The different cell membrane transport mechanisms The cell membrane is referred to as a ‘fluid mosaic model’ because the protein part within the cell membrane used to be though of as an even layer spread over the outside and the inside of the phospholipid. Now we are starting to think that it is spread unevenly, more like a mosaic than a layer. The phospholipid part of the cell membrane is fluid; this means that its molecules are constantly moving about.
Through the molecules constantly moving about it allows for things such as ‘transient gaps’ to occur, these are gaps within the phospholipids which allow molecules to pass through; they are only temporary. Here is a picture of the fluid mosaic model: [pic] Both water-soluble (hydrophilic) and lipid soluble (hydrophobic) substances are able to pass across the cell membrane. It is easier for lipid-soluble compounds to pass relatively quickly through by dissolving in the lipid layer.
Water needs to pass through via osmosis where as water-soluble substances cross the membrane through simple diffusion, facilitated diffusion and active transport. Osmosis is thought of as the diffusion of water from an area of high concentration of water molecules to an area of low concentration, across a partially permeable membrane. To define osmosis more accurately we define it in terms of water potential. Water potential is a measure of how easy it is for water molecules to move. Diffusion occurs because substances attract a ‘cloud’ of polar water molecules around them.
The cloud is held by weak chemical bonds, including hydrogen bonds; this means that these water molecules cannot move freely. This is a picture of osmosis: [pic] Inside cells some solutes have a higher concentration inside than outside of the cell, this means in order to make it equal they need to move across the cell membrane against the concentration gradient. This means that they cannot get in by passive transport; passive transport is the diffusion of substances across a membrane, this is a spontaneous process and cellular nergy is not expended. Molecules will move from where the substance is more concentrated to where it is less concentrated. The rate of diffusion for different substances is not always the same even if the process is spontaneous; this is because membranes are selectively permeable. They must enter by way of a process known as active transport. It is called active transport because unlike passive transport it requires cellular energy. Active transport involves special molecules of the membrane called ‘pumps’.
Pump molecules can pick up molecules and transport them to the other side of the molecule; where they are released by the pump. Pump molecules are globular proteins which span the lipid bilayer. The energy needed for active transport is used here; a reaction with ATP is required by the pump molecules to supply the metabolic energy to the process. Membrane pumps are specific to the particular molecules they work within; this is what makes selective transport. Here is a picture of active transport: [pic]
Simple diffusion can take place in some of the carrier proteins found in the plasma membrane. Carrier proteins have binding sites which pick up specific molecules. They can only function in one direction and they require energy to change shape and move a solute. Simple diffusion is a passive process; this means that substances move down a concentration gradient and they do not need any input of energy from the cell, the rate of diffusion can be affected by a few things. Such as the difference in concentration between the area of high and the area of low concentration.
The distance over which concentration occurs has an affect on the rate of diffusion; for example the thickness of the membrane which separates the areas of high and low concentration. The surface area which separates the area of high concentration from the area of low concentration affects the rate of diffusion. The higher the temperature is the more kinetic energy the particles possess and the faster they move; therefore this affects rate of diffusion. [pic] Facilitated diffusion is when diffusion is speeded up by specific proteins in cell membranes.
These proteins are able to pass substances across the membrane faster than usual. Facilitated diffusion occurs along a concentration gradient and requires no metabolic energy, unlike active transport. There are only two types of proteins involved in facilitated diffusion; specific carrier proteins take particular substances, e. g. glucose, from one side of the membrane to the other. Ion channels are protein pores that can open or close to control the passage of selected ions, e. g. sodium and potassium.
Facilitated diffusion occurs when molecules such as those that are soluble in water cannot pass through the phospholipids in the bilayer. When this happens they are transported across via the carrier proteins. Solute molecules moving about on either side of the membrane will randomly come into contact with their specific binding site. Once they bind, the protein changes shape and the molecules come off the binding site on the other side of the membrane. [pic] Both carrier proteins and ion channels are used during facilitated diffusion.
Carrier proteins transport polar molecules across the membrane. The carrier protein is specific to the diffusing molecule. The molecule binds to the receptor on the carrier protein, and the protein changes shape (after an input of energy). The diffusing molecule ends up on the other side of the cell, where it is released. Ion channels are used for the transport of inorganic ions. They facilitate the diffusion of charged particles. The channels are lined with polar molecules. The charge particles move towards regions of opposite charge. The ion channels are usually specific to one particular ion.