November: Diffusion, Osmosis, Carbohydrates and Photosynthesis

Osmosis, Diffusion and Selective Permeability

Osmosis is the movement of pure water across a selectively permeable membrane. Diffusion is the movement of any substance from an area of high concentration (ie where there's lots of it) to an area of low concentration (where there is less of it). These are important definitions to know.

Selectively permeable membranes, also known as semi-permeable or partially-permeable membranes, have holes of a size which allow smaller molecules to pass through, but not larger ones. Cell membranes are selectively permeable. We used Visking tubing (also known as Dialysis tubing) as an example of such a membrane.

We demonstrated that water can travel through Visking tubing by osmosis. Visking tubing containing sucrose solution was placed in beaker of water. A narrow tube was fixed in the top of the tubing, to allow us to observe changes in the fluid level inside the tubing. The water level rose over time as water osmosised into the concentrated sucrose solution in the tube.

Carbohydrates: Sugars and Starches

Testing for Starch

Alison demonstrated that when iodine solution is added to starch, it turns a blue-black colour.  This colour change shows that starch is present and is the standard test for starch.

Can Starch and Iodine diffuse across Visking tubing?

We took lengths of soaked tubing and knotted them near one end to form a long bag.  Into this we put starch solution, then we closed the tube with a clip to form a sausage shape, and rinsed the outside to remove any deposits of starch which might affect our results.  After patting it dry with paper towels, we measured the mass and recorded it, and we noted the appearance of the tubing.  The starch solution looked milky white.

A beaker was filled with water and iodine solution was added until the liquid in the beaker was a pale golden yellow.  Our starch-filled sausage of Visking tubing was placed in the beaker.

Visking tubing containing starch solution, in beaker of iodine solution

We then observed carefully in a serious, scientific way.

After 5 minutes, some colour change was visible in some of the tubing.  The change happened at different rates and to different degrees.  After an hour, all tubes showed a colour change, ranging from lilac to blue-black, showing that iodine had diffused across the membrane to reach the starch.  However, the solution in the beakers remained pale golden-yellow, showing that no starch had diffused out of the tubing.

Photosynthesis and Molecular Modelling

Imagine if, just using sunlight, water and carbon dioxide, you could make sugar!  Oh hang on, that's what plants do....

We used lego bricks to model atoms and molecules.  A black brick represents carbon, white is hydrogen, and red is oxygen.  We discussed the difference between atoms, molecules, and mixtures.  We made oxygen molecules (O2) , carbon dioxide (CO2) and hydrogen (H), then we made water (H2O), and fizzy water.

Next, we worked as plants; we dismantled water and carbon dioxide molecules and, using sunlight energy provided by Lydia, we reassembled our elements as oxygen and glucose.  

Lydia is being the sun, while our 'plants' make glucose.

Making the molecular models was a very Serious Scientific Process.

Molecular Models and Partially Permeable Membranes

Molecular models help us to understand how molecules work together as mixtures or compounds. We looked at molecular models of iodine, water and starch. Iodine and water molecules are very small, simple sugars such as glucose are larger, other sugars such as sucrose are larger still, and starch molecules are enormous.

Alison showing two lego glucose molecules 

Starch contains many simple sugars joined together in a chain.  Usually over 300 glucose molecules are joined together to make one starch molecule.  We don't have enough lego to do this, so we made some simplified models.

Angie is holding a molecular model of sucrose

Angie built glucose, fructose and sucrose molecular models using a traditional chemistry modelling set, and we discussed the differences between this type of model and the lego models. Starch is harder to model as it contains more than 300 glucose molecules.

Selective Permeability and Passive Transport

To keep the models manageable, we used one small multi-coloured multilink block to represent glucose, and the chains of multilink blocks represent starches.  We demonstrated selective permeability by showing that small blocks representing glucose could pass through the holes in a net, but when several of these 'glucose' blocks were joined together to make a chain representing a starch, they couldn't fit through. In the same way, large starch molecules could not pass through our visking tubing to get out into the iodine solution, but the small iodine molecules could travel through the tubing to get into the sausage of starch solution.

Cell membranes are an example of selectively permeable membranes. There is another important mechanism for transporting substances into and out from cells - Active Transport - but diffusion and osmosis are types of 'passive transport'.  This means no energy input is required, and substances move from an area where they are concentrated, to one in which they are dilute - ie from where there is lots to where there is less of the substance.