Movement of Substances
4. Active Transport
Learning outcomes
- I can define active transport.
- I can explain how active transport differs from diffusion and osmosis.
- I can describe the role of energy in active transport.
- I can identify examples of active transport in living organisms.
- I can explain why active transport is important for cell function.
Active Transport
Cells often move substances by diffusion and osmosis, but these processes only allow net movement down a concentration gradient.
Sometimes a cell needs to move a substance in the opposite direction — from an area where there is relatively little of the substance to an area where there is already more.
To do this, the cell uses active transport.
Active transport is essential for processes such as:
- absorbing mineral ions from the soil
- absorbing nutrients in the small intestine
- maintaining ion concentrations in nerve cells
- controlling the internal conditions of cells
What Is Active Transport?
Active transport is the movement of substances across a cell membrane from an area of lower concentration to an area of higher concentration, using energy.
This means substances move against the concentration gradient.
A simple way to represent this is:
low concentration → high concentration
Energy is required for this movement.
This makes active transport different from diffusion and osmosis.
Moving Against the Concentration Gradient
Recall that a concentration gradient is a difference in the concentration of a substance between two areas.
During diffusion, particles show a net movement:
high concentration → low concentration
This is movement down the concentration gradient.
Active transport can move particles:
low concentration → high concentration
This is movement against the concentration gradient.
Why Does Active Transport Require Energy?
Moving particles against their concentration gradient does not happen spontaneously by diffusion.
The cell must provide energy to make this movement occur.
This energy is usually supplied by ATP.
ATP stands for adenosine triphosphate.
ATP acts as an immediate source of usable energy for many cellular processes.
Cells produce much of their ATP during cellular respiration.
Therefore, active transport indirectly depends on respiration.
More respiration → more ATP available → more energy available for active processes.
Transport Proteins
Active transport occurs using specialized proteins in the cell membrane.
These are often called:
- carrier proteins
- transport proteins
- protein pumps
The proteins recognize and transport particular substances.
A simplified active transport process is:
- A substance binds to a specific membrane protein.
- Energy from ATP is supplied.
- The protein changes shape.
- The substance is moved across the membrane.
- The substance is released.
- The protein returns to its original shape.
This allows the cell to move particular substances against their concentration gradients.
Active Transport Is Selective
Cells do not simply pump every substance across their membranes.
Transport proteins are usually specific to particular substances.
For example, one protein may transport:
- sodium ions
while another transports:
- potassium ions
- calcium ions
- hydrogen ions
- particular nutrients
This contributes to the selective permeability of the cell membrane.
The membrane can control which substances enter or leave the cell.
Active Transport vs Diffusion
Diffusion is the net movement of particles from higher concentration to lower concentration.
Active transport can move particles in the opposite direction.
| Feature | Diffusion | Active Transport |
|---|---|---|
| Direction | High → low concentration | Low → high concentration |
| Concentration gradient | Down gradient | Against gradient |
| Cellular energy required | No | Yes |
| Membrane required | Not always | Yes |
| Transport proteins | Sometimes | Required |
| Example | Oxygen entering cells | Mineral ions entering root cells |
The most important difference is:
Diffusion does not require cellular energy. Active transport does.
Active Transport vs Osmosis
Osmosis is specifically the movement of water across a partially permeable membrane.
Active transport normally moves specific dissolved substances or ions.
| Feature | Osmosis | Active Transport |
|---|---|---|
| Substance moving | Water | Ions or other substances |
| Direction | Down water concentration gradient | Against concentration gradient |
| Energy required | No | Yes |
| Membrane required | Yes | Yes |
| Transport proteins | Water can move through membrane/aquaporins | Specific transport proteins |
| Passive or active? | Passive | Active |
Comparing All Three Transport Processes
Diffusion, osmosis and active transport all allow substances to move across cell membranes, but they work differently.
| Process | What Moves? | Direction | Energy Required? |
|---|---|---|---|
| Diffusion | Particles such as O₂ and CO₂ | High → low concentration | No |
| Osmosis | Water | High → low water concentration | No |
| Active transport | Specific substances or ions | Low → high concentration | Yes |
A useful summary is:
Diffusion = down the gradient
Osmosis = water down its water concentration gradient
Active transport = against the gradient using energy
Active Transport in Plant Roots
One of the most important examples of active transport occurs in root hair cells.
Plants need mineral ions from the soil.
Important mineral ions include:
- nitrate ions
- magnesium ions
- potassium ions
Sometimes the concentration of a mineral ion is lower in the soil than inside the root cells.
Diffusion cannot move the ions into the root because diffusion would move them in the opposite direction.
