Biology Practicals: Osmosis and Plant Physiology

• Biology practicals covering osmosis, potometers, burning food experiment, enzymes, and testing a leaf for starch.

Osmosis

• Osmosis is the movement of water molecules from a region of high water potential to a region of low water potential through a partially permeable membrane.
• In plant cells, osmosis is affected by the cell wall, which creates pressure potential (turgor pressure) that can limit water uptake.
• Osmosis practicals often involve using potato cylinders or other plant tissues placed in solutions of varying concentrations.
• The change in mass or length of the tissue indicates whether water has moved into or out of the cells.
• If a potato cylinder increases in mass, it has gained water because the external solution had a higher water potential.
• If a potato cylinder decreases in mass, it has lost water because the external solution had a lower water potential.
• A control should be used with distilled water, to show osmosis without the presence of an additional solute.
• Serial dilutions of a stock solution (e.g., sucrose) are prepared to create a range of water potentials.
• Measurements should be taken carefully, including initial and final mass or length, to calculate percentage change.
• Percentage change is calculated as [(final value - initial value) / initial value] x 100.
• Anomalous results should be identified and, if possible, repeated for accuracy.
• The water potential of the potato tissue can be estimated by finding the concentration of the external solution which causes no change in mass.

Potometers

• A potometer is a device used to measure the rate of transpiration in a plant.
• Transpiration is the loss of water vapor from the aerial parts of a plant, mainly through the stomata of leaves.
• There are different types of potometers, including mass potometers and bubble potometers.
• A mass potometer measures transpiration by measuring the water uptake, directly displayed by loss of mass.
• With a bubble potometer, a plant shoot is sealed into the apparatus, which is filled with water, with an air bubble introduced into the capillary tube.
• As the plant transpires, it draws water up the stem, causing the air bubble to move.
• The distance the bubble moves in a given time indicates the rate of water uptake, which is assumed to be equal to the rate of transpiration.
• Factors affecting transpiration rate include temperature, humidity, wind speed, and light intensity.
• Higher temperatures increase transpiration by increasing the rate of evaporation.
• Higher humidity decreases transpiration because the water potential gradient between the leaf and the air is reduced.
• Increased wind speed increases transpiration by removing humid air from around the leaf.
• Higher light intensity increases transpiration by causing the stomata to open for photosynthesis.
• A reservoir allows the apparatus to be reset by allowing more water to enter the capillary tube.
• The cut stem should be sealed into the potometer tubing underwater to prevent air bubbles from entering the xylem vessels.
• Measurements should be taken under controlled conditions to ensure reliable results.
• The surface area of the leaves can affect transpiration rate; this should be considered during analysis.

Burning Food Experiment

• The burning food experiment is used to determine the energy content of different food samples.
• The basic principle is to burn a known mass of food and use the heat released to raise the temperature of a known volume of water.
• The amount of energy released by the food is calculated from the temperature increase of the water.
• A known mass of food is placed on a mounted needle and ignited.
• The burning food heats a metal container (e.g., copper calorimeter) holding a known volume of water.
• The temperature of the water is measured before and after the food is burned, using a thermometer.
• The energy released by the food (in joules) can be calculated using the formula: Energy (J) = mass of water (g) x specific heat capacity of water (4.2 J/g°C) x temperature increase (°C).
• The energy content per gram of food can then be calculated by dividing the total energy released by the mass of the food burned.
• Sources of error include incomplete combustion of the food, heat loss to the surroundings, and evaporation of water.
• Insulation around the calorimeter can reduce heat loss, improving accuracy.
• A control experiment without burning food could measure heat gain from the surroundings.
• Ensure all the heat given off goes into the water, with no heat escaping into the surrounding air.
• Nutritional information should be obtained from a reliable resource.

Enzymes

• Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy.
• Enzyme practicals often investigate the effect of factors such as temperature, pH, and enzyme concentration on reaction rate.
• A common enzyme studied is catalase, which catalyzes the breakdown of hydrogen peroxide into water and oxygen.
• The rate of enzyme activity can be measured by monitoring the production of oxygen gas.
• Temperature affects enzyme activity because higher temperatures increase the kinetic energy of molecules, leading to more frequent collisions between enzyme and substrate, up to a point.
• Above the optimal temperature, the enzyme denatures, losing its structure, and activity decreases rapidly.
• pH affects enzyme activity because changes in pH can alter the enzyme's structure and active site, disrupting substrate binding.
• Each enzyme has an optimal pH at which it functions most efficiently.
• Enzyme concentration affects enzyme activity because increasing the enzyme concentration increases the number of active sites available to bind substrate, increasing reaction rate (provided there is sufficient substrate).
• Reaction rate can be measured by collecting the volume of oxygen produced over time.
• Graphs of reaction rate versus temperature, pH, or enzyme concentration can be plotted to determine optimal conditions.
• Safety precautions should be followed when working with enzymes and substrates, as some may be irritants or sensitizers.
• Variables other than the independent variable should be carefully controlled.
• When investigating enzyme activity, a suitable buffer should be used to maintain constant pH.

Testing a Leaf for Starch

• This practical demonstrates that starch is produced during photosynthesis.
• Starch is a storage carbohydrate that is formed when plants convert glucose (produced during photosynthesis) into starch.
• The process involves several steps: killing the leaf, extracting chlorophyll, and testing for starch using iodine.
• First, the leaf is placed in boiling water to kill the cells and stop enzyme activity.
• Second, the leaf is placed in hot ethanol to remove the chlorophyll, as chlorophyll masks the color change of the iodine test.
• Third, the leaf is dipped back into hot water to soften it, as the ethanol would have made it brittle.
• The leaf is then spread out in a petri dish, and iodine solution is added.
• A positive result is indicated by a color change from yellow-brown to blue-black, indicating the presence of starch.
• A negative result means the leaf remains yellow-brown, indicating no starch is present.
• Parts of the leaf that were exposed to light will test positive for starch.
• Parts of the leaf that were covered or kept in the dark because they were unable to perform photosynthesis, will test negative for starch.
• A variegated leaf (with green and white parts) can be used to show that only the green parts (containing chlorophyll) produce starch.
• A leaf that has been destarched (by keeping it in the dark for 24-48 hours) can be used to show that starch is only produced when the leaf is exposed to light.
• Controls can include a leaf kept in the dark or a leaf that has not been exposed to light.
• Iodine solution should be handled with care as it can stain skin and clothing.

Explore osmosis and plant function. Understand water potential through potato cylinder experiments. Learn about the measurement of transpiration rates using potometers and investigate the role of enzymes and starch in leaves.