The major difference between C3 and C4 plants lies in internal leaf anatomy and biochemical pathway, not in obvious external morphology. C4 plants show Kranz anatomy and a CO2-concentrating mechanism, which require anatomical or physiological study.
No, a plant generally cannot be identified as C3 or C4 by external appearance alone.
C4 leaves have large bundle sheath cells arranged around vascular bundles like a wreath. These bundle sheath cells have many chloroplasts, thick walls and no intercellular spaces. C3 leaves lack such prominent Kranz anatomy, and the Calvin cycle occurs in mesophyll cells.
By observing leaf anatomy, especially the presence or absence of Kranz anatomy.
In C4 plants, initial CO2 fixation by PEP carboxylase occurs in mesophyll cells and C4 acids carry CO2 to bundle sheath cells. There, CO2 is released near RuBisCO, so RuBisCO functions mainly as carboxylase. This suppresses oxygenase activity and photorespiration, saves ATP/carbon loss, supports high light and temperature tolerance and increases productivity.
C4 plants are highly productive because they concentrate CO2 in bundle sheath cells and minimise photorespiration.
C4 acids formed in mesophyll cells are transported to bundle sheath cells and broken down to release CO2. This increases local CO2 concentration at the RuBisCO active site and minimises oxygenase activity.
In C4 plants, CO2 concentration around RuBisCO is high in bundle sheath cells, so carboxylation is favoured over oxygenation.
Chlorophyll a forms the reaction centre molecules P700 and P680 and directly participates in photochemical reactions. Chlorophyll b, xanthophylls and carotenoids are accessory pigments: they absorb additional wavelengths, transfer energy to chlorophyll a and protect chlorophyll a from photo-oxidation. They broaden the usable light spectrum but cannot replace chlorophyll a as the main reaction-centre pigment.
Such plants would not carry out normal photosynthesis because chlorophyll a is the chief reaction-centre pigment.
Chlorophyll synthesis and maintenance depend on light. When a leaf is kept in darkness, chlorophyll breaks down, causing the green colour to fade. Carotenoids/xanthophylls persist longer, giving yellow or pale green colour.
In darkness, chlorophyll degrades and is not renewed, so yellow carotenoid pigments become visible; carotenoids are more stable.
(a) In C3 plants, RuBP is the CO2 acceptor, first product is 3-PGA, Calvin cycle occurs in mesophyll cells and photorespiration is significant. In C4 plants, PEP is the first CO2 acceptor, first product is OAA, initial fixation occurs in mesophyll cells, Calvin cycle occurs in bundle sheath cells and photorespiration is negligible. (b) Cyclic photophosphorylation uses PS I only, returns electrons to PS I and produces ATP only. Non-cyclic photophosphorylation uses PS II and PS I, electrons come from water and finally reduce NADP+, producing ATP, NADPH and O2. (c) C3 leaves lack Kranz anatomy; C4 leaves have prominent bundle sheath cells with many chloroplasts around vascular bundles.
C3 and C4 pathways differ in first CO2 acceptor/product, site of Calvin cycle and photorespiration; cyclic and non-cyclic photophosphorylation differ in photosystems used and products; C3 and C4 leaves differ in Kranz anatomy.