Plant Bio Exam 2 Flashcards
Ribulose-bis-phosphate carboxylase-oxygenase [RuBisCO] is what?
‘fickle’; it has an achilles heel
- usually fixes. CO2 (by adding it to ribulose bisphosphate)
- But, under some circumstances it grabs O2 and metabolizes it instead
- This is a big mistake and very costly to the plant
RuBisCO problem
Binding and metabolizing of O2 by RuBisCO
photorespiration
- occurs when CO2 levels are relatively low in the leaf
- this condition is particularly a problem when temperatures are high, and the plant needs to close its stomates
- this favors RuBisCO binding O2 instead
photorespiration
3 scenarios that might favor photorespiration
- Hot dry conditions
- Historical periods of higher O2 in atmosphere
- Crowded conditions for plants with little air movement
Scenario 1: Hot dry conditions
- stomates shut down to conserve water
– O2 waste can’t get out of leaf
– CO2 can’t get into leaf - Relative O2 increases, CO2 goes down
– photorespiration favored
Scenario 3: Crowded conditions for plants with little air movement
this inhibits effective gas exchange and also causes relative increases of O2 in the leaf
Photorespiration involves an alternate what?
metabolic pathway
- O2 is consumed, CO2 is not consumed
- CO2 is released! (as part of complex salvage pathway of glycolate)
- No net useful carbohydrate is produced
- Energy (ATP, NADPH) is wasted on the Glycolate pathway
photorespiration
- Use of O2 instead of CO2 by RuBisCO in the chloroplast
- Wastes RUBP and costs energy
- favored under situations that increase relative concentration of O2 compared to CO2
Photorespiration
Flower plants have evolved two separate (but related) strategies to avoid photorespiration
- C4 photosynthesis
- CAM photosynthesis
- both are adaptations to dry hot climates
- both involve cellular mechanisms to increase the concentration of CO2 around RuBisCO in order to minimize photorespiration and favor CO2 fixation
C4 photosynthesis & CAM photosynthesis
A spatial solution: employs an altered leaf morphology that separates:
- the location of cells where light reactions and carbon capture occur
vs
- the location of cells where the Calvin cycle occurs
C4 Photosynthesis
The light reactions and carbon sequestration occur in what in C4 plants?
mesophyll cells surrounding the bundle sheath
The calvin cycle in C4 plants occur in what?
the bundle sheath cells
CO2 enters through open stomates and diffuses to?
mesophyll cells surrounding the bundle sheath
CO2 (1C) combines with PEP (3C) (via PEP Carboxylase) to form OAA (oxaloacetate; 4C)
Step 1: CO2 Sequestration
Why is the pathway called ‘C4’
because a 4-carbon compound (OAA) is the first compound recovered in CO2 tracer studies
Step 2 of C4 pathway
OAA (4C) is chemically modified to form Malic acid (also 4C)
Step 3 of C4 pathway
Malic acid is then transferred from a mesophyll cell to a bundle sheath cell
Step 4 of C4 pathway
in the bundle sheath cell, CO2 is released from malic acid by a decarboxylase, leaving Pyruvic acid
- Co2 is then captured by RuBisCO for the Calvin Cycle in the bundle sheath cell
Pyruvate (C3) is transported back to what where it is recycled into PEP (3C) – the original CO2 capture molecule
mesophyll cell
Advantages of the C4 pathway
- the bundle sheath cells greatly concentrate CO2 so that [CO2] is much greater than that of [O2]
- this favors the - carboxylase activity of RuBisCO
- photosynthesis is much more efficient, allowing either
1. fewer stomates to be present
2. Stomates able to close more often
Which both conserve water
Disadvantages of the C4 pathway
- energetically very expensive
- C4 photosynthesis therefore not favored in moist cool environments, where C3 is energetically favored
Examples of C4 plants
Many grass species (monocots) including:
- corn, sorghum, sugarcane, millet, switchgrass
Also, many eudicot species adapted to weedy, disturbed hot, dry habitats:
- Amaranths, Chenopods, Euphorbs (some)
About 8,000 flowering plants species use C4 carbon fixation; what percentage is this?
