Chapter 20, Problem 23P

Interpretation Introduction

(a)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Photosynthesis is the process by which plants store energy of sunlight as chemical energy. Process happens in the chloroplast of a plant cell. Photosynthesis happens in two main stages - light reaction and dark reaction.

Answer to Problem 23P

a) Calvin cycle − the dark reactions

Explanation of Solution

Photosynthesis happens via two main stages as light reaction and dark reaction. Light reaction happens in the presence of sunlight. Two photosystems called PS I and PS II absorbs energy of sun light. At the end of light reaction ATP, NADPH and O₂ produce. Next, the dark reaction (which is also known as Calvin cycle) starts which uses ATP and NADPH.

For the dark reaction there is no need of sunlight which happens within the stroma. This is a series of chemical reactions. During the dark reaction, initially CO₂ fixation is catalyzes by an enzyme called RUBISCO (Ribulose 1,5 bisphosphate carboxylase / oxygenase). The fixed CO₂ is converted to glucose. For this reaction to happen ATP provides the energy and NADPH provides the necessary electrons. This cycle is known as Calcin cycle.

Figure 1: Calvin cycle

Interpretation Introduction

(b)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Photosynthesis is the process by which plants store energy of sunlight as chemical energy. Process happens in the chloroplast of a plant cell. Photosynthesis happens in two main stages - light reaction and dark reaction.

Answer to Problem 23P

b) Rubisco − catalyzes CO₂ fixation

Explanation of Solution

Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase), is an enzyme used in the dark reaction of photosynthesis. It catalyzes the carbon fixation process where atmospheric carbon dioxide is converted to glucose by photosynthetic organisms..

The enzyme is capable of fixing both CO₂ and O₂. This enzyme is comparatively less efficient at low CO₂ concentrations and tends to bind O₂. This series of reactions start with O₂ to finally produce CO₂. The process is known as photorespiration.

Interpretation Introduction

(c)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Rubisco activase enzyme is important in activating Rubisco. Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), is an enzyme used in the dark reaction of photosynthesis. It catalyzes the carbon fixation process where atmospheric carbon dioxide is converted to glucose.

Answer to Problem 23P

c) Carbamate − required for the rubisco activity

Explanation of Solution

In plants and some types of green algae, an enzyme called RuBisCO activase is present. This is required to allow the rapid formation of carbamate in the active site of rubisco. RuBisCO activase is required because the ribulose 1,5-bisphosphate (RUBP) substrate binds more strongly to the active sites lacking carbamate and hence slows down the activation process.

Interpretation Introduction

(d)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Starch is a polymeric carbohydrate consist of a large number of glucose monomers joined by glycosidic bonds. So, glucose is the primary unit of starch. Starch is used by most green plants for energy storage.

Answer to Problem 23P

d) Starch − storage form of carbohydrates

Explanation of Solution

Starch is a polymeric carbohydrate produced by joining a large number of glucose units via glycosidic bonds.

Figure 2: Structure of starch

After plants produce glucose by photosynthesis, it is stored in the cells as starch. Starch is insoluble in plant cell. So, unlike starch glucose increases the concentration of the plant cell.

Interpretation Introduction

(e)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Sucrose is a disaccharide of Glucose and fructose, having twelve carbon atoms. Disaccharide consists of two monosaccharides, having six carbon atoms each.

Answer to Problem 23P

a) Sucrose − transport form of carbohydrates

Explanation of Solution

Sucrose consists of a glucose molecule and a fructose molecule joined through a glycosidic bond.

Figure 3: Structure of Sucrose

Plants store glucose (produced in photosynthesis) as starch in different places such as in fruits, seeds, and roots. Starch is insoluble in water and cannot be transported. When the plant needs energy, starch is converted to sucrose and transported. Even though glucose is soluble in water, plant does not convert starch to glucose. Since glucose is highly reactive compared to sucrose, it can transform into other intermediate compounds during transportation. Once it reaches the destination, sucrose is converted to glucose.

Interpretation Introduction

(f)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Amylose is a polysaccharide made up of glucose units. These glucose molecules are bonded to each other through a (1→4) glycosidic bonds. Due to its tightly packed helical structure, amylose is resistant to digestive enzymes and therefore known as a resistant starch.

Answer to Problem 23P

b) Amylose - $\alpha \text{ - 1, 4}$ linkages only

Explanation of Solution

As indicated in the figure, the first and fourth carbon of the glucose monomers react to form a bond between them. This bond is known as 1, 4 glycosidic bond.

