start biology
the soil, and are either aerobes or facultative anaerobes . Bacillus subtilis is a strict aerobe which is used in industry as a source of enzymes such as amylases. Bacillus thuringiensis has become important in genetic engineering because it causes a paralytic disease in many cater-pillars and has been used to produce insect-resistant plants . However, some species of Bacillus are pathogenic, such as Bacillus anthracis which causes the disease anthrax.
Examples: Anabaena cylindrica, Nostoc muscorum, Spirulina platensis Cyanobacteria are prokaryotic microorganisms similar to the true bacteria. They are photo-synthetic but are not true algae because they do not have membrane-bound nuclei, and are consid-ered to be very ancient life-forms. They have been
Structure of cyanobacteria
Blue-greens have a typical prokaryotic cell structure since they have a naked coil of DNA and no true nucleus . The cell wall is similar in structure and composition to that of Gram-negative bacteria. Protein synthesis takes place on 70S ribosomes in the cytoplasm. Blue-greens are photosynthetic. They have chlorophyll and carotenoid pigments incorporated into infoldings of the plasma membrane, called lamellae. They also have photosynthetic pigments, such as phycocyanin and phycoerythrin, which are present in phycobilisomes. These give the cells their distinctive colouration. The cells may occur singly or in colonies, but members of a colony remain independent.
Nitrogen fixation
Only a very few organisms are capable of fixing atmospheric nitrogen by reducing it to ammonia and combining it with organic acids to produce amino acids and proteins. Nitrogen-fixing bacteria can do this and so can some blue-greens. Cells able to fix nitrogen contain the enzyme nitrogen-ase. This enzyme is inactivated by oxygen and so conditions inside the nitrogen-fixing cell have to be anaerobic. Some blue-greens, such as Anabaena, have special thick-walled cells called heterocysts.
(1.8)
Anabaena has filaments made up of many normal photosynthetic cells that produce sugars and oxygen. Scattered along the filaments are a few distinct heterocyst cells that are able to fix atmospheric nitrogen in this way (figure 1.8). Many filamentous blue-greens are also able to produce akinetes, or spores. These are able to survive adverse conditions, such as a period of overpopulation known as an algal 'bloom', and seem to develop from a vegetative cell near to a heterocyst. The cell increases in size and accumu-lates large food reserves. Photosynthesis within the aldnete is reduced and gas vacuoles disappear. This means that the akinete slowly sinks to the bottom of the water. It may survive for several years, and will germinate as soon as conditions become favourable.
The economic importance of blue-greens
Spirulina platensis is a filamentous blue-green found naturally in shallow alkaline lakes in parts of Africa and South America. For thousands of years it has been collected and dried by the local people and used as a food. It is often fried or put in soups and sauces; it is also used as cattle food. Nostoc is another blue-green which is used as a food in Peru and in South-East Asia. In agriculture, nitrogen-fixing blue-greens may be used as organic fertilisers. They are grown on a large scale in China, India, Indonesia and the Philippines, particularly where rice is cultivated in paddy fields. The water may be seeded with a starter culture of blue-greens at the beginning of the growing season. This method has been shown to increase the yield of rice by 15-20%. Research is taking place into the use of blue-greens in a solar energy conversion system. As you have just learnt, Anabaena cylindrica has hetero-cysts to fix nitrogen and is also able to give off oxygen by photosynthesis in the vegetative cells. In the absence of atmospheric nitrogen it gives off hydrogen by nitrogenase-catalysed electron trans-fer to H+ ions in the heterocysts. Both oxygen and hydrogen are in demand industrially.
SAQ 1.4 It is thought that blue-greens may have been the first photosynthetic organisms on Earth and that they represent a very early stage in the evolution of life. Give as many reasons as you can why this might be so.
