A/P Chapter 3 Cellular Form and Function

Robert Hooke

The English scientist who coined the term "cellulae" in 1663 while observing the empty, rigid walls of a cork specimen under a compound microscope.

Theodor Schwann

The 19th-century zoologist who critically analyzed animal tissues and officially concluded that all animals are made up of individual cells.

Tenet 1 of Modern Cell Theory

All organisms are entirely composed of cells and the functional products of cells.

Tenet 2 of Modern Cell Theory

The cell is the absolute simplest, most fundamental structural and functional unit of biological life.

Tenet 3 of Modern Cell Theory

An organism's macroscopic structures and functions are ultimately due to the collective metabolic activities of its individual cells.

Tenet 4 of Modern Cell Theory

Cells arise exclusively from the division of pre-existing cells, meaning life does not spontaneously generate.

Tenet 5 of Modern Cell Theory

The cells of all biological species exhibit a fundamental biochemical and metabolic unity.

Squamous cells

Cells that are thin, flat, and scale-like, often bulging slightly where the nucleus resides; they line the lungs, esophagus, and form the outer epidermis.

Cuboidal cells

Cells that are squarish in cross-section and roughly equal in height and width, typical of liver cells and kidney tubule tracks.

Columnar cells

Cells that are distinctly taller than they are wide, notably lining the inner surfaces of the stomach and intestines.

Polygonal cells

Cells possessing irregular angular profiles featuring four, five, or more distinct sides.

Stellate cells

Cells with a star-like structural morphology featuring multiple long, pointed extensions radiating from the cell body (e.g., neurons).

Spheroidal to ovoid cells

Cells with a round-to-oval geometry, characteristic of female egg cells and human white blood cells.

Discoid cells

Cells shaped like flat discs with dimpled or biconcave centers, maximizing surface area for oxygen transport (e.g., red blood cells).

Fusiform cells

Spindle-shaped cells that are thick in the middle and taper down to narrow, pointed ends, characteristic of smooth muscle cells.

Fibrous cells

Long, slender, thread-like cells, characteristic of skeletal muscle cells and the axons of nerve pathways.

Micrometer (µm)

The standard unit of microscopic measurement used for expressing cell sizes, representing one-millionth of a meter ($10^{-6}$ m).

Human cell size range

Most human cells range from 10 to 15 micrometers in diameter, though human egg cells reach 100 micrometers and nerve axons can stretch over a meter long.

Surface area to volume ratio rule

As a cell grows, its internal volume increases as a cube ($r^3$) while its surface area increases only as a square ($r^2$), meaning a giant cell cannot absorb nutrients or vent wastes fast enough to survive.

Cytoplasm

The entire internal contents of a cell located between the outer plasma membrane and the inner nuclear envelope, housing the cytosol, cytoskeleton, and organelles.

Plasma (cell) membrane

The selectively permeable lipid bilayer that encloses cell contents, acts as a traffic boundary, and controls all structural molecular interactions with other cells.

Intracellular face

The inner structural surface of the plasma membrane that directly faces inward toward the cytoplasm.

Extracellular face

The outer structural surface of the plasma membrane that directly faces outward toward the surrounding environment or tissue fluid.

Membrane lipid composition

The plasma membrane lipids are composed of 75% phospholipids, 20% cholesterol, and 5% glycolipids.

Amphipathic nature of phospholipids

Phospholipids have a polar, hydrophilic phosphate head that faces water on both sides of the membrane, and two nonpolar, hydrophobic fatty acid tails that point inward toward each other.

Cholesterol function in membrane

Cholesterol molecules stiffen the fluid plasma membrane by interacting with phospholipids, or increase fluidity in high concentrations when cold.

Glycolipids

Phospholipids with short, branching carbohydrate chains attached exclusively on the extracellular face, contributing to the glycocalyx coating.

Integral proteins

Membrane proteins that penetrate deep into or completely through the phospholipid bilayer.

Transmembrane proteins

Integral proteins that span entirely across the membrane from the intracellular face to the extracellular face, possessing hydrophilic and hydrophobic domains.

Peripheral proteins

Membrane proteins that do not enter the hydrophobic core of the bilayer but adhere strictly to one face, typically anchored to a transmembrane protein or the cytoskeleton.

Receptor proteins

Membrane proteins with specific binding sites that capture chemical messengers (hormones, neurotransmitters) released by other cells to signal an internal change.

Second-messenger system

A chemical cascade where a cell-surface receptor binds a messenger, activates an internal relay protein, and generates an intracellular molecule to alter cell activity.

