Transport in and out of cells - Intro

A single cell is a tiny room with a wall around it, and everything it lives on has to cross that wall. Oxygen and food have to get in. Waste has to get out. The cell has no hands to reach out and grab any of it. There are three ways across that wall, and they turn up again and again for the rest of biology.

This chapter is about those three ways, and then about the problem of being big. An elephant cannot breathe through its skin the way a single cell can, and living things have built something to get round that. The same idea of particles spreading from where there are lots to where there are few comes back in the leaf, the gut and the kidney. So the rest of the subject quietly leans on this chapter. The videos build on each other, so if this is all new, begin at the top of the list. If part of it already feels familiar, jump to the layer you are missing.

Diffusion and active transport across the cell membrane is where a fresh start belongs. It covers two of the three ways, and the marks go on direction: a common slip is using words like along or across a concentration gradient, which give no direction at all. Osmosis and the change in mass of plant tissue is the third way, with a calculation on top. Press it once the first video makes sense. An osmosis question set in a new situation catches many students out. Exchange surfaces is the big-organism problem, and the video on it is the one on surface area and volume. If you already know the three ways, go straight here, because this is where marks go missing. On one question about how the lungs are adapted for gas exchange, over forty per cent of students scored zero. What sets the rate of diffusion, and Fick's law comes last, because it uses everything before it. Go here when you want to say why one cell, or one pair of lungs, exchanges faster than another. On one question about those factors, fewer than ten per cent of students gained both marks. You can search for any video by its name or its code, and the codes are right there on the list. The quickest way is the scholafly.com website, which indexes every video in every subject.

Next in the chapter is Diffusion and active transport across the cell membrane. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh four, oh one.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.