Cell Structure. Plant Cell. Animal Cell

Transcription

1 Cell Structure Plant Cell Animal Cell

2 Fungal Cell (yeast) Bacterial Cell cytoplasm DNA plasmids capsule cell wall ribosomes cell membrane

3 Structure Nucleus Cytoplasm Cell membrane Cell wall Chloroplast Vacuole Mitochondrion Ribosome Plasmid stores genetic information / controls cell activities site of chemical reactions controls entry and exit of materials gives shape and support to the cell and stops it bursting due to osmosis site of photosynthesis stores water and minerals help support the cell, and when full it pushes outwards against the wall to help provide support site of aerobic respiration site of protein synthesis Function a small circular piece of DNA that controls characteristics which help bacteria adapt to their environment eg. antibiotic resistance. Bacteria swap characteristics with each other by swapping plasmids ( ) The cell wall of a plant cell is made of cellulose, the cell walls of fungal cells and bacterial cells are not.

4 Calculating cell size Cells are too small to be described in terms of millimetres (mm). They are measured in micrometres ( µm ) ( also called microns) 1mm = 1000 (µm) So, 1 µm = 1/1000mm To calculate the size of a cell seen under the microscope; 1. If the diameter of the field of view is given in millimetres, the first thing to do is change it to micrometres 2. Count how many cells you see going from one side to the other 3. Divide the diameter by the number of cells For example, Field of view The diagram shows some cells as observed under a microscope at a magnification of 100X The diameter of the field of view is 1 millimetre. What is the average length of each cell in micrometres? Answer 1mm = 1000 (µm) So field of view = 1000 µm 4 cells stretch across the diameter Average length = millimetre Average length = 250µm

5 Transport across Cell Membranes The cell membrane consists of lipids and proteins. The diagrams show the fluid mosaic model of the cell membrane. A double layer of constantly moving or fluid lipid molecules, with a patchy mosaic of protein molecules. Some protein molecules form channels or pores through the membrane. The membrane can be described as porous. The cell membrane is selectively permeable, that is it selects or controls what substances can enter or leave the cell. Passive transport is when a substance moves across the cell membrane down a concentration gradient. That is, from a high concentration of that substance to a low concentration of it. Passive transport does not require any energy. High concentration Low concentration

6 Diffusion and osmosis are examples of passive transport. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient. In respiration, living cells gain glucose and oxygen by diffusion and release carbon dioxide by diffusion. In photosynthesis, living plant cells take in carbon dioxide by diffusion and release oxygen by diffusion. Osmosis is a special case of diffusion involving water. Osmosis is the movement of water from a high water concentration to a low water concentration across a selectively permeable membrane.

7 Water molecule Solute molecule The water will move from right (high water concentration) to left (low water concentration) However, the solute cannot move as it is too large to fit through the pores in the selectively permeable membrane. Active transport is when a substance moves across a cell membrane against the concentration gradient, from a low concentration to a high concentration. This requires energy from respiration. Protein carriers in the cell membrane move molecules into or out of the cell against the concentration gradient.

8 Chromosomes, Genes and DNA From the Cell to DNA nucleus Cell chromosomes DNA Chromosomes are located in the nucleus of the cell and each chromosome is made up of a molecule of a chemical called DNA which is unique to every individual. A short piece of DNA in a chromosome makes a gene and genes control the different characteristics of living things by producing particular proteins. DNA is passed down from parent to offspring.dna Chromosomes are made up of sections called genes and genes are made up of the nucleic acid DNA (deoxyribonucleic acid). DNA carries the complete genetic information of an organism in the form of a code. This code determines the sequence of amino acids in a protein.

9 Structure of DNA DNA consists of two strands made up of a backbone and bases. The strands are held together by weak hydrogen bonds between the bases forming a structure called a double helix. There are four types of base: adenine (A), thymine (T), guanine (G) and cytosine (C). Base pairing is said to be complimentary since only certain bases can pair: Adenine pairs with thymine (A T) Guanine pairs with cytosine (G C)

10 Unit 1.3 Cell division and its role in growth and repair New cells are produced from existing cells by cell division. Cell division allows organisms to increase the number of cells and these can then be used for growth or repair of damaged body parts (e.g. cuts or broken bones). Cell division is controlled by the nucleus of a cell. A cell Nucleus During cell division, a parent cell will divide to form two new cells. These two new cells will be identical to each other and to the parent cell. parent cell cell division 2 identical cells The nucleus of a cell contains thread-like structures called chromosomes. cell chromosome nucleus Each new cell produced by cell division will contain the same number of chromosomes as the parent cell. Cell division is a controlled cycle; each cell which is made by cell division will become a parent cell and divide itself. If cell division isn t controlled then large numbers of cells can be produced, this is the cause of cancers.

