Transport Across Cell Membrane: Overview, Types, Example

Structure and Function of the Cell Membrane

The plasma membrane is composed of a hydrophilic head i.e., water-attracting and hydrophobic tails i.e., water-repelling. It forms a flexible, semi-permeable barrier around the cell.

Proteins In The Plasma Membrane

The proteins in the membrane take part in signal transduction as well as the structural support of membranes.

• Integral Proteins: They are embedded within the phospholipid bilayer. Facilitate transport and often act as receptors.

• Peripheral Proteins: They are bound to either the inside or the outside of the membrane surfaces.

• Glycoproteins and Glycolipids: They take part in cell recognition and signaling, an event that may trigger immune responses.

Types of Transport Across Cell Membrane

There are major 3 types of transport across the membrane:

Passive Transport

Passive transport refers to the movement of molecules across the membrane with no requirement of energy. It depends on the concentration gradient as well as special proteins. It includes simple diffusion, facilitated diffusion and osmosis.

Simple Diffusion

• Simple diffusion is the movement of small, nonpolar molecules directly through the phospholipid bilayer

• It is driven by concentration gradient, moving from areas of high to those of low concentration

Facilitated Diffusion

• Facilitated diffusion uses carrier proteins and channel proteins for larger or polar molecules transport, such as glucose and ions

• Carrier proteins change shape to transport the molecule, while channel proteins form a tunnel that the molecule can travel through.

Osmosis

• Osmosis is the diffusion of water molecules across a selectively permeable membrane

• It is important in maintaining the turgor pressure, which is vital in maintaining proper cell shape and function.

Active Transport

Active transport requires energy, usually from ATP, to move the molecules against their concentration gradient. This is necessary to maintain the right cellular concentration for ions and other solutes.

Primary Active Transport

• The transport of molecules uses energy from ATP directly.

• Examples include sodium-potassium pump, moves sodium ions out of the cell and potassium ions into the cell across the plasma membrane.

Secondary Active Transport (Co-transport)

• Energy from the movement of one molecule down its gradient drives the transport of another molecule against its gradient.

• In symport, both molecules move in the same direction.

• In antiport, the molecules move in opposite directions.

Bulk Transport

Bulk transport is the transport of large particles or volumes of fluid in or out of the cell involving vesicles and requiring energy.

Endocytosis

• Phagocytosis: Cell eating, engulfing large particles

• Pinocytosis: Cell drinking, engulfing extracellular fluid

• Receptor-mediated Endocytosis: Specific molecules are ingested after binding to receptors.

Exocytosis

• The fusion of vesicles with the plasma membrane releases their contents outside of the cell.

Factors Affecting Membrane Transport

Several factors affect the efficiency and direction of transport across the cell membrane:

Importance Of Transport In Cellular Processes

The importance of transport in cellular processes:

• The various transport mechanisms across the cell membrane play a significant role in several vital processes within the cell.

• Cells take up essential nutrients like glucose, other sugars, amino acids, etc., required for metabolism and energy production.

• Excrete waste products of metabolism to maintain the stability of the internal environment

• Transport signaling molecules and ions are involved in cellular communication and the response of cells to external stimuli.

• Ion concentrations and water balance have to be adequately maintained such that internal conditions remain relatively constant.

• It is important for the proper functioning and survival of all cells.

Transport Across Cell Membrane NEET MCQs (With Answers & Explanations)

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

• Types of transport across cell membrane

• Factors affecting the transport

Practice Questions for NEET

Q1. Diffusion is the result of concentration gradient which refers to

1. Change of concentration with respect to time

2. Change of concentration with respect to space

3. Change of concentration with respect to temperature

4. Change of concentration with respect to gravity

Correct answer: 2) Change of concentration with respect to space

Explanation:

Movement of all substances from the area of their higher free energy to the area of their lower free energy along the concentration gradient. Neutral solutes move across the membranes by diffusion. The net direction of diffusion of particles depends on their concentration in a particular region or space.

Hence, the correct answer is option 2) Change of concentration with respect to space

Q2. Why does an increase in temperature increase the rate of diffusion?

1. The increased thermal energy of the medium reduces the kinetic energy of particles

2. The decreased thermal energy of the medium increases the kinetic energy of particles

3. The increased thermal energy of the medium does not change the kinetic energy of particles

4. The increased thermal energy of the medium increases the kinetic energy of particles

Correct answer: 4) The increased thermal energy of the medium increases the kinetic energy of particles

Explanation:

Higher the thermal energy in a medium: this raises the kinetic energy of its particles resulting in faster diffusion. The more heat there is, the more violent motion the particles perform the more collisions they experience, and the better the diffusion becomes as particles spread out and mix with others. Such a process plays a very significant role in biological and chemical systems, for example, in the transport of molecules, gases, and ions that determine the rate of chemical reactions and dynamics of systems.

Hence the correct answer is option 4) The increased thermal energy of the medium increases the kinetic energy of particles.

Q3. Which of the following has the maximum diffusion pressure?

1. Cell sap

2. Rainwater

3. Ice caps

4. All of these

Correct answer: 2) Rainwater

Explanation:

Pure water has a maximum diffusion pressure of 1236 atm, meaning it has the highest tendency to diffuse and move freely. When solutes are added, the diffusion pressure decreases, leading to a reduction in water potential. This works on the principle that during osmosis, the water moves from a region of higher diffusion pressure (pure water) to a region of lower diffusion pressure (solution).

Hence, the answer is option 2) Rainwater

Also Read:

• MCQs on Diffusion

• Symport, Antiport, Uniport

• Reverse Osmosis

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