Cell level systems Revision Notes

Investigating the Effect of Temperature on Enzyme Activity (using Amylase and Starch)

To investigate the effect of temperature on enzyme activity, a common experiment uses the enzyme amylase and its substrate, starch. Set up a series of test tubes, each containing a mixture of starch solution and amylase solution. Maintain different temperatures for each test tube using water baths (e.g., 10°C, 20°C, 30°C, 40°C, 50°C, 60°C). At regular intervals (e.g., every 30 seconds), take a small sample from each test tube and add it to a drop of iodine solution on a spotting tile. Iodine turns blue-black in the presence of starch. Observe the time it takes for the iodine solution to remain brown/orange, indicating that all the starch has been hydrolysed (broken down) by the amylase. The rate of reaction can be calculated as 1/time. The independent variable is temperature, the dependent variable is the time taken for starch to disappear, and control variables include enzyme concentration, substrate concentration, volume of solutions, and pH. Safety precautions include wearing eye protection.

Investigating the Effect of pH on Enzyme Activity (using Amylase and Starch)

To investigate the effect of pH on enzyme activity, a similar experimental setup is used, but buffer solutions are employed to control the pH. Prepare a series of test tubes, each containing starch solution, amylase solution, and a buffer solution at a specific pH (e.g., pH 4, 5, 6, 7, 8, 9). Maintain a constant temperature for all test tubes using a water bath (e.g., 37°C, which is close to human body temperature for amylase). As before, at regular intervals, take a small sample from each test tube and add it to a drop of iodine solution on a spotting tile. Note the time it takes for the iodine to no longer change colour, signifying the breakdown of all starch. The independent variable is pH, the dependent variable is the time for starch breakdown, and control variables include temperature, enzyme concentration, substrate concentration, and volume of solutions. Plotting reaction rate (1/time) against pH will show the optimum pH for the enzyme. Safety precautions include eye protection and careful handling of buffer solutions, particularly at extreme pH.

Variables and Controls in Enzyme Experiments

In any enzyme investigation, it is crucial to identify and control various factors to ensure valid results. The independent variable is the factor being changed deliberately by the experimenter (e.g., temperature, pH). The dependent variable is what is measured or observed as a result of changing the independent variable (e.g., rate of reaction, time for substrate disappearance). Control variables are all other factors that could affect the enzyme activity, which must be kept constant to ensure a fair test. These typically include enzyme concentration, substrate concentration, volume of reactants, and the presence of any inhibitors or activators. By controlling these variables, any observed change in the dependent variable can be attributed solely to the change in the independent variable. For example, if investigating temperature, pH must be kept constant using buffers. If investigating pH, temperature must be kept constant using a water bath.

Measuring Reaction Rate and Interpreting Results

The rate of an enzyme-catalysed reaction can be determined by measuring how quickly the substrate is used up or how quickly the product is formed. In the amylase-starch experiment, the disappearance of starch is monitored using iodine solution. The end-point is when the iodine no longer turns blue-black. The time taken for this to happen is inversely proportional to the reaction rate (Rate = 1/time). When plotting results, the independent variable goes on the x-axis and the dependent variable on the y-axis. The graph will typically show an optimal point where enzyme activity is highest, with activity decreasing on either side. Beyond a certain temperature or extreme pH, denaturation occurs, leading to a rapid loss of activity because the enzyme's active site changes shape, preventing substrate binding. This highlights the specificity of enzymes and their sensitivity to environmental conditions.

Measuring Oxygen Production

A common experiment to investigate the rate of photosynthesis involves using an aquatic plant, such as pondweed (e.g., Elodea). The plant is submerged in water, and a light source is placed at varying distances to change light intensity. As the plant photosynthesises, it releases oxygen gas, which can be observed as bubbles rising from the cut stem. The rate of photosynthesis can be estimated by counting the number of oxygen bubbles produced per minute, or by collecting the gas in a measuring cylinder and measuring its volume over time. Key controlled variables in this setup include the temperature of the water, the concentration of carbon dioxide (e.g., by adding sodium hydrogen carbonate), and the type and amount of pondweed.

