Cell Biology Unit 2 PDF - Grade 9

Cell biology Unit 2 Contents Section Learning competencies 2.1 The microscope Name different types of microscopes. (page 13) Distinguish between...

Cell biology Unit 2 Contents Section Learning competencies 2.1 The microscope Name different types of microscopes. (page 13) Distinguish between the magnification and resolution of a microscope. State the functions of different types of microscopes. Compare the different resolutions and dimensions of light and electron microscopes. Explain and demonstrate basic techniques using a light microscope. Explain the purpose of staining cells. Use the microscope to study cells. Compare the way materials are prepared for the electron microscope and the light microscope. 2.2 The cell State the cell theory. (page 22) List the structures of cells and describe their function. Draw and label diagrams and compare typical plant and animal cells. Describe the types, shapes and sizes of a variety of cells using diagrams. 2.3 The cell and Describe the permeability of the cell membrane. its environment Describe the process of diffusion and its importance in living organisms. (page 33) Demonstrate diffusion experimentally. Explain the process of osmosis and its importance in living organisms. Demonstrate osmosis experimentally. Show that plant cells become flaccid when they lose water and turgid when they absorb water by osmosis. Explain plasmolysis and turgor pressure. Explain passive and active transport across cell membranes. Discuss the advantages and disadvantages of diffusion, osmosis and active transport for moving substances into and out of cells. 2.1 The microscope By the end of this section you should be able to: Name different types of microscopes. Distinguish between the magnification and resolution of a microscope. State the functions of different types of microscopes. Compare the different resolutions and dimensions of light and electron microscopes. Explain and demonstrate basic techniques using a light microscope. Explain the purpose of staining cells. Use the microscope to study cells. Compare the way materials are prepared for the electron microscope and the light microscope. Grade 9 13 M02_BIOL_SB_ETHG9_0087_U02.indd 13 1/6/10 11:32:07 am  UNIT 2: Cell biology Biologists use different tools to help them study living organisms. Did you know? One of the most important is the microscope. Many important The largest single cell in organisms are very small and biologists need to be able to see them. the world is an ostrich The building blocks of life are called cells and scientists need to be egg – they are about able to see cells to understand living organisms. Most cells cannot 18 cm long and weigh be seen without some kind of magnification. You will be discovering around 1.2 kg. Most cells the secrets of cells revealed with the help of a microscope. In this are much harder to see! section you will learn more about microscopes and how they work. In the next section you will be learning more about the structure of cells and how they work. Seeing cells There are some cells that can be seen very easily with the naked eye. Unfertilised birds eggs are single cells, most cells are much smaller than this. Everything we know about the structure of cells has depended on the development of the microscope. For over 300 years we have been able to look at cells, and as microscopes have improved, so has our knowledge and understanding of cell structure. There are two main types of microscopes in use, the light microscope and the electron microscope. The light microscope uses a beam of light to form the image of an object, while the electron microscope uses a beam of electrons to form an image. You are going to learn about both. Figure 2.1 Ostrich with eggs Magnification and resolving power The reason microscopes are so useful is because they magnify KEY WORDS things, making them look bigger. Magnification means increasing microscope an instrument the size of an object. The best light microscopes will magnify up for magnifying specimens to around 2000 times. Light microscopes have given us a lot of information about the structure of cells, but in the last 50 years or light microscope a so we have also been able to use electron microscopes. An electron microscope that uses a microscope can give you a magnification of around 2 000 000 times. beam of light to form the Using electron microscopes makes it possible for us to learn a lot image of an object more about cells and the ways in which they become specialised for electron microscope a particular functions. microscope that uses a The biggest problem with the light microscope is the limited detail beam of electrons to form it can show. There is a minimum distance between two objects for an image them to be seen clearly as separate. If they are closer together than magnification increase the this they are seen as one thing. This distance is known as the limit size of an object of resolution. Resolution is the ability to distinguish