Table of Contents
Overview: What Hormones Are and What They Do
Hormones are chemical messengers produced by specialized cells and released into body fluids (usually blood or hemolymph) to act on distant target cells. In contrast to rapid and localized nerve impulses, hormonal signaling is typically:
- Slower in onset (seconds to hours or longer)
- Longer-lasting (minutes to months)
- Often widespread, coordinating entire organs or the whole organism
Key, beginner-friendly core ideas:
- Source: Hormones are secreted by endocrine cells or glands.
- Transport: They travel through body fluids.
- Targets: Only cells with the appropriate receptor can “hear” the signal.
- Effects: They change cell activity (e.g., metabolism, growth, gene expression).
You will learn organism-specific details in later subsections (vertebrates, invertebrates, plants, pheromones). Here we focus on principles that apply to all hormonal systems.
Endocrine vs. Exocrine Secretion
Endocrine glands and cells are often contrasted with exocrine ones:
- Endocrine secretion
- No ducts
- Hormones released directly into blood or tissue fluid
- Act on distant targets (e.g., thyroid hormones acting on cells throughout the body)
- Exocrine secretion
- Use ducts
- Secrete to body surfaces or into body cavities
- Products are not hormones in the strict sense (e.g., sweat, saliva, digestive enzymes)
Some organs combine both functions (e.g., the pancreas: digestive enzymes via ducts, hormones like insulin directly into blood).
General Features of Hormonal Signaling
Target Cells and Receptors
A hormone can circulate everywhere, but only target cells with the correct receptor can respond:
- Receptors are usually proteins.
- They recognize a hormone by its shape and chemical properties (similar to a lock and key).
- Binding of a hormone to its receptor triggers changes in the cell, such as:
- Turning genes on or off
- Changing enzyme activity
- Altering transport of ions or nutrients
This receptor-based specificity explains how a single hormone can circulate body-wide yet produce precise, coordinated effects.
Types of Hormone Actions
Depending on where the target is located relative to the hormone’s source, signals can be described as:
- Endocrine: hormone acts on distant cells via the bloodstream (typical “hormone” case)
- Paracrine: signal acts on nearby cells in the same tissue
- Autocrine: cell responds to the hormone it secretes itself
Many signaling molecules can act in more than one of these modes depending on context.
Chemical Classes of Hormones (General Principles)
A later chapter will classify vertebrate hormones in more detail. Here we keep to broad, cross-kingdom categories.
Peptide and Protein Hormones
- Built from amino acids linked in chains
- Range from small peptides (a few amino acids) to large proteins
- Usually:
- Water-soluble
- Stored in secretory vesicles inside cells
- Released by exocytosis when needed
- Bind to receptors on the cell surface because they cannot easily cross the lipid membrane
Consequences:
- Often act quickly (seconds to minutes)
- Frequently work by triggering internal signaling cascades (second messengers), which rapidly modify existing proteins.
Examples (details of specific hormones are covered in later chapters): many vertebrate pituitary hormones, some insect hormones, some plant peptide signals.
Steroid Hormones
- Derived from cholesterol
- Lipid-like, generally:
- Fat-soluble (lipophilic)
- Not stored in large amounts: often synthesized when needed
- Can diffuse through cell membranes
They typically:
- Bind to receptors in the cytoplasm or nucleus
- The hormone–receptor complex often acts as a transcription factor, altering gene expression
Consequences:
- Effects begin more slowly (minutes to hours) because changing gene expression and protein synthesis takes time
- Effects can be long-lasting (hours to days or longer)
Examples: sex hormones and stress hormones in vertebrates; some similar molecules in other groups.
Amine Hormones and Other Small Molecules
- Derived from single amino acids (e.g., tyrosine, tryptophan) or other small molecules
- Can be:
- Water-soluble (acting via surface receptors)
- Or fat-soluble (acting more like steroid hormones)
This group is chemically diverse:
- Some act very quickly (e.g., by altering ion channels)
- Others influence gene expression and development over longer time scales
Plants, animals, fungi, and microbes also use many small molecules as growth regulators or intercellular signals; the term “hormone” is often restricted to signals acting within an organism, while signals between organisms are usually called pheromones (discussed in a dedicated chapter).
