So, you're probably gunna know a lot about osmosis already from the previous unit, so I won't do it in too much detail, but here we go..
Osmosis is the movement of water only, so if a questions talking about exchanging gases, we're not interested.
To make it complicated, we say osmosis is the movement of water particles from a dilute solution (loads of water) to a more concentrated solution (not so much water, more of the other stuff).
This all happens through a partially permeable membrane, so it lets water through, but not solute molecules because well, they're too big.
Therefore, osmosis will gradually dilute the more concentrated solution.
This doesn't just happen in humans, although most of the questions will probs be about that. It also happens in plants, eg, at root hair cells water moves from the soil into the cell by osmosis.
If you're finding the whole dilute thing confusing, just think of robinsons squash. The more water you put in it, the more dilute it is, the less taste it has. If it's got less water in it, it's more concentrated (more yummy).
We say water moves along a concentration gradient, not against it, because it's going from a high to low concentration.
DIFFUSION AND ALVEOLI
Kk, diffusion is pretty much the same as osmosis but it's not about water. It's about gases, so CO2, oxygen etc. It still works along the concentration gradient, going from a high, to a low concentration.
A good example of this happens in our lungs with the alveoli. Call me a geek, but I love the alveoli, they sound complicated but the stuff is so easy to learn you just sound smart.
So there's millions of these alveoli in your lungs, they're kinda like air sacs and they're really close to your blood capillaries. This is really efficient placing for the exchange of carbon dioxide and oxygen in your lungs because being close to the capillaries means there is an excellent blood supply. Alveoli also have a massive surface area (but this can be ruined by smoking, oh dear) and moist surfaces. I'll explain why these things are good on another card.
So we know CO2 is poisonous, so we know that CO2 is diffused out of your blood and into the alveoli, ta for that one alveoli. And we all know oxygen is useful, for respiration and such, so oxygen diffuses from your alveoli into your blood. Your blood then becomes oxygenated and everyone's happy
ALSO, it's important the blood circulation is good because if the blood near the alveoli gets too concentrated in oxygen (even more than in the alveoli), oxygen will start diffusing back into the alveoli! And that would just be terrible.
Pay attention A* people, this is the higher tier stuff.
Some substances are transferred against the concentration gradient, this is called active transport. So, substances move from somewhere dilute, move to somewhere where there's more of them. For example, in humans, sugar can be absorbed from the intenstine through active transport. Or, if you're more of a plant person, there's a low concentration of nitrate ions in the soil, but a high concentration in the root hair cell. But the cell still wants these useful ions so it 'pulls' them in against the concentration gradient anyway.
Seeing as they're working against the gradient, it's like working against a strong current, so they need more energy. This energy comes from respiration.
Just remember, against gradient and uses energy and you'll be fine and dandy.
Because we're so clever we have organ systems which are specialised to help the exchange of materials. Eg, the villi in your small intenstine, y'know, those hairy things and the alveoli in your lungs.
Just remember, it's good to have:
LARGE SURFACE AREA GOOD BLOOD SUPPLY MOIST SURFACES
Large surface area cos, well it's pretty obvious, the bigger the surface area the more room there is for exchange, soooo, more of it happens. Hence the little hairs on villi and the bumpy surface of alveoli (bad description but find a pic and you'll know what i mean).
Good blood supply because of what I explained on the other card about how blood has to keep moving away from the exchange point to ensure the concentration gradient remains the same. And also, more blood supply, more exchange.
Moist surfaces so gases can dissolve more easily.
Fish also have specialised gills. There's loads of them layered over eachother (surface area), they're kept moist under water and instead of good blood supply, they have to have water constantly passing them, so they absorb the oxygen from the water.
EXCHANGE IN PLANTS
Leaves, as we know, are broad, thin and flat with loads of internal air spaces. This gives them a big surface area for efficient photosynthesis.
Leaves have stomata on their undersurface which are a bit like little holes. They let carbon dioxide in (for photosynthesis) and oxygen out (result of photosynthesis). This exchange is reversed when the plant is respiring, obviously..
When a plant photosynthesises it loses water through transpiration. Water vapour from the internal leaf evaporates through the stomata. Transpiration is quicker in hot, dry, windy conditions. The conditions have to be dry because transpiration occurs along the concentration gradient, so it moves from a dilute solution in the leaf, to the air where there is less water. Thus if it's humid or raining, there may be a higher concentration of water in the air than there is in the leaf, meaning less transpiration.
Stomata are really clever. They change their size in order to keep the plant alive. If transpiration is happening too fast (quicker than the root hair cells are taking back up water) then the stomata will close up so less water is lost.The size of the stomata is controlled by a pair of guard cells. If too much water is lost the plant can be dehydrated. The plant will also wilt, to decrease it's surface area.
In a drought, there may be no photosynthesis at all cos the stomata close up.
Just remember, that no transpiration, no photosynthesis.