Cells and Molecules of Life PDF

2 The cell as the basic unit of life Lab-grown meat Meat from a laboratory...

2 The cell as the basic unit of life Lab-grown meat Meat from a laboratory Think about… 1 Cells are too small to be seen This piece of meat was not produced on a farm. It was grown in a with the naked eye. What tool laboratory. Scientists selected some cells from a cow muscle tissue can we use to view them when and put them into a nutrient solution. The cells divided, producing selecting them? new cells. Eventually, a piece of meat was formed. 2 To produce new cells, what materials have to be supplied? Watch more (Answers on p. 36) Acknowledgements and Important Notice: All questions from the HKDSE, HKCEE and HKALE are reproduced by permission of the HKEAA. Unauthorized use of the aforementioned questions in this electronic version is prohibited. I Cells and Molecules of Life 2.1 Chemicals of life Look at the food label of milk powder shown in Fig 2.1. It tells you how much carbohydrates*, fats and proteins* the milk powder contains. But have you ever thought that these chemicals are also the chemical constituents* of your body? Fig 2.1 Food label of milk powder There are many different organisms on earth, but their chemical constituents are similar. The chemical constituents of organisms can be divided into two groups: inorganic* and organic*. Fig 2.2 shows the main inorganic and organic constituents of organisms and their proportions by weight in the human body. Inorganic constituents Organic constituents carbohydrates water lipids* proteins inorganic ions* nucleic acids* Fig 2.2 The main inorganic and organic constituents of organisms and their proportions by weight in the human body carbohydrate 碳水化合物 chemical constituent 化學成分 inorganic ion 無機離子 inorganic 無機的 lipid 脂質 nucleic acid 核酸 organic 有機的 protein 蛋白質 2– 2  2 The cell as the basic unit of life A Inorganic chemical constituents of organisms Water and inorganic ions are the two main inorganic chemical constituents of organisms. 1 Water Organisms consist mostly of water. Water makes up about 60% of our body weight, and even more in organisms like jellyfish (about 98%). Some functions of water in organisms are shown below. 1 As a reactant Water is a reactant* in some chemical reactions. 2 As a medium for chemical reactions e.g. In plants, food is produced from water and carbon dioxide in photosynthesis. Water can dissolve many substances. Water in cells provides an aqueous light carbon dioxide + water food + oxygen medium for chemical reactions to take chlorophyll place. 3 As a medium of transport 4 As a cooling agent* Many substances are transported in Water removes heat when it evaporates organisms by water. from the body of the organisms. e.g. In plants, inorganic ions dissolve e.g. Humans produce more sweat when in water and are carried from the body temperature is high. Evaporation of roots to other parts. sweat helps cool down the body. 5 Provides support 6 As a component of lubricant* Water gives shape and provides support to organisms. Water is the major component of many lubricating fluids in organisms. e.g. When plant cells are full of water, These fluids reduce friction* during they become turgid* and press against movement. one another. This gives support to seedlings so that they can stand upright. e.g. Pleural fluid around human lungs reduces friction during breathing. pleural fluid* turgid cells press against one another seedling stands upright Fig 2.3 Functions of water in organisms cooling agent 冷卻劑 friction 摩擦 lubricant 潤滑劑 pleural fluid 胸膜液 reactant 反應物 turgid 膨脹 2– 3 I Cells and Molecules of Life 2 Inorganic ions Inorganic ions are also called Various inorganic ions are present in organisms. The table below shows minerals* in biology. some of their functions in animals and plants. Inorganic ion Function in animals Function in plants Nitrate* – A source of nitrogen* for the synthesis of proteins Cross-link Magnesium* Activates some A component of The role of enzymes in enzymes*, which regulate chlorophyll* chemical reactions will be chemical reactions in the Activates some discussed in Ch 4. body enzymes Iron* A component of Activates some haemoglobin*, an enzymes oxygen-carrying molecule in red blood cells Activates some enzymes Calcium* A component of bones Helps strengthen cell and teeth walls* Needed for processes like blood clotting, muscle contraction and sending messages in the nervous system* Cross-link Phosphate* A component of bones A component of Phospholipids and nucleic and teeth phospholipids and acids will be introduced in nucleic acids A component of Part B of this section. phospholipids* (which make up cell membranes*) and nucleic acids* (e.g. DNA*) Visit the following website and learn more about the importance of water for life. Discuss with your classmates whether life can exist without water. https://science.howstuffworks.com/environmental/earth/geophysics/water-vital-to- life.htm calcium 鈣 cell membrane 細胞膜 cell wall 細胞壁 chlorophyll 葉綠素 DNA (deoxyribonucleic acid) 脫氧核糖核酸 enzyme 酶 haemoglobin 血紅蛋白 iron 鐵 magnesium 鎂 mineral 礦物質 nervous system 神經系統 nitrate 硝酸鹽 2– 4 nitrogen 氮 nucleic acid 核酸 phosphate 磷酸鹽 phospholipid 磷脂  2 The cell as the basic unit of life B Organic chemical constituents of organisms Carbohydrates, lipids, proteins and nucleic acids are the major Organic substances refer organic chemical constituents of organisms. Thus they are often called to complex molecules biomolecules*. They all contain the element carbon. containing the element carbon. 