In Year 11 we will be concentrating on passive transport across cell membranes, diffusion and osmosis. In Year 12 we will learn more about active transport across cell membranes, which requires the cell to use energy (ATP). There are various forms of active transport across membranes, including:
exocytosis (out of the cell)
endocytosis (into the cell)
phagocytosis (solids, like bacteria or other foreign materials)
Today in Year 11, we completed an experiment using cores of potato in several different concentrations of sugar solution. Weighing the potato discs before and after the experiment, we expected the samples placed in distilled water would increase in mass (due to water moving into the cells by osmosis) and the samples placed in concentrated sugar solution would decrease in mass (due to a net movement of water out of the cells). We also used microscopes to observe thin sections of rhubarb, demonstrating how the cell membrane shrinks away from the cell wall when placed in concentrated sugar solution.
In Year 12, we took identical cores of beetroot and placed them into distilled water in test-tubes in water baths of different temperatures (frozen beetroot core, room temperature, 50C and 70C). After 30 minutes, the beetroot cores are removed and the colour of the remaining water is observed. From this, you can infer that the damage to the cell membrane at 70C is greatest, because the greater amount of pigment has been released from the beetroot cells, giving the water a darker pink colour. Some pigment was also released from the frozen core and at 50C, indicating that the cell membrane has ruptured.
A chicken egg with the shell removed is often used as a model to show how osmosis works – the experiment we did at Federation University, Ballarat, showed how the eggs gain or lose mass depending on the concentration of the solution that they are placed in. It is important to know that the membrane of the egg is not a true biological membrane or plasma membrane. In fact, a chicken egg is a very specialized cell and the membrane is actually composed of keratin fibres – the same protein that makes up human hair, finger nails and rhino horns. Thanks to Andrew Douch for finding this article about chicken egg membranes, with scanning electron micrograph images.
Notice in the image above, the egg in 5% saline solution sinks (indicating that the egg contents are more dense than the solution) and the egg in the 10% saline solution floats (indicating that the egg contents are less dense than the solution). This image should give you a clue as to which egg gains water and which egg loses water by osmosis.
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.
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?
I learnt something new yesterday – Mr Foreman and I were talking about why some people prefer leaf tea to tea bags. Apparently tea bags have salt in them! Remembering some basic biological principles from the beginning of term 1, what could be a reason that manufacturers add small amounts of salt to tea bags? Leave a comment with your thoughts in the comments section above.
This experiment showed how potato discs placed in concentrated sucrose solution overnight lost mass, due to the movement of water out of the tissue into the surrounding environment. The potato in distilled water gained mass, due to the movement of water from the environment into the cells. This diffusion of water through a semi-permeable membrane (the plasma membrane) is called ‘osmosis’. Learn about the three forms of passive transport (diffusion, facilitated diffusion and osmosis) and active transport (including ion pumps, cotransport and endocytosis) at this site: Northland College Biology.
Next week we will start Chapter 3 and on Wednesday, do an experiment with cylinders of beetroot. We will be testing the temperature tolerance limits of beetroot cell membranes.
I found this image on Flickr – do you think they know the scientific meaning of osmosis – “movement of water through a semi-permeable membrane”?
Today we weighed our potato discs that had been placed in distilled water and concentrated sucrose solution overnight. Which samples gained weight and which lost weight? You can try this simple osmosis interactive to check your understanding of what happens to cells in hypotonic, isotonic and hypertonic solutions. Here is another great interactive animation of osmosis and passive transport in living cells.
This site has links to some excellent Biology interactives, from simple cell transport animations and mitosis to DNA replication, photosynthesis and meiosis.