1.6 Prokaryotes vs Eukaryotes

Two fundamentally different types of cell?Prokaryotes vs Eukaryotes

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.

Read More:?1.4-Ultrastructure of an animal cell diagram.

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 topic, but all eukaryotic cells have certain features in common.

A generalised prokaryotic cell is shown in Figure. A comparison of prokaryotic and eukaryotic cells is given below.

Read More:1.2-Cell biology and Light microscopy of Living Cell

Figure 1.30 Diagram of a generalised bacterium showing the typical features of a prokaryotic cell.

Queston: List the structural features that prokaryotic and eukaryotic cells have in common. Briefly explain why each of the structures you have listed is essential.

Prokaryotes vs?Eukaryotes

• average diameter of cell is 0.5–5 μm
• cells commonly up to 40 μm diameter and commonly 1000–10 000 times the volume of prokaryotic cells

• DNA is circular and lies free in the cytoplasm
• DNA is not circular and is contained in a nucleus – the nucleus is surrounded by an envelope of two membranes

• DNA is naked
• DNA is associated with protein, forming structures called chromosomes

• slightly smaller (70S) ribosomes (about 20 nm diameter) than those of eukaryotes
• slightly larger (80S) ribosomes (about 25 nm diameter) than those of prokaryotes

• no ER present
• ER present, to which ribosomes may be attached

• very few cell organelles – no separate membrane-bound compartments unless formed by infolding of the cell surface membrane
• many types of cell organelle present (extensive compartmentalisation and division of labour):

■ some organelles are bounded by a single membrane, e.g. lysosomes, Golgi body, vacuoles■ some are bounded by two membranes (an envelope), e.g. nucleus, mitochondrion, chloroplast■ some have no membrane, e.g. ribosomes, centrioles, microtubules

• cell wall present – wall contains murein, a peptidoglycan (a polysaccharide combined with amino acids)
• cell wall sometimes present, e.g. in plants and fungi – contains cellulose or lignin in plants, and chitin (a nitrogen-containing polysaccharide similar to cellulose) in fungi

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.

Figure shows the structure of a simple virus. It has a very symmetrical shape. Its protein coat (or capsid) is made up of separate protein molecules, each of which is called a capsomere.

Viruses range in size from about 20–300nm (about 50 times smaller on average than bacteria).

All viruses are parasitic because they can only reproduce by infecting and taking over living cells. The virus DNA or RNA takes over the protein synthesising machinery of the host cell, which then helps to make new virus particles.

Read More:?Use of electron microscope - Sohailunny7

Figure? The structure of a simple virus.

Read full chapter:Cell structure

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