Biology: An Interactive Tour
Biology: An Interactive Tour

Biology: An Interactive Tour

Lead Author(s): Robert Pozos

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Biology: An Interactive Tour is for a non-majors audience in technology-enhanced learning and makes the complex world of biological science approachable and relatable.

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Biology: An Interactive Tour introduces the content in a much more approachable way than traditional texts.

Includes homework sets with 30+ questions per chapter.

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Comparison of Introduction to Biology Textbooks

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Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Pricing

Average price of textbook across most common format

Up to 40-60% more affordable

Lifetime access on any device

$63

E-book

$126

Hardcover print text only

$117

E-book

$166.95

Hardcover print text only

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

In-Book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Only available with supplementary resources at additional cost

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Built-In Interactive Assessment Questions

Assessment questions with feedback embedded throughout textbook

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Pricing

Average price of textbook across most common format

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Up to 40-60% more affordable

Lifetime access on any device

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

$63

E-book

$126

Hardcover print text only

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

$117

E-book

$166.95

Hardcover print text only

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Only available with supplementary resources at additional cost

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

About this textbook

Lead Author

Robert PozosUniversity of Minnesota-Duluth School of Medicine

Robert Pozos has extensive experience studying human response to environments resulting in hypothermia and hyperthermia. He established the hypothermia laboratory at University of Minnesota-Duluth School of Medicine and was a part of the chief civilian scientists at Naval Health Research Center in San Diego where he evaluated the thermal effectiveness of military garb for combat operations.

Contributing Authors

Christina AlevrasUniversity of Saint Joseph

Marion McClaryFairleigh Dickinson University

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Chapter 10: Photosynthesis 

Figure 10.1: Plants use photosynthesis to generate metabolic products and in the process release oxygen. [1]

Have you ever wondered about the role that plants have in our world? We take them for granted until a special holiday when we send and receive them as gifts or when we see an exotic jungle or a forest dressed in fall colors. Yet, they serve a greater purpose in our lives than just decorations. They produce the oxygen required for animal survival as well as medicines and drugs. A majority of the world’s population derives their food from plants. In addition, plants are our greatest allies in the fight against climate change and in this chapter, we will learn about their greatest weapon—photosynthesis.   

Concept Map

This chapter is divided into the four different sections as shown in the concept map below.

Photosynthesis: This concept map provides you with a tour of four major concepts dealing with this topic

10.1 Plant Cell Structures

Concept 10.1: Plant cell structures house light and dark reaction.

The term “photosynthesis” is somewhat misleading. It literally means “making light,” which plants do not do. Photosynthesis is a set of chemical reactions that occur in plants to produce energy for the plant in the form of ATP, which is used to power all the anabolic and catabolic activities of the plant cell. What is most remarkable about this process is that the plant cell is able to support anabolic and catabolic activities by initially starting with carbon dioxide, water, and sunlight. It is the ultimate solar gathering system. The ability of plants to take certain wavelengths of sunlight, carbon dioxide, and H20 and subsequently use them to produce all of its chemical needs is a remarkable chemical feat and is the basis of complex biological webs.

Plants and animals are both eukaryotes, therefore, they share many similarities. Both have cytoplasm, Golgi apparatus, mitochondria, smooth and rough endoplasmic reticulum, microtubules, nucleus, and nucleolus. It is commonly thought that plants evolved first since they produce the oxygen needed for oxidative phosphorylation and also decrease the amount of carbon dioxide in the atmosphere. Plants were able to change the concentration of oxygen and carbon dioxide in the atmosphere. Initially, the atmosphere did not have much oxygen until the advent of plants which led to the evolution of animals that were supported by oxidative phosphorylation. Plant and animals have certain characteristics in common suggesting a common ancestor as well as different characteristics. 

Figure 10.2: Plants and animals are both eukaryotes who evolved from a common ancestor

The most important and distinct characteristic of plants is that they have chloroplasts. Chloroplasts synthesize carbohydrates, proteins, and lipids by capturing certain frequencies of sunlight and converting to energy. Then, the energy splits water into oxygen and hydrogen. The hydrogen ion is the key element that powers ATP synthase to make ATP, which is used for the synthesis of all materials for plant cell survival.

Figure 10.3. Differences and similarities between plant and animal cells

Cellular respiration (oxidative phosphorylation) in animal cells depends on photosynthesis in plant cells. Plant cells capture the energy from the sun and subsequently provide organic products and oxygen to animal cells. Animal cells, as well as decaying plant products, produce carbon dioxide, which is required by plants for making organic products in conjunction with the production of ATP.