3%
Light reactions (day time) and carbon capture (night time) occur at different times
- Stomates are closed during day, thus massively conserving water
- stomates open at night and CO2 is sequestered and stored
- Carbon is then released during the daytime when light reactions are producing ATP and NADPH
CAM photosynthesis (Crassulacean acid metabolism)
CAM plants at night
stomates open
- CO2 stored as malate in the central vacuole
- Same chemical pathway as for C4 plants
CAM during day
stomates closed
- malate is transported to the chloroplast
- CO2 is released from malate by a decarboxylase
- CO2 is then fixed by RuBisCO and enters Calvin cycle
CAM leaf anatomy is designed for what?
maximum water conservation
Advantages of the CAM pathway
stomates are closed during the day minimizing water loss
Disadvantages of the CAM pathway
- energetically very expensive
- CAM photosynthesis is not favored except in very extreme, dry environments
Examples of CAM plants
Succulents
- Cacti, Desert Euphorbs
Epiphytes
- Bromeliads, orchids
C4 plant leafs have no what?
Palisade mesophyll layer
About 16,000 plants species use CAM carbon fixation, so about 2x that for C4 photosynthesis. What percentage is that?
6%
Separation of initial CO2 fixation and Calvin cycle: C3 vs C4 vs CAM
- C3: No separation
- C4: Between mesophyll and bundle sheath cells (in space)
- CAM: Between night and day (in time)
Stomata open: C3 vs C4 vs CAM
- C3: Day
- C4: Day
- CAM: night
Best adapted to: C3 vs C4 vs CAM
- C3: cool, wet environments
- C4: Hot, sunny environments
- CAM: Very hot, dry environments
Plants like all other organisms respire in order to what?
generate usable energy (ATP) for metabolism
As for other organisms, plants get energy through the process of
aerobic respiration, in which glucose is broken down in the presence of oxygen to form: CO2, H2O and ATP
Plants make what through photosynthesis?
sugars
In order to extract the energy, the sugars must be what?
broken down by respiration in the mitochondria
Sugars made by photosynthesis are what?
stable energy molecules that can be stored or transported
This energy extracted from sugars (mostly ATP) is used throughout the plant for carrying out its various life processes:
growth, maintenance, defense, all of which utilize long pathways that use ATP to activate intermediates
Breakdown of glucose by aerobic respiration in a eukaryotic cell does not occur inwhat?
a giant explosion of energy
Instead, energy is released how?
in small, incremental steps in 3 main reactions that safely and efficiently maximizes ATP production with minimized loss of energy as heat
3 major steps of aerobic respiration
- Glycolysis
- Pyruvate Oxidation & Citric acid cycle
- Electron transport & ATP synthesis by chemiosmosis
Glycolysis function
splitting of glucose (6C) into 2 x 3 carbon pyruvate molecules
Pyruvate oxidation and citric acid cycle function
Oxidation of pyruvate to CO2, yielding high energy electrons
Electron transport & ATP synthesis by chemiosmosis function
Harvesting of electron energy; reduction of O2 to H2O; coupled with ATP production by a membrane proton pump
Glycolysis location
Cytoplasm
Pyruvate oxidation and citric acid cycle location
Mitochondrion (matrix)
Electron transport & ATP synthesis by chemiosmosis location
mitochondrion (inner membrane)
Net result of glycolysis
- 2 net ATP molecules are produced
- 2 NADH electron carriers produced
most of energy of glucose still remains in pyruvate
To extract remaining energy from pyruvate:
- oxygen must be present
- oxygen acts as an electronegative sink that draws electrons, releasing energy - pyruvate must enter the mitochondrion form cytoplasm
- most of the enzymes required for aerobic respiration of pyruvate are imbedded in mitochondrial membranes
- the mitochondrion (via ancient endosymbiosis) brought the aerobic respiration pathway to the first eukaryotic cells
Pyruvate oxidation and citric acid cycle steps
- pyruvate is transported across the double membranes of the mitochondrion (mt) by specific transport proteins
- In mt, pyruvate is “prepared” for the citric acid cycle:
- CO2 is removed
- Coenzyme A is added
– 1 NAD+ is reduced to NADH
Citric acid cycle occurs where
Occurs in the matrix (inner spaces) of
the mitochondrion
The citric acid cycle is a cyclical series of reaction whereby:
- Acetyl group of acetyl-CoA (2C) is combined with a 4C molecule (oxaloacetate) to make a 6C molecule (citrate)
- Citrate (6C) is systematically oxidized in 8 steps that regenerates oxaloacetic acid (4C)
Both carbons of the acetyl group are ultimately lost as low energy what?
