Midterm 1 Flashcards
What is diffusion
spontaneous movement of solutes from regions of higher to lower concentration
What is osmosis
diffusion of water across a selectively permeable barrier
What is free-energy
represents the potential for performing work, force x distance, in J mol-1
What is Chemical potential
a relative quantitative expression of free energy associated with a substance
What is Water potential
the free energy of water per unit volume
the chemical potential of water divided by the volume of 1 mol of water
What are the major factors influencing water potential in plants
concentration, pressure, gravity
Water potential equation
Yw = Ys + YP + Yg
What is reference state
pure water at ambient temperature and standard atmospheric pressure
How do solutes impact water potential
Reduce free energy of water by diluting the water
Lower Yw
How does pressure impact water potential?
Positive hydrostatic pressure raises Yw
Negative hydrostatic pressure lowers Yw
What is positive pressure called?
Turgor
What is negative pressure called
Tension
What is the water potential of flaccid cells
Negative
Why does water move in and out of cells, or from one plant part to another
In response to a water potential gradient
How is the direction of flow determined
By Yw gradient (from regions of higher to lower Yw) with rate proportional to magnitude
What else influences the rate of movement?
hydraulic conductivity
What do aquaporins do?
Provide water-specific channels to facilitate water movement across membranes
How do solutes in, and drying of, soil lower Yw into the negative range
by lowering Ys and Yp, respectively
What happens as soil dries?
water recedes into intersticies between soil particles
What are root hairs?
outgrowths of epidermal cells that increase surface area (represent ~60%)
What happens to water under a curved surface
develops a negative YP
What is curvature of air-water surfaces a balance of?
Minimizing surface area (surface tension) and attraction of water to soil particles
What is Apoplast?
Continuous system of cell walls, intercellular air spaces, and lumens of non-living cells (xylem and fibres)
What is symplast
Entire network of cell cytoplasm interconnected by plasmodesmata
What does transmembrane pathway alternate between
Apoplast and symplast
What is the casparian strip
a band of hydrophobic suberin in the radial cell walls of the endodermis (inner cell layer of the cortex)
What are the functions of the casparian strip?
- Blocks the apoplastic pathway in the endodermis and requires symplastic movement through these cells
- Forces water and solutes into transmembrane pathway
What is guttation
the formation of liquid droplets (dewdrops) at the edges of leaves through hydrathodes
What does absorption of solutes from the soil leads to?
decreases in Ys and Yw in roots
What does lowering of Yw provide?
driving force for water absorption, leading to positive hydrostatic pressure in root xylem
When does absorption of solutes from the soil occur?
when transpiration is low and soil solute concentration is high
What are tracheids?
Tracheary elements found in all plants.
Long spindle-shaped cells in overlapping vertical files
Pits, pit membranes, tori (singular torus)
Radius less than 50 mm
What are vessel elements?
Tracheary elements found in angiosperms, Gnetales, some ferns
Shorter, wider than tracheids with perforated end walls
Perforation plate
Stacked end-to-end form a vessel
Radius up to 500 mm
How does water move?
Bulk flow
Long-distance transport through the xylem and from the soil
Is water movement extremely sensitive to the radius of the tracheary element?
Yes
Is waster movement independent of solute concentration?
Yes
What is the pressure difference required to overcome frictional drag?
0.01 MPa m-1
What is cohesion tension theory?
- Positive to negative pressure gradients from the base to apex of a plant could move water in the xylem
- a large tension (negative pressure) develops at the top of a plant
- requires cohesive properties of water to sustain this tension
What does water adhere to in the xylem?
hydrophillic components such as cellulose microfibrils
How can air enter the xylem?
to injury, leaf abscission, or adjacent damaged conduits
Can water form if there is breaks in the xylem?
No
Features that reduce cavitation?
Pit membranes
Interconnectivity
Finite lengths of tracheary elements
Reduced tension at night
New growth of xylem tissues
What is the driving force of transpiration?
The difference in water vapor concentration difference between the inside of the leaf and the outside air
Factors that affect rate of transpiration?
- Leaf temperature
- Stomatal resistance (number and diameter)
- Boundary layer resistance (wind speed and leaf size)
Dumbbell shaped guard cells are in what kind of plant?
Grasses
Kidney shaped guard cells are in what kind of plants?
All other plants
What opens the stomata?
Increase in guard cell turgor
What aspect of guard cells is responsible for opening stomata?
Specific alignment of cellulose
How are microfibrils oriented in guard cells?
fan out radially from the pore
What factors influence guard cell opening?
Light intensity and quality
Temperature
Water status
Intracellular CO2 concentration
Why does water loss occur?
as a result of allowing sufficient CO2 uptake for photosynthesis
What is the transpiration ratio due to?
