Movement of Substances Across Cell Membrane PDF

3 Movement of substances across cell membrane A pink salt lake in Australia The pink salt lake...

3 Movement of substances across cell membrane A pink salt lake in Australia The pink salt lake Think about… 1 Why is water with a very high Do you like this pink lake? This pink lake is attractive, but it is not salt content not suitable for suitable for most organisms to live in due to its very high salt content. most organisms to live in? A few microorganisms can live in it by taking in salts from the water. 2 Why can taking in salts from These microorganisms give the characteristic pink colour to the lake. water help the microorganisms survive in the salt lake? Watch more (Answers on p. 31) 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 DSE 13(IA)Q5, 15(IB)Q6 3.1 Structure of the cell membrane In Ch 2, we have learnt that the cell membrane is a differentially Watch this to prepare for permeable membrane mainly made up of phospholipids* and proteins. your class and answer the questions. It controls the movement of substances into and out of the cell. How does the cell membrane carry out this function? To answer this question, Video & first we have to understand the structure of the cell membrane. questions Since 1925, scientists have been proposing different models to illustrate Turn to p. 6 to learn more the structure of the cell membrane. The fluid mosaic model* proposed about the development of by Singer and Nicolson in 1972 is most widely accepted today. It the cell membrane model. suggests that the phospholipid molecules are arranged in a bilayer in the cell membrane. The protein molecules are interspersed* among the phospholipid molecules (Fig 3.1). carbohydrate 3D model 3.1 outside of cell glycoprotein* a cell phospholipid bilayer* inside of cell (cytoplasm) Phospholipid molecules Protein molecules They are arranged in a bilayer. They are interspersed among the They can move laterally* phospholipid molecules in a mosaic pattern (thus the membrane is (thus the membrane is described as ‘mosaic’). described as ‘fluid’). Some of them are attached to the surface of the phospholipid bilayer, some are embedded half-way in the bilayer and others span* the entire bilayer. Carbohydrates are attached to some of them to form glycoproteins. Fig 3.1 The fluid mosaic model of the cell membrane fluid mosaic model 流動鑲嵌模型 glycoprotein 糖蛋白 intersperse 散佈 laterally 橫向地 phospholipid 磷脂 phospholipid bilayer 磷脂雙層 span 貫穿 3– 2  3 Movement of substances across cell membrane A Phospholipids There are different types of lipids. The commonest type is triglycerides (fats and oil). A triglyceride molecule is formed from one glycerol and three fatty acid molecules (Fig 3.3a). A triglyceride molecule has no charged regions, i.e. it is Background chemistry on non-polar*. When triglycerides oil p. 4 explains the meaning of (a triglyceride): are added to water, they do not non-polar polar and non-polar in detail. mix with water (Fig 3.2). This is water: polar because triglyceride molecules are repelled by water, which is polar*. hydrophilic, hydrophobic They are said to be hydrophobic* In Latin, ‘hydro’ means (water-hating). ‘water’, ‘philic’ means Fig 3.2 Triglycerides do not mix with ‘loving’ and ‘phobic’ water means ‘hatred*’. A phospholipid molecule has a structure similar to that of a triglyceride molecule except that one of the fatty acid molecules is replaced by a phosphate group (Fig 3.3b). The phosphate group is polar and is attracted to water which is also polar. Therefore, a phospholipid molecule consists of two parts which exhibit different properties: a hydrophilic*(water-loving) ‘head’ – polar phosphate group and glycerol hydrophobic ‘tails’ – non-polar fatty acids a Triglyceride* molecule b Phospholipid molecule hydrophilic hydrophobic hydrophobic head tails glycerol* fatty acid* phosphate glycerol fatty acid group* Fig 3.3 Basic structure of (a) a triglyceride molecule and (b) a phospholipid molecule fatty acid 脂肪酸 glycerol 甘油 hatred 厭惡 hydrophilic 親水的 hydrophobic 疏水的 non-polar 非極性的 phosphate group 磷酸鹽基團 polar 極性的 triglyceride 甘油三酯 3– 3 I Cells and Molecules of Life Due to the chemical nature of phospholipid molecules described above, the phospholipid molecules are arranged in a bilayer in the cell membrane. The hydrophilic heads point outwards, in contact with the aqueous solutions inside and outside the cells, while their hydrophobic tails point inwards, being protected from contacting the aqueous environments (Fig 3.4). Under an electron microscope, the cell membrane appears as two dark lines with a lighter region in between. The dark lines correspond to the hydrophilic heads, while the lighter region corresponds to the hydrophobic tails (Fig 3.5). aqueous solution phospholipid outside the cell molecule (extracellular fluid) hydrophilic heads hydrophobic tails hydrophilic heads aqueous solution (×130 000) inside the cell Fig 3.4 Arrangement of phospholipid (cytoplasm) Fig 3.5 Cell membrane under an molecules in the cell membrane electron microscope Polar and non-polar molecules Atoms contain positively-charged nuclei and a Non-polar molecule negatively-charged electrons. In a molecule, atoms electron are held together by the attraction between the nuclei and electrons. If the nuclei attract the electrons equally, the molecule will have no charged regions. The molecule is said to be non-polar (Fig 3.6a). nucleus* nucleus On the other hand, in some molecules, one of the no charged regions nuclei attracts the electrons more strongly. The b Polar molecule negatively-charged electrons are ‘pulled’ towards electron pulled towards one nucleus that nucleus. As a result, there is a small negative charge at one end of the molecule and a small positive charge at the other end. These molecules small small positive negative are said to be polar (Fig 3.6b). charge charge nucleus nucleus Fig 3.6 (a) A non-polar molecule and (b) a polar molecule nucleus 原子核 3– 4  3 Movement of substances across cell membrane B Proteins Membrane proteins can be classified into different types according to their functions. Below are some examples. 