Kingdom Protoctista
This kingdom has been created to contain all groups of eukaryotic organisms which are neither animals, plants, fungi nor prokaryotes. These groups are not really related though they do have some similarities. They include all protozoa, all nucleated algae and the slime moulds (figure 1.9 and table 1.4). The protozoa is a collective term for the phyla Rhizopoda, Zoomastigina, Apicomplexa and Ciliophora. They are found wherever moisture is present, in sea water, fresh water and soil. There are commensal, symbiotic and parasitic species in addition to many free-living types. Protozoa are eukaryotic. The nucleus has a nuclear envelope, and movement is by means of a variety of loco-motory structures such as flagella, cilia or pseudopodia. Since the cytoplasm of freshwater protozoans is usually hypertonic to (more concen-trated than) the aqueous environment, they take in water by osmosis. To counteract this, they have contractile vacuoles that act as pumps to remove excess water from the cytoplasm. However, contractile vacuoles may also be found in some marine protozoans. All types of nutrition are found in protozoans: some are autotrophic, others are saprotrophic and many are heterotrophic. Digestion of food takes place in food vacuoles in the cytoplasm. Gas exchange is by diffusion across the plasma membrane. Waste products from cell metabolism diffuse out of the cell. The main nitrogenous waste is ammonia.
The economic importance of protozoa
Many ciliates are saprotrophs and are vital in the recycling of organic wastes, particularly in sewage treatment. Parasitic forms, such as Entamoeba, which causes amoebic dysentery, and Plasmodium, which causes malaria, may cause loss of life.
Malaria, for example, can be devastating to the economy of developing countries, incapacitating millions of workers every year . List three features shared by the four phyla classed as protozoa, and two features which distinguish them from each other.
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Kingdom Fungi
Fungi are obligate aerobes, which are killed in the absence of oxygen, or facultative anaerobes, which grow best when oxygen is available but are able to survive anaerobic conditions. They are found almost everywhere. They are eukaryotic organisms with a cell wall that is usually made of chitin. Fungi are not photosynthetic but heterotrophic, deriving their nutrients by absorption of organic materials. Fungi are usually filamentous or thread-like. The individual threads are called hyphae which are usually multinucleate; these branch profusely, often fusing together to form a tangled mass of branched hyphae called the mycelium. Individual hyphae are surrounded by rigid cell walls and
grow only at their tips. It is this form of apical growth that separates fungi from almost all other organisms, even filamentous ones. Since fungi are eukaryotic, they have distinct nuclei each surrounded by a nuclear envelope with pores; they have chromosomes and a spindle that appears during nuclear division. Mitochondria are found in the cytoplasm and there is extensive endoplasmic reticulum (ER) ; ribosomes are found both free in the cytoplasm and attached to the ER. These ribo-somes are the larger (80S) type which is typical of eukaryotic cells. The cytoplasm also contains numerous vacuoles containing storage materials such as starch, lipid globules and volutin.
(1.4)
The cytoplasm and organelles are surrounded by a selectively permeable, phospholipid unit mem-brane (plasma membrane). The cytoplasm is at its most dense at the tips of the hyphae. The older parts of the mycelium are often metabolically inactive with large vacuoles in the cytoplasm. In septate species there are pores in the cross-walls to allow substances in solution, and also structures such as nuclei, to move freely from one section to another. The growth of hyphae is very rapid under favourable conditions. Each hypha grows at the tip and branches repeatedly along its length to reach new food supplies and grow away from its own waste products
Phylum Zygornycota (zygomycetes) Example: Mucor Mynahs In zygomycetes the hyphae are non-septate and make a large, well-developed branching mycelium. Asexual reproduction is by non-motile spores formed in a sporangium borne on a sporangiophore . Sexual reproduction is shown in figure .
Phylum Ascomycota (ascomycetes)
Examples: Peracillium noratum, Saccharomyces cerevisiae, Aspergillus urger, Neurospora spp. All ascomycetes have septate hyphae. In asexual reproduction, non-motile spores, called conidia, are formed on special hyphal branches called conidiophores. Sexual reproduction is complex. The male gamete is in a struc-ture called the antheridium and the female gamete is in a structure called the asoogonium. If the antherid-ium and ascogonium contact each other, the cell walls break down and the two nuclei fuse. A diploid ascus is formed. Meiosis occurs in the ascus, followed by mitosis, to release eight haploid ascospores. These act as resting cells and
later germinate to produce a new generation. Yeasts, such as Saccharomyces cerevisiae, are unicellular ascomycetes and are industrially important fungi. They have been used for centuries in the making of bread, the brewing of beer and in winemaking. They are uni-cellular organisms, differing from other fungi in that the cell wall is composed largely of polymers of mannan and glucan . Sexual reproduction in yeast cells is by fusion .
