Plant Bio Quiz 2 Flashcards
- similar in leaves and stems
- water tight seal provided by a tight layer
- thick external layer of waxy cutin
- stoma allow for exchange of gases
epidermis
- thick external layer on epidermis
- virtually transparent so as to not block light from penetrating to photosynthetic cells below
cutin
allow for exchange of gases
stoma
across angiosperms, there is great diversity in what?
stoma
across angiosperms, there is a great diversity in stoma in terms of:
shape, size, number, density, position on leaf (upper vs lower surface), and structure
- e.g. some species, leaf stoma are sunken into ‘stomal crypts’; these are cavities that create a region of nonmoving air
Water molecules that bound out may easily re-enter the what just by random molecular motions?
stomata
leaf epidermis is often hairy due to the presence of what?
trichomes
- provide shade on the upper surface of the leaf, deflecting rays of sun
- slow air movement, decreasing water loss from stomata
- make walking or chewing difficult for insects
- if glandular, produce sticky chemical compounds that deter herbivores
trichomes
the tissue interior to the epidermis are the
mesophyll
for most species, mesophyll is divided into:
- palisade layer
- spongy layer
- upper region of the leaf mesophyll, with vertically oriented cells just under the epidermis
- often just one cell layer thick; may be multiple layers thick
- these cells are packed with chloroplasts and are the main photosynthetic tissue of most plants
- loosely packed, exposing surface area for gas exchange
Palisade layer
- lower portion of the leaf mesophyll
- these cells are open, very loose, forming an ‘aerenchyma’ [parenchyma with abundant air spaces] that permits CO2 to diffuse rapidly from stomata (often on bottom of leaf) into the leaf’s interior
spongy layer
Vascular bundles occur where?
between the palisade and spongy layers
A typical eudicot leaf usually has:
- one large midrip (or midvein)
- numerous lateral veins that then branch into narrow minor veins
The veins contain what?
xylem on the upper side and phloem on the lower side
the larger veins of a leaf typically also include:
- a bundle sheath
- a mass of supportive fibers (above, below, or both) that form a bundle sheath extension
- around vascular tissues
- may include parenchyma, collenchyma, and fibers
- provides structural support for the leaf
- also has a regulatory function for transfer of water and sugars between mesophyll cells and vascular cells
bundle sheath
- mass of supportive fibers (above, below, or both)
- these are often more pronounced on the underside of a leaf, leaving the upper surface +/- smooth for light interception
- these provide protection of the bundle and support for the leaf
bundle sheath extension
very small and may include single vessel elements above a sieve cell/companion cell pair
minor, terminal veins
most leaves have a what that connects the leaf blade to the stem?
petioles
one, three, five , or more vascular bundles branch from the stem and diverge toward the petiole
- collenchyma may be present to provide support
leaf traces
where the leaf attaches to the stem is a what?
abscission zone
Cells in the abscission zone are involved in cutting off the leaf in these conditions:
- at the end of the summer (if deciduous)
- if it becomes damaged by weather or herbivores
- if attacked by pathogens
What happens after abscission?
the leaf falls away and remaining undamaged cells become corky, forming a protective leaf scar
distinctive for each species, allowing identification of trees in winter
leaf scars
Which trees do not abscise their dead leaves in the fall?
- these dead leaves stay attached to the tree until bud break the next spring
- known as marcescence
oaks, beeches, and chestnuts
retention of dead plant organs that normally are shed
marcescence
why would a tree hold on to dead leaves?
- buds are present in the axils of leaves
- in winter, stem twigs and buds are a readily available source of nutrition
- trees that retained their old dead leaves may deter herbivores, who don’t want to chew on old dead leaves
Most current tree species evolved when?
the herbivores present were very different than today
- many of these animals were members of the pleistocene megafauna including, mastodon, mammoth and giant ground sloth
advantages of shedding leaves in fall:
- physiologically costly to maintain over winter
- conserves water
- provides opportunity to shed leaf pathogens
- can decrease limb damage from ice
- decreases threat of winter herbivory
- strategy used in junction with early spring flowering (before new leaves appear)
What is the advantage of retaining leaves over winter in temperate zones? (i.e. being evergreen)
- avoids the cost of building entirely new leaves
- especially important in nutrient stressed habitats (including acidic soils):
- evergreens don’t shed these valuable resource
- deciduous trees would have to reacquire these nutrients from environment
Examples of evergreen trees
e.g. laurel cherry, live oak, big-flower magnolia
leaves are initiated by what?
shoot apical meristems
where are younger leaves located? Older ones?
younger: near the top
older: further down
During development, the older developing leaves what?