1 Channel proteins* They provide channels for certain substances to move across the membrane, e.g. ion channels in cells allow ions to enter or leave the cells. channel protein 2 Carrier proteins* They bind to certain substances and transport them to the other side of the membrane, e.g. sugar carriers in the cells in the small intestine help the uptake of sugars into the cells. carrier protein 3 Receptors* They bind to chemical messengers chemical messenger (e.g. hormones) outside cells. The binding may turn on certain activities in the cells. For Cross-link example, insulin* receptors in the liver cells The details of blood glucose bind to insulin and may turn on activities that regulation will be discussed in Bk 2, Ch 18. help lower the blood glucose level. receptor Cross-link 4 Antigens* carbohydrate More about antigens will be antigen discussed in Bk 3, Ch 24. They are glycoproteins for cell recognition*. For example, some white blood cells recognize antigens on our body cells as ‘self’ and do not attack them. Antigens on bacteria are recognized as ‘foreign’ and they are attacked. 5 Enzymes They speed up chemical reactions. For example, some enzymes in the cells in the Cross-link small intestine speed up the breakdown of food The role of enzymes substances. in breakdown of food enzyme substances in the small intestine will be discussed in Ch 6. antigen 抗原 carrier protein 載體蛋白 channel protein 通道蛋白 insulin 胰島素 receptor 受體 recognition 識別 3– 5 I Cells and Molecules of Life Development of the cell membrane model In the past decade, scientists carried out many investigations into the structure of the cell membrane and proposed models based on their findings. Our understanding of cell membrane structure is the collective effort of them. 1925 Gorter and Grendel – The bilayer model The membrane is made up of two layers of ✓ Evidence lipids. Overton found that lipid-soluble substances penetrated cells easily, suggesting the membrane is mainly made up of lipids. Langmuir found that the major component of the membrane exhibited both water-loving and water-hating properties. Gorter and Grendel found that the lipids extracted from a cell could cover twice the area needed to enclose the cell. ? Limitation The model did not include proteins. 1935 Davson and Danielli – The ‘sandwich’ model The membrane is made up of a phospholipid ✓ Evidence bilayer sandwiched between two layers of Scientists observed two dark lines in electron micrographs proteins. of the membrane. They thought that these dark lines protein represented protein layers. (This was later found to be a molecule wrong interpretation.) cell membrane under electron microscope phospholipid (×130 000) bilayer dark lines ? Limitation The proteins extracted from the membrane were mainly hydrophobic. They are not likely to be located at the surfaces of the membrane, where they are in contact with water. 1972 Singer and Nicolson – The fluid mosaic model The membrane is made up of a phospholipid ✓ Evidence bilayer with protein molecules interspersed in a Scientists split the membrane of a frozen cell and observed mosaic pattern. some particles on the inner surfaces, suggesting that some protein proteins are interspersed among the phospholipids. molecules images under electron microscope (×55 000) phospholipid bilayer 3– 6  3 Movement of substances across cell membrane Which of the following aspects of nature of science is/are demonstrated in the development of the cell membrane model? Put a ‘✓’ in the correct box. a Scientific knowledge is tentative and subject to change. b Doing science requires creativity and imagination. c Scientific knowledge is based on or derived from observations of the natural world (i.e. empirically based or evidence based). What is the structure of the cell membrane according to the fluid mosaic model? Phospholipids The phospholipid molecules are arranged in a bilayer, with their hydrophilic heads pointing outwards and hydrophobic tails pointing inwards. The phospholipid molecules can move laterally. Proteins The protein molecules are interspersed among the phospholipid molecules in a mosaic pattern. Some protein molecules are attached to the surface of the phospholipid bilayer, some are embedded half-way in the bilayer and others span the entire bilayer. Carbohydrates are attached to some protein molecules to form glycoproteins. Making a cell membrane model Design and make a model to show the structure of the cell membrane. Compare your model with those of your classmates and discuss the advantages and limitations of each model. Fig 3.7 A cell membrane model You can see how other people make their models at: https://www.youtube.com/watch?v=jVdQe6VeOBg 3– 7 I Cells and Molecules of Life DSE 14(IB)Q7, 16(IA)Q1, 2 3.2 Relationship between the structure, properties and functions of the cell membrane In this section, we will see how the structure of the cell membrane is related to its properties and functions. 