Difference between active and passive transport, Practice Problems and FAQs

How many of you enjoy travelling by the local trains of your city? While there may be mixed feelings regarding this experience, there’s no denying the fact that boarding a heavily crowded train is a task in itself. You have to push through a tide of people while you try to get into the packed compartment and get a secure spot for yourself. This requires a lot of energy and effort.

On the other hand, boarding an empty train compartment is much easier even if there are many passengers waiting to board the train. All you have to do is just mingle in the crowd and as the tide of humans move towards the train compartment, you will also be carried forward, without having to spend too much energy.

Did you know that even our cells follow this principle for the transport of substances across their membranes? Whenever molecules move from their high concentration to lower concentration, they do not require any extra energy and are passively transported. But for movement against a concentration gradient, energy is needed and the process is said to be active transport.

In this article we will try to understand the different aspects which distinguish active transport from passive transport.

Table of Content

• Passive transport
• Active transport
• Difference between active and passive transport
• Practice Problems
• FAQs

Passive transport

When molecules or ions are moved across a semipermeable membrane across their concentration gradient without spending any energy, the process is called passive transport. The major modes of passive transports in plants are as:

• Simple diffusion
• Facilitated diffusion
• Osmosis

Simple diffusion

Simple diffusion is the passive movement of molecules from a higher to lower concentration until equilibrium is attained. It does not involve expenditure of energy and can occur even in the absence of a semipermeable membrane. The exchange of CO₂ and O₂ in plants takes place through simple diffusion.

Fig: Diffusion

Imbibition

The special type of diffusion where the hydrophilic solid colloids absorb the liquid which causes the increase in volume of the solid is called imbibition. The germination of seeds happens by the imbibition of water by seeds.

Fig: Seeds germinating by imbibing water

Facilitated diffusion

If the molecules that are lipid insoluble are moving across the membrane with the help of specialised proteins, then it is called a facilitated diffusion. This process takes place without the help of energy.

GIF: Facilitated diffusion

The facilitated diffusion of a single type of molecule or ion across the membrane in a particular direction is called uniport.

Fig: Uniport GIF: Uniport

Specialised proteins

The specialised proteins involved in the facilitated diffusion are as follows:

• Channel proteins - These are also known as porins and are of two types - aquaporins and ion channels. Aquaporins are water channels and ion channels help in the movement of charged ions. Porins remain open at all times.

GIF: Porins GIF: Aquaporins

• Gated channel proteins - These are channel proteins which resemble gates and have specific binding sites for molecules they transport. They open or close in response to chemical or electrical stimuli. Example: AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors and NMDA (N-methyl-D-aspartate) receptors.

Fig: Gated channel proteins

• Carrier proteins - The specialised proteins which bind the particular solute on one side of the cell membrane and transport it to the other side by undergoing structural changes.

GIF: Carrier proteins

Osmosis

Osmosis is the process of movement of water molecules across a semipermeable membrane from their region of higher concentration to their region of lower concentration. The minimum amount of pressure required to prevent the entry of water by osmosis into a solution is known as the osmotic pressure. Water moves from a solution with lower osmotic pressure to a solution with higher osmotic pressure.

Fig: Osmosis

Exosmosis

Placing a cell in a hypertonic solution which has relatively higher concentration of solute than the cell sap) causes the water to leave the cell by osmosis. This results in shrinking and flaccidity of the cell. This process of water moving out of the cell by osmosis is known as exosmosis.

Fig: Exosmosis

Endosmosis

Endosmosis is the process of water entering into the cell by osmosis when the cell is placed in a hypotonic solution which has relatively less concentration of solute compared to that of the cell sap. It results in swelling of the cell and makes them turgid.

Fig: Endosmosis

Active transport

Active transport is the movement of ions or molecules against their concentration gradient, that is from a lower to a higher concentration, across the cell membrane with the help of special membrane or carrier proteins and by utilising energy.

