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Cambridge. IGCSE. ®. Biology. Third Edition. Biology. Third Edition. 25 . Donald Emslie-Smith portbestbodcero.cf, Textbook of Physiology, 11th Revised. With an emphasis on human biology, the Cambridge IGCSE Biology syllabus helps Textbooks Cambridge IGCSE Biology Student Book Second Edition. Cambridge IGCSE English First Language (Third Edition) Cambridge IGCSE Biology 3rd Edition by D G Mackean and Dave Hayward.
We use the unusual fungus yeast to make ethanol and bread. We obtain antibiotics such as penicillin from various different fungi.
Some fungi, however, are harmful. Fungi do not have chlorophyll and do not photosynthesise. Instead they feed saprophytically, or parasitically, on organic material like faeces, human foods and dead plants or animals.
The kingdom Protoctista Figure 1. They all have cells with a nucleus, but some have plant-like cells with chloroplasts and cellulose cell walls, while others have animal-like cells without these features.
Most protoctists are unicellular made of just a single cell but some, such as seaweeds, are multicellular. Bacteria have cells that are very different from the cells of all other kinds of organism. The most important difference is that they do not have a nucleus.
You will meet bacteria at various stages in your biology course. Some of them are harmful to us and cause diseases such as tuberculosis TB and cholera. Many more, however, are helpful. You will find out about their useful roles in the carbon cycle and the nitrogen cycle, in biotechnology, in the treatment of sewage to make it safe to release into the environment and in making insulin for the treatment of people with diabetes. Some bacteria can carry out photosynthesis.
The oldest fossils belong to this kingdom, so we think that they were the first kinds of organism to evolve. You have almost certainly had an illness caused by a virus. Viruses cause common diseases such as colds and influenza, and also more serious ones such as AIDS. Viruses are not normally considered to be alive, because they cannot do anything other than just exist, until they get inside a living cell.
These new viruses burst out of the cell and invade others, where the process is repeated. The host cell is usually killed when this happens. On their own, viruses cannot move, feed, excrete, show sensitivity, grow or reproduce. It is not made of a cell — it is simply a piece of RNA a chemical similar to DNA surrounded by some protein molecules.
It is hugely magnified in this diagram. The scale bar represents a length of 10 nanometres. In other words, you could line up more than 15 of these viruses between two of the millimetre marks on your ruler. Phylum Vertebrates These are animals with a supporting rod running along the length of the body. The most familiar ones have a backbone and are called vertebrates.
Class Fish The fish Figure 1. Class Amphibians Although most adult amphibians live on land, they always go back to the water to breed.
Frogs, toads and salamanders are amphibians Figure 1. Class Birds The birds Figure 1. Class Reptiles These are the crocodiles, lizards, snakes, turtles and tortoises Figure 1. Reptiles do not need to go back to the water to breed because their eggs have a waterproof shell which stops them from drying out.
Class Mammals This is the group that humans belong to Figure 1. Phylum Arthropods Arthropods are animals with jointed legs, but no backbone. They are a very successful group, because they have a waterproof exoskeleton that has allowed them to live on dry land. There are more kinds of arthropod in the world than all the other kinds of animal put together. Insects Insects Figure 1. They are mainly terrestrial land-living.
Locust head. Arachnids These are the spiders, ticks and scorpions. They are land-dwelling organisms. Crustaceans These are the crabs, lobsters and woodlice.
They breathe through gills, so most of them live in wet places and many are aquatic. Myriapods These are the centipedes and millipedes. We have seen that plants are organisms that have cells with cell walls made of cellulose.
At least some parts of a plant are green. The green colour is caused by a pigment called chlorophyll, which absorbs energy from sunlight. The plant uses this energy to make glucose, using carbon dioxide and water from its environment. This is called photosynthesis. Plants include small organisms such as mosses, as well as ferns Figure 1. These are the plants that are most familiar to us. They can be tiny, or very large — many trees are flowering plants. Monocots have only one cotyledon in their seeds.
They usually have a branching root system, and often have leaves in which the veins run in parallel to one another. Dicots have two cotyledons in their seeds.
They frequently have a tap root system, and their leaves are often broader than those of monocots, and have a network of branching veins. Corn monocot. Ferns Ferns have leaves called fronds. They do not produce flowers, but reproduce by means of spores produced on the underside of the fronds.
