Biology Chapter 1 (Sub 2) PDF

There are more than ten million species of organisms alive on Earth today, and many more that once lived on Earth but are now extinct. In order to make sense of this enormous variety biologists cl...

There are more than ten million species of organisms alive on Earth today, and many more that once lived on Earth but are now extinct. In order to make sense of this enormous variety biologists classify organisms, putting them into groups. Members of each group are related - they are descended from a common ancestor by the process of evolution (see Chapter 19). This common ancestry is reflected in the similarities of structure and function of the members of a group. The five major groups of living organisms are plants, animals, fungi, protoctists and bacteria, PLANTS You will be familiar with flowering plants, such as those shown in Figure 2.1. This group, or kingdom, also contains simpler plants, such as mosses and ferns. All plants are multicellular, which means that their ‘bodies’ are made up of many cells. Their main distinguishing feature is that their cells contain chloroplasts and they carry out photosynthesis - the process that uses light energy o convert simple inorganic molecules such as water and carbon dioxide into complex organic compounds (see Chapter 10). One of these organic compounds is the carbohydrate cellulose, and all plants have cell A Figure 2.1 (a) A pea plant, Its leaves and stem walls made of this material. cells contain chloroplasts, giving them their Plants can make many other organic compounds as a result of photosynthesis. green colour, The white flowers are pollinated by insects. (b) Maize plants are pollinated One of the first to be made is the storage carbohydrate starch, which is often by wind. These are the male flowers, which found inside plant cells. Another is the sugar sucrose, which is transported make the pollen. (c) The female maize around the plant and is sometimes stored in fruits and other plant organs. The flowers produce seeds after pollination, structure and function of flowering plants is dealt with in Unit 3 of this book.  ANIMA You will be even more familiar with this kingdom, since it contains the species Homo sapiens, i.e. humans! The variety of the animal kingdom is also enormous, including organisms such as sponges, molluscs, worms, starfish, insects and crustaceans, through to larger animals such as fish, amphibians, reptiles, birds and mammals (Figure 2.2). The last five groups are all vertebrates, which means that they have a vertebral column, or backbone, All other animals lack this feature, and are called invertebrates. A Figure 2.2 (a) A housefly. (b) A mosguito, feeding on human blood. Houseflies and mosquitoes are both insects, which make up the largest sub-group of all the animals. About 60% of all animal species are insects. (c) This high jumper's movement is coordinated by a complex nervous system. Animals are also multicellular organisms. Their cells never contain chloroplasts, s0 they are unable to carry out photosynthesis. Instead, they gain their nutrition by feeding on other animals or plants. Animal cells also lack cell walls, which allows their cells to change shape, an important feature for organisms that need to move from place to piace. Movement in animals is achieved in various ways, but often involves coordination by a nervous system (see Chapter 6). Another feature common to most animals is that they store carbohydrate in their cells as a compound called glycogen (see Chapter 4). The structure and function of animais is dealt with in Unit 2 of this book. Fungi include mushrooms and toadstools, as well as moulds. These groups of fungi are multicellular. Another group of fungi is the yeasts, which are Because fungi have cell walls, they unicellular (made of single cells). Different species of yeasts live everywhere were once thought to be plants that - on the surface of fruits, in soil, water, and even on dust in the air. The yeast had lost their chiorophyll. We now powder used for baking contains millions of yeast cells (Figure 2.3). The cells know that their cell wall is not made of cellulose as in plants, but of a different of fungi never contain chloroplasts, so they cannot photosynthesise. Their cells chemical called chitin (the same have cell walls, but they are not composed of cellulose (Figure 2.4). material that makes up the outside skeleton of insects). There are many ways that fungi are very different from plants (the most obvious s that fungi do not photosynthesise) and they are not closely related to plants at all. A Figure 2.3 Yeast cells, highly magnified A Figure 2.4 Structure of a yeast cell _ A mushroom or toadstool is the reproductive structure of the organism, called a fruiting body (Figure 2.5), Under the soil, the mushroom has many fine The singular of hyphae Is hypha. thread-like filaments called hyphae (pronounced high-fee). A mould is rather like a mushroom without the fruiting body. It just consists of the network of hyphae (Figure 2.6), The whole network is called a mycelium (pronounced my-sea-lee-um). Moulds feed by absorbing nutrients from dead (or sometimes living) material, so they are found wherever this is present, for example, in soil, rotting leaves or decaying fruit. If you leave a piece of bread or fruit exposed to the air for a few days, it will soon become mouldy. Mould spores carried in the air have landed on the food and grown into a mycelium of hyphae (Figure 2.7). {a) Mycelium of Mucor A Figure 2.5 Toadstools growing on a rotting tree trunk S 3 I 100 wm I J " A Figure 2.6 The ‘pin mould” Mucor growing hyphae ona plece of bread. The dark spots are structures that produce spores for reproduction. (0) Highly magnitied tip of a feeding hypha. | cell wall 1 ’ \ i ‘l—" S _» vacuole - B (ot - s -~ - P B \ - \ ‘A - - enzymes secreted \f cell surface » on to food huclei membrane i) soluble products 0 wm