- Concentration gradient (inside and outside the leaf) of water 50 times greater than than of CO2
- CO2 diffuses 1.6 times slower in air than water
- Assimilation of CO2 requires transport across plasma membrane, cytoplasm and chloroplast envelop
How many essential plant nutrients are there?
17
Why are nutrients required?
Essential for structure or metabolism
Absence causes abnormal growth, development, or reproduction
Macronutrients?
N, K, Ca, Mg, P, S, Si
What is Passive Transport
Spontaneous movement of molecules down a chemical potential gradient
At equilibrium, no further movement occurs without an input of energy
What is active transport?
Movement of substances against a chemical potential gradient
Not spontaneous, requires that energy is applied
What can diffusion of salts across a membrane can produce
electrical membrane potential
What is the membrane potential of plant cells
-200 to -100 mV
Where does energy for proton transport come from?
ATP
What are the types of transport proteins?
channels, carriers, and pumps
What do channels do?
Selective pores that extend completely across membrane and enhance diffusion
What do carriers do?
Do not have pores that extend across membrane, but bind and transport specific molecules
What do you pumps do?
Require energy
Primary active transport is couple directly to ATP hydrolysis
Secondary active transport uses the proton motive force, which is stored energy created by H+ gradients
Two types: symports and antiports
Symport
Both H+ and substrate A travel in the same direction from low to high
Antiport
H+ travels from High to Low
Substrate A travels from low to high
How are most transport processes energized?
by one primary active transport system coupled to ATP hydrolysis by generating ion gradients
Where is the phloem found?
on the outer side of xylem in vascular bundles or In plants with secondary growth, phloem is the inner bark
What do sieve elements do?
conduct sugars and organic compounds
What do sieve cells lack?
nuclei and vacuoles, Golgi bodies, ribosomes, microfilaments and microtubules
Sieve element cell walls
contain pores that interconnect cells
Companion cells
Highly branching plasmodesmata connect cells
Take over critical functions from sieve elements
protein synthesis
ATP supply
What is each sieve element associated with?
One or more companion cells
What are Transfer cells
Companion cells that cells have finger-like wall ingrowths
Efficient for solute transfer with sieve elements
What are Intermediary cells
COmpanion cells that have many connections to surrounding cells, unlike ordinary and transfer cells
What do sources and sinks do?
Defines direction of phloem transport
What are sources?
Exporting organs such as mature leaves and storage roots
What are sinks
Organs that do not produce enough photosynthetic product to support their own growth or storage needs
Roots
Tubers
Developing Fruits
Immature leaves
What is the pressure-flow model
Passive mechanism
Bulk flow of phloem sap driven by an osmotically generated pressure gradient between source and sink
Nevertheless, energy is required in sources and sinks for the synthesis and consumption of photosynthate, which in required for active phloem loading and unloading
What is flow of pholem sap driven by?
pressure gradient between source and sink
What do phloem loading at the source and unloading at the sink establish
Change in pressure potential
What are the three mechanisms generate high sugar concentration in sieve elements of the source
Photosynthesis in mesophyll
Conversion of photoassimilate to sugars in intermediary cells
Active membrane transport
Pressure Gradient at the Sink
phloem unloading lowers sugar concentrations in sieve elements generating higher (more positive) Delta Yp
What maintains change in water potential in the sink?
Cross walls (sieve plates) of sieve elements
Pressure Flow model predictions
Sieve plate pores must be totally unobstructed
Flow cannot be bidirectional at the same time in the same sieve element
Limitations in ATP supply should not immediately stop phloem transport,
Pressure gradient must be greater than the resistance in sieve elements for bulk flow to occur
What happens during phloem loading?
Sucrose moves from mesophyll to vicinity of sieve elements (across only a few cells)
Sugars transported into sieve elements and companion cells
Can occur via symplast or apoplast, or both
Apoplastic phloem loading
Difference results from phloem loading of sucrose by active transport (against its chemical gradient)
ATP-dependent process by a sucrose-H+ symporter
Uses energy generated by the proton pump
How can diffusion-dependent symplastic loading account for selectivity and accumulation of sugars against a concentration gradient?
Polymer-trapping model
Sucrose is converted to raffinose and stachyose in intermediary cells, which diffuse into sieve elements
Larger size compared with sucrose prevents these sugars from diffusing back into mesophyll cells
Phloem unloading
can be symplastic or apoplastic
Sugars move to sink cells
Symplastic pathways predominate
Apoplastic pathways occur in some sinks with high sucrose accumulation
Energy Required for symplastic Phloem Unloading
Sucrose metabolism results in a low sucrose concentration in sink cells, maintaining the required concentration gradient for diffusion
Energy Required for apoplastic Phloem Unloading
Utilize transporters that may require energy
Physiological and anatomical changes of Source to Sink Transition
Plasmodesmatal closure
Fewer plasmodesmata
Reduced symplastic continuity
What is allocation
Regulation of the distribution of fixed carbon into different metabolic processes
Synthesis of storage compounds
Metabolic utilization
Synthesis of transport compounds
What is partitioning
Differential distribution of photosynthates within a plant
Turgor pressure could be the means of communication between sources and sinks, coordinating rates of loading and unloading
Hormones, mineral nutrients, and sugars could be messengers
Important for maximizing crop yield
What is the most active photosynthetic cell
Mesophyll
What is the photosynthesis equation
6 CO2 + 6 H2O -> C6H12O6 + 6 O2
Internal membranes of chloroplasts produce
ATP and NADPH
Where do carbon fixation reactions occur?
