Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
In which type of cells, C4 acids in maize are broken down to release CO2 and 3-carbon molecule?
1. In bundle sheath cells of C3 plant
2. In mesophyll cells of C4 plants
3. In bundle sheath cells of C4 plants
4. In mesophyll cells of C3 plant
Answer:(3)
In bundle sheath cells of C4-plants e.g. Maize C4-Acid (Malic acid) broken down to release CO2 and 3-carbon molecule i.e., pyruvate
Bundle Sheath
👉Monocotyledonous Stem:
The monocot stem has a sclerenchymatous hypodermis, a large number
of scattered vascular bundles, each surrounded by a sclerenchymatous
bundle sheath, and a large, conspicuous parenchymatous ground tissue.
Vascular bundles are conjoint and closed. Peripheral
vascular bundles are generally smaller than the centrally located ones.
The phloem parenchyma is absent, and water-containing cavities are
present within the vascular bundles.
👉Dorsiventral (Dicotyledonous) Leaf
- Dorsiventral leaf
- Three parts: Epidermis, Mesophyll, vascular bundle
- Epidermis : Upper surface=Adaxial epidermis, Lower surface= Abaxial epidermis
- conspicuous cuticle @ lower surface(Abaxial)
- More stomata @ Adaxial surface
- Mesophyll= between adaxial and abaxial epidermis
- Mesophyll carry photosynthasis
- Mesophyll parenchymatous cell
- Mesophyll= Pallisade+ spongy
- pallisade @ adaxial surface, elongated cells
- Spongy @ abaxial surgace, oval & round
- Reticulate venation
- The vascular bundles are surrounded by a layer of
- thick walled bundle sheath cells.
👉THE C4 PATHWAY
- Dry tropical area
- C4 Oxaloacetic acis : 1st CO2 fixation product
- High temperature
- High light intensity
- 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
👉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.
👉C4 plants – maize or sorghum
👉The presence of the bundle sheath would help you identify
the C4 plants.
👉Now study the pathway shown in Figure 13.9.
This pathway that has
been named the Hatch and Slack Pathway, is again a cyclic process. Let
us study the pathway by listing the steps.
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 is formed in the mesophyll cells.
It then forms other 4-carbon compounds like malic acid or aspartic
acid in the mesophyll cells itself, which are transported to the bundle
sheath cells. In the bundle sheath cells these C4 acids are broken down
to release CO2 and a 3-carbon molecule.
The 3-carbon molecule is transported back to the mesophyll where it
is converted to PEP again, thus, completing the cycle.
The CO2 released in the bundle sheath cells enters the C3 or the Calvin
pathway, a pathway common to all plants.
The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase
(RuBisCO), but lack PEPcase.
👉Did you note that the Calvin pathway occurs in all the mesophyll
cells of the C3 plants? In the C4 plants it does not take place in the
mesophyll cells but does so only in the bundle sheath cells.
👉In C4 plants photorespiration does not occur. This is because they
have a mechanism that increases the concentration of CO2 at the enzyme
site. This takes place when the C4 acid from the mesophyll is broken
down in the bundle sheath cells to release CO2 – this results in increasing
the intracellular concentration of CO2. In turn, this ensures that the
RuBisCO functions as a carboxylase minimising the oxygenase activity.
👉Some tropical plants show a special type of photosynthesis called C4 pathway.
In these plants the first product of CO2 fixation that takes place in the mesophyll,
is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried
out for the synthesis of carbohydrates.