Investigating Starch Production

Glucose, produced during photosynthesis, is often converted into starch for storage in plant leaves. The presence of starch can be tested for using iodine solution, which turns blue-black in the presence of starch. To investigate the necessity of light or chlorophyll for photosynthesis, plants can be destarched (by leaving them in the dark for 24-48 hours) before exposing them to experimental conditions. For example, a leaf can be partially covered with foil to demonstrate light's importance, or a variegated leaf (with green and white areas) can show chlorophyll's role. After exposure, the leaf is boiled in water (to break down cell walls), then in ethanol (to remove chlorophyll), rinsed, and then iodine is added. Areas that photosynthesised (produced starch) will stain blue-black.

Controlling Variables in Photosynthesis Experiments

In any experiment investigating photosynthesis, it is critical to identify and control specific variables to ensure a fair test. For example, if investigating the effect of light intensity on oxygen production by pondweed, the independent variable is light intensity (manipulated by distance from the light source). The dependent variable is the rate of oxygen production (number of bubbles per minute). Controlled variables include temperature (use a water bath or keep distance from lamp constant to avoid heating), carbon dioxide concentration (add sodium hydrogen carbonate), and the amount/type of pondweed. Similarly, when investigating carbon dioxide concentration, light intensity and temperature must be kept constant. Failure to control variables would lead to unreliable results and invalid conclusions.

Eukaryotic Cell Structures and Functions (Animal and Plant)

Eukaryotic cells are complex and contain membrane-bound organelles. Both animal and plant cells have a nucleus which contains genetic material and controls cell activities. The cytoplasm is a jelly-like substance where most chemical reactions happen. The cell membrane controls what enters and leaves the cell. Mitochondria are the sites of aerobic respiration, releasing energy for the cell. Ribosomes are responsible for protein synthesis.

Animal cells specifically lack a cell wall, chloroplasts, and a large permanent vacuole. Plant cells, however, possess specialized structures: a cell wall (made of cellulose) provides structural support and protects the cell; chloroplasts contain chlorophyll and are the site of photosynthesis; and a large permanent vacuole filled with cell sap helps maintain turgor pressure against the cell wall, providing support.

Prokaryotic Cell Structures and Functions

Prokaryotic cells are much simpler and smaller than eukaryotic cells. They are characterised by the absence of a nucleus and other membrane-bound organelles. Their genetic material is found as a single loop of DNA in the cytoplasm. They may also contain plasmids, which are small rings of DNA. Like eukaryotic cells, prokaryotes have a cytoplasm, a cell membrane, and ribosomes for protein synthesis. They also possess a cell wall, but it is made of peptidoglycan, which differs from the cellulose cell wall of plants. Some prokaryotic cells may also have a flagellum (tail-like structure) used for movement.

Key Differences and Adaptations

The distinct structures of eukaryotic and prokaryotic cells are directly related to their functions and adaptations. The presence of a nucleus in eukaryotes allows for more complex regulation of gene expression and cell activities. The numerous organelles in eukaryotes specialise further, enabling multicellularity and diverse biological roles. For example, chloroplasts in plant cells are essential for their autotrophic (self-feeding) nature through photosynthesis. The cell wall in plants and prokaryotes provides protection and structural integrity, crucial for organisms living in varying environments. The simpler structure of prokaryotes allows for rapid reproduction and adaptation in many niches.

What is DNA?

DNA, or deoxyribonucleic acid, is the molecule that carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms. It is found within the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. DNA contains the instructions needed for an organism to develop, survive, and reproduce. These instructions are stored as a code made up of four bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

DNA as a Polymer

A polymer is a large molecule (macromolecule) composed of many repeated smaller subunits called monomers. DNA is a prime example of a biological polymer. Its monomers are called nucleotides. Each nucleotide itself has three parts: a pentose sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases (adenine, guanine, cytosine, or thymine). The sequence of these bases along the DNA strand is what encodes genetic information.

Structure of a DNA Strand

Individual nucleotides link together to form a single DNA strand. This linkage occurs between the phosphate group of one nucleotide and the sugar of the next nucleotide. This repeating sugar-phosphate-sugar-phosphate chain forms the backbone of the DNA strand, providing its structural integrity. The nitrogenous bases project inwards from this backbone. These strong covalent bonds between nucleotides make the DNA strand very stable.

The Double Helix

Most commonly, DNA exists as a double helix, which means it consists of two polynucleotide strands twisted around each other, resembling a spiral staircase. These two strands are held together by weaker hydrogen bonds that form between specific base pairs: adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C). This complementary base pairing is essential for DNA replication and protein synthesis.