Photosynthesis Topic 3.8 and 8.2. Autotrophs Plants are autotrophs (meaning “self-feeders” in Greek) in that they make their own food and thus sustain

PhotosynthesisPhotosynthesisTopic 3.8 and 8.2 Topic 3.8 and 8.2

Autotrophs Autotrophs

Plants are Plants are autotrophsautotrophs (meaning “self- (meaning “self-feeders” in Greek) in that they make their feeders” in Greek) in that they make their own food and thus sustain themselves own food and thus sustain themselves without eating other organisms or even without eating other organisms or even organic molecules. organic molecules.

ChloroplastsChloroplasts of plant cells capture light of plant cells capture light energy that has traveled 150 million energy that has traveled 150 million kilometers from the sun and convert it to kilometers from the sun and convert it to chemical energy that is stored in sugar chemical energy that is stored in sugar and other organic molecules. and other organic molecules.

ProducersProducers

Plants, algae, some prokaryotes make their Plants, algae, some prokaryotes make their own organic molecules and are the ultimate own organic molecules and are the ultimate source of organic molecules for almost all source of organic molecules for almost all other organisms. other organisms.

Often referred to as the Often referred to as the producers producers of the of the biosphere because they produce its food biosphere because they produce its food supplysupply

All organisms that produce organic molecules All organisms that produce organic molecules from inorganic molecules using the energy of from inorganic molecules using the energy of light are called light are called photoautotrophsphotoautotrophs..

ChloroplastsChloroplasts All green parts of a plant have All green parts of a plant have chloroplastschloroplasts in in

their cells and can carry out photosynthesis. their cells and can carry out photosynthesis. In most plants, leaves have the most chloroplasts In most plants, leaves have the most chloroplasts

and are the major sites of photosynthesis.and are the major sites of photosynthesis. Chloroplasts are concentrated in the cells of the Chloroplasts are concentrated in the cells of the

mesophyllmesophyll, the green tissue in the interior of the , the green tissue in the interior of the leaf.leaf. Each mesophyll has numerous chloroplastsEach mesophyll has numerous chloroplasts

Carbon dioxide enters the leaf and oxygen exits Carbon dioxide enters the leaf and oxygen exits via tiny pores called stomata. via tiny pores called stomata.

Water absorbed by the roots is delivered to the Water absorbed by the roots is delivered to the leaves in veins. leaves in veins.

ChloroplastsChloroplasts Membranes in the chloroplast form the framework where Membranes in the chloroplast form the framework where

many of the reactions of photosynthesis occur, just as many of the reactions of photosynthesis occur, just as mitochondrial membranes do for cell respiration. mitochondrial membranes do for cell respiration.

Similar to mitochondria, chloroplast has an outer Similar to mitochondria, chloroplast has an outer membrane and an inner membrane, with an membrane and an inner membrane, with an intermembrane space between them.intermembrane space between them.

Inner membrane is filled with a thick fluid called Inner membrane is filled with a thick fluid called stromastroma StromaStroma is where sugars are made from carbon dioxide and is where sugars are made from carbon dioxide and

waterwater Within stroma is a system of interconnected membranous Within stroma is a system of interconnected membranous

sacs called sacs called thylakoidsthylakoids Enclose a third compartment called the thylakoid spaceEnclose a third compartment called the thylakoid space Built into thylakoid membranes are the Built into thylakoid membranes are the chlorophyll chlorophyll molecules molecules

that capture light energy.that capture light energy. Membranes also house much of the machinery that converts light Membranes also house much of the machinery that converts light

energy to chemical energy. energy to chemical energy. In some places, thylakoids are concentrated in stacks In some places, thylakoids are concentrated in stacks

called grana.called grana.

ChloroplastChloroplast

Photosynthesis is a redox Photosynthesis is a redox 6CO6CO22 + 6H + 6H22O O C C66HH1212OO66 + 6O + 6O22

When water molecules are split apart, yielding OWhen water molecules are split apart, yielding O22, they are actually , they are actually oxidized; that is, they lose electrons along with hydrogen ionsoxidized; that is, they lose electrons along with hydrogen ions

Meanwhile, CO2 is reduced to sugar as electrons and hydrogen Meanwhile, CO2 is reduced to sugar as electrons and hydrogen ions are added to it. ions are added to it.

