Terrestrial plants need to maintain their water balance, while still allowing the exchange of gases between the plant cells and the external environment. Gas exchange occurs through stomata, which also allows the escape of water vapour. The image above shows a cross section of a leaf from Marram grass, common on sand dunes, where it is very salty and often dry. You can see how the leaf is rolled, creating an internal micro-climate that is much more humid than the external environment. This reduces water loss and allows stomata to remain open, even in the driest of climates. PIne needles (Pinus) and Casaurina also have cylindrical leaves, an adaptation for dry environments.
The sunken stomata in this image (cross section of a leaf) allows a moist layer of air above the stomata, protecting the leaf from excessive evaporation.
This image shows a cross section of a leaf from a plant adapted to a very arid environment. The stomata are sunken into pits with lots of epidermal hairs, which provide a humid micro-climate, allowing the stomata to remain open, despite very dry external conditions.
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.
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.