Cells, microscopy and biological organisation definitions

Eukaryotic cell structure and organelles

Organelle

A structure inside a cell with a distinct function, usually but not always surrounded by its own membrane.

Ultrastructure

The detail of a cell's internal structure as revealed by an electron microscope.

Cytoplasm

Everything inside the cell-surface membrane except the nucleus: the cytosol and the organelles suspended in it.

Nuclear envelope

The double membrane surrounding the nucleus, continuous with the endoplasmic reticulum and perforated by nuclear pores.

Nucleolus

A dense region within the nucleus where ribosomal RNA is made and ribosomes are assembled.

Rough endoplasmic reticulum

Flattened membrane sacs with ribosomes attached, which make and transport proteins for export or for membranes.

Smooth endoplasmic reticulum

Membrane sacs without ribosomes, which synthesise lipids and steroids and store calcium ions.

Ribosome

A structure of ribosomal RNA and protein, in two subunits, with no membrane, which is the site of translation. 80S in eukaryotic cytoplasm, 70S in prokaryotes and in mitochondria and chloroplasts.

Lysosome

A vesicle containing hydrolytic enzymes, bounded by a single membrane, which digests worn-out organelles and engulfed material.

Crista

A fold of the inner mitochondrial membrane, giving a large surface area for the electron transport chain.

Cytoskeleton

The network of protein fibres (microfilaments, intermediate filaments and microtubules) that gives a cell its shape, anchors its organelles and provides tracks for movement.

Microtubule

A hollow cylinder of tubulin about 25 nm wide, along which motor proteins move vesicles and organelles, and which builds the spindle, cilia and flagella.

Motor protein

A protein such as kinesin or dynein that uses ATP to walk along a cytoskeletal fibre, carrying a vesicle or organelle with it.

Amyloplast

A colourless plastid with a double-membrane envelope that synthesises and stores starch, found in storage tissues such as potato tubers and in root cap cells.

Middle lamella

The layer, mainly calcium pectate, between the walls of neighbouring plant cells, which cements the cells together.

Pit

A thin region of a plant cell wall where no secondary wall has been laid down, usually matched by a pit in the neighbouring cell, allowing water and solutes to pass between them.

Plastid

Any of the family of plant organelles, including chloroplasts and amyloplasts, that have a double-membrane envelope and their own DNA and develop from proplastids.

Microscopy, magnification and resolution

Magnification

How many times larger the image is than the actual object. A ratio, with no units.

Resolution

The minimum distance between two points at which they can still be distinguished as separate. A length, and smaller is better.

Empty magnification

Enlarging an image beyond the detail its resolution contains, so nothing further becomes visible.

Artefact

A structure seen in a prepared specimen that was not present in the living cell, produced by the preparation or imaging process.

Eyepiece graticule

A ruled scale in the eyepiece whose divisions have no fixed length until it is calibrated.

Stage micrometer

A slide bearing a scale of known length, used to calibrate an eyepiece graticule for one particular objective lens.

Prokaryotic cells and viruses

Prokaryotic cell

A cell with no nucleus and no membrane-bound organelles, whose DNA is a single circular molecule free in the cytoplasm.

Plasmid

A small circular molecule of DNA, separate from the main chromosome, which replicates independently and often carries genes such as antibiotic resistance.

Capsule

A layer of slime outside the cell wall which retains water, aids attachment and gives some protection from phagocytosis.

Nucleoid

The region of a prokaryotic cell occupied by its circular DNA, with no surrounding membrane.

Virus

An acellular infectious particle consisting of nucleic acid within a protein capsid, sometimes with a lipid envelope, which can only replicate inside a host cell.

Capsid

The protein coat surrounding a virus's genetic material.

Attachment protein

A protein on a virus's surface with a shape complementary to a receptor on its host cell, which determines which cells it can infect.

Reverse transcriptase

An enzyme carried by retroviruses such as HIV which catalyses the synthesis of DNA from an RNA template.

Lytic cycle

Viral replication in which the virus attaches, injects or releases its nucleic acid, replicates it, assembles new particles and then bursts the host cell to release them.

Latency

A state in which a viral genome remains in a host cell without new virus particles being made, until a trigger reactivates it.

Prophage

The DNA of a phage such as lambda inserted into the chromosome of its host bacterium and copied with it at each division.

Bacteriophage

A virus that infects bacteria, often called a phage.

Antiviral

A drug that inhibits a stage of virus replication, such as entry, copying of the genome, assembly or release, ideally by targeting a molecule coded for by the virus.

Cell specialisation and biological organisation

Differentiation

The process by which a cell develops the structures and characteristics needed to carry out a particular function, through some genes being expressed and others not.

Specialised cell

A cell whose structure is adapted to one particular function.

Totipotent

Able to develop into any cell type in the organism and into the extra-embryonic tissues such as the placenta. Found in the zygote and the cells of the first few divisions.

Pluripotent

Able to develop into any cell type in the body, but not into the placenta. Found in embryonic stem cells in the blastocyst.

Multipotent

Able to develop into a limited range of cell types. Found in adult stem cells, such as those in bone marrow which produce the blood cells.

Cell theory

The unifying concept that all living organisms are made of one or more cells, that the cell is the fundamental unit of structure, function and organisation, and that new cells arise only from existing ones.

Tissue

A group of similar cells working together to perform a particular function.

Organ

A structure made of several different tissues working together to perform a particular function.

Organ system

A group of organs working together to perform an overall life function.

Cartilage

A connective tissue in which chondrocytes, lying in lacunae, are surrounded by a firm extracellular matrix of collagen that they secrete; it has no blood vessels and supports structures such as the trachea, joints, ear and nose.

Meristem

A plant tissue of unspecialised cells that continue to divide, from which specialised cells such as xylem vessels and sieve tube elements differentiate.

Vascular cambium

The meristem layer lying between the xylem and the phloem of a vascular bundle, whose divisions add secondary xylem on its inner side and secondary phloem on its outer side.

Plan diagrams and tissue sections

Plan diagram

A low-power drawing of a section that shows the positions and boundaries of tissues, in correct proportion, without drawing any individual cells.

Transverse section

A section cut across an organ, at right angles to its long axis.

Longitudinal section

A section cut along an organ, parallel to its long axis.

Blood smear

A single-cell-thick film of blood drawn across a slide and stained, used to examine and identify blood cells.

Neutrophil

A phagocytic white blood cell about 12 µm across with a lobed nucleus and granular cytoplasm.

Lymphocyte

A white blood cell about 9 µm across whose round nucleus almost fills the cell, responsible for the specific immune response.

Monocyte

The largest white blood cell of a smear, up to about 18 µm, with a kidney-shaped nucleus, which matures into a macrophage.