Overall, cell respiration harvest energy stored in a glucose Overall, cell respiration harvest energy stored in a glucose molecule by:molecule by:

oxidizing the sugar and reducing Ooxidizing the sugar and reducing O22 to H to H22O, involving a number of O, involving a number of energy-releasing redox reactions,energy-releasing redox reactions,

with electrons losing potential energy as they travel down an energy with electrons losing potential energy as they travel down an energy hill from sugar to Ohill from sugar to O22. .

Along the way, the mitochondria uses some of the energy to synthesize Along the way, the mitochondria uses some of the energy to synthesize ATP.ATP.

In contrast, photosynthesis redox reactions involve an uphill climb. In contrast, photosynthesis redox reactions involve an uphill climb. As water is oxidized and COAs water is oxidized and CO22 is reduced, electrons gain energy by being is reduced, electrons gain energy by being

boosted up an energy hill.boosted up an energy hill. Light energy captured by chlorophyll molecules in the chloroplast Light energy captured by chlorophyll molecules in the chloroplast

provides the boost for the electrons. provides the boost for the electrons. Photosynthesis converts light energy to chemical energy and stores it in Photosynthesis converts light energy to chemical energy and stores it in

sugar molecules. sugar molecules.

Photosynthesis: OverviewPhotosynthesis: Overview

Photo, from the Greek word for light, Photo, from the Greek word for light, refers to the first stage.refers to the first stage.

Synthesis, meaning “putting Synthesis, meaning “putting together” refers to the sugar together” refers to the sugar construction in the second stageconstruction in the second stage

Photosynthesis OverviewPhotosynthesis Overview Occurs in two stages:Occurs in two stages:1.1. Light reactionsLight reactions

Include the steps that convert light energy to Include the steps that convert light energy to chemical energy stored in ATP and NADPH chemical energy stored in ATP and NADPH and produce O2 gas as a waste product.and produce O2 gas as a waste product.

Occur in thylakoid membranesOccur in thylakoid membranes Light energy absorbed by chlorophyll is used Light energy absorbed by chlorophyll is used

to make ATP from ADP and phophate.to make ATP from ADP and phophate. Also used to drive a transfer of electrons Also used to drive a transfer of electrons

from water to NADP+, an electron carrier from water to NADP+, an electron carrier similar to NAD+ that carries electrons in similar to NAD+ that carries electrons in cellular respiration. cellular respiration.

NADP+ gets reduced to NADPH via NADP+ gets reduced to NADPH via enzymes by adding a pair of light-excited enzymes by adding a pair of light-excited electrons along with an H+electrons along with an H+ Reaction temporarily stores energized electrons Reaction temporarily stores energized electrons

which originally came form water that is split and which originally came form water that is split and O2 is released.O2 is released.

Photosynthesis OverviewPhotosynthesis Overview

2. Dark reactions, or Calvin Cycle2. Dark reactions, or Calvin Cycle Occurs in the stroma Occurs in the stroma Does not require light directlyDoes not require light directly Cyclic series of reactions that assembles Cyclic series of reactions that assembles

sugar molecules using COsugar molecules using CO22 and the energy- and the energy-containing products (NADPH and ATP) of the containing products (NADPH and ATP) of the light reactions. light reactions.

Incorporation of carbon from COIncorporation of carbon from CO22 into organic into organic compounds is called carbon fixation. compounds is called carbon fixation.

After carbon fixation, enzymes of the cycle After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon make sugars by further reducing the carbon compounds.compounds.