Stroma
What is Absorption spectrum of chlorophyll a ?
the portion used by plants
Why does chlorophyll appear green
Absorbs mainly red and blue light
Only light enriched in green (550 nm) is reflected
What does absorption of a photon cause?
transition to a higher-energy state
What are the 4 ways to dispose of extra energy
- Re-emit a photon, known as fluorescence
- Convert excitation energy to heat
- Participate in energy transfer
- Photochemistry, or transfer of energy directly to chemical reactions
What chlorophyll is in green plants
A and B
What chlorophyll is in protists /cyanobacteria
C and D
what is the porphyrin ring?
structure related to hemoglobins and cytochromes
Involved in electronic transition and redox
What are the components of chlorpophyll
Central magnesium atom surrounded by a nitrogen-containing structure called a porphyrin ring; attached to the ring is a long carbon–hydrogen side chain, known as a phytol chain
What are carotenoids?
An accessory pigment
Linear molecules with many conjugated double bonds
Absorb between 400 – 500 nm
Characteristic orange to yellow color
What is an action spectrum?
shows response of a biological system to light as a function of wavelength
What serves as an antenna complex?
Pigments
What does some light energy absorbed by Chl get stored as?
Chemical bonds
What is the role of antenae?
Collect light and transfer energy to reaction centers
What are reaction centers the site of?
Redox reactions that lead to long-term energy storage
What is quantum yield?
(ϕ) is a definitive measure of the energetic efficiency of photoautotrophy
0 if process does not respond to light
1 if every absorbed photon contributes to the process
0.95 in dim light with fluorescence at 0.05
What reduces NADP+ to NADPH
light
NADP+ reduction is what kind of process
redox reaction where Electrons are moved from one chemical species to another
What is the recycling agent for carbon reactions like the Calvin-Benson cycle
NADP+
What led to the discovery of PS1 and PS2
Red drop and enhancement effect
What does PS1 prefer?
Far red > 680nm
What does PS1 produce?
Produces strong reductant; can reduce NADP+
And a weak oxidant
What does PS2 prefer?
Red 680nm
What does PS2 produce?
Produces a strong oxidant; can oxidize water
And a weak reductant
What is the Z scheme?
Reductant of PSII re-reduces the oxidant of PSI
Where do light reactions occur
Thylakoids
What is a stack of thylakoids called?
grana lamellae
What is a non-stacked thylakoid called?
Stroma lamellae
What do thylakoids contain?
Contain Chl and integral membrane proteins
Reactions centers, antenna pigment-protein complexes and electron transport chain proteins
Where do carbon reactions (dark) occur?
Stroma
How many lipid bi-layers do chloroplasts contain?
Two
What do chloroplasts also contain?
DNA RNA and ribosomes
WHat is the role of PS1 in thylakoids
antennae Chl, e transfer proteins and ATP synthase in stroma lamellae
What is the role of PS2 in thylakoids
antennae Chl e transfer proteins in grana lamellae
What are the diffusible carriers of electrons?
Plastocyanin and Plastoquinone
What is Plastocyanin
Blue-colored copper protein
What is plastoquinone
Organic redox cofactor
Electron Transport
- PSII oxidizes H2O to O2 in thylakoid lumen releasing protons
- Excited PSII P680 transfers e to pheophytin
- Cytochrome b6f oxidizes plastohydroquinone (PQH2)
- Pheophytin transfers e to plastoquinones QA and QB
- Cytochrome b6f complex transfers e to plastocyanin (PC)
- PC reduces P700 of PSI
- e transferred to a Chl (A0), a quinone (A1), a series of iron sulfer proteins (FeSX, FeSA, FeSB), and to ferredoxin (Fd)
- Ferredoxin-NADP reductase reduces NADP+ to NADPH
- ATP synthase functions as H+ diffuses from lumen to stroma
PSII is only biochemical system capable of doing this reaction
2 H2O -> O2 + 4 H+ + 4 e
What is PS2 damaged by
Excess light (photoinhibition)
WHat are the three stages of Calvin-Benson Cycle
Carboxylation of CO2 acceptor
Reduction of triose phosphate 3-phosphoglycerate (NADPH)
Regeneration of CO2 acceptor