CO2
Oxidation during cycle yields: (per acetyl group)
- 3 NADH
- 1 FADH2
- 1 ATP
Net production from one glucose molecule through the end of the citric acid cycle
4 ATP + 10 NADH + 2 FADH2
→ Electrons are transferred from NADH and FADH2 to the E.T.C. embedded in the inner mitochondrial membrane
→ As electrons move down chain, they drive the pumping of protons (H+) to other opposite side of mitochondrial membrane.
→ Electrons ultimately are drawn at end of chain to O2 , which is reduced to H2O
Electron Transport Chain
Protons at high concentration on outside of membrane flow through ATP Synthase along their gradient back to the matrix, providing the energy to phosphorylate ADP
Chemi-osmosis
Final energy production from aerobic respiration of one glucose molecule
30-32 ATP
In plants, lacking a circulatory system, each plant part independently undergoes gas exchange, whether it be by:
stoma (leaves) or lenticels (bark)
Tissues in a single plant may have different what?
energy balances
Different energy balances of plants:
Leaves: photosynthesis > respiration
* CO2/O2 gas exchange strikes a balance between the two processes
Roots: Respiration»_space;» photosynthesis
* O2 needed to respire carbohydrates imported from other parts of plant
Plants can tolerate much lower concentration of what?
O2
Current atmospheric levels of O2
20.95%
Plants can survive in _ oxygen without difficulty
- higher O2 levels are bad for plants due to photorespiration
2%
Humans can only tolerate (without supplementation) _ O2
19.5%
Roots of plants may experience what conditions during flooding?
anaerobic
Flooding conditions what?
s fills in the air spaces in soil, reducing oxygen levels to anaerobic levels, and preventing root hairs from taking in O2
Plant roots can undergo what part of respiration, but with only minimal energy extraction
glycolysis
prolonged flooding leads to plant death through what?
starvation and anaerobic alcohol production
Some tree species produce specialized root structures that emerge above the water line in order to what? (Bald cypress ‘knees’)
access O2
Some early spring and alpine plants generate heat via what to melt snow?
respiration
Advantages of plant heat generation
- Gives the plant an early growth start
- Attracts early pollinators
- Heat volatilizes pollinator attracting scents
How is heat generated?
NADH from the Citric Acid Cycle passes electrons
to alternative carriers in the membranes of the
mitochondrion
* No proton gradient and no ATP is formed
* Energy in NADH is converted entirely to heat in a big blast
Water is of central importance to many aspects of plant life:
- Obtaining water from soil
- Transporting water throughout the plant body
- Maintaining cell turgor pressure via the central vacuole
- Concentrating solutes and metabolites for transport
Therefore much of the study of the physiology of plants concerns how plants what?
interact with and employ different
ways to utilize water movement for transporting substances
Vascular tissues and long distance transport evolved under what?
intense competition to grow upright and to maximize light interception.
The vascular system allows:
- Photosynthetic products to be effectively transported to locations distant from their source— from leaves to roots, or concentrated in structures such as fruits.