1 The cell membrane is differentially permeable The core of the phospholipid bilayer is hydrophobic. It is permeable to non-polar substances but impermeable to polar substances and Ions are particles which carry ions. Polar substances and ions can be transported across the membrane charges. by channel proteins or carrier proteins. The table below shows how different substances move across the cell membrane. How they move across Substance Examples the membrane ❶ Small, Oxygen, carbon They dissolve in the non-polar dioxide, glycerol, phospholipid bilayer and molecules fatty acids, vitamin A, move across the membrane. vitamin D ❷ Small, polar Water, urea, amino They are repelled by the molecules acids, glucose phospholipid bilayer and cannot move through it. ❸ Small ions Sodium ion, calcium They can be transported by ion channel proteins or carrier proteins. ❹ Large Starch, triglycerides, They cannot move across the molecules proteins membrane. ❶ small, non-polar ❷ small, polar molecules ❹ large molecules ❸ small ions molecules phospholipid bilayer channel carrier protein protein Fig 3.8 Substances move across the membrane along different paths 3– 8  3 Movement of substances across cell membrane Factors affecting the permeability of the cell membrane Temperature and organic solvents affect the structure of the cell membrane and hence its permeability. Factor How it affects membrane permeability Temperature When temperature increases, the phospholipid molecules in the cell membrane have more kinetic energy. They move faster and pack less closely together. Therefore, substances can move across the phospholipid bilayer of the cell membrane more easily, i.e. the permeability of the membrane increases. Boiling can even damage the membrane, causing it to become fully permeable. Organic solvents Organic solvents dissolve the phospholipids, (e.g. alcohol) causing damage to the membrane. As a result, the permeability of the membrane increases. Effects of temperature and organic solvents on the 3.1 permeability of cell membrane Introduction Beetroot* cells contain a red pigment. If the permeability of the cell membrane Practical 3.1 increases, or if the membrane is damaged, the pigment will leak out. Procedure A Effect of temperature 1 Add 5 cm3 of distilled water into 12 test tubes. Label six of them A1 to The cork borer and knife are very sharp. F1, and six of them A2 to F2. Handle them with care. 2 Prepare six groups of beetroot discs as shown below. Make six beetroot strips Cut each strip to 1.5 cm long. Cut each strip into five with a cork borer. Ensure that no peel is left on discs of approximately the strips. equal thickness. cork borer beetroot beetroot strip knife 1.5 cm 3 Rinse each group of discs in running water to wash off the pigment on the surface. Blot them with tissue paper. cont. beetroot 甜菜根 3– 9 I Cells and Molecules of Life 4 Use a water bath to heat tube A1 to 30 °C. Put one group of discs into the tube and wait for 1 minute. Transfer the discs to tube A2. Keep it at room temperature and wait for 20 minutes. A1 beetroot after A1 water bath disc (30 °C) A1 1 minute A2 distilled water distilled water distilled water at at 30 °C room temperature 5 Remove the beetroot discs from tube A2. Shake the tube gently and observe the colour intensity of the solution. Record the result. 6 Repeat the steps for tubes B1 to F1 which are heated to 40 °C, 50 °C, 60 °C, 70 °C and 80 °C respectively. B Effect of alcohol 1 Add 5 cm3 of distilled water, 10% alcohol, 30% alcohol and 50% alcohol into four test tubes. Alcohol is flammable. Use it 2 Prepare four groups of beetroot discs as stated in steps 2 and 3 of Part A. only in the absence 3 Put one group of discs into each tube. Wait for 20 minutes. of naked flame. The cork borer and knife are very sharp. Handle them with care. beetroot disc distilled 10% 30% 50% water alcohol alcohol alcohol 4 Remove the beetroot discs from the tubes. Shake the tubes gently and observe the colour intensities of the solutions. Record the results. Results and discussion The higher the temperature, the higher the intensity of the red colour in the solution. This indicates that membrane permeability increases as temperature increases. Alcohol is an organic solvent. Alcohol damages the cell membrane, causing the red pigment to leak out. The higher the alcohol concentration, the higher the intensity of the red colour in the solution. This indicates that membrane permeability increases as alcohol concentration increases. 3– 10  3 Movement of substances across cell membrane 2 The cell membrane is fluid in nature Since the phospholipid molecules in the cell membrane can move laterally, the cell membrane is fluid in nature. This allows the cell membrane to change shape and fuse with one another during processes like phagocytosis* and cell division (Fig 3.9). Photomicrographs of cells undergoing cell division (×1000) Drawings of the Cell Cell Two cells cell membrane membrane membrane are formed. changes fuses shape as together. the cell begins to divide. Fig 3.9 Cell membrane changes shape and fuses during cell division How is the structure of the cell membrane related to its properties and functions? Structure of How it is related to the properties and the cell membrane functions of the cell membrane The phospholipid This makes the cell membrane molecules are arranged differentially permeable. in a bilayer. The core of Small, non-polar molecules can the phospholipid bilayer dissolve in the phospholipid bilayer is hydrophobic. and move across the membrane. Some protein molecules Small, polar molecules and small act as channels or ions are transported by channel carriers. proteins or carrier proteins. The phospholipid This makes the cell membrane fluid in molecules can move nature. This allows the cell membrane laterally. to change shape and fuse with one another during processes like phagocytosis and cell division. phagocytosis 吞噬 3– 11 I Cells and Molecules of Life Level 1 1 The diagram on the right shows the fluid X mosaic model of the cell membrane. Which of the following parts of the cell Y membrane prevent(s) polar substances and ions from entering the cell? Z A X only B Y only C Z only D Y and Z only p. 8 2 Which of the following correctly describe(s) the possible way in which an ion moves across the cell membrane? (1) through the phospholipid bilayer (2) through a channel protein (3) transport by a carrier protein A (1) only B (1) and (2) only C (2) and (3) only D (1), (2) and (3) p. 8 Level 