Cell Membranes Lecture Flashcards

Flashcards covering the key concepts of cell membranes, including lipid-protein bilayers, membrane fluidity, transport mechanisms (passive and active), and vesicle-mediated transport.

Fluid mosaic model

The general structure of biological membranes where phospholipids form a bilayer which acts like a "lake" in which a variety of proteins "float."

Phospholipid bilayer

A membrane structure composed of phospholipids with polar, hydrophilic "heads" facing outward and hydrophobic fatty acid "tails" facing inward.

Hydrophilic

The property of phospholipid heads that are polar and face outward toward the aqueous environment.

Hydrophobic

The property of fatty acid tails that face inward in the membrane, away from water.

Membrane fluidity factors

Determined by lipid composition (cholesterol and fatty acid saturation) and temperature.

Saturated fatty acids

Fatty acids that pack tightly, resulting in a less-fluid membrane; cells require more of these at higher temperatures to maintain constant fluidity.

Unsaturated fatty acids

Long-chain fatty acids with "kinks" that pack less densely, making the membrane more fluid.

Integral membrane proteins

Proteins that have hydrophobic R groups which interact with the hydrophobic core of the membrane and hydrophilic R groups that interact with aqueous environments.

Peripheral membrane proteins

Proteins that lack hydrophobic regions and do not penetrate the bilayer; they are located on only one side of the membrane.

Anchored membrane proteins

Proteins that are covalently attached to fatty acids or other lipids.

Transmembrane proteins

Proteins that extend all the way through the phospholipid bilayer, with one or more transmembrane domains.

Glycolipids

A combination of a carbohydrate and a lipid found on the outer surface of the membrane.

Glycoproteins

A combination of a carbohydrate (oligosaccharide) and a protein; they serve as recognition sites for other cells.

Proteoglycans

A type of protein-carbohydrate molecule with a higher percentage of carbohydrates than glycoproteins.

Tight junctions

Specialized structures that form a "quilted" seal between cell membranes, barring the movement of dissolved materials and ensuring directional movement.

Desmosomes

Specialized structures that act like "spot welds" to link adjacent cells tightly while permitting materials to move around them in the intercellular space.

Gap junctions

Specialized structures containing connexins (channel proteins) that allow communication between adjacent cells by letting molecules pass through hydrophilic channels.

Integrin

A transmembrane protein that binds to the extracellular matrix outside epithelial cells and to actin filaments inside the cells through noncovalent and reversible binding.

Passive transport

The movement of substances across a membrane from regions of greater concentration to lesser concentration without the requirement of energy input.

Active transport

The movement of substances against a concentration and/or electrical gradient, requiring energy usually in the form of ATP.

Selective permeability

The property of membranes that allows some substances to pass through while preventing others.

Diffusion

The process of random movement toward equilibrium, where the net movement is from regions of greater concentration to lesser concentration.

Osmosis

The diffusion of water depends on the relative concentrations of water molecules across a membrane.

Isotonic

A solution with an equal solute concentration compared to another solution.

Hypertonic

A solution with a higher solute concentration compared to another solution.

Hypotonic

A solution with a lower solute concentration compared to another solution.

Plasmolysis

The process in a plant cell watered with salt water where the cell shrinks and the cell membrane pulls away from the cell wall.

Facilitated diffusion

Passive transport of polar and charged molecules across a membrane facilitated by integral membrane protein channels or carrier proteins.

Channel proteins

Integral membrane proteins that form a tunnel for substances to cross the membrane.

Carrier proteins

Membrane proteins that bind specific substances and speed their diffusion through the bilayer.

Uniporter

A transport protein that moves one substance in one direction.

Symporter

A transport protein that moves two different substances in the same direction.

Antiporter

A transport protein that moves two different substances in opposite directions.

Primary active transport

Active transport that requires the direct hydrolysis of ATP to move substances.

Secondary active transport

Active transport that uses energy from an ion concentration gradient established by primary active transport.

Sodium–potassium ($Na^+$–$K^+$) pump

An integral membrane glycoprotein and antiporter that export $3\,Na^+$ ions and brings $2\,K^+$ ions into the cell using ATP.

Endocytosis

The process by which the cell membrane folds inward (invaginates) to bring molecules and cells into a eukaryotic cell through a vesicle.

Phagocytosis

A form of endocytosis where large molecules or entire cells are engulfed to form a food vacuole or phagosome.

Pinocytosis

A form of endocytosis involving the formation of small vesicles to bring fluids or small dissolved substances into the cell.

Receptor mediated endocytosis

A highly specific process where macromolecules bind to receptor proteins at sites on the cell membrane coated with proteins such as clathrin.

Exocytosis

The process where materials packaged in vesicles, such as digestive enzymes or neurotransmitters, are secreted from a cell as the vesicle membrane fuses with the cell membrane.