Lipids, Membranes, and the First Cells

Comprehensive vocabulary flashcards covering the structure and function of lipids, the properties of membrane bilayers, and the mechanisms of substance transport including passive and active transport.

Plasma membrane

A selective barrier that separates life from nonlife, allowing entry of needed materials and keeping out damaging ones while facilitating necessary chemical reactions.

Lipids

Carbon-containing compounds that are insoluble in water due to a high proportion of nonpolar bonds.

Hydrocarbons

Nonpolar, hydrophobic molecules composed only of carbon and hydrogen, characterized by equal electron sharing.

Fatty acid

A hydrocarbon chain bonded to a carboxyl functional group, typically containing $14 \text{--} 20$ carbon atoms.

Saturated hydrocarbon

Hydrocarbon chains consisting only of single bonds between carbons, containing the maximum number of hydrogen atoms.

Unsaturated hydrocarbon

Hydrocarbon chains with one or more double bonds, which creates a "kink" in the chain.

Steroids

A family of lipids distinguished by a bulky, four-ring structure, including cholesterol and hormones like estrogen or testosterone.

Fats (Triacylglycerols)

Lipids composed of three fatty acids linked to a glycerol, primarily serving the role of energy storage.

Ester linkage

A covalent bond formed by condensation reactions between the hydroxyl group of glycerol and the carboxyl group of a free fatty acid.

Phospholipids

Molecules consisting of glycerol linked to a phosphate group and two hydrocarbon chains; their primary role is to form cell membranes.

Amphipathic

Substances that contain both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

Micelles

Tiny spherical aggregates formed from free fatty acids when placed in water.

Lipid bilayers

Structures created when lipid molecules align in paired sheets, forming spontaneously in water without an input of energy.

Liposomes

Artificial membrane-bound vesicles formed from phospholipids in a laboratory setting.

Selective permeability

The property of a membrane that allows some substances to cross more easily than others; high permeability for small nonpolar molecules like $O_2$ and low for small ions like $Cl^-$ or $Na^+$.

Diffusion

The spontaneous net movement of molecules and ions from regions of high concentration to regions of low concentration.

Passive transport

The movement of substances across a membrane through diffusion in the absence of an outside energy source.

Osmosis

A special case of diffusion involving the movement of water across selectively permeable membranes from regions of low solute concentration to high solute concentration.

Hypertonic solution

An outside solution with a higher concentration of solutes than the inside of the cell.

Hypotonic solution

An outside solution with a lower concentration of solutes than the inside of the cell.

Isotonic

A condition where the solute concentrations on the outside and inside of the cell are equal.

Protocells

Simple vesicle-like structures harboring nucleic acids, considered possible intermediates in the evolution of the cell.

Integral membrane proteins

Also known as transmembrane proteins, these span the membrane and have segments facing both the interior and exterior surfaces.

Peripheral membrane proteins

Proteins that bind to membrane lipids or other proteins without passing through the lipid bilayer.

Electrochemical gradient

Established when ions build up on one side of a membrane, creating both a concentration gradient and a charge gradient.

Aquaporins

Selective channel proteins that permit water to cross the plasma membrane.

Gated channels

Membrane proteins that open or close in response to specific signals, such as the binding of a molecule or a change in electrical voltage.

Facilitated diffusion

The process where transmembrane proteins (channels or carriers) assist the passive transport of substances across the lipid bilayer.

Carrier proteins

Specialized membrane proteins that selectively pick up a solute on one side of the membrane and drop it on the other side by changing shape.

GLUT-1

A specific carrier protein that increases membrane permeability to glucose.

Active transport

The movement of substances against their concentration gradient, requiring an input of energy, often from $ATP$.

Na+/K+-ATPase

A sodium–potassium pump that uses $ATP$ to move $Na^+$ and $K^+$ ions against their concentration gradients.

Secondary active transport

Also called co-transport; a process where electrochemical gradients established by pumps provide the potential energy to move another molecule against its gradient.