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8D: Photosynthesis - The Light Reaction - Biology

8D: Photosynthesis - The Light Reaction - Biology


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Learning Objectives

  • described the mechanistic similarities between mitochondrial oxidative/phosophorylation in which NADH and FADH2 are regenerated on reduction of O2 and the light reaction of photosynthesis in which O2 and a reducing agent, NADPH are produced;
  • describe similarities in fluorescence resonance energy transfer and exciton transfer;
  • describe the difference in properties between chlorophylls acting as antennae and chlorophylls at the reaction center;
  • describe how sunlight driven excitation of chlorophyll molecules at the reaction center produces an oxidzing agent strong enough to oxide water and form O2, itself a powerful oxidizing agent;
  • explain the general flow of electrons from dioxgen to NADP+ through a series of mobile and membrane protein bound electron carriers in the Z scheme of electron transport in the chloroplast thylacoid membranes;
  • explain with picture diagrams how oxidation of H2O and phosphorylation reactions (to produce ATP) are coupled in in the Z scheme;

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What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy in a spectrum from very short gamma rays to very long radio waves). Humans can see only a tiny fraction of this energy, which we refer to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength (shorter wavelengths are more powerful than longer wavelengths)—the distance between consecutive crest points of a wave. Therefore, a single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure).

The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure). The difference between wavelengths relates to the amount of energy carried by them.

The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength, the less energy it carries. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, which explains why both X-rays and UV rays can be harmful to living organisms.


Biology 110

What happens in photosynthesis? Reagents and products.

Why is photosynthesis important for life on earth?

Highlights of the history of photosynthesis research:

  • 1772 England Priestly
  • 1779 Netherlands Ingen Housz
  • 1804 Switzerland de Saussure
  • 1930's Stanford van Niel
  • 1940's Berkeley Calvin and colleagues

Photosynthesis consists of two groups of reactions, one that requires light (photochemical "light reaction") and the other that doesn't (biochemical "dark reaction").

Light reaction: Light is absorbed by a pigment molecule and the light energy is converted into useable chemical energy.

Dark reaction: The useable chemical energy from the light reaction is used to reduce carbon dioxide to sugar (photosynthetic carbon reduction cycle - PCR cycle).

Photosynthesis takes place in chloroplasts .

  • endosymbiont origin of chloroplasts
  • structure of chloroplasts: outer membranes thylakoids stroma

Light is electromagnetic radiation in the visible region of the spectrum (l 400 - 700 nm).

Pigments are molecules that absorb energy of photons at particular wavelengths - absorption spectra. Electrons in the molecules become excited to higher energy levels when photons are absorbed.

  • Chlorophyll a and b
  • Carotenoids
  • Xanthophylls
  • (Why are leaves changing colors now?)

Photosynthetic pigment molecules are clustered in groups of about 200 molecules in "antennae" in thylakoid membranes.

Photochemistry of the light reaction : Know the reagents and products of the light reaction and the three main results of the electron transport chain.

  • PSI donates electron to reduce NADP+ to NADPH (in stroma)
  • PSII donates electron to restore PSI and is restored by electron from water which is split to produce electrons, protons, and oxygen.
  • Protons (H+) accumulate in thylakoid proton gradient from thylakoid to stroma powers the synthesis of ATP (in stroma) from ADP and Pi using membrane-bound enzyme ATP synthase.

Biochemistry of the dark reaction : Know the reagents, products and the three main steps of the PCR cycle.

  • Carboxylation - role of Rubisco enzyme
  • Reduction - role of ATP and NADPH from light reaction
  • Regeneration of RUBP, the initial substrate for CO2

Photorespiration is a loss of fixed carbon resulting from the oxygenation of RUBP by Rubisco.

Oxygen competes with carbon dioxide for active site of Rubisco enzyme molecule particularly at:

  • High temperature.
  • High light levels causes intercellular CO2 to decline and O2 to increase.
  • Water is limiting causing stomata to close.

C4 photosynthesis and CAM are alternative photosynthetic pathways that eliminate photorespiration by concentrating CO2 at the site of sugar synthesis by the PCR cycle.

  • C4 spatial separation of CO2 capture and PCR cycle.
  • CAM temporal separation of CO2 capture and PCR cycle.
  • C4 and CAM plants conserve water.

Sample question from past exams:

a. The light and dark reactions of photosynthesis are mutually interdependent.

b. The sac-like structure of chloroplast thylakoids is essential for ATP synthesis.


Understanding Pigments

Different kinds of pigments exist, and each has evolved to absorb only certain wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts and will be the focus of the following discussion.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the absorption spectrum . The graph in Figure shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.