Biologists need to be able to look closely at specimens — which might be whole organisms, or just part of an organism — and note significant features of them. It is also important to be able to make simple drawings to record these features. A biological drawing needs to be simple but clear. You will be provided with a specimen of an animal to draw. Jot down the features of the organism that helped you to classify it. Here are some points to bear in mind when you draw.
However, do leave space around it so that you have room for labels. You could also label any features that help the organism to survive in its environment. These are called adaptations. Here are some points to bear in mind when you label a diagram.
Drawings of biological specimens are usually made at a different size from the real thing. It is important to show this on the diagram. The magnification of a diagram is how much larger it is than the real thing. You should find that it is 40 mm long. The following are two very important things to notice. Usually, millimetres are the best units to use. Magnification does not have a unit. Write your answer in millimetres. A2 The real length of the appendage was 10 mm.
Use this, and your answer to question A1, to calculate the magnification of the drawing of the centipede. Study tip Be prepared to use the magnification equation organised in a different way: If you want to identify an organism whose name you do not know, you may be able to find a picture of it in a book.
However, not every organism may be pictured, or your organism may not look exactly like any of the pictures. If this happens, you can often find a key that you can use to work out what your organism is.
A key is a way of leading you through to the name of your organism by giving you two descriptions at a time, and asking you to choose between them.
Each choice you make then leads you on to another pair of descriptions, until you end up with the name of your organism. This kind of key is called a dichotomous key.
Here is a key that you could use to identify the organisms shown in Figure 1. To use the key, pick one of the animals that you are going to identify. Decide which description in step 1 matches your organism.
It has jointed limbs, so the key tells us to go to step 2. Decide which description in step 2 matches organism B. It has more than 5 pairs of jointed limbs, so it is a centipede. Constructing keys Using a key is quite easy, but writing your own key is much more of a challenge. First, make a list of features that clearly vary between the flowers.
They should be features that cannot possibly be mistaken. Remember that the person using the key will probably only have one of the flowers to look at, so they cannot necessarily compare it with another kind of flower. Now choose one of these features that can split the flowers into two groups.
Now concentrate on a group that contains more than one flower. Choose another feature that will allow you to split the flowers into two further groups. Now go back and refine your key. Think carefully about the wording of each pair of statements. Make sure that each pair is made up of two clear alternatives.
Try to reduce your key to the smallest possible number of statement pairs. Finally, try your key out on a friend. If they have any problems with it, then try to reword or restructure your key to make it easier to use.
Summary You should know: End-of-chapter questions 1 a Without looking back at the beginning of this chapter, decide which five of these characteristics are found in all living things. Carpodiptera africana, Commiphora africana, Commiphora angolensis Which two of these species do biologists consider to be the most closely related?
Explain your answer. Try to meet these criteria:. When you have finished, swap your key with someone else to check if it works. If not, make adjustments to it. You do not need to label your diagram. Show your working, and remember to include the unit. With reference to the diagram, and your own knowledge, discuss whether or not viruses can be considered to be living organisms.
Measure the length of the scale bar. Use this, and the label on the scale bar, to calculate the magnification of the diagram. Show your working. In this chapter, you will find out about: Cells from deep time If a long, thin spike of limestone hanging down from the roof of a cave is called a stalactite, what do you call a long, thin drip of bacteria-filled slime?
Caver Jim Pisarowicz decided to call them snottites, and the name stuck Figure 2. Snottites are studied by biologists interested in organisms that can live in environments so strange that almost. Snottites are found in caves where the atmosphere contains large amounts of the smelly, toxic gas hydrogen sulfide.
The bacteria in the slimy threads, far from being poisoned by the gas, actually use it to make their food.
In the middle of the threads, there is virtually no oxygen, yet some kinds of bacteria live even here. At that time, the cells of all organisms were much less complex than those of plants and animals which did not appear on Earth until around 2 billion years ago. They had no nucleus, for example. Yet bacteria made of these seemingly simple cells are clearly very successful, if they have managed to survive almost unchanged through such an unimaginably long period of time.
All organisms are made of cells. Cells are very small, so large organisms contain millions of cells. Some organisms are unicellular, which means that they are made of just a single cell. Bacteria and yeast are examples of single-celled organisms. Microscopes To see cells clearly, you need to use a microscope Figure 2. The kind of microscope used in a school laboratory is called a light microscope because it shines light through the piece of animal or plant you are looking at.
