BIO103 - B05 VOCAB - membranes

homeostasis

The steady-state physiological condition of the body.

plasma membrane

The membrane at the boundary of every cell that acts as a selective barrier, regulating the cell's composition. This structure separates the interior of the cell from the outside environment (the extracellular space) which protects the cell from its environment. It consists of a phospholipid bilayer, cholesterols (maintain their fluidity at various temperatures), and membrane proteins (integral proteins and peripheral proteins). This structure is also found on the outside of organelles controlling movement of substances in and out of different parts within cells.

phospholipid

A lipid made up of a glycerol jointed to two fatty acids and a phosphate group. The hydrocarbon chains of the fatty acids act as nonpolar, hydrophobic tails, while the rest of the molecule acts as a polar, hydrophilic head. These molecules form bilayers that function as biological membranes.

hydrophilic

Having an affinity for water.

hydrophobic

Having no affinity for water; tending to coalesce and form droplets in water.

fluid mosaic model

The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.

saturated fatty acid

This fatty acid has all carbons in the hydrocarbon tail connected to one another by single covalent bonds, thus maximizing the number of hydrogen atoms that are attached to the carbon skeleton.

unsaturated fatty acid

This fatty acid has one or more double covalent bonds between carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton.

cholesterol

A steroid that forms an essential component of animal cell membranes; also acts as a precursor molecule for the synthesis of other biologically important steroids, such as many hormones.

integral protein

A transmembrane protein with hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on one or both side of the membrane (or lining the channel in the case of a channel protein).

peripheral protein

A protein loosely bound to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer.

selective permeability

A property of biological membranes that allows them to regulate the passage of substances across them.

transport protein

A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.

aquaporin

A channel protein in a cellular membrane that specifically facilitates osmosis, the diffusion of free water across the membrane.

diffusion

The random thermal motion of particles of liquids, gases, or solids. In the presence of a concentration

or electrochemical gradient, this process results in the net movement of a substance from a region where it is more concentrated to a region where it is less concentrated.

concentration gradient

A region along which the density of a chemical substance increases or decreases.

electrochemical gradient

The diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force).

membrane potential

The difference in electrical charge (voltage... think of a battery) across a cell's plasma membrane due to the differential distribution of ions. This property of membranes affects the activity of excitable cells (i.e. nerve cells or neurons) and the transmembrane

movement of all charged substances since there exists an electrochemical gradient.

passive transport

The diffusion of a substance across a biological membrane with no expenditure of energy.

osmosis

The diffusion of free water across a selectively permeable membrane.

solution

A liquid that is a homogeneous mixture of two or more substances.

solute

A substance that is dissolved in a solution.

solvent

The dissolving agent of a solution. Water is the most versatile _________ known.

water potential

The physical property predicting the direction in which water will flow, governed by solute

concentration and applied pressure.

tonicity

The ability of a solution surrounding a cell to cause that cell to gain or lose water.

isotonic

Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell.

hypertonic

Referring to a solution that, when surrounding a cell, will cause the cell to lose water.

hypotonic

Referring to a solution that, when surrounding a cell, will cause the cell to take up water.

ion channel

A transmembrane protein channel that allows a specific ion to diffuse across a membrane down its concentration or electrochemical gradient.

facilitate diffusion

The passage of molecules or ions down their concentration or electrochemical gradient across a biological membrane with the assistance of specific transmembrane

proteins, requiring no energy expenditure.

sodium-potassium pump

This is a transport protein in the plasma membrane of animal cells that actively transports sodium ions out

of the cell and potassium ions into the cell. It does so in a ratio of three (3) sodium ions out for every two (2) potassium ions brought in. In the process, the pump helps to stabilize the membrane potential, and thus is essential in creating the conditions necessary for the firing of neurons (= nerve cells) via action potentials.

osmoregulation

Regulation of solute concentrations and water balance by a cell or organism.

active transport

The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins

and requiring an expenditure of energy.

bulk transport

This is a general phrase for the active transport of large solid or liquid molecules across the cell's

plasma membrane. Again, these are active processes and thus require ATP. It includes endocytosis, where substances enter the cell by forming an invagination in the plasma membrane and eventually a vesicle

surrounds the incoming substance. It also includes

exocytosis, basically the opposite of endocytosis, in which the vesicle containing the substance fuses with the plasma membrane to release the substance outside the cell. Substances requiring this type of transport include hormones, polysaccharides, etc.

phagocytosis

A type of endocytosis in which large particulate substances or small organisms (i.e. bacterium) are taken up by a cell. It is carried out by some protists (type of unicellular eukaryote) and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells). This literally means "cell eating."

pinocytosis

A type of endocytosis in which the cell ingests

extracellular fluid and its dissolved solutes. This literally means "cell drinking" and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this process takes in dissolved molecules, including the solvent (= water), which the cell needs from the extracellular fluid. This form of endocytosis

results in a much smaller vesicle than does

phagocytosis, and the vesicle does not need to merge with a lysosome for subsequent digestion.

receptor-mediated endocytosis

The movement of specific molecules into a cell by the infolding of vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of a specific substance. This targeted type of bulk transport employs receptor proteins on the plasma membrane that have a specific binding affinity for certain substances. Typically used for bulk transport of specific macromolecules.

exocytosis

The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane. This process is the opposite of

endocytosis since its purpose is to expel material (i.e. waste products) from the cell into the extracellular fluid.

endocytosis

Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane. This process is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. Phagocytosis (the condition of "cell eating"),

pinocytosis (the condition of "cell drinking"), and receptor-mediated _________ are types of this general process of bulk cellular uptake.