Unit 2: Cell Structure and Function

Cell

The basic unit of structure and function in living things.

Structure–function principle

The idea that how a biological structure is built helps explain what it can do.

Prokaryotic cell

A cell (Bacteria/Archaea) that is generally small and lacks membrane-bound organelles; DNA is in a nucleoid.

Eukaryotic cell

A cell (animals, plants, fungi, protists) containing internal membranes that form membrane-bound organelles.

Nucleoid

A non-membrane-bound region in a prokaryote where the (usually circular) DNA is located.

Cell wall

A protective, supportive outer layer outside the plasma membrane (present in plants, fungi, and many bacteria; absent in animals).

Peptidoglycan

A structural polysaccharide-protein material that composes most bacterial cell walls.

Compartmentalization

The organization of eukaryotic cells into membrane-bound regions that maintain distinct conditions to improve efficiency, protection, and control.

Cytoplasm

The internal cell region between the plasma membrane and nucleus (in eukaryotes), containing cytosol and organelles.

Plasma membrane

A selectively permeable boundary that regulates transport, communication, and gradient maintenance between a cell and its environment.

Phospholipid bilayer

The core structure of the plasma membrane, formed by two layers of phospholipids with heads outward and tails inward.

Fluid mosaic model

A model describing membranes as a laterally fluid “mosaic” of lipids, proteins, and carbohydrates.

Cholesterol

A membrane component in animal cells that buffers fluidity, helping prevent membranes from becoming too rigid or too fluid with temperature changes.

Selective permeability

The property of membranes that allows some substances to cross more easily than others based on size, polarity, and charge.

Integral (transmembrane) protein

A membrane protein embedded in the bilayer, often spanning it, that provides functions such as transport and signaling.

Channel protein

A transport protein that forms a hydrophilic passageway, allowing specific ions or molecules to cross the membrane down a gradient.

Carrier protein

A transport protein that binds a solute and changes shape to move it across the membrane (often used in facilitated diffusion).

Glycoprotein

A membrane protein with a carbohydrate chain used for cell recognition, adhesion, or communication; carbohydrates face the extracellular side.

Nucleus

A usually large eukaryotic organelle that stores DNA and helps direct cell activities and reproduction.

Nuclear envelope

A double membrane surrounding the nucleus.

Nuclear pores

Regulated openings in the nuclear envelope that control traffic (e.g., RNA out, proteins in).

Nucleolus

A dense nuclear region where rRNA is produced and ribosome subunits begin to assemble.

Ribosome

A non-membrane-bound structure made of rRNA and proteins that carries out protein synthesis (translation).

Free vs. bound ribosomes

Free ribosomes typically make cytosolic proteins, while ribosomes bound to rough ER often make proteins for secretion, membranes, or certain organelles.

Rough endoplasmic reticulum (RER)

An ER region studded with ribosomes that synthesizes and processes proteins for secretion, membranes, and lysosomes (including folding/initial modifications).

Smooth endoplasmic reticulum (SER)

ER lacking ribosomes that synthesizes lipids (including steroids), helps detoxify substances, participates in carbohydrate metabolism, and stores Ca2+.

Golgi apparatus

An organelle that modifies, sorts, and packages products from the ER into vesicles for specific destinations (including secretion).

Vesicle

A small membrane-bound sac used to transport or store materials, especially in the endomembrane system.

Lysosome

A membrane-bound digestive compartment with enzymes that break down macromolecules, old organelles, and ingested material (acidic interior).

Apoptosis

Programmed cell death; lysosomes can play essential roles in this controlled breakdown process.

Vacuole

A fluid-filled membrane-bound sac used for storage (water, food, wastes, salts, pigments) and other functions, especially in plants and protists.

Central vacuole

A large plant vacuole that stores water/solutes and contributes to support and growth by generating pressure against the cell wall.

Turgor pressure

Pressure of plant cell contents (especially the central vacuole) pushing against the cell wall, supporting the plant and aiding cell expansion.

Mitochondrion

An organelle that converts energy from organic molecules into ATP and contains its own DNA and ribosomes (endosymbiosis evidence).

Chloroplast

A photosynthetic organelle in plants/algae containing chlorophyll and its own DNA and ribosomes (endosymbiosis evidence).

Endosymbiotic theory

The hypothesis that mitochondria and chloroplasts originated as prokaryotes that entered a host cell and formed a mutualistic relationship.

Peroxisome

An organelle that breaks down fatty acids and detoxifies harmful substances, converting H2O2 to water and oxygen to isolate damaging chemistry.

Cytoskeleton

A dynamic network of protein fibers that maintains cell shape, organizes components, and enables movement and intracellular transport.

Microtubule

A hollow cytoskeletal tube made of tubulin that functions in cell division (spindle), intracellular transport, and the core of cilia/flagella.

Microfilament (actin filament)

A thin cytoskeletal rod made of actin involved in cell shape changes, movement, and muscle contraction via rapid assembly/disassembly.

Surface area-to-volume ratio (SA:V)

A relationship showing that as size increases, volume grows faster than surface area, reducing exchange efficiency and limiting cell size.

Diffusion

Net movement of particles from higher concentration to lower concentration due to random molecular motion (net change is zero at equilibrium).

Facilitated diffusion

Passive transport down a concentration gradient using membrane proteins (channels or carriers) without ATP.

Osmosis

Diffusion of water across a selectively permeable membrane from higher free water (lower solute) to lower free water (higher solute).

Aquaporin

A water-specific channel protein that increases a membrane’s permeability to water.

Tonicity

A description of how a solution affects cell volume based on nonpenetrating solutes (hypotonic, hypertonic, isotonic).

Water potential (Ψ)

A quantitative measure predicting water movement (water moves from higher Ψ to lower Ψ), where Ψ = Ψs + Ψp.

Sodium-potassium pump

A primary active transport pump using ATP to move 3 Na+ out of the cell and 2 K+ into the cell, maintaining ion gradients.

Endocytosis

Bulk transport that brings materials into a cell by plasma membrane engulfment to form an internal vesicle.

Exocytosis

Bulk transport that exports materials when vesicles fuse with the plasma membrane, releasing contents outside the cell.