closely surround and protect the delicate meristem
At the base of the shoot apical meristem, cells interior to the what bulge outward?
protoderm (immature epidermis)
- leaf protoderm
- leaf ground meristems
leaf primordium
In the leaf primordium, a strand of cells in the center of the primordium differentiation into what?
pro-vascular tissues, then into primary xylem and phloem, forming a direct connection with the vascular bundles in the stem
The young leaf consists of what?
a midrib and two small, then wings of lamina tissue
All cells in the wings are what?
meristematic (capable of dividing)
What enlarges the lamina rapidly?
division of leaf wings
- stomata, trichomes, and vascular bundles differentiate
- the petiole becomes distinct from the midrib
- when all the structures are in place, the leaf is still very small, much less than one-tenth is mature size
- no new mitotic division will ever occur
Lamina expansion
Full expansion to mature size, typically in spring with bud break, occurs as what?
water is taken up and expands the cells
Leaf structure in what are highly varied in composition?
angiosperms
Leaves with a single blade are what?
simple
leaves divided into many separate leaflets are what?
compound
For the development of compound leaves, the leaf primordium consists of what?
lateral regions that expand into separate leaflet lamina
In many perennial plants, leaves and shoots for _ initiate development in summer or autumn
next year
What becomes dormant within the resting terminal or axillary bud until the growing season begins?
leaves and shoot for the next year
what surround and protects the embryonic shoot?
bud scales
What happens when the bud opens?
the young shoot (and often flowers) rapidly expands
In _, meristem cells adjacent to the leaf primordium grow upward along with it, encircling the apical meristem
monocots
the primordium has what shape in monocots?
hood-like shape
The leaf primordium becomes tubular and forms a _ leaf base, while the upper portion forms the _ (blade)
sheathing; lamina
Linear, strap-shaped leaves (like grasses) grow continuously from a unique what?
intercalary meristem
Where is the intercalary meristem located?
between the sheath and blade
- allows for continuous lengthening of the blade
- remains active so that if lamina is consumed, it can grow
- likely an adaption in response to herbivory
intercalary meristem
- leaves are thick and fleshy
- adaptations to survive in desert habitats
- reduced surface-to-volume ratios, favoring water conservation
- mesophyll contains few air spaces, and is more transparent so photosynthesis may occur deeper in the leaf
- plants also frequently employ modified photosynthesis
succulent
- tough often spiny leaves
- leaves have a sclerenchyma layer below the epidermis and in the bundle sheets
- their structure provides a high level of protection
- common in harsh dry climates, such as the California chaparral, and a few other biomes
sclerophyllous
- small modified leaves that protect young buds in the winter from extremes in weather and herbivores
- often numerous and tightly overlapping
- frequently pubescent, and also with a thin, corky bark-like layer
bud scales
What are common adaptions in plants to prevent herbivory?
sharp pointy structures: thorns & spines
a modified, sharp-pointed stem. It occurs in the axil of a leaf where an axillary bud and shoot would normally develop
thorns
modified, sharp-pointed leaf. it will have a bud in its axil, signifying that the spine occurs in the relative position of a leaf
spines
- modified leaves
- needle-sharp projections that are protective and made primarily of fiber sclerenchyma
- lignified cell walls make them especially hard and presistent
spines
common pars of a leaf, typically found at the base of the petiole as little flaps
- serve a protective function when the leaf is young
stipules
In some species, the stipules develop into what?
spines; “stipular spines”
In several Acacia species (legume family), the stipular spines are what and provide shelter for who?
hollow; Pseudomyrmex ants
The leaves in Acacia species produce what?
Beltian bodies for the ants
- modified leaves or leaflets
- common in climbing plants
- the tendrils are touch sensitive
- upon contact with an object, they tightly coil and use the object for support
Tendrils
- many plant species growing in habitats poor in nitrates have evolved these
- leaf appears highly modified, but is actually similar to many foilage leaves, except: the lamina secretes watery digestive fluid & epidermis is absorptive
Leaf traps
Insects are a source of what?
nitrogen
Leaf of leaf traps appear highly modified, but is actually similar to many foliage leaves, except:
- the lamina (which may be tubular) secretes a watery digestive fluid
- the epidermis is absorptive
Trap can be elaborate, as in what?
pitcher plants
Plants may form active traps as in the what?
