Passive Transport: An Overview of Simple Diffusion, Imbibition, Facilitated Diffusion, Osmosis, Practice Problems and FAQs

You must be aware of how nutritious sprouts are for your body. Many of us prefer having sprout salads as a part of our meal. But do you know how sprouts are prepared? We usually soak the seeds of pea, bengal gram, kidney beans, corns etc. overnight in water and gradually the young seedlings sprouts out. These can then be cooked or eaten raw.

GIF: Sprouting

But how is this sprouting happening? What is the process involved? The seeds soaked in water absorb water with the help of a process called imbibition which causes them to swell up, rupture the seed coat and the young shoot to emerge.

Let us think of another scenario. How do plants take in CO₂ during photosynthesis? We know that CO₂ enters the leaves through tiny pores called stomata but do you know what process is involved in this? The process involved is called diffusion and it allows the CO₂ gas to travel from its higher concentration in the atmosphere to its lower concentration in the mesophyll cells of the leaves.

All the above scenarios are examples of short distance transport of substances in plants. Transporting molecules across short distances in plants may or may not require energy. The processes which utilise energy for the transportation of substances are called active transport and those which do not utilise any energy for transport of substances are called passive transport. We are going to discuss different types of passive transports in plants in this article.

Table of contents:

• Passive transport
• Simple diffusion
• Facilitated diffusion
• Osmosis
• Practice Problems
• FAQs

Passive transport

The membrane transport which does not use energy to move across the semipermeable membrane is called passive transport. The major passive transports seen in plants are as follows:

• Simple diffusion
• Facilitated diffusion
• Osmosis

Simple diffusion

The movement of molecules of gases, liquids and dissolved substances from the higher concentration to lower concentration until it attains equilibrium is called simple diffusion. It is a slow process which does not require energy for the movement of molecules. The absorption of CO₂ and O₂ in plants takes place through simple diffusion.

Fig: Diffusion

Rate of diffusion

The change in the number of diffusing molecules inside the cell over time is called the rate of diffusion. It is represented as dn/dt.

Where

dn = Change in the number of diffusing molecules

dt = Change in time

Fig: Rate of diffusion

Many factors can affect the rate of diffusion and they are as follows:

• Concentration gradient
• Temperature
• Pressure
• Density
• Surface area of the membrane

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 molecules. Only few and specialised molecules are transported through facilitated diffusion.

GIF: Facilitated diffusion

The unidirectional movement of a single type of molecule across the membrane by facilitated diffusion is called uniport.

Fig: Uniport GIF: Uniport

Specialised proteins

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

• Channel proteins - The pore-like proteins present in the cell membrane. Examples: ion channels, aquaporins. These remain open at all times.

GIF: Porins GIF: Aquaporins

• Gated channel proteins - The specialised proteins with a structure of gate and having specific binding sites for each molecule they transport are called gated channels. They work under the influence of some chemical or electrical stimuli. Example: NMDA (N-methyl-D-aspartate) receptors and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors.

Fig: Gated channel proteins

• Carrier proteins - The specialised proteins which can undergo structural changes according to the molecules that are to be transported.

GIF: Carrier proteins

Osmosis

The diffusion of water across a selectively permeable or differentially permeable membrane from its region of higher concentration to its region of lower concentration is called osmosis. The minimum amount of pressure required to prevent osmosis is known as osmotic pressure.Two factors that are responsible for the direction and rate of osmosis are as follows:

• Concentration gradient
• Pressure gradient

Fig: Osmosis

Endosmosis

It is the process by which water enters into the cell by osmosis. This occurs when the cell is placed in a solution that has a relatively lower solute concentration compared to the cell sap, that is, a hypotonic solution. Endosmosis results in swelling up of the cell.

Fig: Endosmosis

Exosmosis

It is the process by which water leaves the cell by osmosis. This occurs when the cell is placed in a solution that has a relatively higher solute concentration compared to the cell sap, that is, a hypertonic solution. Exosmosis results in shrivelling up of the cell.

Plasmolysis

When a plant cell is placed in a hypertonic solution the water moves out of the cell by exosmosis. Due to the presence of a rigid cell wall, the plant cell does not shrink but the cell membrane and protoplasm draw away from the cell wall. This phenomenon is known as plasmolysis.

