Transport in Cells (AQA GCSE Biology): Revision Notes
Active transport
What is active transport?
Active transport is a special way that cells move substances across their membranes. Unlike diffusion and osmosis, active transport needs energy to work.
Key features of active transport:
- Moves substances from a dilute solution to a concentrated solution
- Works against the concentration gradient (uphill)
- Energy from respiration is needed
- Uses special transport proteins in the cell membrane
Think of it like pushing a ball uphill - you need energy to do it!
How does active transport work?
Active transport happens at the cell membrane using transport proteins. The process involves several key steps:
- Transport proteins sit in the cell membrane
- Energy from respiration makes these proteins change shape
- This shape change allows them to grab substances and move them across the membrane
- Substances move from where there are fewer of them to where there are more of them
This is the opposite of diffusion, which moves substances downhill without energy.
Active transport in the gut
Your digestive system uses active transport to absorb glucose from food. This process is essential for getting nutrients from your digestive system into your bloodstream.
Example: Glucose Absorption in the Small Intestine
The situation:
- Glucose from food must get into your bloodstream
- Sometimes there's more glucose in your blood than in your small intestine
- Active transport can still absorb glucose even when this happens
- This ensures your cells get the glucose they need for respiration
Without active transport, you might not absorb enough glucose from your food.
Active transport in plant roots
Plants use active transport to absorb mineral ions from soil water. This is crucial for plant nutrition and growth.
Examples of mineral ions plants need:
- Nitrate ions (for making proteins)
- Other ions for healthy growth
Example: Mineral Ion Uptake from Soil
Why active transport is needed:
- Soil water has very low concentrations of mineral ions
- Plant roots contain higher concentrations of these ions
- Active transport moves ions against the concentration gradient
- This helps plants get enough minerals even from poor soil
Comparing transport processes
The three main transport processes work in different ways:
| Process | Direction | Energy needed? | Membrane needed? |
|---|---|---|---|
| Diffusion | High to low concentration | No | Not needed |
| Osmosis | Dilute to concentrated (water only) | No | Partially permeable needed |
| Active transport | Low to high concentration | Yes (from respiration) | Yes |
Notice how active transport is the only process that can move substances from low to high concentration - this is why it needs energy!
Key Points to Remember:
- Active transport needs energy from respiration to work
- It moves substances against the concentration gradient (from low to high concentration)
- Transport proteins in the cell membrane do the work by changing shape
- Important examples include glucose absorption in the gut and mineral ion uptake in plant roots
- Unlike diffusion and osmosis, active transport works uphill and needs energy