Venus fly trap
What is the primary source of all energy on earth?
the sun
Every second, in the sun, 600 million tons of hydrogen are converted into Helium in a what yielding high energy gamma radiation?
proton-proton fusion nuclear reaction
From the sun’s surface, sunlight takes about _ minutes to reach the Earth
8.3
_ produced by the solar fusion reaction typically only rarely leave the sun; instead they spontaneously convert to lower energy radiation including visible light
Gamma rays
The total amount of energy received at Earth ground level from the sun at the zenith is _ watts per square meter
1004 watts
The total amount of energy received at Earth ground level from the sun at the zenith is 1004 watts per square meter, which is composed of:
- 527 (52.5%) watts of infrared radiation
- 445 (44.3%) watts of visible light
- 32 (3.2%) watts of ultraviolet radiation
Destabilizing at the molecular level
high energy
Too low to be used at the molecular level
low energy
Light has both _ and _ properties
wave; particle
The fundamental particle of light is the what?
photon (or quantum)
How many photons are in sunlight?
10^17 photons per second per square centimeter
In photosynthesis: Photons of light are absorbed by what?
pigments in leaves
In photosynthesis: These pigments cause what?
a flow of energized electrons through reaction chains
In photosynthesis: the electron flow produce chemical energy in the form of what?
ATP and NADPH
In photosynthesis: ATP and NADPH are used to what?
fix atmospheric CO2 to make sugars (carbohydrates)
Why is photosynthesis important?
- primary entry of useable energy into the ecosystem
- consumers absolutely rely either directly on these carbohydrates by eating plants or consuming other consumers that ate plants
- Insights from understanding how sunlight is used to produce chemical energy informs solar capture technology
- Photosynthesis study informs the field of photobiology
Photosynthesis is divided into two distinct steps
- Light reactions
- Carbon fixation reactions (Calvin cycle)
- require light & occurs in internal membranes (thylakoid) of chloroplasts
- electrons are energized by photons of light
- Water split to provide electrons, while also yielding H+ and O2
- protons used to makes charge gradient
- O2 released as waste
light reactions
- are not dependent on light energy; may occur in light or dark
- use ATP and NADPH from light reaction to chemically fix atmospheric CO2 to commence the process of reduction to form sugars
- Occurs in the stoma, the cytoplasmic spaces of chloroplast (not a membrane bound process)
Carbon fixation reactions (Calvin cycle)
Where does photosynthesis take place?
Mostly chloroplast packed leaf parenchyma cells of:
- palisade layer
- spongy layer
Photosynthesis takes place in thylakoid membranes that form grana stack in interior of chloroplasts
light reactions
Photosynthesis takes place in the stoma surrounding the grana in the chlorplasts
Carbon fixation reactions
Stacks of thylakoid vesicles
grana
comprise the grana, and house many of the components of the light reactions, including chlorphyll pigments
thylakoid membranes
the interior of the thylakoid vesicles
lumen
the cytoplasm of the chloroplast, surrounding the thylakoid membranes
stroma
Photons of light are absorbed by pigments where?
embedded in the thylakoid membranes
Pigments are organized into distinct clusters =
antennae complexes; million of these are in each chloroplast
In the middle of each antenna complex lies a what that contains 2 special Chlorophyll A molecules?
reaction center
Photons of light hit antenna pigment molecules which what?
elevate the orbital levels of electrons
The energy is transferred from pigment molecule to pigment molecule toward the reaction center by what?
resonance
Resonance allows what?
neighboring pigment molecules to transfer energy
The transferred energy arrives at the reaction center causing what?
emission of one e- form the ‘special pair’ in the reaction center to an electron receptor
Three main pigments of photosynthesis
- Chlorphyll a
- Chlorophyll b
- Carotenoids
Chlorphyll a (blue-green) and Chlorphyll b (Olive-green) composed of what?
- Flat Porphyrin rings with central Mg ion
- hydrophobic tail
- diverse group of molecules
- linear with rings at either end & consisting of about 40 carbon atoms
- yellow, orange, red
Carotenoids
Different pigments _ different wavelengths of light
absorb
Green wavelengths what by pigments?
reflected; not absorbed
In an antenna complex, photon energy transfers how?
down a gradient of pigments that have absorption maxima that are progressively shifted toward longer, less energetic wavelengths, finally arriving at the reaction center
Significance of antenna energy transfer
- This shift of absorption maxima means that lower-energy pigments are closer to the reaction center than higher-energy pigments
- This energy gradient ensures that excitation transfer toward the reaction center is energetically favorable
- Thus, it is more energetically favorable for the reaction center to absorb at longer wavelengths (680-700 nm)
Electron transfer to a primary acceptor: step 1
- accumulated energy from photons energize electrons in the special pair of chlorphyll a molecules in the reaction center
Electron transfer to a primary acceptor: step 2
- The energized electron is passed from the special Chlorophyll a to a primary electron acceptor molecule (Phaeophytin)
Electron transfer to primary acceptor: step 3
- The electron is subsequently transferred down a chain of subsequent acceptor molecules
Electron transfer to primary acceptor: step 4
- This pathway is essentially the generation of an electrical current; the flow of electrons initially energized by light
Light reaction 4 main components
- Photosystem II
- Photosystem I
- Oxygen Evolving Complex
- Oxidative phosphorylation (ATP production)
PSII occurs _ in the chain of events
first
PSII was discovered after what?