2 3 Which of the following are the most possible paths along which fatty acids and amino acids move across the cell membrane? X Y phospholipid bilayer protein molecules Fatty acids Amino acids A X X B X Y C Y X D Y Y p. 8 3– 12  3 Movement of substances across cell membrane 3.3 Movement of substances across membranes Substances move across the cell membrane by four main mechanisms: diffusion*, osmosis*, active transport* and phagocytosis. DSE A Diffusion 19(IA)Q8, 11 1 What is diffusion? When a tea bag is put into a cup of hot water, brown colour spreads out Animation 3.1 from the tea bag. Eventually the whole cup of water becomes brown. Why does this happen? Substances are made up of particles. In liquids and gases, particles move randomly in all directions and thus they tend to distribute evenly. When there is a difference in the concentrations of particles between two regions (i.e. a concentration gradient* exists), there will be a net movement of particles from the region of higher concentration to the region of lower concentration (i.e. down the concentration gradient), until the particles are evenly distributed. This is called diffusion (Fig 3.10). region of region of brown particles evenly distributed net (equilibrium* state) higher lower movement no net movement concentration concentration brown particle particles move randomly Fig 3.10 Diffusion When the particles become evenly distributed (i.e. the concentration gradient no longer exists), we can say that an equilibrium state is reached. There is no net movement of particles between the two regions, but the particles are still moving randomly in all directions. Diffusion depends on the spontaneous and random movement of particles. It does not require energy. It is a passive process. active transport 主動轉運 concentration gradient 濃度梯度 diffusion 擴散 equilibrium 平衡 osmosis 滲透 3– 13 I Cells and Molecules of Life 2 Factors affecting the rate of diffusion across membranes Factor How it affects the rate of diffusion Concentration The steeper the concentration gradient between two gradient regions, the higher the rate of diffusion. Surface area The larger the surface area over which diffusion occurs, the higher the rate of diffusion. Distance The shorter the distance between two regions of different concentrations, the higher the rate of diffusion. Temperature At higher temperatures, particles have more kinetic energy. This results in a higher rate of diffusion. Size of the Smaller particles diffuse faster than large particles. particles Polar molecules and ions Nature of the Non-polar substances usually diffuse faster than diffuse across the cell particles polar substances because they move across the membrane with the help of membrane through the phospholipid bilayer directly. channel proteins and carrier proteins. 3 Importance of diffusion Diffusion enables cells to waste nutrients (e.g. carbon dioxide) exchange materials with the environment. Cells obtain useful oxygen materials like oxygen and cell nutrients and remove waste like carbon dioxide by diffusion fluid surrounding the cell (Fig 3.11). Fig 3.11 Cells exchange materials with Cross-link Diffusion is also involved in many the environment by diffusion Human small intestine life processes like absorption and human lungs are well adapted for diffusion. For of nutrient in human small example, they have large intestine and gas exchange in surface areas. The details will human lungs. be discussed in Ch 6 and Bk 1B, Ch 7. What is diffusion? Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (i.e. down the concentration gradient). 3– 14  3 Movement of substances across cell membrane DSE B Osmosis – a special kind of diffusion 13(IA)Q23–25, 16(IA)Q24, 25, 17(IB)Q2, 18(IB)Q2, 1 What is osmosis? 19(IA)Q8 Like particles of other substances in liquids and gases, water molecules also move around randomly and tend to distribute evenly. If some substances are dissolved in water, the tendency of water molecules to move decreases because the solute particles attract the water molecules. We can describe the tendency of the water molecules to move from Water potential is measured one place to another using the term water potential*. The more the in kilopascals (kPa). solute particles in a solution (i.e. the higher the concentration of the solution), the lower the tendency of the water molecules to move. We can say the solution has a lower water potential. When two solutions of different water potentials are separated by a Animation 3.2 differentially permeable membrane which does not allow the solute particles to move across, more water molecules will move from the solution of higher water potential to the solution of lower water potential (Fig 3.12). The net movement of water molecules from a region of higher water potential to a region of lower water potential across a differentially permeable membrane is called osmosis. Like diffusion, osmosis does not require energy. It is a passive process. region of region of higher water potential differentially lower water potential permeable less concentrated solution more concentrated solution membrane (fewer solute particles per (more solute particles per unit volume) unit volume) water molecule solute particle can move across cannot move across net movement of water molecules Fig 3.12 Osmosis water potential 水勢 3– 15 I Cells and Molecules of Life Water potentials Water molecules in pure water have the highest tendency to move. Thus pure water has the highest water potential, which is defined as zero. All solutions have a water potential lower than that of pure water, i.e. a negative value. The higher the concentration of the solution, the Which of the following lower (more negative) its water potential (Fig 3.13). ? solutions has a higher water potential? a a 0.5% sucrose solution and a 2% sucrose solution b a solution with a pure water 