How Hormones Influence Target Cells
Details of intracellular signaling will be expanded when specific hormonal systems are discussed. Here we focus on the main routes:
Cell-Surface Receptors and Second Messengers
Water-soluble hormones (e.g., most peptide hormones) cannot cross the cell membrane easily. They typically:
- Bind to a receptor protein embedded in the cell membrane.
- Cause the receptor to change shape, activating intracellular signaling proteins.
- Trigger second messenger molecules (such as cyclic AMP, Ca²⁺, or others).
- These second messengers:
- Modify enzyme activity
- Open or close ion channels
- Affect cytoskeleton and membrane transport
- Can eventually influence gene transcription as well
Features:
- Signal amplification: one hormone–receptor interaction can activate many molecules in a cascade, producing a large response from a small signal.
- Rapid onset and adjustable intensity of response.
Intracellular Receptors and Gene Regulation
Fat-soluble hormones (e.g., steroid hormones) often:
- Diffuse through the cell membrane.
- Bind to a receptor inside the cytoplasm or nucleus.
- The hormone–receptor complex binds specific DNA regions.
- This modulates transcription of certain genes, changing which proteins the cell makes.
Features:
- Responses are slower to appear.
- They often cause qualitative changes in cell behavior (e.g., differentiation, long-term metabolic shifts).
Hormones in Homeostasis and Regulation
Hormones are central to homeostasis—keeping internal conditions relatively stable despite changing external conditions. They help coordinate:
- Metabolism and energy balance (e.g., regulating blood sugar or fat storage)
- Growth and development (e.g., controlling body size, metamorphosis, flowering)
- Reproduction (e.g., timing of mating, pregnancy, seed formation)
- Water and salt balance (e.g., fluid volume, ion concentrations)
- Stress responses (e.g., “fight-or-flight” reactions, long-term adaptation to stress)
In nearly all organisms, hormonal signals work in networks rather than isolation. Often:
- One hormone can influence the secretion of others.
- Several hormones can act together on the same tissue (synergistically or antagonistically).
Feedback Control of Hormone Levels
Hormonal systems are usually regulated by feedback loops to avoid under- or overreaction.
Negative Feedback
Negative feedback stabilizes a system:
- A change in a variable triggers hormones that counteract the change.
- When the variable returns toward its set point, hormone secretion is reduced.
Generic example structure:
- If some variable (e.g., $X$) becomes too high:
- Hormone A is released.
- Hormone A causes tissues to lower $X$.
- As $X$ falls back, the stimulus for Hormone A secretion diminishes, and its release decreases.
This keeps variables such as temperature, osmotic pressure, or nutrient levels within a survivable range.
Positive Feedback
Positive feedback amplifies a change:
- A hormone causes changes that increase its own further release (directly or indirectly).
- Usually occurs in special situations that require rapid, self-reinforcing processes (e.g., certain reproductive events).
Because positive feedback can quickly run out of control, it is typically time-limited and eventually stopped by:
- Exhaustion of the initial trigger
- Activation of strong inhibitory mechanisms
- Structural changes in the organ or tissue
Coordination of Hormonal and Nervous Systems
This topic is explored in more detail in the chapter on coupling of nervous and endocrine systems; here are just the general principles:
- The nervous system is:
- Fast
- Precise and localized
- Often transient in its effects
- The hormonal system is:
- Slower to start
- More diffuse and long-lasting
- Well suited for long-term adjustments and broad coordination
In many animals:
- Certain brain regions produce or control hormones.
- Nerve impulses can trigger hormone release.
- Hormones can in turn modify the excitability and function of neurons.
In plants and simpler animals without a centralized nervous system, chemical signals play a particularly dominant role in coordinating development and responses to the environment.
General Patterns of Hormonal Effects Across Organisms
Details about vertebrates, invertebrates, plants, and pheromones appear in separate chapters. Here we highlight common patterns:
- Conserved themes:
- Use of small molecules and peptides as signals
- Specific receptors determining which cells respond
- Feedback loops maintaining internal balance
- Integration of environmental information (e.g., light, food availability, stress)
- Different implementations:
- Specialized endocrine glands vs. scattered hormone-producing cells
- Use of circulating fluids (blood, hemolymph, sap) to distribute signals
- Species-specific hormones and receptors, yet often based on similar chemical building blocks
Across life, hormones help transform simple chemical signals into organized behavior at the level of tissues, organs, whole organisms, and—even via pheromones—interactions between individuals.