Phylum Basidiomycota (basidiomycetes)
Examples: Agaricus campestris, Puccinia graminis All basidiomycetes have septate hyphae. Asexual repro-duction is usually absent. Sexual reproduction is usually by the fusion of vegetative hyphae from two mating types. The nuclei of the two hyphae do not fuse, and the fungus exists for most of its life as a dikaryon, with two types of nucleus in its hyphae . Later a complex fruiting body is formed. This basidiocarp contains many basidia in which the nuclei fuse and meiosis takes place. Four haploid basidiospores are produced from each basid-ium to complete the life cycle. The mushroom Agaricus is cul-tivated for food on a large scale. There are many other fungi which are nutritious and edible although some are notoriously poisonous.
Fungal spores
At some stage in all fungal life cycles, spores are produced .................
Anabaena has filaments made up of many normal photosynthetic cells that produce sugars and oxygen. Scattered along the filaments are a few distinct heterocyst cells that are able to fix atmospheric nitrogen in this way (figure 1.8). Many filamentous blue-greens are also able to produce akinetes, or spores. These are able to survive adverse conditions, such as a period of overpopulation known as an algal 'bloom', and seem to develop from a vegetative cell near to a heterocyst. The cell increases in size and accumu-lates large food reserves. Photosynthesis within the aldnete is reduced and gas vacuoles disappear. This means that the akinete slowly sinks to the bottom of the water. It may survive for several years, and will germinate as soon as conditions become favourable.
The economic importance of blue-greens
Spirulina platensis is a filamentous blue-green found naturally in shallow alkaline lakes in parts of Africa and South America. For thousands of years it has been collected and dried by the local people and used as a food. It is often fried or put in soups and sauces; it is also used as cattle food. Nostoc is another blue-green which is used as a food in Peru and in South-East Asia. In agriculture, nitrogen-fixing blue-greens may be used as organic fertilisers. They are grown on a large scale in China, India, Indonesia and the Philippines, particularly where rice is cultivated in paddy fields. The water may be seeded with a starter culture of blue-greens at the beginning of the growing season. This method has been shown to increase the yield of rice by 15-20%. Research is taking place into the use of blue-greens in a solar energy conversion system. As you have just learnt, Anabaena cylindrica has hetero-cysts to fix nitrogen and is also able to give off oxygen by photosynthesis in the vegetative cells. In the absence of atmospheric nitrogen it gives off hydrogen by nitrogenase-catalysed electron trans-fer to H+ ions in the heterocysts. Both oxygen and hydrogen are in demand industrially.
SAQ 1.4 It is thought that blue-greens may have been the first photosynthetic organisms on Earth and that they represent a very early stage in the evolution of life. Give as many reasons as you can why this might be so.
Kingdom Protoctista
This kingdom has been created to contain all groups of eukaryotic organisms which are neither animals, plants, fungi nor prokaryotes. These groups are not really related though they do have some similarities. They include all protozoa, all nucleated algae and the slime moulds (figure 1.9 and table 1.4). The protozoa is a collective term for the phyla Rhizopoda, Zoomastigina, Apicomplexa and Ciliophora. They are found wherever moisture is present, in sea water, fresh water and soil. There are commensal, symbiotic and parasitic species in addition to many free-living types. Protozoa are eukaryotic. The nucleus has a nuclear envelope, and movement is by means of a variety of loco-motory structures such as flagella, cilia or pseudopodia. Since the cytoplasm of freshwater protozoans is usually hypertonic to (more concen-trated than) the aqueous environment, they take in water by osmosis. To counteract this, they have contractile vacuoles that act as pumps to remove excess water from the cytoplasm. However, contractile vacuoles may also be found in some marine protozoans. All types of nutrition are found in protozoans: some are autotrophic, others are saprotrophic and many are heterotrophic. Digestion of food takes place in food vacuoles in the cytoplasm. Gas exchange is by diffusion across the plasma membrane. Waste products from cell metabolism diffuse out of the cell. The main nitrogenous waste is ammonia.
