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Introduces the chemical messengers of the body and how they interact with specific target cells.
Imagine your body is a massive city where every citizen needs to stay synchronized without using a single phone or internet connection. How does a signal from the 'command center' reach a specific 'suburb' miles away without getting lost?
In the human body, chemical messengers called hormones travel different distances to deliver their instructions. Endocrine signaling involves hormones secreted into the bloodstream to reach distant target cells. In contrast, paracrine signaling acts on nearby neighboring cells, like a whisper across a desk. Finally, autocrine signaling occurs when a cell secretes a hormone that binds to receptors on its own surface, effectively talking to itself. The effectiveness of these signals depends entirely on the presence of specific receptors, which are protein structures designed to recognize only one type of hormone molecule.
Quick Check
If a cell releases a growth factor that stimulates its own division, which signaling pathway is it using?
Answer
Autocrine signaling.
Why doesn't adrenaline make your fingernails grow? The answer lies in target cell specificity. A hormone can circulate throughout the entire body, but it only affects cells that possess the matching receptor. This is often called the 'Lock and Key' model. The number of receptors is not static; cells can undergo up-regulation (increasing receptors to become more sensitive) or down-regulation (decreasing receptors to become less sensitive). The binding affinity, or the 'strength' of the bond between hormone and receptor, also determines the magnitude of the cellular response.
1. After a meal, the pancreas releases insulin into the blood. 2. Insulin travels to muscle and fat cells. 3. These specific cells have insulin receptors (). 4. Other cells, like certain neurons, may lack these receptors and remain unaffected by the insulin surge.
Quick Check
What process describes a cell increasing its number of receptors in response to low hormone levels?
Answer
Up-regulation.
Hormones are divided into two chemical classes that dictate how they interact with cells. Steroid hormones (e.g., testosterone, cortisol) are lipid-soluble. They easily diffuse through the phospholipid bilayer of the cell membrane and bind to intracellular receptors. This complex then enters the nucleus to act as a transcription factor, directly altering gene expression. Non-steroid hormones (e.g., epinephrine, insulin) are water-soluble and cannot cross the membrane. Instead, they bind to extracellular receptors, triggering a second messenger cascade, such as the (cyclic Adenosine Monophosphate) pathway, to activate existing enzymes.
1. A non-steroid hormone (First Messenger) binds to a membrane receptor. 2. The receptor activates a -protein on the inner membrane. 3. The -protein activates the enzyme adenylate cyclase. 4. This enzyme converts into (Second Messenger). 5. activates protein kinases, leading to the final cellular response.
Consider the hormone Aldosterone: 1. It diffuses into kidney tubule cells. 2. It binds to a cytoplasmic receptor, forming a Hormone-Receptor Complex (). 3. The moves into the nucleus and binds to specific DNA sequences. 4. This triggers the transcription of for sodium-potassium pump proteins. 5. New proteins are synthesized, increasing reabsorption.
Which of the following is a characteristic of steroid hormones?
In the second messenger system, what is the role of the hormone?
Paracrine signaling involves hormones traveling through the systemic circulatory system to reach target organs.
Review Tomorrow
In 24 hours, try to sketch the difference between a steroid hormone entering a cell and a non-steroid hormone binding to the surface. Can you name one 'second messenger'?
Practice Activity
Research the hormone 'Epinephrine.' Determine if it is steroid or non-steroid based on its speed of action and receptor location.