- Water and nutrients from roots to effectively reach distant stems and leaves, even in the top canopy of tall trees
The evolution in the ancestor of what distinguishes them from the Bryophytes, which lack vascular tissues
ferns, gymnosperms, angiosperms
Bryophytes include:
mosses, liverworts hornworts
- They represent the first group of plants to colonize land
- These groups are diverse and occupy numerous niches, but all lack vascular tissue
- Plants usually small and are often in moist habitats
Bryophytes
Plants have also evolved several means to what?
conserve, exclude or control water movement within the plant body
Plants have also evolved several means to conserve, exclude,
or control water movement within the plant body
* These include (among others):
- The cuticle: which prevents water from being lost through plant surfaces
- Casparian strips: which regulates diffusion of substances in water in the roots
In plants what and what play important roles
diffusion and active transport
the random movement of particles in
solution from areas of high concentration to areas of low concentration.
Diffusion
Diffusion through a selectively permeable membrane is known as what?
osmosis
membranes allow only certain substances to pass through; e.g., cell membranes
selectively permeable
is the movement of molecules against their gradient, using energy (ATP)
Active transport
Any volume of water has free energ, a capacity to do work, called what?
chemical potential
In plant physiology, the chemical potential of water is referred to as what?
water potential (symbolized as ψ; psi)
- When water adheres to a substance, the water molecules form hydrogen bonds to the material and are not as free to diffuse as are other water molecules.
- Their capacity to do work has what?
decreased
- They have less water potential than water in free solution
Water potential has three components
- ψπ = osmotic potential
- ψp = pressure potential
- ψm = matric potential
Water potential equation
ψ = ψπ + ψp + ψm
(Psi Phi) is the effect that solutes have on ψ
* Pure distilled water has a ψπ = 0
* Adding solutes decreases water’s free energy, so ψπ is always negative.
ψπ = osmotic potential
(Psi P) the effect that pressure has on ψ
* Water can be compressed within its volume,
thereby increasing its water potential
* Water can also be stretched, which decreases its
water potential
ψp = pressure potential
(Psi M) refers to adhesion of water to structures such as cell walls, membranes, and soil particles
* Adhesion can only decrease water’s free energy, so ψm is always negative.
* In many circumstances, this value can be negligible
ψm = matric potential
Values of water potential are measured in what?
MPa (megapascals)
1 Pa =
1 N/m^2
1MPa is _ pascals
1 million
Water diffuses from regions where water potential is relatively _ to regions where water potential is relatively more _
positive; negative
If the water potentials of two regions are equal, the regions are in _ , and there is no net movement of water.
equilibrium
- If a cell with a ψ of –0.1 MPa is placed in a solution o the same: then
no net movement of water
- If a cell with a ψ of –0.1 MPa is placed in a solution with a ψ of –0.3
MPa:
water moves from the cell into the solution until
the cell’s ψ is –0.3 MPa.
Cell walls, either primary or secondary, are strong enough to resist breakage (bursting) by
water absorption
Growing cells with weak, deformable walls _ rather than burst; this is one mechanism by which cells grow/expand
enlarge
However, _ _ is a serious problem, causing wilting
through contraction of the central vacuole.
water loss
the point at which the protoplast has lost enough water to pull slightly away from the wall.
Incipient plasmolysis
if the cell continues to lose water, the protoplas pulls _ away from the wall and shrinks
completely
- The cell is plasmolyzed, and will likely not recover
Living plant cells pass many materials to each other through:
Plasmodesmata that directly interconnect cells (and form the symplast)
* Only very large molecules are restricted from passage
A typical plant cell may have between _ plasmodesmata connecting it with adjacent cells!
10^3 and 10^5
Transport may also be across the plasma membrane via various means, including……:
1) Osmosis
Most small molecules, like water, can move easily through both the wall
and the intercellular spaces, and across cell membranes!!!
2) Active transport via molecular pumps in the membrane
3) Fusion between transport vesicles and the plasma
membrane.