(a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum.

Many photosynthetic organisms have a mixture of pigments using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation (Figure).

Plants that commonly grow in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments. (credit: Jason Hollinger)


Problem: Compare and contrast the light reactions and the Calvin cycle of photosynthesis. Identify 2 similarities and 2 differences.

Light reaction and Dark reaction similarities includes the following:

  • Both are necessary process of photosynthesis
  • Both occur in the chloroplasts however it occurs in different specific part of the chloroplast
Problem Details

Compare and contrast the light reactions and the Calvin cycle of photosynthesis. Identify 2 similarities and 2 differences.

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Stages of Photosynthesis concept. You can view video lessons to learn Stages of Photosynthesis. Or if you need more Stages of Photosynthesis practice, you can also practice Stages of Photosynthesis practice problems.

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Based on our data, we think this problem is relevant for Professor Phelan's class at UCLA.


Photosynthesis Lab - Assignment

Gizmo Warm-up During ​ photosynthesis ​, plants use the energy of light to produce ​ glucose (C​ 6 ​H​ 12 ​O​ 6 ​) from ​ carbon dioxide ​ (CO​ 2 ​), and water (H​ 2 ​O). Glucose is a simple sugar that plants use for energy and as a building block for larger molecules.

A by-product of photosynthesis is oxygen. Plants use some of the oxygen they produce, but most of it is released. In the ​ Photosynthesis Lab ​Gizmo, you can monitor the rate of photosynthesis by measuring oxygen production.

Observe the left pane closely. What do you think the bubbles are?

Select the BAR CHART tab. On the graph, notice the ​ Oxygen production ​bar. Move the ​ Light intensity slider back and forth. How does light intensity affect oxygen production?

Experiment with the vertical ​ Temperature ​ slider (upper left) and the ​ CO2level ​ slider.

Name: Sivaareni Selvakumar Date: March 3rd, 2021

● To survive, what gas do we need to breathe in? Oxygen

● Where is this gas produced? Photosynthesis

I think the bubbles are bubbles of oxygen

When the light intensity increases the oxygen production also increases

A. How does temperature affect oxygen production?

oxygen production decreases when the temperature is over 31 degrees and increases between 17 and 31 degrees

B. How does CO​ 2 ​ level affect oxygen production? When the CO2 level is under 230ppm oxygen production decreases, when the CO2 levels are over 230ppm the

Question: In the Gizmo, what are the ideal conditions for photosynthesis?

  1. Form hypothesis​: During photosynthesis, light energy is used to synthesize carbon dioxide (CO​ 2 ​) and water (H​ 2 ​O) into glucose (C​ 6 ​H​ 12 ​O​ 6 ​) and oxygen (O​ 2 ​). The complex series of chemical reactions is summarized by the following formula:

6CO​ 2 ​ + 6H​ 2 ​O + light energy ฀ C​ 6 ​H​ 12 ​O​ 6 ​ + 6O​ 2

In the Gizmo, what light intensity and CO​ 2 ​ level do you think will maximize the rate of photosynthesis?

Experiment​: Use the Gizmo to find the ideal conditions for photosynthesis. Use any method you like. When you think you have the answer, list the conditions below.

Revise and repeat​: One way to test if you’ve found the ideal conditions is to change each variable slightly from the value that you recorded above. If the oxygen production decreases with each change that you make, it is likely you have found the ideal conditions. If a small change causes oxygen production to increase, continue to experiment.

If necessary, revise your numbers in the table above.

oxygen production doesn’t change

C. How does oxygen production relate to the rate of photosynthesis?

When the rate of photosynthesis increases the oxygen production increases

Ideal conditions

Get the Gizmo ready​: ● Be sure that the BAR CHART tab is selected. ● Turn on ​ Show numerical values ​.

I think the light intensity will go up to about 80-90% and the CO2 level will go up to about 700ppm

Temperature Light intensity CO2level Oxygen production 24.0 88.0 620 51.

A. Why would it be hard to find the ideal light intensity if the temperature were very hot or cold?

If it’s too cold or too hot photosynthesis would not occur because enzymes would be denatured

B. Which colors were absorbed the worst?

  1. Think and discuss​: When we look at a leaf, we see the colors of light that are reflected off its surface. How does this explain the relatively low flow of oxygen in green light?

Introduction: ​Photosynthesis requires light, water, and CO​ 2 ​ to work. When one of these factors is in short supply, it is called a ​ limiting factor ​. Temperature can also be a limiting factor when it is too hot or too cold for photosynthesis to work well.

Question: What is the effect of limiting factors on photosynthesis?

