BIO 201 Exam 2: Set 2 (H2O/sug trans--sensory sys)

Water potential

The potential energy of water in a particular environment (the tendency of water to move to a new position)

ψs

• solute potential

• Always negative (more solute more neg)

• Relative to solute potential in pure water

Ψp

• pressure potential

• tendency of water to move due to physical pressure on the water

• Pos or neg

  • Neg pressure = tension (pulling)

What is the role of the cell wall in determining pressure potential in plants? 

• Cell wall exerts back pressure when water moves in 

  • Causes inner pressure to build up as turgor pressure

Turgor pressure

Outward pressure of the protoplast against the cell wall

How turgor pressure functions to support the primary plant body

Allows plant cell to hold its shape, otherwise wilting happens

What is considered a turgid cell?

Cells that are exerting enough turgor pressure to induce wall pressure (cell wall pushing back with equal opposite force)

What is the role of the stomata?

Gas exchange – where transpiration occurs (water regulation) + CO2 taken in

Transpiration

Evaporation of water from surface of mesophyll cells and diffusion out of leaf via stomata

When are stomata open/closed

Open when guard cells have full turgor pressure on them, close when guard cells are flaccid

Why is water transport necessary in plants (3 reasons)?

• To move water for photosynthesis, turgor pressure, and cellular processes

• To move minerals carried from root to shoot

• To cool the plant through evaporative cooling

How does water enter the root?

• Water enters root through root hairs via osmosis 

  • Soil ψw almost always higher than in roots

How can water move through the root to the xylem (3 options)?

• Symplastic

  • Path through cytoplasm moving between cells via plasmodesmata 

  • Symplast = continuous cytoplasm in plants

• Transmembrane

  • Path through water channels in the plasma membrane (go through pores in plasma membrane)

• Apoplastic 

  • Path outside of plasma membrane through cell wall matrix 

Apoplast

Continuous cell wall matrix

What is the role of the casparian strip in mediating water uptake in roots?

• Blocks the apoplastic route 

  • Water (and solutes) must pass through cytoplasm of endodermal cell to enter vascular tissue

  • Limits Na+, heavy metals, and pathogens

Casparian strip

band of suberin (waxy substance) on the endodermis

What is root pressure?

The positive upward force generated in plant roots by actively pumping nutrients in that then diffuses water in after it into the xylem

Cohesion-Tension Theory (passive process)

• 1. Water exits mesophyll cells via evaporation and diffuses out of leaves

• 2. Drop in water potential inside of mesophyll cell causes net water movement into those cells from surrounding cells

• 3. Cells bordering the xylem elements replace water with water from xylem

• 4. Cohesion (H2O molecules sticking together from H-bonds) and adhesion (sticking of H2O to cell walls) lifts water column in the xylem

• 5. Negative pressure (pulling) reaches to tip of roots, so water moves passively into xylem, then is pulled up the xylem to replace water lost by transpiration

How do plants prevent extensive water loss (3 ways)?

• Reduce water loss through stomata

  • Sunken stomata

  • Trichomes 

    • Help maintain high humidity at air/leaf interface to reduce water potential gradient 

• 2. Reduce water loss via diffusion across epidermis 

  • Thick cuticle

  • Multiple layers of epidermis

  • Reduce SA

• 3. Timing of stomata opening

  • CAM (Crassulacean Acid Metabolism)

  • Stomata are open to allow CO2 uptake at night

Pressure flow model (general description)

Sugars flow in from sources to sinks that are on the same side and region along a pressure gradient (high to low pressure)

How does pressure flow model work?

• High pressure at the source

  • Sugars from source cells are actively transported to and loaded into companion cells and then into sieve tube members

    • Phloem loading

  • Water from adjacent xylem cells passively moves into sieve tube element due to more solute entering (turgor pressure increases)

• Low pressure at the sink

  • Sugars are moved out of the sieve tube element and into sink cells

    • Phloem unloading

  • Water follows as solute concentration drops in phloem at sink

    • Turgor pressure falls

How does water flow in pressure flow model?

• Water in sieve tube flows from high pressure at source to low pressure at sink

  • Carries sugar and other solutes with it 

Source vs sink

Place where sugars enter system (where they’re either made or stored) vs where sugars exit phloem (where they’re used)

How do sources/sinks change seasonally?

• In fall, sinks = roots, trunk, branches, tubers etc. 