It uses glass lenses to magnify and focus the image. A very good light microscope can magnify about times, so that all the structures in Figures 2. Photomicrographs of plant and animal cells are shown in Figure 2. A photomicrograph is a picture made using a light microscope. To see even smaller things inside a cell, an electron microscope is used.
This means that a lot more detail can be seen inside a cell. We can see many structures more clearly, and also some structures that could not be seen at all with a light microscope.
Questions 2. With a light microscope you can see some structures inside a cell, such as a nucleus. With an electron microscope much more detail can be seen. Figure 2. Cell membrane Whatever sort of animal or plant they come from, all cells have a cell membrane sometimes called the cell surface membrane around the outside.
Inside the cell membrane is a jelly-like substance called cytoplasm, in which are found many small structures called organelles. The most obvious of these organelles is usually the nucleus. In a plant cell, it is very difficult to see, because it is right against the cell wall. The cell membrane is a very thin layer of protein and fat. It is very important to the cell because it controls what goes in and out of it.
A level Biology
It is said to be partially permeable, which means that it will let some substances through but not others. Cell wall All plant cells are surrounded by a cell wall made mainly of cellulose. Paper, which is made from cell walls, is Animal cells never have cell walls made of cellulose.
Cellulose belongs to a group of substances called polysaccharides, which are described in Chapter 4. Cellulose forms fibres which criss-cross over one another to form a very strong covering to the cell Figure 2. This helps to protect and support the cell. If the cell absorbs a lot of water and swells, the cell wall stops it bursting. Because of the spaces between fibres, even very large molecules are able to go through the cellulose cell wall.
It is therefore said to be fully permeable.
Cytoplasm Cytoplasm is a clear jelly. It contains many substances dissolved in it, especially proteins. Many different metabolic reactions the chemical reactions of life take place in the cytoplasm.
Vacuoles A vacuole is a space in a cell, surrounded by a membrane, and containing a solution. Plant cells have very large vacuoles, which contain a solution of sugars and other substances, called cell sap. A full vacuole presses outwards on the rest of the cell, and helps S to keep it in shape. Animal cells have much smaller membrane-bound spaces, called vesicles, which may contain food or water.
Chloroplasts Chloroplasts are never found in animal cells, but most of the cells in the green parts of plants have them. They contain the green colouring or pigment called chlorophyll. Chlorophyll absorbs energy from sunlight, and this energy is then used for making food for the plant by photosynthesis Chapter 6. Chloroplasts often contain starch grains, which have been made by photosynthesis.
Animal cells never contain starch grains. Some animal cells, however, do have granules tiny grains of another substance similar to starch, called glycogen. These granules are found in the cytoplasm, not inside chloroplasts.
Nucleus The nucleus is where the genetic information is stored. This helps the cell to make the right sorts of proteins. The chromosomes are made of DNA. Chromosomes are very long, but so thin that they cannot easily be seen even using the electron microscope. However, when the cell is dividing, they become short and thick, and can be seen with a good light microscope.
Table 2. Mitochondria Photographs of cells taken using an electron microscope, called electronmicrographs, show tiny structures that are almost invisible with a light microscope. They are called mitochondria singular: Mitochondria are found in almost all cells, except those of prokaryotes. Figures 2. Mitochondria are the powerhouses of the cell.
Inside them, oxygen is used to release energy from glucose, in the process called aerobic respiration. You will find out more about aerobic respiration in Chapter Not surprisingly, cells that use a lot of energy have a lot of mitochondria. Muscle cells, for example, are tightly packed with mitochondria.
Sperm cells, which need energy to swim to the egg, and neurones nerve cells , which need energy to transmit impulses, also have large numbers of mitochondria. The black spots in the electron micrograph in Figure 2. This is similar to starch. Starch is never found in animal cells — they store glycogen instead. Glycogen is a reserve fuel. When required, it can be broken down to glucose, to be used as a fuel by the mitochondria in the liver cell, or transported in the blood to other cells that need it.
Even tinier structures than mitochondria can just be seen with an electron microscope Figure 2. They are called ribosomes. They look like tiny dots attached to a network of membranes that runs throughout the cytoplasm. This network is called the rough endoplasmic reticulum. Ribosomes may also just be scattered freely in the cytoplasm. Ribosomes are found in all types of cells — bacteria, protoctists, fungi, animals and plants all have ribosomes in their cells.
