Exam 4 - Biology (Osmoregulation & Internal Transport in Plants)

What is transpiration?

Evaporation of water from leaf surfaces

What are properties of water that aid transport?

• Cohesion: Water molecules stick to each other —> conytinuous water column

• Adhesion: Water molecules stick to xylem walls —> help resist gravity

• Tension: Negative pressure from transpiration pulls water upward

Result: Water moves root —> stem —> leaves in the xylem

Plant vs. Animal Vascular Systems

Vessel Structure:

• Plants: Xylem (tracheids, vessel elements), Phloem (sieve tube elements/specialized cell that transport sugars thorughout the plant)

• Anmals: Arteries, veins, capillaries

Fluid:

• Plants: Xylem (water & minerlas), Phloem (sugar solution)

• Animals: Blood

Flow Direction:

• Pants: Xylem (up, roots—> leaves), Phloem (bidirectional, source —> sink/any place where sugar concentration is low)

• Animals: Circulation (heart —> body —> heart)

Motive Force:

• Plants: Xylem (transpiration pull, root pressure), Phloem (osmotic pressure from sugar loading)

• Animals: Heart pumps blood —>pressure gradient

What is thge pressure-flow hypothesis in phloem transport?

A mechanism where:

• sugar actively loaded into sieve tube elements —> lowers water potential

• water flows from xylem by osmosis —> creates high turgor pressure (hypotonic)

• pressure pushes sap from source (high pressure) —> sink (low pressure)

Local vs. Long-Distance Signaling

Signal Molecule:

• Local/paracrine: ligand

• Long-distance/endocrine: hormone

Delivery:

• Local/paracrine: Diffusion to nearby cells (diffuses through extracellular fluid, NOT the bloodstream, short-lived/ high conc. short -lasting effects)

• Long-distance/endocrine: circulation in blood

Target:

• Local/paracrine:Neighboring cells

• Long-distance/endocrine: Distant cells

Example:

• Local/paracrine: Growth factors, neurotransmitters

• Long-distance/endocrine: insulin, glucagon

What are the cellular responses to hormones?

• Change in gene transcription

• Activation of enzymes

• Altered ion channel activity

• Secretion of molecules

What is a gastrin?

• Hormone

• Stimulus: Food in stomach

• Stomach G cells (organ)

• Target: stomach (parietal cells)

• Effect: stimulates HCl secretion

What is secretin?

• Hormone

• Stimulus: Acid in the duodenum.

• Duodenum S cells

• Target: Pancreas

• Effect: Stimulates bicarbonate secretion

What is Cholecystokinin (CCK)?

• Hormone

• Stimulus: Fat/protein in the duodenum

• Duodenum I cells

• Target: Gallbladder/pancreas

• Effect: stimulates bile and enzyme secretion

What is the timing regulation for hormones?

Hormones are released in response to specific nutrients or pH changes —> ensures digestive secretions match food arrival

How does signal transduction work in water-soluble hormone (hydrophilic)?

• Hormone binds plasma membrane receptor

• Activates second messenger (cAMP, Ca2+)

• Activates kinases, transcription factors, or enzymes —> cellular response

• Insulin, Epinephrine, Glucagon

How does signal transduction work in lipid-soluble hormone (hydrophobic)?

• Hormone diffuses through cell membrane

• Binds to intracellular receptor (cytoplasm or nucleus)

• Hormone receptor complex —> binds DNA —> alters gene transcription

• Steroid hormones (estrogen, testosterone, cortisol)

Water-soluble → membrane receptors →

fast response

Lipid-soluble → intracellular receptors →

slow, transcription-based response

How does the Na/K pump work at resting potential?

• pumps 3 Na+ out, 2 K+ in —> maintains conc. gradients

• Passive K+ channels —> K+ leaks out —> inside negative (-70 mV)

• Resting potential = stable inside-negative membrane

What happens during action potential?

• Depolarization: Voltage-gated Na channels open —> Na enters —> membrane becomes positive

• Repolarization: Na channels close, voltage-gated K channels open —> K leaves —> membrane becomes negative again

• Hyperpolarization: membrane slightly more negative before stabilizaing

• Return to resting potential: Na/K pump restores gradients

• ACtion poetntial happens in the axon

What are the effects of altered ion permeability?

• Na channels blocked —> no depolarization —> no AP

• K+ channels blocked —> delayed repolarization —> prolonged AP

What are the different components of a neuron anatomy?

• Dendrites: Receive stimulus

• Cell body (soma): Integrates signals

• Axon: Conducts action potential

• Axon terminals: Communicate with target cells (muscle, gland, neuron)

Key Vocabulary

• Transpiration: Evaporation of water from leaves

• Xylem: Water/mineral transport

• Phloem: Sugar transport

• Tracheid: Xylem cell, tapered ends

• Vessel element: Xylem cell, open-ended

• Sieve tube element: Phloem cell, lacks nucleus

• Companion cell: Supports sieve tube

• Plasmodesmata: Cytoplasmic channels between plant cells

• Water potential (Ψ): Potential energy of water

• Solute potential (Ψs): Effect of solutes on Ψ

• Pressure potential (Ψp): Turgor pressure effect

• Cohesion: Water sticks to water

• Adhesion: Water sticks to walls

• Ligand: Molecule that binds receptor

• Hormone: Chemical signal transported in blood

• Paracrine: Local signaling

• Endocrine: Long-distance signaling

• Pheromone: Chemical signal between individuals

• Second messenger: Intracellular signal (cAMP, Ca²⁺)

• Kinase: Enzyme that phosphorylates proteins

• Gene transcription: DNA → RNA

• Cholecystokinin (CCK): Stimulates bile & enzyme secretion

• Secretin: Stimulates bicarbonate secretion

• Gastrin: Stimulates stomach acid