Topic 2 Cell Structure and Function

Microtubules

Hollow tubes (\sim 25 \text{ nm} diameter) made of α- and β-tubulin protofilaments, crucial for cell shape, organelle movement, chromosome segregation, and the structure of cilia/flagella.

Protofilaments

Linear chains of tubulin dimers that assemble to form a microtubule.

Centrosome

The microtubule-organizing center near the nucleus; in animal cells it contains a pair of centrioles and is a major site of microtubule nucleation.

Centriole

One of a pair of cylindrical structures in the centrosome, each with nine triplets of microtubules arranged in a ring.

Spindle apparatus

Microtubule-based structure formed during cell division that organizes and segregates chromosomes

MTOC (microtubule-organizing center)

Sites in the cell where microtubules nucleate and are organized; examples include the centrosome and other cellular locations.

Cilia

Membrane-bound, slender projections extending from a cell; can be non-motile (sensory) or motile; core structure driven by microtubules with a typical 9+2 arrangement in motile cilia.

Flagella

Longer cellular projections used for locomotion; have a similar microtubule-based core to cilia and typically beat with a different motion; commonly found on sperm.

Basal body

A structure that anchors a cilium or flagellum to the cell; derived from a centriole.

9+2 arrangement

Ultrastructure of motile cilia/flagella: nine outer doublets of microtubules surrounding a central pair.

9+0 arrangement

Ultrastructure of a primary (non-motile) cilium: lacks the central pair of microtubules.

Dynein

Motor protein that drives bending movements of cilia and flagella by causing sliding between microtubule doublets.

Kinesin

Motor protein that moves along microtubules (usually toward the plus end) and mediates intracellular transport.

Actin

Globular actin (G-actin) monomers polymerize into filamentous actin (F-actin), forming two intertwined strands; diameter ~7 nm.

Microfilaments

Actin filaments (F-actin) built as a twisted double chain of actin subunits; ~7 nm in diameter; support cell shape, enable movement, participate in muscle contraction and cytokinesis, and drive cytoplasmic streaming.

G-actin

Globular actin monomer that polymerizes to form F-actin.

F-actin

Filamentous actin; polymerized form of actin making up microfilaments, ~7 nm in diameter.

Myosin

Motor protein that interacts with actin to cause muscle contraction and other cellular movements.

Cytoplasmic streaming

Circular flow of cytoplasm within cells, propelled by actin–myosin interactions, aiding distribution of materials.

Muscle

________ contraction results from the interaction between actin and myosin filaments sliding past one another.

Intermediate filaments

Fibrous keratin-family proteins 8–12 nm in diameter that provide mechanical strength, anchor the nucleus, and form the nuclear lamina.

Keratin

Protein subunits of intermediate filaments, diverse by cell type.

Nuclear lamina

A meshwork of intermediate filaments lining the inner surface of the nuclear envelope, providing structural support.

Extracellular matrix (ECM)

Glycoproteins and other macromolecules outside animal cells that provide support, adhesion, movement, and regulation; major components include collagen, fibronectin, and integrins that link to the cytoskeleton.

Collagen

Primary structural protein of the ECM, forming fibrous networks that provide tensile strength.

Fibronectin

ECM glycoprotein that binds to integrins and other ECM components to promote adhesion.

Integrin

Transmembrane receptors that connect the ECM to the cytoskeleton and mediate adhesion and signaling.

Cell wall

Extracellular structure in plants that protects the cell, maintains shape, and limits water uptake; composed mainly of cellulose; may have primary, middle lamella, and secondary layers.

Primary cell wall

Thin, flexible outer wall formed during cell growth.

Middle lamella

Thin layer rich in pectin between primary walls of adjacent plant cells, helping glue cells together.

Secondary cell wall

Additional rigid layer laid between the plasma membrane and the primary cell wall in some cells for extra strength.

Plasmodesmata

Membrane-lined channels that connect plant cells, enabling transport of ions, hormones, and metabolites; establish the symplast (shared cytoplasm).

Symplast

Continued cytoplasm of connected plant cells via plasmodesmata, allowing intercellular transport.

Tight junctions

Junctions that seal neighboring cells at the apical region to prevent leakage of extracellular fluid; involve transmembrane proteins like claudins and JAMs, connected to the actin cytoskeleton by ZO proteins.

Desmosomes

Anchoring junctions that fasten cells together into strong sheets; composed of desmoglein and desmocollin cadherins linked to intermediate filaments via plakins and catenins.

Gap junctions

Communicating - provide cytoploasmic channels

Intercellular junctions

Connections between cells that include tight junctions, desmosomes, and gap junctions, linking membranes to the cytoskeleton.

Plasma membrane

The phospholipid bilayer that encloses the cell, with embedded proteins, separating cytoplasm from the outside.

Phospholipid bilayer

Two-layer arrangement of phospholipids forming the cell membrane; hydrophobic core restricts movement of water-soluble substances.

Inner leaflet

The cytosolic-facing half of the phospholipid bilayer.