1% sucrose solution 2% sucrose solution water potential highest water potential (0 kPa) of –600 kPa and a solution with a increasing concentration water potential of decreasing water potential (more negative) –300 kPa Fig 3.13 Relationship between concentration and water potential of a solution 3.2 Demonstration of osmosis using dialysis tubing Introduction Dialysis tubing* is differentially permeable. It has many small pores which Practical 3.2 allow small molecules (e.g. water molecules) to pass through but not large ones (e.g. sucrose molecules). It can be used to demonstrate osmosis. Procedure Animal tissues like a 1 Wet two dialysis tubing of 15 cm long. Tie a knot at one end of each chicken crop*, a pig tubing. bladder* and a fish swim bladder* are also 2 Fill one of the dialysis tubing with 20% sucrose solution and another differentially permeable with distilled water. Tie each tubing to a capillary tube with thread. and can also be used in this experiment. Watch the video below capillary tubes which demonstrates the experiment. thread Video 3.1 dialysis tubing 20% distilled sucrose water solution bladder 膀胱 crop 嗉囊 dialysis tubing 透析管 swim bladder 鰾 3– 16  3 Movement of substances across cell membrane 3 Rinse the outside of the tubing with distilled water. 4 Immerse each tubing in a beaker of distilled water as shown below. Mark the initial liquid levels. Observe any changes in the liquid levels after 30 minutes. experimental set-up control set-up capillary tubes initial liquid levels distilled water 20% sucrose dialysis tubing distilled solution water Results and discussion The liquid level in the experimental set-up rises. The final liquid level of the experimental set-up is higher than that of the control set-up. In the experimental set-up, distilled water has a higher water potential than 20% sucrose solution. There is a net movement of water molecules from distilled water in the beaker to the sucrose solution in the dialysis tubing. Which of the following changes in the experimental set-up would affect the rate at Video 3.2 ? which the liquid level rises? Which would affect the final liquid level? a using a longer dialysis tubing b using a larger volume of 20% sucrose solution c using a larger volume of distilled water d increasing the temperature e using a more concentrated sucrose solution 3– 17 I Cells and Molecules of Life 2 Osmosis and cells Solutions surrounding cells may cause the cells to lose or gain water by osmosis. The table below shows what happens when animal cells are put into solutions of different water potentials. hypotonic, isotonic, Surrounding Hypotonic* Isotonic* solution Hypertonic* hypertonic solution solution (same water solution In Greek, ‘hypo’ means (higher water potential as the (lower water ‘below’, ‘iso’ means potential than the cytoplasm) potential than the ‘equal’ and ‘hyper’ means cytoplasm) cytoplasm) ‘above’. Net Enters the cells No net movement Leaves the cells movement of water Changes in Swell and may No change Shrink* and the animal finally burst* become wrinkled* cells The bursting of red blood a red blood b c cells with the release of cell burst haemoglobin is known as haemolysis*. Why do some red ? blood cells burst while others remain intact in Fig 3.14a? Fig 3.14 Red blood cells in a (a) hypotonic, (b) isotonic and (c) hypertonic solution (×400) When a cell is put into an isotonic solution, there is no water movement across its cell membrane. When a cell is put into an isotonic solution, there is no net water movement across its cell membrane. Water enters and leaves the cell at the same rate. burst 爆裂 hypertonic 高滲的 hypotonic 低滲的 haemolysis 溶血 isotonic 等滲的 shrink 萎縮 wrinkle 皺褶 3– 18  3 Movement of substances across cell membrane The table below shows what happens when plant cells are put into solutions of different water potentials. Surrounding Hypotonic solution Isotonic solution Hypertonic solution solution Net Enters the cells No net movement Leaves the cells movement of water Changes in Become turgid* No change Become flaccid* the plant and plasmolysed*; cells vacuoles shrink What is the ? substance between the cell wall and the cell membrane in a vacuole plasmolysed cell? cytoplasm is pushed shrinks against the cell wall cell membrane is pulled away from the cell wall (plasmolysed) In hypotonic solutions, plant cells gain water by osmosis. As plant cells are surrounded by a rigid cell wall, the swelling of cytoplasm causes a pressure to build up on the cell wall which prevents further entry of water. Eventually, water will stop entering the cells. The cytoplasm is pushed against the cell wall and the cells are said to be turgid. On the other hand, in hypertonic solution, plant cells lose water by osmosis. The cytoplasm is no longer pushed against the cell wall. The cells are said to be flaccid. If water loss continues, the cell membrane will be pulled away from the cell wall. This phenomenon is called plasmolysis (Fig 3.15). a b cell wall cell membrane chloroplast Fig 3.15 (a) Normal plant cells and (b) plasmolysed plant cells (×400) flaccid 軟縮 plasmolysis 質壁分離 turgid 膨脹 3– 19 I Cells and Molecules of Life When water supply to a plant is plentiful, plant cells are surrounded by hypotonic solutions. The cells are turgid and press against one Cross-link another. This is important for the support of the plants, e.g. keeping The details of support in the leaves stretching out. When a plant does not have enough water, the plants will be discussed in Bk 1B, Ch 10. cells become flaccid and fail to press against one another. The leaves droop* (Fig 3.16). When water supply is plentiful When the plant does not have enough water leaf droops leaf stretches out Turgid cells press against Flaccid cells fail to press each other to give support against each other and give to the plant. no support to the plant. Fig 3.16 How cell turgidity