Light Reactions:Light Reactions:Converting Solar Energy to Chemical EnergyConverting Solar Energy to Chemical Energy

Electromagnetic energyElectromagnetic energy type of energy that is sunlighttype of energy that is sunlight Travels in space as rhythmic waves analagous to those Travels in space as rhythmic waves analagous to those

made by a pebble dropped in a puddle of watermade by a pebble dropped in a puddle of water Distance between the crests of two adjacent waves is Distance between the crests of two adjacent waves is

called a called a wavelengthwavelength. . In the electromagnetic spectrum, shorter wavelengths In the electromagnetic spectrum, shorter wavelengths

have more energy than longer ones. have more energy than longer ones. Visible light- the radiation your eyes can see as different Visible light- the radiation your eyes can see as different

colors, consists of wavelengths from about 380 nm to colors, consists of wavelengths from about 380 nm to 750 nm750 nm


Figure 7.6B shows what happens to Figure 7.6B shows what happens to visible light in the chloroplast. visible light in the chloroplast. Light absorbing molecules called pigments, Light absorbing molecules called pigments,

built into the thylakoid membranes, absorb built into the thylakoid membranes, absorb some wavelengths of light and reflect or some wavelengths of light and reflect or transmit other wavelengths.transmit other wavelengths.

We do not see the absorbed wavelengths; We do not see the absorbed wavelengths; their energy has been absorbed by their energy has been absorbed by pigment moleculespigment molecules

We see green wavelengths when we look We see green wavelengths when we look at plants that the pigments transmit and at plants that the pigments transmit and reflect.reflect.


Pigments of chloroplast:Pigments of chloroplast: Chlorophyll aChlorophyll a

Absorbs mainly blue-violet and red lightAbsorbs mainly blue-violet and red light Participates directly in the light reactionsParticipates directly in the light reactions Looks grass-green because it reflects mainly green lightLooks grass-green because it reflects mainly green light

Chlorophyll bChlorophyll b Absorbs mainly blue and orange light and reflects (appears) Absorbs mainly blue and orange light and reflects (appears)

yellow-green.yellow-green. Broadens the range of light that a plant can use by Broadens the range of light that a plant can use by

conveying absorbed energy to chlorophyll a, which then conveying absorbed energy to chlorophyll a, which then puts the energy to work in the light reactionsputs the energy to work in the light reactions

CarotenoidsCarotenoids Absorb mainly blue-green light and reflects yellow-orangeAbsorb mainly blue-green light and reflects yellow-orange Some may pass energy to chlorophyll a, as chlorophyll b Some may pass energy to chlorophyll a, as chlorophyll b

doesdoes Have a protective function: absorb and dissipate excessive Have a protective function: absorb and dissipate excessive

light energy that would other-wise damange chlorophyll light energy that would other-wise damange chlorophyll


The theory of light as waves explains The theory of light as waves explains most of light’s properties.most of light’s properties.

However, light also behaves as discrete However, light also behaves as discrete packets of energy called packets of energy called photonsphotons:: A fixed quantity of light energy, and the A fixed quantity of light energy, and the

shorter the wavelength, the greater the shorter the wavelength, the greater the energy. energy.

Each type of pigment absorbs certain Each type of pigment absorbs certain wavelengths of light because it is able to wavelengths of light because it is able to absorb the specific amounts of energy in absorb the specific amounts of energy in those photons.those photons.


PhotosystemsPhotosystems Clusters of chlorophyll molecules along with other Clusters of chlorophyll molecules along with other

pigments and proteins in the thylakoid membranepigments and proteins in the thylakoid membrane Consists of a number of light-harvesting complexes Consists of a number of light-harvesting complexes

surrounding a reaction center.surrounding a reaction center. Have chlorophyll a, chlorophyll b, and carotenoid Have chlorophyll a, chlorophyll b, and carotenoid

pigments that function collectively as a light-gathering pigments that function collectively as a light-gathering antenna. antenna.

Pigments absorb photons and pass the energy from Pigments absorb photons and pass the energy from molecule to molecule until it reaches the molecule to molecule until it reaches the reaction reaction centercenter. .