1 Carbohydrates Carbohydrates contain the elements carbon, hydrogen and oxygen. Glucose*, starch*, glycogen* and cellulose* are four common carbohydrates found in organisms. Glucose is the main energy source for cells. It is directly broken down in respiration to release energy for chemical reactions in cells and various activities of organisms. Starch acts as an energy reserve* in plants and glycogen acts as an energy reserve in animals (Fig 2.4 and 2.5). They are broken down to glucose to provide energy when needed. Cellulose is a major component of plant cell walls. starch grain glycogen granule photomicrograph of potato cells (×180) electron micrograph of a liver cell (×10 000) Fig 2.4 Starch stored as starch grains in potatoes Fig 2.5 Glycogen stored as glycogen granules in (starch grains are stained so that they the human liver appear orange) 2 Lipids Lipids also contain the elements carbon, hydrogen and oxygen, but with a higher hydrogen-to-oxygen ratio. Triglycerides* and phospholipids are two common lipids found in organisms. Triglycerides (fats and oil) act as an energy reserve in organisms. They also have the following functions in animals: a Fats stored in adipose tissues* under the skin act as an insulator to reduce heat loss from the body. b Fats stored in adipose tissues around the internal organs act as a shock absorber, which protects the internal organs. Phospholipids are a major component of cell membranes. adipose tissue 脂肪組織 biomolecule 生物分子 cellulose 纖維素 energy reserve 能量儲備 glucose 葡萄糖 glycogen 糖原 starch 澱粉 triglyceride 甘油三酯 2– 5 I Cells and Molecules of Life 3 Proteins Cross-link Proteins contain the elements carbon, hydrogen, oxygen and nitrogen. The structures and functions Some also contain sulphur. Some functions of proteins in organisms are of carbohydrates, lipids and proteins will be discussed in shown below. detail in Ch 5. Some proteins make up body tissues. Hair, muscles and skin are mainly made up of proteins (Fig 2.6). hair skin muscle Fig 2.6 Hair, muscles and skin are mainly made up of proteins Some proteins act as enzymes. They regulate chemical reactions in organisms. For example, enzymes are involved in regulating certain reactions in respiration. Some proteins act as hormones*. They help regulate body processes in organisms. For example, growth hormone* regulates growth. Pathogens are viruses* Some proteins act as antibodies*. They help protect the body or organisms that cause against pathogens*. diseases. Some proteins are involved in the transport of substances, e.g. haemoglobin is an oxygen-carrying protein in red blood cells. 4 Nucleic acids Cross-link Nucleic acids contain the elements carbon, hydrogen, oxygen, nitrogen The structures and functions and phosphorus. There are two types of nucleic acids, deoxyribonucleic of nucleic acids will be discussed in detail in Bk 4, acid (DNA) and ribonucleic acid* (RNA). Ch 25 and Ch 26. DNA is the genetic material* in organisms. It carries genetic information* which controls activities of cells and determines the features of organisms. RNA is involved in the synthesis of proteins. antibody 抗體 genetic information 遺傳信息 genetic material 遺傳物質 growth hormone 生長激素 hormone 激素 pathogen 病原體 virus 病毒 ribonucleic acid 核糖核酸 2– 6  2 The cell as the basic unit of life 1 What are the inorganic chemical constituents of organisms? The inorganic chemical constituents include water and inorganic ions (e.g. nitrate, magnesium, iron, calcium and phosphate). 2 What are the organic chemical constituents of organisms? What are their functions? Organic chemical Functions constituent Carbohydrates Glucose The main energy source for cells Starch An energy reserve in plants Glycogen An energy reserve in animals Cellulose A major component of plant cell walls Lipids Triglycerides An energy reserve in organisms (fats and oil) Fats stored in adipose tissues reduce heat loss and protect internal organs in animals Phospholipids A major component of cell membranes Proteins Structural proteins Make up body tissues Enzymes Regulate chemical reactions Hormones Help regulate body processes Antibodies Help protect the body against pathogens Haemoglobin Carries oxygen Nucleic acids Deoxyribonucleic Carries genetic information acid (DNA) Ribonucleic acid Involved in the synthesis of proteins (RNA) Level 1 Level 2 1 Which of the following about inorganic 2 Which of the following are the functions of ions in organisms is correct? carbohydrates in organisms? A Magnesium is a component of teeth. (1) Makes up cell walls B Nitrate is a source of nitrogen for the (2) Provides energy synthesis of proteins. (3) Regulates chemical reactions C Calcium is a component of chlorophyll. A (1) and (2) only B (1) and (3) only D Iron is a component of bones. p. 4 C (2) and (3) only D (1), (2) and (3) p. 5 2– 7 I Cells and Molecules of Life 2.2 Discovery and early studies of cells Watch this animation to Cells are the basic unit of life. All organisms are made up of cells. get some idea of how big Some organisms (e.g. Amoeba) are made up of one cell only, while others a cell is. (e.g. plants and animals) are made up of more than one cell (Fig 2.7 http://learn.genetics.utah. edu/content/cells/scale and 2.8). Most cells are very small. They cannot be seen with the naked eye. How were they discovered? animal cells plant cells (×280) (×140) (×60) Fig 2.7 Amoeba is made up of one cell Fig 2.8 Plants and animals are made up only of more than one cell A Discovery of cells In 1590, lens makers Hans Janssen and his son invented the first microscope. This quickly led to the discovery of cells. In 