To further complicate matters, intercellular movement, without even entering a cell may occur _ (along cell walls)
apoplastically
In sum: Transport pathways may be what?
diverse and complex
Mechanisms of transport are broken down into
two categories:
- short-distance transport
- long-distance transport
transfer of nutrients or molecules from cell to cell
- distances of a few cm or less
short-distance transport
usually via xylem and phloem
- overlong distances in plant body
long-distance transport
Specialized Examples of Short (Cell to Cell)
Transport:
1) Opening and closing of stomates
2) Folding and unfolding of leaves in response to environmental signals
3) Transfer cells
When stomatal pores are to open, what are actively transported from surrounding cells into guard cells?
potassium ions (K+)
- Guard cell ψ becomes _ and the adjacent cells become _ ; this results in a net movement of water into the guard cell.
more negative; less negative
The guard cell swells, causing what?
bending and opening of the pore
Once guard cells open, what happens?
active pumping stops and water movement brings guard cells and adjacent cells into water potential equilibrium, and net water movement stops.
When the stomatal pore closes…
K+ are pumped out of the guard cells, and water
follows, decreasing the turgor pressure, and the guard cells close the pore
There are many examples whereby leaves move by flexing or folding in response to a stimulus.
* This includes plants that have _ ; leaves
that fold in response to water stress, touch stimulation, or light levels
‘sensitive’ leaves
Plants that have sensitive leaves include:
- Prayer plants (oxalis)
- Sensitive plants. (mimosa sp.)
The location of flexure is either the entire_ or points of _ _, “joints,” or pulvini
midrib; petiole attachment
these joints include
motor cells
In transfer cells, the inner surface of the cell wall
has numerous what?
finger-like and ridge-like outgrowths.
Consequently, the plasma membrane is pressed firmly against these ridges and has what?
a much larger surface area.
Larger SA provides room for many what?
molecular pumps
High-volume transport can occur across transfer walls, including in plants:
- Where glands secrete excess salt (like salt flats)
- In areas that pass nutrients to embryos in seeds
- Regions where sugar is loaded into or out of phloem, for instance into fruits
What is an important factor in considering overall water balance in plants?
ψp (pressure potential)
Transport of carbohydrates (sugars) through
phloem is a matter of flow from source to sink
Long distance transport: phloem
Long distance transport: phloem leaf source?
Source: during most of year is direct production of
sugars via photosynthesis in leaves
Leaves sink in long-distance transport via phloem
- meristems and growing shoots
- flowers and fruits
- roots
Long distance transport: phloem source for roots
Source: For temperate woody plants in the spring, the roots (from storage) are the source
Long distance transport: phloem sink for roots
- root meristems
- flowers (if early)
- shoot meristem and growing shoots
In a source, phloem loading occurs:
- Companion cells (and phloem parenchyma)
actively (using ATP) pump sugars into sieve
elements of the phloem - Sugars are usually too large to pass freely
through cell membranes, so transport across
membranes requires energy - Membrane-bound pumps bind to sugars and
concentrate them in the sieve elements
What actively. pumps sugars into sieve elements of the phloem?
companion cells
Why does sugar transport require energy?
sugars are usually too large to pass freely through cell membranes
Membrane-bound pumps bind to sugars and concentrate them in what?
sieve elements
What explains the movement of water and nutrient through the phloem?
pressure flow hypothesis
What is the predominant sugar transported in the phloem?
sucrose
Sucrose arrives either directly from photosynthesizing
cells or?
results from the breakdown of starch
Step 1 of pressure flow hypothesis
Step 1) Sucrose is pumped into phloem cells
Step 2 of pressure flow hypothesis
Step 2) As the phloem water-potential becomes more negative, water is drawn in from surrounding parenchyma and xylem
Step 3 of pressure flow hypothesis
Step 3) Pressure produced by water in
the Sieve Tube Members causes the
entire contents of these cells to
squeeze through the sieve pores and
into the next Sieve Tube cell.