G protein

An intracellular peripheral protein that binds guanosine triphosphate (GTP); acts as a molecular relay switch activated by a surface receptor.

Adenylate cyclase

An enzyme embedded within the plasma membrane that is triggered by an active G protein to rapidly convert ATP into cyclic AMP (cAMP).

Cyclic AMP (cAMP)

The classic intracellular second-messenger molecule that travels through the cytosol to turn on metabolic enzymes called kinases.

Kinases

Cytoplasmic enzymes activated by cAMP that covalently add phosphate groups (phosphorylation) to other cellular proteins, activating or silencing them.

Enzymes (membrane-bound)

Membrane proteins that catalyze chemical reactions directly at the cell surface, such as breaking down digested nutrients or destroying used chemical messengers.

Channel proteins

Transmembrane proteins featuring central fluid-filled tunnels that allow water and small hydrophilic ions to cross the hydrophobic membrane.

Leak channels

Channel proteins that stay open continuously, allowing specific ions to drift down their concentration gradients without interruption.

Gated channels

Channel proteins that possess a molecular gate that opens and closes at precise times in response to specific environmental stimuli.

Ligand-gated channels

Gated channels that open or close strictly when a specific chemical messenger binds to its surface receptor.

Voltage-gated channels

Gated channels that open or close in response to direct fluctuations in the electrical potential (voltage) across the plasma membrane.

Mechanically gated channels

Gated channels that open or close in response to physical trauma or stress on the cell, such as stretching, tissue pressure, or vibration.

Carriers / Transporters

Transmembrane proteins that physically bind a specific solute particle on one side, change their shape, and release the solute on the opposite side.

Pumps

A specialized sub-category of carrier proteins that consume cellular metabolic energy (ATP) to force solutes up or against their concentration gradients.

Cell-identity markers

Glycoproteins on the extracellular face that act like structural identification tags, allowing the immune system to distinguish healthy self-cells from foreign invaders.

Cell-adhesion molecules (CAMs)

Membrane proteins that physically bind and link a cell to adjacent cells or to the extracellular matrix, which is crucial for building stable tissues.

Glycocalyx

A fuzzy, carbohydrate-rich coating on the outer surface of all animal cell membranes; functions in body cell identification, tissue binding, embryo development, and cushioning.

Microvilli

Short, dense, finger-like extensions of the plasma membrane that maximize surface area for optimal chemical absorption, forming a "brush border" on cells like intestinal epithelium.

Cilia

Hair-like projections from the cell surface; can be nonmotile (acting as cellular antennas or sensors) or motile (beating in waves to sweep mucus and fluids across tissue tracks).

Axoneme

The structural core of a cilium consisting of a specific array of microtubules, showing a 9+2 arrangement in motile cilia and a 9+0 arrangement in primary nonmotile cilia.

Dynein arms

Motor protein claws that use ATP to crawl along adjacent microtubules, causing a motile cilium to bend and beat.

Flagellum

A long, whip-like singular membrane extension structurally similar to a cilium, used exclusively for cell locomotion; found only in sperm cells within humans.

Selectively permeable

The characteristic of a membrane that allows certain substances to pass through easily while excluding or restricting others.

Passive transport

Mechanisms of movement across a membrane that require no expenditure of cellular metabolic energy (ATP), such as diffusion or osmosis.

Active transport

Mechanisms of movement across a membrane that consume cellular ATP energy to force solutes up their concentration gradients.

Carrier-mediated transport

Any transport process across a membrane that requires the physical assistance of a transmembrane carrier protein.

Filtration

A passive transport mechanism where physical pressure forces water and dissolved solutes across a membrane through small gaps or pores.

Simple diffusion

The net passive movement of particles down their concentration gradient from an area of higher concentration to lower concentration due to spontaneous kinetic motion.

Factors affecting diffusion rate

Temperature (higher increases), molecular weight (larger decreases), steepness of gradient (greater increases), membrane surface area (larger increases), and membrane permeability.

Osmosis

The net passive diffusion of water through a selectively permeable membrane from an area of higher water concentration to lower water concentration.

Aquaporins

Specialized transmembrane channel proteins optimized to accelerate the rate of water flow across a cell membrane.

Hydrostatic pressure

The physical fluid force exerted against a membrane surface due to the weight or pressure of water.

Osmotic pressure

The pulling force exerted by non-penetrating solute particles in a solution that drives the osmotic movement of water into that compartment.

Reverse osmosis

A physical process where mechanical pressure is applied to a solution to override osmotic pressure, forcing water backward through a membrane against its concentration gradient.