1665, English scientist Robert Hooke (1635–1703) used a microscope designed by himself to examine a thin slice of cork*, a tissue of bark*. He observed that cork seemed to be made up of many small irregular boxes. He called these boxes ‘cells’ (Fig 2.9). Fig 2.9 Robert Hooke’s drawing of cork ‘cells’ bark 樹皮 cork 木栓 2– 8  2 The cell as the basic unit of life Although the boxes Robert Hooke observed were actually the cell walls of dead cork cells, his study raised the interest of other scientists in the microscopic examination of different materials. B The Cell Theory As microscopes were improved and more materials were observed, scientists noticed that there was a basic structural pattern in organisms. Matthias Schleiden discovered that plant parts are made up of cells and Theodor Schwann discovered that animal parts are made up of cells (Fig 2.10 and 2.11). Fig 2.10 Matthias Schleiden (1804–81) Fig 2.11 Theodor Schwann (1810–82) Based on the above findings and the work of other scientists, Schwann proposed the Cell Theory* in 1839. This theory became one of the foundations of biology. The Cell Theory (with subsequent modifications by other scientists) states that: all organisms are made up of one or more cells. the cell is the basic unit of life; it is the smallest unit that shows all Visit the following website and learn more the characteristics of life. about the contribution of the development all cells come from pre-existing cells. of microscopes to the understanding of cells. Apart from the discovery of cells and the formulation* of the Cell https://bitesizebio. Theory, some other major events in cell biology since the 1500s are com/166/history-of-cell- shown in the timeline on p. 10. All these were made possible by the biology improvement of microscopes, which allowed more details of cells to be observed. From the timeline, we can see that scientific knowledge advances with improvement in technology. Cell Theory 細胞學說 formulation 構想 2– 9 I Cells and Molecules of Life Some major events in cell biology and the development of microscopes Major events in cell biology Development of microscopes 1590 The first light microscope Lens makers Hans Janssen and his son made the first microscope. magnification*: 3–9 times 1665 Discovery of cells Robert Hooke observed many small irregular boxes in a slice of cork using microscope of his own design. He called them ‘cells’. Robert Hooke’s microscope magnification: ~50 times 1674 First observation of living cells under a microscope Antoni van Leeuwenhoek observed microorganisms* in pond water with his Leeuwenhoek’s microscope simple single-lens microscope. magnification: ~200 times 1831 Discovery of the nucleus Robert Brown discovered nuclei* in cells of an orchid* leaf. 1839 Formulation of the Cell Theory Theodor Schwann proposed the Cell Theory. 1886 Discovery of mitochondria The first modern light microscope Scientists discovered mitochondria* in Light microscopes human muscle cells. Mitochondria are of design similar to those structures with a diameter of about one we use in schools today tenth of that of nuclei. were made. a light microscope used nowadays magnification: 1600 times 1933 The first electron microscope The first electron microscope* was made. 1950 Discovery of ribosomes Using an electron microscope, scientists an electron microscope discovered ribosomes* in cells. Ribosomes used nowadays are small granules about a hundred times magnification: smaller than mitochondria. ~12 000 times electron microscope 電子顯微鏡 magnification 放大率 microorganism 微生物 mitochondrion 線粒體 nucleus 細胞核 orchid 蘭 ribosome 核糖體 2– 10  2 The cell as the basic unit of life Which of the following aspects of nature of science is/are demonstrated in the discovery of cells and the development of the Cell Theory? Put a ‘✓’ in the correct box. a Scientific knowledge is based on and/or derived from observations of the natural world. b Science is affected by the technology and the types of equipment available at the time. c Scientists build on the work of other scientists. 1 How did the development of microscopes contribute to our understanding of cells? The development of microscopes led to the discovery of cells and the formulation of the Cell Theory. With improved microscopes, more details of the cells were observed. 2 What is the Cell Theory? The Cell Theory states that: all organisms are made up of one or more cells. the cell is the basic unit of life. all cells come from pre-existing cells. Level 1 Level 2 1 Which of the following is stated in the Cell 2 CE Bio 2006 II Q39 Theory? Sir Robert Hooke is the first scientist who A All organisms have similar chemical used the light microscope to observe cells. constituents. His study led to B Cells vary in size and shape. A the discovery of virus. C All cells contain DNA as their genetic B the formulation of the Cell Theory. material. C the discovery of bacteria as a D All cells come from pre-existing cells. disease-causing agent. p. 9 D the discovery of the fine structure of cell organelles. p. 9, 10 2– 11 I Cells and Molecules of Life 2.3 Microscopes: tools for studying cells A Types of microscopes Nowadays, many different types of microscopes are available for observing cells. Light microscopes and electron microscopes are two types of commonly used microscopes. The table below shows their differences. Electron microscopes Light microscopes Transmission electron Scanning electron microscopes* (TEM) microscopes* (SEM) Working Light passes through a Electron beams* pass Electron beams scan over principle specimen or a