Cell Biology: Plasma Membrane Structure and Transport Mechanisms

Comprehensive vocabulary flashcards covering membrane structure, lipids, proteins, passive and active transport mechanisms, osmosis, tonicity, and vesicular bulk transport.

Plasma Membrane Structure

A lipid bilayer containing phospholipids, cholesterol, glycolipids, and embedded integral and peripheral proteins that separates the cytoplasm from extracellular fluid.

Cholesterol

A membrane lipid referred to as 'the glue' that embeds within the hydrophobic core of the bilayer to stabilize membrane structure and regulate fluidity.

Glycocalyx

A superficial layer of extracellular carbohydrates consisting of glycolipids and integral glycoproteins projecting from the outer surface of the plasma membrane.

Integral Proteins

Membrane proteins that span or are deeply embedded within the lipid bilayer, including channel proteins and carrier proteins.

Peripheral Proteins

Membrane proteins attached loosely to the inner or outer membrane surfaces rather than spanning the lipid bilayer, frequently anchoring to the cytoskeleton microfilaments.

Membrane Permeability Types

The three classifications of membrane permeability: freely permeable (everything crosses), selectively permeable (only specific/smaller items cross), and impermeable (nothing crosses).

Selectively Permeable Membrane

A biological membrane that allows specific substances such as water and lipids to pass through while restricting others such as proteins, ions, and large carbohydrates.

Passive Processes

Membrane transport mechanisms that do not require $ATP$ energy input, moving substances down their concentration gradients via diffusion or carrier-mediated transport.

Active Processes

Membrane transport mechanisms that require $ATP$ energy, including carrier-mediated active transport and vesicular transport.

Diffusion

The net passive movement of solute molecules down a concentration gradient from an area of higher concentration to an area of lower concentration.

Concentration Gradient

The physical difference in solute concentration between two regions (such as $ECF$ and $ICF$), acting as a driving force that moves solutes from high concentration to low concentration.

Chemical Disequilibrium ($Na^+$ and $K^+$)

The unequal ion distribution across the membrane where $Na^+$ is high outside and low inside, while $K^+$ is high inside and low outside.

Resting Membrane Potential

The electrical disequilibrium across the plasma membrane, typically measuring $-70\text{ mV}$ inside the cell, caused by an excess of negative ions inside and more positive ions outside.

Osmosis

The passive diffusion of water molecules across a selectively permeable membrane from an area of low solute concentration (high water) to an area of high solute concentration (low water).

Osmolarity vs. Tonicity

Osmolarity measures total solute concentration (isosmotic, hyperosmotic, hyposmotic), whereas tonicity describes how a surrounding solution affects cell volume (isotonic, hypotonic, hypertonic).

Hypotonic Solution

A solution containing a lower solute concentration than the intracellular fluid, causing net osmotic water influx that leads to cell swelling.

Hypertonic Solution

A solution containing a higher solute concentration than the intracellular fluid, causing net osmotic water efflux that leads to cell shrinkage.

Isotonic Solution

A solution that has an equal solute concentration relative to the cell cytoplasm, resulting in no net movement of water.

Membrane Transporters

Membrane-spanning proteins categorized into channel proteins that create water-filled pores and carrier proteins that never form an open channel between both sides of the membrane.

Gated Channels

Channel proteins that open and close in response to chemical, electrical, or mechanical signals rather than remaining permanently open.

Open Channels (Pores)

Channel proteins that create water-filled pores and are usually open to allow continuous movement of specific ions or water.

Uniport Carriers

Carrier proteins that transport only one kind of substrate across the plasma membrane.

Symport Carriers

Cotransporters that bind and move two or more substrates across the plasma membrane in the same direction.

Antiport Carriers

Cotransporters that move two or more substrates across the membrane in opposite directions.

Primary ($1^\circ$) Active Transport

Carrier-mediated transport that directly hydrolyzes $ATP$ on the transport protein to move substrates against their concentration gradients.

Secondary ($2^\circ$) Active Transport

Active transport that indirectly relies on $ATP$ by using the potential energy stored in the concentration gradient of another ion (such as $Na^+$) to move a substrate against its gradient.

Transcellular Glucose Transport

The polarized epithelial transport process combining apical $Na^+$/glucose symport, basolateral GLUT facilitated diffusion, and basolateral $Na^+/K^+$ ATPase pumping.

Transcytosis

A process requiring energy where caveolae concentrate proteins to form endocytic vesicles that cross the cell via the cytoskeleton and are released on the opposite side through exocytosis without being broken down.

Phagocytosis

A form of endocytosis ('cell eating') wherein cytoplasmic extensions called pseudopodia surround an object, fuse to form a phagosome, and fuse with lysosomes for enzymatic breakdown.

Pinocytosis

A form of endocytic vesicular transport also known as 'cell drinking' that involves non-specific uptake of extracellular fluid and dissolved solutes.

Exocytosis

An active vesicular transport process wherein intracellular vesicles fuse with the plasma membrane to release hormones, proteins, or undigested residues into the extracellular fluid.