Photosystems:Photosystems: Reaction centerReaction center

A protein complex that contains a A protein complex that contains a chlorophyll a molecule and a molecule called chlorophyll a molecule and a molecule called the the primary electron acceptorprimary electron acceptor::

Captures a light-excited electron from the Captures a light-excited electron from the reaction-center chlorophyll molecule and passes it reaction-center chlorophyll molecule and passes it to an electron transport chainto an electron transport chain


Photosystems:Photosystems: Two types: Photosystem I and Two types: Photosystem I and

Photosytem II:Photosytem II: Photosystem I:Photosystem I:

Occurs second in light reactionsOccurs second in light reactions Reaction center is called P700 because the Reaction center is called P700 because the

wavelength of light it absorbs best is 700 nmwavelength of light it absorbs best is 700 nm Photosystem II:Photosystem II:

Occurs first in light reactionsOccurs first in light reactions Chlorophyll a molecule in reaction center is called Chlorophyll a molecule in reaction center is called

P680 because the light it absorbs best is red light P680 because the light it absorbs best is red light with a wavelength of 680nmwith a wavelength of 680nm



Light Reactions:Light Reactions: Light energy is transformed into the Light energy is transformed into the

chemical energy of ATP and NADPHchemical energy of ATP and NADPH In this process, electrons removed from In this process, electrons removed from

water molecules pass from photosystem water molecules pass from photosystem II to photosystem I to NADP+II to photosystem I to NADP+

Between the two photosystems, the Between the two photosystems, the electrons move down an electron electrons move down an electron transport chain and provide energy for transport chain and provide energy for ATP production.ATP production.


Flow of electrons in light reactions (Figure Flow of electrons in light reactions (Figure 7.8A):7.8A):1.1. A pigment molecule in a light-harvesting A pigment molecule in a light-harvesting

complex absorbs a photon of light. The energy complex absorbs a photon of light. The energy is passed to other pigment molecules and is passed to other pigment molecules and finally to the reaction center of Photosystem II, finally to the reaction center of Photosystem II, where it excites an electron of chlorophyll P680 where it excites an electron of chlorophyll P680 to a higher energy level. to a higher energy level.

2.2. The electron is captured by the primary The electron is captured by the primary electron acceptor.electron acceptor.

3.3. Water is split, and its electrons are supplied one Water is split, and its electrons are supplied one by one to P680, replacign those lost to the by one to P680, replacign those lost to the primary electron acceptor. The oxygen atom primary electron acceptor. The oxygen atom compbines with an oxygen from another split compbines with an oxygen from another split water molecule to form a molecule of O2. water molecule to form a molecule of O2.


Flow of electrons in light reactions (Figure Flow of electrons in light reactions (Figure 7.8A):7.8A): 4. each photoexcited electron passes from photosystem 4. each photoexcited electron passes from photosystem

II to photosystem I via an electron transport chain. The II to photosystem I via an electron transport chain. The exergonic “fall” of electrons provides energy for the exergonic “fall” of electrons provides energy for the synthesis of ATP.synthesis of ATP.

5. Meanwhile, light energy excites an electron of 5. Meanwhile, light energy excites an electron of chlorophyll P700 in the reaction center of photosystem I. chlorophyll P700 in the reaction center of photosystem I. The primary electron acceptor captures the excited The primary electron acceptor captures the excited electron and an electron from the bottom of the electron electron and an electron from the bottom of the electron transport chain replaces the lost electron in P700.transport chain replaces the lost electron in P700.

6. The excited electrons of photosystem I is passed 6. The excited electrons of photosystem I is passed through a short electron transport chain to NADP+, through a short electron transport chain to NADP+, reducing it to NADPHreducing it to NADPH


ChemiosmosisChemiosmosis Drives ATP synthesis using the potential Drives ATP synthesis using the potential

energy of a concentration gradient of energy of a concentration gradient of hydrogen ions across a membranehydrogen ions across a membrane

Gradient is created when an electron Gradient is created when an electron transport chain pumps hydrogen ions across transport chain pumps hydrogen ions across a membrane as it passes electrons down the a membrane as it passes electrons down the chain.chain.