thin slide of through a very thin slide of a the surface of a specimen it to form an image. specimen to form an image. to form an image. Maximum 1600 times 1 500 000 times 200 000 times magnification Appearance Coloured image Black and white Black and white of the image two-dimensional image three-dimensional image produced showing the internal showing the external structures of the specimen structures of the specimen Most electron micrographs in guard cell this book are in colour because ‘false colour’ is added to them using computer software. (×160) (×2000) (×2800) Advantages Living specimens can Magnifications and resolution* of the images be observed. produced are higher, so more details can be seen. Specimens can be prepared easily. electron beam 電子束 resolution 分辨率 scanning electron microscope 掃描電子顯微鏡 transmission electron microscope 透射電子顯微鏡 2– 12  2 The cell as the basic unit of life Resolution of images One of the advantages of electron microscopes over light microscopes is that they can produce images with higher resolution. Fig 2.12 shows images of the same type of cells with the same magnification produced using a light microscope and an electron microscope. We can see that the image produced using an electron microscope, which has a higher resolution, is clearer and shows more details. light microscope electron microscope (×750) (×750) Fig 2.12 Images of the same type of cells produced using different microscopes Due to advances in technology, microscopes have become more (×3 000 000) and more powerful. Nowadays, some microscopes have such a high Fig 2.13 Protein molecules resolution that they allow observation of individual molecules in cells seen under a super-resolution (Fig 2.13). Using these microscopes, scientists can now study biological microscope* (red) structures and processes at a molecular level. What can you observe under a light microscope and an electron microscope? height of width of width of width of thickness diameter width of a diameter width of diameter a 5-year- a hand a finger an ant of a human of an mitochondrion of a a DNA of an old child hair animal ribosome molecule atom cell or a plant cell 1m 0.1 m 0.01 m 1 mm 0.1 mm 0.01 mm 1 μm 0.1 μm 0.01 μm 1 nm 0.1 nm (0.001 m) (0.001 mm) (0.001 μm) naked eye light microscope electron microscope Fig 2.14 Size range of objects that can be seen with the naked eye, under a light microscope and under an electron microscope super-resolution microscope 超高解析度顯微鏡 2– 13 I Cells and Molecules of Life DSE B Light microscopes 14(IA)Q3, 15(IA)Q1, 16(IA)Q12 1 Different parts of a light microscope The light microscopes used in the school laboratory are compound microscopes*. They use two sets of lenses, an eyepiece and an objective, to produce magnified images. Fig 2.15 shows the different parts of a compound microscope and their functions. 1 Eyepiece* It is a magnifying lens which our eyes look through. 8 Arm Eyepieces with different We hold it when magnifications are often carrying the microscope available. 5x 10x 16x from place to place. 2 Body tube* 9 Coarse adjustment knob* It holds the eyepiece and the objectives*. We turn it to raise or lower the stage to get a rough focus*. 3 Nosepiece* (Some microscopes focus by moving the We can rotate it to choose body tube instead.) the objective required. Turning it causes a larger movement of the stage*. 4 Objective It is a magnifying lens 10 Fine adjustment knob* pointing to the specimen. We turn it to raise or Objectives with different lower the stage (or the magnifications are held on body tube) to get a the nosepiece. sharp focus. Turning it causes a smaller movement of the stage. We often use it when the specimen is in rough focus. 5 Condenser* 11 Stage It is a lens that focuses light onto the specimen. We clip the slide here for observation. 6 Diaphragm* It can be raised or lowered to focus. We can adjust it to control the amount of light shone onto the specimen. 7 Light source 12 Base It provides light for viewing the It supports the whole specimen. (Some microscopes have Fig 2.15 Light microscope microscope. a mirror for reflecting light from an external light source instead.) body tube 鏡筒 coarse adjustment knob 粗調節器 compound microscope 複式顯微鏡 condenser 聚光器 diaphragm 光欄 eyepiece 目鏡 fine adjustment knob 微調節器 focus 聚焦 nosepiece 物鏡轉換器 objective 物鏡 stage 載物台 2– 14  2 The cell as the basic unit of life 2 How a light microscope works When we observe a specimen under a light microscope, light from the light source penetrates the specimen and enters the objective. It then passes through the eyepiece and enters our eye, so that we can see an image. The image observed is inverted. For example, if you observe the letter ‘p’ under the microscope, the image becomes ‘d’. Observation with a light microscope 2.1 Procedure Practical 2.1 A Observation at low-power magnification 1 Place a microscope on the bench. 2 Insert a low-power eyepiece (e.g. 5X) into the body tube. Select a low-power objective (e.g. 4X) by rotating the nosepiece. 3 Turn on the light source. Look through the eyepiece. Adjust the diaphragm until the light is sufficient. 4 Clip a prepared slide of onion epidermal cells* onto the stage. Make sure the specimen is directly over the hole of the stage. 