Module 3 Review - Chemical and Electrical Signals

Juxtacrine (direct)

Cells communicate through direct contact, often through gap junctions in animals or plasmodesmata in plants. Touching or connected cells

Autocrine

A cell releases a signal that acts on the same cell. Acts on itself

Paracrine

A signal diffuses locally to nearby cells. Neural signaling across a synaptic cleft is a specialized example. Nearby cells

Endocrine

Hormones travel through the vascular system to distant target cells. Long-distance signaling

Neuroendocrine

Neurons release signals that travel through the vascular system to distant targets. Neuron + bloodstream

Pheromones

Signals are released into the environment and act on another individual. Between individuals

traits of a hormone

- Is secreted into the vascular system, or into the environment in the case of single-celled organisms.

- Travels to distant cells.

- Produces a large effect even at a low concentration because of signal amplification.

- Acts only on specific target cells that have the appropriate receptor.

- Produces a characteristic response under a given set of circumstances.

- Participates in a positive or negative feedback loop.

peptide hormones class

Hydrophilic; cannot cross the hydrophobic plasma membrane. located on cell surface

Amino acid-derived hormones class

Hydrophilic; cannot cross the membrane. located on cell surface

steroid hormones class

Hydrophobic, generally nonpolar; can cross the membrane. located on cytoplasm or nucleus

gas hormones class

Hydrophobic; can cross the membrane; intracellular

true or false: hydrophilic hormones need a receptor on the cell surface, while hydrophobic hormones can bind to receptors inside the cell.

true

signal reception

The hormone binds to its specific receptor. The receptor may be inside the cell or on the cell surface.

signal transduction

The signal is converted from one form to another and relayed inside the cell. This step is required for hormones that cannot cross the membrane and bind to cell-surface receptors.

signal response

The cell changes its activity, such as its gene expression, metabolism, growth, or division.

Steps for hydrophobic hormones (steroid and gas)

1. The hormone diffuses through the plasma membrane.

2. It binds to an intracellular receptor in the cytoplasm or nucleus.

3. Binding changes the receptor's shape.

4. The hormone-receptor complex enters the nucleus if it is not already there.

5. The complex binds specific DNA sequences and activates or represses gene transcription.

6. The resulting changes in gene expression produce a cellular response.

Signal amplification: A single gene can produce hundreds of mRNA copies, and each mRNA can be translated into hundreds of protein copies.

Steps for hydrophilic hormones (peptide and amino acid-derived)

These hormones cannot cross the plasma membrane, so they bind to cell-surface receptors.

1. The hormone binds to the extracellular portion of its receptor.

2. The receptor changes shape.

3. The intracellular portion activates a signaling pathway or cascade.

4. The pathway relays the signal through molecules inside the cell.

5. The pathway produces a cellular response, which may include altered gene expression.

Two important mechanisms relay the signal:

Second messengers: Small, non-protein signaling molecules, such as calcium or cyclic AMP (cAMP), spread the signal inside the cell.

Phosphorylation cascades: A series of proteins gain phosphate groups, changing their activity and passing along the signal.

Signal amplification: One hormone-receptor interaction can lead to thousands of second-messenger molecules being produced or thousands of proteins being activated.

Compare features of hydrophobic and hydrophilic hormones

HYDROPHOBIC:

-Crosses plasma membrane: yes

-receptor: inside cell

-main signaling mechanism: Hormone-receptor complex regulates DNA

-signal transduction cascade required: no

-amplification: Transcription and translation

HYDROPHILIC:

-Crosses plasma membrane: no

-receptor: cell surface

-main signaling mechanism: Intracellular messengers and signaling cascades

-signal transduction cascade required: yes

-amplification: Second messengers and phosphorylation cascades

describe positive feedback

The response increases or amplifies the original signal.

Example: the rock concert. The musicians turn up the music, which excites the crowd. The crowd's excitement encourages the musicians to play even louder, further increasing excitement.

describe negative feedback

-The response decreases or counteracts the original signal.

-The cycle helps regulate conditions rather than allowing the signal to keep increasing

Example: the thermostat. When the room becomes too hot, the thermostat activates the air conditioning. The temperature decreases, reducing the original problem.

How can a hormone produce different responses in different cell types?

Case 1: Same hormone, same receptor type, different cellular functions

- Adrenaline causes liver cells to release glucose into the bloodstream and blood vessels in skeletal muscle to dilate.