provides support to a plant Spring onion ‘flower’ Spring onion ‘flowers’ for decorating dishes are easy to make. You just need to cut several vertical slits at one end of a spring onion section and put it into water. When the spring onion section is put into water, water enters its cells by osmosis. The cells increase in size, but the outer waxy layer of the spring onion prevents the cells in the outer layer from stretching. The cells in the inner layer expand more than those in the outer layer. As a result, the cut tips of the spring onion section curl outwards, and a spring onion ‘flower’ is formed. cells in inner layer expand more waxy outer layer Fig 3.17 A spring onion ‘flower’ Fig 3.18 The cut tips curl outwards because the cells in the inner layer expand more droop 垂下 3– 20  3 Movement of substances across cell membrane 3.3 Study of osmosis at cellular level Introduction Zebrina* and Rhoeo discolour* contain red pigments in their leaf epidermal cells. Practical 3.3 They are often used in the study of osmosis because the epidermal cells can be observed clearly under a microscope. Procedure The forceps are very A Observe plant cells in a concentrated sucrose solution sharp. Handle them 1 Obtain a leaf of Zebrina or Rhoeo discolour. Prepare a temporary mount of with care. the lower epidermis of the leaf as shown. Tear the leaf diagonally. Peel Lay the piece of lower epidermis Add a drop of concentrated off a small piece of the lower flat on a slide. sucrose solution and put a cover epidermis of the leaf. slip on it. cover slip slide lower epidermis lower epidermis concentrated sucrose solution 2 Wait for 3 minutes. Observe the lower epidermis with a microscope under high-power magnification. Draw a labelled high-power diagram of the cells. B Observe plant cells in a less concentrated sucrose solution 1 Using the same temporary mount in Part A, dilute the concentrated sucrose solution slowly with distilled water as shown below. add distilled water slowly at this side draw distilled water slowly to the right by tissue paper water flow at this side 2 Observe the epidermis again. Draw a labelled high-power diagram of the cells. C Observe plant cells in a very dilute sucrose solution 1 Using the same temporary mount in Part B, further dilute the sucrose cont. solution with distilled water. 2 Observe the epidermis again. Draw a labelled high-power diagram of the cells. Rhoeo discolour 紫萬年青 Zebrina 水竹草 3– 21 I Cells and Molecules of Life Results and discussion The appearances of Zebrina epidermal cells in different liquids are shown on the right. Fig 3.19 Zebrina epidermal cells in concentrated sucrose solution (left) and very dilute sucrose solution (right) (×100) The concentrated sucrose solution is hypertonic to the epidermal cells. In this solution, the cells lose water by osmosis and become plasmolysed. Dilute sucrose solution is hypotonic to the epidermal cells. When the sucrose solution is slowly replaced by distilled water, the cells gain water by osmosis and become turgid again. Plasmolysis is usually reversible without causing permanent damage to the cell. 3.4 Study of osmosis at tissue level Introduction In this practical, we will study the changes in texture, length and mass of Practical 3.4 potato strips when they are immersed in distilled water and sucrose solutions of different concentrations. Procedure 1 Add 20 cm3 of distilled water, 10% sucrose solution and 20% sucrose solution into boiling tubes A1 to A3, B1 to B3 and C1 to C3 respectively. 2 Prepare nine potato strips as shown below. Make nine potato strips Cut each strip to 5 cm long. The cork borer and with a cork borer. Ensure that no peel is left. knife are very sharp. Handle them with cork borer care. potato potato strip knife 5 cm cont. 3 Blot the strips with tissue paper. Weigh them with an electronic balance. 3– 22  3 Movement of substances across cell membrane 4 Put one potato strip into each boiling tube as shown below. Cover the tubes with plastic food wrap and leave them for 1 hour. In distilled water In 10% sucrose solution In 20% sucrose solution plastic food A1 A2 A3 B1 B2 B3 C1 C2 C3 wrap potato strip 5 Remove the strips from the boiling tubes. Blot each strip with tissue paper. Feel the texture of each strip and measure their length and mass immediately. 6 Calculate the percentage changes in length and mass for each strip. The change in length or mass of the potato Percentage change = final length – initial length × 100% strips can also be in length (%) initial length expressed in the ratio of final to initial Percentage change = final mass – initial mass × 100% length or mass. in mass (%) initial mass Calculate the average values of the percentage changes in length and mass for the three strips in each solution. Results and discussion The following table summarizes the changes in the potato tissues. In 10% sucrose In 20% sucrose In distilled water solution solution Change in texture Become harder No change Become softer Change in length Become longer Change only slightly Become shorter Change in mass Become heavier Change only slightly Become lighter In distilled water, the potato strips become harder, longer and heavier. This shows that there is a net movement of water into the cells by osmosis. Distilled water is hypotonic to the potato tissue. In 10% sucrose solution, the potato strips show no change in texture and change only slightly in length and mass. This shows that the net amount of water moving into or out of the cells by osmosis is very small. The 10% sucrose solution is nearly isotonic to the potato tissue. In 20% sucrose solution, the potato strips become softer, shorter and lighter. This shows that there is a net movement of water out of the cells by osmosis. The 20% sucrose solution is hypertonic to the potato tissue. 