chapter 4 - cells

Cell

The smallest unit of a living thing and the basic building block of all organisms.

Microscope

An instrument that magnifies an object so it can be studied, derived from micro- ('small') and -scope ('to look at').

Micrograph

A photograph or image of an object or cell taken using a microscope.

Magnification

The process of enlarging an object in appearance.

Resolving power

A microscope's ability to distinguish two adjacent structures as separate, which determines the clarity and detail of an image.

Light microscope

A microscope that passes and bends visible light through a lens system to enable viewing of specimens, commonly magnifying up to 400 times (or 1,000 times using oil immersion).

Electron microscope

A microscope that uses a beam of electrons in a vacuum instead of light, providing higher magnification and resolving power but killing the specimen during preparation.

Scanning electron microscope (SEM)

An electron microscope that moves a beam of electrons back and forth across a cell's surface to produce detailed images of surface characteristics.

Transmission electron microscope (TEM)

An electron microscope where the electron beam penetrates the cell to provide detailed images of its internal structures.

Animalcules

The collective term used in the 1600s by Antony van Leeuwenhoek to describe the movements of single-celled organisms he observed through crafted lenses.

Unified cell theory

The theory proposed by Matthias Schleiden and Theodor Schwann stating that one or more cells comprise all living things, the cell is the basic unit of life, and new cells arise from existing cells.

Cytotechnologist

A professional who studies cells via microscopic examinations and laboratory tests to detect normal and abnormal cellular changes.

Prokaryote

A simple, predominantly single-celled organism that lacks a nucleus or any other membrane-bound organelle, categorized into Bacteria and Archaea.

Nucleoid

The central region of a prokaryotic cell where its chromosomal DNA is localized.

Capsule

A polysaccharide outer structure present in many bacteria that enables the cell to attach to environmental surfaces.

Conjugation

The process by which one bacterium transfers genetic material to another through direct contact using specialized structures called pili.

Fimbriae

Cellular appendages used by bacteria to attach to a host cell.

Flagella

Cellular structures used by some prokaryotes and single-celled organisms for locomotion.

Eukaryote

An organism whose cells possess a membrane-bound nucleus, multiple rod-shaped chromosomes, and membrane-bound organelles.

Plasma membrane

A phospholipid bilayer with embedded proteins and cholesterol that separates the cell's interior contents from its surrounding environment.

Microvilli

Fingerlike foldings of the plasma membrane on absorptive cells, such as those lining the small intestine, that increase surface area for absorption.

Cytoplasm

The entire region of a cell between the plasma membrane and the nuclear envelope, comprising organelles suspended in gel-like cytosol, the cytoskeleton, and dissolved chemicals.

Nuclear envelope

A double-membrane structure punctuated with pores that constitutes the outermost portion of the nucleus, with both inner and outer layers formed by phospholipid bilayers.

Nucleoplasm

The semi-solid fluid inside the nucleus where chromatin and the nucleolus are situated.

Chromatin

The unwound protein-chromosome complexes resembling jumbled threads found during the growth and maintenance phases of a cell's life cycle.

Nucleolus

A darkly staining area within the nucleus that aggregates ribosomal RNA with associated proteins to assemble ribosomal subunits.

Ribosome

A large cellular complex of protein and RNA consisting of large and small subunits responsible for synthesizing proteins.

Mitochondria

Oval-shaped, double-membrane organelles termed the 'powerhouses' or 'energy factories' of the cell that produce ATP via cellular respiration.

Cristae

The folds of the inner mitochondrial membrane that increase surface area for cellular respiration and ATP synthesis.

Mitochondrial matrix

The space enclosed by the inner membrane and cristae of a mitochondrion.

Peroxisome

A small, round, single-membrane organelle that carries out oxidation reactions to break down fatty acids and amino acids and safely converts $H_2O_2$ into oxygen and water.

Glyoxysome

A specialized peroxisome in plant cells responsible for converting stored fats into sugars.

Centrosome

A microtubule-organizing center found near the nuclei of animal cells that contains a pair of perpendicular centrioles.

Centriole

A structure inside an animal cell's centrosome consisting of a cylinder made of nine triplets of microtubules held together by nontubulin proteins.

Lysosome

An organelle in animal cells containing digestive enzymes active at an acidic pH that breaks down macromolecules and worn-out organelles, functioning as the cell's 'garbage disposal.'

Cell wall

A rigid external covering that protects the cell, provides structural support, and gives shape to plant, fungal, and some protistan cells.

Cellulose

A polysaccharide composed of glucose units connected by 1-4 linkages that forms the primary organic structural component of plant cell walls.

Chloroplast

A double-membrane plant and algal cell organelle that contains its own DNA and ribosomes and carries out photosynthesis.

Thylakoid

An interconnected and stacked fluid-filled membrane sac within the chloroplast where light-harvesting photosynthetic reactions take place.

Granum

A stack of thylakoids located inside a chloroplast.

Stroma

The aqueous fluid enclosed by the inner membrane of a chloroplast surrounding the grana where sugar synthesis takes place.

