Hormones are chemical messengers that are secreted into the blood stream by endocrine cells. They can be peptide hormones, steroid hormones, or messenger molecules called cytokines.
Hormones can influence cells in the blood stream, in other organs, and in distant tissues. This is how hormones coordinate complex cellular and physiological responses throughout the body.
The secretion of a hormone is regulated by a special cell membrane protein called a receptor. Receptors are activated by specific hormones, facilitating a chemical reaction thatsecretes the hormone.
Because receptors are membrane proteins, however, how could a hormone be secreted if it needs a receptor? The answer lies in finding a receptor-less hormone!
There are some hormones that exist as free peptides or molecules in the blood plasma. These unbound (or free) forms of the hormone can then cross the cell membrane via passive diffusion.
Fat-soluble molecules can cross biological membranes because they are able to combine with lipid molecules
As mentioned earlier, hormones are a type of small-molecule signal that is produced and released by one cell and received by a different cell.
Some hormones are able to pass through the cell membrane that separates the cell that produces it from other cells. How can this be?
There are two main properties of hormones that allow them to pass through the cell membrane. The first is whether or not the hormone is water-soluble or fat-soluble, and the second is whether or not it can interact with proteins in the cell membrane.
If a hormone is fat-soluble, like some hormones such as testosterone and progesterone, then it can pass straight through the cell membrane because of how similar it is to the lipid molecules in the membrane.
If a hormone interacts with proteins in the cell membrane, like many receptors do, then it can attach to the protein layer and get pulled through with it.
Helps hormones travel long distances in the body
A second important feature of hormones is that they can help transmit a message from one cell to another cell in the body. Some hormones, like estrogen, testosterone, cortisol, and adrenal hormones like deoxycorticosterone (DOC) and glucocorticoids, can affect cells in other parts of the body.
How is this possible? Hormones can act at a distance by facilitating communication between the cells that make up different parts of the body.
For example, estrogen produced in one part of the body can influence other cells to produce certain proteins that function as receptors for the hormone. These receptors are located on the cell surface where they can receive a chemical message from outside the cell.
If this message is received, it may trigger a response in the cell. For example, it may stimulate the cell to start growing or changing its function or state. This is how estrogen can help prevent bone loss or osteoporosis by triggering cells inside bones to grow and rebuild them.
Allows hormones to pass through the plasma membrane
A second important feature of a hormone is its ability to pass from one cell or tissue type to another. Many hormones, such as insulin and testosterone, are produced in specific cells or organs and then distributed to other cells and tissues where they have an effect.
How can a molecule made in one cell gain access to all the other cells in the body? How can it bypass the protective barrier that separates one cell or tissue type from another?
As explained above, hormones are large, complex molecules that consist of a protein part plus some kind of chemical component. One part of the hormone that plays an important role in its function is the amino acid cysteine.
Cysteine is found in proteins, and when two cysteine molecules come into contact, they can connect by forming a disulfide bond. This bond is essentially a link between the two proteins.
Makes the hormone more effective
Another reason a cell would export a hormone is to make the cell more effective. For example, neural crest cells produce pigment cells in the skin when they are stimulated by a hormone.
This process requires energy and materials, so it is advantageous for the cell to be able to pass the pigment cells through the plasma membrane without having to invest in more internal transport systems.
Thus, the function of transporting hormones out of the cell and into the blood stream or lymphatic system can also help the cell function more effectively.
Some hormones are produced in such high quantities that it would be wasteful for the cell to invest in transportation systems to move it outside of the cell. Thus, having the ability to pass it through the membrane is a more efficient way to dispel it.
Allows hormones to pass through the plasma membrane
A second important property of hormones is that they can pass from one cell to another cell through the plasma membrane that separates the two cells.
Not all substances in a cell can do this. Only substances that are very watery in nature can pass from one cell to another through the protective layer between them called the plasma membrane.
Hormones are of immense importance as they regulate many functions of our body. As we have seen above, there are two main categories of hormones – endocrine and paracrine. The next question we will address is: how do these hormones regulate body functions?
Paracrine hormones function in very close proximity to each other within a specific area of the body. For example, immune cells called mast cells release paracrine chemicals that trigger inflammation in the area surrounding them. This inflammation helps fight off pathogens like bacteria and viruses.
Endocrine hormones function at a distance from one another within the body. One cell sends a hormone into the blood or lymph system where it can then go to other cells and/or back to the same cell it originated from to activate a response. This response could be anything from regulating metabolism, to changing how a cell grows or moves, to fighting off pathogens.
Helps cells communicate with each other
Hormones are molecules that are produced, stored, and released by a specific group of cells. These cells include endocrine, digestive, and neural cells.
Endocrine cells are specifically defined as a cell type that produces and releases hormones into the blood or lymphatic system. Hormones travel in the blood to other parts of the body to perform functions.
Digestive cells also produce hormones, which aid in digestion. For example, serotonin is a hormone produced by nerve fibers in the stomach that helps control digestion.
Neural hormones are produced and released directly from the nerve cell itself. These hormones help regulate many different processes in the body.
The characteristics of a hormone depend on its property. A question regarding which property of a hormone would allow it to pass unassisted through a plasma membrane is therefore valid.
Molecules can interact with protein receptors on cell surfaces
As discussed above, hormones are molecules that are secreted into the blood or intercellular fluids, and that can regulate the function of cells far away from their source.
Some hormones, such as insulin, act directly on cell surfaces by binding to receptors located there. This interaction leads to a change in the cell, either by turning it on or off, increasing or decreasing its activity.
How is this possible? How can a molecule that is floating around in the blood enter the cell and interact with its inner components?
The answer lies in another property of hormones: They are lipophilic, which means they have an affinity for fatty layers—or membranes—in cells.
Insulin enters cells through a process called pinocytosis, which involves the cell taking in a large particle containing the hormone. This process occurs when insulin levels in the blood are low. When insulin levels are high, it binds to receptors on the surface of the cell or internal cellular components directly, activating the cell.
Lipids help transport some steroid and thyroid hormones around the body
A second way that hormones can be transported is as a lipid-associated molecule. Some hormones, including glucocorticoids and mineralocorticoids, are fat-soluble.
This means they can be dissolved in triglycerides, or fat molecules. When a hormone is bound to a lipid, it is more easily transported through the plasma membrane due to a property of lipids.
Namely, the plasma membrane is not very selective about what it lets pass through it. Anything that can combine with things outside of the cell can pass through – it’s quite permeable.
A lipid-associated hormone cannot pass through the plasma membrane alone, but when it is attached to a lipid molecule it can. This allows for easy transportation of the hormone within the cell.