Chemiosmosis (ctd)Chemiosmosis (ctd) Relationship between chloroplast structure Relationship between chloroplast structure

and function in light reactions:and function in light reactions: The two photosystems and e.t.c. are all The two photosystems and e.t.c. are all

located in the thylakoid membrane of a located in the thylakoid membrane of a chloroplast.chloroplast.

As photoexcited electrons are passed down As photoexcited electrons are passed down the e.t.c. connecting the two photosystems, the e.t.c. connecting the two photosystems, H+ are pumped across the membrane from H+ are pumped across the membrane from the stroma into the thylakoid space. This the stroma into the thylakoid space. This generates a concentration gradient across the generates a concentration gradient across the membrane. membrane.


Chemiosmosis (ctd)Chemiosmosis (ctd) Similar ATP synthase complex in Similar ATP synthase complex in

mitochondriamitochondria Energy of concentration gradient drives Energy of concentration gradient drives

H+ back across the membrane through H+ back across the membrane through ATP synthaseATP synthase

ATP synthase couples the flow of H+ to ATP synthase couples the flow of H+ to the phosphorylation of ADP: called the phosphorylation of ADP: called photophosphorylationphotophosphorylation


Chemiosmosis (ctd)Chemiosmosis (ctd) In photosynthesis, light energy is used to drive electrons In photosynthesis, light energy is used to drive electrons

to the top of the transport chain (whereas, cell to the top of the transport chain (whereas, cell respiration, high-energy electrons pass down the e.t.c. respiration, high-energy electrons pass down the e.t.c. coming from oxidation of food molecules)coming from oxidation of food molecules)

Chloroplasts transform light energy into the chemical Chloroplasts transform light energy into the chemical energy of ATP (whereas, mitochondria transfer chemical energy of ATP (whereas, mitochondria transfer chemical energy from food to ATP)energy from food to ATP)

In photosynthesis, the final electron acceptor is NADP+ In photosynthesis, the final electron acceptor is NADP+ (whereas, in cell respiration, O2 is)(whereas, in cell respiration, O2 is)

In photosynthesis, electrons are stored in at a high state In photosynthesis, electrons are stored in at a high state of potential energy in NADPH (whereas, in cell of potential energy in NADPH (whereas, in cell respiration, they are at a low energy level in H20)respiration, they are at a low energy level in H20)


Dark Reaction (Calvin Cycle)Dark Reaction (Calvin Cycle)Converting COConverting CO22 to sugars to sugars

Figure 7.10A: Overview of Calvin Figure 7.10A: Overview of Calvin CycleCycle CO2 (from air), energy from ATP and CO2 (from air), energy from ATP and

high energy electrons from NADPH (both high energy electrons from NADPH (both generated by light reactions) , the generated by light reactions) , the Calvin Cycle constructs an energy-rich, Calvin Cycle constructs an energy-rich, three-carbon sugar, glyceraldehyde-3-three-carbon sugar, glyceraldehyde-3-phosphate (G3P).phosphate (G3P).

A plant cell uses G3P to make glucose A plant cell uses G3P to make glucose and other organic molecules as needed. and other organic molecules as needed.


Figure 7.10B: Details of the Calvin Figure 7.10B: Details of the Calvin CycleCycle1.1. Carbon fixation: Carbon fixation: the enzyme rubisco the enzyme rubisco

attaches CO2 to RuBP (5-C). The unstable 6-C attaches CO2 to RuBP (5-C). The unstable 6-C product splits into two molecules called 3-product splits into two molecules called 3-PGA.PGA.

1.1. For three CO2, six 3-PGA resultFor three CO2, six 3-PGA result

2.2. Reduction: Reduction: NADPH reduces the organic acid NADPH reduces the organic acid six 3-PGA to six molecules G3P with the six 3-PGA to six molecules G3P with the assistance of ATPassistance of ATP


3. Release of one molecule of G3P: 3. Release of one molecule of G3P:

1.1. Five G3Ps remain in the cycle, and one Five G3Ps remain in the cycle, and one G3P will leave. Plant cells use two G3P G3P will leave. Plant cells use two G3P molecules to make one molecule of molecules to make one molecule of glucose.glucose.