• In spring, sources are sinks from fall

  • Roots, trunk, branches, tubers send sugars from starch to buds and new shoots

    • Buds and new shoots = sinks

Phloem loading

Done actively, with cotransporters (secondary transport)

• Movement of sucrose coupled with movement of H+ down gradient moving in same direction (symporter)

Phloem unloading

Passively or actively depending on where its being unloaded

• Passively

  • Sucrose moves along concentration gradient out of sieve-tube member to companion cell then developing leaf cell where it’ll be used for photosynthesis

• Actively

  • Sucrose moves along gradient out of sieve-tube member to companion cell then root cell where it is stored in vacuole

    • High concentration in vacuole, so active transport

Why plants need to uptake mineral nutrients (1 reasons, 4 examples)

• To synthesize molecules necessary for growth and reproduction

  • Amino acids

  • Nucleic acids

  • Enzymes

  • Chlorophylls

Examples of macronutrients

N, P, and K

Micronutrients

Nutrients plants need in relatively small quantities that act as cofactors for specific enzymes (not part of enzyme structure, but required for enzyme to work)

Micronutrients examples

E.g. Cl, Fe, Mn, Zn, etc.

Examples of typical limiting nutrients

Often, N, P, and K

Mobile nutrients

Elements that can move inside the plant tissue to areas of new growth when supplies are low

Mobile nutrient symptoms

• Old leaves exhibit nutrient deficiency of these

• Plant moves limited nutrients to younger parts

Mobile nutrient examples

N, P, K, and Mg

Immobile nutrient symptoms

• Young leaves exhibit nutrient deficiency of immobile nutrients

• Nutrients are stuck in old leaves

Immobile nutrient examples

Fe, Ca

Immobile nutrients

Elements that CANNOT move inside the plant tissue to areas of new growth when supplies are low

How does the plant uptake nutrients (describe process)?

• SIMPLY: 1) pump H+ out of cell, use secondary transport for + and - ions 2) move inner to reach endo

• 1. Proton pumps establish electrochemical gradient by moving protons out of cell

  • Builds up voltage across membrane (membrane potential)

    • Cations move along gradient, anions move into cell against gradient by coupling movement with protons

• 2. Ions move from epidermal cells through root cortex to reach endodermis 

  • Endodermis has casparian strip, so all materials must pass thru endodermal cell

What is passive exclusion?

• Ions are excluded if no transporter is present

• The number of transporters affects how many ions can reach the xylem

Nitrogen fixation

The conversion of N₂ to ammonium, nitrate, or nitrite by bacteria and archaea

Why is nitrogen fixation essential?

Eukaryotes can’t use N₂, so they need form they can use

How does nitrogen fixation in plants work?

• Plants of legume family enter symbiotic relationship with rhizobia (nitrogen fixing bacteria) in root nodules

What is the root nodule?

Bumps that form on the roots of legumes that host rhizobia

What is the role of leghemoglobin?

• Binds O₂ and keeps O₂ concentrations low inside nodules

  • Protects oxygen-sensitive enzyme nitrogenase needed for fixation

• Provides source of O₂ for cellular respiration

George Washington Carver’s contributions

• Proposed planting crops like peanuts in rotation with cotton grown in the south

  • allows for a restoration of nutrients to depleted soil

• Communicated these strategies with poor farmers

  • helped increase productivity and reduce poverty in the South

The three sisters

Agricultural system used by native americans involving squash, maize, and beans

How does the three sisters system work?

• Planted squash, maize, and beans together

  • Corn supports beans (trellis for beans to grow on)

  • Beans fix nitrogen

  • Squash acts as weed control and shade to the soil

What is the purpose of crop rotation?

• To allow for the restoration of nutrients in depleted soil

  • Rotate with legumes to restore nitrogen in soil

What is a parasitic plant?

Plant that lives in close physical contact with plants from another species and lowers the fitness of those plants by obtaining water or nutrients from them

Haustoria

Structures that penetrate host plant’s vascular tissue to extract water and nutrients

Are parasitic plants heterotrophic?

• Can be heterotrophic but most are photosynthetic still

  • Just use host for water and nutrients

What is an epiphyte?

Plants that grow on top of other plants but are not parasitic (commensal usually) and absorb water and nutrients from collected rainwater, dust, and particles that collect on them

Carnivorous plants

• Trap and kill insects, digest them, and absorb nutrients 

• Found in bogs or habitats with limited nitrogen

• Make their own carbs (photosynthetic) but need nitrogen supplement

The general mechanisms by which signals are perceived and transduced.