Figure 10.4: The Animal and Plant worlds interact. The plant cell has chloroplasts which produce metabolic products such as glucose and oxygen that are subsequently used by the animal cells' mitochondria to produce metabolic products such as carbon dioxide that is used by the plant.

The chemistry in this section is similar to what was presented in Chapter 7 dealing with oxidative phosphorylation. In oxidative phosphorylation, hydrogen ion and its associated electron generate ATP by way of the electron transfer system and ATP synthase. Each of these major reactions occurs in specific parts of the mitochondria. The same occurs with plants. Plants use the same strategy in photosynthesis as in oxidative phosphorylation in that they use electrons and hydrogen ions to produce ATP. The difference is that the electrons and hydrogen ions originate from the water. Plants also have an electron transport system and ATP synthase to generate ATP similar to the mitochondria. 

Overall, photosynthesis results in the production of energy-rich products such as carbohydrates, proteins, lipids utilizing water, and light. Light enters the cell by way of interacting with the chlorophyll molecules. The anatomy of the leaf is important in understanding how plants survive. A specialized structure in plant leaves called the stomata play a major role in both water and gas exchange. Stomata are openings that are controlled by small cells called guard cells which open and close the stomata based on environmental factors. The plant requires carbon dioxide which enters through the stomata, and it releases oxygen which leaves through the stomata.

Figure 10.5: Stomata in the leaf: The entry point for carbon dioxide and the exit point for oxygen and water


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Water and minerals which are essential for photosynthesis are transferred up from the roots. The water route is from the soil to roots to stems to leaves in land plants. Approximately 1% of water is used for photosynthesis with the rest being evaporated. This evaporation which is called transpiration occurs chiefly in the leaves while their stomata are open for the passage of carbon dioxide and oxygen during photosynthesis. In some ways, transpiration may be considered to be a form of sweating in that it is more active with higher environmental temperatures. The overall reaction for photosynthesis involves the capture of carbon dioxide to make carbohydrates and other products for the plant to us. The energy for the production of these organic compounds is ATP, which is formed when the chlorophyll captures the wavelengths of light and splits water into oxygen and hydrogen. It is the electrons and hydrogen ions that power the processes to make carbohydrates and other compounds that the plant uses. 

There are two major processes involved in photosynthesis which are contained in one structure called the chloroplast. The first one, called the light reaction, is the capture of energy from the sun. This involves photons from the wavelengths of light activating the photosensitive pigment such as chlorophyll to energize electrons. This step is the secret of life on earth. The light reaction involves the excited electrons producing two products needed for the production of products needed by the plant. These products are ATP and NADPH. However, to make these two products, chlorophyll takes the excited electron and splits waters which then produces oxygen. The light reaction is housed in specialized cells organelles called the thylakoids. The second process called the dark reaction, also called the Calvin cycle, takes the ATP and NADPH and captures carbon dioxide from the atmosphere to make various organic compounds. The Calvin Cycle attaches carbon dioxide to other carbon molecules by a process called carbon dioxide fixation. The reason it is called the dark reaction is that light is not needed for carbon dioxide fixation. It occurs in the stroma of the cell. 

Biological life is replete with many examples of how plants have been able to survive and thrive in different ecological niches ranging from arid to tropical zones. Most importantly, plants are now considered one of the weapons to be used in decreasing the increasing carbon dioxide levels in the atmosphere. Protection of jungles, forests and other areas of vegetation is important to minimize the increases in atmospheric carbon dioxide. 

Figure 10.6: The chloroplast is an organelle that contains light and dark reactions which have different locations and functions in the cell.


Question 10.02

Which of the following captures photons from wavelengths of light?

A

Light Reaction

B

Calvin Cycle

C

Both Light Reaction and Calvin Cycle

D

Chlorophyll in the mitochondria of the plant


10.1.1 Plant Cell Anatomy: The Home of Photosynthesis

The plant cell is the functional unit of plants. It differs from animal cells in that it has a cell wall in addition to a plasma membrane. It serves a variety of functions from protecting the cell to regulating the life cycle of the plant. In addition, plant cells have chloroplasts which are the structures that have the chemical structure (chlorophyll) that converts light energy into chemical energy. The chlorophyll is located in the membrane of sacs called thylakoids.

Figure 10.7: The granum in the chloroplasts have the thylakoids in which the light reaction occurs, and the stroma is where the dark reaction (Calvin Reaction) occurs.