Although they are so tiny that we can scarcely see them even with an electron microscope, ribosomes have a very important function in a cell. They are the places where proteins are made, by joining amino acids together in a long chain. You can read more about this in Chapter 4. Micrometres Cells, and structures inside them such as mitochondria and ribosomes, are so small that we need a very small unit in which to measure them.
Record your measurement in mm millimetres. How many of these mitochondria could you line up end to end between two of the mm marks on your ruler? Looking at animal cells Skills AO3. Wash your hands thoroughly after handling the trachea and cells. Some simple animal cells line the mouth and trachea or windpipe.
If you colour or stain the cells, they are quite easy to see using a light microscope see Figure 2. You will probably not be able to see anything at all at this stage. A1 Which part of the cell stained the darkest blue? A2 Is the cell membrane permeable or impermeable to methylene blue? Explain how you worked out your answer.
Looking at plant cells Skills AO3. Take care with the sharp blade when cutting the onion. To be able to see cells clearly under a microscope, you need a very thin layer. It is best if it is only one cell thick.
An easy place to find such a layer is inside an onion bulb. Do not let it get dry. Put the piece of epidermis into it, and spread it flat.
What sort of cells are surrounded by a cell membrane? What are plant cell walls made of? What does fully permeable mean? What does partially permeable mean? What is the main constituent of cytoplasm? What is a vacuole? What is cell sap?
Chloroplasts contain chlorophyll. What does chlorophyll do? What is stored in the nucleus? Why can chromosomes be seen only when a cell is dividing? A1 Name two structures which you can see in these cells, but which you could not see in the tracheal cells Activity 2. A2 Most plant cells have chloroplasts, but these onion cells do not. Suggest a reason for this. A3 Iodine solution turns blue-black in the presence of starch. Did any of the onion cells contain starch? Do remember not to colour your drawing.
Very carefully place some iodine solution next to the edge of the coverslip. The iodine solution will seep under the edge of the coverslip. To help it do this, you can place a small piece of filter paper next to the opposite side of the coverslip, which will soak up some of the liquid and draw it through. Note any differences between what you can see now and what it looked like before adding the iodine solution. Which types of cells contain mitochondria?
Outline the function of mitochondria. Which types of cells contain ribosomes?
IGCSE Biology Revision Guide
Outline the function of ribosomes. A large organism such as yourself may contain many millions of cells, but not all the cells are alike.
Almost all of them can carry out the activities which are characteristic of living things, but many of them specialise in doing some of these better than other cells do. Muscle cells, for example, are specially adapted for movement. Most cells in the leaf of a plant are specially adapted for making food by photosynthesis.
Tissues Often, cells which specialise in the same activity are found together. A group of cells like this is called a tissue. An example of a tissue is a layer of cells lining your stomach.
These cells make enzymes to help to digest your food Figure 2. The stomach also contains other tissues. For example, there is a layer of muscle in the stomach wall, made of cells which can move. This muscle tissue makes the wall of the stomach move in and out, churning the food and mixing it up with the enzymes. Plants also have tissues.
You may already have looked at some epidermis tissue from an onion bulb. Inside a leaf, a layer of cells makes up the palisade tissue, in which the cells are specialised to carry out photosynthesis.
Organs All tissues in the stomach work together, although each has its own job to do. A group of tissues like this makes up an organ. The stomach is an organ. Other organs include the heart, the kidneys and the lungs. In a plant, an onion bulb is an organ. A leaf is another example of a plant organ.
Organ systems The stomach is only one of the organs which help in the digestion of food. The mouth, the intestines and the stomach are all part of the digestive system.
The heart is part of the circulatory system, while each kidney is part of the excretory system. The way in which organisms are built up can be summarised like this: For example, the ciliated cells in Figure 2. You should know: For each of the following, state whether it is an organelle, a cell, a tissue, an organ, an organ system, or an organism. State which part of a plant cell: Distinguish between each pair of terms.
Use only words from the list to copy and complete the statements about cell B. Cell B has a thick outer layer called the cell This is made of The cytoplasm of cell B contains many The large permanent Diffusion spreads a deceptive scent Like most brightly-coloured flowers, fly orchids rely on insects to transfer their pollen from one flower to another Figure 3.
The pollen contains the male gametes, so the insects help the male gametes to reach the female gametes in another flower, so that fertilisation can take place. But insects do not perform this service out of kindness. Many flowers persuade insects to pollinate them by providing sweet nectar, or lots of spare protein-rich pollen for the insects to eat.
