Cell Membrane Structure & Passive Transport

Vocabulary practice flashcards covering cell membrane composition, the fluid mosaic model, membrane proteins, transport mechanisms, diffusion factors, and tonicity.

Plasma Membrane

The gatekeeper of the cell's interior that separates the cell from its surrounding environment and regulates the passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell.

Phospholipid Bilayer

A two-layered membrane structure consisting of hydrophilic outer head groups and a hydrophobic core formed by fatty acid tails, separating intracellular fluid from extracellular fluid.

Fluid Mosaic Model

A model describing the structure and function of the plasma membrane as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates that interact to give the membrane its fluid character.

Phospholipid

The primary structural lipid forming the main membrane fabric, composed of a glycerol molecule linked to two fatty acids and a phosphate group.

Cholesterol

A type of lipid composed of four fused carbon rings located between phospholipids and between the two phospholipid layers (with other sterols serving this role in plant cells).

Integral Proteins

Proteins embedded within the phospholipid layer(s) that integrate completely into the membrane structure and have hydrophobic and hydrophilic regions to span the membrane (e.g., integrins).

Peripheral Proteins

Proteins located on the inner or outer surfaces of the phospholipid bilayer that are not embedded in the phospholipids; they act as enzymes, cytoskeletal filament anchors, or recognition sites.

Glycoprotein

A membrane component consisting of a carbohydrate covalently attached to a protein, located exclusively on the exterior surface of the plasma membrane to aid in cell recognition.

Glycolipid

A lipid molecule with an attached carbohydrate found on the exterior surface of the plasma membrane that functions in cell identification and recognition.

Glycocalyx

The collective term for the carbohydrate coating of glycoproteins and glycolipids on a cell's exterior surface; it is highly hydrophilic, attracts water, and enables the immune system to identify foreign cells.

Selectively Permeable

A property of the plasma membrane that allows certain substances to pass through freely while restricting the movement of others.

Passive Transport

The naturally occurring movement of substances across the plasma membrane from areas of high concentration to areas of lower concentration without the expenditure of cellular energy.

Concentration Gradient

The difference in the concentration of a chemical substance between two distinct regions.

Diffusion

The unassisted passive movement of substances from an area of higher concentration to an area of lower concentration until concentration is equal throughout.

Dynamic Equilibrium

The state reached when a substance is evenly distributed across a space, eliminating the concentration gradient so that molecules continue to move without any net directional movement.

Extent of the Concentration Gradient

A factor affecting diffusion rate where a larger concentration difference produces faster diffusion, which subsequently slows down as dynamic equilibrium is approached.

Mass of Diffusing Molecules

A factor affecting diffusion rate where heavier molecules move more slowly and lighter molecules move faster.

Solvent Density

A factor affecting diffusion rate where an increase in solvent density decreases the rate of diffusion by hindering molecular movement.

Solubility (Membrane Transport)

A factor determining diffusion across the membrane where nonpolar or lipid-soluble materials cross the bilayer much more easily than polar materials.

Facilitated Transport

The passive movement of polar substances across the plasma membrane down their concentration gradient using the assistance of integral membrane proteins (channels or carriers).

Channel Protein

An integral membrane protein that features hydrophilic outer regions and an open hydrophilic core that allows specific polar compounds to cross the hydrophobic membrane interior.

Carrier Protein

An integral membrane transport protein that specifically binds a target substance and undergoes a conformational shape change to shuttle the molecule across the membrane.

Osmosis

The specific diffusion of water across a semipermeable membrane from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration).

Tonicity

The measure of the relative solute concentrations across the plasma membrane that determines the direction and extent of water movement.

Hypotonic

A solution that has a lower solute concentration than the cytoplasm inside the cell, driving water to enter the cell and causing animal cells to lyse and plant cells to become turgid.

Isotonic

A solution that contains an equal concentration of solutes and water relative to the cell's interior, resulting in no net movement of water (normal for animal cells, flaccid for plant cells).

Hypertonic

A solution that has a higher solute concentration than the cytoplasm inside the cell, causing water to leave the cell, leading to shriveling in animal cells and plasmolysis in plant cells.

Turgid

The normal, firm condition of a plant cell when placed in a hypotonic environment due to osmotic water intake pushing the plasma membrane against the rigid cell wall.

Plasmolyzed

The condition of a plant cell in a hypertonic solution where water loss causes the plasma membrane to constrict and pull away from the cell wall.