Glossary
Active Transport
The movement of substances across a cell membrane against their concentration gradient, requiring an input of cellular energy (ATP).
Example:
Moving sodium ions out of a cell, where they are already less concentrated, is a classic example of active transport.
Cell Membrane
A flexible barrier that surrounds the cell, controlling the movement of substances into and out of the cell.
Example:
The cell membrane acts like a bouncer at a club, deciding who gets in and who stays out.
Endocytosis
A process by which cells engulf substances from their external environment by forming a vesicle from the cell membrane.
Example:
When an amoeba 'eats' a bacterium by surrounding it with its membrane, it's performing endocytosis.
Exocytosis
A process by which cells release substances to their external environment by fusing a vesicle with the cell membrane.
Example:
Nerve cells releasing neurotransmitters into the synapse to communicate with other cells is a prime example of exocytosis.
Hydrophilic heads
The water-loving phosphate portions of phospholipids that face the aqueous environments inside and outside the cell.
Example:
The hydrophilic heads of the phospholipids are like tiny magnets, attracted to the water molecules surrounding the cell.
Hydrophobic tails
The water-fearing fatty acid portions of phospholipids that face inward, away from water, forming the core of the membrane.
Example:
The hydrophobic tails huddle together in the center of the membrane, avoiding the watery cellular environment like oil avoids water.
Integral proteins
Proteins that are permanently embedded within the phospholipid bilayer, often spanning the entire membrane.
Example:
A channel protein that allows ions to cross the membrane is an integral protein, deeply integrated into the cell's boundary.
Passive Transport
The movement of substances across a cell membrane down their concentration gradient, without the expenditure of cellular energy.
Example:
Oxygen diffusing from the lungs into the bloodstream is a perfect illustration of passive transport, as it moves from high to low concentration.
Peripheral proteins
Proteins that are temporarily associated with the cell membrane, typically attached to the surface rather than embedded within it.
Example:
An enzyme that temporarily binds to the inner surface of the membrane to catalyze a reaction is an example of a peripheral protein.
Phospholipid Bilayer
The fundamental structure of the cell membrane, composed of two layers of phospholipid molecules arranged tail-to-tail.
Example:
Imagine a double-layered fence around a cell; that's essentially the phospholipid bilayer forming the boundary.
Primary Active Transport
A type of active transport that directly uses ATP hydrolysis to move substances against their concentration gradient.
Example:
The sodium-potassium pump, which directly breaks down ATP to move ions, is a key example of primary active transport.
Proteins (membrane)
Macromolecules embedded within or associated with the cell membrane, performing various functions like transport, signaling, and adhesion.
Example:
Just as a building has doors and windows, the cell membrane uses various proteins to allow specific molecules to pass through or to receive signals.
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 move a different substance.
Example:
Glucose entering intestinal cells alongside sodium ions, which are moving down their gradient, demonstrates secondary active transport.
Sodium-potassium pump
A crucial integral protein that uses ATP to actively transport three sodium ions out of the cell and two potassium ions into the cell.
Example:
The sodium-potassium pump is vital for maintaining the resting potential of neurons, allowing nerve impulses to fire.