between two separate points and it is the resolving power of a microscope that resolution ability to affects how much detail it can show. The greater the resolving power distinguish between two of the microscope, the more detail it can show. For the optical light separate points microscope the limit of resolution is approximately 200 nanometres resolving power how much (1 nm = 1 × 10–9 m). In comparison, the human eye can only resolve detail the microscope is down to about 0.1 mm (1 mm = 1 × 10–2 m) (see figure 2.2). Objects able to show closer than 0.1 mm are seen as one by human eyes. The magnification we can get with a light microscope is limited by the resolving power possible using the wavelength of light. To see 14 Grade 9 M02_BIOL_SB_ETHG9_0087_U02.indd 14 1/6/10 11:32:08 am  UNIT 2: Cell biology more detail clearly we need an electron microscope where an electron Did you know? beam is used to make the image. As the wavelength gets smaller, the resolving power is increased. An electron microscope has a resolving If you magnified an power around a thousand times better than a light microscope, about average person by the 0.3 nm. Objects that are 0.3 nm apart can be seen as separate by an same amount as the best electron microscope, demonstrating that the resolving power of an light microscopes (×2000) electron microscope is greater than that of a light microscope. they would be about 3.5 kilometres tall. Magnified by an electron microscope Functions of different types of microscopes (×2 000 000), the average person becomes about We will now look in more detail at the different types of microscope 3500 kilometres tall! and how they are used. The light microscope KEY WORD To look at a biological specimen using a light microscope you will stains chemicals added to often use a slide of cells, tissues or individual organisms. These are slide tissues to make the often very thin slices of biological material that have been specially cells easier to see treated and stained, but you can look at living material directly through a light microscope as well. Often chemicals known as stains are added to the tissue on the slide to make it easier to see particular cells, or parts of a cell. When you are looking at stained cell samples it is important to remember that the cells are dead. The cells have been treated with chemicals or ‘fixed’ so they do not decay. The tissue has also been sliced very thinly. These things can damage or change the cells. Living cells have not been treated in this way, but are less easy to see. Below is a list of commonly used stains. Table 2.1 Application of commonly used stains Type of stain Type of cells Main organelles stained Haematoxylin Animal and plant Nuclei stained blue/ cells purple or brown Methylene Animal cells Nuclei stained blue blue Acetocarmine Animal and plant Staining the Figure 2.2 The lines that make up cells chromosomes in dividing this diagram are actually a mass nuclei of dots on the page. The resolving power of your eyes means that Iodine Plant cells Any material containing you see the dots merged together starch to make lines because you can’t resolve the dots individually. If you magnify the line, you can How does a light microscope work? see the dots. In the same way, In a light microscope, a specimen is placed on the stage and what you can see through the illuminated (lit) from underneath. The light passes through the light microscope is limited by the specimen and on through the lenses to give an image at the eyepiece resolving power of the microscope lens which is greatly magnified, upside down and right to left. itself. Grade 9 15 M02_BIOL_SB_ETHG9_0087_U02.indd 15 1/6/10 11:32:09 am  UNIT 2: Cell biology To calculate the magnification of the specimen, you multiply the Did you know? magnification of the objective lens by the magnification of the Electron beams have a eyepiece lens. So if the magnification of the objective lens is ×10, shorter wavelength than and the eyepiece lens is also ×10, the overall magnification of the light. microscope is 10 × 10 = ×100. If you move the objective lenses round and use the ×40 lens, the overall or total magnification will become 40 × 10 = ×400. eyepiece lens tube coarse focusing knobs rotating nosepiece fine Did you know? objective lenses stage stage clips A light microscope with iris diaphragm (under stage) two lenses – the eyepiece lens and the objective mirror lens – is known as a compound microscope. It produces much better magnification than is Figure 2.3 A compound microscope has two sets of lenses (objective possible with a single lens. and eyepiece lenses) which are used to magnify the specimen. These microscopes are widely used for looking at cells. Activity 2.1: Learning to use a microscope You will need: 4. Now look through the eyepiece lens and a microscope adjust the iris diaphragm until the light a lamp is bright but doesn’t dazzle you. The illuminated area you can see is known as a piece of graph paper the field of view. a prepared