11 Unit Producing new cells A diploid cell is a cell that contains 2 matching sets of chromosomes. These cells divide in a process known as mitosis The Stages of Mitosis Just before a cell divides, each chromosome doubles up to form two identical chromatids joined by a centromere. The chromosomes shorten and thicken and become visible. They move to the equator of the cell and attach to spindle fibres by the centromere. The chromatids are pulled to the opposite ends of the cell. Nuclear membranes form around the new chromosomes and the cytoplasm divides. There are now two new cells each with the same number of chromosomes as the original cell. The stages of mitosis are shown below: Mitosis

12 Production of Proteins To make proteins the nucleic acid mrna (messenger RNA) is needed. mrna is a single stranded molecule which carries a copy of the code from DNA. Proteins are assembled at the ribosome so mrna carries the code from the nucleus to the ribosome. Proteins are made by joining amino acids together. Remember the sequence of bases on DNA determines the sequence of amino acids in a protein.

13 1.5 Protein and Enzymes Part A There are many different proteins. Proteins are made of units called AMINO ACIDS. There are twenty different amino acids (aa). These are often depicted by using different shapes. Each amino acid can join with any other amino acid e.g aa 1 aa2 aa3 aa4 aa 5 The structure of a protein depends on the SEQUENCE in which the amino acids are joined together. Proteins, therefore, have DIFFERENT SHAPES e.g. STRUCTURAL PROTEIN and GLOBULAR PROTEIN. These shapes determine protein FUNCTION. Part B COLLAGEN has a super-coiled helical shape that is long, stringy and strong and resembles a rope. This structure provides support. Collagen is an example of a STRUCTURAL PROTEIN. Collagen gives STRENGTH to BONES, TEETH, CARTILAGE, TENDONS and SKIN. It also gives skin ELASTICITY and is used in COSMETIC / BURNS SURGERY HAEMOGLOBIN is a protein, found in blood. It is folded and compact. Its spherical shape allows it to move through blood vessels. Haemoglobin is an example of a GLOBULAR PROTEIN. Haemoglobin carries oxygen in the blood. ENZYMES, HORMONES and ANTIBODIES are all globular proteins. Each has a different function. ENZYMES speed up chemical reactions, HORMONES act as messengers in the body, helping to coordinate body activities and ANTIBODIES defend the body from foreign invaders

14 Part C Enzymes are made by all living cells. Their function in a cell is to work as a BIOLOGICAL CATALYST. A catalyst is a substance which SPEEDS UP A CHEMICAL REACTION and REMAINS UNCHANGED at the end of the reaction. Without enzymes, the reactions which go on inside ALL LIVING CELLS would be so slow that life would simply grind to a halt. Enzymes act with a particular substance known as a SUBSTRATE. Specificity of Enzymes The substance on which an enzyme acts is called its SUBSTRATE. Normally an enzyme will only act upon ONE SUBSTRATE. Each enzyme has a characteristic shape on its surface called an active site. A substrate molecule will fit into this active site. A reaction then takes place while the enzyme and the substrate are joined together. The products of the reaction leave the active site, freeing it for another molecule of the substrate to join. Each enzyme has a different shaped active site which is SPECIFIC to its substrate. The shape of the active site can be described as COMPLEMENTARY to that of its substrate. Substrate moves towards active site Product released from enzyme. Reaction occurs On the enzyme molecule Active site enzyme molecule

15 Part D Effect of Temperature on Enzyme Activity The graph below shows the effect of temperature on the activity of an enzyme: The rate of activity of an enzyme is LOW at LOW TEMPERATURES, increasing to a maximum at the OPTIMUM TEMPERATURE and then further DECREASING RAPIDLY as the temperature is increased further. Most enzymes will STOP WORKING if the temperature rises above 45 o C. This is because enzymes are PROTEINS and are therefore DENATURED by heat (i.e. the shape of the enzyme molecule is altered). This CANNOT BE REVERSED by cooling the enzyme down again. HEAT

