Chapter 1: Cell structure
additional structures sometimes present
structures always present
flagellum for locomotion; very simple structure
X
capsule additional protection infolding of cell surface membrane may form a photosynthetic membrane or carry out nitrogen fixation
Figure 1.29 Chloroplasts (× 16 000). Thylakoids (yellow) run through the stroma (dark green) and are stacked in places to form grana. Black circles among the thylakoids are lipid droplets. See also Figure 13.6, page 291. Chloroplast X is referred to in Question 1.2.
Two fundamentally different types of cell At one time it was common practice to try to classify all living organisms as either animals or plants. With advances in our knowledge of living things, it has become obvious that the living world is not that simple. Fungi and bacteria, for example, are very different from animals and plants, and from each other. Eventually it was discovered that there are two fundamentally different types of cell. The most obvious difference between these types is that one possesses a nucleus and the other does not. Organisms that lack nuclei are called prokaryotes (‘pro’ means before; ‘karyon’ means nucleus). They are, on average, about 1000 to 10 000 times smaller in volume than cells with nuclei, and are much simpler in structure – for example, their DNA lies free in the cytoplasm. Organisms whose cells possess nuclei are called eukaryotes (‘eu’ means true). Their DNA lies inside a nucleus. Eukaryotes include animals, plants, fungi and a group containing most of the unicellular eukaryotes known as protoctists. Most biologists believe that eukaryotes evolved from prokaryotes, 1500 million years after prokaryotes first appeared on Earth. We mainly study animals and plants in this book, but all eukaryotic cells have certain features in common. A generalised prokaryotic cell is shown in Figure 1.30. A comparison of prokaryotic and eukaryotic cells is given in Table 1.2.
plasmid small circle of DNA; several may be present pili for attachment to other cells or surfaces; involved in sexual reproduction
cell wall containing murein, a peptidoglycan cell surface membrane cytoplasm circular DNA sometimes referred to as a chromosome ribosomes
Figure 1.30 Diagram of a generalised bacterium showing the typical features of a prokaryotic cell.
QUESTION 1.6 List the structural features that prokaryotic and
eukaryotic cells have in common. Briefly explain why each of the structures you have listed is essential.
Viruses
In 1852, a Russian scientist discovered that certain diseases could be transmitted by agents that, unlike bacteria, could pass through the finest filters. This was the first evidence for the existence of viruses, tiny ‘organisms’ which are much smaller than bacteria and are on the boundary between what we think of as living and non-living. Unlike prokaryotes and eukaryotes, viruses do not have a cell structure. In other words, they are not surrounded by a partially permeable membrane containing cytoplasm with ribosomes. They are much simpler in structure. Most consist only of: ■■
■■
a self-replicating molecule of DNA or RNA which acts as its genetic code a protective coat of protein molecules.
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