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Kranz Anatomy, Practice Problems and FAQs

Kranz Anatomy, Practice Problems and FAQs

We saw a vast variety of plants around us and each plant species has its own adaptations to survive in their habitat. Can you tell me what are the physiological adaptations, which plants gained during evolution? Yes, you are correct. Desert plants have some adaptations like specialised root structures to thrive in little rainfall. They also have a waxy coating over the leaves or possess the prickly spines to reduce evaporation and resist drought.

Fig: Adaptations in cactus

Fig: Adaptations in cactus

If we consider the under canopy layer plants living in tropical rainforests, they are adapted to have large leaves to absorb more sunlight. 

Fig: Under canopy layer plants

Fig: Under canopy layer plants

Now think about the aquatic plants. They are also adapted to survive in their aquatic environment. Plants are able to compete with the adverse conditions due to physiological adaptations mainly. Many possess the aerenchyma tissues which help in buoyancy. Some plants show resistance to root rot. Some aquatic plants can thrive due to their height.

Fig: Lotus plant

Fig: Lotus plant

These many adaptations are there for a plant to live and survive in a particular environment. What do you think, only the physiological adaptations are essential for the effective survival of a plant? The answer is no. They need some anatomical adaptations along with the physiological adaptations for survival.

Just like we always try to improve the ways of tasks which we used to do, plants also try to improve the pathways used for photosynthesis. So that they can increase their productivity. The C3 pathway is the process used by the C3 plants to produce sugar in dark reactions. C3 plants are usually seen in temperate regions. But one of their drawbacks is the loss of photorespiration which is a wasteful process. Hence, to avoid this, plants adapted to another pathway called C4 pathway and those plants are called C4 plants. C4 plants did not directly evolve from C3 plants. Those C3 plants which had leaf anatomical properties for C4 pathways later evolved as C4 plants.

So for the C4 pathways to take place the anatomical adaptations are also required. One of the adaptations that happened was the Kranz anatomy observed inside the leaves. So now we are going to discuss more about the Kranz anatomy. Let’s check out how this adaptation is helping the plants for better productivity and survival.

Table of contents

  • C4 plants
  • Kranz anatomy 
  • Significance of Kranz anatomy
  • Practice Problems
  • FAQs

C4 Plants

While studying the process of photosynthesis M.D. Hatch and C.R. Slack discovered a special type of pathway in the leaves on sugarcane. This pathway was different from the C3 pathway or Calvin cycle which is carried out by most of the plants. 

Fig: Discoverers of C4 pathway

Fig: Discoverers of C4 pathway

The first stable compound of the C3 cycle was a three-carbon compound called 3-Phosphoglyceric acid or 3-PGA. But they found that in the sugar cane the first stable compound was a four carbon compound called oxaloacetic acid or OAA. Hence they called this cycle, the C4 cycle and those plants which show the C4 cycle are called C4 plants. 

The examples of C4 plants are maize, sugarcane, pearl millet, Sorghum, crabgrass, Amaranthus, Euphorbia etc. C4 cycle can be observed in both monocot plants and dicot plants which are mostly present in the tropical and subtropical regions. 

Fig: C4 Plants

Fig: C4 Plants

Requirement of C4 cycle

Greater productivity than the ancestor C3 photosynthetic type is provided by the complicated physiological adaptations in C4 photosynthesis. So what makes these C4 plants special than the C3 plants? It is the absence of photorespiration which is a wasteful process. 

The enzyme called RuBisCO, combines sugars with oxygen in the atmosphere during photorespiration instead of carbon dioxide. This was considered to be a waste of energy and also reduce sugar synthesis. There will be loss of water also in the C2 pathway. So the C4 plants have to minimise the water loss in hot and dry environments.

Hence the C4 plants have adapted a special mechanism which can deliver the CO2 directly to RuBisCO. As a result they can avail of the high rate of photosynthesis and less rate of photorespiration. The C4 biochemical pathway depends on a particular set of functional leaf characteristics known as Kranz anatomy. These include the presence of distinct compartments with varying connections to the atmosphere, intimate contact between these compartments, and a separate compartment to conduct the Calvin cycle.

Fig: Photorespiration

Fig: Photorespiration

Kranz anatomy

The leaf anatomy of C4 plants are different from the C3 plants. But how are they different? So let’s check it out.

If we consider the anatomy of leaves, the ground tissue system of leaves consists of mesophyll cells. They are of two types such as palisade parenchyma and spongy parenchyma. 

Fig: Anatomy of C3 leaf

Fig: Anatomy of C3 leaf

But in a C4 plant, the mesophyll tissues are undifferentiated and palisade tissues are absent. But spongy mesophyll tissues are present with some adaptations. 

