Question

Figure 1 represents a plasma membrane and molecules present on either side.

 

Figure 1. A model of a plasma membrane demonstrating selective permeability

(a) Describe the biological need for cells to be surrounded by a membrane that is selectively permeable for different materials. 

(b) Explain how the model shows selective permeability of the membrane to specific ions. 

 

(c) Describe the characteristics of the phospholipid bilayer that permit small hydrophobic lipid molecules to pass directly across the membrane.

(d) Based on the model, explain whether the molecules shown crossing the membrane require energy to do so. 

Answer 1

Answer a: Plasma membrane/cell membrane is the membrane system of the cells which help in separation (physical barrier) of cells from outside/external environment. It regulates the entry and exit of the cells. It also provides a fixed environment inside the cells for the proper functioning of the cell organelles. The plasma membrane is selectively permeable in nature, that can allow certain molecules and ions to pass through it by means of either active (expend of energy) or passive (without the utilisation of cellular energy) transport. The selective permeability of the plasma membrane is due to the presence of phospholipids bilayer in the plasma membrane.

Answer b: The selective permeability of the plasma membrane is due to the presence of phospholipids bilayer. Basically, the side and charge of the molecules determine the entry of molecules inside the cells. This bilayer can permit the entry of small hydrophobic molecules (O₂, Co₂, N₂) small uncharged polar molecules (H₂O, ethanol, glycerol), but not permit the direct entry of large uncharged polar molecules (amino acids, glucose, nucleotides) and charged ions (H⁺, Na⁺, HCO3^-, K⁺, Ca²⁺, Cl^-, Mg²⁺). For the entry of these molecules require some transporter molecule present on the cell membrane.

Answer c: The phospholipids bilayer of the plasma membrane is composed of hydrophilic-head and hydrophobic-tail of plasma membrane structure. This hydrophilic-hydrophobic nature of the components of the plasma membrane makes it selective permeable. The hydrophobic molecules, small polar and charged (+/-) molecules can easily diffuse through the plasma membrane

Answer d: For active transport of molecules inside the cell requires energy (ATP). The movement of molecules from a region of lower concentration to a region of higher concentration