Photosynthesis and translocation Flashcards
Photosynthesis
Photosynthesis is a process that converts light energy
into biochemical energy which is then used to drive
the assimilation of low energy inorganic carbon
(CO2) into high energy organic biochemicals
Photoautotrophic organisms
Pro - some bacteria, cyanobacteria
Euk - algae, bryophytes, vascular plants
Characteristics of photosynthesis in green plants
Chloroplasts:- grana (thylakoid membranes)
stroma (soluble matrix)
Primary pigment:- chlorophyll a
Accessory pigments:- chlorophyll b
carotenoids (carotenes and
xanthophylls)
Light essential for photosynthesis
Light is a small part of the electromagnetic spectrum: 400 nm 700
nm (0.4 m 0.7 m)
* Light behaves as waves and particles
* Particles of light are called photons
* Each photon contains a quantum of energy
* Pigments absorb photons and become energised
Photosynthetically active radiation
That part of the spectrum which drives
photosynthesis is called photosynthetically active
radiation (PAR)
* This is measured as a flux of photons in units of:-
mol m-2 s-1
* This is called the photon flux density (PFD)
* 1 mol of photons = 6 x 1017 photons
* On a bright sunny day, PAR = 2000 mol m-2 s-1
Pigments
- Pigments give colour to leaves (and flowers)
- Can absorb light of different
wavelengths - Transfer energy to chlorophyll a
- ETC
- Quench excess energy
Light phase of photosynthesis
In grana
Light energy absorbed by pigments is funnelled to reaction centres and used to drive the production of:
ATP (metabolic energy)
NADPH (reducing energy)
Oxygen is also formed during the light phase
Dark phase of photosynthesis
In stroma
Uses the ATP and NADPH formed in the LIGHT PHASE in a series of enzyme catalysed reactions to assimilate CO2 into high energy organic form (e.g. hexose sugar -
glucose, fructose)
Thylakoid structure
e – and H+ transfer in thylakoid membranes carried out by 4 protein complexes: PSI & II, cytochrome b6f, ATP synthase enzyme. Water is oxidised to generate O2 plus H+. H+ released into lumen by PSII. H+ diffuse down electrochemical gradient through ATP synthase and generate ATP. ETC generates NADPH
Photosystem organisation
Each PS = 250-400 pigment molecules in antenna complex with a reaction centre of specialised chlorophyll a molecules. Efficient energy capture.
PSI and PSII are linked by the ETC and work simultaneously and continuously. Cyclic and non-cyclic light driven production of ATP (photophosphorylation)
PSI = P700*
PSII = P680*
Photophosphorylation
Light driven production of ATP
Two types of photophosphorylation, both driven by
proton motive force:-
Non-cyclic photophosphorylation: ATP generated in an
open electron transfer system, linked with oxygen
evolution in PSII, electron transfer to PSI and NADPH
formation
Cyclic photophosphorylation: ATP generated in a closed
system as electron is cycled from ferredoxin to PQ and
then back to PSI, via the cytochrome complex
3 types of photosynthesis in plants
C3 photosynthesis - most plants
C4 photosynthesis - mostly plants of arid
climates
Crassulacean Acid Metabolism (CAM) - mostly
cacti and succulents in arid climates
Calvin cycle phases
Fixation
Reduction
Regeneration
- light phase in thylakoid generators chemical energy to power Calvin cycle
Photorespiration
Ribulose 1,5 bisphosphate (a 5C sugar) has O2 added to it by the enzyme (RuBisCO), instead of CO2 during
photosynthesis
Complex network of enzyme reactions that exchange metabolites between chloroplasts, peroxisomes
and mitochondria
Reduces efficiency of photosynthesis in C3 plants
C4 photosynthesis
In C4 plants, the first, stable, organic compounds
formed during photosynthesis are C4 acids such as
oxaloacetic acid, malic acid and aspartic acid: (in C3
plants it is PGA, which is C3)
The initial carboxylation reaction is catalysed by
phosphoenol pyruvate (PEP) carboxylase and takes
place in the cytoplasm of the mesophyll cells
Requires 2 additional ATP to regenerate PEP, therefore
lower quantum yield than C3 photosynthesis
The CO2 fixed as C4 acid is imported into the bundle-
sheath chloroplasts from the mesophyll
* In the bundle-sheath chloroplasts, C4 acids are
decarboxylated and the chloroplasts are enriched with
CO2
* This CO2 is then fixed by the RUBISCO reaction to give
2 x PGA which enters the Calvin Cycle as in C3
photosynthesis
* PEP has to be regenerated (using ATP)
Some characteristics of C4 plants
Kranz anatomy
No (or very little) photorespiration
High productivities at warm temperatures and high
irradiance
Low CO2 compensation point, steep CO2 diffusion gradient
High water use efficiency
More common in tropical and subtropical arid
environments
Some C4 plants
At least 3000 species known, distributed in
about 18 families (Monocots and Dicots) e.g.
