Question

Assertion: Molecules up to small size of protein can pass through outer membrane of plastids. Reason: Outer membrane of plastids contain porins

Short answer

Both the assertion and reasoning are true and the reasoning is a correct explanation for the assertion.

Step-by-step solution

Understand the Nature of Plastids

Plastids are double membraned organelles found in the cells of plants and algae. They are responsible for important tasks like photosynthesis and synthesis and storage of starch.

Analyze the Function of Outer Membrane

The outer membrane of the plastids is semi-permeable meaning it allows certain particles to pass through. This membrane acts as a barrier to the cytosol (liquid within the cell).

Understand the Role of Porins

Porins are proteins that form pores in the outer membrane of the plastids. They facilitate the movement of ions and small molecules, including small-sized proteins, through the membrane.

Evaluation of Assertion and Reasoning

Given the role of porins in matter interchange between plastids and cell cytosol, the assertion that 'small-sized protein can pass through outer membrane of plastids' is true. Similarly, the reason that 'outer membrane of plastids contain porins' is also true and is a valid explanation for the assertion.

Key concepts

Plastid Structure

Plastids are crucial components of plant and algal cells, contributing significantly to a variety of cellular functions, including photosynthesis, storage of starches, lipids, and proteins, as well as the synthesis of many types of molecules.

They are characterized by a double membrane structure that consists of an inner and an outer membrane. The outer membrane is less selective and has a variety of proteins called porins that allow molecules of certain sizes to pass through freely. The inner membrane, by contrast, is highly selective in what it allows to enter and exit the internal compartments of the plastid.

Inside the plastid, there are distinct areas such as the stroma, the liquid matrix containing enzymes for synthesis processes, and in some types, thylakoid membranes where photosynthesis takes place. Plastids can differentiate into various forms, including chloroplasts, which are specialized for photosynthesis, amyloplasts for starch storage, and chromoplasts for pigment synthesis and storage, reflecting their dynamic nature in plant cell physiology.

Understanding the layers and spaces within plastids is integral to appreciating their function and the biochemical processes they support in plant life.

Porins Function

Porins are proteins found in the membranes of various organelles, including plastids, and they play a pivotal role in controlling the passage of molecules. Unlike other membrane proteins that may have a selective binding site for a specific molecule, porins form large channels that permit passive diffusion.

This means that molecules do not require energy to move through porins; they simply move according to their concentration gradient, from an area of higher concentration to lower concentration. The sizes of the pores created by porins are such that they allow the passage of ions, nutrient molecules, and small proteins while still restricting larger molecules and cellular components.

Because of porins, the outer membrane of plastids is considered to be semi-permeable. Small molecules that are essential for the processes in the plastid, such as amino acids, sugars, ATP, and ions can pass through, enabling communication and material exchange with the rest of the cell. Without porins, the outer membrane would be a significant barrier to these molecules, impeding the plastid’s ability to function effectively within the cell.

Semipermeable Membranes

Semipermeable membranes are fundamental to cellular life, allowing cells to maintain homeostasis by controlling the internal environment's composition. These membranes are selective barriers; they permit some substances to pass through while blocking others based on factors such as size, charge, and solubility.

A classic example of a semipermeable membrane in plant cells is the outer membrane of plastids, which utilizes porins to regulate molecular traffic. Because of these structures, the membrane achieves subtle control over what enters and leaves the organelle, which is vital for maintaining the necessary conditions for the plastid's functions.

It is important to note that while semipermeable membranes allow passive movement of materials (i.e., diffusion and osmosis), other mechanisms such as active transport, facilitated diffusion, and bulk transport involve different proteins and additional energy sources to move substances against their concentration gradients. The balance of passive and active processes is what allows cells to respond to the ever-changing internal and external environments.