The_Working_Cell__5_

THE WORKING CELL - Chapter 5

MEMBRANE STRUCTURE

• Phospholipid Bilayer:

  • Basis of metabolic pathways in cell membranes.

  • Embedded components include cholesterol, proteins, glycoproteins, and glycolipids (carbohydrates).

  • Components generally move fluidly within the membrane.

  • Exhibits selective permeability allowing certain substances to pass.

• Fluid Mosaic Model:

  • Describes the membrane as a two-dimensional fluid with mixed composition.

  • Lipid bilayer exhibits lateral movement; not chemically bonded to one another.

  • Proteins within the lipid bilayer can move laterally.

MEMBRANE FUNCTION

• Types of Molecules:

  • Small nonpolar molecules (e.g., O2, CO2) can diffuse across the membrane.

• Key Functions:

  • Enzymes: Facilitate metabolic activities.

  • Intercellular Junctions: Connect adjacent cells.

  • Glycoproteins: Allow for recognition of neighboring cells.

  • Transport Proteins: Move ions and molecules into/out of the cell.

  • Receptor Proteins: Bind to signaling molecules and relay messages.

  • Attachment Proteins: Connect the ECM (extracellular matrix) and cytoskeleton, providing membrane support.

DIFFUSION AND MEMBRANES

• Basic Principle:

  • Spontaneous spreading of molecules/ions is essential for movement into and out of cells.

  • Molecules/ions experience jiggling and collisions that facilitate movement along concentration gradients.

  • Net Diffusion occurs when substances move from areas of high concentration to low concentration until equilibrium is reached.

• Passive Transport:

  • Movement that does not require energy; e.g., O2 and CO2 diffuse passively across membranes.

OSMOSIS

• Definition:

  • Diffusion of water across a selectively permeable membrane.

  • Water moves toward areas of higher solute concentration, impacting cell volume and shape.

TONICITY

• Concept:

  • Determines how surrounding solutions affect cell water balance.

  • Hypotonic: Lower solute concentration than the cell; may cause cell swelling.

  • Isotonic: Equal solute concentration; no change in cell volume.

  • Hypertonic: Higher solute concentration than the cell; may cause cell shrinkage.

• Osmoregulation: The process of regulating water balance in cells.

TURGOR PRESSURE

• Created by osmosis until the concentrations are equal.

• Defined as the pressure exerted by fluid against the structure that contains it.

• Prevents cells from bursting and helps maintain structure.

MEMBRANE TRANSPORT

• Facilitated Diffusion:

  • Passive; involves transport proteins that aid movement.

  • Aquaporins: Channels that speed up water diffusion.

  • Carrier Proteins: Bind and transport substances across the membrane.

ACTIVE TRANSPORT

• Definition:

  • Requires energy to move solutes against concentration gradients.

• Mechanisms:

  • Calcium Pump and Co-transporters (e.g., sodium-potassium pumps) actively transport ions.

• Involves four steps:

  1. Solute bindings.

  2. ATP provides energy.

  3. The transport protein reverts to its original shape.

  4. Prepares for the next solute.

TRANSPORT OF LARGE MOLECULES

• Exocytosis:

  • Exports large materials by vesicle fusing with the plasma membrane and expelling contents.

• Endocytosis:

  • Involves the uptake of substances in bulk through invagination of the membrane.

    • Receptor-Mediated Endocytosis: Specific uptake of molecules.

    • Phagocytosis: Cell “eating” to engulf large particles.

ENERGY

• The capacity to do work exists in various forms: light, heat, electricity, potential, motion.

• Energy can be transformed from one form to another (e.g., electrical to light).

• Types include:

  • Kinetic Energy: Energy of motion.

  • Thermal Energy: Kinetic energy associated with atomic movement.

  • Potential Energy: Stored energy based on position.

  • Chemical Energy: Potential energy available in chemical bonds.

THERMODYNAMICS

• Concept:

  • Study of energy transformations, retains that total energy remains constant.

• First Law of Thermodynamics: Energy cannot be created or destroyed; it can only change form.

• Entropy: Describes the degree of disorder, which increases with energy transformations, leading energy to spread out.

LAWS OF THERMODYNAMICS

• Discuss energy conversion across biological reactions (e.g., cellular respiration).

POTENTIAL ENERGY

• Stored in the arrangement of particles, especially in chemical bonds.

CHEMICAL BOND ENERGY

• Reactants and Products: Molecules undergo reactions, maintaining atom count before and after.

METABOLISM

• All chemical reactions in organisms to gain and use energy, including:

  • Metabolic Pathway: Series of reactions often stepwise; involves energy coupling.

ATP DRIVES CELLULAR WORK

• ATP Hydrolysis: Breaks bonds to release energy.

• Phosphorylation: Transfers phosphate groups for energy transfer.

ATP CYCLE

• Continuous process to use and replenish ATP; main driving force for cellular reactions.

ACTIVATION ENERGY

• Minimum energy needed to initiate reactions; prevents unwanted reactions from occurring spontaneously.

ENZYMES

• Vital for metabolic processes, lowering reaction activation energy.

• Active Site: Where substrates bind and reactions occur; enzymes can modify their shape (Induced Fit Model).

• Enzymes are not consumed in reactions.

ENZYME HELPERS

• Cofactors: Non-protein molecules aiding enzyme function.

• Coenzymes: Organic cofactors (often vitamins) involved in reactions.

METABOLISM CONTROL

• Cells regulate metabolic pathways based on need; rates depend on reactant/product concentrations.

INHIBITION

• Competitive Regulation: Molecules compete for the active site.

• Non-competitive Regulation: Molecules bind elsewhere, altering enzyme function.

FEEDBACK INHIBITION

• Regulates pathways based on product availability; excess product acts as an inhibitor.