Question

The plant cell cytoplasm is surrounded by both cell wall and cell membrane. The specificity of transport of substances is mostly across the cell membrane, because (a) cell membrane is impermeable (b) cell membrane is selectively permeable (c) cell membrane is fully permeable (d) cell wall is impermeable.

Short answer

The correct answer is (b) because the cell membrane is selectively permeable, which allows it to control the transport of specific substances in and out of the cell.

Step-by-step solution

Understanding Cell Structures

Recognize that plant cells have both a cell wall and a cell membrane. The cell wall provides structure and protection, while the cell membrane regulates the entry and exit of substances into and out of the cell.

Understanding Permeability

Understand the concept of permeability. An impermeable membrane does not allow any substances to pass through, a fully permeable membrane allows all substances to pass through, and a selectively permeable membrane allows only certain substances to pass through based on various factors such as size, charge, and solubility.

Determining the Correct Option

Analyzing the given options with the knowledge that the cell membrane controls the specificity of transport, the correct option is (b) cell membrane is selectively permeable because it allows only certain substances to cross it, hence controlling the specificity of substance transport.

Key concepts

Plant Cell Structure

Understanding the intricacies of a plant cell is vital for students starting on their journey learning about biology. A plant cell, unlike animal cells, possesses both a cell wall and a cell membrane with each performing distinct functions. The cell wall is the outermost layer, providing structural support and rigidity to the plant cell. It is primarily comprised of cellulose and is porous, allowing molecules such as water and oxygen to pass through freely.

In contrast, the cell membrane, also known as the plasma membrane, lies just inside the cell wall. Its role is pivotal for maintaining homeostasis within the cell. It is a thin, flexible barrier made up of a double layer of lipid molecules interspersed with various proteins and carbohydrates. This sophisticated structure not only protects the cell from its external environment but also plays a key role in the transport of substances, a concept we'll delve deeper into a later section.

Selective Permeability

Delving into the concept of selective permeability reveals why some substances can enter a cell while others cannot. Selective permeability is a feature of the cell membrane that allows it to act as a gatekeeper, controlling which molecules can cross into or out of the cell. This selection is based on the size, charge, and solubility of the molecules.

For example, small nonpolar molecules like oxygen and carbon dioxide can easily pass through the lipid bilayer, while ions and large polar molecules often require specialized transport proteins or channels. Selective permeability is crucial for the survival of the cell, as it ensures that essential nutrients are absorbed, wastes are eliminated, and that the chemical composition inside the cell stays balanced, also known as the cell’s internal environment.

Transport of Substances

The transport of substances across cell membranes is a fundamental process in biology, necessary for cells to acquire nutrients, excrete waste, and communicate with their environment. There are several modes of transport, each adapted for specific types of substances.

Passive Transport: This process does not require energy and relies on the concentration gradient, moving substances from areas of high concentration to areas of low concentration. Passive transport includes diffusion, osmosis, and facilitated diffusion.

Active Transport: Unlike passive transport, active transport requires energy, usually in the form of ATP, to move substances against their concentration gradient. This is usually carried out by transport proteins, like pumps, within the cell membrane.

Endocytosis and Exocytosis: These processes involve the cell membrane to transport larger molecules or particles. Endocytosis allows a cell to engulf external substances within vesicles, while exocytosis enables the expulsion of substances from the cell's interior. It's through these mechanisms that cells maintain the delicate balance of their internal environment amidst the constantly changing external conditions.