LESSONS 2-3: Movement of Substances Across Membranes By the end of these lessons you should be able to: Define diffusion, osmosis and active transport. Explain how different concentrated solutions can effect the movement of water. Explain the biological importance of osmosis and diffusion.

Concentration of Solutions Hypertonic: has a higher concentration of dissolved substances than the cell or solution that it is being compared to. Isotonic: has the same concentration of dissolved substances as the cell or solution it is being compared to. Hypotonic: has a lower concentration of dissolved substances than the cell or solution that it is being compared to.

Surrounding Fluid Animal Cell Plant cell Isotonic solution Water moves in and out at same rate. Water moves in and out at the same rate. Hypotonic solution Water enters the cell causing it to swell. The cell may burst. Water enters the cell and into the vacuole causing it to swell. The membrane pushes against the cell wall. The cell is turgid. Hypertonic solution There is movement of water out of the cell. The cell decreases in size. There is movement of water out of the cell. Cell contents decrease in size. The cell wall keeps the shape. Plasmolysis.

A turgid cell A plasmolysed cell

What type of salt solution are these cells in? B Hypertonic. High salt concentration Isotonic. Same conc. as inside the cell Hypotonic. Low salt concentration Answer:

Movement across Membranes All cells must be able to take in and expel various substances across their membranes in order to survive, grow and reproduce. Cells do this via a number of different processes……

Diffusion Is the net movement of a substance from a region of high concentration to a region of low concentration. Movement occurs down a concentration gradient. No energy is required = passive process. For gases or lipid-soluble molecules, diffusion occurs directly across the membrane. For large, lipid-insoluble, polar molecules and ions, diffusion across a membrane can only occur with the aid of a carrier protein or protein channel.

Why do cells need diffusion? For survival… Cells obtain nutrients such as oxygen and glucose. Cells excrete wastes as carbon dioxide and urea.

Osmosis Osmosis is the movement of water particles from a dilute solution to a more concentrated one through a partially permeable membrane. Movement occurs down a concentration gradient. No energy is required = passive process.

“dilute” This is either a low concentration of the solute particles sucrose/salt or pure water. In each case there is a lot of water molecules in comparison to the solute molecules.

“A more concentrated solution” This is a mixture of a solute like salt/sugar, and a solvent like water, where there is a high amount of solute particles in comparison to the water particles.

“Partially permeable membranes” Very thin layers of material which allow some things, but prevent other things from passing through them. Cell membranes will allow small molecules like oxygen, water, carbon dioxide, ammonia, glucose, amino-acids, etc. to pass through. They will not allow larger molecules like sucrose, starch, protein, etc. to pass through.

Cell Medium Low concentration of solute High concentration of solute

Water molecules are free to pass across the cell When you put an animal or plant cell into a liquid containing water …… If the medium surrounding the cell is more dilute than inside the cell, then it will ________________________. gain water by osmosis Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.

Cell Medium High concentration of water and low concentration of solute Low concentration of water and high concentration of solute

Water crosses the cell membrane in both When you put an animal or plant cell into a liquid containing water …… If the medium is a more concentrated solution than inside the cell the cell will __________________. lose water by osmosis Water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.

Cell

no net movement of water across the cell membrane When you put an animal or plant cell into a liquid containing water …… If the medium is exactly the same concentration as the cell there will be _____________________________________ ______________________. no net movement of water across the cell membrane Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

Why do cells need osmosis? Osmosis is vitally important to plants. Plants gain water by osmosis through their roots, and it is osmosis that moves water into plant cells, making them turgid, and thus able to hold the plant upright.

A turgid cell A plasmolysed cell

Does water move in or out?

Comparing and Contrasting Diffusion and Osmosis Random movement of particles Random movement of water From higher to lower concentration Down the concentration gradient No energy needed from the cell

Other types of movement Facilitated diffusion, active transport and bulk transport

Facilitated Diffusion Channel Mediated Some substances need to move through protein channels in membrane. Carrier Mediated Some substances need a protein carrier in addition to a protein channel. Neither require energy and both occur down a concentration gradient.

Active Transport Is the movement of substances from a region of low concentration to a region of high concentration Energy is required as movement is against a concentration gradient. Carrier molecules assist the movement

Bulk Transport Endocytosis: bulk transport of material ______ a cell Exocytosis: bulk transport of material ______ of a cell

Question Time!

Surface Area: Volume Ratio The surface area of an object in comparison to it’s volume. Important for the movement of substances. The LARGER the SA:V ratio the FASTER the rate of exchange.

Why are cells so small? To enhance the rate at which molecules are exchanged across the cell membrane. If cells were large diffusion would occur at such a slow rate that the cell would not be able to survive. So, cells are miniscule.

Transport Systems Multicellular organisms do not rely on diffusion alone. Transport systems such as the circulatory system enables cells to have a ready supply of nutrients and oxygen. Transport systems also enable an organism to excrete wastes at an efficient rate.

Question Time! Explain the difference in size between the Little Penguins on coastal Victoria and the Emperor Penguins from Antarctica. __________________________________ __________________________________ __________________________________ __________________________________ __________________________________ __________________________________ __________________________________

Osmosis Experiment: method Use a cork borer to cut out 6 lengths of potato chips. Use a ruler and scalpel to cut each chip to 20mm. Measure and record the mass of each chip. Place each chip into a separate specimen tube. Label the specimen tubes with 0, 0.2, 0.4, 0.6, 0.8, 1. Add 20cm3 of each salt solution to a different tube. Leave the chips in the salt solution for 30 minutes. Remove the chips and re-measure the mass. Record the change in mass, indicating whether the chips have increased or decreased in size.