All of the green structures in plants, including stems and un-ripened fruit, contain chloroplasts, but the majority of photosynthesis activity in most plants occurs in the leaves. On an average, the chloroplast density on the surface of a leaf is about one-half million per square millimeter.

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The chloroplast is the organelle responsible for photosynthesis. It is very similar to the mitochondrion in structure. It contains a permeable outer membrane, a less permeable inner membrane, an intermembrane space, and an inner section a fluid-filled matrix called the stroma. In addition, the chloroplasts like the mitochondrion have its own unique DNA separate from that of the plant nucleus. However, there are differences between plant and animal cells. The chloroplast is larger than the mitochondrion since its membrane is not folded into cristae. Also, the inner membrane is not used for the electron transport chain. The chloroplast has stacks of flattened membrane systems each one of which looks like pouches that contain the light-absorbing system(chlorophyll), the electron transport chain, and ATP synthase in an inner membrane compartment. These pouches are called thylakoids which means "pouch like". Each thylakoid stacked on top of each other to form a granum. There are a large number of granum in the leaf, which are collectively called grana in plural form.

Figure 10.8: Leaf(1) has the chloroplast(2) which has the grana(3) that are composed of thylakoids where (light reaction of photosynthesis) occurs in its membranes. The Dark Reaction(Calvin cycle) occurs in the (4)Stroma.

The reason that this anatomy is important is that the light reaction occurs in the membrane of the thylakoid. The structure shown in Figure 10.9 is repeated on the membrane. 

Figure 10.9. The membrane of the thylakoid has the photosynthetic pigments and ATP synthase. Notice the hydrogen ions inside the thylakoid space which will be used to power ATP synthase.​


Question 10.04

The oxygen that animals use for oxidative phosphorylation arises from which of the following?

A

Light reaction in which oxygen is derived directly from sunlight

B

Dark reaction in which oxygen is derived from carbon dioxide

C

Light reaction in which oxygen is derived from the breakup of water

D

Dark reaction in which ATP is generated directly from sunlight


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10.2 Light Reaction

Concept 10.2: Light reaction results in oxygen, ATP, and NADPH.

So how does photosynthesis occur in a plant? Van Helmont (1588-1644) took a tree, measured its length as a sapling and the amount of soil that it was in, gave it water, and monitored its weight over a five year period of time. At this point, it weighed 170 lbs., but the amount of soil lost was only two ounces. The conclusion of the experiment was that the plant grew using water since the amount of soil did not change. This experiment demonstrates how difficult it was to understand the process of photosynthesis since Van Helmont was not aware of the gases in the atmosphere that also played a role in the growth of the tree. In some ways, studying animals is easier than studying plants since animals have a quick reaction if there is not enough oxygen. However, with plants, they do not respond in the same manner and in many ways their changes seem invisible to us.

10.2.1 The Light Reaction or Light-Dependent Phase: The Role of the Chlorophyll Molecule

Photosynthetic pigments are divided into two groups: carotenoids and chlorophylls. Carotenoids absorb light energy and pass it on to the chlorophyll molecules that convert light into chemical energy. These two molecules work together in that carotenoids protect the chlorophylls from photo (light) damage, acting like sunglasses for chlorophylls. These pigments absorb different wavelengths of light; chlorophylls absorb blue and red wavelengths, whereas carotenoids absorb red and green wavelengths. There are six chlorophyll molecules ranging from chlorophyll a to chlorophyll f. Of these, chlorophyll a and b are found in land plants and green algae.

Figure 10.10: Pigments in plants: Plants have many different pigments involved in different functions.

Chlorophyll gives plants their green color, but more importantly, it captures photons from the various wavelengths of light coming from the sun. It has a major role in capturing a small part of the sun's energy, which is then transferred to all living systems in the world. The chlorophyll molecule has two components: the head and tail. The head contains the light absorbing porphyrin ring while the tail attaches to the thylakoid membrane.

Figure 10.11: Chlorophyll molecule is embedded in the thylakoid membrane. Its head is the light absorbing site.​

The color of plants is determined by their photosynthetic pigments. The pigments absorb the various wavelengths of light to activate the electrons, which is the first steps in the photosynthetic process. Those wavelengths are absorbed by the pigment and those wavelengths not absorbed are the ones that we see. Thus, the green color of plants indicates that the pigment is absorbing other wavelengths of light to conduct photosynthesis. There are different kinds of energy capturing pigments, but the two most important are chlorophyll a and b. They absorb light in the blue and red wavelengths, and as such, they emit the green color. In addition, chlorophyll a and b have slightly different absorbing wavelengths, so that they can utilize the lights effectively. 