4. Regeneration of RuBP4. Regeneration of RuBP

energy from ATP drives a series energy from ATP drives a series of chemical of chemical reactions to rearrange the reactions to rearrange the atoms in the five G3P atoms in the five G3P molecules to form molecules to form three RuBP molecules. These can three RuBP molecules. These can start start another turn of the cycle.another turn of the cycle.


Absorption spectrumAbsorption spectrum As light meets matter, it may be As light meets matter, it may be

reflected, transmitted, or absorbed.reflected, transmitted, or absorbed. PigmentsPigments

Substances that absorb visible lightSubstances that absorb visible light Different pigments absorb light of different Different pigments absorb light of different

wavelengthswavelengths Chlorophylls absorb red and blue-violet light and Chlorophylls absorb red and blue-violet light and

appear greenappear green Carotenoids absorb blue-violet and appear orange, Carotenoids absorb blue-violet and appear orange,

yellow, or redyellow, or red Measured with a spectrophotometer.Measured with a spectrophotometer.

Absorption spectrumAbsorption spectrum

Absorption spectrumAbsorption spectrum Light absorption vs. the wavelengthLight absorption vs. the wavelength Absorption spectrum of different Absorption spectrum of different

photosynthetic pigments provides clues to photosynthetic pigments provides clues to their role in photosynthesis, since light can their role in photosynthesis, since light can only perform work if it is absorbed.only perform work if it is absorbed.

Accessory pigments (chlorophyll b and Accessory pigments (chlorophyll b and carotenoids) absorb wavelengths of light that carotenoids) absorb wavelengths of light that chlorophyll a cannot, pass the energy to chlorophyll a cannot, pass the energy to chlorophyll a, broadening the spectrum that chlorophyll a, broadening the spectrum that can effectively drive photosynthesis. can effectively drive photosynthesis.

Absorption SpectrumAbsorption Spectrum

Action spectrumAction spectrum Profiles the effectiveness of different Profiles the effectiveness of different

wavelength light in fueling wavelength light in fueling photosynthesis.photosynthesis.

It is obtained by plotting wavelength It is obtained by plotting wavelength against some measure of photosynthetic against some measure of photosynthetic rate.rate.

Action SpectrumAction Spectrum

Abiotic factors impact on Abiotic factors impact on photosynthetic ratephotosynthetic rate

Photosynthetic rate is depended on Photosynthetic rate is depended on environmental factors:environmental factors: Amount of light availableAmount of light available Level of carbon dioxideLevel of carbon dioxide temperaturetemperature

Light intensityLight intensity

Up to a certain intensity, Up to a certain intensity, photosynthesis increases as more photosynthesis increases as more light is available to the chlorophyll.light is available to the chlorophyll.

When all the chlorophyll molecules When all the chlorophyll molecules are activated (saturated) by the light, are activated (saturated) by the light, more light has no further effect.more light has no further effect.

Light IntensityLight Intensity

TemperatureTemperature

Increased temperature increases Increased temperature increases photosynthetic rate until an optimal photosynthetic rate until an optimal temperature is reached.temperature is reached.

Above the optimal temperature, Above the optimal temperature, enzymes cannot function properly enzymes cannot function properly and photosynthesis will decrease.and photosynthesis will decrease.

TemperatureTemperature

Carbon Dioxide levelsCarbon Dioxide levels

Increased carbon dioxide levels Increased carbon dioxide levels increases photosynthesis, unless increases photosynthesis, unless limited by another factor, then levels limited by another factor, then levels off.off.

Carbon DioxideCarbon Dioxide

Measuring photosynthesisMeasuring photosynthesis

Production of oxygen or uptake of Production of oxygen or uptake of carbon dioxidecarbon dioxide

Increase in biomassIncrease in biomass

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Photosynthesis Topic 3.8 and 8.2. Autotrophs Plants are autotrophs (meaning “self-feeders” in Greek) in that they make their own food and thus sustain