Diffusion Importance of the Substance Transportation It is vital for substances to be transported in and out of a range of organisms. These substances include: 1. 2. 3. 4. 5. 6. oxygen carbon dioxide water dissolved food molecules mineral ions urea Cells must remove waste substances and to take in useful substances in order for the cell to function. These substances exchange through the cell membrane through diffusion, __osmosis __and __active transport. __ Surface Area Volume Ratio Multicellular organisms__ require exchange surfaces and a transport system__ in order for substances to pass in and out of the cells. The larger the surface area is compared to the volume, the quicker and more substances can pass through the membranes. 1. Small and thin structures have a larger surface area compared to the volume. 2. Large and thick structures have a smaller surface area compared to the volume. Multicellular organisms have specialised exchange surfaces__ __- such as lungs, gills or villi in the intestines so that there is a larger surface area for the transport of substances to take place. Surface area = Number of sides x (length x length) Volume = length x length x length So, as the volume increases, the surface area does not increase at the same rate. If there was a sugar cube that was 5g, it will have a larger SA:V ratio to 5g of powdered sugar. Alveoli Adaptations Alveoli__ __are highly adapted for gas exchange by diffusion between air in the lungs, and blood in the capillaries. 1. They are folded in order to maximise the surface area to volume ratio. 2. This allows more gas exchange to take place. 3. The alveoli wall linings are moist, allowing oxygen and carbon dioxide to dissolve easily. 4. It has__ thin walls__ so that the distance in which the gases have to diffuse across is short. 5. Capillaries have a rich blood supply, which intensifies the concentration gradient. Rate of Diffusion The following factors have an affect on the rate of diffusion: Surface Area 1. The larger the surface area to volume ratio, the quicker the rate of diffusion takes place. 2. The substance simply has more area to diffuse across, despite being the same volume. Concentration Gradient 1. The higher the concentration differences in the gradient, the higher the diffusion rate. 2. If there is an equal concentration on both sides of the membrane, the rate of diffusion will be extremely slow. 3. E.g. if the alveoli has a much higher concentration of oxygen than in the blood, then the oxygen will diffuse into the blood much quicker. Diffusion Distance 1. Shorter the distance between the material in which the substance is diffusing through, the faster the diffusion rate. It simply has a shorter journey to travel. Temperature 1. By increasing the temperature, the rate of diffusion increases as there is more energy in each particle. 2. These particles will now bounce against each other more frequently. Ficks Law The rate of diffusion can be calculated using Fick’s Law: __Rate of diffusion is proportional to: (surface area × concentration difference ) / thickness of membrane __ This will mean that the rate of diffusion will double if the surface area / concentration difference is doubled, or if the thick of membrane in which exchange takes place is halved. Based on Ficks Law, what will happen to the rate of diffusion if the surface area or concentration difference is doubled? Your answer should include: Diffusion / Double Explanation: Rate of diffusion will double What will happen to the rate of diffusion if the temperature is increased? Increase Explanation: Increase, as the particles have more energy What is the advantage of the alveoli’s thin walls? Short diffusion distance Effect of surface area:volume ratio on rate of diffusion 2/19/2016 0 Comments Equipment required; A block of stained agar Single edged razor blade/scalpel Vernier callipers 5cm3 syringe White tile Ruler Stopwatch Hydrochloric acid 2M (1M in our experiment) Forceps Method 1. Cut the agar into 5 different-sized cubes, use the razor blade to cut. The measurements should be made with vernier callipers or a clear ruler. 2. Use the razor blade to place the agar block into equally sized test tubes 3. Use the syringe to immerse the blocks in 3cm3 of 1M HCl 4. As the HCl is added start the stopwatch 5. When the block becomes colourless, immediately record the time. 6. Record the results in a table and draw a graph. Safety Wear safety googles, HCl is an irritant. Take care with the razor blade cut away from the body. Experiment Results The results show that as the surface area:volume ratio the time it takes for the agar block to become colourless decreases, therefore the rate of diffusion increases. This is due to the surface area being greater compared to the volume of the agar jelly. More HCl molecules can diffuse into the jelly at the same time, therefore the HCl molecules can reach the whole of the agar in a shorter amount of time. Resulting in the agar block becoming colourless more quickly with a greater SA:Vol ratio. Questions Identify 3 factors controlled; 1. Temperature 2. Size of reciprocal HCl is in 3. Concentration difference of HCl to the agar. Increase reliability and accuracy; To increase the reliability the experiment should be repeated at least 3 times for each size of block. Then averaging the results and discounting any anomalies. To increase accuracy the agar blocks should be cut more accurately, a laser cutter would be ideal as it would have much more accurate cutting than a razor blade. As you can see in the pictures of the experiment our agar