The vascular bundles are concentrated on the veins of the leaf here. The vascular bundles have a prominent bundle sheath around it. Around the vascular bundles the mesophyll cells are present. The mesophyll cells of the C4 plants will form a ring outside the bundle sheath around the vascular bundles. The term ‘Kranz anatomy’ refers to this kind of arrangement. Kranz is a German word which means wreath or halo. The concentric layers of mesophyll cells around the bundle sheath makes it look like a wreath, hence it is called Kranz anatomy. 

Fig: Leaf anatomy of C4 plants

Fig: Leaf anatomy of C4 plants

Transfer of gases between cells in C4 leaves

The mesophyll and bundle sheath cells are connected by specialised channels known as plasmodesmata. Plasmodesmata are cytoplasmic channels connecting two plant cells. The transport of CO2 occurs between the mesophyll to the bundle-sheath cells via plasmodesmata. The cell walls of bundle sheath cells are thick and they are impervious to gaseous exchange. 

Fig: Plasmodesmata

Fig: Plasmodesmata

Dimorphic chloroplast

Bundle sheath cells and mesophyll cells have chloroplast present in them. Since the chloroplasts seen in mesophyll cells and bundle sheath cells are different they are called dimorphic chloroplasts. 

All the C4 Plants possess the dimorphic chloroplasts. Thylakoid architectures differ according to energy requirements, which causes them to display structural dimorphism. This dimorphic nature of chloroplast separates the site of evolution of oxygen from the site of CO2 fixation. The chloroplasts are divided into two according to the presence and absence of grana in them. Grana is the stalk of thylakoids where the light reactions of photosynthesis takes place. So the two types of chloroplasts are: granal chloroplasts and agranal chloroplasts. 

Granal chloroplast

Granal chloroplast is present in mesophyll cells. The chloroplast will always be small with the well developed grana here. It is specialised to perform light reactions. Oxygen is released through non-cyclic photophosphorylation.

Fig: Granal chloroplast

Fig: Granal chloroplast

Agranal chloroplast

The agranal chloroplast is present in bundle-sheath cells. This chloroplast will be large and without or with less developed grana. The bundle-sheath cells have thick walls that are impervious to gaseous exchange to prevent the entry of oxygen.

Fig: Agranal chloroplasts

Fig: Agranal chloroplasts

Since there is no grana in agranal chloroplast, they lack a photosystem II and are not involved in the Hill reaction. So we can say that in the bundle sheath cells non cyclic photophosphorylation is highly diminished and cyclic photophosphorylation is predominant. As a result no oxygen is released. But the C3 cycle or the dark reaction takes place here. 

Fig: Steps in agranal chloroplast

Fig: Steps in agranal chloroplast

Major differences between bundle sheath cells and mesophyll cells in Kranz anatomy

The following are the major differences between mesophyll cells and bundle sheath cells

Mesophyll cells

Bundle sheath cells

Well developed and large grana

Small and poorly developed grana or grana are absent

RuBP carboxylase is absent

RuBP carboxylase is present

Carry out light dependent and independent reactions 

Majorly carry out light independent reactions

High activity of photosystem II

Low activity of photosystem II

No C3 cycle 

C3 cycle occurs

Evolution of oxygen occurs

No evolution of oxygen

Thin cell walls

Thick cell walls

Absence of starch granules

Presence of starch granules

CO2 acceptor is PEP

CO2 acceptor is RuBP

Key enzymes for starch synthesis are absent

Key enzymes for starch synthesis are present

Significance of Kranz anatomy 

The following are the significance of Kranz anatomy. 

More efficient photosynthesis

Kranz anatomy enhances the efficiency of photosynthetic processes in plants. The Kranz anatomy creates a layer of light-absorbent cells around the veins in the leaves. As a result, the plant is able to produce more sugar and oxygen for utilisation by the plant.

More biomass in plants

Plants with Kranz anatomy are able to produce greater biomass than plants with other leaf arrangements. This means they are able to transform or fix more light energy into useful chemical energy.

Reduces photorespiration 

Carbon dioxide is continuously pumped into the bundle sheath cells by the mesophyll cells. As a result, there is always more carbon dioxide in the area around RuBisCO. Thus, photorespiration is reduced.

Most plants use Ribulose bisphosphate carboxylase oxygenase (RuBisCO) to fix carbon dioxide into a three-carbon molecule. RuBisCO can also catalyse a reaction with oxygen that results in photorespiration which is a wasteful process. To get rid of this, the C4 pathway uses the enzyme phosphoenolpyruvate carboxylase to fix atmospheric carbon dioxide. 

Practice Problems

Q1. C4 plants do not exhibit photorespiration because ________________.

A. of enzyme RuBisCO
B. the first primary acceptor is phosphoenolpyruvate
C. they do not require CO2
D. they have a mechanism that makes the CO2 concentration at the enzyme site higher

Solution: C4 plants do not undergo photorespiration because of a process that raises the CO2 concentration at the enzyme site. Malic acid is eventually produced in the mesophyll cells as part of the C4 pathway through carbon fixation. In the bundle sheath, the malic acid is broken down to release CO2. The bundle sheath cells do not allow CO2 to escape because of their thick walls. Due to this, the concentration of CO2 inside the cells rises. As a result, photorespiration is avoided, because the RuBisCO in the bundle sheath cells works as a carboxylase rather than an oxygenase. Hence the correct option is d. 

