Chapter 10, Problem 7STP

Summary Introduction

Introduction:

All cells have a special membrane called plasma membrane that acts as a thin flexible boundary between a cell and its watery environment. It controls what enters and leave the cell. The cells of our skin and that of a plant stem, all have a plasma membrane. It also helps in maintaining homeostasis in an organism. Plasma membranes are selectively permeable as they allow only certain substances to pass through while keeping others out.

Answer to Problem 7STP

Correct answer :

The correct answer is option D. transport proteins in the cell membrane

Explanation of Solution

Explanation/justification for the correct answer:

Option D. transport proteins in the cell membrane- An important property of plasma membrane is selective permeability, by which a membrane allows some substances to pass through while keeping others out. This property is important to the survival of a cell. Proteins that are found on the outer surface of plasma membrane called receptors transmit signals to the inside of the cell. Some proteins span the entire membrane and act as tunnels through which certain substances enter and exit the cell. These transport proteins contribute to the selective permeability of the plasma membrane. They transport the needed substances and waste materials through the membrane. Calcium and sodium ions are also transported in and out of cardiac cells by these transport proteins.

Hence, this is the correct option.

Explanation for incorrect answer:

Option A. charged particles in the phospholipid bilayer- The phospholipids are arranged in such a way, that the polar heads are closest to water molecules. So the fatty acid tails form the interior of the plasma membrane and the heads face the watery environment found inside and outside the cell. This allows the plasma membrane to permit only water and uncharged molecules like oxygen and carbon dioxide.They do not help in transport of calcium and sodium ions in and out of cardiac cells as lipid bilayers are not permeable to charged ions. Hence, this is not the correct option.

Option B. cholesterol molecules in the phospholipid bilayer- Cholesterol molecules are nonpolar and are positioned among the phospholipids in such a way that they prevent the fatty acid tails of the phospholipid bilayer from sticking together. This causes fluidity in the plasma membrane. They do not help in transport of calcium and sodium ions in and out of cardiac cells.Hence, this is not the correct option.

Option C. diffusion channels in the cell membrane- Diffusion channels allow only small ions like chloride ions and sugar to move across plasma membranes. Calcium and sodium ions are not transported in and out of cardiac cells by diffusion. Transport proteins help in movement of charged ions across plasma membranes. Hence, this is not the correct option.