16 Effect of ph on Enzyme Activity Most enzymes work best within a NARROW ph RANGE. The graph below shows how the activity of the enzymes PEPSIN and CATALASE is affected by ph. pepsin catalase Rate of Enzyme Activity ph PEPSIN (found in acidic conditions of the stomach) works BEST in the range around ph1 - ph4 with an OPTIMUM ph about ph2. CATALASE has a working range of around ph5.5 - ph9 with an OPTIMUM ph value around ph7. Properties of Enzymes ALL enzymes have the following properties: They are always PROTEINS They are UNCHANGED by the reaction in which they take part and, therefore, can be used over again. They are destroyed (DENATURED) by HEAT They are sensitive to ph They are SPECIFIC in their action. They work best in OPTIMUM CONDITIONS.

17 Genetic engineering and therapeutic uses of cells A gene is a small section of a chromosome that codes for a particular protein e.g. insulin. Insulin is a hormone that regulates the blood sugar levels. Insulin is required by people who suffer from diabetes, as their bodies do not produce enough. Bacterial cells Bacterial cells contain a large circular chromosome which controls the cells activities and other smaller circular DNA called Plasmids. Allow bacteria to swap characteristics The plasmids can be extracted and used in genetic engineering.

18 Genetic engineering Genetic engineering is the transfer of genes from one organism to another e.g. human to bacteria. Genetic engineering can be used to produce large quantities of insulin and other proteins. Genetic engineering involves transferring the desired gene (insulin gene) into a bacterial cell. Once inside, the bacterial cell will reproduce rapidly producing many bacterial cells with the insulin gene. The bacterial cells will produce the insulin which can then be extracted and purified. Steps involved in genetic engineering to produce insulin. 1.Chromosome extracted and insulin gene identified 3. Plasmid extracted 2. Gene cut out 5. Gene inserted into plasmid 4.Plasmid cut open 6. Plasmid inserted into bacterial host cell 7. Bacterium grows and multiplies 8. Insulin mass produced by duplicates of plasmid

19 How does this work? 1. The required gene is identified in the correct organism. 2. This gene is cut out and removed from its chromosome using and enzyme. 3. A plasmid is extracted from a bacterial cell. 4. The same enzyme as used in step 2 is used to cut open the plasmid. 5. The required gene is inserted into the plasmid using a different enzyme to the one used in steps 2 and 4 to seal it in. 6. The plasmid containing the new gene is inserted back into the bacterial cell it was removed from. 7. The altered plasmid duplicates inside the cell and the cell multiplies. 8. The bacterial cell mass produces insulin, which is then taken for purification.

20 Other proteins and substances can be produced in the same way such as; Medical Product Insulin Factor VIII Growth Factor Need Regulates blood sugar levels Required for normal blood clotting Required for normal cell growth Genetically Modified Plants These are organisms which have had genes inserted from another organism. These organisms are said to be genetically modified and are important in the development of new varieties of animals, plants and micro-organisms, GM organism Modification Benefit Soya bean Resistance to weed killer Increased yield Tomato Prevent softening Stays ripe longer Oilseed rape Produce oils Used in plastics Brewer s yeast Higher alcohol content and lower carbohydrate content Light beers Yeast Produce rennin Vegetarian cheese

21 Crops can also be modified, to develop Drought tolerance Disease resistance Pest resistance Easier harvesting Richer in vitamins Genetic Engineering Advantages Disadvantages Completely different species can be combined Only desired characteristics are produced Only takes one generation to get desired results Large quantities of protein produced Easier to purify Less contamination Inserting genes into animals and plants has proved to be very difficult Some complex proteins can only be made by plants or animals. Only a few plants have been found that will accept the plasmids carrying the foreign gene. Research and implementation are very expensive. Can go wrong, e.g. in production of factor VIII too much may be produced at a time. There is a question of ethics.

22 Therapeutic Uses of Cells Fighting Disease Genetic engineering can produce proteins which act as vaccines, or even the antibodies themselves. These proteins can then be adapted so that they can be taken orally, resist the human digestive system and then be absorbed into the bloodstream. Gene Therapy Gene therapy is the replacement of a defective gene with a fully functional gene. Cystic fibrosis is one of the most common genetic disorders A defective gene is unable to produce a protein required for normal cell function and so the linings of the airways and pancreas become blocked with very thick mucus. This defective gene has been identified and isolated. Scientists are currently investigating ways of inserting the correct gene into the body cells. If this can be achieved, cystic fibrosis could be cured by gene therapy. Gene therapy is also being investigated for cancer. Special genes called suicide genes are placed into the cancer cells. These genes are only active in cancer cells not normal cells.

