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.
Students enjoyed this class, consolidating their knowledge of carbohydrates, lipids and nucleic acids using soft lollies and toothpicks today. They were able to show that disaccharides are made up of two monosaccharides and polysaccharides are made up of many sugar units (using fruit pastilles) and that lipids consist of a glycerol unit (jersey caramel) and 3 fatty acid chains (jelly snakes). Phospholipids consist of a phosphate group (marshmallow), a glycerol (jersey caramel) and two fatty acid chains. DNA consists of a sugar (fruit pastille) and phosphate (marshmallow) backbone and pairs of nitrogenous bases (jelly joiners), arranged in a double helix. Some showed the two hydrogen bonds between Adenine and Thymine and the three hydrogen bonds between Cytosine and Guanine.
As our next lesson will not be until after the VCE Study Camp, please continue to read through Chapter 2 (Membranes and Cell Organelles), as the practical experiment we are doing at Federation University is all about cell membranes. You will need to understand the following terms and definitions:
prokaryotic and eukaryotic
diffusion and osmosis (passive transport)
hydrophilic and hydrophobic (or lipophilic)
channel-mediated and carrier-mediated
phagocytosis, pinocytosis, exocytosis and endocytosis
ribosomes, endoplasmic reticulum and golgi complex
lysosomes, peroxisomes and endosomes
chloroplasts – including lamella, grana and thylakoids
I suggest you create a set of Quizlet flashcards using these terms and definitions.
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?