Organelle Health Dictionary

The basic structural unit of all living organisms. The human body consists of billions of cells, structurally and functionally integrated to perform the complex tasks necessary for life. In spite of variation in size and function, most human cells have a similar basic structure. Each cell is an invisibly small bag containing liquid cytoplasm, surrounded by a cell membrane that regulates the passage of useful substances (such as oxygen and nutrients) into the cell; and waste materials (such as carbon dioxide) and manufactured substances (such as hormones) out of the cell. Some cells, such as those lining the small intestine, have microvilli, projections that increase the cells’ surface area to facilitate absorption.

All cells, except red blood cells, have a nucleus, a control centre that governs all major cell activities by regulating the amount and types of proteins made in the cell. Inside the nucleus are the chromosomes, which are made of the nucleic acid DNA. This contains the instructions for protein synthesis, which are carried into the cytoplasm by a type of RNA, another nucleic acid, and are decoded in particles called ribosomes. The nucleus also contains a spherical structure called the nucleolus, which plays a role in the production of ribosomes.

The cell also contains various organelles, each with a specific role.

Energy is generated from the breakdown of sugars and fatty acids by mitochondria.

Substances that would damage the cell if they came into contact with the cytoplasm are contained in particles called lysosomes and peroxisomes.

A system of membranes in the cytoplasm called the endoplasmic reticulum transports materials through the cell.

Flattened sacs called the Golgi complex receive and process proteins dispatched by the endoplasmic reticulum.

Products for export, such as enzymes and hormones, are secreted by vesicles at the cell surface.

Other materials, water, and waste products are transported and stored in the cytoplasm by vacuoles.

The cytoplasm itself has a network of fine tubes (microtubules) and filaments (microfilaments) known as the cytoskeleton, which gives the cell a definite shape.... cell

The study of all aspects of micro-organisms (microbes) – that is, organisms which individually are generally too small to be visible other than by microscopy. The term is applicable to viruses (see VIRUS), BACTERIA, and microscopic forms of fungi, algae, and PROTOZOA.

Among the smallest and simplest microorganisms are the viruses. First described as ?lterable agents, and ranging in size from 20–30 nm to 300 nm, they may be directly visualised only by electron microscopy. They consist of a core of deoxyribonucleic or ribonucleic acid (DNA or RNA) within a protective protein coat, or capsid, whose subunits confer a geometric symmetry. Thus viruses are usually cubical (icosahedral) or helical; the larger viruses (pox-, herpes-, myxo-viruses) may also have an outer envelope. Their minimal structure dictates that viruses are all obligate parasites, relying on living cells to provide essential components for their replication. Apart from animal and plant cells, viruses may infect and replicate in bacteria (bacteriophages) or fungi (mycophages), which are damaged in the process.

Bacteria are larger (0·01–5,000 µm) and more complex. They have a subcellular organisation which generally includes DNA and RNA, a cell membrane, organelles such as ribosomes, and a complex and chemically variable cell envelope – but, unlike EUKARYOTES, no nucleus. Rickettsiae, chlamydia, and mycoplasmas, once thought of as viruses because of their small size and absence of a cell wall (mycoplasma) or major wall component (chlamydia), are now acknowledged as bacteria; rickettsiae and chlamydia are intracellular parasites of medical importance. Bacteria may also possess additional surface structures, such as capsules and organs of locomotion (?agella) and attachment (?mbriae and stalks). Individual bacterial cells may be spheres (cocci); straight (bacilli), curved (vibrio), or ?exuous (spirilla) rods; or oval cells (coccobacilli). On examination by light microscopy, bacteria may be visible in characteristic con?gurations (as pairs of cocci [diplococci], or chains [streptococci], or clusters); actinomycete bacteria grow as ?laments with externally produced spores. Bacteria grow essentially by increasing in cell size and dividing by ?ssion, a process which in ideal laboratory conditions some bacteria may achieve about once every 20 minutes. Under natural conditions, growth is usually much slower.