Figure 4: Structure of amylose

Interpretation Introduction

(g)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Amylopectin is a highly branched polymer of glucose.

Answer to Problem 23P

c) Amylopectin − includes $\alpha \text{ - 1,6}$ linages

Explanation of Solution

Amylopectin is a highly branched polymer. This is also known as polysaccharide. The primary unit is glucose. Like amylose, the glucose units are linked in a linear way with a (11→4) glycosidic bonds. This linear polymer consists of branching made up of a (11→6) bonds. Comparing to amylose (consists of only a (11→4) bonds), amylopectin takes more time to hydrolyze and solubilize

Figure 5: Structure of amylopectin

Interpretation Introduction

(h)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Photosynthesis is the process by which plants store energy of sunlight as chemical energy. Process happens in the chloroplast of a plant cell. Photosynthesis happens in two main stages - light reaction and dark reaction. The dark reaction consists either of the two metabolic pathways, that is, C₃ and C₄ cycle.

Answer to Problem 23P

d) C₃ plants − 3 − Phosphoglycerate is formed after carbon fixation

Explanation of Solution

The dark reaction of photosynthesis consists of two metabolic pathways, that is, C₃ and C₄ cycle. All the green plant uses one of these two pathways. The nomenclature is based on the number of carbon atoms present in the first metabolite produced after CO₂ fixation.

The first metabolite produced in C₃ cycle composed of three carbons while a four carbon metabolite is produced in C₄ cycle.

The light reaction is common to both C₃ and C₄ cycle. But the type of enzymes and intermediate produced during the dark reaction are different.

RUBP (Ribulose Bisphosphate) carboxylase is the enzyme of C₃ cycle which fixes CO_2, present in the atmosphere. After Co₂ fixation, a five-carbon molecule, RUBP (Ribulose bisphosphate) reacts with CO₂ to form a six-carbon complex. The six-carbon molecule break immediately and form two 3- phosphoglycerate. 3- phosphoglycerate consists of three carbon atoms.

So, this is the first stable product of CO₂ fixation. This is the reason why the mechanism is known as C₃ cycle.

Figure 6: Calvin cycle of C₃photosynthesis representing the carbon fixation.

Interpretation Introduction

(i)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Photosynthesis is the process by which plants store energy of sunlight as chemical energy. Process happens in the chloroplast of a plant cell. Photosynthesis happens in two main stages - light reaction and dark reaction. The dark reaction consists either of the two metabolic pathways, that is, C₃ and C₄ cycle.

Answer to Problem 23P

e) C₄ plants − carbon fixation results in oxaloacetate formation

Explanation of Solution

The dark reaction of photosynthesis consists of two metabolic pathways, that is, C₃ and C₄ cycle. All the green plant uses one of these two pathways. The nomenclature is based on the number of carbon atoms present in the first metabolite produced after CO₂ fixation.

As explained earlier, the first metabolite produced in C₃ cycle composed of three carbons while a four-carbon metabolite is produced in C₄ cycle.

Out of the two steps of photosynthesis (light and dark reaction), light reaction is common. But the type of enzymes and intermediate produced during the dark reaction are different.

PEP (Phosphoenolpyruvate) carboxylase is the primary enzyme of C₄ mechanism which catalyzes the fixation of CO₂ in chloroplast. For this, a three-carbon molecule (PEP) reacts with CO₂ to form a four-carbon molecule which is known as oxaloacetate. Oxaloacetate is the first stable product of C₄ cycle.

This is the reason why the mechanism is called C₄ mechanism. The reactions take place in mesophyll cells.

Interpretation Introduction

(j)

Interpretation:

The most suitable matching pair of answers should be selected.

Concept introduction:

Stomata are pores, found in the epidermis of leaves, stems, and other organs that facilitate gaseous exchange.

Answer to Problem 23P

f) Stomata − allow exchange of gases

Explanation of Solution

Stomata are the pores which are mostly found in the epidermis of leaves and stems. It facilitates gaseous exchange. This stomatal pore is bordered by the guard cells which regulates the stomatal opening and closing.

Atmospheric air consists of carbon dioxide and oxygen enters the plant through these openings. These gases are crucial for photosynthesis and respiration, respectively. Also, water vapor releases to the atmosphere through the stomata. The process is known as transpiration.

So, stomata are the structures, present in the plant leaves crucial for all types of gaseous exchanges.

Figure 8: Microscopy view of stomata