blocks did not have very straight edges which would affect our surface area:volume ratio. Multiple stopwatches should be used, and each experiment should be done one at a time, so that readings for time are closer to the true value. Explain effect of SA:Vol ratio on rate of diffusion, relate to living organisms As the surface area:volume ratio increases rate of diffusion also increases. This allows single celled organisms that have a large SA:Vol, such as amoeba, to rely on just diffusion to provide nutrients for cell processes. Whereas multicellular organisms whom have a small SA:Vol have evolved transport systems as they cannot rely on skin surface diffusion to provide nutrients to the cells in the body quickly. Limitations of experiment to compared to a living organism The experiment uses a lifeless agar jelly block, which lacks a cell membrane which allows diffusion to take place in all living cells. The cell membrane is a complex mix of a phospholipid bilayer, protein carrier channels, ion channels, carbohydrates, proteins. The agar block lacks this important feature. Other than that its is a good way to show the comparison between surface area:volume ratio and rate of diffusion 5.2C: Diffusion 1. Last updated Nov 19, 2019 o 2. 5.2B: Selective Permeability o 5.2D: Facilitated transport 3. picture_as_pdf Readability Donate Contributed by Boundless General Microbiology at Boundless Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane. Key Points Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as dynamic equilibrium. Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled. Key Terms diffusion: The passive movement of a solute across a permeable membrane concentration gradient: A concentration gradient is present when a membrane separates two different concentrations of molecules. Diffusion Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle; its lowest concentration is at the edges of the room. The ammonia vapor will diffuse, or spread away, from the bottle; gradually, more and more people will smell the ammonia as it spreads. Materials move within the cell ‘s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion. Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated. Figure 5.2C.15.2C.1: Diffusion: Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm). Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium. Factors That Affect Diffusion Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion: Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes. Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules. Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion. Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density will inhibit the movement of the materials. An example of this is a person experiencing dehydration. As the body’s cells lose water, the rate of diffusion decreases in the cytoplasm, and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect. Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells. Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion. Surface area and thickness of the plasma membrane: Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it. Distance travelled: The greater the distance that a substance must travel, the slower the rate of diffusion. This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the center of the cell. Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes. A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly. This occurs in the kidney where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules. The rate of diffusion in this instance is almost totally dependent on pressure. One of the effects of high blood pressure is the appearance of protein in the urine, which is “squeezed through” by the abnormally high pressure. Lab 1 - Surface Area to Volume Ratios Lab 1 - Surface Area to Volume Ratio WORKSHEET Major Concepts 1. 2. 3. 4. 5. 6. As the surface area and the surface -to-volume ratio of a cell increases, so does its ability to exchange materials. The rate increased almost linearly with increasing surface -tovolume ratio. As cells perform their normal functions, they need to exchange materials with the world outside of the cellular membrane. Cells with larger surface -to-volume ratios are able to exchange materials more efficiently. The smaller a cell, the larger its surface -to-volume ratio. As a cell grows larger, the volume increases faster than the surface area. Small cells are more efficient at exchanging materials than large cells because they have more surface area to work with in relation to their size. In the small intestine, th e surface is covered by epithelial cells that have increased their surface areas by forming tiny folds of their cellular membrane called microvilli. The microvilli dramatically increase the surface area of these cells and their ability to absorb nutrients. In general, cells adapt to the task of absorption and excretion by increasing the size of their cellular membrane while keeping their volume low. As cells grow larger, their volume increases faster than their surface area. This leads to a decrease in the efficiency of the cell at exchanging materials across the cell membrane. To be able to efficiently exchange materials, cells must divide by mitosis and cytokinesis when they become too large. An experiment to investigate the effect of surface area : volume ratio on the diffusion rate. 2/9/2016 3 Comments The objectives of this experiment was to calculate the surface area to volume to ratio of pieces of agar and determine what effect this ratio had on the rate of diffusion. Equipment: