B1 Revision

B1 Revision:

Cells and Microscopy:  Eukaryotes: all animals and plants made from complex cells.  Prokaryotes: bacteria are smaller and simpler cells.  Both eukaryotic and prokaryotic cells contain sub-cellular structures.    Eukaryotic Cells:  Nucleus - contains DNA in the form of chromosomes that controls the cell’s activities.  Cytoplasm - gel substance where most chemical reactions occur.  Mitochondria - the site of cellular respirations and contain the enzymes that are needed for the reactions involved.  Cell Membrane - holds the cell together and controls what goes in and out by providing a selective barrier.  Cell Wall - made of cellulose for support for the cell.  Chloroplasts - where photosynthesis occurs. Contain chlorophyll.    Prokaryotic Cells:  DNA - loose DNA controls the cell’s activities and replication. It floats free in the cytoplasm.  Plasmids - small loops of extra DNA that aren’t part of the chromosome. Contain genes for things like drug resistance and can be passed between bacteria.  Cell membrane - controls what goes in and out. Cell is supported by a cell wall. 

Light Microscopy:  Parts of the light microscope:  Eyepiece lens - looked through to see the image and also magnifies the image.  Objective lens - magnifies the image. Usually there are three different objective lenses.  Stage - supports the slide.  Clips - holds the slide in place.  Handle - to carry the microscope with.  Lamp - shines light through the slide so the image can be seen more easily.  Focusing knobs - move the stage up and down to bring the image into focus.  Specimen Preparation:  The specimen needs to let light through it for you to be able to see it clearly. If you’ve got a thick specimen, you’ll need to take a thin slice of it to start with.  Next, take a clean slide and use a pipette to put one drop of water in the middle of it to secure the specimen in place.  Use tweezers to place the specimen on the slide.  Add a drop of stain if needed (if your specimen is completely transparent or colourless). A drop fo stain is added to make the specimen easier to see. Place a cover slip at one end of the specimen, holding it at an angle with a mounted needle.  Carefully lower the cover slip onto the slide. Press it down gently with the needle so that no air bubbles are trapped under it.  Using the microscope:  Start by clipping the slide containing your specimen onto the stage.  Select the lowest-powered objective lens.  Use the coarse adjustment knob to move the stage up to just below the objective lens. Then, looking down the eyepiece, move the stage downwards until the specimen is in focus.  Then, still looking down the eyepiece, adjust the focus with the fine adjustment knob, until with get a clear image of your specimen.  If you need to see the specimen with a greater magnification, swap to a higher-powered objective lens and refocus.    total magnification = eyepiece lens magnification x objective lens magnification  magnification = image size/real size

DNA:  Chromosomes = long molecules of coiled up DNA.  Genes = short sections of DNA.  DNA is a double helix. Each of the two strands is made up of lots of nucleotides joined in a long chain, making DNA a polymer.  Each nucleotide contains a small molecule called a base. The bases are A, C, G and T. A always pairs with T, C always pairs with G.  Each DNA nucleotide has the same sugar and a phosphate group. The base on each nucleotide is the only part of the molecule that varies. The base is attached to the sugar.    Polymers = large, complex molecules composed of long chains of monomers joined together.  Monomers = small, basic molecular units. 

Enzymes in depth:  If the temperature is too high, the enzyme’s active site denatures and the substrate can no longer fit into the active site. This reduces catalysed reactions.  If the pH of the enzyme is too high or low, it interferes with the bonds holding the enzyme together which denatures the active site. pH is usually a neutral 7, but not always.  The more enzymes there are in a solution, the more likely a substrate with meet with one and join. In summary, increasing the concentration of the enzyme increases the rate of reaction.    Investigating enzyme activity:  How does temperature affect enzyme activity?  Add a buffer solution with a different pH level to a series of different tubes containing the enzyme-substrate mixture.  Vary the initial concentrations of the substrate to investigate the effect of substrate concentration.  Vary the initial concentrations of the enzyme to investigate the effect of enzyme concentration.  How fast does a product appear?  The enzyme catalyse catalyses the breakdown of hydrogen peroxide into water and oxygen.  Collect the oxygen and measure how much is produced in a set time.  Use a pipette to add a set amount of hydrogen peroxide to a boiling tube. Put the tube in a water bath at 10 degrees Celsius.  Set up the rest of the apparatus with the source of catalase e.g. potato in a test tube full of hydrogen peroxide attached to a delivery tube which goes under the upside down measuring cylinder that is filled with water in a bowl of water. Use a water bath for constant temperature.  Record how much oxygen is produced in the first minute by counting bubbles that appear. Repeat three times and calculate a mean.  Repeat at 20-40 degrees Celsius.  Control variables = pH, potato used, size of potato etc.  Calculate the mean rate of reaction at each temperature. Mean rate of reaction = mean volume of oxygen produced/time taken.  How fast does a substrate disappear?  The enzyme amylase catalyses the breakdown of starch to maltose.  It is easy to detect starch using iodine solution - if starch is present, the iodine solution will change from browney-orange to blue-black.  Set up the apparatus with a starch solution + amylase solution in a water back. Every ten seconds, drop a sample of the mixture into a well using a pipette. When the iodine solution remains browney-orange, record the total time taken.  Repeat with the water bath at different temperature to investigate how it affects the time taken for the starch to be broken down. 

