Test: Means of Transport (Old NCERT) - Grade 12 MCQ

The movement of the molecules of solids, gases or liquids from the region of their higher concentration to the region of their lower concentration is known as diffusion.

The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport proteins without involving energy expenditure, is called facilitated diffusion. Facilitated diffusion is very Specific as it allows cell to select substances for uptake. It is sensitive to inhibitors as well as show saturation effect. Two major types of transport proteins are known viz, carrier proteins carriers, transporters) and channel proteins. Carrier proteins bind the particular solute to be transported and deliver the same to the other side of the membrane.

Diffusion keeps the cell walls of the internal plant tissues moist. It is a means of spreading of ions and other substances throughout the protoplast. Transpiration or loss of water in vapour forms is a diffusion process. Exchange of gases (CO₂ and O₂) during photosynthesis and respiration between the plant interior and outside air also occurs through diffusion.

Some carries proteins allow transport only if two types of molecules move together. This is called cotransport. It is of two type: symport and antiport method. In symport method of cotransprot, both molecules cross the membrance in the same direction at the same time. In antiport method of cotransport, both molecules move in opposite direction. When a molecules moves across a membrane independent of other molecule, the process is called uniport.

Particles which are lipid soluble (i.e., lipophilic or hydrophobic) can easily diffuse through the cell membrane as the lipid is a major constituent of the membrane. Where as hydrophilic substances (i.e., the substances which are soluble in water, e.g., glucose) do not pass through a biological membrane by simple diffusion. They are, therefore, transported across a cell membrane by facilitated diffusion (i.e., carrier mediated diffusion), which takes place with the help of certain transport proteins.

Active transport is the transport of solute linked with some metabolic reaction (or reactions) involving expenditure of energy. The molecules or ions, in active transport, cross a protoplasmic membrane (viz, plasma membrane, tonoplast, - etc.) against the chemical potential (concentration) gradient. This kind of transport is carried out by membrane proteins.

The given figure represents facilitated type of diffusion as molecules are moving from their higher concentration to lower concentration across the membrane with the help of special transport proteins.

Statement 1: The process of diffusion does not require any input of energy.
- Diffusion is the spontaneous movement of particles from an area of higher concentration to an area of lower concentration.
- It is a passive process that occurs due to the random motion of particles.
- No external energy input is required for diffusion to take place.
- Therefore, statement 1 is correct.
Statement 2: Diffusion involves movement of particles from a region of higher concentration to a region of lower concentration.
- Diffusion is driven by the concentration gradient, which is the difference in concentration between two regions.
- Particles move from an area of higher concentration to an area of lower concentration in order to equalize the concentration on both sides.
- This movement occurs due to the random motion of particles.
- Therefore, statement 2 is also correct.
Conclusion: Both statements 1 and 2 are correct.
- The process of diffusion is a passive process that does not require any input of energy.
- It involves the movement of particles from a region of higher concentration to a region of lower concentration in order to achieve equilibrium.

In active transport, the movable carrier proteins are called pumps. They employ ATP energy for transport across the membrane. It is uphill transport, i.e., against concentration gradient and is faster than passive transport. The rate of active transport reaches the maximum when all the protein pumps are being used in transport (saturation effect). Carrier proteins are highly specific like enzymes. They are also sensitive to inhibitors that react with protein side chains.