Sugar cane (Saccharum officinarum), Maize (Zea
mays), Sorghum, Millet, Cord grass (Spartina
spp),Tumbleweed (Salsola kali)
Note that some genera contain C4 and C3 species
The C4 strategy arose several times during
evolution and is now represented in many
unrelated taxa
CAM plants
Crassulacean Acid Metabolism
* CAM plants open stomata at night but close during the day
* CO2 enters plant at night and is fixed into organic acids (via PEP carboxylase) in cytoplasm
* Malic acid (C4) is stored in the vacuole
* During the day, stomata close, malic acid is released
from vacuole and decarboxylated to liberate CO2, which is used as substrate for photosynthesis (Calvin Cycle) in chloroplast
* The CAM strategy is a water saving adaptation
Examples of CAM plants
- Perhaps about 30,000 species in about 20 families,
Monocots and Dicots: - Pineapple
- Members of the Crassulaceae
- Members of the Cactaceae
- Members of the Euphorbiaceae
- Note the Euphorbiaceae contains C3 species, C4
species and CAM species
C4 VS CAM plants
CO2 incorporation spatial or temporal separation
Factors that limit photosynthesis
Light
CO2
Temperature
Mineral deficiencies
Herbicides
Pollutants
Light saturation curve
- At low PAR, photosynthesis is masked by respiration
(O2 uptake/CO2 output). Note difference between
gross and net photosynthesis - The PAR level where CO2 uptake (photosynthesis) is
equal to CO2 output (respiration) is called the
compensation point - During the light limitation phase, photosynthesis is
limited by the light phase - At light saturation, photosynthesis is limited by the
dark phase (temp, CO2 availability)
Photoinhibition
Photoinhibition is a decrease in photosynthesis induced by high fluxes of PAR (400-700nm) caused by
(i) exposure to excess irradiance
(ii) exposure to chilling under normal irradiance
(iii) exposure to conditions that decrease CO2 fixation
under normal irradiance
End products of photosynthesis
- The photosynthesis equation leads us to believe
that hexose sugar (e.g. glucose) is the end
product - Sucrose (transport sugar), starch and fructans
(storage carbohydrates) are more important - all
are made from hexose sugar - Sucrose is translocated to growing regions and
storage tissues - Starch is synthesised and stored in leaves, stems
(including underground stems), roots and seeds - In storage tissues, starch is made from sucrose and
stored in amyloplasts - Fructans are found in forage grasses and other plants.
They are polymers of fructose, stored in vacuoles, e.g.
inulins (Asteraceae) and levans (Poaceae)
Translocation
- Sugars (e.g. sucrose) manufactured during
photosynthesis move out of the leaf and are
translocated in the assimilate stream of the phloem - Transport is bidirectional
- Translocation is from sources to sinks
- Leaves are sources (exporters) in the assimilate stream
- Sugars are transported to sinks such as growing areas of
shoot and root (e.g. apical regions, young leaves) and
storage areas (e.g. roots, stems, fruits, seeds) - Storage areas may be sources or sinks
Phloem
- Sieve elements do not have rigid
cell walls and they contain living
protoplasm with mitochondria - The protoplasts of contiguous
sieve elements are interconnected
through sieve areas in adjacent
walls - Sieve plates are sieve areas with
large pores which ensure
protoplasmic continuity between
consecutive sieve tube members
Phloem parenchyma
Also part of phloem, parenchyma
cells with complete living
protoplasm:-
* Companion cells - provide
metabolic support for sieve
elements
* Transfer cells - perhaps involved
in solute exchange between sieve
elements and leaf mesophyll (not
seen in all plants)
P-protein and callose
- P-protein often arranged in tubular filaments in sieve
tubes and has been implicated in the phloem
translocation mechanism - P-protein has also been observed plugging sieve pores
- Callose is a glucan which becomes deposited on the
surface of sieve plates - P-protein and callose may both be involved in
protecting and sealing sieve plates - i.e. maintaining
hydrostatic pressure after damage
Phloem sap
- Mostly carbohydrate (90%) (usually sucrose, but
raffinose, sugar alcohols in some species) amino
acids - Small amounts of minerals (particularly K+, but no
nitrate) - Hormones (growth regulators)
- ATP
- pH = 7.2 8.5
Phloem translocation mechanism
- Explanation must explain rates of flow, bidirectionality,
role of living cells - As yet, not understood completely
- Most favoured hypothesis is the Mass Flow (or
Pressure Flow) Hypothesis, usually attributed to Munch
(1930) - There are other hypotheses
Mass flow mechanism
Phloem translocation from sources (leaves) to sinks (roots)
Sugar enters sieve tubes, water follows by osmosis = high turgor pressure
Sugar leaves sieve tubes, water moves into xylem into transpiration stream