• Step 1: Sensory cells receive external signal

  • External stimulus is detected by sensory cell, typically by signal receptor proteins 

    • Signal receptor proteins change shape, converting signal to intracellular one

  • External signal examples

    • Light, gravity, pressure, day-length, temperature

• Step 2: The signal moves through the plant body to target cells 

  • Chemical signals move in the form of hormones (usually)

• Step 3: Target cells receive the signal and change their activity in response 

  • Cells must have the appropriate receptor for a response to occur 

  • Most receptor molecules are located in cell membrane 

  • This initiates signal transduction

    • Involves phosphorylation cascades

    • Involves secondary messengers

The general characteristics of hormones (4)

• Active in small quantities 

• Can have more than one action

• Can act in combo or with non hormonal factors (Ca+)

• Can inhibit or stimulate action or growth

What is a hormone?

Organic molecule synthesized by plant in one place that acts in another to initiate physiological response

Phototropic response (stimulus, perception, and signal)

• Stimulus

  • Blue light wavelength

• Perception 

  • Perceived at the tip of coleoptile in receptor PHOTOTROPIN 1 (PHOT1)

  • Growth stimulus (auxin) is produced in tip and transmitted to the growth zone

• Signal

  • Diffuses downward and causes elongation of cells

  • Called auxin meaning “to grow” 

    • Later identified as Indoleacetic acid (IAA)

Cholodny-Went hypothesis

• Phototropic response results from the asymmetrical distribution of auxin

  • Auxin moves to shaded side in response to light

    • More auxin transported down shaded side of coleoptile than down side exposed to light

    • Auxin concentrated on shaded side causes shaded cells to elongate to greater degree

      • Plant bends toward light

How does the acid growth hypothesis explain cell expansion?

• 1. IAA (auxin) stimulates proton pumps in membrane

• 2. Proton pumps secrete proton into cell wall and decreases pH to 4.5

• 3. Stimulates pH dependent enzyme (expansin) that breaks bonds in cellulose of plant wall

• 4. Breaking of bonds causes wall to loosen, and turgor pressure causes cell to expand

How do stomata open in response to blue light?

• 1. Blue light strikes photoreceptor (phototropin)

• 2. Proton pumps are stimulated

  • Protons pumped out of guard cells, and an electrochemical gradient builds up

• 3. K+ and Cl- enter down electrochemical gradient through K+ channels and H+ / Cl- cotransporter 

• 4. Water enters by osmosis

• 5. Cells swell and pore opens

How does phytochrome act as a molecular switch?

• Exhibits photoreversibility

  • Reversible change in shape in response to light 

  • Switches between P_r and P_fr form depending on what light it absorbs

P_r vs P_fr

• P_r (phytochrome red)

  • Absorbs red light

  • Inactive form

• P_fr (phytochrome far-red) 

  • Absorbs far-red light 

  • Active form that triggers biological response

What responses is phytochrome involved in?

• Seed germination

  • P_r absorbs red light, changes form to P_fr, which activates biological response 

• Flowering

  • Phytochrome triggers flowering based on daylength

    • Controls switch from the vegetative to reproductive meristem

  • Photoperiodism

• Etiolation 

  • Conversion to P_fr switches developmental pattern

    • Photomorphogenesis

Photomorphogenesis

light-mediated development and growth pattern of plants and other organisms separate from photosynthesis (i.e. germination & flowering) AFTER etiolation

3 categories of plants when it comes to flowering

• 3 categories 

  • Long day plants

    • Flower when nights are shorter than certain length

  • Short-day plants

    • Flower when nights are longer than certain length

  • Day-neutral plants 

    • Flower without regard to photo period

Photoperiodism

any response by an organism that is based on photoperiod (relative lengths of day and night)

What is etiolation?

Response by seedling that results in abnormal elongation of stem and no chloroplast development

Significance of etiolation/why its adaptive

Produces maximum growth in length allowed by food reserved to attempt to reach light source in the absence of light

What is gravitropism?

Directional growth of a plant in response to gravity

Auxin’s role in gravitropism

• In vertical root, auxin distributed normally

  • When root tip moved to horizontal position, auxin is redistributed by gravity-sensing cells so that more goes to bottom side

    • Root bends because higher auxin concentration inhibits cell growth on lower side but stimulates growth on upper side, leading to bending

Statolith hypothesis

• Gravity is perceived in the root cap

• Root cap cells contain amyloplasts (plastids full of starch)

  • Amyloplasts accumulate on the bottom of the cell due to gravity (denser than rest of cell parts)

    • Acts as a statolith - structure that senses gravity

    • Puts pressure on the cell membrane

How do plants respond to wind and touch?