5 Follow the steps below to focus on the specimen. a Watch the stage from the side. Raise the stage (or lower the body tube) by turning the coarse adjustment knob until the objective is Never raise the stage at a position closest to the slide. (or lower the body b Look through the eyepiece again. Lower the stage (or raise the body tube) with the coarse adjustment knob tube) slowly by turning the coarse adjustment knob until the image when you are looking of the specimen is roughly in focus. through the eyepiece. c Turn the fine adjustment knob to get a sharp focus. raise the lower the adjust stage stage the focus cont. onion epidermal cell 洋葱表皮細胞 2– 15 I Cells and Molecules of Life B Observation at high-power magnification 1 Carry out the steps in Part A so that the specimen is in focus at Always start with low- low-power magnification. power magnification because the wider 2 Search the field and select a part of the specimen to observe in detail. field of view allows the specimen to be Move that part to the centre of the field of view. located more easily. 3 Select a high-power objective (e.g. 40X) by rotating the nosepiece. 4 The specimen should be in rough focus now. Turn the fine adjustment knob to get a sharp focus. Watch the stage from the side when rotating the nosepiece to prevent the adjust objective from touching the slide. the Do not use the coarse adjustment knob when using a high-power objective. focus 5 Adjust the diaphragm to brighten the view if necessary. 6 If you cannot get a clear image, follow the steps below: a Keep watching the stage from the side. Raise the stage (or lower the body tube) by turning the coarse adjustment knob until the objective nearly touches the slide. b Look through the eyepiece. Focus the image by turning the fine adjustment knob. Adjust the diaphragm to brighten the view if necessary. 7 Compare the observations at low-power and high-power magnifications. Results and discussion Low-power magnification High-power magnification e.g. ×100 e.g. ×400 Area of specimen observed Larger Smaller (More cells are observed) (Fewer cells are observed) Details of specimen observed Less More Brightness of image Brighter Dimmer 2– 16  2 The cell as the basic unit of life 3 Magnification of a light microscope Magnification represents how many times an image is larger than the object. The total magnification* of a light microscope depends on the objective and the eyepiece used. It can be calculated by Total magnification magnification of magnification of = × of a microscope eyepiece objective For example, if a 10X eyepiece and a 40X objective are used, the total magnification of the microscope is 400X. On a photomicrograph or a biological drawing, magnification is often indicated. We can find out the actual size of the object shown using the magnification given. Skill builder below shows how this can be done. Skill builder Skill practice Calculating the actual size of an object To calculate the actual size of an object shown in a photomicrograph or a biological drawing, we can use the equation shown below. size of the image Magnification = size of the object For example, to calculate the actual size of the white blood cell shown X on the right in Fig 2.16, we can do the steps as follows: (×400) Fig 2.16 A white blood cell Steps Workings 1 Measure the length of the cell in the Length of the cell in the photomicrograph (X) = 0.6 cm photomicrograph using a ruler. Magnification = 400 2 Find out the magnification of the size of the image Magnification = photomicrograph. size of the object 3 Substitute these values into the equation. 0.6 cm 400 = size of the object 4 Give the answer using a suitable unit. 0.6 cm size of the object = 400 size of the object = 0.0015 cm = 15 μm total magnification 總放大率 2– 17 I Cells and Molecules of Life Skill builder Skill practice The electron micrographs show two structures found in cells, X and Y. Calculate their actual lengths. length of X length of Y (×12 000) (×50 000) (4 marks) Q7 (p. 39) 1 What are the types of microscopes commonly used today? What are the differences in the magnifications and resolution of their images? Light microscopes and electron microscopes are commonly used today. Compared to light microscopes, electron microscopes can produce images with higher magnifications and resolution. 2 What are the differences between observations at low-power magnification and high-power magnification under a microscope? Low-power High-power magnification magnification Area of specimen Larger (more cells Smaller (fewer cells observed are observed) are observed) Details of specimen Less More observed Brightness of image Brighter Dimmer 3 How can we calculate the total magnification of a compound microscope? Total magnification of magnification of magnification of = × a microscope eyepiece objective 2– 18  2 The cell as the basic unit of life Level 1 Questions 1 and 2: A student is going to observe a cell using a light microscope under high-power magnification. State whether the following statements about the procedure are true or false. 1 She should start with a high-power objective. p. 16 2 She should turn the coarse adjustment knob to focus when using a high-power objective. p. 16 3 Some steps in observing a specimen using a light microscope under low-power magnification are shown below. Arrange them into the correct sequence. (1) Turn the course adjustment knob to lower the stage. (2) Turn the course adjustment knob to raise the stage. (3) Turn the fine adjustment knob. (4) Rotate the nosepiece to select a low-power objective. (5) Clip the slide on the stage. _____ _____ _____ _____ _____ p. 15, 16 Level 2 4 A student is observing a tissue under a light microscope. Which of the following combinations of eyepiece and objective allows him to see the largest number of cells in the field of view? A 10X eyepiece and 10X objective B 10X eyepiece and 40X objective C 15X eyepiece and 4X objective D 15X eyepiece and 40X objective p. 17 5 DSE Bio 2015 IA Q1 Which of the following parts of the microscope should be adjusted to obtain a clear and sharp image when you switch 4 from low-magnification to high-magnification observation? 