Bilayer Properties

The lipid bilayer is composed of amphipathic molecules, primarily phospholipids, which have both hydrophilic and hydrophobic regions.

• Phospholipids: Hydrophilic phosphate/choline head and hydrophobic fatty acid tails.

• Glycolipids: Non-polar lipid region with carbohydrate chains.

• Sterols: Cholesterol (animals) and phytosterols (plants); stabilize membrane fluidity.

Membrane Proteins: Location and Function

Proteins by Location

• Integral Proteins: Span the bilayer or are embedded within it.

• Peripheral Proteins: Attached to the surface of the membrane, often associated with integral proteins.

Protein Function

• Transport Proteins: Move substances across the membrane.

  • Carriers: Bind and move molecules across the membrane.

  • Channels: Allow movement of ions or small polar molecules.

  • Regulated: Open/close in response to signals.

  • Non-regulated: Always open.

• Cell Recognition Proteins: Glycoproteins that function as cellular fingerprints.

• Enzyme Proteins: Catalyze chemical reactions.

• Receptor Proteins: Bind signaling molecules and initiate cellular responses.

• Attachment Proteins: Anchor the cell to the cytoskeleton or extracellular matrix.

Membrane Permeability

Membranes are selectively permeable, allowing some substances to cross more easily than others.

• Permeable: Allows substance to move across.

• Impermeable: Does not allow substance to cross.

• Semi-permeable: Allows selective movement of substances.

Physical or Passive Methods for Crossing the Membrane

• Diffusion: Random movement of particles from high to low concentration. $J = -D \frac{dC}{dx}$

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Dialysis: Diffusion of solute across a selectively permeable membrane.

Isotonic, Hypertonic, and Hypotonic Solutions

These terms describe the relative concentration of solutes in solutions compared to the cell.

• Isotonic Solution: Equal solute concentration inside and outside the cell.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out.

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in.

Turgor

Turgor pressure is the internal pressure in plant cells due to water intake, which helps maintain cell rigidity and support.

• Results from water diffusing into the cell, causing the vacuole to press against the cell wall.

• Responsible for support and firmness in plant tissues.

Physiological (Active) vs. Passive Transport

• Passive Transport: Does not require energy; substances move down their concentration gradient.

• Active Transport: Requires energy (usually ATP); substances move against their concentration gradient.

Carrier-Mediated Transport

• Facilitated Diffusion: Passive transport via protein carriers; no energy required.

• Active Transport: Protein carriers move substances against their gradient; energy required.

Exocytosis and Endocytosis

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: Cell takes in substances by engulfing them in vesicles.

  • Phagocytosis: Engulfing large particles ('cell eating').

  • Pinocytosis: Engulfing fluids ('cell drinking').

  • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors.

Co-transport

Co-transport involves the simultaneous transport of two substances across the membrane, often using the energy from one to drive the movement of the other.

Ground Rules of Metabolism

Metabolism and Metabolic Pathways

Metabolism is the sum of all chemical reactions in a cell. Metabolic pathways are series of reactions leading from reactants to products.

• Anabolism: Synthesis of complex molecules from simpler ones.

• Catabolism: Breakdown of complex molecules into simpler ones.

Types of Energy: Potential and Kinetic

• Energy: Capacity to do work or cause change.

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

• Kinetic Energy: Energy of motion.

Thermodynamics in Biology

• First Law of Thermodynamics: Energy cannot be created or destroyed; only transformed.

• Second Law of Thermodynamics: Every energy transfer increases the disorder (entropy) of the universe.

Free Energy, Entropy, and Enthalpy

• Entropy (S): Measure of disorder.

• Free Energy (G): Energy available to do work.

• Enthalpy (H): Total heat content of a system.

Relationship: $\Delta G = \Delta H - T \Delta S$

Chemical Reactions

• Reactants: Substances entering a chemical reaction.

• Products: Substances produced by a chemical reaction.

• Activation Energy: Energy required to start a reaction.

• Exergonic Reaction: Releases energy; products have less energy than reactants.

• Endergonic Reaction: Absorbs energy; products have more energy than reactants.

• Reversible Reaction: Can proceed in both directions.

• Chemical Equilibrium: Rate of forward reaction equals rate of reverse reaction.

Factors Influencing Chemical Reactions

• Temperature

• Pressure

• Concentration

• Catalysts/Enzymes

Coupled Reactions

Endergonic and exergonic reactions are often coupled in biological systems to drive necessary processes.

Enzymes and Catalysts

• Catalyst: Substance that speeds up a chemical reaction without being consumed.

• Enzyme: Biological catalyst, usually a protein, highly specific for its substrate.

• Induced Fit Hypothesis: Enzyme changes shape to fit substrate, facilitating reaction.

Enzyme Properties and Factors Affecting Activity

• Enzymes are specific to their substrates.

• Factors affecting activity: temperature, pH, substrate concentration, inhibitors.

Type of Transport	Energy Required?	Direction Relative to Gradient	Example
Passive Transport	No	Down gradient	Diffusion, Osmosis
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Facilitated Diffusion	No	Down gradient	Glucose transporters
Co-transport	Indirect (uses gradient)	Varies	Sodium-glucose co-transporter

Example: The sodium-potassium pump uses ATP to move Na+ out of the cell and K+ into the cell, maintaining cellular electrochemical gradients.

Additional info: These notes expand on the original content by providing definitions, examples, and equations for key biological concepts relevant to membrane structure, function, and metabolism.