Thigmomorphogenesis, Thigmotropism, and thigmonasty

Thigmomorphogenesis

• Change in development usually due to wind 

  • Whole plant phenotype changes

Thigmotropism

• Bending due to touch that is directional

  • E.g. tendrils on herbaceous vines

Interesting observations about thigmotropism

• 1. Response can be rapid 

  • 5-10 minutes for tendril to enclose

• 2. Response is long lasting 

  • Brief contact with tendril can induce response that lasts for days

• 3. Tendrils touched in dark can retain “memory” 

  • Response only occurs later in light, but doesn’t require continued stimulus to occur

Thigmonasty

• Very rapid

• Involves the generation of an electrical signal by building up membrane potential 

• Involves rapid changes in turgor pressure 

• E.g. venus flytrap closing

What is a nastic movement?

• Movements that don’t involve growth 

  • Reversible

  • Nondirectional

What responses is Auxin involved in?

• Maintaining apical dominance (inhibits axillary buds)

  • Why if you cut shoot tip, you cut off production of auxin, so with no auxin to inhibit axillary buds, they start growing

• Stimulates formation of adventitious roots

• Leaf abscission (inhibits abscission zone)

  • When leaf damaged, less auxin produced, abscission zone activated, and leaf falls off

• cell elongation (e.g. phototropism)

What response is ethylene involved in?

• Fruit ripening

• Stem elongation

What response is ABA (abscisic acid) involved in?

• Seed dormancy

  • Viviporous (giving live birth) mutants lack ABA gene and therefore never go dormant 

• Closing stomata in response to drought

How do stomata close?

• 1. Water stress (condition where plant loses water faster than can take up) stimulates synthesis of the hormone ABA in roots

• 2. ABA moves up the plant via xylem

• 3. Pumping by proton pumps in guard cells stops pushing H+ out

• 4. Changes in membrane potential open Cl- channels

  • Cl- exits along electrochemical gradient

• 5. Changes in membrane potential also trigger opening of K+ channels

  • K+ exits along the electrochemical gradient

• 6. Water follows by osmosis, guard cells shrink, and stoma closes

Mechanical defense strategies in plants

• Cuticle

• Trichomes, thorns, and spines

Chemical defense strategies in plants

• Secondary metabolites

  • Usually bi products of metabolic pathways

  • Peppermint, basil, and sage oil

  • Pitch (tar like substance from pines and firs)

  • Latex (milkweeds)

  • Tannins (cloves, cinnamon, oak)

  • Opium, caffeine, cocaine, nicotine, THC

Inducible defense

Defensive compounds are EXPENSIVE, so only produce defensive compounds when under attack

Salicylic acid/methyl salicylate (SA)

Involved in pathogen attack response in triggering SAR that is activated by the HR

Systemin

Stimulates production of jasmonic acid

Jasmonic acid (JA)

Stimulates production of gene products that include proteinase inhibitors

How do plants respond to pathogen attack?

HR (hypersensitive response) and SAR (systemic acquired resistance)

HR (Hypersensitive response)

• Pathogen enters through wound or stoma 

• If pathogens recognized by protein receptors, initiates signaling cascade resulting in HR

  • Stomata closure

  • Production of toxins

  • Reinforcement of neighboring cell walls

  • Rapid cell death of infected cells

SAR (Systemic Acquired Resistance)

• Triggered by SA that was triggered by HR

  • Slower acting, long-term defensive events

  • Results in changes in gene transcription and increases defense in whole plant 

  • Trigger hormone: Salicylic Acid (SA) derivative Methyl Salicylate (MeSA)

How do plants respond to herbivore attack?

Insect damage → systemin → JA → gene transcription → proteinase inhibitors

• Damaged cells produce hormone Systemin that travels thru plant and stimulates Jasmonic Acid production

  • JA stimulates the production of new gene products including proteinase inhibitors

    • Block digestive enzymes of herbivores and makes them sick

      • Herbivores avoid plant that produces proteinase inhibitors

Which responses/processes involve a proton pump?

• Nutrient uptake

• Stomata opening/closing

• Acid-growth hypothesis