Transport in Cells (AQA GCSE Biology Combined Science): Revision Notes
Active transport
What is active transport?
Active transport is a way that cells move substances across their membranes. Unlike diffusion and osmosis, active transport needs energy from respiration to work.
Active transport moves substances from areas where there are fewer of them to areas where there are more of them. This is the opposite direction to diffusion.
Active transport lets substances move against the concentration gradient.
Think of it like pushing a ball uphill - you need energy to do it! This analogy helps explain why active transport requires energy from respiration to work against the natural tendency of substances to move down concentration gradients.
How active transport works
Cell membranes contain special proteins called transport proteins. These proteins act like tiny pumps that use energy to move substances.
Here's how it works:
- Energy from respiration powers the transport protein
- The protein changes shape
- This pushes substances from the dilute side to the concentrated side
- This happens even when there are already more substances on the other side
Active transport in the gut
When you eat food, glucose enters your small intestine. Your body needs to absorb this glucose into your blood, even when there's already lots of glucose in your blood.
Worked Example: Glucose Absorption
Sometimes there's more glucose in your blood than in your small intestine. Active transport can still move glucose from your intestine into your blood because it uses energy to work against the concentration gradient.
This process ensures your cells get the glucose they need, regardless of concentration differences.
This is really important because your cells need glucose for respiration. The glucose goes to your mitochondria to make energy.
Active transport in roots
Plants need mineral ions like nitrate ions () to grow properly and make proteins.
The problem is that soil water contains very few mineral ions - it's a very dilute solution. But plant cells need lots of these ions.
Active transport solves this by:
- Using energy to absorb mineral ions from soil water
- Moving ions from the dilute soil water into plant cells
- Working even when plant cells already have more ions than the soil
If plants don't get enough nitrate ions, they become stunted and can't grow properly. This shows why active transport is essential for plant survival in nutrient-poor soils.
Comparing transport processes
| Transport type | Direction | Energy needed | What moves |
|---|---|---|---|
| Diffusion | High to low concentration | No | Any substances |
| Osmosis | Dilute to concentrated solution | No | Water only |
| Active transport | Low to high concentration | Yes (from respiration) | Any substances |
Key difference: Only active transport can move substances against the concentration gradient.
Remember!
Key Points to Remember:
- Active transport needs energy from respiration to work
- Moves substances against the concentration gradient (from low to high concentration)
- Uses transport proteins in cell membranes that act like pumps
- Essential in the gut for absorbing glucose when blood glucose is already high
- Essential in plant roots for absorbing mineral ions from very dilute soil water
- Different from diffusion and osmosis because it requires energy and can work against gradients