3– 23 I Cells and Molecules of Life Learning through examples Skill builder Skill practice In an investigation, potato strips were immersed in boiling tubes containing sucrose solutions of different concentrations for one hour. The initial and final mass of the potato strips were measured and a graph was plotted using the results. The graph is shown below. 1.1 1.0 ratio of final mass to initial mass 0.9 0 5 10 15 concentration of sucrose solution (%) a Using the graph, determine the water potential of the potato tissue in terms of sucrose solution concentration. (1 mark) Determining water potential from a b Describe the texture of the potato strips after they were immersed in graph 20% sucrose solution for one hour. Explain your answer. (4 marks) Refer to p. 25. c State two modifications that can be made to the experiment so that the results can be obtained faster. (2 marks) Suggested answers a Water potential of the potato tissue equals that of 7% sucrose Online tutorial 3.1 solution. 1 b The mass of the strips decreased after they were immersed in 20% sucrose solution for one hour. 1 That means there was a net loss of water from the potato strips. 1 The potato cells became flaccid and failed to press against one The term ‘flaccid’ another. 1 should be used instead of ‘plasmolysis’ here. The potato strips became soft. 1 ‘Plasmolysis’ describes c Cut the potato strips into several discs to increase the surface area for the separation of a cell membrane from osmosis. 1 the cell wall. It is not Immerse the boiling tubes into a water bath to increase the temperature sufficient to explain the at which the experiment is carried out. 1 texture of the tissue. 3– 24  3 Movement of substances across cell membrane Learning through examples Skill builder Skill practice Determining water potential from a graph A net movement of water into or out of the potato strips would cause a change in mass of the strips. To determine the water potential of the potato tissue in terms of sucrose solution concentration, we have to find the concentration of the solution in which the mass of potato strips remains unchanged after immersion, i.e. the ratio of final mass to initial mass equals 1. The ratio of final mass to initial mass of potato strips after being immersed into sucrose solutions of different concentrations 2 Draw a horizontal line from the 1.1 y-axis until it hits the graph line. 1 Find the value ‘1’ on the y-axis. 3 Draw a vertical line from the point on the 1.0 graph line down to the ratio of final mass x-axis. Read the value to initial mass on the axis. 0.9 0 5 7 10 15 concentration of sucrose solution (%) Learning through examples Skill builder Skill practice The graph below shows the percentage changes in the mass of sweet potato strips after they were immersed in salt solutions of different concentrations for one hour. 10 percentage change 0.2 0.4 concentration of in mass (%) 0 salt solution (M) Determine from the graph the water potential of the sweet potato tissue, in terms of salt concentration. (1 mark) Q19 (p. 37) 3– 25 I Cells and Molecules of Life 3 Importance of osmosis Osmosis is the main mechanism by which water enters and leaves cells in all organisms. Cross-link Absorption of water in human intestines and absorption of water The details of absorption from the soil into plant roots are also carried out by osmosis. of water in humans and in plants will be discussed in Ch 6 and Bk 1B, Ch 10 respectively. Preservation of food Growth of microorganisms on food may cause food decay*. We can preserve* foods by immersing them in concentrated salt or sugar solutions (Fig 3.20). Since these solutions are hypertonic to the microorganisms, water is drawn out from the microorganisms by osmosis. The microorganisms will die or stop Fig 3.20 Peaches preserved growing due to a lack of water. in a concentrated sugar solution 1 What is osmosis? Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a differentially permeable membrane. 2 What happens to animal cells and plant cells if they are put into a hypotonic, an isotonic or a hypertonic solution? Hypotonic solution Isotonic solution Hypertonic solution (higher water potential (same water potential (lower water potential than the cytoplasm) as the cytoplasm) than the cytoplasm) Net movement Enters the cells No net movement Leaves the cells of water Changes in the Swell and may finally No change Shrink and become animal cells burst wrinkled Changes in the Become turgid No change Become flaccid and plant cells plasmolysed; vacuoles shrink food decay 食物腐壞 preserve 保存 3– 26  3 Movement of substances across cell membrane Level 1 Questions 1 and 2: State whether the statements are true or false. 1 When a cell is placed in a concentrated sucrose solution, sucrose solution enters the cell. p. 18, 19 2 An animal cell shrinks when it is placed in a hypertonic solution. p. 18 3 The diagrams below show the appearances of the cells in three pieces of leaf epidermis before and after they were immersed into solutions. X Y Z before after before after before after Which piece of epidermis was likely to be immersed in a distilled water? b dilute sucrose solution? c concentrated sucrose solution? p. 19 Level 2 4 The appearances of three potato strips before and after they were immersed in three sucrose solutions (X, Y and Z) are shown below. X Y Z Before immersion After immersion The water potentials of the solutions from the lowest to the highest are Many people drink A X, Z, Y. B Y, X, Z. isotonic drinks after C Y, Z, X. D Z, Y, X. p. 22, 23 exercise. Visit the following website and 5 In the set-up on the right, the weight of the dialysis learn more about isotonic tubing changed from 10 g to 11.2 g in 30 minutes. drinks. Discuss with your dialysis Which of the following may be X and Y? tubing classmates the possible benefits of drinking X Y X isotonic drinks. A distilled water 5% sucrose solution Y https://blog.hiddit.com/ B 5% sucrose solution 5% sucrose solution isotonic-for-triathlon C 5% sucrose solution 10% sucrose solution D 10% sucrose solution distilled water p. 16, 17 3– 27 I Cells and Molecules of Life DSE C Active transport 19(IA)Q8 1 What is active transport? We have learnt on p. 5 that in the cell membrane, there are carrier proteins which help the transport of substances across the cell membrane. Some of these carrier proteins can help move substances from a region of lower concentration to a region of higher concentration (i.e. against the concentration gradient) using energy. The transport mechanism is called active transport (Fig 3.21). carrier protein Fig 3.21 shows the uptake lower higher of a particle into the cell as concentration concentration an example. Substances can 1 The particle binds to also be transported out of a carrier protein. the cells by active transport. 