slide of stained human cheek 5. Looking at your microscope from the side cells (see figure 2.4), or look on page (not through the eyepiece lens) and 18 to find out how to make a slide for using the coarse focusing knob, move yourself the objective lens down slowly so it is Method as close as possible to the paper without Remember, microscopes are delicate pieces of touching it. equipment so always take care of them and 6. Now look through the eyepiece lens again. handle them safely. Turn the coarse focusing knob very gently 1. Set up your microscope with the lowest in the opposite direction to move the power lens (the smallest lens) in place. objective lens away from the slide. Do this while you are looking through the eyepiece 2. Clip the prepared slide into place on the lens and the lines on the graph paper will stage using the stage clips. Position the gradually appear in focus. Once you can piece of graph paper over the hole in see the specimen clearly, use the fine the stage. focusing knob to get the focus as sharp as 3. If your microscope has a built-in lamp, you can. switch it on. If it has a mirror, adjust the 7. You may find that if you now shut the iris angle of the mirror until the specimen is diaphragm down further, so that the hole illuminated. for the light to pass through gets smaller, you will see the specimen better (the contrast is greater). 16 Grade 9 M02_BIOL_SB_ETHG9_0087_U02.indd 16 1/6/10 11:32:25 am  UNIT 2: Cell biology 8. To use the higher magnifications, rotate 10. Return the microscope lenses to their the nosepiece so that the next lens clicks original positions. Now look at a slide into place. Do not adjust the focusing of stained human cheek cells and practise knobs at this point as the specimen should focusing on what you see. still be in focus and, with the coarse focusing knob in particular, it is very easy to break a slide. It is good to practise this using graph paper, which will not break! If you do need to adjust the focus, use the fine focusing knob only with higher magnifications. Take great care to avoid letting the lens touch the slide/paper. You may want to adjust the iris diaphragm as well. 9. Make simple drawings to show how much of the graph paper you can see at each magnification. This will help you to get an idea of how much the microscope is magnifying what you are seeing. Notice Figure 2.4 Human cheek cells stained with how the appearance of the smooth lines methylene blue (×100) changes as you see them at greater magnification. Advantages and disadvantages of the light microscope One of the biggest advantages of using a light microscope is that we can see living plants and animals or parts of them directly. It is very important to observe living cells. It lets us check if what we see on prepared slides of dead tissue is at all like the real living thing. Any biologist working in a hospital, industrial or research lab will have a light microscope readily available to use at any time. School and university students around the world also rely on light microscopes to enable them to learn about the living world of cells. Light microscopes can also be used without electricity, which means they can be used anywhere in the world. Light microscopes are relatively small and not very heavy, so Figure 2.5 (a) Typical green they can be moved around easily. They are quite delicate so they plant cells seen under the light need to be protected, but with care biologists can even take light microscope microscopes out into the field with them to do their research. The biggest disadvantage of light microscopes is that their resolving power is limited by the wavelength of light. As you saw earlier, this limits their powers of magnification. Also we can’t usually magnify living cells as much as we can dead tissue, which limits what we can learn from living cells. Grade 9 17 M02_BIOL_SB_ETHG9_0087_U02.indd 17 1/6/10 11:32:26 am  UNIT 2: Cell biology Using the light microscope Figure 2.5 (b) Using the light In the next section of this book you will learn how the light microscope. microscope can be used to examine many different types of animal and plant cells. It is important to learn how to mount a specimen on a slide to use with a light microscope. Sometimes you may need to add stain to the specimen so that it can be seen more easily. The activity below explains exactly how to carry out this process. Activity 2.2: Making a slide of plant cells The prepared slide you looked at in Activity 3. Using the mounted needle (or a sharp 2.1 showed animal cells that were dead and pencil), lower the cover slip very gently stained to make them easier to see. In this over the first specimen. Take great care activity you are going to learn how to make a not to trap any air bubbles – these will slide of living tissue and stain it so that the show up as black ringed circles under the cells are easier to see. microscope. You will need: a microscope lower cover slip slowly to mounted needle avoid air bubbles becoming trapped beneath