Eukaryotic micro-organisms comprise fungi, algae, and protozoa. These organisms are larger, and they have in common a well-developed internal compartmentation into subcellular organelles; they also have a nucleus. Algae additionally have chloroplasts, which contain photosynthetic pigments; fungi lack chloroplasts; and protozoa lack both a cell wall and chloroplasts but may have a contractile vacuole to regulate water uptake and, in some, structures for capturing and ingesting food. Fungi grow either as discrete cells (yeasts), multiplying by budding, ?ssion, or conjugation, or as thin ?laments (hyphae) which bear spores, although some may show both morphological forms during their life-cycle. Algae and protozoa generally grow as individual cells or colonies of individuals and multiply by ?ssion.

Micro-organisms of medical importance include representatives of the ?ve major microbial groups that obtain their essential nutrients at the expense of their hosts. Many bacteria and most fungi, however, are saprophytes (see SAPROPHYTE), being major contributors to the natural cycling of carbon in the environment and to biodeterioration; others are of ecological and economic importance because of the diseases they cause in agricultural or horticultural crops or because of their bene?cial relationships with higher organisms. Additionally, they may be of industrial or biotechnological importance. Fungal diseases of humans tend to be most important in tropical environments and in immuno-compromised subjects.

Pathogenic (that is, disease-causing) microorganisms have special characteristics, or virulence factors, that enable them to colonise their hosts and overcome or evade physical, biochemical, and immunological host defences. For example, the presence of capsules, as in the bacteria that cause anthrax (Bacillus anthracis), one form of pneumonia (Streptococcus pneumoniae), scarlet fever (S. pyogenes), bacterial meningitis (Neisseria meningitidis, Haemophilus in?uenzae) is directly related to the ability to cause disease because of their antiphagocytic properties. Fimbriae are related to virulence, enabling tissue attachment – for example, in gonorrhoea (N. gonorrhoeae) and cholera (Vibrio cholerae). Many bacteria excrete extracellular virulence factors; these include enzymes and other agents that impair the host’s physiological and immunological functions. Some bacteria produce powerful toxins (excreted exotoxins or endogenous endotoxins), which may cause local tissue destruction and allow colonisation by the pathogen or whose speci?c action may explain the disease mechanism. In Staphylococcus aureus, exfoliative toxin produces the staphylococcal scalded-skin syndrome, TSS toxin-1 toxic-shock syndrome, and enterotoxin food poisoning. The pertussis exotoxin of Bordetella pertussis, the cause of whooping cough, blocks immunological defences and mediates attachment to tracheal cells, and the exotoxin produced by Corynebacterium diphtheriae causes local damage resulting in a pronounced exudate in the trachea.

Viruses cause disease by cellular destruction arising from their intracellular parasitic existence. Attachment to particular cells is often mediated by speci?c viral surface proteins; mechanisms for evading immunological defences include latency, change in viral antigenic structure, or incapacitation of the immune system – for example, destruction of CD 4 lymphocytes by the human immunode?ciency virus.... microbiology

Small organelles that are found inside cells, in which cell respiration takes place.

The mitochondrial wall consists of 2 membranes, and the inner one is highly folded to provide a surface for the respiration reactions.

Cells that use a lot of energy, such as muscle cells, contain many mitochondria.... mitochondria

n. a microscopic spherical membrane-enclosed vesicle or sac (20–30 nm in diameter) made artificially in the laboratory by the addition of an aqueous solution to a phospholipid gel. The membrane resembles a cell membrane and the whole vesicle is similar to a cell organelle. Liposomes can be incorporated into living cells and may be used to transport relatively toxic drugs into cancer cells, where they can exert their maximum effects. The cancerous organ is at a higher temperature than normal body temperature, so that when the liposome passes through its blood vessels the membrane melts and the drug (e.g. *doxorubicin) is released. Liposomes are also undergoing clinical trials as vehicles in *gene therapy for cystic fibrosis.... liposome