2 The carrier protein changes its shape using energy. 3 The particle is released on the other Animation 3.3 side of the membrane. Fig 3.21 Active transport Active transport is an active process. It requires energy. The energy for active transport comes from respiration of the cells. Therefore, active transport occurs only in living cells. The cells with high rates of active transport usually have a high respiration rate and a large number of mitochondria. Conditions that lower the rate of respiration would decrease the rate of active transport. Examples include low temperature, low oxygen concentration and presence of chemicals that inhibit respiration (e.g. cyanide*). 2 Importance of active transport Active transport enables cells to take up additional useful substances which are already high in concentration in the cells. Cross-link Active transport is involved in many processes in organisms. The details of these processes Examples include the absorption of nutrients (e.g. glucose and will be discussed in Ch 6 and Bk 1B, Ch 10 respectively. amino acids) in the human small intestine, and the absorption of minerals from the soil into plant roots. cyanide 氰化物 3– 28  3 Movement of substances across cell membrane What is active transport? Active transport is the movement of substances across the cell membrane from a region of lower concentration to a region of higher concentration (i.e. against the concentration gradient) using energy. DSE D Phagocytosis 19(IA)Q8 1 What is phagocytosis? Diffusion, osmosis and active transport are mechanisms by which small molecules or ions are transported across the cell membrane. Sometimes Phagocytosis cells also take in large particles. They do this by packaging the In Greek, ‘phago’ means ‘eat’ and ‘cyto’ means particles into vacuoles formed from the cell membrane. This transport ‘cell’. mechanism is called phagocytosis (Fig 3.22). 1 Foot-like extensions of cytoplasm called pseudopodia are formed to surround the particle to be taken in. nucleus 2 Cell membrane particle to be fuses to form a taken in vacuole which encloses the pseudopodium* cell membrane particle. How does the property ? of the cell membrane allow the cells to carry out phagocytosis? digested products 4 The digested products are absorbed into the cytoplasm. digestive enzyme Animation 3.4 3 The vacuole is fused with some other vacuoles which contain digestive enzymes. The particle Video 3.3 is broken down with the help of the enzymes. Fig 3.22 Phagocytosis pseudopodium 偽足 3– 29 I Cells and Molecules of Life Amoeba Phagocytosis is an active process. It requires energy. Energy is needed for changing the shape of the cell membrane, fusing the cell membrane and moving the vacuoles. 2 Importance of phagocytosis food particle Phagocytosis is carried out in certain types of cells. For example: (×100) Some unicellular organisms (e.g. Amoeba) engulf food particles by Fig 3.23 Amoeba engulfing food particles by phagocytosis (Fig 3.23). Phagocytosis is important for their nutrition. phagocytosis In humans, certain types of white blood cells engulf harmful microorganisms by phagocytosis. Phagocytosis is important for body defence against diseases. What is phagocytosis? Phagocytosis is the uptake of large particles into cells by packaging the particles into vacuoles formed from the cell membrane. Level 1 1 The table below shows a comparison of diffusion, osmosis, active transport and phagocytosis. Complete the table. (4 marks) Energy Movement of particles needed? Diffusion Net movement of particles from a region of (a) (c) concentration to a region of (b) concentration Osmosis Net movement of water molecules from a region of (f) (d) water potential to a region of (e) water potential Active Movement of substances across the cell membrane from a region (i) transport of (g) concentration to a region of (h) concentration Phagocytosis Uptake of large particles into the cells by packaging them into (j) vacuoles formed from the cell membrane cont. p. 13, 15, 28, 29 3– 30  3 Movement of substances across cell membrane Level 2 2 Plants absorb ions such as nitrate and sulphate from the soil into the root hair cells in their roots. The table below shows the concentrations of nitrate and sulphate in the root hair cells of a plant and in the soil. Concentration in the Concentration in the Ion root hair cells (ppm) soil (ppm) Nitrate 20 000 1000 Sulphate 1200 800 Which of the following are most likely to be the mechanisms by which the ions are absorbed? Nitrate Sulphate A diffusion diffusion B diffusion active transport C active transport diffusion D active transport active transport p. 13, 28 Recall Think about... (p. 1) 1 The highly salty water has a lower water potential than the cytoplasm of the cells. Water moves out of the cells due to osmosis. Organisms may die due to excessive water loss. 2 Taking in salts reduces the water potential of the cytoplasm. This decreases the water potential gradient between the salty water and the cytoplasm.