it microscope slides cover slips cover-slip forceps a mounted needle specimen glass slide mounting medium a pipette Figure 2.6 Making a slide a lamp 4. Remove any excess liquid from the slide a piece of onion skin and place it under the microscope. iodine solution 5. Repeat this process with the other slide, adding a drop of iodine solution instead Method of water. Remember, microscopes are expensive and 6. Starting with the slide mounted in water delicate pieces of equipment so always take and using the lowest power objective lens, care of them and handle them safely. follow the procedure for looking at cells Onion cells (the sample taken) do not contain described in activity 2.1. Use the higher any chlorophyll so they are not coloured. You power lenses to look at the cells in as can look at them as they are, or stain them much detail as possible. You can judge how using iodine, which reacts with the starch in well you have mounted the tissue – the cells and turns blue-black. it should be a single layer thick and there 1. Take your piece of onion and remove should be NO air bubbles! a small piece of the thin skin (inner 7. Repeat this process looking at the cells epidermis) on the inside of the fleshy part stained with iodine solution. What using your forceps. It is very thin indeed difference does the stain make? and quite difficult to handle. 8. Make a labelled drawing of several of 2. Place the epidermis onto a microscope the cells you can see. When you make a slide and add a drop of water. Make drawing of cells, you try and show another identical slide and add a drop of clearly and simply what is seen under the iodine very gently from a pipette. microscope (see figure 2.7). 18 Grade 9 M02_BIOL_SB_ETHG9_0087_U02.indd 18 1/6/10 11:32:27 am  UNIT 2: Cell biology Use a pencil for your drawing and always show the magnification. nucleus cell wall (a) mag x100 (b) Figure 2.7 (a) Onion epidermis cells stained with iodine x100 (b) Illustration of some sample onion epidermis cells You can get even more information from the light microscope by KEY WORD using the light in different ways. Dark-field illumination, which is where the background is dark and the specimen illuminated, can be wavelength the distance useful for showing tiny structures inside cells. between neighbouring wave There is one big problem to bear in mind when you are working with crests microscopes. Unless you are looking at living material, or have the use of a scanning electron microscope (see below), all the cells that you see appear flat and two-dimensional. But cells are actually three- dimensional – spheres, cylinders and strange three-dimensional (3-D) shapes. You need to use your imagination when you look at cells and see them as the living things that they really are. The electron microscope The electron microscope was developed in the 1930s and came into regular use in the 1950s. It has greatly increased our biological knowledge. Instead of relying on light with its limit of resolving power, an electron beam is used to form an image. The electrons behave like light waves, but with a much smaller wavelength. The resolving power is increased as the wavelength gets smaller, and as a result, the electron microscope can resolve detail down to 0.3 nm. Samples of material have to be specially prepared for the electron microscope. They are fixed, stained and sliced very thinly in a similar way to the preparation of samples for the light microscope but the materials and stains used are very different. How does an electron microscope work? The image in an electron microscope is formed as electrons, which cannot be seen by the human eye, scattered by the biological material, in much the same way as light is scattered in the light microscope. The electron beams are focused by magnetic lenses. A series of magnifications gives you an image. However, you do not simply look into an electron microscope. Complex electronics Grade 9 19 M02_BIOL_SB_ETHG9_0087_U02.indd 19 1/6/10 11:32:28 am  UNIT 2: Cell biology cathode (–) produce the image on a television screen, which can then be These produce the beam of electrons recorded as a photograph known as an electron micrograph or EM. anode (+) The most common type of electron micrograph you will see is magnetic lens produced by a transmission electron microscope, but the scanning electron microscope produces spectacular images of the surfaces of cells and organisms. It shows the surface of structures, greater depth specimen of focus, and a three-dimensional view of the object (see figure 2.9). magnetic lens – the magnification can be varied by altering Advantages and disadvantages of the electron microscope the current in these lenses We can see much more detail using an electron microscope than with a light microscope. It gives us much higher magnification and resolution. This is its biggest advantage. Biologists have discovered many structures inside cells since electron microscopes magnetic lens were developed. The electron microscope