In the egg osmosis experiment we used a chicken egg as a model of an animal cell to demonstrate the movement of water across a semi-permeable membrane. We have learned about the different components of the plasma membrane, including the phospholipid bilayer, integral and peripheral proteins, glycoproteins and glycolipids. We know that it is sometimes referred to as a “fluid mosaic” referring to it’s flexible structure of different parts.
Today we are going to test the effect of temperature and solvents on the cell membrane, using cores of raw beetroot. Beetroot is brightly coloured due to the presence of betalain and this coloured pigment can assist us to determine the effect of different temperatures and concentrations of solvents on the cell membrane. When the cell membrane is damaged, the pigment leaks out, so the more damage that occurs, the darker the colour of the medium into which the pigment leaks.
Your task is to design and complete an experiment with an aim, hypothesis, list of materials and equipment, method, results, discussion and conclusion. Your variable can be temperature OR concentration of a solvent (ethanol, for example) or detergent.
This is a picture of the experiment “Shaping Up – the relationship between shape and diffusion”. It shows the agar stained with phenolphthalein indicator, which turns clear in acid. When the different size and shaped agar blocks are placed in an acid solution, the aicd diffuses into the jelly, causing to change from pink to clear. The time taken for a block to totally decolourise is a measure of the rate of diffusion of acid into the jelly. We will be doing this experiment on Friday, during periods 1 and 2.
Please make sure you have read all of chapters 1 and 2 and finished all the relevant Chapter Review questions. If you have finished this work, you can start reading chapter 3 “Composition of Cells” (pages 52 to 73). This chapter is designed for you to develop a knowledge and understanding of the composition of cells, understand the relationship between the nature of various substances found in cells and the functions they perform in those cells and learn more about inputs and outputs, enzymes and biochemical processes.
All living organisms consist of cells and all cells are surrounded by a membrane. One of the major functions of the membrane is to regulate the passage of materials into and out of the cell. These materials include dissolved gases, sugars, salts and water. Cell membranes are partially-permeable which means that some substances can easily pass through them whereas others can not. Most materials move by simple diffusion from high concentration on one side of the membrane to a lower concentration on the other. Substances which will not move by passive diffusion require energy and are actively transported.
Water is the most abundant and one of the most important substances in cells. The diffusion of water across a partially-permeable membrane is called osmosis. An egg is a large cell containing mainly water, proteins and salts for the possible benefit of the growing embryo. It is surrounded by a shell, and inside that, a membrane. It provides an excellent model to assist the understanding of the structure and function of membranes. Our experiment will use hen’s eggs, with the shell removed by dissolving the calcium carbonate in acetis acid (vinegar). We will then record the mass of each egg and place them in different concentrations of saline soution (distilled water, 1%, 5%, 10% and 20%). After several hours we will remove the eggs and weigh each to record the mass gained or lost in the solution.Download the practical investigation here: investigating-osmosis-in-chickens-eggs-ss-28
Graph your results to show which eggs gained and lost mass due to the movement of water through the membrane. Did you get any unexpected results? What may have caused any irregularities?
Create a table listing the different forms of transport through cell membranes (passive diffusion, facilitated diffusion, active transport) and describe how each of these methods worked and what materials may be transported using each of these methods.
Concepts in Biochemistry – Cellular Transport
Teacher’s Domain – Transport of substances through the cell membrane
Cell membrane animation on You Tube
This week’s practical experiment involves using chicken’s eggs as a model for the cell – even though the egg is not a single animal cell, it is a good model because it has a semi-permeable membrane that shows the effect of osmosis on animal tissue.
“The plasma membrane of the cell is essential for separating the extracellular and intracellular environments. Made of a semipermeable bilayer of phospholipids embedded with proteins, the plasma membrane acts as a molecular gatekeeper to prevent certain substances from crossing, while granting access to others. Simple elements and compounds, like water, oxygen, and carbon dioxide may easily pass through. Larger, more complex molecules like carbohydrates and proteins must seek aid from the carrier proteins within the bilayer in a process known as facilitated diffusion.
Diffusion is the movement of molecules down a concentration gradient from an area of higher concentration to an area of lower concentration. Simple diffusion is an example of passive transport, which occurs without energy input from the cell. Similarly, osmosis, or the movement of water molecules across a membrane from an area of higher concentration to an area of lower concentration, does not require energy input from the cell. Cells existing in an extracellular environment that has a higher solute concentration than inside of the cell are in a hypertonic solution. When the extracellular solute concentration is lower than intracellular solute concentration, the cell exists in a hypotonic solution. In an isotonic solution, the extracellular and intracellular solute concentrations are the same.” from http://www.sd5.k12.mt.us/ghs/sci/young/documents/Lab–EggOsmosis.pdf
In this experiment, which solutions will cause water to move into the egg (cell) and which solutions will cause water to move out of the egg?
More pictures of this experiment here: An Egg-sellent Osmosis Experiment
What’s beetroot got to do with cell biology? Well, today we did a practical experiment to investigate the effect of temperature on the plasma membrane of beetroot tissue. Why beetroot? Because it is relatively easy to identify the amount of damage to cell membranes by observing the pigment leakage from the tissue. We used four 5mm x 15mm cylinders of beetroot and four different treatments – room temperature (control), freezing overnight, 50 degrees Celcius and 70 degrees Celcius for 2 minutes each. Then each sample was placed in a test tube with 5 ml of water. What did our results show?
One of the questions in this practical asks why some people put hessian bags over their garden plants in winter and how some plants can survive freezing temperatures. Hessian bags can help to insulate plants against the effects of freezing – some wineries use large fans to keep air circulating over their crops for the same reason. Some plants are genetically more tolerant of frost. This article, from the CSIRO, describes how some plants can empty water out of the cell into the extracellular spaces, where the water can form ice without damaging the cell membrane. Another method is increasing solute concentration, for example by storing monosacharides in place of disacharides, which effectively lowers the freezing point of the cell contents. This is a bit like using anti-freeze in the car radiator during winter.
Revision questions (with answers!) for cell membranes at Biology-Questions-and-Answers.