1 A 1 and 4 only 3 2 B 2 and 3 only C 1, 3 and 4 only D 2, 3 and 4 only p. 16 2– 19 I Cells and Molecules of Life DSE 14(IA)Q5, 17(IB)Q4 2.4 Structure of cells A Animal cells and plant cells Watch this to prepare for your class and answer the There are hundreds of different types of cells in organisms. Fig 2.17 questions. shows some cells in humans and plants. They vary in shape and size. Are there any similarities between them? Video Questions neurone* in the guard cell* in the brain (×400) leaf (×200) cardiac muscle cell* palisade mesophyll in the heart (×100) cell* in the leaf (×100) white blood cell in cortex cell* in the the blood (×400) root (×200) Fig 2.17 Different cells in humans and plants Plant cells and animal cells generally share the same basic structure. The major part of both of them is a jelly-like fluid called cytoplasm*. The cytoplasm is bounded by a cell membrane. Various organelles* 3D model 2.1 (e.g. nucleus, endoplasmic reticulum*, mitochondria, vacuoles*, ribosomes and chloroplasts*) are held in the cytoplasm (Fig 2.18 on the 3D model 2.2 next page). cardiac muscle cell 心肌細胞 chloroplast 葉綠體 cortex cell 皮層細胞 cytoplasm 細胞質 endoplasmic reticulum 內質網 guard cell 保衞細胞 neurone 神經元 organelle 細胞器 palisade mesophyll cell 柵狀葉肉細胞 vacuole 液泡 2– 20  2 The cell as the basic unit of life Animal cell cell membrane cytoplasm nucleus rough endoplasmic reticulum* smooth endoplasmic reticulum* mitochondrion (plural: mitochondria) (×5000) vacuole ribosome Plant cell cell wall chloroplast large central vacuole cell membrane cytoplasm nucleus rough endoplasmic reticulum smooth endoplasmic reticulum mitochondrion (×4000) ribosome Key: structures that can be found in plant cells but not in animal cells Fig 2.18 Drawings (left) and electron micrographs (right) of an animal cell and a plant cell Plant cells are generally larger than animal cells and have a more regular shape. They have a cell wall while animal cells do not. Some plant cells also have a large central vacuole and chloroplasts. rough endoplasmic reticulum 粗糙內質網 smooth endoplasmic reticulum 光滑內質網 2– 21 I Cells and Molecules of Life We will take a closer look at each of the sub-cellular structures* and see what functions they have. 1 Nucleus Some cells (e.g. muscle cells Most cells have one nucleus (Fig 2.19). and some human liver cells) have more than one nucleus. Mature human red blood cells have no nucleus. nucleus cytoplasm (×400) Fig 2.19 Plant cells with a nucleus under a light microscope It is a spherical structure bounded by a double membrane called the nuclear membrane* (Fig 2.20). There are pores in the nuclear membrane, which allow the exchange of materials between the nucleus and the cytoplasm. It contains DNA, the genetic material of the cell. DNA carries genetic information, which controls the activities of the cell. a b nuclear membrane nuclear pore DNA Nucleolus is involved in the making of ribosomes. nucleolus* (×5000) Fig 2.20 (a) Drawing and (b) electron micrograph of a nucleus 2 Cytoplasm It is a jelly-like fluid consisting of mainly water and proteins. It holds many organelles (e.g. nucleus and mitochondria). It is the site for many chemical reactions. It allows the movement and transport of materials inside the cell. nuclear membrane 核膜 nucleolus 核仁 sub-cellular structure 亞細胞構造 2– 22  2 The cell as the basic unit of life 3 Cell membrane Cross-link It is a thin and flexible membrane mainly made up of phospholipids The structure and functions and proteins. of the cell membrane will be discussed in detail in Ch 3. It encloses the cell and separates the cell contents from the outside environment. It is differentially permeable*, i.e. it only allows certain substances to pass through. It controls the movement of substances into and out of the cell. 4 Cell wall cell wall It is present in all plant cells but not in animal cells. It is a thick, rigid outermost layer mainly made up of cellulose. It is fully permeable, i.e. it allows water and all dissolved substances to pass through. (×400) Fig 2.21 Photomicrograph of plant It protects, supports and gives cells, showing the cell wall shape to the plant cell. a b cell membrane cell wall cell wall of adjacent cell (×4000) Fig 2.22 (a) Drawing and (b) electron micrograph of cell wall and cell membrane Visit the following website to watch a video of an Amoeba eating a microorganism. You can see how flexible the cell membrane is! https://www.youtube.com/watch?v=mv6Ehv06mXY differentially permeable 差異透性的 2– 23 I Cells and Molecules of Life 5 Endoplasmic reticulum (ER) It is a network of interconnected membrane-bounded sacs. It is continuous with the outer nuclear membrane and extends throughout the cytoplasm. There are two types of ER, rough ER and smooth ER. Rough ER Smooth ER drawing of a section of rough ER drawing of a section of smooth ER ribosome electron micrograph of rough ER electron micrograph of smooth ER ribosome o u g rh (×20 000) E R (×20 000) With ribosomes attached No ribosomes attached Cross-link A site for the synthesis of proteins A site for the synthesis of lipids The details of protein synthesis will be discussed in Abundant in cells that produce a Abundant in cells that produce a Bk 4, Ch 26. large amount of proteins, e.g. large amount of lipids, e.g. saliva-secreting cells which the cells in testes that secrete produce the enzymes in saliva male sex hormones which are pancreatic cells which secrete lipids enzymes for digestion pancreatic cells which secrete hormones for regulating blood glucose level 6 Ribosome It is a small particle not surrounded by a membrane. Some ribosomes are attached to rough ER, while others are lying free in the cytoplasm. It is involved in the synthesis of proteins. 