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# Glossary

actin
  Protein component of microfilaments of the cytoskeleton. Also found as part of the contractile apparatus of muscles.

adherens junctions
  Junction between adjacent animal cells whose actin filaments are linked, via transmembrane proteins (cadherins) and intracellular proteins.

anchoring junctions
  (Called desmosomes and hemidesmosomes) Intercellular junctions that are present in many types of animal tissues and serve to hold the constituent cells to each other and to the surrounding extracellular matrix respectively, via transmembrane proteins, cadherins. The link is to intermediate filaments.

autophagy
  The degradation and recycling of the components of a cell's organelles and structures by the action of lysosomes. This process occurs when cell components are old or damaged, or in order to release nutrients under conditions of cell starvation.

capping protein
  A protein that attaches to and stabilises microtubules and prevents disassembly.

centrosome
  The microtubule organising centre (MTOC) found in animal cells that is involved in assembly and maintenance of the cytoskeleton. During cell division, the centrosome divides to organise the two poles of the mitotic spindle.

cell fractionation
  Technique for the separation of the components of cells, by disruption followed by ultracentrifugation.

chromatin
  Complex of DNA and histone proteins, which makes up eukaryotic chromosomes.

collagen
  A protein that is abundant in the extracellular matrix. There are several types of collagen; it can form long thin fibres and provide structure, strength and flexibility to many tissues.

cotranslational localisation
  The process whereby a ribosome with attached mRNA is moved from the cytosol to the endoplasmic reticulum (if the protein that is being encoded is destined for lysosomes, or to be secreted from the cell, or to be embedded in a cell membrane).

cristae
  (Sing. crista) Folds of the inner mitochondrial membrane where electron transport takes place.

cytoskeleton
  System of long filament-like protein polymers, which confer shape and support and mediate many types of cellular movement. The three types of cytoskeletal filaments are the microfilaments, microtubules and intermediate filaments.

elastin
  A flexible protein that is found in the extracellular matrix of tissues such as blood vessels.

endospores
  Dormant structures made by some bacteria within their cells, which can later germinate to new individual bacteria.

euchromatin
  Regions of chromatin where the DNA is less condensed and appear pale after staining; thought to contain many genes that are being actively transcribed.

exocytosis
  The release of materials, which may be dissolved or particulate and are enclosed within a vesicle, from a cell. Vesicles are transported to the cell surface where the cell membrane and vesicle membrane fuse, releasing the vesicle contents to the outside.

gap junctions
  A type of specialised cell junction that allows intercellular communication, by passage of molecules and electrical signals between cells.

glycosylation
  A type of post-translational protein modification where sugar residues are added to proteins to produce glycoproteins.

Golgi apparatus
  A group of disc-like sacs found in eukaryotic cells, within which polysaccharides and proteins are processed (e.g. to form glycoproteins) and packaged for delivery to the cell surface for secretion to the outside.

grana
  (Sing. granum) Stacks of thylakoids within the chloroplast. The grana are linked by stroma lamellae, which join the stacks of grana together to form a single compartment.

granum
  See grana.

heterochromatin
  Regions of chromatin where DNA is highly condensed, thought to be regions of chromosomes that are not transcriptionally active.

histones
  Proteins around which DNA winds and binds to in the chromosome. The complex of DNA and histones is chromatin.

integrins
  Type of cell surface proteins that are involved in binding to extracellular matrix components such as fibronectin, which links to collagen. On the inside of the cell, integrins link, via other proteins, to actin filaments.

intermediate filaments
  Type of cytoskeletal filament; there are several different types of intermediate filament protein. All intermediate filaments are about 8-10 nm in diameter, and they play a role in cell support.

intermembrane space
  The space between the inner and outer membranes in mitochondria, into which protons are pumped as a result of the activities of the mitochondrial electron transport chain (located in the inner mitochondrial membrane).

lamins
  The type of intermediate filament found inside the nucleus.

lysate
  The name given to the mixture of components that results when a cell/cells are disrupted, e.g by lysis (being lysed). Lysis is the term given to the rupture of the cell membrane and release of the contents of the cell.

lysosomes
  Small spherical organelles with a single membrane. Lysosomes contain degradative enzymes and break down old organelles and also ingested material. They have a low internal pH.

