Question

List the constituents of plasma membrane

Answer 1

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The plasma layer is comprised of 3 of the 4 sorts of macromolecules found in the body; Lipids, Proteins, and Carbohydrates. There are 2 sorts of Lipid present in the plasma membrane, Phospholipids, and Cholesterol.

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Answer 2

THE CELL MEMBRANE

Structure of the plasma membrane

The fluid mosaic model of the plasma membrane. Protein, lipid, and carbohydrate components of the membrane.

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Introduction

Each cell of your body is encased in a tiny bubble of membrane. This membrane has about the consistency of...salad oil^11start superscript, 1, end superscript. The first time I read that factoid, I didn't find it very reassuring! Salad oil seems like an awfully fragile boundary to place between a cell and the rest of the world. Luckily, the plasma membrane turns out to be very well-suited to its job, salad oil texture and all.

What exactly is its job? The plasma membrane not only defines the borders of the cell, but also allows the cell to interact with its environment in a controlled way. Cells must be able to exclude, take in, and excrete various substances, all in specific amounts. In addition, they must able to communicate with other cells, identifying themselves and sharing information.

To perform these roles, the plasma membrane needs lipids, which make a semi-permeable barrier between the cell and its environment. It also needs proteins, which are involved in cross-membrane transport and cell communication, and carbohydrates (sugars and sugar chains), which decorate both the proteins and lipids and help cells recognize each other.

Here, we’ll take a closer look at the different components of the plasma membrane, examining their roles, their diversity, and how they work together to make a flexible, sensitive, and secure boundary around the cell.

Fluid mosaic model

The currently accepted model for the structure of the plasma membrane, called the fluid mosaic model, was first proposed in 1972. This model has evolved over time, but it still provides a good basic description of the structure and behavior of membranes in many cells.

According to the fluid mosaic model, the plasma membrane is a mosaic of components—primarily, phospholipids, cholesterol, and proteins—that move freely and fluidly in the plane of the membrane. In other words, a diagram of the membrane (like the one below) is just a snapshot of a dynamic process in which phospholipids and proteins are continually sliding past one another.

Interestingly enough, this fluidity means that if you insert a very fine needle into a cell, the membrane will simply part to flow around the needle; once the needle is removed, the membrane will flow back together seamlessly.



Image of the plasma membrane, showing the phospholipid bilayer with peripheral and integral membrane proteins, glycoproteins (proteins with a carbohydrate attached), glycolipids (lipids with a carbohydrate attached), and cholesterol molecules.

Image modified from OpenStax Biology.

The principal components of the plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrate groups that are attached to some of the lipids and proteins.

A phospholipid is a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group. Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer.

Cholesterol, another lipid composed of four fused carbon rings, is found alongside phospholipids in the core of the membrane.

Membrane proteins may extend partway into the plasma membrane, cross the membrane entirely, or be loosely attached to its inside or outside face.

Carbohydrate groups are present only on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins, or lipids, forming glycolipids.

The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary between different types of cells. For a typical human cell, however, proteins account for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent, and the remaining 10 percent comes from carbohydrates.

Phospholipids

Phospholipids, arranged in a bilayer, make up the basic fabric of the plasma membrane. They are well-suited for this role because they are amphipathic, meaning that they have both hydrophilic and hydrophobic regions.



Chemical structure of a phospholipid, showing the hydrophilic head and hydrophobic tails.

Image credit: OpenStax Biology.

The hydrophilic, or “water-loving,” portion of a phospholipid is its head, which contains a negatively charged phosphate group as well as an additional small group (of varying identity, “R” in the diagram at left), which may also or be charged or polar. The hydrophilic heads of phospholipids in a membrane bilayer face outward, contacting the aqueous (watery) fluid both inside and outside the cell. Since water is a polar molecule, it readily forms electrostatic (charge-based) interactions with the phospholipid heads.