Communication in Animals and Plants: Hormones – Study Notes

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Hormones and the Endocrine System

Overview of the Endocrine System

The endocrine system consists of glands and tissues throughout the animal body that produce and secrete hormones to initiate and maintain body functions and activities. Hormones are chemical messengers that travel through the bloodstream to target cells.

Endocrine signaling: Long-distance communication via blood.
Paracrine signaling: Local communication to nearby cells (common in immune/inflammatory signaling).
Autocrine signaling: A cell signals itself.

Key Point: Only cells with the appropriate receptors can respond to a specific hormone.

Soluble vs. Insoluble Hormones

Water-Soluble Hormones

Cannot cross the plasma membrane.
Bind to receptors on the cell surface.
Trigger signal transduction pathways inside the cell.
Examples: Epinephrine, insulin.

Mechanism: Hormone binds noncovalently and reversibly to the receptor, activating intracellular signaling cascades.

Lipid-Soluble Hormones

Can diffuse across the plasma membrane.
Bind to receptors in the cytoplasm or nucleus.
The hormone-receptor complex acts as a transcriptional activator for specific genes, increasing gene expression.
Examples: Steroid hormones such as cortisol, estrogen.

Key Point: One hormone can exert a variety of actions throughout the body depending on the target cell type.

Types of Hormone Molecules

Class	Chemical Properties	Location of Target Cell Receptors	Mechanism of Action	Examples
Amines	Derived from tyrosine or tryptophan; small, water-soluble (except thyroid hormones, which are lipophilic)	Plasma membrane (except thyroid hormones, which act via intracellular receptors)	Stimulate second-messenger pathways (except thyroid hormone, which acts via changes in gene transcription)	Epinephrine, norepinephrine, dopamine, thyroid hormone, melatonin
Polypeptides	Water-soluble	Plasma membrane	Stimulate second-messenger pathways	Insulin, glucagon, leptin
Steroids	Synthesized from cholesterol; lipid-soluble	Cytosol or nucleus	Usually stimulate gene transcription directly	Aldosterone, cortisol, androgens, estrogens

Endocrine Glands and Their Functions

The Hypothalamus and Pituitary Gland

Hypothalamus: Main control center of the endocrine system; receives neural input about internal and external conditions and controls the pituitary gland.
Pituitary gland: Has anterior and posterior lobes.
Posterior pituitary: Stores and releases hormones (oxytocin and antidiuretic hormone/ADH) produced by neurosecretory cells in the hypothalamus.
Anterior pituitary: Releases hormones that regulate sexual maturation, ovulation, milk production, immunity, growth, stress responses, metabolism, digestion, muscle control, and bone growth.

Thyroid Gland

Controls metabolic rate, heart rate, breathing, digestion, body temperature, brain development, skin and bone maintenance, and fertility.
Hormones: Thyroxine (increases metabolic rate) and calcitonin (decreases blood calcium levels).
Thyroid hormone binds to intranuclear receptors, activating genes for increased metabolic rate and thermogenesis.

Pineal Gland

Regulates circadian rhythms by secreting melatonin.
Circadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle, primarily responding to light and dark.

Thymus

Site of T-cell maturation (lymphocytes from bone marrow mature into specialized T-cells).
Mature T-cells enter the bloodstream and help fight disease and infection.
Active in children until maturity.

Adrenal Glands

Located on top of each kidney; each gland has an inner adrenal medulla and an outer adrenal cortex.
Adrenal medulla: Secretes epinephrine and norepinephrine for short-term stress responses ("fight or flight").
Adrenal cortex: Secretes corticosteroids (glucocorticoids and mineralocorticoids) for long-term stress responses.

Short-term stress response: Increased blood glucose, increased blood pressure, increased breathing rate, and decreased digestive/kidney activity.

Long-term stress response: Glucocorticoids mobilize cellular fuel; mineralocorticoids act on salt and water balance.

Plant Hormones

Major Types and Functions

Plants use hormones to control body-wide responses, including growth, development, and defense.
Each hormone's effect depends on its site of action, concentration, and the plant's developmental stage.
Most plant hormones act in balance with others to regulate growth and development.

Ethylene

A gaseous plant hormone that triggers fruit ripening and programmed cell death.
Ethylene production increases during ripening, creating a positive feedback loop.
Also produced in response to stress (drought, flooding, injury, infection).
Induces the "triple response": short, thickened stem (hypocotyl) and exaggerated apical hook.

Plant Defenses Against Herbivores and Microbes

Herbivores are animals that eat plants; plants produce jasmonic acid (JA) as a defense hormone.
Plants are also subject to infection by bacteria, viruses, and fungi.
The epidermis is the first line of defense.
Systemic acquired resistance (SAR): A defense response at the infection site produces signal molecules that spread to other tissues, warning the rest of the plant and triggering additional defensive chemicals.
Salicylic acid (SA): A hormone associated with SAR; its volatile form (MeSA) can warn neighboring plants.

Study Guide Questions – Key Concepts

What is a hormone? A chemical messenger produced by glands/tissues that regulates physiological processes in target cells.
Why do water-soluble hormones usually bind receptors on the cell surface? Because they cannot cross the plasma membrane.
Which type (water-soluble vs. lipid-soluble) tends to act faster, and why? Water-soluble hormones act faster because they trigger rapid signal transduction pathways at the cell surface.
Match the class to the example: Amines (epinephrine), polypeptides (insulin), steroids (cortisol). Amines and polypeptides commonly work through second messengers.
Name the three major types of stimuli that can make an endocrine gland release a hormone: Humoral (blood levels of ions/nutrients), neural (nerve fibers), and hormonal (other hormones).
What is the basic difference between anterior pituitary control and posterior pituitary control? Anterior pituitary synthesizes and releases its own hormones; posterior pituitary stores and releases hormones made by the hypothalamus.
For oxytocin: name one target tissue shown on the slide: Uterine muscles or mammary glands.
What gland releases growth hormone (GH)? The anterior pituitary gland.
What hormone does the pineal gland secrete? Melatonin.
What is one major difference between animal hormone signaling and plant hormone signaling? Animal hormones are often produced in specialized glands and travel through the bloodstream; plant hormones are produced in many tissues and can act locally or systemically.
What is "positive feedback" in ethylene-driven ripening? Ethylene triggers more ethylene production, amplifying the ripening process.
What is systemic acquired resistance (SAR) and what hormone is most associated with SAR? SAR is a plant-wide defense response; salicylic acid (SA) is the hormone most associated with SAR.

Additional info: These notes integrate animal and plant hormone signaling, highlighting both similarities and unique features of each system. Understanding these mechanisms is essential for topics in physiology, development, and defense in both kingdoms.