has also shown us the complicated structures inside cell organelles (see next section) and this helps us understand how they work. There are several disadvantages to the electron microscope. All the specimens are examined in a vacuum because air would scatter the electron beam. This means it is impossible to look at living material. final image – formed on a screen or photographic plate Some scientists question how useful the images are because the tissue is dead, sliced very thinly, treated with strong chemicals and Figure 2.8 A diagram of an put in a vacuum before we look at it. electron microscope and how it works Electron microscopes are very expensive. They take up a lot of space and are usually kept in a separate room. They have to be kept at a constant temperature and pressure and have an internal vacuum. They rely on a constant source of electricity. Few scientists outside of the top research laboratories have access to electron microscopes and so their use for the majority of biologists is limited. Figure 2.9 The transmission Preparing samples for microscopes electron microscope shows the Materials must be prepared in different ways depending on what inside details of a cell (right) and type of microscope you are using. the scanning electron microscope can show us three-dimensional Tissue has to be prepared and stained in different ways for light and shapes (blood cells above). electron microscopes: for light microscopes staining is done using 20 Grade 9 M02_BIOL_SB_ETHG9_0087_U02.indd 20 1/6/10 11:32:29 am  UNIT 2: Cell biology coloured dyes to reflect light, whereas for electron microscopes heavy metals such as lead and uranium are used to reflect electrons. For light microscopes only non-living materials need fixation, while living materials are not fixed: specimens are always fixed with electron microscopes. Summary In this section you have learnt that: is around 1000 times greater than the Light microscopes and electron microscopes resolving power of a light microscope. are widely used by biologists. Using a light microscope takes skill and Microscopes magnify both living and dead practice. tissue so you can observe the features of Dead specimens are fixed, stained and the cells and tissue. sliced before mounting on slides to be Magnification involves increasing the size observed under the microscope. Living of an object. To work out the magnification specimens are mounted on slides and stains of a microscope you multiply the may be added. magnification of the objective lens by the Stains are used to make parts of cells (e.g. magnification of the eyepiece lens. the nucleus) or types of cells show up Resolution is the ability to distinguish better under the microscope. between two separate points. Tissue has to be prepared carefully before The resolving power of a microscope is it can be used in the electron microscope. dependent on the wavelength used, so the Only dead tissue can be used in the resolving power of an electron microscope electron microscope. Review questions Select the correct answer from A to D. 1. The maximum magnification of a light 3. Which of the following is not an advantage of microscope would make a person: the light microscope? A 3.5 m tall A It can be used anywhere without B 35 m tall electricity. C 3.5 km tall B Its resolving power is limited by the D 35 km tall wavelength of light. 2. The largest single cell is: C It is relatively light so can be carried out into the field for research. A an amoeba D It is relatively cheap. B a jelly fish 4. Which of the following is the main advantage C an unfertilised ostrich egg of the electron microscope? D an unfertilised human egg A It’s very expensive. B Specimens are examined in a vacuum so must be dead. C It needs a constant temperature and pressure. D It gives a greatly increased magnification and resolution over the light microscope. Grade 9 21 M02_BIOL_SB_ETHG9_0087_U02.indd 21 1/6/10 11:32:29 am  UNIT 2: Cell biology 2.2 The cell By the end of this section you should be able to: State the cell theory. List the structures of cells and describe their function. Draw and label diagrams and compare typical plant and animal cells. Describe the types, shapes and sizes of a variety of cells using diagrams. The planet we live on is covered with a wide variety of living organisms, including animals, plants and microbes. All living organisms are made up of units called cells. Some organisms, such as amoeba, consist of single cells. Others, such as ourselves, are made up of many millions of cells all working together. Organisms that contain more than one cell are known as multicellular. Cell theory Figure 2.10 An organism like Cells were first seen over 300 years ago. In 1665, the English this Paramecium carries out all scientist Robert Hooke designed and put together one of the first the characteristic reactions of life working optical microscopes. He examined many different things within a single cell. including thin sections of cork. Hooke saw that these sections