Not so the fly orchid. This flower uses deception to attract male digger wasps. Female digger wasps produce a chemical whose molecules diffuse through the air for long distances. The chemical, called a pheromone, is sensed by male digger wasps, which follow it up its concentration gradient to its source.
There, hopefully, they will find a female wasp with which they can mate. Fly orchids produce a very similar chemical, which diffuses outwards from the flower. Male digger wasps sense and react to it just as they do to the pheromone of the female wasps.
As they try to mate, the wasps pick up pollen from the flower. Atoms, molecules and ions are always moving. The higher the temperature, the faster they move. In a solid substance the particles cannot move very far, because they are held together by attractive forces between them.
In a liquid they can move more freely, knocking into one another and rebounding. In a gas they are freer still, with no attractive forces between the molecules or atoms. Molecules and ions can also move freely when they are in solution. When they can move freely, particles tend to spread themselves out as evenly as they can Figure 3.
This happens with gases, solutions, and mixtures of liquids. Imagine, for example, a rotten egg in one corner of a room, giving off hydrogen sulfide gas. To begin with, there will be a very high concentration of the gas near the egg, but none in the rest of the room. However, before long the hydrogen sulfide molecules have spread throughout the air in the room.
Soon, you will not be able to tell where the smell first came from — the whole room will smell of hydrogen sulfide. The hydrogen sulfide molecules have spread out, or diffused, through the air. Diffusion and living organisms.
Living organisms obtain many of their requirements by diffusion. They also get rid of many of their waste products in this way. For example, plants need carbon. Key definition diffusion — the net movement of molecules and ions from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement dioxide for photosynthesis. This diffuses from the air into the leaves, through the stomata.
It does this because there is a lower concentration of carbon dioxide inside the leaf, as the cells are using it up. Outside the leaf in the air, there is a higher concentration. Carbon dioxide molecules therefore diffuse into the leaf, down this concentration gradient. Oxygen, which is a waste product of photosynthesis, diffuses out in the same way. There is a higher concentration of oxygen inside the leaf, because it is being made there.
Oxygen therefore diffuses out through the stomata into the air. Diffusion is also important in gas exchange for respiration in animals and plants Figure 3. Cell membranes are freely permeable to oxygen and carbon dioxide, so these easily diffuse into and out of cells.
Some of the products of digestion are absorbed from the ileum of mammals by diffusion page 85—86 , and we have already seen that flowering plants use diffusion to attract pollinators like bees and wasps.
Fill a gas jar with water. Leave it for several hours to let the water become very still. You can try this with other coloured salts as well, such as copper sulfate or potassium dichromate. A1 Why was it important to leave the water to become completely still before the crystal was put in? A2 Why had the colour spread through the water at the end of your experiment?
A3 Suggest three things that you could have done to make the colour spread more quickly. Water is one of the most important compounds in living organisms.
It has many functions, including acting as a solvent for many different substances. For example, substances are transported around the body dissolved in the water in blood plasma. Various substances are dissolved in this water, and their concentrations may be different inside and outside the cell. This creates concentration gradients, down which water and solutes will diffuse, if they are able to pass through the membrane.
Figure 3. The membrane has holes or pores in it which are very small. An example of a membrane like this is Visking tubing. Water molecules are also very small. Each one is made of two hydrogen atoms and one oxygen atom. Sugar molecules are many times larger than this. In Visking tubing, the holes are big enough to let the water molecules through, but not the sugar molecules.
Visking tubing is called a partially permeable membrane because it will let some molecules through but not others. There is a higher concentration of sugar molecules on the right-hand side of the membrane in Figure 3. If the membrane was not there, the sugar molecules would diffuse from the concentrated solution into the dilute one until they were evenly spread out. However, they cannot do this because the pores in the membrane are too small for them to get through.
There is also a concentration gradient for the water molecules. On the left-hand side of the membrane, there is a high concentration of water molecules. On the. You can see that osmosis is really just a kind of diffusion. It is the diffusion of water molecules, in a situation where the water molecules but not the solute molecules can pass through a membrane.
It is much better to use a different term instead. We say that a dilute solution where there is a lot of water has a high water potential. A concentrated solution where there is less water has a low water potential. In Figure 3. There is a water potential gradient between the two sides. The water molecules diffuse down this gradient, from a high water potential to a low water potential. Over time, there will be an overall, or net, movement of water from left to right.