Outer leaflet

The extracellular-facing half of the phospholipid bilayer.

Cytoplasm

All material within the cell membrane, excluding the nucleus (in eukaryotes).

Cytosol

The aqueous component of the cytoplasm containing water, ions, and small molecules.

Chromosome

The DNA molecule that stores hereditary information.

Ribosome

Ribonucleoprotein particle that carries out protein synthesis; can be free in cytosol or bound to membranes.

Cytoskeleton

Protein filament network that maintains cell shape, anchors organelles, and aids in movement.

Prokaryote

A cell type lacking a nucleus and membrane-bound organelles (Bacteria and Archaea).

Eukaryote

A cell type with a nucleus and membrane-bound organelles.

Nucleoid

Region where prokaryotic DNA is located, not enclosed by a membrane.

Nucleus

Membrane-bound organelle containing most of the cell's DNA; site of transcription; enclosed by a nuclear envelope with pores.

Nuclear envelope

Double-membrane barrier surrounding the nucleus, with nuclear pores.

Nuclear pore

Protein-lined channel regulating traffic between nucleus and cytoplasm.

Nucleolus

Nuclear substructure where ribosome assembly begins.

Chromatin

DNA-protein complex that forms chromosomes within the nucleus.

Glycocalyx

Sugar-rich coating on cell surfaces; in bacteria includes slime layer or capsule depending on attachment to the cell.

Slime layer

Loosely attached glycocalyx; a non-tight protective layer.

Capsule

Firmly attached glycocalyx; protective layer around some bacteria.

Pili

Hairlike appendages that mediate attachment and genetic exchange in bacteria.

Flagellum

Long whip-like tail used for propulsion by rotation.

Bacterial cell wall

Rigid outside layer of bacteria providing shape and protection, typically containing peptidoglycan.

Endomembrane system

Interconnected membranes (ER, Golgi, lysosomes, vacuoles, plasma membrane) that modify, package, and transport lipids and proteins; does not include mitochondria or plastids.

Endoplasmic reticulum

Network of membrane-bound tubules and sacs; continuous with the nuclear envelope; two forms: rough (with ribosomes) and smooth (without).

Rough ER

ER region studded with ribosomes; synthesizes proteins and membranes; products move to the Golgi.

Smooth ER

ER region lacking ribosomes; synthesizes lipids, detoxifies compounds, and stores calcium.

Golgi apparatus

Stack of flattened membranes that modifies, sorts, and ships proteins and lipids via vesicles; cis, medial, and trans cisternae.

Cis face

Golgi’s receiving side facing the ER.

Trans face

Golgi’s shipping side where vesicles depart.

Vesicles

Small membrane-bound sacs that transport cargo between organelles or to the plasma membrane.

Lysosome

Membrane-bound organelle with hydrolytic enzymes for digestion and autophagy; acidic interior.

Phagocytosis

Cellular process of engulfing large particles into a phagosome that fuses with a lysosome.

Autophagy

Lysosome-dependent degradation and recycling of cellular components.

Mitophagy

Autophagy of mitochondria.

Vacuole

Membrane-bound sac with varied functions; in plants central vacuole; in animals lysosomal/storage vacuoles.

Central vacuole

Large plant cell vacuole for storage and turgor; surrounded by tonoplast.

Tonoplast

Membrane surrounding the central vacuole.

Mitochondria

Organelle for cellular respiration; produces ATP; double membrane; contains its own DNA and ribosomes.

Cristae

Folds of the inner mitochondrial membrane increasing surface area for enzymes.

Chloroplast (plastids)

Site of photosynthesis in plants/algae; double membrane; contains thylakoids, granum, and stroma; contains its own DNA.

Thylakoid

Flattened membrane sacs within chloroplasts where light reactions occur.

Granum

Stack of thylakoids within a chloroplast.

Stroma

Fluid inside chloroplast surrounding thylakoids; site of the Calvin cycle.

Peroxisome

Oxidative organelle; contains enzymes to break down fatty acids and detoxify; produces hydrogen peroxide and converts it to water and oxygen.

Glyoxysome

Specialized peroxisome in plants/molds converting stored lipids into carbohydrates.

Endosymbiosis

Theory that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by a host cell.

Mitochondrial DNA

Circular DNA present in mitochondria, separate from nuclear DNA.

Double membrane

Two phospholipid bilayers enclosing organelles such as mitochondria and chloroplasts.

Desmosome

Anchoring junction that mechanically links adjacent cells in tissues.

Plasmodesmata

Channels through plant cell walls that connect cytoplasm of neighboring cells.

Cell wall (plants)

Rigid layer outside the plasma membrane; in plants primarily cellulose; provides support and protection.

Cytoskeleton components

Network of protein filaments (microtubules, microfilaments, intermediate filaments) that supports the cell, enables movement, and organizes organelles.

Semiautonomous

organelles that can grow and divide independently within a cell, such as mitochondria and chloroplasts.

Which locations in the cell do not contain DNA?

Ribosomes