were made up of many tiny, regular compartments, which he called cells. It took many years of further work for the importance of cells to be recognised. In 1839 Matthias Schleiden and Theodore Schwann introduced an idea known as the cell theory. The cell theory states that cells are the basic units of life and by the 1840s this idea was KEY WORDS accepted by most biologists. cells the basic structural All living organisms have certain characteristics, which they carry and functional units in all out regardless of whether they have one cell or millions. When we living organisms look at cells we can see how all of these functions are carried out. cell theory states that cells The seven life processes that are common to most living organisms are the basic units of life are: nutrition food substances Nutrition – all living organisms need food to provide them with needed by the body the energy used by their cells. Plants make their own food by photosynthesis, whereas animals eat other organisms. respiration process whereby living organisms Respiration – the process by which living organisms get the obtain energy from their energy from their food. food Excretion – getting rid of the waste products produced by the excretion removal of cells. poisonous waste products Growth – living organisms get bigger. They increase in both size produced by cells and mass, using chemicals from their food to build new material. growth increase in size and Irritability – all living organisms are sensitive to changes in their mass of an organism surroundings. 22 Grade 9 M02_BIOL_SB_ETHG9_0087_U02.indd 22 1/6/10 11:32:30 am  UNIT 2: Cell biology Movement – all living organisms need to move to get near to KEY WORDS things they need or away from problems. Animals move using muscles, plants move more slowly using growth. irritability sensitivity of Reproduction – producing offspring is vital to the long-term an organism to changes in survival of any type of living organism. surroundings movement the need to get near to or away from Cell structures and functions things reproduction the There are some basic similarities between all cells, animal and plant alike. For example, almost all cells have a nucleus, a cell membrane, production of offspring to mitochondria, ribosomes, endoplasmic reticulum and cytoplasm. ensure the survival of a Other features are often seen in plant cells, particularly from type of organism the green parts of the plants, but not in animal cells. This has nucleus controls all cell led scientists to develop a picture of the basic structure of an activity and contains unspecialised animal cell and an unspecialised green plant cell. chromosomes Although there are not many cells which are quite this simple, the cell membrane outer idea of unspecialised animal and plant cells gives us a very useful layer of living cell that base point with which to compare other, more specialised cells. controls the movement of substances in and out Structures and functions in unspecialised mitochondria carry out animal cells cellular respiration All cells have some features in common and we can see them clearly ribosomes organelles in typical unspecialised animal cells (like the ones on the inside involved in protein synthesis of your cheek). They contain small units called organelles. Many of these organelles contain enzymes and chemicals to carry out endoplasmic reticulum specialised jobs within the cell. links the nucleus of a cell The nucleus controls all the activities of the cell. It also contains with the cell membrane the instructions for making new cells or new organisms in the cytoplasm liquid gel which form of long threads known as chromosomes. This is the genetic contains all the organelles material. You will find out more about this in Grade 10. of a cell The cytoplasm is a liquid gel in which most of the chemical organelles the small units reactions needed for life take place. About 70% of the cytoplasm inside a cell of a cell is actually water! The cytoplasm contains all the other chromosome strand of DNA organelles of the cell where most of the chemical reactions take carrying genetic information place. The cell membrane forms a barrier like a very thin ‘skin’ around the outside of the cell. The membrane controls the passage of Did you know? substances such as carbon dioxide, oxygen and water in and out of the cell. Because it lets some substances through but not Human beings contain an others it is known as selectively permeable. enormous number of cells. Estimates range from The mitochondria (singular: mitochondrion) are the 10 million million cells powerhouse of the cell. They carry out most of the reactions (1012) to 100 million of respiration, whereby energy is released from the food in a million (10012) cells – no form your cells can use. Whenever cells need a lot of energy – one has counted accurately! such as muscle cells and secreting cells – you will see a lot of mitochondria. Grade 9 23 M02_BIOL_SB_ETHG9_0087_U02.indd 23 1/6/10 11:32:30 am