From unicellular microorganisms to all higher life forms, all life on Earth is cellular (Fig. 4.1). Prokaryotes are single-celled organisms. The cells are simple and usually devoid of any internal membrane structures (except for the photo-synthetic cyanobacteria, which possess intensive photosynthetic membranes). The cells are spherical, ellipsoid, elongated, or rod-shaped (see Chapter 5) and of sizes varying from 0.1 |im to 600 |im, depending on the species.
All eukaryote cells have intensive internal structure. Cell organelles include
• the nucleus (contains the chromosomes or the genomic DNA);
• mitochondria (energy-producing organelles);
• the endoplasmic reticulum (internal membranes),
• the Golgi apparatus (membranous vesicles for protein processing and transport),
• lysosomes and peroxisomes (for containment of degradative enzymes and wastes); and
• chloroplasts (photosynthetic organelles; in plant cells only).
The cells of all eukaryotes are very similar to each other, except that the plant cells have thick and rigid cell walls and contain numerous chloroplasts (or other types of plastids). Typical sizes vary from 10 |im to several hundred micrometers, but some large cell types occur (of a size up to more than 1 m for some nerve cells).
Multicellular organisms may be conceived as tightly bound colonies ofindividual cells. The cells are entities contained within the cell membrane, which separates the cell from its environment but also allows sophisticated communication with the environment. The communication includes the intake of water and necessary nutrients, excretion of waste products, receiving of chemical signals via different receptor molecules, and sending off signals via secretion of different molecules. Thus, the cells dynamically communicate with their environment. All communication through the membrane is mediated via specific proteinaceous channels and structures embedded in the membrane, because the membrane itself is a hydro-phobic barrier and therefore impermeable to water-soluble molecules.
Inside the cells, life is a chemical process where multiple biochemical reactions take place in aqueous solution. Water functions as the solvent and carrier for ionic and polar molecules, and also is a reacting component in many reactions. Water also serves as the exclusion medium to allow the lipid molecules to assemble and form membranes, and as the medium that interacts with proteins, helping them to fold into their three-dimensional forms. Therefore, water provides a suitable environment for the different cellular reactions and for the assembly of important cellular structures. It provides turgor (internal pressure) and strength to the cells. Water is such an essential environment and requirement for all the life functions that it seems likely that the origin oflife had to happen in an aqueous environment (see Chapter 1).
Proteins, or gene products, have a multitude of essential roles in the cells. They form several different structures, such as the cytoskeletons in eukaryotes, components of cell walls, and the porous structures in cell membranes. They regulate reactions such as gene expression and genome replication, and they function in signal transduction by interacting with different target molecules. Still another area where proteins are essential for life is in the (regulated) catalysis of all the reactions going on inside the cells. Most cellular molecules are both synthesized and degraded via multi-step reaction pathways - and none of these reactions would happen, at least at any desired rate, without the assistance of proteinaceous enzymes. Even the energy conversion and -releasing reactions have to be mediated via enzymatic catalysis so that they happen in a controlled and regulated
At adequate substrate level, the maximum reaction rate is determined by enzyme saturation
Fig. 4.2 Chemical reaction rates in dependence of substrate concentration, with and without enzyme catalysis. Enzymes speed up reaction rates very strongly as compared to non-catalyzed reactions and make them happen efficiently even at very low substrate concentrations. The maximum rate of reaction is reached when the enzyme becomes saturated with the substrate molecules.
Reaction rate without enzyme catalysis
Reaction rate with enzyme catalysis
fashion. The enzymes themselves do not participate in the reactions but associate with the reacting molecules and hold them in such a conformation that the reaction can proceed. Enzymatic catalysis is capable of raising reaction rates by many orders of magnitude, from nearly zero to very high, and in practice most biological reactions would not happen in physiological conditions without enzyme catalysis (Fig. 4.2).
Proteins form the tools and the structures in the cells, and genetic information, encoded in the genomic deoxyribose nucleic acid (DNA), specifies how these tools and structures are made. Life depends on the function of the proteins and thus on the genetic information coding for them. Proteins are composed of long chains of different amino acids, and the sequence of the amino acids in these chains is determined by the genes coding for them. From the DNA, the protein-coding sequences are first copied into ribose nucleic acid (RNA) and from this are converted into amino acid sequences in peptide chains. RNA is a polymer molecule very similar to DNA; it is composed of similar nucleotide components, with only the small difference of an OH group, rather than H, being connected to the carbon in position 2 (2') in the pentose sugar ring (thus, the sugar is ribose instead of deoxyribose) (Fig. 4.3). However, this OH group is very reactive and, differing from the DNA, makes the RNA polymers very fragile, unstable, and short-lived.
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