The economic importance of protozoa
Many ciliates are saprotrophs and are vital in the recycling of organic wastes, particularly in sewage treatment. Parasitic forms, such as Entamoeba, which causes amoebic dysentery, and Plasmodium, which causes malaria, may cause loss of life.
Kingdom Fungi
Fungi are obligate aerobes, which are killed in the absence of oxygen, or facultative anaerobes, which grow best when oxygen is available but are able to survive anaerobic conditions. They are found almost everywhere. They are eukaryotic organisms with a cell wall that is usually made of chitin. Fungi are not photosynthetic but heterotrophic, deriving their nutrients by absorption of organic materials. Fungi are usually filamentous or thread-like. The individual threads are called hyphae which are usually multinucleate; these branch profusely, often fusing together to form a tangled mass of branched hyphae called the mycelium. Individual hyphae are surrounded by rigid cell walls and
grow only at their tips. It is this form of apical growth that separates fungi from almost all other organisms, even filamentous ones. Since fungi are eukaryotic, they have distinct nuclei each surrounded by a nuclear envelope with pores; they have chromosomes and a spindle that appears during nuclear division. Mitochondria are found in the cytoplasm and there is extensive endoplasmic reticulum (ER) ; ribosomes are found both free in the cytoplasm and attached to the ER. These ribo-somes are the larger (80S) type which is typical of eukaryotic cells. The cytoplasm also contains numerous vacuoles containing storage materials such as starch, lipid globules and volutin.
The cytoplasm and organelles are surrounded by a selectively permeable, phospholipid unit mem-brane (plasma membrane). The cytoplasm is at its most dense at the tips of the hyphae. The older parts of the mycelium are often metabolically inactive with large vacuoles in the cytoplasm. In septate species there are pores in the cross-walls to allow substances in solution, and also structures such as nuclei, to move freely from one section to another. The growth of hyphae is very rapid under favourable conditions. Each hypha grows at the tip and branches repeatedly along its length to reach new food supplies and grow away from its own waste products
Phylum Zygornycota (zygomycetes) Example: Mucor Mynahs In zygomycetes the hyphae are non-septate and make a large, well-developed branching mycelium. Asexual reproduction is by non-motile spores formed in a sporangium borne on a sporangiophore . Sexual reproduction is shown in figure .
Phylum Ascomycota (ascomycetes)
Examples: Peracillium noratum, Saccharomyces cerevisiae, Aspergillus urger, Neurospora spp. All ascomycetes have septate hyphae. In asexual reproduction, non-motile spores, called conidia, are formed on special hyphal branches called conidiophores. Sexual reproduction is complex. The male gamete is in a struc-ture called the antheridium and the female gamete is in a structure called the asoogonium. If the antherid-ium and ascogonium contact each other, the cell walls break down and the two nuclei fuse. A diploid ascus is formed. Meiosis occurs in the ascus, followed by mitosis, to release eight haploid ascospores. These act as resting cells and
later germinate to produce a new generation. Yeasts, such as Saccharomyces cerevisiae, are unicellular ascomycetes and are industrially important fungi. They have been used for centuries in the making of bread, the brewing of beer and in winemaking. They are uni-cellular organisms, differing from other fungi in that the cell wall is composed largely of polymers of mannan and glucan . Sexual reproduction in yeast cells is by fusion .
Phylum Basidiomycota (basidiomycetes)
Examples: Agaricus campestris, Puccinia graminis All basidiomycetes have septate hyphae. Asexual repro-duction is usually absent. Sexual reproduction is usually by the fusion of vegetative hyphae from two mating types. The nuclei of the two hyphae do not fuse, and the fungus exists for most of its life as a dikaryon, with two types of nucleus in its hyphae . Later a complex fruiting body is formed. This basidiocarp contains many basidia in which the nuclei fuse and meiosis takes place. Four haploid basidiospores are produced from each basid-ium to complete the life cycle. The mushroom Agaricus is cul-tivated for food on a large scale. There are many other fungi which are nutritious and edible although some are notoriously poisonous.
Fungal spores
At some stage in all fungal life cycles, spores are produced .................
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