Observe​: Set ​ Temperature ​to 24°C, ​ Light intensity ​ to 50%, and ​ CO2level ​ to 200 ppm.

Analyze​: In this situation, what was the limiting factor?

There’s low flow of oxygen in green light due to the denaturation of the enzymes so photosynthesis wouldn’t occur because there’s no energy resulting in little oxygen production

Limiting factors

● Select the WHITE tab and the BAR CHART tab. ● Turn on ​ Show numerical values ​.

A. Move the ​ Temperature ​slider up and down. Were you able to increase oxygen production? (Return the slider to 24°C when finished.)

The oxygen production was only able to decrease or stay the same not increase

B. Move the ​ Light intensity ​slider back and forth. Were you able to increase oxygen production? (Return the slider to 50% when finished.)

The oxygen production was only able to decrease or stay the same not increase

C. Move the ​ CO2level ​slider back and forth. Were you able to increase oxygen production? (Return the slider to 200 ppm when finished.)

When the CO2 levels increased the oxygen production increased

The CO2 was the limiting factor

Challenge​: In each of the situations below, use the Gizmo to find the limiting factor.

Think and discuss​: Suppose you were a farmer trying to grow plants in a greenhouse. Why would it be important to know what the limiting factor is?

Because only when the CO2 levels were changed it would affect the oxygen production

Temperature Light intensity CO2level Limiting factor 25°C 60% 700 ppm light intensity 15°C 20% 200 ppm temperature 30°C 50% 400 ppm none

As a farmer knowing what the limiting factor is can help the farmer know what to make adjustments to allow the plants to grow better.


CHLOROPLAST

Chloroplast is the cell organelle where photosynthesis takes place in plants and algae. A typical plant cell may contain about 10 to 100 chloroplasts.

Chloroplast is enclosed by a membrane. This membrane is composed of an inner, outer and an intermediate membrane. An aqueous fluid called stroma is present within the membrane.

Stacks of thylakoids are present in the stroma. A stack is called a granum. Thylakoids are the sites of photosynthesis.

A thylakoid is a flattened disc. It is bound by a membrane. The lumen or thylakoid space is present within the membrane. The tyhlakoid membrane is the site of photosynthesis. It contains integral and peripheral membrane protein complexes. Pigments which absorb light energy are also present on the membrane. The protein complexes and the pigments form the photosystems.

Chlorophyll is the main pigment to absorb light. Additionally, carotenes and xanthophylls are also used by plants to absorb light energy. Algae also use chlorophyll for absorbing light.

These pigments are embedded in plants and algae in special antenna-proteins. The pigments are ordered in these proteins so that they can work in perfect coordination. Such a protein is also called a light-harvesting complex.

All the cells in the green parts of a plant have chloroplasts but most of the energy is captured in the leaves. The mesophyll of the leaf can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle which protects the leaf from excess evaporation of water. It also decreases the absorption of ultraviolet or blue light to reduce heating. The epidermal layer of the leaf is transparent. It allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.


The Light Reactions of Photosynthesis

Historically, the role of light in photosynthesis has been ascribed either to a photolysis of carbon dioxide or to a photolysis of water and a resultant rearrangement of constituent atoms into molecules of oxygen and glucose (or formaldehyde). The discovery of photophosphorylation demonstrated that photosynthesis includes a light-induced phosphorus metabolism that precedes, and is independent from, a photolysis of water or CO2. ATP formation could best be accounted for not by a photolytic disruption of the covalent bonds in CO2 or water but by the operation of a light-induced electron flow that results in a release of free energy which is trapped in the pyrophosphate bonds of ATP.

Photophosphorylation is now divided into (a) a non-cyclic type, in which the formation of ATP is coupled with a light-induced electron transport from water to ferredoxin and a concomitant evolution of oxygen and (b) a cyclic type which yields only ATP and produces no net change in the oxidation-reduction state of any electron donor or acceptor. Reduced ferredoxin formed in (a) serves as an electron donor for the reduction of NADP by an enzymic reaction that is independent of light. ATP, from both cyclic and noncyclic photophosphorylation, and reduced NADP jointly constitute the assimilatory power for the conversion of CO2 to carbohydrates (3 moles of ATP and 2 moles of reduced NADP are required per mole of CO2).

Investigations, mainly with whole cells, have shown that photosynthesis in green plants involves two photosystems, one (System II) that best uses light of “short” wavelength (λ < 685 nm) and another (System I) that best uses light of “long” wavelength (λ > 685 nm). Cyclic photophosphorylation in chloroplasts involves a System I photoreaction. Noncyclic photophosphorylation is widely held to involve a collaboration of two photoreactions: a short-wavelength photoreaction belonging to System II and a long-wavelength photoreaction belonging to System I. Recent findings, however, indicate that noncyclic photophosphorylation may include two short-wavelength, System II, photoreactions that operate in series and are joined by a “dark” electron-transport chain to which is coupled a phosphorylation site.