PSI
The initial Antennae complex (P680) and associated membrane-bound electron transport proteins
Photosystem II
PSII: Special pari of chlorophylls from the Reaction center of P680 donates an energized electron to _ which is transferred to _
Phaeophytin (Ph); Plastocyanin (PC)
The second antennae complex (P700) and associated membrane-bound electron transport proteins
Photosystem I
- provides a second photon boost for electrons received from PSII
- contains the enzymes that reduce NADP+ to NADPH using the electrons energized by PSII and PSI
Photosystem I
molecules associated with PSII that split water. This generates low energy electrons that are passed to the P680 reaction center
- it is the source of electrons for electron flow in photosynthesis
Oxygen Evolving Complex
The oxygen evolving complex location
- closely associated with PSII
- Position in the lumen (inside) of thylakoid is critical to its function
The oxygen evolving complex structure
comprises three main proteins - PsbO, PsbP & PsbQ (Psb= PSII) plus:
- a core comprising Ca, Mn, and O (Mn4Ca1Ox) that binds 2 water molecules
Different AA of the three proteins of the OEC
Glu333, Glu354, His332
As electrons are ejected from the reaction center of PSII, they are sequentially what?
replaced by electrons shared within the Mn4Ca1Ox core
Electron loss from the P680 reaction center is thus closely coupled with the what?
OEC (oxygen evolving complex)
Electrons are not lost directly from what to PSII? Where instead?
Not lost from water; lost from OEC core
Each electron lost from the OEC core to PSII increases what?
molecular tension on the two bound water molecules
After a _ electron is donated, the stress reaches a maximum
4th
When the core reaches maximum molecular tension, it causes:
- both water molecules to be split
- 4 electrons produced replace those previously lost by the OEC core
- tension in the core dissipates
- 2 new water molecules enter the core and bind… process repeats
Water splitting
photolysis in the OEC
Water splitting steps
- 4 e- go to OEC core to replace those lost to PSII
- 4H+ stay in lumen of chloroplast (helps create a gradient used to make ATP)
Water splitting equation
2 H2O -> 4H+ + 4e- + O2
What happens to the energized electrons that leave PSII?
‘Mobile’ electron carriers in membrane (phaeophytin (Ph) then plastoquinone (PQ) transfer electrons via energetically favorable re-dox reactions to the Cytochrome b6 /f complex
- catalyzes the transfer of electrons from PQ to mobile carrier plastocyanin (PC)
- Uses energy lost by electron transfer to pump a proton (H+) to the lumen from the stroma (joining those produced by splitting water)
Cytochrome b6/f complex
What transfers the electron to the reaction center special pair of chlorophyll A in PSI?
PC
The electron receives a what?
second boost of energy harvested by the PSI antenna complex
The electron is then passed from the R.C. of PSI to a cluster of what?
iron-sulfur (Fe-S) proteins, incl. Ferrodoxin [Fd]
Fd transfers electrons to NADP+ in stroma with help of the enzyme _
FNR [ferrodoxin-NADP+ reductase]
Reduction of NADP+ by electrons
NADP+ + 2e- + H+ ———–> NADPH
(low energy) (PSI) (stroma) (high energy)
Powerful reducing agent
NADPH
- referred to as “non-cyclic electron flow”
- this path is also referred to as the ‘Z-Scheme’
flow of electrons
The light reactions result in what?
protons accumulating in the thylakoid lumen
Light reaction steps/parts:
1) Splitting of 2H2O ➔ 4H+
2) E.T.C. of every 4 e- ➔ 8 H+ (via Cytb6f in PSII)
Each pair of water molecules is responsible for the addition of _ to the lumen
12H+
- creates strong pH gradient across the thylakoid membranes
The pH/proton gradient is used to do what?
molecular work
H+ accumulation creates an electrochemical proton gradient, a source of what?
potential energy
The thylakoid membrane is impermeable to what?
H+
Instead, protons pass through what?
membrane bound channel to produce ATP
ATP production via what?
chemiosmotic coupling (oxidative phosphorylation)
Protons pass through an ATP synthase complex located in the thylakoid:
- Protons pass from lumen to
stroma dissipating the gradient - Protons passing through the
ATP synthase complex provides
the energy to produce ATP
Each split pair of water molecules produce how many ATP?
3-4 ATP
