Photosynthesis in Higher pLants 4 Flashcards
what are c4 plants?
Plants that are adapted to dry tropical regions have the C4
pathway
mentioned earlier. Though these plants have the C4
oxaloacetic acid as
the first CO2
fixation product they use the C3
pathway or the Calvin cycle
as the main biosynthetic pathway.
what is kranz anatomy
The particularly large cells around the vascular bundles of the C4
plants are called bundle sheath cells, and the leaves which have such
anatomy are said to have ‘Kranz’ anatomy. ‘Kranz’ means ‘wreath’ and
is a reflection of the arrangement of cells. The bundle sheath cells may
form several layers around the vascular bundles; they are characterised
by having a large number of chloroplasts, thick walls impervious to
gaseous exchange and no intercellular spaces.
They are surrounded by mesophyl cells. Bundle sheath cells have large chlotoplast but mesophyll have fewer smaller cholorplast
what happens in 1st step: fixation
The primary CO2
acceptor is a 3-carbon molecule phosphoenol
pyruvate (PEP) and is present in the mesophyll cells. The enzyme
responsible for this fixation is PEP carboxylase or PEPcase. It is important
to register that the mesophyll cells lack RuBisCO enzyme. The C4
acid
OAA(Oxoaloacetic acid) is formed in the mesophyll cells. It is the first stable product of C4 cycle.
OAA is then converted to Malic acid.
what happens in 2nd step: decarboxylation
The malic acid moves he bundle sheath cells view cytoplasmic connections called plasmodesmata. Over here it is decarboxylated( co2 removed) to give pyruvic acid. As a result, the carbon dioxide concenrtation increases within bundle sheath cells. Bundle sheath cells also have RuBisCO. DUe to higher conc of co2, they bind with 02 and run through the rest of calcin cycle to produce suugar.
at is the 3rs step
The pyruvic acid is shifted back to mesophyll cells where it is used to regernate Phosphoenol pyruvate carboxylase using 2atp
what are the factors which photosynthesis depends upon
The plant factors include
the number, size, age and orientation of leaves, mesophyll cells and
chloroplasts, internal CO2
concentration and the amount of chlorophyll.
The plant or internal factors are dependent on the genetic predisposition
and the growth of the plant.
The external factors would include the availability of sunlight,
temperature, CO2
concentration and water. As a plant photosynthesises,
all these factors will simultaneously affect its rate
how do the factors affect the rate of photosynthesis
Hence, though several
factors interact and simultaneously affect photosynthesis or CO2
fixation,
usually one factor is the major cause or is the one that limits the rate.
Hence, at any point the rate will be determined by the factor available at
sub-optimal levels.
For example, despite the presence of a green
leaf and optimal light and CO2
conditions, the
plant may not photosynthesise if the temperature
is very low. This leaf, if given the optimal
temperature, will start photosynthesising
state blackman’s law of limitng factors
When several factors affect any [bio] chemical process, Blackman’s
(1905) Law of Limiting Factors comes into effect. This states the following:
If a chemical process is affected by more than one factor, then its
rate will be determined by the factor which is nearest to its minimal
value: it is the factor which directly affects the process if its quantity is
changed.
what is the factor: light
We need to distinguish between light quality, light
intensity and the duration of exposure to light,
while discussing light as a factor that affects
photosynthesis. There is a linear relationship
between incident light and CO2
fixation rates at
low light intensities. At higher light intensities,
gradually the rate does not show further increase
as other factors become limiting (Figure 11.10).
What is interesting to note is that light saturation
occurs at 10 per cent of the full sunlight. Hence,
except for plants in shade or in dense forests, light
is rarely a limiting factor in nature. Increase in incident light beyond a point causes the breakdown of chlorophyll and a
decrease in photosynthesis.
how is co2 concentration a major factor
Carbon dioxide is the major limiting factor for photosynthesis. The
concentration of CO2
is very low in the atmosphere (between 0.03 and
0.04 per cent). Increase in concentration upto 0.05 per cent can cause an
increase in CO2
fixation rates; beyond this the levels can become damaging
over longer periods.
how do c3 and c4 plants respond differently to co2
The C3
and C4
plants respond differently to CO2
concentrations. At
low light conditions neither group responds to high CO2
conditions. At
high light intensities, both C3
and C4 plants show increase in the rates of
photosynthesis. What is important to note is that the C4
plants show
saturation at about 360 µlL-1 while C3
responds to increased CO2
concentration and saturation is seen only beyond 450 µlL-1. Thus, current
availability of CO2
levels is limiting to the C3
plants.
how are c3 plants grown in greenhouse
The fact that C3
plants respond to higher CO2
concentration by
showing increased rates of photosynthesis leading to higher productivity
has been used for some greenhouse crops such as tomatoes and bell
pepper. They are allowed to grow in carbon dioxide enriched atmosphere
that leads to higher yields.
what is temperature as a factor
The dark reactions being enzymatic are temperature controlled. Though
the light reactions are also temperature sensitive they are affected to a
much lesser extent. The C4
plants respond to higher temperatures and
show higher rate of photosynthesis while C3
plants have a much lower
temperature optimum.
The temperature optimum for photosynthesis of different plants also
depends on the habitat that they are adapted to. Tropical plants have a
higher temperature optimum than the plants adapted to temperate
climates.
how does water affect photosynthesis
Even though water is one of the reactants in the light reaction, the effect of
water as a factor is more through its effect on the plant, rather than directly
on photosynthesis. Water stress causes the stomata to close hence reducing
the CO2
availability. Besides, water stress also makes leaves wilt, thus,
reducing the surface area of the leaves and their metabolic activity as well.
