Passive Transport: Definition, Overview, Types, Examples, Difference

What is Passive Transport?

The transfer of molecules across cell membranes without energy input is called passive transport. It builds on the tendency of molecules, by diffusion, to move from an area of high concentration to an area of low concentration. Since no external energy is required in passive transport, it makes up a basic mechanism that helps the cell maintain equilibrium and carry out important functions.

Passive transport in biological systems is important in maintaining homeostasis as it allows the interchange of substances between a cell and its environment without expending energy. The basic principles involve concentration gradient, which means the difference in concentration between two regions. Movement down this gradient i.e., high to low concentration brings out the energy independence concept in passive transport.

Types of Passive Transport

Passive transport includes several means by which molecules travel through cell membranes :

Simple Diffusion

• Movement of molecules from an area of high concentration to one of low concentration across a lipid bilayer.

• The molecules move directly across the phospholipid bilayer of the membrane.

• Examples: Oxygen and carbon dioxide exchange in lungs and tissues.

Facilitated Diffusion

• It enables the movement of molecules that cannot directly go through the lipid bilayer.

• Carrier proteins bind to specific molecules and have a conformational change to transport. Channel proteins form a pore through which specific ions or small molecules pass.

• Examples: Glucose is brought into the cell via GLUT transporters.

Osmosis

• The diffusion of water molecules across a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration.

• Water moves through aquaporins or directly through the membrane, thus equalizing solute concentrations on both sides.

• It helps regulate cell volume and turgor pressure and also plays a crucial role in many cellular activities in the maintenance of cellular structure.

• Example: uptake of water by plant roots, maintenance of water balance in red blood cells.

Filtration

• The movement of water and solutes through a selectively permeable membrane due to hydrostatic pressure.

• Hydrostatic pressure forces fluid and dissolved substances through porous membranes.

• It is essential in processes like blood filtration in the kidneys, removing waste products while retaining larger molecules like proteins.

The table below shows the differences between the different types of passive transport:

Factors Affecting Passive Transport

The efficiency of passive transport can be affected by many factors:

Concentration Gradient

This is the difference in concentration of some substance in two different regions. The greater the difference in concentration i.e., the steeper the gradient, the faster the molecules will move from high to low concentration.

Temperature

The temperature increases the kinetic energy of molecules. Hence, raising the temperature increases the speed of the molecules. While the molecular speed increases, so does the rate of diffusion.

Molecule Size

Smaller molecules diffuse faster than larger ones since of their smaller size, they meet less resistance while diffusing across the medium. The larger the molecule, the less its ability to easily pass through the spaces, hence a slower diffusion rate. Also, lipophilic molecules readily pass through the lipid bilayer.

Transport Proteins

Facilitated diffusion only occurs in the presence of specific transport proteins that work correctly. The higher number of transport proteins elevates the diffusion rate. Saturation occurs because there are a limited number of carrier proteins. When molecules are high and the carrier proteins become occupied, the rate of transport cannot be increased further.

Nature of Medium

The nature of the medium through which transport takes place affects the rate too. In gases, the rate is higher because their density is lower and the movement of their molecules is more as compared to liquids and solids. In liquids, the rate is slow because the molecules are more closely packed and thus face more resistance.

Passive Transport in Prokaryotic and Eukaryotic Cells

The passive transport occurs in all cell types but differs in each:

Mechanism in Prokaryotes

• Primarily, the simple and facilitated diffusion flow because of the simpler design of the cell.

• Simple diffusion allows gases and small molecules to move across the plasma membrane without using energy.

• Facilitated diffusion mediated by channel proteins or porins, helps in the transport of ions and nutrients in bacteria.

Mechanism in Eukaryotes

• In eukaryotic cells, it involves more advanced mechanisms that include special proteins and organelles.

• Passive transport regulates the exchange of substances such as oxygen, carbon dioxide, and water across the membranes.

• Facilitated diffusion aids the movement of essential molecules like glucose and ions, ensuring cellular homeostasis without using ATP.

Plant vs Animal Cell

Plant cells often have extra mechanisms, including specialised transport proteins in vacuoles and cell walls, though these transport processes were much more dynamic and divergent in an animal cell.

Examples and Applications of Passive Transport

Passive transport plays a significant role in many physiological and ecological processes:

In Human Physiology (Gas exchange, filtration)

• Oxygen and carbon dioxide diffuse across alveolar membranes during gas exchange.

• Filtration processes in kidneys use passive transport to regulate blood and urine composition.

In Plants (Water uptake, nutrient transport)

• The uptake of water by the plant roots occurs through the process of osmosis, which is important in transporting nutrients and turgidity pressure in plant cells.

• The passive flow of nutrients through the plant tissues helps in maintaining the health and growth of plants.

Passive Transport NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Types of Passive Transport

• Factors affecting passive transport

Practice Questions for NEET

Q1. Water moves into the root hairs during passive absorption due to the _____ that takes place in the upper part of the plant.

1. Photosynthesis

2. Diffusion

3. Transpiration

4. Respiration

Correct answer: 3) Transpiration

Explanation:

Transpiration in the upper portion of the plant causes water to travel into the root hairs during passive absorption.

When water evaporates from the stomata of leaves, transpiration produces a pull. As a result, water flows upward via the xylem when the water pressure in the upper portion of the plant drops. By travelling from the higher water potential in the soil to the lower water potential inside the root hairs use osmosis to take water from the soil to replace the lost water.

Hence, the correct answer is option 3) Transpiration.

Q2. Which of the following molecules can enter the cell passively?

1. Water

2. Urea

3. Ca2+

4. Both 1 and 2

Correct answer: 4) Both 1 and 2

Explanation:

O2 and CO2, two small nonpolar molecules, readily dissolve in lipid bilayers and diffuse across them at high rates. Water and urea are examples of small, uncharged polar molecules that can diffuse through a bilayer as well, but considerably more slowly.

Option (1) is correct because water can enter cells passively.

Option (2) is correct because urea can enter cells passively.

Option (3) is incorrect because metal ions including Na+, K+, Mg2+, and Ca2+ are among the substances that are transferred via active transport across the cell membrane. Ion pumps or ion channels are needed to move these charged particles through membranes and throughout the body.

Hence, the correct answer is option 4) Both 1 and 2.

Q3. Passive absorption of water by the root system is the result of

1. Osmotic force in the shoot system

2. Tension on the cell sap due to transpiration

3. Forces created in the cells of the root

4. Increased respiratory activity in root cells

Correct answer: 2) Tension on the cell sap due to transpiration

Explanation:

Negative pressure from transpiration in the shoot system leads to the passive absorption of water in plants. As leaves lose water, osmotic pressure in the mesophyll cells increases, and thus it pulls water from the adjacent cells with tension. This tension tugs water out of the elements of the midrib of a leaf down through the stem, and lastly from the roots, which serve as passive pathways only to absorb water into the stem in an energy-saving mode.

Hence, the correct answer is option (2) Tension on the cell sap due to transpiration

Also Read:

• MCQs on Passive Transport

• Transport in Plants

• Reverse Osmosis

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