Chlorophyll

A green pigment contained within chloroplasts that captures the light energy driving the reactions of photosynthesis.

Endosymbiosis

A mutually beneficial relationship in which one organism lives inside another; accounts for the evolutionary origin of mitochondria and chloroplasts from ingested bacteria.

Central Vacuole

A large plant cell organelle that occupies most of the cell's volume, regulates the cell's concentration of water in changing environmental conditions, and aids in cell expansion.

Endomembrane System

A group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins.

Lumen (Cisternal Space)

The hollow portion of the endoplasmic reticulum's tubules.

Rough Endoplasmic Reticulum (RER)

A region of the endoplasmic reticulum studded with ribosomes on its cytoplasmic surface that engages in protein modification and phospholipid synthesis for cellular membranes.

Smooth Endoplasmic Reticulum (SER)

A region of the endoplasmic reticulum with few or no ribosomes that synthesizes carbohydrates, lipids, and steroid hormones, detoxifies medications and poisons, and stores calcium ions.

Sarcoplasmic Reticulum

A specialized smooth endoplasmic reticulum in muscle cells responsible for storing calcium ions needed to trigger coordinated muscle contractions.

Golgi Apparatus

A eukaryotic organelle composed of a series of flattened membranous sacs that sorts, tags, packages, and distributes lipids and proteins.

Cis Face

The side of the Golgi apparatus located closer to the endoplasmic reticulum that fuses with incoming transport vesicles.

Trans Face

The opposite side of the Golgi apparatus from which vesicles bud off to deliver lipids and proteins to their destinations.

Lysosome

An organelle in animal cells containing hydrolytic enzymes that breaks down macromolecules, recycles worn-out organelles, and destroys pathogens.

Phagocytosis

A process of endocytosis in which a section of the plasma membrane invaginates and engulfs a pathogen or particle, pinching off to form an intracellular vesicle.

Cytoskeleton

A network of protein fibers in the cytoplasm that maintains cell shape, secures organelles in place, facilitates vesicle movement, and enables cellular locomotion.

Microfilaments

The narrowest protein fibers of the cytoskeleton (diameter about $7\,nm$), comprised of two intertwined actin strands, that function in movement, rigidity, and cell shape.

Actin

The globular protein that polymerizes into intertwined strands forming microfilaments, serving as a track for myosin.

Myosin

A motor protein that moves along actin microfilaments during motion events such as eukaryotic cell division and muscle contraction.

Cytoplasmic Streaming

The circular movement of the cell cytoplasm in plant cells, enabled by actin microfilaments.

Intermediate Filaments

Cytoskeletal elements consisting of several intertwined strands of fibrous proteins (diameter $8\text{ to }10\,nm$) that bear tension, maintain cell shape, and anchor organelles.

Keratin

A fibrous protein component of intermediate filaments that strengthens hair, nails, and the skin's epidermis.

Microtubules

Small hollow tubes with a diameter of about $25\,nm$ made of polymerized dimers of $\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$ that resist compression and guide vesicle transport.

Centrosome

A region in animal cells composed of two centrioles that serves as the microtubule-organizing center.

Flagella

Long, hair-like structures extending from the plasma membrane in small numbers ($1$ or a few) that function in cell locomotion.

Cilia

Short, hair-like structures extending in large numbers across the plasma membrane surface that move entire cells or sweep substances across the cell surface.

9 + 2 Array

The structural arrangement of microtubules in eukaryotic flagella and cilia, consisting of a ring of nine outer microtubule doublets surrounding a single central doublet.

Extracellular Matrix (ECM)

A complex network of materials secreted by animal cells—primarily collagen and proteoglycans—that holds cells into tissues and allows intercellular communication.

Proteoglycans

Carbohydrate-containing protein molecules that interweave with collagen fibers in the animal extracellular matrix.

Tissue Factor

A protein receptor displayed by damaged blood vessel lining cells that binds the extracellular matrix to trigger platelet adhesion and clotting.

Plasmodesmata

Channels passing through adjacent plant cell walls that connect the cytoplasm of neighboring cells and allow transport of materials throughout the plant.

Tight Junction

A watertight seal between adjacent animal cells created by claudins and occludins that prevents fluids and materials from leaking between cells.

Desmosomes

Spot weld-like junctions between adjacent animal epithelial cells formed by cadherins linked to intermediate filaments, maintaining tissues in sheet-like arrangements.

Cadherins

Short proteins in the plasma membrane that connect to intermediate filaments to form desmosomes between adjacent cells.

Gap Junction

A protein-lined pore between adjacent animal cells that allows water, ions, nutrients, and low molecular weight substances to pass freely between cells.

Connexon

An elongated, doughnut-like arrangement of six connexin proteins in the plasma membrane that aligns with another connexon to create a gap junction channel.

Peroxisome

A small, round organelle that oxidizes and breaks down fatty acids and amino acids, contains hydrogen peroxide, and detoxifies poisons.

Unified Cell Theory

A biological concept stating that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.