- These different responses occur even though both cell types have the same type of receptor. The cells have different functions and different intracellular proteins that respond to the signal.

Case 2: Same hormone, different receptors, different responses

- Adrenaline causes blood vessels in skeletal muscle to dilate and blood vessels in the intestine to constrict.

- Here, the different responses occur because the blood vessels have different types of adrenaline receptors, which activate different signaling pathways.

***different cell functions can cause diff responses even with the same receptor, while different receptors can activate different pathways***

what is signaling crosstalk

- when different signaling pathways interact, changing the outcome of a cellular response.

- This can happen because the same signaling proteins participate in multiple pathways

example of signaling crosstalk

1. Blood glucose levels are high.

2. Insulin signals muscle cells to take up excess glucose from the blood.

3. Adrenaline is also present.

4. The adrenaline signaling pathway blocks the insulin signaling pathway in muscle cells.

5. As a result, the muscle cells do not take up the excess glucose in response to insulin while adrenaline is present.

different outcomes related to hormones

- Same hormone, same receptor type, different cell types -> Different cellular functions and intracellular proteins

- Same hormone, different receptor types -> Different signaling pathways are activated

- Two signaling pathways interact -> Crosstalk changes the outcome

what is quorum sensing

type of cell signaling in which individual single-celled organisms monitor the density of their population by measuring the concentration of a constantly-secreted signal. They collectively change their gene expression and behavior when the population reaches a threshold density

how does quorum sensing work?

Bacteria continuously release signaling molecules called autoinducers.

1. (low cell density) Few bacteria release autoinducers, so the molecules remain at a low concentration. The quorum-sensing response is not activated.

2. (population grows) More bacteria release autoinducers, causing their concentration in the environment to increase.

3. (threshold reached) Once autoinducers reach a sufficient concentration, they activate the appropriate signaling pathway and trigger a coordinated cellular response.

what are some behaviors controlled by quorum sensing?

1. Symbiosis and bioluminescence: the autoinducer activates genes involved in producing light and the gene for the enzyme that makes the autoinducer itself. This reinforces signaling through positive feedback.

2. Biofilm formation: a community of bacterial cells that stick to one another and to a surface. Ex: dental plaque and bacterial communities on catheters and contact lenses. This is crucial since:

- Their sticky material helps protect bacteria from antibiotics and disinfectants.

- Cells can organize into complex structures, sometimes with specialized roles.

- Some cells form channels that help remove toxins, including antibiotics.

- Established biofilm infections can be extremely difficult to eliminate.

3. Virulence: a microorganism's ability to cause disease

why is quorum sensing useful?

- allows bacteria to coordinate activities that may be ineffective or wasteful when performed by only a few cells.

For example:

- Producing light is more useful when many bacteria glow together inside a squid.

- Forming a biofilm creates a protective community.

- Activating virulence genes together may help a bacterial population overcome host defenses.

describe the hypothalamus and its functions

- located in the brain

- Connects the nervous and endocrine systems.

- Receives information from the body and brain.

- Initiates endocrine responses to environmental changes.

- Produces hormones stored and released by the posterior pituitary.

- Produces regulatory hormones that control the anterior pituitary.

describe the pituitary gland

- located in the base of the brain

- Often called the "master gland" because it regulates other endocrine glands and body processes.

- consists of the anterior pituitary and posterior pituitary

what is the anterior pituitary

- from the pituitary gland

- responds to hypothalamic releasing or inhibiting hormones and produces hormones such as ACTH.

what is the posterior pituitary

- from the pituitary gland

- stores and releases hormones made by the hypothalamus in response to neural signals.

describe the thyroid gland and its functions

- located in the neck

- Produces thyroxine (T4) and triiodothyronine (T3).

- increases metabolic rate and heart rate; promotes growth.

- In amphibians, surges of T3 help initiate metamorphosis.

describe the pancreas and its functions

- located near the stomach and small intestine

- Beta cells: produce insulin, which lowers blood glucose.

- Alpha cells: produce glucagon, which raises blood glucose.

- These hormones have opposing effects that help maintain glucose homeostasis.

what do beta cells do

- Found in the pancreas

- produce insulin, which lowers blood glucose

what do alpha cells do

- Found in the pancreas

- produce glucagon, which raises blood glucose.

describe the adrenal glands and its functions

- located on top of the kidneys

- Adrenal medulla (inner region): produces epinephrine and norepinephrine for short-term stress responses.

