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New unit: Investigating CELLS - Cells are the basic units of all living things. Activity: Glue diagram of animal and plant cells and label it using your book for help. The functions (i.e. the jobs) of the different parts of the cell. Part of cell Nucleus Cytoplasm Cell Membrane Vacuole Cell wall Chloroplasts Found in all animal cells? Found in all plant cells? Observing cells with a microscope: - Cell needs to be stained to make the different part easier to see with a microscope. - How to do a wet mount: - How to measure the size of cells: The length of a cell is 1000m = 200m 5 Investigating cell diffusion Particles in gases and liquids move from where they are tightly packed (high concentration) to areas where they are less tightly packed (low concentration). Diffusion is the name given to the movement of particles from an area of high concentration to an area of low concentration. The substance moves down its concentration gradient. Experiment on the model cell 1- Using a spotting tile, test the starch and sugar mixture for the presence of starch and sugar. Testing the starch and sugar mixture Substance to be tested Sugar Starch Testing solution Colour change Conclusion Benedict’s reagent (to be incubated in hot water) Iodine 2- Place the mixture into visking tubing, this will be your “model cell”. 3- Place this “model cell” into a test tube half filled with water and leave until the next lesson. 4- After at least 24h, test the water in the boiling tube to find out whether sugar or/and starch has diffused through the selectively permeable membrane. Testing the water in the boiling tube after 24h Substance to be tested Sugar Starch Testing solution Colour change Conclusion Benedict’s reagent (to be incubated in hot water) Iodine 5- Write down a conclusion using the words: high concentration, low concentration, concentration gradient, diffuse, selectively permeable membrane. The importance of diffusion to organisms: Diffusion: allows the movement of gases (i.e. oxygen and carbon dioxide) to move in and out of cells. The cell membrane is selectively permeable: size of membrane holes determines which molecules can go through it. For small organisms: no specialised organs (e.g. amoeba), no specialised organ needed, diffusion through cell membrane is enough. For large organisms; specialised organs needed: - lungs in mammals - gills in fish - leaves in plants. The special case of diffusion of water The diffusion of water is called OSMOSIS, from the area of High Water Concentration (HWC) to Low Water Concentration (LWC Experiment: osmosis in potatoes Potato cell directly in contact with water: - When cells are placed in water, the water moves by osmosis from the area of high concentration in the water to the area of low concentration in the cell. As a result, the cell swells up and this is called a turgid cell. It does not burst as the cell wall helps it keep its shape. - When the cell is placed in a concentrated salt solution, the water moves by osmosis from the area of high concentration in the cell to the area of low concentration in the salt solution. As a result, the cell shrinks or shrivels. For plant cells, this is called a plasmolysed cell. - If the cell was placed in a dilute salt solution,, the concentration of water in and out of the cell are similar and as a result, the cell does not shrink or swell, it stays the same. Osmosis in red blood cells QuickTime™ and a decompressor are needed to see this picture. - When cells are placed in water, the water moves by osmosis from the area of high concentration in the water to the area of low concentration in the cell. As a result, the cell swells up and eventually bursts. - When the cell is placed in a concentrated salt solution, the water moves by osmosis from the area of high concentration in the cell to the area of low concentration in the salt solution. As a result, the cell shrinks or shrivels. - If the cell was placed in a dilute salt solution,, the concentration of water in and out of the cell are similar and as a result, the cell does not shrink or swell, it stays the same. Cell division Cells divide to increase the number of cells in an organism so that growth and cell replacement can take place. Cell division is also called MITOSIS. A cell divides to produce 2 identical cells called daughter cells. The hereditary information (also called genetic material) is held in the DNA contained in the nucleus. The DNA is sub-divided in chromosomes. After mitosis, each daughter cell has an identical set of chromosomes that carries the same genetic information as the original cell. The number of chromosomes in a nucleus is called the chromosome complement and it changes depending on species. To obtain two identical sets of chromosomes, the genetic material contained in a nucleus is replicated (copied) before mitosis. If a cell inherits fewer or more chromosomes than the normal number, it will function abnormally or die. Cell division in animal cells 1- Chromosomes get shorter and fatter. The cell is ready for mitosis. 2- Chromosome are attached to spindle fibres and positioned at the equator (plane at the centre of the cell). They can be seen to be made of two chromatids. 3- Spindle fibres contract separating the chromatids which are pulled apart towards opposites poles. 4- Two nuclei form at each end of the cell 5- The cytoplasm start to divide 6- Two daughter cells are formed Cell division in plant cells Same as for animal cells except for 5: 5- New cell wall forms on a plane at the centre of a cell. Investigating Enzymes A chemical reaction is when a substrate is chemically altered into a product. A catalyst is a substance that speeds up the rate of a chemical reaction without being changed or used up (i.e. a catalyst is neither a substrate nor a product as it is unaffected by chemical reactions). In a cell there is a large number of chemical reactions that take place continuously. These would not happen fast enough to sustain life without the help of catalysts. Enzymes are “biological catalysts” produced by the cells themselves. In biology, it is important for experiments to include a control. A control is a test where all variables are kept the same except for the variable which is deliberately changed i.e. the input variable or the dependent variable. This can be done in two ways: - using material in which biological activity has been destroyed by boiling, cooking, or sterilising the biological sample. - using an equivalent mass or volume of glass beads. 1/ Study of catalase .Catalase speeds up the following reaction: H2O2 Hydrogen Peroxide H2O + 1/2O2 Water + Oxygen Catalase is a very common enzyme found in most organisms. In mammals, it is especially concentrated in the liver. When doing experiments in order to compare the activity of catalase in various biological samples, which variables should be carefully controlled? Class test: Test First value Second value Mass Values (g) Height of bubbles (cm) Surface Values (cm2) area Height of bubbles (cm) Tempera ture Value (°C) Height of bubbles (cm) pH Height of bubbles From these experiments, we can conclude that, mass, surface area, temperature and pH affect catalase activity. 2/ Other enzymes to know: C-L-A-P-P-P - Catalase: breaks down hydrogen peroxide into water and oxygen - Lipase: breaks down fats into fatty acids and glycerol - Amylase: It breaks down starch into maltose. - Pepsin: breaks down proteins into polypeptides - Potato Phosphorylase: in potatoes, joins molecules of Glucose-1-phosphate to form starch. 3/ Summary of important facts to consider in reaction catalysed by enzymes - To make a fair experiments: - mass - surface area - volume - origin of biological material - temperature - pH are variables (or factors) that need to be kept constant. - Variables affecting the speed of the reaction: - Temperature - pH The effect of temperature on enzyme activity At low temperatures, enzymes do not work effectively (molecules move too slowly). Enzymes increase the rate of reaction (i.e. speed up) most effectively at a temperature called the optimum temperature Beyond that temperature, an enzyme becomes denaturated, i.e. it is irreversibly damaged. Examples of enzyme optimal temperatures: - animal: body temperature - plant: in our climate, about 20°C The effect of pH on enzyme activity Each enzyme has a specific pH, i.e. a pH at which it works most effectively (faster rate of reaction). Enzyme may work at other pH but the rate of the chemical reaction that they control is usually not as fast. Examples of enzyme optimal pH: - catalase: pH 7 - pepsin: pH 2.8 Conditions (temperature and pH) at which enzymes work most effectively are called optimum conditions. The conditions at which the enzymes work is called the working range. Specificity of enzymes: Each enzyme only works on one substrate. E.g. Amylase only breaks down starch. Enzymes and substrates have matching shapes like a “lock and key”. Enzymes are proteins.