Chapter 4: Cell Membrane and Transport

The basic structure of cell membrane is a 7nm thick phospholipid layer with protein molecules spanning the bilayer or within one or other layer. Phospholipids and some protein move within layers. Hence the structure is described as a fluid mosaic. Phospholipid bilayers are a barrier to most water soluble substance because the interior of the membrane is hydrophobic. The main function of the membrane is controlling the movement of substances in and out of cell.

Fluid Mosaic Model

In 1972, two scientists Singer and Nicolson this model and it is called fluid mosaic model because bot the phospholipid and protein can move about by diffusion.

Structure of Cell Membrane

The following are the structure of cell membrane:

  • Glycoproteins: They are the form of antigens that acts as cell recognition markers. It helps to interact with other cells.
  • Phospholipids: The phospholipid bilayers that make up cell membranes also contain proteins
    • The proteins can either be intrinsic or extrinsic 
    • Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions
    • Extrinsic proteins are found on the outer or inner surface of the membrane
  • Glycolipids: Glycolipids are lipids with a carbohydrate attached by a glycoside (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues. they also help in cell signaling.
  • Cholesterol: They are the molecules among the phospholipids. At low temperature, cholesterol increases the fluidity of the membrane.


Cell Signaling

Cell signaling is the ability of a cell to receive, process, and transmit signals with its environment and with itself. … Receptors play a key role in cell signaling as they are able to detect chemical signals or physical stimuli.

Movement of Substance in and out of cell

The following factors affect the movement of substance in and out of cell:

  1. Diffusion

It can be defined as the net movement of molecules and ions from a region higher concentration to a region of lower concentration down a gradient, as a result of random movement of particles. Oxygen, carbon dioxide and water cross membranes by diffusion through the phospholipid bilayer. Diffusion of ions and larger polar molecules through membrane is allowed by transport proteins. This process is called facilitated diffusion.

  1. Osmosis

It is the net diffusion of water molecules from an area of low solute concentration to an area to high solute concentration through permeable membrane.

  1. Water potential and Solute Potential

Water potential is the pressure exerted by water molecules that are free to move in a system. It is measured in kPa. The pure water has a water potential of zero. The higher the water potential, the larger number of water molecules that are free to move.

Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water.


Active Transport

Active transport is the process of moving molecules across a cellular membrane through the use of cellular energy. … Active transport is used by cells to accumulate needed molecules such as glucose and amino acids. Active transport powered by adenosine triphosphate (ATP) is known as primary active transport. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.