Figure 10.12: Wavelengths of light that are captured by different chlorophyll molecules. Notice that wavelengths in the green frequency range are not captured by the pigments resulting in the plant leaf looking green

A chlorophyll molecule has a hydrophobic “tail" that inserts the molecule into the thylakoid membrane. The "head" of a chlorophyll molecule has a ring structure called a porphyrin.

The key point about the wavelengths of light is that they are composed of photons. These photons, when they interact with photopigments, activate electrons in the photopigment molecule in photosystem II. The electron traverses a number of photopigment molecules until it reaches a reaction center. The reaction center is where the electron is activated and where light energy is converted into chemical energy.

Figure 10.13: Activation of chlorophyll results in the activation of an electron in the reaction center. ​

As a result of the light, the photopigment in photosystem II (PSII) has excited electrons which act as an oxidizing agent. The electrons oxidize water, splitting it into hydrogen, oxygen and an electron.

Figure 10.14. Splitting of Water: Excited electrons will split water into electrons, oxygen, and protons (hydrogen ions). (Faded area shows the fate of electrons, oxygen and protons)

Three specific outcomes occur due to the breakup of water by the excited electron. Oxygen leaves the cell; hydrogen stays in the stroma to power ATP synthase and the electron moves onto the electron transport system. Figure 10.15 shows the diffusion of oxygen from the plant into the atmosphere. This is the oxygen that we breathe. It is derived from the splitting of water. 

Figure 10.15: Splitting of water results in oxygen being released into the atmosphere. (Faded area shows what will happen to electrons and hydrogen ions)

The electrons will go to an electron transfer center which will be activated so that hydrogen will also enter the stroma. (This hydrogen is not from the splitting of water!) Thus there will be an abundance of hydrogen ions inside the stroma coming from two sources: 

  • The breakup of water
  • The movement of the electron through the electron transport system
Figure 10.16: Excited electrons promotes movement of hydrogen ion into Thylakoid interior. (Faded area shows the fate of electrons)

The hydrogen ions from water and electron transport system will activate ATP synthase to make ATP. Thus chemiosmosis will occur. The ATP will power reactions in the Calvin Cycle.

Figure 10.17: Hydrogen ions will power ATP production which will be used in the Calvin Cycle. (Faded areas show the steps of hydrogen production that preceded ATP synthase activation)

The electrons are not through with their journey. They will be activated again by sunlight as they enter a secondary pigment (photosystem I, PSI) and this time they will cause NADP to become NADPH. Why is this important? If the electrons are not able to complete their journey from the pigment of PSII to NAPDH, there will not be enough hydrogen in the stroma to activate ATP synthase. In some ways, NADP is similar to oxygen's function in the mitochondria. Oxygen must accept electrons so that hydrogen ion will activate our ATP synthase. In plants, NADP must accept the electrons so Hydrogen ions will also activate ATP synthase. The NADPH will power the production of carbohydrates in the Calvin Cycle.

Figure 10.18: NADPH will power reactions in the Calvin Cycle. (Faded areas show the steps involved in the other aspects of photosynthesis)


Question 10.07

Photons of light will do the following?

A

Directly produce ATP

B

Activate electrons in the chlorophyll molecule

C

Directly cause the splitting of water into hydrogen and oxygen

D

Directly cause the production of NADPH


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Question 10.09

The oxygen that you breath is a product of?

A

Light Reaction

B

Dark Reaction

C

Oxidative phosphorylation

D

Glycolysis


10.2.2 New and Old forms of Photosynthesis: Non-cyclic and Cyclic Photophosphorylation

There are two distinct forms of photosynthesis that utilize the above approach: Cyclic phosphorylation and noncyclic photophosphorylation. The term photophosphorylation refers to adding a phosphate group to ADP by way of light activation of pigments. Cyclic photophosphorylation is the older form of photosynthesis and is found in bacteria, whereas the noncyclic photophosphorylation is found in all other plants.

In noncyclic photophosphorylation, light activates Photosystem II first and subsequently Photosystem I. Photosystem II, when activated by sunlight, separates hydrogen and oxygen from water. This is called splitting water. This process involves the extraction of four electrons and four protons from two molecules of water. Oxygen is expelled into the atmosphere and hydrogen ions are placed in the thylakoid space to power ATP synthase by chemiosmosis. Recall that chemiosmosis is the movement of hydrogen ions through ATP synthase which also occurs in mitochondria.

