Module 6: Plant Biology Flashcards
what are autotrophic organisms?
> sufficient without eating other living organisms
- plants, algae, certain other protists and some prokaryotes
what are heterotrophs?
live on compounds produced by other organisms
- us
where does photosynthesis occur?
> in leaves
- chloroplasts
- contain thylakoids
- these are stacked into grana or one granum
- stroma outside the thylakoids
what are the different reactions in photo synthesis?
Light reactions and the calvin cycle (dark reactions)
what are the products of the light reaction in photosynthesis?
ATP and NADPH
describe the linear electron flow during light reactions to produce ATP and NADPH
1) a photon of light strikes a pigment molecule in a light-harvesting complex of PSII, boosting one of its electrons to a higher energy level. As this electron falls back to its ground state, an electron of a nearby pigment molecule is simultaneously raised to an excited state. the process continues, with energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the reaction centre complex. It excites an electron in this pair of chlorophylls to a higher energy state.
2) This electron is transferred from the excited P680 to the primary electron acceptor
3) An enzyme catalyses the splitting of a water molecule into two electrons, two hydrogens, and an oxygen atom. The electrons are supplied one by one to the P680+ pair to replace the electrons lost to the primary acceptor. P680+ is the strongest oxidizing agent known; its electron hole must be filled.
the H+ are released into the thylakoid lumen. The oxygen atom immediately combines with other available oxygens to form an oxygen molecule.
4) each photoexcited electron passes from the primary acceptor of PSII to PSI via an electron transport chain. This is made up of the electron carrier plastoquinone (pq), a cytochrome complex, and a protein called plastocyanin (Pc).
5) the exergonic ‘fall’ of electrons to a lower energy level provides energy for the synthesis of ATP. As electrons pass through the cytochrome complex, H+ are pumped into the thylakoid lumen, contributing to the proton gradient that is subsequently used in chemiosmosis
6) meanwhile, light energy has been transferred via light-harvesting complex pigments to the PS I reaction-centre complex, exciting an electron of the P700 pair of chlorophyll a molecules located there. the photoexcited electron was then transferred to PS I’s primary electron acceptor, creating a ‘hole’ in P700+, which is filled by an incoming electron from the electron transport chain from PS II.
7) Photoexcited electrons passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd).
8) The enzyme NADP+ reductase catalyses the transfer of electrons from Fd to NADP+. two electrons are required for its reduction to NADPH. This molecule is at a higher energy level than water, and its electrons are more readily available for the reactions of the calvin cycle than were those of water. This process also removes an H+ from the stroma.
what are the pigments involved in photosynthesis?
>chlorophyll a - is the main photosynthetic pigment >chlorophyll b - is an accessory pigment - absorb different wavelengths of light - pass energy to chlorophyll a > other accessory pigments - carotenoids
describe photosystems
- composed of a reaction centre surrounded by a number of light-harvesting complexes
- molecule in the thylakoid membrane
- energy absorbing
describe light-harvesting complexes
- consist of pigment molecules bound to proteins
- funnel the energy of photons of light to the reaction centre
describe the phases of the calvin cycle in C3 plants
PHASE 1) CARBON FIXATION:
- incorporates each CO2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (RuBP).
- rubisco catalyses RuBP and the CO2 to form a six-carbon intermediate so unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed).
PHASE 2) REDUCTION:
- 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-biphosphoglycerate.
- pair of electrons donated form NADPH reduce to G3P, which stores more potential energy
- 6 molecules of G3P are formed from three CO2 molecules, however only one can be used by the plant.
- OUTPUT = 1 G3P molecule
PHASE 3) REGENERATION OF THE CO2 ACCEPTOR:
- 5 G3P molecules are rearranged into RuBP, using 3 molecules of ATP.
- RuBP can accept CO2 again.
what is the net usage and production of the calvin cycle?
INPUT > 9 ATP molecules > 6 NADPH molecules OUTPUT > 1 G3P molecule (sugar)
what happens to plants when it gets too hot?
> stomata open
- to allow CO2 to enter and O2 to exit
But when stomata open, water evaporates
in hot, dry weather, plants close their stomata
- conserves water, but limiting access to CO2
- causing oxygen to build up
- O2 build up results in photorespiration
what is photorespiration?
> rubisco starts incorporating O2 instead of CO2
energy consumed instead of produced
photosynthetic rate is reduced
what are C4 plants?
> minimise the cost of photorespiration
spatially confine the calvin cycle to very internal cells
1) CO2 incorporated into PEP by PEP carboxylase in the mesophyll cells to form oxaloacetate.
- PEP carboxylase has a much higher affinity for CO@ thn rubisco, and it has no affinity for O2, therefore it can fix carbon effiiently where rubisco cannot.
2) the mesophyll cells export malate/4-carbon compound to bundle sheath cells through plasmodesmata.
3) in the bundle-sheath cells, CO@ is released and incorporated into the calvin cycle by rubisco. Pyruvate is generated in this process and is transported to mesophyll cells.
- ATP converts pyruvate to PEP, cycle starts again.
are C4 plants better than C3 plants?
> C4 photosynthesis uses two extra ATPs
C4 plants have a lot less photorespiration
optimum temperature for C4 photosynthesis is higher than C3 plants
at optimal temps (high) C4 photosynthesis is 2-3 times more efficient than C3.
at low temperatures C3 photosynthesis is more efficient than C4.
what are CAM plants?
> uses temporal separation
open their stomata at night, incorporating CO2 into organic acids (malate) stored in their vacuoles.
during the day the stomata closes, and the CO2 is released from the organic acids for use in the calvin cycle.
what is the hierarchy of plant components?
> organs -roots -stems (flowers modified version) - leaves > tissues - dermal - vascular - ground > cells
what are roots?
> anchors the plant > absorbs minerals and water - mainly through root hairs - vast number of tiny root hairs increase the surface area of the root - near the root tips > often stores organic nutrients
what are stems?
Consist of:
> an alternating system of nodes were leaves are attached
> internodes, the stem segments between nodes
> axillary buds
- structures with the potential to form a lateral shoot or branch
> terminal bud
- located near the shoot tip; causes elongation of the shoot
what are leaves?
> the main photosynthetic organ of most vascular plants
describe tissues
> each organ has dermal, vascular and ground tissues > dermal tissue - protection > vascular tisse - long-distance transport of materials - two tissues, xylem and phloem > ground tissue - specialised cells for functions such as storage, photosynthesis and support
what are the types of tissue organisation of stems?
> eudicots
- two cotyledons
- the vascular tissue consists of vascular bundles arranged in a ring
monocots
- single cotyledon
- the vascular bundles are scattered throughout the ground tissue, rather than forming a ring
what are xylem and phloem?
Xylem:
> empty dead cells forming tubes
> bottom to top
> conveys water and dissolved minerals upward from roots into the shoots
> once the cells reach maturity, they commit suicide and form empty tubes that act as the xylem.
Phloem:
> live cells
> transports organic nutrients from where they are made to where they are needed
> can change directions
what is the tissue organisation of leaves?
> the epidermal barrier in leaves - waxy
- impermeable cuticle (to liquid and gas)
- is interrupted by the stomata
- to allow CO2/O2 exchange between the surrounding air and the photosynthetic cells within a leaf
the ground tissue in leaf
- is sandwiched between the upper and lower epidermis
- very active in photosynthesis
the vascular tissue of each leaf
- is continuous with the vascular tissue of the stem
- import export from stem to leaves
DIAGRAM