23 Stem Cell Therapy Stem cell therapy is a set of techniques that aim to replace cells damaged or destroyed by d... with healthy functioning ones. If successful, the healthy replacement stem cells will integrate into the body and give rise to more cells that can take on the necessary functions for a specific tissue. What are Stem Cells? Stem cells have several unique properties that separate them from other cells: They are unspecialised cells. They can self-renew, which means they are capable of replenishing themselves for long periods of time by dividing. They can differentiate into specialised cells such as a nerve or heart cell. Sources of Stem Cells Stem cells may be derived from several sources: 1. Embryonic stem cells: they are extracted from e Adult stem cells: these are present in adult tissues such as the b... m..., brain and blood. 3. Cord blood stem cells: this source of stem cells is derived from u... cord blood. Benefits of Stem Cells Stem cells are currently used to treat cancers such as l.... You may be familiar with the concept of bone marrow transplants, which have been used for decades now to provide a healthy source of cells in the body. Other diseases that stem cells may help include: Parkinson's disease Stroke Spinal cord injuries Retinal diseases Alzheimer's disease Type I diabetes

24 Ethical Debate The use of stem cells from an embryo has prompted massive debate amongst the public, politicians, scientists and religious groups. Because an embryo is destroyed after stem cells are extracted, opponents argue that this is the equivalent of killing a potential life. Fortunately, newer techniques are currently being investigated which will allow for embryonic stem cell extraction without either destroying an embryo or creating one. Artificial Organs Stem cells have been touted as the treatment of the future for many diseases. They have even made it possible to rebuild areas of the body that have suffered from tissue destruction eg. the growth of a windpipe for a lady whose windpipe had been destroyed by tuberculosis. Growing Artificial Organs

25 Yeast Yeast are single-celled fungi which need an organic material to grow on. Their preferred food source would be sugar, for example sucrose. They can respire aerobically (in the presence of oxygen) or anaerobically (in the absence of oxygen). In industry we use yeast in anaerobic conditions as they give the most useful products. sugar carbon dioxide + ethanol raw material products Baking In the baking industry, yeast is added to dough with some sugar to make the dough rise. It does this by making carbon dioxide in the dough. This collects as small bubbles, making the dough bigger in volume and the bread lighter in texture. Bubbles of carbon dioxide At start After 2 hours Like all living things, yeast has a range of temperatures that it is active in. If the temperature is too low the yeast won t respire and make carbon dioxide. However, if the temperature is too high, the yeast will die and will no longer be able to respire.

26 This is important for industry to know, especially in brewing. Brewing Brewing is where the yeast is grown on sugar to produce alcohol (ethanol). malting Hops added for flavour Filtration Fermentation

27 Bacteria Bacteria such as E. coli can complete a life cycle and make another E. coli within 20 minutes in a laboratory. This quick growth means you don t need to wait long for a small number to become a very large number. The more bacteria you have, the more products you can make. In industry, this means you spend less money growing the bacteria. There are several types of bacteria which give us different products. Lactic acid bacteria This group of bacteria produce lactic acid as a product when grown on a carbohydrate such as a basic sugar. The sugar is called lactose (some people are lactose intolerant). The lactic acid can be used to turn milk into yoghurt as it causes the milk proteins to clump together making the milk thicken. The steps of making yoghurt are designed to make sure only the bacteria we want to grow in the milk can. 1. Use fresh pasteurised milk to ensure there is as few bacteria there as possible. 2. Heat the milk up to 73 o C to kill any remaining bacteria. 3. Cool the milk to 44 o C and add lactic acid bacteria. This could be from a packet or using live yoghurt in natural yoghurt from the supermarket. 4. Leave at 44 o C for four hours to let the bacteria grow on the lactose to make lactic acid 5. Store at 4 o C to slow growth of the bacteria.