Osmolarity

The total concentration of non-penetrating solute particles per liter of a solution, expressed in milliosmoles per liter (mOsm/L).

Tonicity

The ability of a surrounding extracellular solution to influence the fluid volume and pressure inside a cell.

Hypotonic solution

A solution with a lower concentration of non-penetrating solutes than the intracellular fluid, causing water to rush into the cell and induce swelling or bursting.

Lysis

The rupture or bursting of a body cell due to excessive water intake in a hypotonic environment.

Hypertonic solution

A solution with a higher concentration of non-penetrating solutes than the intracellular fluid, causing water to exit the cell and induce shriveling.

Crenation

The shriveling, scalloped distortion of a cell (such as a red blood cell) placed in a hypertonic environment due to water loss.

Isotonic solution

A solution possessing a solute concentration identical to the internal cellular fluid, yielding no net movement of water and no structural changes to the cell.

Specificity (in carriers)

The trait of a carrier protein meaning it binds only to a distinct solute ligand and will not transport structurally different molecules.

Saturation

The property of carrier transport where all available binding sites are occupied, causing the transport rate to plateau at a maximum velocity.

Transport maximum (Tm)

The absolute ceiling rate of carrier-mediated transport that occurs when all membrane carriers are fully saturated with solute.

Uniport

A carrier protein that moves only one specific solute species across the membrane at a time.

Symport / Cotransport

A carrier protein that moves two or more different solutes simultaneously through the membrane in the same direction.

Antiport / Countertransport

A carrier protein that moves two or more different solutes through the membrane in opposite directions.

Facilitated diffusion

The passive carrier-mediated transport of a solute down its concentration gradient without using cellular energy.

Primary active transport

A process where a carrier protein binds a solute and directly hydrolyzes ATP to pump the substance up its concentration gradient.

Sodium-potassium (Na+-K+) pump

A primary active antiport carrier that consumes one ATP to pump 3 sodium ions out of the cell and 2 potassium ions into the cell against their gradients.

Functions of Na+-K+ pump

Regulation of cell volume, secondary active transport, maintenance of a negative resting membrane potential, and heat production.

Secondary active transport

A process where a carrier moves a solute up its gradient without directly consuming ATP, instead utilizing the potential energy of a sodium gradient maintained by a separate pump.

Vesicular transport

The active, energy-intensive movement of large particles, fluid droplets, or multiple molecules across a membrane enclosed within spherical membrane sacs called vesicles.

Endocytosis

Vesicular transport mechanisms that bring matter from the outside environment into the interior of the cell.

Phagocytosis

A form of endocytosis ("cell eating") where a cell extends false feet (pseudopods) to engulf solid foreign particles or cell debris and trap them in a phagosome vesicle.

Pinocytosis

A form of endocytosis ("cell drinking") where the plasma membrane dimples inward to sample droplets of extracellular fluid containing dissolved solutes.

Receptor-mediated endocytosis

A highly selective form of endocytosis where specific extracellular molecules bind to cluster receptors, triggering the membrane to sink inward coated in clathrin proteins.

Exocytosis

Vesicular transport mechanisms that discharge materials from the interior of a cell out into the extracellular fluid.

Transcytosis

The transport of a substance across a cell layer by capturing it via endocytosis on one side, moving it through the cytoplasm, and discharging it via exocytosis on the opposite side.

Cytoskeleton

An interconnected network of protein filaments, cylinders, and tubules that provides structural support, organizes organelles, and drives cellular motion.

Microfilaments

The thinnest elements of the cytoskeleton, composed of actin protein; they form a supportive terminal web on the inside of the cell membrane.

Intermediate filaments

Thicker, tougher protein cords that give a cell its physical shape, resist mechanical stress, and anchor cells to neighbors.

Microtubules

Hollow cylinders made of tubulin protein protofilaments; they radiate from the centrosome to act as intracellular railroad tracks for motor proteins and form the core of cilia/flagella.

Organelles

Internal, specialized structures within a cell that carry out distinct metabolic tasks.

Nucleus

The largest, central membrane-bound organelle acting as the control center of the cell; houses genetic material (DNA).

Nuclear envelope

The double-membrane boundary enclosing the nucleus, perforated by nuclear pores.

Nuclear pores

Protein rings spanning the nuclear envelope that regulate the transport of raw materials and RNA between the nucleus and cytoplasm.

Nuclear lamina

A dense web of intermediate protein filaments lining the inside of the nuclear envelope that supports its structure and anchors chromatin.

Nucleoplasm

The fluid material inside the nucleus, housing the nucleolus and chromatin.