Transport Across the Cell Membrane
Senior 2 · Biology
Transport Across the Cell Membrane
Lesson 2 of 5
18 min
Let's begin
When someone in the kitchen starts frying onions for isombe, you can smell it across the compound within a minute or two, though nobody carried the smell to you. When you put a crystal of potassium permanganate in a glass of still water, a purple colour slowly spreads until the whole glass is evenly pink.
Both are diffusion. It is how oxygen reaches your blood in the lungs, how carbon dioxide enters a leaf, and how digested food reaches your cells.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement.
| Where | What diffuses | From → to |
|---|---|---|
| Air sacs (alveoli) of the lungs | oxygen | air in the alveolus → blood |
| Air sacs of the lungs | carbon dioxide | blood → air in the alveolus |
| Leaf in daylight | carbon dioxide | air → leaf cells, through the stomata |
| Small intestine | digested food, such as glucose | gut → blood |
| Fish gills | oxygen | water → blood |
Facilitated diffusion is diffusion through a channel or carrier protein in the membrane. It helps substances such as glucose and ions that cannot cross the oily bilayer on their own. It still goes down the concentration gradient and still uses no energy from the cell.
Question: A student cuts three cubes of pink agar jelly with sides of 1 cm, 2 cm and 4 cm and puts them in acid, which turns the agar colourless as it diffuses in. Calculate each cube's surface area to volume ratio and predict which becomes fully colourless first.
Solution: For a cube of side x: surface area = 6x², and volume = x³.
| Side | Surface area (cm²) | Volume (cm³) | Ratio |
|---|---|---|---|
| 1 cm | 6 × 1 = 6 | 1 | 6 : 1 |
| 2 cm | 6 × 4 = 24 | 8 | 3 : 1 |
| 4 cm | 6 × 16 = 96 | 64 | 1.5 : 1 |
The 1 cm cube goes colourless first. It has the most surface for each cm³ of volume, and its centre is only 0.5 cm from the surface. As an organism gets bigger, its surface area to volume ratio falls. This is why large animals cannot rely on diffusion through their skin alone, and need lungs, gills and a blood system.
Tip
Memory hook: diffusion is fast when the gradient is steep, the temperature is high, the area is large and the distance is short.
Exam technique: always say "from a higher to a lower concentration" or "down a concentration gradient". The word "net" earns credit too.
Define diffusion.
Solution: The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement. It needs no energy from the cell.
Give three features of the lungs that make diffusion of gases fast.
Solution: A very large surface area (millions of alveoli); a very thin surface (alveolus and capillary walls one cell thick); and a steep concentration gradient kept up by breathing and blood flow. A moist surface also helps, because gases dissolve before they diffuse.
Explain why sugar dissolves and spreads faster in hot tea than in cold tea.
Solution: At a higher temperature the particles have more kinetic energy and move faster, so they diffuse through the tea more quickly.
Calculate the surface area to volume ratio of a cube with sides of 3 cm.
Solution: surface area = 6 × 3² = 54 cm²; volume = 3³ = 27 cm³; ratio = 54 : 27 = 2 : 1.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