8.7 Critical Thinking Questions

How does the equation relate to both photosynthesis and cellular respiration?
  1. Photosynthesis utilizes energy to build carbohydrates while cellular respiration metabolizes carbohydrates.
  2. Photosynthesis utilizes energy to metabolize carbohydrates while cellular respiration builds carbohydrates.
  3. Photosynthesis and cellular respiration both utilize carbon dioxide and water to produce carbohydrates.
  4. Photosynthesis and cellular respiration both metabolize carbohydrates to produce carbon dioxide and water.
  1. When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll a molecules that excite an electron, which is then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS-II to PS-I. The products of the light dependent reaction are used to power the Calvin cycle to produce glucose.
  2. When photons strike photosystem (PS) I, pigments pass the light energy to chlorophyll, molecules that excite electrons, which is then passed to the electron transport chain. The cytochrome complex then transfers protons across the thylakoid membrane and transfers electrons from PS-II to PS-I. The products of the light dependent reaction are used to power the Calvin cycle to produce glucose.
  3. When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll molecules that in turn excite electrons, which are then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS-I to PS-II. The products of the light dependent reaction are used to power the Calvin cycle to produce glucose.
  4. When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll molecules that excite electrons, which is then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS II to PS I. The products of the light independent reaction are used to power the Calvin cycle to produce glucose.
  1. UV and X-rays are high energy waves that penetrate the tissues and damage cells.
  2. UV and X-rays are low energy waves that penetrate the tissues and damage cells.
  3. UV and X-rays cannot penetrate tissues and thus damage the cells.
  4. UV and X-rays can penetrate tissues and thus do not damage the cells.
  1. Photosynthesis does not take place.
  2. The rate of photosynthesis increases sharply.
  3. The rate of photosynthesis decreases drastically.
  4. The rate of photosynthesis decreases and then increases.
  1. After splitting water in PS-II, high energy electrons are delivered through the chloroplast electron transport chain to PS-I.
  2. After splitting water in PS-I, high energy electrons are delivered through the chloroplast electron transport chain to PS-II.
  3. After the photosynthesis reaction, the released products like glucose help in the transfer of electrons from PS-II to PS-I.
  4. After the completion of the light dependent reactions, the electrons are transferred from PS-II to PS-I.
  1. no effect on the rate of photosynthesis
  2. Photosynthesis will slow down or stop possibly.
  3. Photosynthesis will increase exponentially.
  4. Photosynthesis will decrease and then increase.
  1. The product of the Calvin cycle is glyceraldehyde-3 phosphate and RuBP is regenerated.
  2. The product of the Calvin cycle is glyceraldehyde-3 phosphate and RuBisCO is regenerated.
  3. The product of the Calvin cycle is a 3-PGA molecule and glyceraldehyde-3 phosphate is regenerated.
  4. The product of the Calvin cycle is glyceraldehyde-3 phosphate and oxygen is regenerated.
  1. by using CAM photosynthesis and by closing stomatal pores during the night
  2. by using CAM photosynthesis and by opening of stomatal pores during the night
  3. by using CAM photosynthesis and by keeping stomatal pores closed at all times
  4. by bypassing CAM photosynthesis and by keeping stomatal pores closed at night
  1. because the prey of lions are generally herbivores which depend on heterotrophs
  2. because the prey of lions are generally smaller carnivorous animals which depend on non-photosynthetic organisms
  3. because the prey of lions are generally herbivores which depend on autotrophs
  4. because the prey of lions are generally omnivores that depend only on autotrophs.
  1. To fix enough carbon to export one G3P molecule.
  2. To fix enough oxygen to export one G3P molecule.
  3. To produce RuBisCO as an end product.
  4. To produce ATP and NADPH for fixation of G3P.
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This text is based on Openstax Biology for AP Courses, Senior Contributing Authors Julianne Zedalis, The Bishop's School in La Jolla, CA, John Eggebrecht, Cornell University Contributing Authors Yael Avissar, Rhode Island College, Jung Choi, Georgia Institute of Technology, Jean DeSaix, University of North Carolina at Chapel Hill, Vladimir Jurukovski, Suffolk County Community College, Connie Rye, East Mississippi Community College, Robert Wise, University of Wisconsin, Oshkosh

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 Unported License, with no additional restrictions


Watch the video: Introduction to Photosynthesis (November 2022).