The_Working_Cell__5_

THE WORKING CELL - Chapter 5

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.

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.

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.

Definition:
- Diffusion of water across a selectively permeable membrane.
- Water moves toward areas of higher solute concentration, impacting cell volume and shape.

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.

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.

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.

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.

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.

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.

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.

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

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

All chemical reactions in organisms to gain and use energy, including:
- Metabolic Pathway: Series of reactions often stepwise; involves energy coupling.

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

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

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.

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

Competitive Regulation: Molecules compete for the active site.
Non-competitive Regulation: Molecules bind elsewhere, altering enzyme function.

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