Diffusion, Osmosis and Transpiration: How Substances Move Without Energy

Every living cell constantly exchanges water, gases, and nutrients with its surroundings — and most of this movement happens without the cell spending any energy at all. In this part of the Transport in Organisms chapter, we look at four passive processes that make this possible: diffusion, imbibition, osmosis, and transpiration. Each follows its own rules — some need a membrane, some don't; some move any substance, others move only water — and SSC board exams frequently test students on telling them apart. This note breaks down the definitions, real biological examples, and the key distinctions examiners look for.

Diffusion

Importance of diffusion:

1. Many physiological processes of living body is caused by diffusion.
2. During photosynthesis, plants intake carbon dioxide and expel oxygen by this process.
3. During respiration in animal, oxidation occurs due to this process.
4. Excess water is expelled out from plant by this process.
5. Nutrition element from blood is carried to the cell by this process.

Factors of Diffusion

1. Distance: shorter distance increases the rate of diffusion.

2. Concentration gradient: steeper concentration gradient means faster diffusion rate.

3. Surface area: A larger surface area increases the rate of diffusion.

Diffusion in Living Systems

When diffusion happens in living cells , the cells themselves do not have to expend any energy for it to take place. Rather the random movement of the particles is due to their own kinetic energy.

Many key processes in living organisms are dependent on diffusion. For example: respiration, photosynthesis, transpiration.

Some important diffusion related terms for SSC exam

Diffusion pressure is the potential ability of the molecules or ions of any substance to diffuse from an area of higher concentration to an area of lower concentration of solution.

DPD or Diffusion Pressure Deficit is the difference between the diffusion pressure of the solution and its solvent at a particular atmospheric condition.  

A Special Case: Imbibition

Imbibition is a special type of diffusion, where seeds absorb water before germination.

📌 Imbibition is what allows a dry seed to swell and soften before germination begins — it's the first step of absorbing the water needed to activate the seed's metabolic processes.

Osmosis

Importance of Osmosis:

1. It helps the cell to do many biochemical reactions.
2. Plants absorb water and mineral salt by this process.
3. Stem and leaves become fresh due to this process.
4. Stomata of leaves opens and closes due to this process.

💡Remember: Osmosis is NOT simply diffusion of water — it specifically requires a semi-permeable membrane and involves solvent moving toward higher solute concentration, while diffusion needs no membrane and applies to any substance.

Transpiration

Importance of Transpiration

1. The metabolic activities of a living plant cell are dependent on transpiration.
2. Transpiration helps keep the temperature in the cells of leaves within a range.

Types of transpiration

Based on the aerial parts through which transpiration occurs, it is categorized into three types.

1. Stomatal transpiration: When transpiration occur through stomata, it is called stomatal transpiration. 95% of the total transpiration in a plant occurs through the stomata. Stomata are special openings with two guard cells in the leaves, young stems, sepals, and petals of flowers.
2. Cuticular transpiration: When tranpiration occur through the thin cuticles of plant, it is called cuticular transpiration. This type of transpiration remains functional in dry environment where other types had already stopped.
3. Lenticular transpiration: When transpiration occur through lenticels, it is called lenticular transpiration. Lenticular transpiration accounts for only 0.1% of the total transpiration in plants. Generally, It is observed among the dicotyled plants.

Harmful Effect of Transpiration on Plants

If the rate of loss of water is greater than the rate of its absorption, it will cause deficiency of water and minerals in the plant. In extreme case, the plant may die.

💡Remember: Despite this risk, transpiration is often called a "necessary evil" — the water loss is unavoidable because the same stomata that let CO₂ in for photosynthesis also let water vapour out, but this loss is what generates the pulling force (transpiration pull) that draws water all the way up from the roots to the leaves.

Experiment to Demonstrate Transpiration in Plants

Take a healthy, medium-sized, and leafy potted plant. Pour sufficient water in the plant pot a few hours before starting the experiment. Place it in the sunlight cause sunlight increases the transpiration rate. Wrap the surface of the soil with a plastic sheet to prevent water evaporating from soil into the air. Cover a leafy branch of the plant with a cellophane bag and tie it with thread to make it air tight. No air circulation should occur between the environment and the cellophane bag inside. Leave the flowerpot in the sunlight for an hour. After an hour you can observe water droplets inside the bag. Since the bag was airtight, the water must have come from the plant itself. It proves transpiration in plants.

📌 The upward pull created by transpiration is strong enough to lift water many meters against gravity, through a continuous, unbroken column of water inside the xylem — this is known as the cohesion-tension mechanism.

Previous Board Questions

• Describe different types of transpiration. Sylhet Board-2022 
• Transpiration process is 'necessary evil'—Analyze it with logic. Sylhet Board-2022 
• Transpiration process has both advantages and disadvantages in case of plant—Analyze it. Mymensingh Board-2020 
• Explain the absorption process of water from Soil to Leaf. Mymensingh Board-2020