Fig: Active transport

Types of Active Transport

Active transport is of two types -

• Primary active transport uses energy in the form of ATP (adenosine triphosphate).
• Secondary active transport uses the energy from an electrochemical gradient to transport molecules from a low to high concentration

Difference between active and passive transport

Practice Problems

1. In active transport, movement of molecules

1. is along concentration gradient
2. is against concentration gradient
3. does not require energy
4. Both b and c

Solution: Active transport is the movement of ions or molecules against their concentration gradient, that is from a lower to a higher concentration, across the cell membrane with the help of special membrane or carrier proteins and by utilising energy. Thus, the correct option is c.

2. Endosmosis occurs when the cell is placed in a solution which has

1. Higher solute concentration compared to cell sap
2. Same solute concentration compared to cell sap
3. Lower solute concentration compared to cell sap
4. Both b and c

Solution: Endosmosis is the process of water entering into the cell by osmosis when the cell is placed in a hypotonic solution which has relatively less concentration of solute compared to that of the cell sap. It results in swelling of the cell and makes them turgid. Thus, the correct option is c.

3. Which of the following statements are correct?

1. Passive transport requires pumps while active transport does not.
2. Passive transport is faster than active transport
3. Active transport is an uphill transport but passive transport is not.
4. Simple diffusion is a type of passive transport but facilitated diffusion is a type of active transport.

Solution: When molecules or ions are moved across a semipermeable membrane along their concentration gradient (downhill from high to low concentration) without spending any energy, the process is called passive transport. The three types of passive transport are simple diffusion, facilitated diffusion and osmosis. Active transport is the movement of ions or molecules against their concentration gradient, that is uphill from a lower to a higher concentration, across the cell membrane with the help of special membrane or carrier proteins and by utilising energy.

Active transport is a faster process than passive transport and is carried out using pumps while passive transport does not involve pumps. Thus, the correct option is c.

4. Which of the following is a gated channel?

1. Aquaporin
2. NMDA (N-methyl-D-aspartate) receptors
3. Porins
4. Permeases

Solution: The movement of lipid insoluble molecules and ions across the membrane occurs the help of specialised proteins. This process is called facilitated diffusion. This process takes place without the help of energy. The specialised proteins involved in the facilitated diffusion are as follows:

• Channel proteins - These are also known as porins and are of two types - aquaporins and ion channels. Aquaporins are water channels and ion channels help in the movement of charged ions. Porins remain open at all times.
• Gated channel proteins - These are channel proteins which resemble gates and have specific binding sites for molecules they transport. They open or close in response to chemical or electrical stimuli. Example: AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors and NMDA (N-methyl-D-aspartate) receptors.
• Carrier proteins - The specialised proteins which bind the particular solute on one side of the cell membrane and transport it to the other side by undergoing structural changes.

Thus, the correct option is b.

FAQs

1. What is diffusion pressure deficit?

Answer: The pressure generated by molecules during the diffusion process is called diffusion pressure. It is highest in pure solvent such as water and reduces as solutes are added to it. This difference in the diffusion pressure of water in solution compared to its pure state is called diffusion pressure deficit or DPD.

2. How does the Na⁺-K⁺ pump work?

Answer: The Na⁺-K⁺ pump is an actively transporting pump which utilises ATP to pump out three Na⁺ ions from the cell in exchange for pumping in two K⁺ ions into the cell.

3. What is water potential?

Answer: Water potential is the measure of free energy and concentration of water in per unit volume of a system. Water potential of pure water is considered to be highest and zero. Thus, the water potential of a solution is always negative.

4. How is osmosis useful in plants?

Answer: Plants absorb water from the soil through the root hair cells by the process of osmosis. The cell sap of root hair cells has relatively higher concentration of solutes compared to the soil solution and hence water from the soil crosses the semipermeable cell membrane of the root hair cells and enters them by the process of endosmosis.