Chapter 5, Problem 1TY

Which of the following statements best describes the chemical composition of biological membranes?

1. Biological membranes are bilayers of proteins with associated lipids and carbohydrates.
2. Biological membranes are composed of two layers: one layer of phospholipids and one layer of proteins.
3. Biological membranes are bilayers of phospholipids with associated proteins and carbohydrates.
4. Biological membranes are composed of equal numbers of phospholipids, proteins, and carbohydrates.
5. Biological membranes are composed of lipids with proteins attached to the outer surface.

Summary Introduction

Introduction:

Every organism present in the environment is composed of a cell. Cells perform a number of important activities that help in the survival of an organism. These activities are known as cellular activities. The cell contains three major parts. These are plasma membrane, cytoplasm, and DNA.

Answer to Problem 1TY

The correct answer is option (c) Biological membranes are bilayers of phospholipids with associated proteins and carbohydrates.

Explanation of Solution

Explanation/justification for the correct answer:

Option (c) Biological membranes are bilayers of phospholipids with associated proteins and carbohydrates. The phospholipid bilayer is present in a cell membrane. The phospholipid bilayer has selective permeability which allows some substances to pass through it. A double layer of phospholipids is formed because the head of this molecule is charged and considered as polar end whereas the region of the tail is uncharged, so it is considered as the nonpolar end. The hydrophobic heads of the phospholipids face the aqueous region of the cell, and hydrophilic tails of phospholipids face each other within the plasma membrane. Proteins and carbohydrates are associated with the lipid bilayer. Protein is one of the components that make the membrane molecules. Two categories of membrane proteins are integral proteins and peripheral proteins. Glycoproteins are formed when carbohydrates are combined with proteins. These biomolecules help in the process of cell surface recognition. Glycolipids are formed when carbohydrates are combined with lipids. Its carbohydrate portion is bind to the polar head and protrudes in the extracellular environment.So, the correct answer is option (c).

Explanation for incorrect answer:

Option (a) Biological membranes are bilayers of proteins with associated lipids and carbohydrates. The biological membrane is made up of two layers of lipids, but the proteins and carbohydrates are associated with it. Proteins that are associated with this structure are categorized into integral proteins and peripheral proteins. So, this is an incorrect option.

Option (b) Biological membranes are composed of two layers: one layer of phospholipids and one layer of proteins. There are two layers of the phospholipids in a biological membrane. The hydrophilic tails of phospholipids cannot face the aqueous region. Therefore, two layers of phospholipids are present in which hydrophobic heads of the phospholipids face the aqueous region of the cell, and hydrophilic tails of phospholipids face each other within the plasma membrane. So, this is an incorrect option.

Option (d) Biological membranes are composed of equal numbers of phospholipids, proteins, and carbohydrates. The molecules that are majorly involved in the composition of the plasma membrane are lipids and proteins. Around 45-50% of the total weight of the plasma membrane is accounted for lipids, and other 45-50% consist of proteins. The proportion of lipids and protein in cell membrane may change according to the specific type of cell analyzed. Carbohydrates are also a component of the plasma membrane, and it makes up around 4-8% of the weight of its total weight. Hence, biological membranes are composed of an equal number of phospholipids, proteins, and carbohydrates. So, this is an incorrect answer.

Option (e) Biological membranes are composed of lipids with proteins attached to the outer surface. Protein is one of the components that make the membrane molecules. Two categories of membrane proteins are integral proteins and peripheral proteins. Integral proteins are not present on the outer surface. So, this is an incorrect answer.

Conclusion

Biological membranes are made up of phospholipid bilayer, which is associated with proteins and carbohydrates. Hence, the correct answer is option (c) Biological membranes are bilayers of phospholipids with associated proteins and carbohydrates.