Question

Choose the incorrect statement regarding cell membrane. (a) Generally smaller molecules pass easily and readily than large molecules. (b) Water soluble substance pass through it less readily than lipid soluble substances. (c) Membranes are selectiveiy permeable. (d) None of these.

Short answer

The incorrect statement regarding the cell membrane is (d) None of these.

Step-by-step solution

Understanding the Concept of Cell Membrane Permeability

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that controls the movement of substances into and out of the cell. It allows certain molecules to pass through more easily than others based on their size, solubility, and other factors.

Analyzing Statement (a)

Statement (a) says that smaller molecules pass more easily and readily than large molecules. This is generally true, as the cell membrane allows small molecules and ions to pass through its pores, while larger molecules require special transport mechanisms.

Analyzing Statement (b)

Statement (b) suggests that water-soluble substances pass through the cell membrane less readily than lipid-soluble substances. This is also true because the lipid bilayer of the cell membrane is hydrophobic (water-fearing) and thus non-polar or lipid-soluble substances can diffuse through it more easily.

Analyzing Statement (c)

Statement (c) mentions that membranes are selectively permeable. This statement is accurate as it reflects the property of the cell membrane to allow some substances to pass through while restricting others.

Identifying the Incorrect Statement

Given that statements (a), (b), and (c) are all accurate descriptions of the cell membrane's properties, the incorrect statement must be (d) 'None of these,' because the other statements are correct.

Key concepts

Selective Permeability

Imagine the cell membrane as a discerning bouncer at the entrance of a club, deciding who gets to enter and who must stay out. This is the core idea behind selective permeability, a fundamental property of cell membranes that allows them to control which substances can move in and out. By being selectively permeable, cell membranes sustain a stable internal environment within cells, which is crucial for maintaining cellular health and function.

The structure of the cell membrane itself aids in this selectivity. Composed mainly of a lipid bilayer with embedded proteins, the membrane is adept at admitting small, non-polar molecules like oxygen and carbon dioxide simply due to their size and compatibility with the lipid environment. In contrast, larger molecules or those that are polar and charged, such as ions and glucose, typically require assistance from transport proteins to cross the membrane.

This selectivity is not arbitrary but is tuned to the specific needs of the cell. For instance, a red blood cell must allow oxygen in and carbon dioxide out, and its membrane has specialized proteins to facilitate this.

Lipid Soluble Substances

When discussing the permeability of the cell membrane, the term 'lipid soluble' frequently crops up. Lipid soluble substances are, as the name suggests, those that can dissolve in lipids or fats. Thanks to the predominantly lipid composition of the cell membrane, these substances can slip through with relative ease, like a ghost passing through walls.

The concept of lipophilicity (fat-loving) is essential here. Molecules such as steroid hormones or fat-soluble vitamins (A, D, E, and K) have this lipophilic quality, allowing them to penetrate the membrane without the need for any transport mechanisms.

On the chemistry side, these substances are non-polar and hydrophobic, meaning they do not mix well with water but mesh seamlessly with the fatty acids' tails in the lipid bilayer. As a result, their solubility in the lipid environment is what dictates their ability to enter or exit the cellular space unescorted.

Membrane Transport Mechanisms

Cells have developed sophisticated strategies to transfer materials that can't simply drift through the lipid bilayer. These membrane transport mechanisms are the tools cells use to move these more complex substances across the barrier. There are two main categories of transport mechanisms: passive and active transport.

Passive Transport

Passive transport does not require cellular energy. Substances move from an area of high concentration to an area of low concentration, following their concentration gradient. Diffusion and osmosis are classic examples of passive transport. Additionally, facilitated diffusion is a type of passive transport that relies on carrier proteins in the membrane to help move substances down their concentration gradient without energy input.

Active Transport

In contrast to passive transport, active transport is an energy-demanding process where substances are moved against their concentration gradient. The cell must use ATP (adenosine triphosphate), the currency of cellular energy, to power transport proteins that act like pumps. These pumps are vital for maintaining the correct concentrations of ions and molecules that cells need to function. For example, the sodium-potassium pump actively exchanges sodium for potassium across the plasma membrane, which is crucial for nerve impulse transmission and muscle contraction.