A block of stained agar Scalpel/single razor blade 4 test tubes 2M hydrochloric acid 5 cm3 syringe White tile Callipers Stopwatch Method: Cut the agar into 4 different sized cubes (10mm, 7mm, 5mm and 1mm) using a sharp razor, callipers and a white tile to provide a neutral background to aid cutting. Using the razor blade to lift the agar blocks, place each different sized cube into different test tubes and label clearly. Using a syringe, immerse the blocks in 2M HCl by adding 3 cm3 of the acid. Immediately start the stopwatch and time how long the pink colour takes to disappear and for each block to turn colourless. To ensure reliability, repeat this experiment 3 times. Safety precautions: HCl is an irritant so eye protection should be worn. Cut away from oneself and take care with sharp edges. What happens when the agar is placed in the HCl? As time goes on the colour of the agar block becomes increasingly orange as it loses its pink colour. The reason for this is because of the hydrochloric acid diffusing into the block and making it lose its colour. The acid travels from an area of high concentration of HCl (in the test tube) to an area of low concentration of HCl (the agar cube) along a concentration gradient and the point at which the original colour has completely gone is when an even distribution of particles has been achieved. This experiment was designed to then see how this diffusion process occurred in different sized cubes and to investigate the difference I looked at the diffusion rate, i.e. how long it took for the pink colour to disappear. My prediction would be that the rate of diffusion would speed up as the surface area to volume ratio was increased, i.e. the cube was made smaller. Conclusion From these results I can see that the surface area to volume ratio ad an effect on how fast diffusion occurred. As the agar cubes were cut smaller the ratio of surface area to volume became larger and this affected the diffusion rate by making it faster. The results show a clear trend that supports my knowledge on how the diffusion rate speeds up when there is a larger surface area to volume ratio. The time decreased which highlights how diffusion occurred quicker as there was more surface area in relation to its volume over which the HCl could diffuse over. Another reason for this increase in rate was due to the fact that the HCl particles had to travel longer distances in those agar cubes which had a larger volume in comparison to its surface area. Overall my results support the idea that as the surface area to volume ratio increases so does the rate of diffusion. This experiment relates to real life because single-celled organisms use diffusion as their means of getting the substances they need and expelling those they don’t. Diffusion works efficiently for single-celled organisms such as amoeba due to a large surface area to volume ratio as there is a relatively large surface area over which substances can diffuse into or out of the organism. This is demonstrated in my experiment as the smaller cubes with a larger surface area to volume ratio resemble single-celled organisms. My results also support the idea that large organisms cannot use diffusion as their means of transporting substances in and out, as shown by the 10mm by 10mm cube which took 10 minutes and 10 seconds for the HCl to diffuse all the way through. Diffusion would not be efficient enough for larger organisms as the surface area is not large enough for substances to diffuse over, as a result they require a complex transport system. This experiment does have its limitations as the organism’s cell is represented by a lifeless agar cube. It lacks a semi-permeable membrane and only allows for simple diffusion unlike real-life membranes. Cell surface membranes are much more sophisticated as they consist of a combination of phospholipids, carbohydrates and carrier/channel proteins. As well as this the shape of the agar cube does not resemble an organism’s cell. Apart from these limitations this experiment is a good way of demonstrating the effect of surface area to volume ratio on the rate of diffusion. jor Concepts 1. 2. 3. 4. 5. 6. As the surface area and the surface -to-volume ratio of a cell increases, so does its ability to exchange materials. The rate increased almost linearly with increasing surface -tovolume ratio. As cells perform their normal functions, they need to exchange materials with the world outside of the cellular membrane. Cells with larger surface-to-volume ratios are able to exchange materials more efficiently. The smaller a cell, the larger its surface -to-volume ratio. As a cell grows larger, the volume increases faster than the surface area. Small cells are more efficient at exchanging materials than large cells because they have more surface area to work with in relation to their size. In the small intestine, the surface is covered by epithelial cells that have increased their surface areas by forming tiny folds of their cellular membrane called microvilli. The microvilli dramatically increase the surface area of these cells and their ability to absorb nutrients. In general, cells adapt to the task of absorption and excretion by increasing the size of their cellular membrane while keeping their volume low. As cells grow larger, their volume increases faster than their surface area. This leads to a decrease in the efficiency of the cell at exchanging materials across the cell membrane. To be able to efficiently exchange materials, cells must divide by mitosis and cytokinesis when they become too large.