The plant therefore uses active transport.
For example:
Soil nitrate concentration: low
Root cell nitrate concentration: high
Nitrate ions still need to enter the root.
Therefore:
soil → root cell
This movement is:
low concentration → high concentration
The plant must use energy to make this happen.
Why Do Plants Need Mineral Ions?
Mineral ions are essential for healthy plant growth.
For example:
Nitrate ions are needed to make amino acids and proteins.
Magnesium ions are needed to make chlorophyll.
Potassium ions are involved in several important cellular processes.
A plant cannot simply stop absorbing these ions when their concentration in the soil becomes low.
Active transport allows the plant to continue absorbing essential minerals even against a concentration gradient.
Root Hair Cells Are Adapted for Transport
Root hair cells have long projections called root hairs.
These increase the surface area available for absorption.
Root hair cells also contain many mitochondria.
Mitochondria carry out aerobic respiration and help supply ATP.
This is important because active transport requires energy.
This is another example of the relationship between structure and function.
Root hair cells need active transport.
Active transport needs ATP.
ATP is supplied by cellular respiration.
Therefore, cells involved in high rates of active transport often have many mitochondria.
Active Transport in the Small Intestine
Active transport also occurs in the small intestine.
After food is digested, small molecules such as glucose need to be absorbed into the body.
Glucose can sometimes move down a concentration gradient using transport proteins.
However, when glucose concentration in the intestine becomes relatively low, the body still needs to absorb as much useful glucose as possible.
Transport mechanisms involving active transport allow glucose absorption to continue even when simple passive movement would not be sufficient.
The small intestine is well adapted for absorption because it has:
- many villi
- microvilli
- a large surface area
- a good blood supply
- cells containing many mitochondria
These features help nutrients move efficiently into the body.
Active Transport in Nerve Cells
Nerve cells depend on carefully controlled concentrations of ions.
Important ions include:
- sodium ions, Na⁺
- potassium ions, K⁺
The concentrations of these ions are different inside and outside nerve cells.
A membrane protein called the sodium-potassium pump helps maintain these differences.
The pump uses energy from ATP to transport sodium and potassium ions across the membrane.
Maintaining these concentration differences is essential for the electrical activity of nerve cells.
Without active transport, nerve cells would gradually lose the ion gradients required for normal function.
Why Active Transport Is Important
Active transport allows cells to maintain conditions that are different from their surroundings.
This is extremely important.
Without active transport, cells would be limited to substances that could move naturally down concentration gradients.
Active transport allows cells to:
- absorb substances even when external concentrations are low
- remove substances when necessary
- maintain ion gradients
- regulate internal conditions
- absorb nutrients
- support nerve and muscle function
- maintain homeostasis
Active Transport and Homeostasis
Homeostasis is the maintenance of relatively stable internal conditions.
Cells need to control the concentrations of many substances.
For example, a cell may need:
- more potassium ions inside than outside
- less sodium inside than outside
- particular concentrations of calcium ions
- suitable concentrations of nutrients
Diffusion alone would eventually reduce many of these differences.
Active transport allows cells to create and maintain concentration gradients.
This makes active transport an important part of cellular homeostasis.
Active Transport and Respiration
Because active transport requires ATP, its rate can depend on cellular respiration.
Imagine two groups of root hair cells.
Group A receives plenty of oxygen.
Group B receives very little oxygen.
With sufficient oxygen, Group A can carry out aerobic respiration efficiently and produce ATP.
Group B may produce less ATP.
As a result, active transport of mineral ions may decrease in Group B.
This explains why poorly aerated or waterlogged soils can sometimes cause problems for plant roots.
Roots need oxygen for respiration as well as water and mineral ions.
Worked Example: Root Hair Cell
The concentration of magnesium ions in the soil is:
2 units
The concentration inside a root hair cell is:
8 units
The plant still absorbs magnesium ions from the soil.
Direction:
soil → root
Concentration:
2 → 8
This is movement from lower concentration to higher concentration.
Therefore, the process requires:
active transport
and the cell must supply:
energy from ATP
Worked Example: Diffusion or Active Transport?
A substance has the following concentrations:
Outside cell: 12 units
Inside cell: 4 units
The substance moves into the cell.
Direction:
12 → 4
This is movement from higher concentration to lower concentration.
Therefore, it could occur by:
diffusion
assuming the membrane is permeable to the substance.
Now consider:
Outside cell: 3 units
Inside cell: 10 units
The substance still moves into the cell.
Direction:
3 → 10
This movement is against the concentration gradient.