This is called osmosis. Cell membranes Cell membranes behave very much like Visking tubing. They let some substances pass through them, but not others. They are partially permeable membranes. There is always cytoplasm on one side of any cell membrane. Cytoplasm is a solution of proteins and other substances in water.
There is usually a solution on the other side of the membrane, too. Inside large. In the soil, the roots of plants are often surrounded by a film of water. If the solutions are of different concentrations, then osmosis will occur. Skills AO3.
You are going to investigate diffusion of two different substances dissolved in water solutes. When a substance is dissolved, its particles are free to move around. In this investigation, you will use starch solution and iodine solution. The solutions will be separated by a membrane made out of Visking tubing. Visking tubing has microscopic holes in it.
The holes are big enough to let water molecules and iodine molecules through, but not starch molecules, which are bigger than the holes. Collect a piece of Visking tubing. Moisten it and rub it until it opens.
Tie a knot in one end of the tubing. Using a pipette, carefully fill the tubing with some starch solution.
Tie the top of the tubing very tightly, using thread. Rinse the tubing in water, just in case you got any starch on the outside of it. Put some iodine solution into a beaker. Gently put the Visking tubing into the iodine solution, so that it is completely covered, as shown in the diagram. Leave the apparatus for about 10 minutes. A1 What colour were the liquids inside and outside the tubing at the start of the experiment?
A2 What colour were the liquids inside and outside the tubing at the end of the investigation? A3 When starch and iodine mix, a blue-black colour is produced. Where did the starch and iodine mix in your experiment? A4 Did either the starch particles or the iodine particles diffuse through the Visking tubing?
How can you tell? A5 Copy and complete these sentences. At the start of the experiment, there were starch molecules inside the tubing but none outside the tubing. Starch particles are too At the start of the experiment, there were iodine molecules The iodine molecules diffused into the tubing, down their When the starch and iodine molecules mixed, a Osmosis and animal cells Figure 3.
The cytoplasm inside the cell is a fairly concentrated solution. The proteins and many other substances dissolved in it are too large to get through the cell membrane. Water molecules, though, can get through. If you compare this situation with Figure 3.
The dilute solution in Figure 3. Therefore, osmosis will occur. Water molecules will diffuse from the dilute solution into the concentrated solution. What happens to the cell? As more and more water enters the cell, it swells. The cell membrane has to stretch as the cell gets bigger, until eventually the strain is too much, and the cell bursts. If this solution is more concentrated than the cytoplasm, then water molecules will diffuse out of the cell.
Look at Figure 3. As the water molecules go out through the cell membrane, the cytoplasm shrinks. The cell shrivels up. Osmosis takes place. Water diffuses out of the cell through the partially permeable cell membrane. Osmosis and plant cells Plant cells do not burst in pure water.
Plant cells are surrounded by a cell wall. This is fully permeable, which means that it will let any molecules go through it. Water diffuses into the cell through the partially permeable cell membrane. Water diffuses into the cytoplasm and vacuole through the partially permeable cell surface membrane.
The cell swells and becomes firm. Although it is not easy to see, a plant cell also has a cell surface membrane just like an animal cell. The cell membrane is partially permeable. A plant cell in pure water will take in water by osmosis through its partially permeable cell membrane in the same way as an animal cell.
As the water goes in, the cytoplasm and vacuole will swell. However, the plant cell has a very strong cell wall around it. The cell wall is much stronger than the cell membrane and it stops the plant cell from bursting.
The cytoplasm presses out against the cell wall, but the wall resists and presses back on the contents. It is said to be turgid. The turgidity of its cells helps a plant that has no wood in it to stay upright, and keeps the leaves firm. Plant cells are usually turgid. Like the animal cell in Figure 3. The cytoplasm shrinks, and stops pushing outwards on the cell wall. Like a tyre when some of the air has leaked out, the cell becomes floppy.
It is said to be flaccid. If the cells in a plant become flaccid, the plant loses its firmness and begins to wilt. Water diffuses out of the cytoplasm and vacuole through the partially permeable cell membrane.
First, the cell shrinks slightly and becomes flaccid. Then the cell membrane pulls away from the cell wall, and the cell is plasmolysed. If the solution is very concentrated, then a lot of water will diffuse out of the cell. The cytoplasm and vacuole go on shrinking. The cell wall, though, is too stiff to be able to shrink much. As the cytoplasm shrinks further and further into the centre of the cell, the cell wall gets left behind. The cell membrane, surrounding the cytoplasm, tears away from the cell wall.
