Biological Science (1995) PDF

Chapter Two Variety of life — prokaryotes, viruses and fungi Chapter 2 is concerned with three groups of organisms: 2.1 Prokaryotes compared with bacteria (prokaryotes), viruses and fungi. Although, as eukaryotes will become clear...

Chapter Two Variety of life — prokaryotes, viruses and fungi Chapter 2 is concerned with three groups of organisms: 2.1 Prokaryotes compared with bacteria (prokaryotes), viruses and fungi. Although, as eukaryotes will become clear, they are placed in three very different groups taxonomically, they are often studied together in All cellular organisms so far studied fall the branch of biology known as microbiology. This is naturally into one of two major groups, the prokaryotes because the techniques used in their study are similar in and eukaryotes. Prokaryotes appeared about 3 500 million many respects, involving particularly microscopy and years ago and comprise a variety of organisms collectively sterile (aseptic) procedures. They are of tremendous known as bacteria. Eukaryotes include protoctists, fungi, ecological and economic importance, and are intimately green plants and animals. Eukaryotes appeared first in the associated with the developing areas of biotechnology and late Pre-Cambrian period, about 2000 million years ago, genetic engineering. and probably evolved from prokaryotes. The cells of prokaryotes (pro, before; karyon, nucleus) lack true nuclei. In other words, their genetic material (DNA) is not enclosed by nuclear membranes, and lies free in the cytoplasm. The cells of eukaryotes (eu, true) are Table 2.1 Major differences between prokaryotes and eukaryotes. Feature Prokaryote Eukaryote Cell size Average diameter 0.5—Sum. Up to 40 um diameter common; commonly 1 000-10 000 times volume of prokaryotic cells. Form Unicellular or filamentous. Unicellular, filamentous or truly multicellular. Genetic material Circular DNA lying naked in the cytoplasm. No true Linear DNA associated with proteins and RNA to nucleus or chromosomes. No nucleolus. form chromosomes within a nucleus. Nucleolus in nucleus. Protein synthesis 70S ribosomes (smaller). No endoplasmic reticulum 80S ribosomes (larger). involved. (Many other details of protein synthesis Ribosomes may be attached to endoplasmic reticulum. differ, including susceptibility to antibiotics, e.g. prokaryotes inhibited by streptomycin.) Organelles Few organelles. Many organelles. None are surrounded by an envelope (2 membranes). Envelope-bound organelles present, e.g. nucleus, mitochondria, chloroplasts. Internal membranes scarce; if present usually Great diversity of organelles bounded by single associated with respiration or photosynthesis. membranes, e.g. Golgi apparatus, lysosomes, vacuoles, microbodies, endoplasmic reticulum. Cell walls Rigid and contain polysaccharides with amino acids. Cell walls of green plants and fungi rigid and contain Murein is main strengthening compound. polysaccharides. Cellulose is main strengthening compound of plant walls, chitin of fungal walls. Flagella Simple, lacking microtubules. Extracellular (not Complex, with ‘9+2’ arrangement of microtubules. enclosed by cell surface membrane). Intracellular (surrounded by cell surface membrane). 20 nm diameter. 200 nm diameter. Respiration Mesosomes in bacteria, except cytoplasmic Mitochondria for aerobic respiration. membranes in blue-green bacteria. Photosynthesis No chloroplasts. Takes place on membranes which Chloroplasts containing membranes which are usually show no stacking. stacked into lamellae or grana. Nitrogen fixation Some have the ability. None have the ability. eee MMW WYsIix