AP Biology: Cell Structure, Membrane Transport, and Bioenergetics

Comprehensive vocabulary flashcards covering membrane structure, active and passive transport, water balance, cell organelles, and the endomembrane system.

Tonicity

The ability of an extracellular solution to cause a cell to gain or lose water, depending on the concentration of solute that cannot pass through the cell membrane.

Osmoregulation

The process by which cells regulate their internal solute concentrations and maintain water balance.

Isotonic Solution

A solution in which the concentration of nonpenetrating solutes is equal inside and outside the cell, resulting in no net movement of water because water diffuses into and out of the cell at the same rate.

Hypertonic Solution

A solution where the concentration of nonpenetrating solutes outside the cell is higher than inside, causing cells to lose water to their extracellular surroundings; animal cells shrivel and die, while plant cells undergo plasmolysis.

Hypotonic Solution

A solution where the concentration of nonpenetrating solutes outside the cell is lower than inside, causing water to enter the cell; animal cells swell and undergo cytolysis (burst open), while plant cells become turgid and operate in their optimal working condition.

Plasmolysis

The shrinking of a plant cell's vacuole and cytoplasm in response to the diffusion of water out into a hypertonic solution, causing the plasma membrane to pull away from the cell wall.

Water Potential

A physical property that predicts the direction in which water will flow ($\psi = \psi_p + \psi_s$); water moves from areas of high water potential to low water potential, low solute concentration to high solute concentration, and high pressure to low pressure.

Pressure Potential ($\psi_p$)

In a plant cell, the physical pressure exerted by the rigid cell wall that limits further water uptake.

Solute Potential ($\psi_s$)

The effect of solute concentration on water potential, calculated as $\psi_s = -iCRT$ (where $i$ is the ionization constant, $C$ is the solute concentration, $R = 0.0831\,\text{liter bar/mole K}$, and $T$ is the temperature in $\text{K}$). Pure water at atmospheric pressure has a value of zero, and adding solute makes it more negative.

Exocytosis

The process by which vesicles fuse with the plasma membrane to release materials out into the extracellular fluid, such as nerve cells releasing neurotransmitters.

Endocytosis

The cellular uptake of materials by forming vesicles derived from the inward folding and fusion of the plasma membrane.

Phagocytosis

A form of endocytosis ('cellular eating') in which a cell surrounds and engulfs large particles, packaging them into a food vacuole to later be digested by lysosomes.

Pinocytosis

The nonspecific uptake ('cellular drinking') of extracellular fluid containing dissolved molecules packaged into protein-coated vesicles.

Receptor-Mediated Endocytosis

The specific uptake of molecules by the cell triggered by solutes binding to specific membrane receptors, causing them to cluster into a coated vesicle to import large quantities of target substances.

Active Transport

The transport of a molecule across a membrane that requires cellular energy (such as ATP) because it moves a solute against its concentration gradient.

Adenosine Triphosphate (ATP)

An energy source molecule used by cells that transfers a phosphate group to a transport protein, altering its shape to facilitate the active movement of substances across the membrane.

Membrane Potential

An electrical charge differential (electrochemical gradient) across a membrane caused by unequal concentrations of ions, rendering the cytoplasm relatively negative in comparison to the extracellular fluid.

Sodium-Potassium Pump

An electrogenic transport pump in animal cells that maintains membrane potential by pumping $3\,\text{Na}^+$ out of the cell and $2\,\text{K}^+$ into the cell, resulting in a net $+1$ charge in the extracellular fluid.

Proton Pump

An integral membrane protein found in plants, fungi, and bacteria that uses ATP to pump $\text{H}^+$ ions out of the cell, building up a proton gradient across the membrane.

Selective Permeability

The property of cellular membranes allowing certain substances (small, nonpolar, hydrophobic molecules such as hydrocarbons, $\text{CO}_2$, $\text{O}_2$, and $\text{N}_2$) to cross freely, while restricting hydrophilic, polar, large molecules and charged ions.

Passive Transport

The movement of a molecule across a membrane without the expenditure of cellular energy because the solute moves down its concentration or electrochemical gradient.

Diffusion

The spontaneous movement of substance molecules down their concentration gradient from an area of high concentration to an area of low concentration as a result of constant molecular motion.

Osmosis

The diffusion of water down its concentration gradient across a selectively permeable membrane, moving from areas of low solute concentration to areas of high solute concentration.

Facilitated Diffusion

The passive transport of molecules (such as small ions, water, and carbohydrates) across a biological membrane via specific transport proteins down their concentration gradient without requiring energy.

Channel Proteins

Membrane transport proteins that provide a hydrophilic tunnel allowing specific ions or molecules to pass; many function as gated channels that open only in response to a stimulus.

Aquaporins

Specific channel proteins that facilitate the rapid passage of water molecules across the cell membrane.

Carrier Proteins

Membrane transport proteins that undergo conformational shape changes to move bound substances across the lipid bilayer.