Figure 10.19: Non -Cyclic Photosynthesis: Light Reaction


Question 10.10

Place the following steps of noncyclic photophosphorylation in order.

A

NADP+ turns into NADPH

B

Light energy and H2O activates chlorophyll

C

Electrons go from chlorophyll to the electron transport chain

D

ATP is created

The light reaction results in the breakup (splitting) of water and generates excited electrons, which will be used to generate NADPH. The hydrogen ion resulting from the breakup of water will be used to power ATP Synthase. 

10.2.3 NADP = Ultimate Electron Acceptor

Recall how oxygen is the ultimate electron acceptor promoting the proton imbalance that powered the ATP synthase pump in animal mitochondria. In plants, the ultimate electron acceptor is NADP which is reduced to NADPH in LDP. If NADP is not available, the ATP synthase pump will not function, and as a consequence, the plant will die.

So how is ATP produced? During the electron movement from photosystem II to photosystem I, the electron that is removed from photosystem II is replaced by electrons from water. Water is oxidized so that the electron and its associated proton is separated from oxygen. Thus oxygen is produced as a result of the oxidation of water. The electron from water is used to replace the electrons that are activated in photosystem II. The hydrogen ion is used to power ATP synthase to make ATP. 

Figure 10.20: First part of light reaction: Photosystem II is activated by sunlight which causes 1. electrons to split water resulting in hydrogen being available for chemiosmosis and 2. electrons move through the cytochrome complex which also causes hydrogen ion to go to the thylakoid interior. The electron will be reactivated in photosystem I.

The hydrogen ion will not power ATP synthase unless the electron is ultimately captured by NADP. Thus NADP acts in a similar fashion to oxygen in oxidative phosphorylation in animals in that oxygen picks up the electron associated with the proton. ​​

Figure 10.21. NADPH and ATP production: NADPH is generated by activated electrons that originated from the splitting of water.

10.2.4 Cyclic Photophosphorylation

Cyclic photophosphorylation involves only photosystem I to generate ATP and no other molecules including NADPH are generated. It is considered to be a very primitive system since photosynthetic bacteria use this approach. Since there is no NADPH produced, complex carbohydrates are not produced, and more importantly, the system itself does not produce much ATP. Also, cyclic photophosphorylation does not oxidize water – thus it can survive during periods of drought. However, it does not produce oxygen since water is not involved. It is the noncyclic photophosphorylation that is responsible for the evolution of aerobic living forms like us. 

Figure 10.22: Cyclic photophosphorylation utilizes only photosystem I to produce ATP. The process is as follows: (1) Light activates pigments which cause (2) electrons to be activated that contributes (3) electrons to electron acceptors FD (ferrodoxin) which are (4) carried to cytochromes which promote (5) hydrogen ions to enter the thylakoid interior to activate (6) ATP synthase. The electrons are returned to the pigment thus completing the cycle.


Question 10.11

The light reaction generates ATP by using hydrogen ions from which source?

A

Sugars

B

Water

C

Water, Hydrogen from the stroma

D

Sunlight

Non-cyclic and cyclic photophosphorylation are compared in the following table. Cyclic photophosphorylation will not produce oxygen, and it does not generate organic compounds. 

Figure 10.23: Comparison of the two photophosphorylation systems. (PS I and PS II represent photosystems I and II)​

10.3 Mitochondria in Plants and Animals

Concept 10.3: The Calvin Cycle (or the Dark Reaction) produces metabolic products.


10.3.1 The Calvin Cycle

The light-independent reactions do not require light and are commonly called the Calvin Cycle named after the chemist who described it. This set of reaction is responsible for the production of all products made by plants. 

Figure 10.24: The Calvin cycle has three stages: Stage 1. Carbon dioxide fixation which combines carbon dioxide to existing 5 carbon structures (RuBP) making six carbon structures using the enzyme, rubisco. Stage 2. Reduction of 6 carbon structures into 2-3 carbon structures, and Stage 3. regeneration of 5 carbon sugar. Stage 2 requires ATP and NADPH that originated from the light reaction.​

