Glossary
ATP
Adenosine triphosphate, the primary energy currency of the cell, used to power most cellular processes, including active transport.
Example:
Muscle cells rapidly break down ATP to provide the energy needed for muscle contraction.
Active Transport
Movement of molecules across a cell membrane against their concentration gradient, from an area of low concentration to an area of high concentration, requiring energy input, typically from ATP.
Example:
Plants use active transport to absorb mineral ions from the soil into their roots, even when the concentration of ions is higher inside the root cells.
Aquaporins
Specific channel proteins that facilitate the rapid movement of water molecules across cell membranes, significantly increasing the rate of osmosis.
Example:
Kidney cells have many aquaporins to efficiently reabsorb water from urine back into the bloodstream, maintaining the body's fluid balance.
Carrier Proteins
A type of membrane protein that binds to specific molecules, undergoes a conformational change, and then releases the molecule on the other side of the membrane.
Example:
The transport of amino acids into a cell often involves carrier proteins that bind to the amino acid and then change shape to move it across the membrane.
Channel Proteins
A type of membrane protein that forms a hydrophilic pore or channel through the lipid bilayer, allowing specific ions or small molecules to pass through rapidly.
Example:
Nerve impulses rely on the rapid opening and closing of channel proteins to allow sodium and potassium ions to flow across the neuron's membrane.
Concentration Gradient
The difference in the concentration of a substance across a space or membrane, driving the movement of molecules from an area of higher concentration to an area of lower concentration.
Example:
When you spray air freshener, the scent molecules spread out from a high concentration gradient in the bottle to a lower concentration throughout the room.
Facilitated Diffusion
A type of passive transport where molecules move across the cell membrane down their concentration gradient with the help of specific membrane proteins.
Example:
Glucose enters red blood cells via facilitated diffusion, using specific carrier proteins because it's too large and polar to pass directly through the lipid bilayer.
Gated Ion Channels
Channel proteins that open or close in response to specific stimuli, such as changes in voltage, binding of a ligand, or mechanical force, regulating ion flow across the membrane.
Example:
In muscle contraction, the release of neurotransmitters triggers the opening of gated ion channels on the muscle cell, leading to depolarization.
Membrane Proteins
Proteins embedded within or associated with the cell membrane, performing various functions such as transport, enzymatic activity, signal transduction, and cell-cell recognition.
Example:
Many hormones bind to specific membrane proteins on the cell surface to trigger a response inside the cell.
Passive Transport
Movement of molecules across a cell membrane down their concentration gradient, from an area of high concentration to an area of low concentration, without the cell expending energy.
Example:
Oxygen moving from the high concentration in the lungs into the lower concentration in the bloodstream is an example of passive transport.
Phospholipid Bilayer
The double layer of phospholipids that forms the basic structure of a cell membrane, with hydrophilic heads facing outward and hydrophobic tails facing inward.
Example:
The cell membrane's phospholipid bilayer acts as a selective barrier, allowing small, nonpolar molecules to pass through easily while blocking larger or charged ones.
Primary Active Transport
A type of active transport that directly uses ATP hydrolysis to power the movement of specific molecules against their concentration gradient.
Example:
The proton pump in the stomach lining uses primary active transport to pump H+ ions into the stomach lumen, creating a highly acidic environment for digestion.
Secondary Active Transport
A type of active transport that uses the energy stored in an existing electrochemical gradient (often created by primary active transport) to indirectly move another substance against its concentration gradient.
Example:
In the small intestine, glucose is absorbed into cells via secondary active transport, piggybacking on the sodium gradient established by the sodium-potassium pump.
Sodium-Potassium Pump
A crucial primary active transport protein that pumps three sodium ions out of the cell and two potassium ions into the cell for every ATP molecule consumed, maintaining electrochemical gradients.
Example:
The continuous action of the sodium-potassium pump is vital for nerve impulse transmission and maintaining cell volume.