How is step 3 of the pressure flow hypothesis possible?
This is possible because:
* The sieve pores are exceptionally large
* The central vacuole disintegrates and its contents mixes with the sugars to create a very watery phloem sap
pressure builds and a large volume of sap moves through phloem quickly (up to several m/hr)
Because phloem sap is under high pressure, the danger exists of what?
uncontrolled “bleeding” if phloem is severed by chewing insects, browsing animals, or by weather breakage
Two concurrent mechanism function to rapidly seal off compromised phloem:
- P-protein (‘P’ = phloem)
- Callose
- always present as a fine mesh network on the inner surface of sieve tube element membranes
- When phloem is ruptured, the rush loosens and sweeps this toward the sieve plate, where it forms a plug
P-protein
- A complex polysaccharide normally in solution in phloem cells under pressure
- With the pressure change with a breach, this precipitates and forms a tangled mass that joins P-protein in forming the plug
callose
Assuming no interference, pressure flow brings sugary sap to what?
the sink tissue
Within sinks, , sugars are_ unloaded from
sieve elements into surrounding cells where the sucrose may be used for various purposes:
actively
The polar covalent bonds of water molecules are
responsible for both its cohesive and adhesive nature.
long distance transport: Xylem
Partial charges occur because of differences in _ of O vs H
electronegativity
Electronegativity makes water molecules what?
sticky,
both to other molecules of water and to other substances
Even in soil, water adheres firmly to what?
soil particles
water can most easily be absorbed by roots from molecules filling the space between soil particles in what soil?
moist soil
water is held tightly by the soil and cannot be easily absorbed in what soil?
Dry soil
what explains the transport of water through xylem?
cohesion-tension hypothesis
Water unavoidably escapes through stomata; this is
called what?
trans-stomatal transpiration
As water moves out of the leaf into the air, the tissues
dry and a what becomes
established.
water potential gradient
Water flows from the xylem, where water potential is
_ , toward air, where water potential is
_
least negative; most negative
The amount of water lost to transpiration is impressive:
- Large oak tree: ?
- Tomato plant: ?
- Large oak tree: 40,000 gallons per year
- Tomato plant: 97-99% of water taken up by roots
As H2O diffuses out of xylem in the leaves, _ forces pull H2O upward through the xylem, all the way from the roots.
cohesive
What from stretching is on the column of water, and
consequently, the pressure potential is a negative number.
tension
The negative water potential in xylem helps what?
draw in water from soil through the root-hairs.
Problem with water column
The pull at the top of the water column must
overcome gravity in order to lift the weight of the entire H2O column
- especailly a problem in tall trees
To overcome gravity and friction, the water potential of plant tissues receive water must be at least what?
0.2 MPa
more negative than that of roots for every 10 meters of height separating them
Therefore: in a 30m tall elm tree, leaf water potential must be at least_ MPa more negative than root water potential
0.6 Mpa
If the soil water potential is highly negative (as in drought condition), the draw/pull on the water column _ until a breaking point is reached
increases
The cohesive properties of water are overcome, hydrogen bonding is broken over a large area, and the water column breaks!
* This breaking is called _ , and the open space is an
_ .
cavitation; embolism
what often means that that the tracheid or vessel can
never conduct water again……..there’s no repair
mechanism
cavitation
After tracheary elements cavitate, surrounding parenchyma cells may block them off with what or by what?
tyloses; secreting gums and resins to seal them off
What can help prevent cavitation, and is most effective in narrow elements (more surface area in contact with the water column to secure it).
Adhesion between water and the cell wall
Therefore, what have an advantage especially in drier parts of the year
narrower tracheary elements
water is less abundant; smaller vessel elements predominate
Summer wood
water is abundant; large vessel elements
Spring wood
In _ _ , most wide vessels cavitate, and most water conduction is provided by the later, smaller vessels!
late summer