2– 24  2 The cell as the basic unit of life 7 Mitochondrion It is bounded by a double membrane. The inner membrane is highly folded (Fig 2.23). It is the main site of respiration. It converts chemical energy in food into energy that the cell can use. The number of mitochondria in a cell is related to the energy requirement of the cell. Generally, more mitochondria are present in cells that use a lot of energy. For example, Cell with a large number of Energy is required for… mitochondria Enzyme-secreting cell Synthesis of enzymes Cross-link Liver cell High level of metabolic activities The functions of the liver will be discussed in Ch 6. Epithelial cell* in the inner wall of the Absorption of nutrients small intestine Muscle cell Contraction Root hair cell Absorption of minerals a double membrane b infolding of inner membrane (×37 000) Fig 2.23 (a) Drawing and (b) electron micrograph of a mitochondrion Appearance of mitochondria in electron micrographs Mitochondria often appear in various shapes and sizes mitochondrion in electron micrographs. This is because mitochondria 1 1 may be spherical, rod-shaped, or even branched. The 2 2 position of the section obtained for microscopic 3 3 examination also determines the appearance of the mitochondria (Fig 2.24). Fig 2.24 The appearance of a mitochondrion in different positions of sectioning epithelial cell 上皮細胞 2– 25 I Cells and Molecules of Life 8 Chloroplast Plant cells such as the It is present in green plant cells, epidermal cells of onion and e.g. palisade mesophyll cells and root hair cells do not have chloroplast chloroplasts. guard cells in leaves. It is not present in animal cells. It is bounded by a double membrane, with a network of membrane inside (Fig 2.26). (×400) It contains a green pigment called Fig 2.25 Plant cells with chloroplasts under a light microscope chlorophyll, which captures light energy and converts it into chemical energy in food in photosynthesis. Starch grains are often present inside. This is because some of the glucose produced during photosynthesis is converted into starch and temporarily stored in chloroplast. a b outer membrane inner membrane network of membrane starch grain (×15 000) Fig 2.26 (a) Drawing and (b) electron micrograph of a chloroplast 9 Vacuole large central single It is a fluid-filled sac bounded by a vacuole membrane single membrane. Most animal cells have only a few small vacuoles and some do not have any. Their vacuoles may contain water, enzymes and food. Plant cells often have a large central vacuole. It contains cell sap*, (×4000) which is a liquid containing water Fig 2.27 Electron micrograph of Cross-link and dissolved substances such as a plant cell with a large How turgidity of plant cells central vacuole glucose, pigments and waste. provides support to plants will be discussed in Ch 3. Plant cells become turgid when the large central vacuole is full of water. This provides support to the plant. cell sap 細胞液 2– 26  2 The cell as the basic unit of life Learning through examples Skill builder Skill practice A student is observing a prepared slide of liver tissue using a light microscope. Fig P is a photomicrograph which shows what he is observing. Fig Q is an electron micrograph of part of a liver cell. X Y (×400) (×5000) Fig P Fig Q a With reference to Fig P above, draw a labelled diagram of the cells enclosed with the dotted line. (4 marks) b Name organelles X and Y shown in Fig Q. (2 marks) c The liver carries out many chemical reactions. To regulate these reactions, liver cells produce many enzymes. Describe how organelles X and Y shown in Fig Q work together so that liver cells can perform their functions. (2 marks) Suggested answers a nucleus Drawing high-power biological diagrams cytoplasm Refer to p. 28. cell membrane Online tutorial 2.1 Liver cells (×400) Title 1 Resemblance of drawing 1 Labels (any 2) 1×2 b X: rough endoplasmic reticulum 1 Y: mitochondrion 1 Naming a structure When naming a c X is the site for synthesis of enzymes, which regulate the reactions. 1 structure, make sure Y provides energy for synthesis of enzymes / for the chemical the spelling is correct. reactions to occur. 1 2– 27 I Cells and Molecules of Life Learning through examples Skill builder Skill practice Drawing high-power biological diagrams Scientists often draw biological diagrams to record what they observed. To make a clear drawing of the specimen, you should note the following: Use a sharp HB pencil. Draw only a few representative cells. The drawing should resemble the specimen. The structures should be in proportion. Label the relevant structures. Give a title for your drawing. State the magnification of the drawing. Below are two biological diagrams of the plant cell shown in Fig 2.28. We can see the differences between poor and good biological diagrams. Fig 2.28 A plant cell (×180) Poor biological diagram Good biological diagram Label lines Drawing lines should not cross should be smooth cell one another. and continuous. membrane cell wall cell membrane cell wall chloroplast chloroplast vacuole vacuole nucleus Label lines should Title and be straight. Do not Do not magnification A