Therefore:
active transport is required
Worked Example: Energy Supply
A chemical prevents mitochondria from producing enough ATP.
What would happen to active transport?
The amount of energy available to transport proteins would decrease.
Therefore:
active transport would decrease or stop.
Diffusion, however, could continue because diffusion does not require ATP from the cell.
Worked Example: Identifying Transport Processes
Consider three situations.
Situation A
Oxygen moves from a high concentration outside a cell to a lower concentration inside.
Process:
Diffusion
Situation B
Water moves across a partially permeable membrane from a dilute solution toward a more concentrated solution.
Process:
Osmosis
Situation C
Mineral ions move from a low concentration in the soil to a higher concentration inside a root hair cell.
Process:
Active transport
The direction of movement and whether energy is required are important clues.
Why Cells Cannot Depend Only on Diffusion
Imagine a root hair cell that needs nitrate ions.
At first:
Soil nitrate concentration: high
Root nitrate concentration: low
Nitrate may be able to move into the root down its concentration gradient through suitable transport mechanisms.
Eventually, however:
Soil nitrate concentration: low
Root nitrate concentration: high
Passive movement can no longer produce net movement into the root.
But the plant still needs nitrate.
Active transport allows the plant to continue absorbing nitrate even under these conditions.
This is one of the major advantages of active transport.
Surface Area and Active Transport
Cells that absorb large quantities of substances often have structures that increase their surface area.
Examples include:
Root hair cells – Long projections increase contact with soil.
Small intestine cells – Microvilli increase the membrane surface available for absorption.
More membrane surface means more transport proteins can be present.
This can increase the amount of material transported.
Active Transport and Cell Structure
Cells specialized for active transport often have two important adaptations:
Large membrane surface area
This provides space for many transport proteins.
Many mitochondria
These provide ATP through cellular respiration.
This creates a clear structure-function relationship:
large surface area + many mitochondria → efficient active transport
Common Misconceptions
Active transport means particles are moving because the organism is active.
Incorrect. "Active" refers to the use of cellular energy.
Active transport moves substances from high concentration to low concentration.
Usually this would be passive movement. Active transport allows movement against a concentration gradient.
Active transport does not require a membrane.
Active transport uses specialized proteins in cell membranes.
Diffusion and active transport both require ATP.
Diffusion does not require cellular energy. Active transport does.
Osmosis is a type of active transport.
Osmosis is passive and does not require ATP.
Plants only absorb water through their roots.
Roots also absorb important mineral ions. Many of these can be taken up using active transport.
Mitochondria directly pump substances across the membrane.
Mitochondria do not perform the pumping. They help provide ATP that membrane proteins can use.
Did You Know?
One of the most important active transport proteins in animal cells is the sodium-potassium pump.
A single cycle of the pump typically moves:
3 Na⁺ ions out of the cell
and
2 K⁺ ions into the cell
while using energy from ATP.
This helps maintain the ion gradients needed by nerve cells and many other cells.
Your cells continually spend a significant amount of their available energy maintaining these ion differences.
Key Terms
Active transport – Movement of substances across a membrane against a concentration gradient using energy.
Concentration gradient – A difference in concentration between two regions.
ATP – A molecule that provides usable energy for cellular processes.
Transport protein – A membrane protein that helps move a particular substance across the membrane.
Protein pump – A membrane protein that uses energy to transport substances.
Cellular respiration – A process through which cells release energy from food molecules and produce ATP.
Passive transport – Movement that does not require cellular energy.
Diffusion – Net movement of particles from higher concentration to lower concentration.
Osmosis – Net movement of water across a partially permeable membrane.
Root hair cell – A specialized plant cell adapted for absorbing water and mineral ions from the soil.
Homeostasis – Maintenance of relatively stable internal conditions.
Key Takeaways
- Active transport moves substances across cell membranes using energy.
- It can move substances from lower concentration to higher concentration.
- This means active transport can work against a concentration gradient.
- Energy for active transport is usually supplied by ATP.
- ATP is produced through cellular respiration.
- Specialized membrane proteins carry out active transport.
- Diffusion and osmosis are passive and do not require cellular energy.
- Plants use active transport to absorb mineral ions through their roots.
- Cells in the small intestine use transport mechanisms involving active transport to help absorb nutrients.
- Nerve cells use active transport to maintain important sodium and potassium ion gradients.
- Cells performing large amounts of active transport often contain many mitochondria.
- Increased membrane surface area can provide more space for transport proteins.
- Active transport allows cells to maintain internal conditions that are different from their surroundings.
- This makes active transport essential for cell function and homeostasis.