A cell like this is said to be plasmolysed. This does not normally happen because plant cells are not usually surrounded by very concentrated solutions. However, you can make cells become plasmolysed if you do Activity 3. Plasmolysis usually kills a plant cell because the cell membrane is damaged as it tears away from the cell wall.
What happens to an animal cell in pure water? Explain why this does not happen to a plant cell in pure water. Which part of a plant cell is: What is meant by a turgid cell? What is plasmolysis? How can plasmolysis be brought about?
Describe the events shown in Figures 3. There are many occasions when cells need to take in substances which are only present in small quantities around them. Root hair cells in plants, for example, take in nitrate ions from the soil.
Very often, the concentration of nitrate ions inside the root hair cell is higher than the concentration in the soil. The diffusion gradient for the nitrate ions is out of the root hair, and into the soil. Despite this, the root hair cells are still able to take nitrate ions in. They do it by a process called active transport. Active transport is an energy-consuming process by which substances are transported against their concentration gradient.
The energy is provided by respiration in the cell. In the cell membrane of the root hair cells are special transport proteins. These proteins pick up nitrate ions from outside the cell, and then change shape in such a way that they push the nitrate ions through the cell membrane and into the cytoplasm of the cell. As its name suggests, active transport uses energy. The energy is provided by respiration inside the root hair cells. You can find out about respiration in Chapter Energy is needed to produce the shape change in the transport protein.
You can think of active transport as a process in which chemical energy that has been released from glucose by respiration is converted into kinetic energy of molecules and ions. S Most other cells can carry out active transport. In the human small intestine, for example, glucose can be actively transported from the lumen of the intestine into the cells of the villi. In kidney tubules, glucose is actively transported out of the tubule and into the blood. The energy needed for it to do this is provided by respiration in the cell.
Measuring the rate of osmosis Skills AO3. Moisten it and rub it between your fingers to open it. Tie one end tightly. Use a dropper pipette to put some concentrated sugar solution into the tubing.
Place a long, narrow glass tube into the tubing, as shown in the diagram. Tie it very, very tightly, using thread. Place the tubing inside a beaker of water, as shown in the diagram. Every 2 minutes, record the level of the liquid in the glass tube. Collect a sheet of graph paper. Draw a line graph of your results. Put time in minutes on the x-axis, and height in mm on the y-axis.
A1 Describe what happened to the liquid level inside the glass tube. A2 Explain why this happened.
A3 Use your graph to work out the mean average rate at which the liquid moved up the tube, in mm per second. Ask your teacher for help if you are not sure how to do this. A4 Predict what would have happened to the rate of osmosis in this experiment if you had used a kind of Visking tubing with ridges and grooves in it, giving it a larger surface area. A5 When temperature rises, particles move more quickly.
Describe how you could use this apparatus to carry out an experiment to investigate the effect of temperature on the rate of osmosis. Think about the following things. End-of-chapter questions 1 Which of a—d below is an example of i diffusion, ii osmosis, or iii neither? Explain your answer in each case. Saliva flows out of the salivary glands into your mouth. A spot of blue ink dropped into a glass of still water quickly colours all the water blue.
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Each one contains at least one error. Decide what is wrong with each statement, and rewrite it correctly. S d Animal cells plasmolyse in a concentrated sugar solution.
Diffusion happens faster when the temperature rises. Oxygen diffuses out of a plant leaf during daylight hours. Our aim at Vedantu is to hand out every enabler to the students which will aid them to prepare and score well in the exams. Perhaps the most authentic NCERT textbook solutions, the answers are curated by our team of expert teachers of various subjects who ensure that the pattern of the answers is in line with CCE marking scheme pattern.
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We at Vedantu provide online classes to students at an affordable cost.End-of-chapter questions 1 For each of these carbohydrates, state: These enzymes are also found in plants — for example, in germinating seeds, where they digest the food stores for the growing seedling. They look like tiny dots attached to a network of membranes that runs throughout the cytoplasm.
Species are grouped into larger groups called genera singular: genus. A5 Copy and complete these sentences. You will find out more about aerobic respiration in Chapter As the cytoplasm shrinks further and further into the centre of the cell, the cell wall gets left behind. What happens to an animal cell in pure water?
They also get rid of many of their waste products in this way.
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