- Adrenal cortex (outer region): produces cortisol for long-term stress responses and aldosterone for sodium and water retention.

what is the adrenal medulla (inner region)

- from the kidneys

- produces epinephrine and norepinephrine for short-term stress responses

what is the adrenal cortex (outer region)

- from the kidneys

- produces cortisol for long-term stress responses and aldosterone for sodium and water retention.

describe the gonads and its functions

- located in the ovaries and testes

- Produce sex steroid hormones.

- Promote development of secondary sexual characteristics.

- Help regulate reproductive function.

T3 and T4

- produced by the thyroid

- Increases basal metabolic rate; regulate growth and metabolism

insulin

- produced by pancreatic beta cells

- lowers blood glucose

glucagon

- produced by pancreatic alpha cells

- raises blood glucose

Epinephrine and norepinephrine

- produced by the adrenal medulla

- Short-term fight-or-flight response

cortisol

- produced by the adrenal cortex

- Long-term stress response; increases glucose availability

aldosterone

- produced by adrenal cortex

- Increases kidney reabsorption of sodium and water

ACTH

- produced by anterior pituitary

- Stimulates adrenal cortex to release corticosteroids

juvenile hormone

- produced by Corpus allatum in insects

- Maintains the larval state

ecdysone

- produced by Prothoracic gland in insects

- Promotes molting or metamorphosis, depending on juvenile hormone levels

functions of hormones in the hypothalamus and pituitary gland

The hypothalamus coordinates endocrine responses, while the pituitary relays signals that regulate other glands.

- Hypothalamus: detects information from the body and brain and initiates hormonal responses.

- Anterior pituitary: releases regulatory hormones, including ACTH, in response to hypothalamic signals.

- Posterior pituitary: releases hormones synthesized by the hypothalamus.

- ACTH: stimulates the adrenal cortex to release corticosteroids during long-term stress.

functions of hormones produced by thyroid gland

- T3 and T4 increase the basal metabolic rate and affect protein synthesis and other metabolic processes. They also help regulate long-bone growth in coordination with growth hormone.

- ***T3 & T4 LEVELS VARY THROUGHOUT THE DAY***

- In amphibians, a surge of T3 initiates developmental changes associated with metamorphosis.

functions of hormones produced by pancreas

Insulin and glucagon act in opposite directions to regulate blood glucose.

1. Insulin lowers blood glucose

- Released by beta cells when glucose is high, such as after a meal.

- Increases glucose uptake and use by target cells.

- Stimulates conversion of glucose into glycogen for storage.

- Promotes conversion of glucose into fat and supports protein synthesis.

2. Glucagon raises blood glucose

- Released by alpha cells when glucose is low, such as between meals or during exercise.

- Stimulates glycogen breakdown and glucose release by the liver.

- Promotes uptake of amino acids by the liver for glucose production.

- Stimulates adipose tissue to release fatty acids.

functions of hormones produced by the adrenal glands

1. Epinephrine and norepinephrine: Increase heart rate, blood pressure, breathing rate, and metabolic rate; redirect blood toward the muscles, heart, and brain; increase energy availability.

2. Cortisol: Promotes fat breakdown, increases glucose production from fats and proteins, and inhibits immune function to conserve energy.

3. Aldosterone: Increases sodium and water retention by the kidneys, helping increase blood volume and pressure.

functions of hormones produced by the gonads

The ovaries and testes produce sex steroid hormones that promote secondary sexual characteristics and regulate reproductive function.

Hormone pathways for blood glucose

The body tries to keep blood glucose near its normal target. The human target is approximately 90 mg per 100 mL of blood.

BlOOD GLUCOSE RISES AFTER EATING:

1. Blood glucose rises

2. pancreatic beta cells release insulin

3. cells take up glucose; liver stores glucose as glycogen

4. blood glucose falls toward normal

(negative feedback reduces further insulin release)

BLOOD GLUCOSE FALLS DURING FASTING OR EXERCISE

1. Blood glucose falls

2. pancreatic alpha cells release glucagon

3. liver breaks down glycogen and releases glucose

4. blood glucose rises toward normal

(negative feedback reduces further glucagon release)

what happens after eating a carb-rich meal

Insulin rises; glucose uptake and storage increase; blood glucose falls.

what happens after fasting between meals

Glucagon rises; glycogen breaks down; blood glucose rises.

what happens after rapid glucose use during exercise

Glucagon release helps restore blood glucose.

what happens because of too little insulin or reduced cell sensitivity to insulin

Glucose uptake is impaired, potentially causing hyperglycemia and diabetes mellitus.

what happens after excessive insulin release

Blood glucose may fall too low, causing hypoglycemia.

