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BOTANY AND ECOLOGY > Photosynthesis and translocation > Flashcards

Photosynthesis and translocation Flashcards

1
Q

Photosynthesis

A

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

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2
Q

Photoautotrophic organisms

A

Pro - some bacteria, cyanobacteria
Euk - algae, bryophytes, vascular plants

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3
Q

Characteristics of photosynthesis in green plants

A

Chloroplasts:- grana (thylakoid membranes)
stroma (soluble matrix)
Primary pigment:- chlorophyll a
Accessory pigments:- chlorophyll b
carotenoids (carotenes and
xanthophylls)

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4
Q

Light essential for photosynthesis

A

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

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5
Q

Photosynthetically active radiation

A

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

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6
Q

Pigments

A
  • Pigments give colour to leaves (and flowers)
  • Can absorb light of different
    wavelengths
  • Transfer energy to chlorophyll a
  • ETC
  • Quench excess energy
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7
Q

Light phase of photosynthesis

A

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

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8
Q

Dark phase of photosynthesis

A

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)

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9
Q

Thylakoid structure

A

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

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10
Q

Photosystem organisation

A

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*

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11
Q

Photophosphorylation

A

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

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12
Q

3 types of photosynthesis in plants

A

C3 photosynthesis - most plants
C4 photosynthesis - mostly plants of arid
climates
Crassulacean Acid Metabolism (CAM) - mostly
cacti and succulents in arid climates

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13
Q

Calvin cycle phases

A

Fixation
Reduction
Regeneration
- light phase in thylakoid generators chemical energy to power Calvin cycle

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14
Q

Photorespiration

A

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

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15
Q

C4 photosynthesis

A

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)

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16
Q

Some characteristics of C4 plants

A

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

17
Q

Some C4 plants

A

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

18
Q

CAM plants

A

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

19
Q

Examples of CAM plants

A
  • 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
20
Q

C4 VS CAM plants

A

CO2 incorporation spatial or temporal separation

21
Q

Factors that limit photosynthesis

A

Light
CO2
Temperature
Mineral deficiencies
Herbicides
Pollutants

22
Q

Light saturation curve

A
  • 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)
23
Q

Photoinhibition

A

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

24
Q

End products of photosynthesis

A
  • 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)
25
Q

Translocation

A
  • 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
26
Q

Phloem

A
  • 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
27
Q

Phloem parenchyma

A

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)

28
Q

P-protein and callose

A
  • 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
29
Q

Phloem sap

A
  • 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
30
Q

Phloem translocation mechanism

A
  • 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
31
Q

Mass flow mechanism

A

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

BOTANY AND ECOLOGY (22 decks)

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