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/CELLS, TISSUES AND SYSTEMS/CELL STRUCTURE

Section 1 – CELLS, TISSUES AND SYSTEMS

CHAPTER 1 – Basic structure and function of cells

CELL STRUCTURE

GENERAL CHARACTERISTICS OF CELLS

The shapes of mammalian cells vary widely depending on their interactions with each other, their extracellular environment and internal structures. Their surfaces are often highly folded when absorptive or transport functions take place across their boundaries. Cell size is limited by rates of diffusion, either that of material entering or leaving cells, or of diffusion within them. Movement of macromolecules can be much accelerated and also directed by processes of active transport across membranes and by transport mechanisms within the cell. According to the location of absorptive or transport functions, apical microvilli (Fig. 1.1) or basolateral infoldings create a large surface area for transport or diffusion.

  

Fig. 1.1  The main structural components and internal organization of a generalized cell.

Motility is a characteristic of most cells, in the form of movements of cytoplasm or specific organelles from one part of the cell to another. It also includes: the extension of parts of the cell surface such as pseudopodia, lamellipodia, filopodia and microvilli; locomotion of entire cells as in the amoeboid migration of tissue macrophages; the beating of flagella or cilia to move the cell (e.g. in spermatozoa) or fluids overlying it (e.g. in respiratory epithelium); cell division and muscle contraction. Cell movements are also involved in the uptake of materials from their environment (endocytosis, phagocytosis) and the passage of large molecular complexes out of cells (exocytosis, secretion).

Cells rarely operate independently of each other and commonly form aggregates by adhesion, often assisted by specialized intercellular junctions. They may also communicate with each other either by generating and detecting molecular signals that diffuse across intercellular spaces, or more rapidly by membrane contact, which may involve small, transient, transmembrane channels or interactions between membrane-bound signalling molecules. Cohesive groups of cells constitute tissues and more complex assemblies of tissues form functional systems or organs.

Most cells are between 5–50 μm in diameter: e.g. resting lymphocytes are 6 μm across, red blood cells 7.5 μm and columnar epithelial cells are 20 μm tall and 10 μm wide (all measurements are approximate). Some cells are much larger than this: e.g. megakaryocytes of the bone marrow are more than 200 μm in diameter. Large neurones and skeletal muscle cells have relatively enormous volumes because of their extended shapes, some of the former being over 1 metre in length.

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