Chapter 5: The Working Cell - Membrane Transport and Cellular Energetics

Fluid Mosaic Model

A model describing the plasma membrane's structure as diverse protein molecules suspended in and drifting through a fluid phospholipid bilayer.

Cholesterol (in cell membranes)

A lipid embedded within animal cell membranes that helps stabilize the membrane at warmer temperatures and prevents tight packing to maintain fluidity at lower temperatures.

Attachment Proteins

Membrane proteins that bind to the extracellular matrix (ECM) and cytoskeleton to help support membrane architecture and coordinate internal and external cellular changes.

Receptor Proteins

Membrane proteins that bind specific extracellular signaling molecules and transmit information into the cell by activating internal relay molecules.

Junction Proteins

Membrane proteins that form intercellular connections to attach adjacent cells to one another.

Glycoproteins

Proteins embedded in the plasma membrane with attached carbohydrate chains that serve as cellular identification tags recognized by other cells.

Selective Permeability

A property of biological membranes that allows some substances to cross more easily than others while excluding or restricting others.

Diffusion

The tendency of solute particles to spread out evenly into an available space, moving down their concentration gradient from an area of higher concentration to lower concentration until dynamic equilibrium is reached.

Passive Transport

The movement of substances across a biological membrane down their concentration gradient without the expenditure of cellular energy ($\text{ATP}$).

Osmosis

The diffusion of water across a selectively permeable membrane down its concentration gradient toward a region with a higher solute concentration.

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water, depending on the relative concentration of nonpenetrating solutes on either side of the membrane.

Isotonic Solution

A solution whose solute concentration is equal to that inside the cell, resulting in no net movement of water and maintaining cell volume.

Hypotonic Solution

A solution with a lower solute concentration than the inside of a cell, causing water to enter the cell and potentially leading to bursting (lysis) in animal cells or turgidity in plant cells.

Hypertonic Solution

A solution with a higher solute concentration than the inside of a cell, causing water to flow outward and leading to cell shriveling (crenation in animal cells, plasmolysis in plant cells).

Turgor Pressure

The back-pressure exerted by a plant cell wall that prevents excessive uptake of water and bursting when the plant cell is placed in a hypotonic environment.

Plasmolysis

The process occurring in plant cells in a hypertonic environment where the cell loses water, causing the plasma membrane to pull away from the cell wall and shrivel.

Facilitated Diffusion

The passive transport of polar or charged substances across a membrane down their concentration gradient with the assistance of specific transport proteins.

Channel Protein

A transport protein that provides a hydrophilic passageway across the membrane through which specific ions or small molecules can diffuse rapidly.

Carrier Protein

A transport protein that binds to a specific solute, changes its three-dimensional conformation, and releases the solute on the opposite side of the membrane.

Active Transport

The process by which a cell moves a solute across a membrane against its concentration gradient (from low to high concentration), requiring cellular energy in the form of $\text{ATP}$.

Exocytosis

A transport vesicle-mediated mechanism used by cells to export large or bulky molecules, such as proteins and polysaccharides, out into the extracellular space.

Endocytosis

A cellular process in which the plasma membrane folds inward to form vesicles, importing macromolecules or extracellular fluids into the cell.

Phagocytosis

A type of endocytosis known as 'cellular eating,' where a cell engulfs solid particles or food by extending pseudopodia around them to form a vacuole.

Pinocytosis

A form of endocytosis known as 'cellular drinking,' in which the cell non-specifically gulps droplets of extracellular fluid and dissolved solutes into tiny vesicles.

Receptor-Mediated Endocytosis

A highly selective endocytic process that uses specific membrane receptor proteins to bind target molecules before forming an inward vesicle.

Kinetic Energy

The form of energy associated with the relative motion of matter.

Potential Energy

Energy stored within matter as a consequence of its spatial location or chemical arrangement, including the chemical energy held in molecular bonds.

First Law of Thermodynamics

The principle of conservation of energy stating that energy in the universe is constant; it can be transferred and transformed, but cannot be created or destroyed.

Second Law of Thermodynamics

The principle stating that every energy conversion increases the disorder (entropy) of the universe, with a portion of the energy converted into unusable heat.

Exergonic Reaction

A spontaneous chemical reaction that releases energy because the covalent bonds of the reactants store more potential energy than those of the products.

Endergonic Reaction

A non-spontaneous chemical reaction that requires an input of net energy, yielding products that possess more potential energy than the starting reactants.

Metabolism

The totality of all chemical reactions carried out within a living organism, including both anabolic and catabolic pathways.

Activation Energy ($E_A$)

The amount of initial energy required to contort reactant molecules so their chemical bonds can break, initiating a chemical reaction.

Enzyme

A macromolecule (usually a protein) that acts as a biological catalyst, speeding up a reaction rate by lowering the activation energy barrier without being consumed.

Substrate

The specific reactant molecule that an enzyme binds and acts upon during a chemical reaction.

Active Site

The specific pocket or groove on an enzyme's surface where the substrate binds and catalysis takes place.

Induced Fit

The slight conformational