Hormone pathway for metamorphosis in amphibians

- Thyroid → T3 surge → developmental changes → metamorphosis

- T3 helps initiate the development of new structures and reorganizes internal organ systems.

Hormone pathway for metamorphosis in insects

- Juvenile hormone (JH): released by the corpus allatum; maintains the larval state.

- Prothoracicotropic hormone (PTTH): released by another endocrine gland in the brain as the larva grows; stimulates the prothoracic gland.

- Ecdysone: released by the prothoracic gland; promotes molting or metamorphosis.

Ecdysone's effect depends on the level of juvenile hormone.

- If juvenile hormone remains high, PTTH stimulates ecdysone release + high JH, leading to another larval stage. The insect sheds its exoskeleton and continues larval development.

- If juvenile hormone remains low, PTTH stimulates ecdysone release + low JH, leading to metamorphosis into an adult. Ecdysone promotes the transition to the adult form.

Hormone pathways for stress (short-term stress: fight-or-flight)

(coordinated by hypothalamus)

1. Stressful stimulus

2. Hypothalamus activates sympathetic nervous system

3. Neural impulses stimulate adrenal medulla

4. Epinephrine and norepinephrine released

5. Rapid energy release and increased cardiovascular and respiratory activity

Effects include:

- Increased heart rate, blood pressure, breathing rate, and metabolic rate.

- Breakdown of glycogen to make glucose available.

- Increased blood flow to skeletal muscles, the heart, and brain.

- Decreased blood flow to the digestive system, skin, and kidneys.

- Prediction: If an animal suddenly encounters a predator, its adrenal medulla releases epinephrine and norepinephrine, rapidly preparing the animal to fight, flee, or freeze.

Hormone pathways for stress (long-term stress)

(coordinated by hypothalamus)

1. Persistent stress

2. Hypothalamus releases CRH

3. Anterior pituitary releases ACTH

4. ACTH stimulates adrenal cortex

5. Cortisol and other corticosteroids released

The two major corticosteroid groups have different functions:

- Glucocorticoids such as cortisol: promote fat breakdown, stimulate glucose synthesis from fats and proteins, and inhibit immune function to conserve energy.

- Mineralocorticoids such as aldosterone: promote sodium and water retention by the kidneys, increasing blood volume and pressure.

- Prediction: If an animal experiences a prolonged illness/injury, cortisol helps mobilize energy reserves after short-term glycogen stores become depleted. Aldosterone helps conserve salt and water.

describe the Human Growth Hormone (HGH) in human metamorphism

produced in anterior pituitary with varying level throughout lifetime

what is negative feedback

occurs when the response reduces the original stimulus, helping restore homeostasis.

how does negative feedback work in insulin?

High glucose triggers insulin release; insulin lowers glucose, reducing the stimulus for further insulin release.

how does negative feedback work in glucagon?

Low glucose triggers glucagon release; glucagon raises glucose, reducing the stimulus for further glucagon release.

how does negative feedback work in cortisol regulation?

Cortisol exerts negative feedback on the hypothalamus and pituitary, reducing further stimulation of cortisol release.

Cortisol pathway negative feedback loop:

Hypothalamus (CRH) → anterior pituitary (ACTH) → adrenal cortex (cortisol) → inhibition of hypothalamus and pituitary.

positive feedback

occurs when the response reinforces or increases the original signal.

what is crosstalk

- occurs when one signal changes the effect of another signal or pathway.

- Ex: insect metamorphosis

- Ecdysone promotes molting or metamorphosis.

- Juvenile hormone changes which developmental outcome occurs.

- High juvenile hormone plus ecdysone results in another larval stage.

- Low juvenile hormone plus ecdysone results in metamorphosis into an adult.

- The same ecdysone signal therefore produces different outcomes depending on the other hormonal signal present.

how do hormones travel in plants?

Hormones can travel through the xylem and phloem or from cell to cell through plasmodesmata.

describe auxin hormone (plant)

***auxin = cell elongation***

- Promotes cell elongation in shoots during phototropism and gravitropism.

- Helps regulate gravitropism in roots, where high auxin inhibits elongation.