Fig: Plasmolysed cell

De-plasmolysis

When the plasmolysed cell is placed in a hypotonic solution or pure water, the water moves into the cell and the cell protoplast starts swelling. This process is called de-plasmolysis.

Fig: De-plasmolysis

Reverse Osmosis

The movement of solvent molecules in the opposite direction of the natural osmosis is called reverse osmosis. This process happens when a pressure more than the osmotic pressure is applied in the region where osmosis happens. The RO filtration system works on this principle.

Fig: RO water filtration system

Practice Problems

1. Which of the following is not a passive transport process in plants?

1. Simple diffusion
2. Facilitated diffusion
3. Osmosis
4. Antiport transport

Solution: The membrane transport which does not use energy to move across the semipermeable membrane is called passive transport. The major passive transports seen in plants are simple diffusion, facilitated diffusion and osmosis. The movement of two types of molecules in the opposite direction is called antiport transport. Even though it is similar to the process of diffusion, antiport is a type of secondary active transport. In this one of the solute moves from high concentration to low concentration and an entropic energy is generated. By using this entropic energy, the other molecules move from lower to higher concentration, which is the opposite direction. Hence the correct option is d.

2. Which of the following statements are wrong about the simple diffusion?

1. The movement of molecules of gases, liquids and dissolved substances from the higher concentration to lower concentration.
2. It is a slow process which does not require energy.
3. Lipid insoluble molecules are transported across the cell membrane by simple diffusion.
4. Imbibition is a special type of diffusion.

Solution: The movement of molecules of gases, liquids and dissolved substances from the higher concentration to lower concentration until it attains equilibrium is called simple diffusion. It is a slow process which does not require energy for the movement of molecules. 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. Only lipid soluble molecules can be transported across the cell membrane by simple 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 using energy. Hence the correct option is c.

3. Which of the following are the specialised proteins involved in the facilitated diffusion?

1. Channel proteins
2. Gated channel proteins
3. Carrier proteins
4. Na⁺ - K⁺ pump

1. I and II
2. II and IV
3. I, II and III
4. Only III

Solution: 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 molecules. Only few and specialised molecules are transported through facilitated diffusion. The specialised proteins involved in the facilitated diffusion are channel proteins, gated channel proteins, and carrier proteins. The pore-like proteins present in the cell membrane are called channel proteins. The specialised proteins with a structure of gate and have specific binding sites for each substance to be transported are called gated channel proteins. The specialised proteins which can undergo structural changes according to the molecules that are to be transported are called carrier proteins. Na⁺ - K+ pump helps in active transport.

Hence the correct option is c.

4. The process of movement of water from the cell to outside when it is placed in a hypertonic solution is called

1. Exosmosis
2. Imbibition
3. Reverse osmosis
4. Diffusion

Solution: The movement of molecules of gases, liquids and dissolved substances from the higher concentration to lower concentration until it attains equilibrium is called simple diffusion. 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 movement of solvent molecules in the opposite direction of the natural osmosis is called reverse osmosis. This process happens when a pressure higher than the osmotic pressure is applied in the region where osmosis happens. When a cell is placed in a hypertonic solution (higher concentration of solute than protoplasm) the water movement is from the cell to outside and as a result the cell shrinks. This process is called exosmosis. Hence the correct option is a.

FAQs

1. Find out the differences between passive and active transport?

Answer: The major difference between passive and active transport are as follows:

2. How does temperature affect the rate of diffusion?

Answer: Rate of diffusion and temperature are directly proportional to each other. When temperature increases, the kinetic energy of the molecules also increases. This causes them to readily move from their region of higher concentration to their region of lower concentration. Easy dissolving of sugar in hot water is an example for this.

3. What is filtration in the human body?

Answer: The movement of water and solute molecules across the membrane because of the hydrostatic pressure generated by the cardiovascular system is called Filtration. The solute molecules passing through the membrane is limited and depends on the membrane pores.

4. What is MFS or major facilitator superfamily?

Answer: The superfamily of membrane transport proteins which can facilitate the movement of small solutes across the plasma membrane is called MFS or major facilitator superfamily. The solutes move in response to chemiosmotic gradients.

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