10.3.2 The Three Components of the Calvin Cycle

  • Carbon fixation: One molecule of carbon dioxide combines with a 5-carbon sugar (RuBP) to form a 6-carbon sugar. The important step is catalyzed by the enzyme, Rubisco. This step is very important since it captures the carbon dioxide from the atmosphere. Since there are so many plants, and carbon fixation is key to all forms of photosynthesis, Rubisco is considered to be the most common enzyme as well as the most important. Without it, carbon dioxide would not be fixed resulting in the lack of production of organic compounds that animals rely on for survival. Life as we know it would cease. In addition, carbon dioxide fixation by plants is one of the major ways to reduce the amount of carbon dioxide in the atmosphere. Hence the preservation of forests is critical to minimize the increase in carbon dioxide. 
  • Reduction: The newly formed 6-carbon sugar is subsequently split into two molecules of a 3-carbon sugar known as phosphoglyceric acid (3-PGA). For every CO2 molecule fixed, two 3 PGA molecules are formed This step utilizes ATP and NADPH to convert the 3-PGA into another 3-carbon molecule called G3P. This is called the reduction step since NADPH donates electrons to the 3-PGA which is, therefore, reduced. This step is critical since some of the 3-carbon G3P molecules are eventually converted into carbohydrates and other organic compounds that animals use
  • Regeneration: This phase is for the regeneration of the five-carbon sugar, RuBP, so that the Calvin cycle can accept another carbon dioxide molecule and the cycle repeats 


Question 10.12

Which of the phases of the Calvin Cycle is associated with the production of carbohydrates?

A

Fixation

B

Reduction

C

Regeneration


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Figure 10.25: Calvin Reaction: Three key steps are involved in the Calvin Cycle: carbon fixation, reduction and regeneration.

All plants go through these three steps of the Calvin Cycle; however, plants use different strategies to bring CO2 to RubisCO. In this following section, we will focus on two types of plants, C3 and C4, that utilized different forms of fixation.

10.3.3 Dark Reaction: Different ways of Carbon Dioxide Fixation

10.3.3.1 C3 Plants 

These types of plants undergo what’s called the C3 process in which 3-carbon structures (G3P) are made from carbon dioxide. This is a very successful adaptation since 85% of all plant species utilize this method including, rice, wheat, soybeans and all trees. However, there is a problem with this metabolism. C3 plants, on a hot, dry day, will close their stomata to retain water. The oxygen concentration inside the leaf rises since the chloroplasts are using the carbon dioxide and rubisco combines oxygen with other organic compounds. Interestingly, in this situation, rubisco combines oxygen with other chemicals. Rubisco can either combine carbon dioxide with other carbons when it is in high concentration, called carbon fixation, or combine with oxygen when it is in high concentration. This latter process, which is called photorespiration, is wasteful since it uses energy and the plant is not able to synthesize its organic products. However, certain plants have produced a way to thrive in hot, dry climates and avoid photorespiration. 

Figure 10.26: Rubisco can either be involved in carbon fixation or photorespiration.


Question 10.14

Rubisco is an enzyme that _____________.

A

Attaches oxygen to carbon

B

Attaches hydrogen ion to carbon

C

Attaches carbon dioxide to a 5 carbon chain

D

Is used in medicine to treat persons with carbon dioxide poisoning


10.3.3.2 C4 Plants Separate Carbon Fixation by Position

Photorespiration is not an efficient system for plants since it reduces the anabolic processes of the plant to make chemical and physical structures. So how does the plant get around this problem? In the case where there is too much oxygen, plants found a way to minimize photorespiration. It stores carbon dioxide in the form of a chemical called malic acid (malate), a C4 carbon structure that is transferred deep in the leaf so oxygen cannot diffuse into it. The malic acid is broken down into carbon dioxide to enter the Calvin cycle. C4 plants are well adapted to high daytime temperatures and intense sunlight. Corn, sugar cane, crabgrass are examples. What is important is that the process of carbon dioxide fixation is physically separate from the Calvin Cycle.

Figure 10.27. Various methods of Carbon Dioxide Fixation : C3 the process occurs immediately, C4, the process is delayed by carbon dioxide being converted to Malate in mesophyll cells and subsequently converted to carbon dioxide in another part of the plant to be used in the Calvin cycle; CAM process is delayed by Malate being placed in vacuole and utilized during the daylight.


10.3.3.3 CAM Plants-Separate Carbon Fixation by Time

Another C4 plant group found another way to counter photorespiration. It is called the Crassulacean Acid Metabolism (CAM) and was discovered in the plant family Crassulaceae (succulents). In the CAM group of plants, photosynthesis is divided temporally (night and day) CAM plants take in carbon dioxide and convert it to malic acid which accumulates in the central vacuole of the plant.