- Maintains apical dominance by suppressing lateral bud growth.

- Inhibits leaf abscission during the growing season.

- Helps cytokinins promote growth; synthetic auxin can stimulate root formation in cuttings.

***Key behaviors: Phototropism, gravitropism, apical dominance***

describe cytokinin hormone (plant)

***cytokinin = cytokinesis (cell division)***

- Promotes cell division, especially in growing roots, embryos, and fruits.

- Works with auxin to regulate branching and growth.

- Promotes lateral bud growth when auxin-mediated apical dominance is reduced.

***key behaviors: Growing tissues, lateral bud growth***

describe gibberellin hormone (plant)

***growth and germination***

- Stimulates stem elongation.

- Promotes seed germination and breaks seed dormancy.

- Stimulates fruit growth and flower maturation.

- Promotes production of amylase during water-triggered germination, helping release stored energy from starch.

***key behaviors: Stem elongation, seed germination, fruit growth***

describe abscisic acid (ABA) hormone (plant)

***avoid water loss; dormancy and stress response***

- Accumulates during dehydration, cold, and shortened days.

- Closes stomata during drought to reduce water loss.

- Inhibits stem elongation and induces dormancy in seeds and lateral buds.

- Inhibits gibberellin activity, preventing germination when conditions are unsuitable.

- Can contribute to leaf abscission.

***key behaviors: Stomatal closing, seed dormancy, growth inhibition***

describe ethylene hormone (plant)

***ethylene = end of life cycle; aging and ripening***

- A volatile gas that promotes fruit ripening.

- Promotes flower wilting and leaf abscission.

- Helps initiate senescence (aging) and programmed cell death at the leaf attachment site.

- May regulate localized cell death during prolonged drought.

- Commercially used to control fruit ripening; ventilation can remove ethylene and delay leaf dropping.

***key responses: Fruit ripening, flower wilting, leaf abscission***

distinction between auxin and gibberellin

Auxin and gibberellin both promote growth, but auxin is especially important for differential cell elongation during tropisms, whereas gibberellin promotes stem elongation and germination.

receptor/signal for blue light

Phototropins, Phot1 and Phot2

receptor/signal for red light

Phytochrome switches from Pr to Pfr

receptor/signal for far-red light

Converts active Pfr toward inactive Pr

receptor/signal for gravity

Amyloplasts/statoliths settle within cells

receptor/signal for water uptake

Activates gibberellin signaling in certain seeds

receptor/signal for water stress/drought

Abscisic acid (ABA) accumulates

receptor/signal for touch

Auxin redistribution in slow responses; electrical signals in fast responses

behavior/outcome for blue light

Phototropism: shoots bend toward light; stomata open

Blue light activates Phototropins 1 & 2 > auxin accumulation for cell elongation through acid growth hypothesis > cell wall expansion on shaded side leading plant to bend towards from light

behavior/outcome for red light

Promotes growth and can trigger seed germination

behavior/outcome for far-red light

Reduces phytochrome activity; can inhibit red-light responses

behavior/outcome for gravity

Gravitropism: roots grow down, shoots grow up

behavior/outcome for water uptake

Germination begins

behavior/outcome for water stress/drought

Stomata close, growth is inhibited, dormancy may occur

behavior/outcome for touch

Thigmotropism: vines wrap around supports; some plants move rapidly

phototrophism

growth or bending in response to light.

Gravitropism

growth in response to gravity.

Germination

the seed begins growing into a seedling.

Positive gravitropism

roots grow toward gravity (downward).

Negative gravitropism

shoots grow against gravity (upward).

Positive phototropism

shoots bend toward light

describe the pathway of phototropism (specifically blue light making shoots bend toward light)

1. Blue light hits the shoot tip. Phototropins Phot1 and Phot2 detect blue light.

2. Auxin accumulates on the shaded side. The signal moves from the tip toward the growing region.

3. Cells elongate more on the shaded side. Auxin activates proton pumps, causing acid growth.

4. The shoot bends toward the light

What is the acid growth hypothesis?

1. Auxin activates proton pumps that move \(H^+\) ions into the space outside the cell membrane but within the cell wall.

2. The lower pH activates expansins, which loosen connections between cellulose fibers in the cell wall.

3. Sugars move into the cell, water follows by osmosis, and the cell expands.

4. Because the shaded side elongates more than the illuminated side, the shoot bends toward the light.