plant cell (×180) use arrows. shade. are missing. Learning through examples Skill builder Skill practice Draw a labelled biological diagram of the cells in each of the photomicrographs below. (12 marks) a Human cheek cells b Human white blood cell c Leaf cells (×400) (×400) (×200) Q17 (p. 41) 2– 28  2 The cell as the basic unit of life Preparation of temporary mounts and observation of 2.2 animal cells Introduction Practical 2.2 We can prepare temporary mounts* of animal cells or tissues for microscopic examination. As many cell structures are colourless when observed under the microscope, animal cells and tissues are often stained with methylene blue solution* so that the cell structures can be observed more clearly. Procedure Methylene blue solution may cause haemolysis in people with G6PD deficiency*. Do not use it if you have G6PD deficiency. Safranine solution* can be used instead. Methylene blue solution is harmful. Avoid contact with skin. Cover any exposed wounds with sterile dressings and wear disposable gloves. 1 Touch the ox cornea* gently with the middle of a clean slide. 2 Add a drop of methylene blue solution to the touched area to stain the Adding sufficient methylene blue cells. solution can help 3 Use a pair of forceps to place a cover slip over the cells. This flattens the minimize the chance cells for observation, prevents the cells from drying out and prevents the of trapping air bubbles in the next step. objective lens from touching the specimen and getting dirty. Let the edge of a cover slip touch Slowly lower the cover slip. Make Use tissue paper to soak up any the methylene blue solution. sure no air bubbles are trapped. excess methylene blue solution. forceps tissue paper edge of cover slip touching methylene blue solution 4 Observe the cells with a microscope under high-power magnification. Draw a labelled high-power diagram of the ox corneal cells. 5 Dispose of the ox eye, the slide and the gloves properly after the practical. Wash your hands thoroughly. cornea 角膜 G6PD deficiency 葡萄糖 -6- 磷酸脫氫酶缺乏症 methylene blue solution 亞甲藍溶液 safranine solution 番紅溶液 temporary mount 臨時裝片 2– 29 I Cells and Molecules of Life Preparation of temporary mounts and observation of 2.3 plant cells Procedure Practical 2.3 Prepare the temporary mounts of the following cells or tissues. Observe the cells with a microscope under high-power magnification and draw a labelled high-power diagram of the cells observed. A Onion epidermis Iodine solution is an irritant. Avoid 1 Peel off the inner epidermis of the fleshy layer of an onion. contact with skin. 2 Cut out a small piece of epidermis. Wear disposable gloves. 3 Mount it with a drop of iodine solution. inner inner inner epidermis epidermis epidermis fleshy layer of onion scissors cover slip iodine forceps solution B Hydrilla leaf Mount a Hydrilla* leaf with a drop of water. water cover slip Hydrilla leaf C Pollen grains 1 Touch a piece of sticky tape with the anther* of a flower (e.g. a Gladiolus* Do not perform part C flower or a lily flower). if you are allergic to pollen grains. 2 Stick the tape onto a slide. pollen grains* anther cont. sticky tape stuck sticky tape on a slide anther 花藥 Gladiolus 劍蘭 Hydrilla 黑藻 pollen grain 花粉粒 2– 30  2 The cell as the basic unit of life D Onion root tip cells 1 Prepare onion root tip tissue as follows. a Put small pieces of onion root tips into freshly prepared macerating fluid* for 2 to 3 days to soften the root tips. The macerating fluid b Pour the contents into a watch glass. Tear the tissues apart. is corrosive*. Avoid c Filter off the macerating fluid. Wash the macerated onion root tip contact with skin gently with water. 2 Mount the macerated onion root tip with a drop of water. E Banana tissue 1 Take a small amount of tissue from the soft white middle part of a banana. 2 Put the tissue into a drop of water on a slide. 3 Separate the cells with a toothpick. banana toothpick tissue stir banana water 4 Mount the cells with a drop of iodine solution. Results and discussion The appearance of the cells is shown below. nucleus cell wall cell wall chloroplast cell wall (×400) (×400) (×400) Fig 2.29 Onion epidermal cells Fig 2.30 Hydrilla leaf cells Fig 2.31 Pollen grain cell wall nucleus starch grain cell wall nucleus (×400) (×400) Fig 2.32 Onion root tip cells Fig 2.33 Cells in banana tissue corrosive 腐蝕性 macerating fluid 浸離液 2– 31 I Cells and Molecules of Life What are the sub-cellular structures found in animal cells and plant cells? What are their functions? Can be found in Sub-cellular structure Function animal cells plant cells Nucleus ✓ ✓ Contains DNA, which controls the activities of the cell Cytoplasm ✓ ✓ Holds many organelles A site for many chemical reactions Allows the movement and transport of materials inside the cell Cell membrane ✓ ✓ Encloses the cell and separates the cell contents from the outside environment Controls the movement of substances into and out of the cell Cell wall ✗ ✓ Protects, supports and gives shape to the plant cell Rough ✓ ✓ A site for the synthesis of proteins endoplasmic reticulum (rough ER) Smooth ✓ ✓ A site for the synthesis of lipids endoplasmic reticulum (smooth ER) Ribosome ✓ ✓ Involved in the synthesis of proteins Mitochondrion ✓ ✓ The main site of respiration. It converts chemical energy in food into energy that the cell can use Chloroplast ✗ ✓ (in green Contains chlorophyll which plant captures light energy and converts cells it into chemical energy in food in only) photosynthesis Vacuole ✓ (small or ✓ (often May contain water, enzymes, food absent) a large