At night, the stomata open up to capture the moisture from the nighttime air as well as the carbon dioxide. The carbon dioxide is converted into malic acid and is not metabolized until the daylight.

During daylight, the stomata close, preserve moisture, and reduce the inward diffusion of oxygen. The malic acid is metabolized into CO2 which is taken into the Calvin cycle. This adaption works very well for plants with high daytime temperatures, intense sunlight and low soil moisture. Examples of CAM plants are the cacti, the pineapple and all its relatives the bromeliads, and the “ice plant.”

Figure 10.28. C3, C4, and CAM plants are adapted for specific environments.


Question 10.15

Which of the following separates its carbon dioxide from the dark reaction based on time such as day and night.

A

CAM

B

C3 plants

C

C4 plants

D

All of the above


10.3.3.4 Products of the Dark Reaction: Starch, Cocaine, Nicotine, Proteins, Lipids, etc.

Figure 10.29: Products of Plant Dark Reaction [2]

Photosynthesis produces many products such as hormones for the plant to direct its growth and various chemicals to ward off or kill any animal that wants to consume it. Certain plant chemicals can be lethal to insects as well as vertebrates. We use these toxins today.

Cocaine is an insecticide. It causes a number of reactions such as shakiness in invading insects. What is important to stress is that the cocaine produced by plants coincidentally activates our pleasure center while being a poison.

Figure 10.30: The Calvin Cycle produces chocolate, which has health benefits. [3]

The reaction to the insecticide in humans varies depending on a number of factors such as dosage, tolerance (e.g., whether the person has been previously exposed to the drug or not), the method of introduction, gender, age, race, and more. From a biological point of view, the insecticides produced by plants have their effect by activating specific nerve cells in an area of the brain called the nucleus accumbens, also known as the reward center.

This site gives us the sense of well being and being happy. Once activated by a “happy” chemical stimulus, the nucleus accumbens sends signals to various parts of our brain. The neurons release a chemical called dopamine that gives the sensation of pleasure. This chemical is similar to the neurochemical called octopamine in insects which is the target of cocaine. Thus, cocaine produced by the LIP in plants will affect the chemical metabolism of octopamine in insects as well as the dopamine in humans.

In another example, marijuana's active ingredient, tetrahydrocannabinol (THC), affects the pleasure center of the brain by interacting with receptors on the brain cells that control thinking, memory, pleasure and coordination. Once the receptors are activated, they will influence these processes. The nucleus accumbens, with its metabolism of the neurochemical dopamine, is one of the targets by THC.


Question 10.16

The Dark reaction also called the Calvin Reaction has a number of phases. Which of the phases is responsible for generating sugars and other organic molecules?

A

Carbon fixation

B

Reduction

C

Regeneration

D

All three phases are directly involved


10.4 Herbicides

Concept 10.4: Herbicides attack various components of the photosynthetic pathway.

Herbicides, poisons that kill plants, are designed to inhibit some components of the photosynthetic pathway. In this section we will focus on four types of inhibitors: precursor inhibitors, photosystem I inhibitors, photosystem II inhibitors and environmental inhibitors.

10.4.1 Precursor Inhibitors

Lactofen is an inhibitor to the common precursor molecules for the synthesis of chlorophyll that is needed for photosynthesis and heme that is needed for the electron transport system. In addition to these reactions, reactive oxygen species are also produced and causes damage to the cell membrane, resulting in death.

10.4.2 Photosystem I Inhibitors

These herbicides accept electrons from photosystem I and form herbicide radicals that readily destroy membrane lipids, chlorophylls, and cell membranes. The destruction causes the cytoplasm to leak. This causes the leaf to wilt and dry out (desiccation). Paraquat is an example of this type of herbicide as it is used for killing green plants.

10.4.3 Photosystem II Inhibitors

Photosystem II inhibitors bind to photosystem II in the thylakoid membrane and block electron transport, carbon dioxide fixation and the production of energy needed for plant growth. Blocking electron transport strongly promotes the formation of reactive oxygen species that cause lipid and membrane destruction. An example of a photosystem II poison is pyrazon (pyramin), a selective herbicide to control weeds.

10.4.4 Environmental Inhibitors

Any condition that blocks the sun's rays can be lethal to plants. Volcanic eruptions or comet collisions that spew particulate matter into the atmosphere prevent the sun rays from reaching plants. In addition, air pollution will also decrease photosynthesis.