microfilaments
  Cytoskeletal proteins composed of sub-units of the protein actin. They are the thinnest of the cytoskeletal filaments, having a diameter of about 6nm, and mediate many cell movements.

microtubules
  Tubular cytoskeletal structures composed of subunits of the protein tubulin. They are the largest of the cytoskeletal filaments, having a diameter of about 25 nm. Microtubules play an important role in maintaining cell shape and mediate the transport of vesicles and other cellular components.

mitochondrial DNA
  The DNA that is localised in mitochondria (in the mitochondrialmatrix). It encodes most of the proteins of the inner mitochondrial membrane.

mitochondrial matrix
  The innermost part of the mitochondrion, surrounded by a double membrane. It contains mitochondrial DNA, and the TCA cycle takes place in the mitochondrial matrix.

nuclear pores
  Small gaps in the nuclear envelope that form channels of about 9 nm in diameter, allowing the free passage of small water-soluble molecules into and out of the nucleus. Movement of specific proteins into the nucleus, and RNA out of the nucleus is regulated by proteins of the nuclear pore complex.

nucleoli
  See nucleolus.

nucleolus
  Electron-dense structures in the nucleus that are the site of ribosomal RNA transcription and ribosome assembly.

peptidoglycan
  Major component of bacterial cell walls. It is a polymer of high molecular mass, composed of two complex monosaccharides derived from glucose, which are linked together by amino acids, including three amino acids that are not found naturally in proteins.

peroxisomes
  Small spherical organelles, bound by a single membrane. Peroxisomes contain enzymes that oxidise fatty acids and amino acids.

phagocytosis
  The term used for engulfment of large particles or bacteria by endocytosis. Some cells of the immune system, known as phagocytes or macrophages engulf bacteria by phagocytosis.

protein sorting
  Also called protein targeting. The delivery of newly synthesised proteins to the correct destination within the cell or, in the case of secreted proteins, to the exterior.

protein targeting
  See protein sorting

rough endoplasmic reticulum (RER)
  Part of the endoplasmic reticulum where protein synthesis takes place. Ribosomes are hence attached to the RER, giving it a 'rough' appearance. It is the site of synthesis of membrane and lysosomal proteins, and also proteins that are to be exported from the cell.

scanning electron microscopy (SEM)
  A type of microscopy used to study the surface of intact cells and tissues. A sample is coated with a thin metallic layer that deflects an electron beam onto a detector.

signal recognition particle (SRP)
  A protein complex that recognises a specific sequence of translated amino acids emerging from a free ribosome. The sequence is a signal that the protein is to be translated on the membrane of the rough endoplasmic reticulum (RER). Translation is interrupted whilst the whole complex (including the ribosome, mRNA and partially translated polypeptide) is moved to the RER.

signal sequence
  A short, N-terminal sequence at the end of a protein molecule that serves as its 'address label', directing proteins to specific targets in the cell.

smooth endoplasmic reticulum (SER)
  Part of the endoplasmic reticulum where phospholipids and steroids are produced, and where detoxification of drugs occurs.

thylakoids
  Internal membrane system of chloroplasts, where photosynthesis takes place.

tight junctions
  A type of junction that forms tight contacts between cells in animal tissues such as epithelia. Tight junctions prevent the lateral movement of other membrane proteins, thus allowing different regions of the cell membrane to have different functional properties (e.g. transport characteristics).

transmission electron microscopy (TEM)
  A type of microscopy used to study the internal organisation of cells. A beam of electrons is passed through a very thin tissue section and an image obtained by focussing them on a fluorescent screen that emits light when electrons hit it.

transport vesicles
  Small membrane-bound sacs that bud off from one membrane, move through the cytosol and fuse with another membrane. For example, vesicles move from ER to Golgi and from Golgi to the cell membrane and to lysosomes.

tubulin
  Protein subunit of microtubules.

vacuole
  Membrane-bound space, prominent in all plant and fungal cells (but usually not in animal cells), which acts as a storage compartment for water, ions and small organic molecules. In mature plant cells the vacuole fills most of the centre of the cell and together with the cell wall plays a role in determining the shape of the cell.

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OpenLearn - A tour of the cell
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