Multicellular organisms, cells

Cell death is a necessary event in the life of a multicellular organism. Cells predominantly die via... 

Cells are the fundamental units of life. They are functional entities, each of which is enclosed in a semipermeable membrane that varies in composition and function both over a single cell surface and between different cell types. There are two basic forms of cell prokaryotic and eukaryotic. Prokaryotes are most noted for their small sizes and relatively simple structures. Presumably because of these traits, in addition to their remarkably rapid reproduction rates and biochemical diversity, various prokaryotic species occupy virtually every ecological niche in the biosphere. In contrast, the most conspicuous feature of the eukaryotes is their extraordinarily complex internal structure. Because eukaryotes carry out their various metabolic functions in a variety of membrane-bound organelles, they are capable of a more sophisticated intracellular metabolism. The diverse metabolic regulatory mechanisms made possible by this complexity promote two important lifestyle features required by multicellular organisms cell specialization and intercellular cooperation. Consequently, it is not surprising that the majority of eukaryotes are multicellular organisms composed of numerous types of specialized cells. 

This section will focus on cell-substratum adhesion, although in multicellular organisms, cell-cell adhesion may be an important process for cell motility in some circumstances [60, 243]. For Dictyostelium, the major contacts appear to be broad close contacts with the substratum together with dynamic eupodia that enable relatively rapid motility. This is consistent with the actomyosin system structure in Dictyostelium—a... 

Cell culture The in vitro growth of cells isolated from multicellular organisms. These cells are usually of one type. 

The abundance of many protein kinases in cells is an indication of the great importance of protein phosphorylation in cellular regulation. Exactly 113 protein kinase genes have been recognized in yeast, and it is estimated that the human genome encodes more than 1000 different protein kinases. Tyrosine kinases (protein kinases that phosphorylate Tyr residues) occur only in multicellular organisms (yeast has no tyrosine kinases). Tyrosine kinases are components of signaling pathways involved in cell-cell communication (see Chapter 34). 

This process occurs in the mitochondria, organelles present in the cells of all multicellular organisms (see Figs 6.8 and 6.26). Mitochondria have two membranes. The invaginations of the internal membrane into the inner space of the organelle (matrix space) are termed crests (from the Latin, cristae). 

In this section, two models of development were presented, a complex model consisting of a multioperon genome and a cytoplasm, and a simple model based on random Boolean networks. The simpler model was explained in more detail, as it is the basis for the extended example described here. This model utilizes both development and evolution to get to a cell that can develop into a multicellular organism able to seek a chemical trace. 

Deliberate degradation of biopolymers, even of cells, is very developed in multicellular organisms. We have already stressed the need for it externally in the constant remodelling of organisms during growth. There is also the obvious need for... 

Most multicellular organisms exploit mechanoreceptors — specialized cells that detect external mechanical forces — to help construct their internal view of the external world. The senses of hearing, balance and touch all rely on mechanoreceptors, as do the proprioceptive sensations that tell an organism how it is situated in the environment. Mechanoreception is probably one of the most ancient of the senses. 

Multicellular organisms are especially constrained to allow cell division just at that time and place where it is necessary to build up new or enhance present functional structures during growth and differentiation. 

For a complex multicellular organism to function, cells must rapidly respond to messages from other regions of the organism. These messages, often in the form of hormones, arrive at the cell membrane through the aqueous environ-... 

Cells in multicellular organisms communicate with each other in order to harmonize their individual actions. These signal transduction pathways control... 

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