WJEC Biology for AS Level Student Book: 2nd Edition

Key terms Prokaryote: A single-celled organism lacking membrane-bound organelles, such as a nucleus, with its DNA free in the cytoplasm. Eukaryote: An organism containing cells that have membrane- bound organelles, with DNA in chromosomes within the nucleus. Link Archaea are described on p141. Link The classification of living organisms is described on p137. Exam tip Make sure you can draw and label a diagram of a simple prokaryotic cell. Prokaryotic cells, eukaryotic cells and viruses The cells of prokaryotes may resemble the first living cells. The oldest fossil prokaryotes are from rocks formed 3.5 billion years ago, so they must have evolved before then, within the first billion years of Earth’s history. The cells of eukaryotes probably evolved from prokaryotic cells and the oldest fossilised examples are from rocks about 2.1 billion years old. Fungi, Protoctista, plants and animals all have eukaryotic cells. Viruses are not made of cells and are not classified with living organisms. They seem to exist at the interface between living and non-living systems. Prokaryotic cells Examples of prokaryotic cells are bacteria and Archaea. The major distinguishing feature of prokaryotic cells is that they have no nucleus, or any internal membranes, so, unlike eukaryotic cells, they have no membrane-bound organelles. In some prokaryotes, infolding of the cell membrane in a mesosome or photosynthetic lamellae increases the membrane’s surface area. Prokaryotes rarely form multicellular structures and are often described as ‘unicellular’. Their cells are not subdivided, so they are sometimes described as ‘acellular’. The table summarises their appearance. All prokaryotes Some prokaryotes DNA molecule loose in cytoplasm Slime coat Peptidoglycan (murein) cell wall Flagella (one, some or many) 70S ribosomes Photosynthetic lamellae holding photosynthetic pigments Cytoplasm Mesosome – possible site of aerobic respiration Cell membrane Plasmids The table below summarises the differences between eukaryotic and prokaryotic cells. Prokaryotes Eukaryotes Length small: 1–10 µm larger: 10–100 µm Organelles none membrane-bound DNA free in cytoplasm combined with protein in chromosomes Nuclear envelope none double membrane Plasmids may be present absent Cell wall peptidoglycan (murein) cellulose in plants; chitin in fungi Chloroplasts none, but may use photosynthetic lamellae for photosynthesis in plants and some Protoctista Mitochondria none, but may use mesosome for aerobic respiration present Mesosome present in some absent Ribosomes 70S; free in cytoplasm 80S; free in cytoplasm or attached to ER Viruses Viruses are so small that they cannot be seen in the light microscope. They pass through filters that can trap bacteria and although experiments in the late nineteenth century suggested their existence, they could not be seen until the electron microscope was invented. Diagram of generalised bacterial cell Fig 30 cell membrane cytoplasm plasmid agellum cell wall slime coat in some bacteria ribosome mesosome DNA WJEC Biology for AS Level 40