3.2 - Concept of Movement of Substances across a Plasma Membrane

Characteristics of Substances That Can Move Across a Plasma Membrane

The three factors can affect the permeability of substances across the plasma membrane are molecular size, polarity as well as ionic charge.

Figure 5 - Lipid-soluble substances can move freely across the plasma membrane. Lipid-insoluble substances cannot pass the membrane, however small and uncharged molecules such as water, oxygen and carbon dioxide can still move through the membrane. Large molecules and ions cannot pass freely through the membrane but they can move through with the help of transport proteins.

Passive transport

Passive transport is a type of membrane transport that does not require energy for the movement of substances across the cell membrane. The three examples of passive transport are simple diffusion, osmosis and facilitated diffusion.

Simple diffusion refers to the process where the molecules or ions move from a higher concentration region to a lower concentration region ( down the concentration gradient ). The process continues until a dynamic equilibrium is reached. This process can happen with or without a plasma membrane. The examples of molecules that travel across the plasma membrane by simple diffusion include gases (oxygen, carbon dioxide etc.), fatty acids, glycerol, urea and ethanol.

Figure 6 - When you can smell the bad odor coming from your socks, that is because the (smelly) gas particle diffuses through the air from a higher concentration region (your socks) to a lower concentration region (the surrounding) until it reaches your nose. The same explanation goes when you can smell something delicious cooking in the kitchen.

Osmosis is the process of movement of water or solvents across a cell membrane. The water molecules move from a higher water potential region to a lower water potential region ( down water potential gradient ) through a semi-permeable membrane. The membrane is permeable to water molecules, but not larger molecules such as sucrose and amino acids.

Figure 7 - Osmosis is commonly used to describe the movement of water molecules.

Facilitated diffusion as its name implies, means the diffusion happens with the help of the membrane proteins. Large, uncharged molecules are not able to pass through the membrane because of their size. Despite so, these substances are still essential to the cell for life. Therefore, they pass through the membrane with the help of the carrier or channel proteins. No energy is required as the diffusion happens down the concentration gradient.

Figure 8 - Large, polar molecules and ions require the help of transmembrane integral proteins to move across the phospholipid bilayers.

Active transport

Some molecules require energy in the form of ATP when they move across the plasma membrane. This is because the molecules move against the concentration gradient , which means they move from a region of lower concentration to a region of higher concentration. They also require the help of specific carrier proteins for transport. The examples include the sodium-potassium pump and the proton pump.

Figure 9 - ATP molecules decompose into adenosine diphosphate (ADP) and phosphate (P). The phosphate group then binds to the carrier protein. The phosphate bond provides energy which changes the shape of the carrier protein.