Respiration:  Respiration = process of transferring energy from them breakdown of glucose.  Respiration is controlled by enzymes so the rate of respiration is affected by both temperature and pH. It is an exothermic reactions.    Aerobic Respiration:  Happens when there is plenty of oxygen available. “Aerobic” means “with oxygen” and it is the most efficient way to transfer energy from glucose.  GO COW  Glucose + Oxygen —> Carbon Dioxide + Water  C6H12O6 + 6O2 —> 6CO2 + 6H2o    Anaerobic Respiration:  “Anaerobic” means “without oxygen” and it is not the best way to transfer energy from glucose because it transfers much less energy than aerobic respiration.  The process of anaerobic respiration is slightly different in different organisms:  Animals produce lactic acid during anaerobic respiration. This is because, when you’re doing exercise, your body can’t supply enough oxygen. In anaerobic respiration, glucose is only partly broken down and lactic acid is also produced: glucose —> lactic acid. An advantage is that you can keep on using your muscles. After resorting to anaerobic respiration, you will be in an oxygen debt so you need more extra oxygen to break down all of the lactic acid that is built up in your muscles.  Plants and fungi produce ethanol and carbon dioxide. Glucose —> ethanol + carbon dioxide.   

Biological Molecules:  Biological molecules can be broken down to fuel respiration.    Carbohydrates:  They are made up of simple sugars. Their smallest units are monomers and can be joined together in long chains to form polymers.  In the body, carbohydrates are broken down (digested) by enzymes in the mouth and the small intestine.    Proteins:  They are made up of amino acids. They are polymers that are made up of long chains of monomers called amino acids.  In the body, proteins are broken down by enzymes in the stomach and the small intestine.    Lipids:  They are made up of fatty acids and glycerol. They are not polymers because they do not form a long chain of repeating units.  In the body, lipids are broken down by enzymes in the small intestine. 

Photosynthesis:  Photosynthesis happens in the chloroplasts - they contain chlorophyll which absorbs light. Energy is transferred to the chloroplasts from the environment by light.  COW GO  Carbon Dioxide + Water —> Glucose + Oxygen  6CO2 + 6H2O —> 6H12O6 + 6O2    How does oxygen production show the rate of photosynthesis? - PRACTICAL! The rate of photosynthesis is affected by light intensity, concentration of carbon dioxide and the temperature. These can become limiting factors meaning that they can stop photosynthesis from happening any faster. Using pondweed and measuring oxygen production, you can investigate how much these factors affect photosynthesis rates.  The rate at which the pondweed produces oxygen corresponds to the rate at which it is photosynthesising - the faster the rate of production of oxygen, the faster the rate of photosynthesis.  Place the pondweed in a test tube full of water with a bung at the end with a capillary tube running to a syringe.  The pondweed is left to photosynthesise for a set amount of time. As it does this, the oxygen released with collect in the capillary tube.  At the end of the experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas bubble is measured. This is proportional to the volume of oxygen that is produced.  The experiment is then repeated to test a range of values for the factor being investigated e.g. a range of different temperatures.  Variable other than the one being investigated should be kept the same e.g. the other limiting factors and the time in which the pondweed is left for. 

The Rate of Photosynthesis:  Not enough light slows down the rate of photosynthesis:  As the light level is raised, the rate of photosynthesis increases steadily until reaching a plateau.  Beyond that, it won’t make any difference.  You can investigate light intensity by moving a lamp closer to or further away from the plant.  You need to measure the light intensity at the plant using a light meter or light intensity = 1/distance^2 (halving the distance —> intensity is 2x2 = 4 times greater. Tripling distance —> intensity = 3x3 = 9 times smaller).   

The Rate of Photosynthesis:  Too little carbon dioxide also slows it down:  As with light intensity, concentration of carbon dioxide will only increase the rate of photosynthesis to a point. After this, the graph flattens out showing that carbon dioxide is no longer the limiting factor.  As long as light and carbon dioxide are in supply, the factor limiting photosynthesis must be temperature. 

The Rate of Photosynthesis:  The temperature has to be perfect!  It is usually the limiting factor because it is too low - the enzymes needed for photosynthesis work slower at low temperatures. However, if the plant overheats then the enzymes will denature and the rate of photosynthesis decreases majorly!  This usually happens at about 45 degrees Celsius, especially in greenhouses.  The best way to control temperature of the boiling tube in experiments is to place it in a water bath.