Integral Proteins

Amphipathic proteins embedded directly into the hydrophobic interior of the lipid bilayer that allow hydrophilic molecules to cross the cell membrane.

Peripheral Proteins

Proteins that are not embedded in the lipid bilayer but are loosely bonded to the external or internal surface of the membrane.

Glycolipids

Membrane lipids with covalently attached carbohydrates that play an important role in cell-to-cell recognition.

Glycoproteins

Membrane proteins with covalently attached carbohydrates (the most abundant surface markers) essential for cell-to-cell recognition.

Cell Wall

An extracellular protective structure covering the plasma membrane in plants, fungi, algae, and some bacteria; composed of cellulose in plants, it provides shape, structure, protection, and regulation of water intake.

Plasmodesmata

Hole-like channels through plant cell walls filled with cytosol that connect the cytoplasm of adjacent plant cells.

Fluid Mosaic Model

A model describing cell membrane structure as a mosaic of macromolecules in a fluid phospholipid bilayer held together by weak hydrophobic interactions that allow components to move and shift.

Unsaturated Hydrocarbon Tails

Kinked fatty acid tails of phospholipids that prevent tight packing, thereby maintaining membrane fluidity at lower temperatures.

Cytoskeleton

A network of fibers extending throughout the cytoplasm that provides structural and mechanical support, anchors organelles, and interacts with motor proteins to facilitate vesicle and cellular movement.

Microtubules

Hollow, rod-like cytoskeleton fibers composed of tubulin protein subunits that grow from the centrosome, serve as tracks for organelle movement with motor proteins, assist in chromosome separation during cell division, and form cilia and flagella.

Microfilaments

Thin, solid cytoskeletal rods made of actin protein that maintain cell shape, interact with myosin for muscle contraction and cell motility, and form the contractile ring of the cleavage furrow in dividing animal cells.

Intermediate Filaments

Permanent, fibrous structural proteins of the cytoskeleton made of varying subunits that maintain cell shape, anchor the nucleus and other organelles, and form the nuclear lamina lining the nuclear envelope.

Smooth Endoplasmic Reticulum (Detoxification Role)

The organelle responsible for detoxifying alcohol, drugs, and organic chemicals in liver tissue.

Lysosomes (Organelle Recycling Role)

Membranous organelles that hydrolyze macromolecules and degrade damaged cellular organelles, releasing recycled organic materials back to the cell for renewal.

Mitochondria

Double-membrane energy organelles that serve as the site of cellular respiration to produce ATP; their number within a cell correlates directly with the cell's metabolic activity.

Cristae

Folds of the inner mitochondrial membrane that divide the organelle into two internal compartments and increase surface area for ATP synthesis.

Mitochondrial Matrix

The internal compartment enclosed by the inner mitochondrial membrane containing mitochondrial DNA, ribosomes, and metabolic enzymes.

Chloroplasts

Double-membrane photosynthetic organelles containing the green pigment chlorophyll, serving as the site of photosynthesis where light energy is converted into glucose.

Thylakoids

Membranous sacs inside chloroplasts that organize into stacks called grana.

Stroma

The fluid compartment surrounding thylakoids inside the inner membrane of a chloroplast, containing chloroplast DNA, ribosomes, and enzymes.

Secretory Protein Pathway

The route taken by a protein synthesized on rough ER ribosomes: dehydration synthesis of amino acids into the rough ER lumen, compartmentalization into a transport vesicle, entry at the cis face of the Golgi complex for modification, sorting, and tag addition, exit from the trans face in a secretory vesicle, and fusion with the cell membrane.

Golgi Complex

An organelle consisting of flattened membranous sacs (cisternae) with directionality (cis face and trans face) that receives transport vesicles from the ER, modifies, correctly folds, sorts, packages, and tags proteins before dispatching them.

Cisternae

Flattened membranous sacs that comprise the Golgi apparatus; they are not physically connected to each other, allowing distinct biochemical reactions to occur in sequence.

Central Vacuole

A large vacuole found exclusively in plant cells that stores inorganic ions and water, playing a critical role in generating turgor pressure.

Contractile Vacuole

A specialized vacuole in certain animal-like cells and protists that collects and pumps excess water out of the cell to regulate osmotic balance.

Ribosomes

Non-membrane-bound complexes of ribosomal RNA (rRNA) and proteins that carry out protein synthesis; they exist as free ribosomes in the cytosol (producing proteins that function in the cytosol) or bound ribosomes attached to the rough ER or nuclear envelope (producing proteins for secretion or membranes).

Rough Endoplasmic Reticulum

A network of membranous sacs and tubes studded with bound ribosomes that compartmentalizes the cell to keep synthesized proteins separate before shipping them in transport vesicles.

Smooth Endoplasmic Reticulum (Biosynthetic Role)

A network of membranous tubules devoid of ribosomes that synthesizes lipids and phospholipids for membranes and detoxifies harmful chemicals.