Now we’re going to see another difference between then. In contrast a steroid, like cholesterol, can be swallowed. Any protein you swallow will be broken down into amino acids, so you have to inject it. What are the differences between the two? You could swallow a steroid hormone but not a protein hormone. Remember there are two classes of hormones. So now we understand why hormones and neurotransmitters affect some cells and not others.
The cell with the insulin receptor site (called the target cell) will be affected by it. If insulin is secreted by an endocrine cell (the beta cell of the pancreatic islets), this hormone will go out of the blood stream and if a cell doesn’t have any insulin receptor sites, the insulin will have not affect it. The type of receptor site it has, determines what type of hormone or neurotransmitter can affect it. One cell might have 20 different types of receptor sites while another might have just one. These receptor sites are specific: There are different receptor sites for different hormones. (Etymology of ligand: From Latin ligandus, gerundive of ligare ‘to bind’.) These signal molecules (hormones, neurotransmitters, chemicals) are called ligands. These receptor sites are how they affect the cells. We know hormones affect the activity of the cells. This is where hormones and neurotransmitters and other chemicals attach to the surface of the cell. The receptor sites are always found on the outer cell membrane surface (unless it’s for a protein hormone, which we’ll discuss soon). Receptor site proteins (this is the most important part). These proteins are associated with movement and affect the shape of the cell.ĥ. The cytoskeleton refers to the matrix of proteins in the cytoplasm. The linker proteins are always attached to the cytoskeleton of the cell. Linker proteins are always facing the cytoplasmic fluid. These are found in the cell membrane as well.Ĥ. Enzymes are chemicals that catalyze biochemical proteins. Interestingly, the transport of glucose across a cell membrane uses a transport protein but doesn’t require ATP, so it’s called passive transport, or alternatively, facilitated diffusion.ģ. Amino acids are actually transported by active transport. When no energy is needed, that’s called passive transport. When energy is required to transport/carry a protein across, that’s known as active transport. Remember a glucose molecule (C 6H 12O 6) is 24 atoms large, so it needs help getting across and amino acids are even bigger than that. They are too large to go through ion channels. They are embedded in the cell membrane to help transport glucose and amino acids across the membrane. Some of you may have heard of a class of drugs called “calcium channel blockers” which are important in regulating blood pressure, for example.Ģ. They can open and close and are very important in understanding the rest of physiology. Each ion channel is specifically for specific ions (Na +, K +, Cl –, Ca +2, etc). They are only large enough to allow an ion to go through. Ion channels are a very narrow tube-shaped protein that help establish a tiny pore in the cell membrane. There are globular proteins in between this bilayer that help transport things in and out of the cell.ġ. Imagine the lipids are the creamy filling, a double layer of fat, and since fats and water don’t mix, this effectively creates a partition between the two. So the phosphate is always facing water.Īnalogy: Imagine the phospholipid bilayer as an oreo cookie. We see that the balloon part is facing the tissue fluid outside the cell and the inner part of the cell. As we mentioned previously, the balloon part is phosphate that’s hydrophilic and the two strings are hydrophobic and hate water. The phospholipid bilayer that makes up the cell membrane is made of phospholipids that are commonly depicted like a balloon with two strings. As we know already, the fluid inside a cell is called the intracellular fluid and all living cells have to be surrounded by fluid and we call that the extracellular fluid.