10.4.5 Mother Nature's Poison Box: Plant Insecticides

Plants for millions of years have had to defend themselves from every kind of animal that wants to eat them. As a result, plants have evolved different chemicals to ward off insects, herbivores, etc. Mankind has taken this treasure trove of chemicals hand have used them for its benefit. Here are a couple of examples: 

1. Foxglove: The plant produces a family of drugs called digitalis. Digitalis compounds are used for increasing the strength of cardiac contractions in humans who have weak cardiac muscle. It works by inhibiting a cellular pump that controls sodium/potassium levels resulting indirectly in an increase in calcium levels which increases the strength of contraction. In too great a quantity, the drug causes the heart to stop beating. As a product of the dark reaction, digitalis protects the plant since herbivores such as deer and rabbits avoid eating it. 

2. Deadly Nightshade (Aropa belladonna). Atropine is the common name for this drug that was used in the middle ages to dilate a woman's pupils. The juice from the berries was applied to the eyes and the pupil dilated supposedly making the woman more attractive. However, in high amounts, it can increase heart rate. It works by blocking the action of a neurotransmitter, acetylcholine which is a major transmitter that controls heart rate. Interestingly, atropine is the antidote for chemical weapons. Troops inject themselves with atropine in their thighs if they suspect a chemical attack since the neurotoxin will cause a decrease in heart rate.

3. Monkshood (Aconitum nepallus). Aconitum is a highly toxic lethal neurotoxin that affects both the nervous and cardiovascular systems. It works by altering the sodium channels in the membranes of both neural and cardiovascular tissue causing the heart to decrease its contraction. It was initially used to coat arrows to poison the victim. According to one report, Eskimoes would hunt whales with a poison-tipped lance, paralyzing it and causing it to drown. Atropine is the antidote to this poison as it will increase heart rate. 

Plants have produced a large number of chemicals such as cocaine, marijuana, nicotine that were synthesized for their protection and their properties of activating various organ has been used as medicines or addicting drugs.

10.4.6 Feedback Loop Video

Figure 10.31: Feedback loop of the hydrogen and oxygen molecules that are a product of the noncyclic electron transport system.

The feedback loop above demonstrates a process in which the setpoint calls for the production of 2 hydrogen molecules and 1 oxygen molecule. In this case, the output equals the setpoint which makes this a negative feedback loop. 

Question 10.17

Which of the following is the most lethal in terms of killing plants?

A

Photosystem 1 inhibitor

B

Photosystem 11 inhibitor

C

Environmental disasters that allow constant sunlight

D

Environmental disasters that block sunlight


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Question 10.19

What is the key process that occurs when photons of light activate chlorophyll?

A

ATP is directly generated

B

Energizes electrons

C

NADPH is directly generated

D

Water is split


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Question 10.21

Plant poisons are used by the plant to do which of the following?

A

Discourage herbivores from eating them

B

Develop compounds that humans like

C

Affect some organ system of any animals that eat it

D

Both A and C


Figure 10.32: Chapter overview of photosynthesis.

10.5 Vocabulary Questions


Vocabulary Question 10.01

A set of chemical reactions in plants to produce energy in the form of ATP, which is used to power all the anabolic and catabolic activities of the plant cell.


Vocabulary Question 10.02

A plant organelle that contains the pigment chlorophyll and is the site of photosynthesis.


Vocabulary Question 10.03

A fluid-filled area in chloroplasts that is enclosed by the inner membrane.


Vocabulary Question 10.04

Disk-shaped membrane sacs inside chloroplasts that contain chlorophylls and enzymes for the light dependent reactions of photosynthesis.


Vocabulary Question 10.05

A green photosynthetic pigment found in plants, algae, and cyanobacteria.


Vocabulary Question 10.06

A mechanism whereby electrons are energized and subsequently used in the synthesis of ATP and NADPH.

Vocabulary Question 10.07

Chemical reactions powered by ATP and NADPH. The end result of the reactions is that carbon dioxide from the atmosphere is transformed into an organic compound by using ATP and NADPH.


Vocabulary Question 10.08

The primitive form of photophosphorylation.

Vocabulary Question 10.09

The most abundant protein on earth, which catalyzes the first step in the Calvin Cycle.


Vocabulary Question 10.10

A ring on the "head" of a chlorophyll molecule.

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10.7 Image Credits

[1] Image courtesy of Stocksnap in the Public Domain.

[2] Image courtesy of H Zell under CC BY-SA 3.0.

[3] Image courtesy of Jing in the Public Domain.