Q2. Which of the following plants exhibits Kranz anatomy?

A. Rice
B. Banyan
C. Potato
D. Sugarcane

Solution: In C4 plants, such as sugarcane, the Kranz anatomy is visible. Kranz anatomy is a unique structure found in C4 plants, where the mesophyll cells are arranged in a ring-like pattern around the bundle sheath. The existence of the C4 pathway and the Kranz anatomy in C4 plants raises the CO2 levels near the RuBisCO enzyme and prevents photorespiration. Rice, banyan, and potato are C3 plants. Hence the correct option is d.

Q3. In which parts of C4 plants does the Calvin cycle occur?

A. Bundle sheath
B. Mesophyll cells
C. Phloem
D. Epidermal cells

Solution: Calvin cycle process happens in mesophyll cells in C3 plants as opposed to the bundle sheath cells in C4 plants. Phosphoenolpyruvate (PEP) accepts CO2 in the mesophyll cells of C4 plants. RuBisCO is not present in mesophyll cells. When PEP is exposed to CO2, PEP is transformed into oxaloacetic acid in the presence of the enzyme phosphoenolpyruvate carboxylase (PEPcase). Malic acid is then generated from oxaloacetic acid. The bundle sheath receives the malic acid. Malic acid can pass through the plasmodesmata that exist between the bundle sheath cells and the mesophyll cells. The RuBisCO enzyme is abundant in the bundle sheath cells, while PEPcase is absent. In the bundle sheath cells, malic acid undergoes decarboxylation and emits CO2. After being released, the CO2 enters the C3 cycle. Hence the correct option is a.

Q4. The bundle sheath cells in Kranz anatomy have ________________.

A. several chloroplasts, thick walls, and no intercellular spaces
B. thick walls, no chloroplasts, and no intercellular spaces
C. many intercellular spaces, many chloroplasts, and thin cell walls
D. no chloroplasts, many intercellular spaces, and thin cell walls

Solution: The bundle sheath cells of C4 plants exhibit a unique type of anatomy known as Kranz anatomy, which is characterised by thick walls, no intercellular spaces, and an abundance of chloroplasts. These cells form many layers around the vascular bundles. Between the bundle sheath cells, there are no intercellular spaces and thick walls that block the passage of CO2 and oxygen. Hence the correct option is a. 

FAQs

Question 1. Do C4 plants withstand high temperatures?
Answer: Even though the optimum temperature of C4 plants is higher than that of C3 plants, photosynthesis is typically inhibited when leaf temperatures rise above roughly 38°C. 30 - 40oC is the ideal temperature for C4 plants. This occurs due to solarisation. The inhibition of photosynthesis due to photo-oxidation of chlorophylls at higher intensities is commonly called solarisation.

Question 2. What type of evolution is applicable to C4 plants?
Answer: C4 carbon fixation is a prime example of convergent evolution because it has evolved up to 61 times independently in 19 different plant groups. Convergent evolution is the autonomous development of comparable traits in species from several epochs or eras of time. Convergent evolution produces homologous structures that have a similar form or function but were absent from those groups' most recent common ancestor. This convergence might have been facilitated by the existence of numerous possible evolutionary routes leading to the C4 phenotype, many of which entail early evolutionary events unrelated to photosynthesis. C4 plants first appeared in the Oligocene, 35 million years ago, but did not reach ecological significance until the Miocene, 6 – 7 million years ago.

Question 3. Can we convert a C3 plant into a C4 plant?
Answer: Yes, there is a possibility that a C4 plant could develop from a C3 plant inside a lab. Given the benefits of C4, a team of researchers from many institutions are collaborating on the C4 rice project to create a breed of rice that is unavoidably a C3 plant. According to the scientists, C4 rice might produce up to 50% more grain while using fewer water and nutrients. The development of a prototype C4 rice plant is being pursued by the researchers, who have already discovered the genes required for C4 photosynthesis in rice.

Question 4. Without Kranz anatomy, can plants carry on C4 photosynthesis?
Answer:  Bienertia cycloptera from Chenopodiaceae family is an example of a plant that exhibits C4 photosynthesis without Kranz anatomy. This unique plant, which thrives in the salty depressions of the Central Asian semi-deserts, lacks Kranz anatomy yet possesses C4 plant-like photosynthetic characteristics.

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Related Topics

Photorespiration, Practice Problems and FAQs

The Calvin cycle, Practice Problems and FAQs

Dual nature of RuBisco and CAM pathway, Practice Problems and FAQs

The C4 pathway, Practice Problems and FAQs

Dark reaction and Significance of 6 Calvin cycles, Practice Problems and FAQs

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