28 Lactic acid bacteria are also used in the first stage of making cheese. Again the lactic acid starts clumping the proteins together before a special enzyme called rennet is added. This makes the solids collect together, becoming curds. This leaves a liquid behind called whey. Biofuels A biofuel is produced by living cells and will burn to give energy. These are produced so successfully some cars are designed to be run on them. They are important to society as they can replace the fossil fuels, coal, oil and gas which are running out as they are finite. Biofuels can be produced from many waste products from industry so they also help find a use for material that would otherwise be rubbish. The waste material could be sugar cane, which is a rich source of sugar for yeast to grow on, producing ethanol. This gets mixed with normal petrol and used in cars. The waste can also include oils produced from sunflower, rapeseed and soya. This undergoes a process where it is made suitable to go into a diesel engine for a car to use. This called biodiesel. Filtered vegetable oil can also be used to make biodiesel, instead of being put out with food waste.

29 Biogas Biogas is another type of biofuel, but is produced by micro-organisms breaking down organic waste in anaerobic conditions (no oxygen). The gas produced is methane which burns well and can be used for heating, cooking etc. Sewage Treatment Sewage needs to be treated before it can be released into rivers and streams. This prevents disease-causing bacteria growing on it in our water ways, carrying disease throughout the local community. A wide range of specific bacteria are used for this as they use the organic waste in sewage as a food source and none are the type to cause disease. They all need oxygen to break down sewage into harmless products. The wide range of bacteria allows all the different substances to be broken down at the same time.

30 Bacteria Bioremediation Bioremediation is when bacteria are used to help us remove harmful substances which are released into our environment. They can break them down (biodegrade) into safer products. Examples can include breaking down oil in oil spills, or in biodegrading things like polystyrene which was through to be non-biodegradable.

31 Respiration Summary Notes National 4 Respiration is the chemical process where we release energy from our food (glucose). Every cell needs to release energy from glucose and there are two types of respiration; Aerobic with oxygen Anaerobic without oxygen The equation for aerobic respiration can be summarised as follows: Glucose + oxygen Carbon dioxide + water + ENERGY Raw materials Products All animals and plants make the same products when there is oxygen present. In anaerobic respiration, glucose is still required but no oxygen. For this reason, less energy is produced. In Animals and Bacteria, anaerobic respiration produces not Carbon Dioxide and Water, but Lactic Acid. Lactic acid is what makes your muscles sore when you are exercising. Once you stop exercising and your breathing catches up to deliver enough oxygen, you can return to aerobic respiration and the lactic acid is removed. Glucose Lactic acid (Remember lactic acid was made by the bacteria in the milk to produce yoghurt) In plants and fungi, anaerobic respiration produces Carbon Dioxide and Alcohol.

32 energy Glucose Carbon Dioxide + Alcohol + little Raw material Products The carbon dioxide is released and cannot be regained. (Remember yeast (fungus) makes Carbon Dioxide and Alcohol for making dough rise and in alcohol production) Respiration Summary Notes National 5 Adenosine triphosphate (ATP) is a molecule made up from an adenosine molecule bound to three inorganic phosphates (Pi). Adenosine P i P i P i Terminal phosphate Energy which is stored in ATP is released when the bond attaching the terminal phosphate is broken by enzymes. Adenosine P i P i Energy Once the terminal phosphate is broken off we are left with ADP + Pi (adenosine di-phosphate and inorganic phosphate). During respiration ATP is regenerated from ADP + Pi, in an enzyme controlled process called phosphorylation. P i this reaction can be summarised as: ATP High energy state ADP + Pi Low energy state

33 When an energy rich substances such as glucose are broken down they produce energy which is used to produce ATP. If glucose is burned in a dish it releases its energy quickly as heat and light. However in a living cell respiration releases energy gradually through a series of enzymes controlled steps. There are many molecules of ATP in each living cell. As ATP is broken down to produce ADP + Pi, the energy released is used to fuel biological processes such as muscle contraction, transmission of nerve impulses, cell division and protein synthesis. In order to regenerate ATP, cells require glucose which is derived from digested carbohydrates. Glucose is carried, dissolved in the blood plasma, to every cell in the body where it is required. As the glucose is used up in the cells the concentration levels in the cell remain low while the concentration in the blood is higher allowing for diffusion of glucose from the blood into the cells. Respiration can happen with or without oxygen, however it is more efficient if oxygen is available. Aerobic respiration is the term used to describe respiration with oxygen. Glucose + Oxygen Carbon dioxide + water Oxygen is transported to the cells bound to a special protein called haemoglobin in red blood cells; oxygen is used by the cells maintaining a low concentration inside the cell.

34 As we breathe, more oxygen enters the blood, maintaining a high concentration of oxygen in the blood reaching cells. This allows oxygen to move into the cells by diffusion. Stage 1 Glycolysis Respiration begins in the cytoplasm of a cell and involves the breakdown of one glucose molecule into two molecules of pyruvate. In order to do this the cell must use energy from 2 ATP molecules; at the same time producing 4 ATP molecules giving a net gain of 2 ATP molecules. Glucose 2 x Pyruvate Stage 2 Kreb s cycle (with oxygen only) Pyruvate enters into the Kreb s cycle a series of enzyme controlled reactions which takes place in the mitochondria of the cell. During this part of the process hydrogen is removed by high energy carrier molecules. Carbon dioxide is also given off as a by-product. Stage 3 The hydrogen is used in the hydrogen transfer system to produce ATP before finally combining with oxygen to form water. Including glycolysis, each molecule of glucose produces 38 ATP. Carbon dioxide and water are breathed out as waste products. Fermentation There are occasions when oxygen is not available. Under these conditions fermentation takes place.

35 Pyruvate cannot enter the Kreb s cycle in the absence of oxygen so only glycolysis can take place and only 2 ATP can be produced overall. The pyruvate therefore must enter an alternative pathway. This is different in animals and plants. In animals pyruvate is converted to lactic acid which causes fatigue and cramp in muscle tissue. When oxygen becomes available lactic acid is converted back to pyruvate which can then enter the Kreb's cycle. The reaction is said to be reversible. Pyruvate Lactic Acid In plants pyruvate is broken down to carbon dioxide and ethanol (an alcohol). This is a non-reversible reaction. Pyruvate Carbon dioxide Fermentation is less efficient than aerobic respiration producing 20 times less energy. Fermentation takes place in the cytoplasm and only involves energy produced in glycolysis. + Ethanol

36 Photosynthesis Photosynthesis is the process which allows green plants to make their own food. It can be summarised with the word equation Carbon Dioxide + Water Raw materials Light Chlorophyll Glucose + Oxygen Products This takes place in the chloroplast of the plant cell. Testing for Starch The glucose the plant makes during photosynthesis can be stored in the leaves as starch. Testing a leaf for starch can show if the plant has been photosynthesising.

37 Chemistry of Photosynthesis Light Reactions (Photolysis) This is the first stage of photosynthesis and involves the splitting of water into oxygen and hydrogen using energy captured from sunlight by chlorophyll. This reaction also produces ATP. Light energy from the sun captured by chlorophyll Light energy Water ADP + Pi ATP Oxygen Released as a by-product Hydrogen Used in the second stage Carbon Fixation. Due to requirement of light energy to break apart water molecules this reaction is classified as light dependent, meaning it will only occur in the presence of light. Carbon Fixation This is the second stage of photosynthesis. It involves a series of enzyme controlled reactions joining together carbon dioxide and hydrogen to form glucose. This reaction requires ATP and hydrogen, passed on from photolysis, and cannot occur without the products of photolysis. Carbon Dioxide Enzyme controlled Glucose Hydrogen ATP ADP + Pi The glucose produced during photosynthesis can be used in various ways by the plant cell

38 1. Used immediately for energy to power cell process, such as mitosis or protein synthesis. 2. It can be converted to starch for long term storage. 3. It can be converted to cellulose and used to create cell walls 1. Limiting Factors A limiting factor is a factor that by its presence or absence controls the rate at which a reaction happens. For photosynthesis there are various factors that can affect the rate of photosynthesis Light Intensity Temperature Carbon dioxide concentration Increasing light intensity or carbon dioxide concentration should increase the rate of photosynthesis, until the plant is receiving as much as it can utilise at which point they cease being a limiting factor C % CO 2 Y B % CO 2 X A % CO 2 At the point labelled Y the limiting factor is the light intensity as the rate of photosynthesis increases as the light intensity increases. At the points labelled X the rate of photosynthesis shows no further increase even if the light intensity is increased, therefore another factor such as carbon dioxide concentration must be limiting the rate of photosynthesis. Temperature usually works in the same way. Remember, however, that above a certain temperature the enzymes controlling photosynthesis will become denatured and the rate of photosynthesis will actually start decreasing. This is shown on the following graph.

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