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IB DP Biology HL C1.3 Photosynthesis Flashcards

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[h] IB DP Biology HL C1.3 Photosynthesis Flashcards

 

[q] Photosynthesis

[a] The conversion of light energy into chemical energy in plants.

 

[q] Light Dependent Reactions (LDR)

[a] (Photo) Uses light energy to produce ATP and to split water molecules into H+ ions.

Takes place in the thylakoid membrane.

 

[q] Light Independent Reactions (LIR)

[a] (Synthesis) Calvin Cycle, uses ATP and H+ ions to ‘fix’ carbon dioxide. makes glucose.

Occurs in the stroma.

[q] Pigment

[a] Absorbs useful wavelengths of light that contain energy appropriate for photolysis in light dependent reactions

 

[q] Chlorophyll

[a] A chemical pigment that changes the color of reflected light stored in chloroplasts

 

[q] Action Spectrum

[a] Indicates which wavelengths of light can be used by a plant for photosynthesis

[q] Absorption Spectrum

[a] The spectrum used to measure absorption where various wavelengths of light represent different colors of light

 

[q] Photolysis

[a] The splitting or decomposition of a chemical compound by means of light energy or photons

 

[q] Chloroplast

[a] Plastid organelle containing chlorophyll and other pigments. 30-40 in a typical leaf cell.

 

[q] Stroma

[a] A thick fluid between grana where various enzymes, molecules, and ions are found, and where the light independent reaction of photosynthesis occurs

[q] Thylakoid

[a] A disk-like structure in the chloroplast that contains chlorophyll.

Where the light dependent reaction occurs.

Stacked as grana.

Site for electron flow.

The generation of a proton gradient and chemiosmosis.

 

[q] Grana (Granum)

[a] A stack of thylakoids within the chloroplast of plant cells

 

[q] Non-cyclic Photophosphorylation

[a] light dependent, an electron donor is required, oxygen is produced as a waste product.

It consists of two photoreactions, resulting in the synthesis of ATP and NADPH

 

[q] Cyclic Photophosphorylation (electric flow)

[a] Photosystem 1. An electron is excited by light and is used to transform ADP into ATP.

The same electron can be used to repeat the process

 

[q] Photosystem I

[a] Excited e- from PSI enters second electron chain (NADP reductase- add e-).

Redox reaction. Generate excited electrons.

 

[q] Photosystem II

[a] Absorbs light for use to drive the oxidation of water.

Photolysis of H2O. O2 is released.

Builds concentration gradient inside thylakoid.

E- contribute to generate a proton gradient for H+.

 

[q] Photoactivation

[a] The initial stage of photosynthesis in chlorophyll molecules where the presence of light energy raises the total energy

 

[q] Chemiosmosis

[a] The movement of high energy e- through the electron transport chain.

Releases energy to run proton (H+) pump.

Pulls H+ from a high to low concentration gradient (ATP synthase enzyme).

Photophosphorylation.

 

[q] RuBP (Ribulose Biphosphate)

[a] A 5 carbon sugar found in the Calvin Cycle

 

[q] G3P (Glycerate-3-phosphate)

[a] A phosphorylated three-carbon sugar that is an intermediate in the Calvin Cycle

 

[q] TP (Triose Phosphate)

[a] A compound derived from G3P in the light independent reaction and will be used to create glucose

 

[q] ATP (Adenosine Triphosphate)

[a] An organic compound composed of adenosine and 3 phosphates and is the energy molecule for the cell.

 

[q] NADP+

[a] Nicotinamide adenine dinucleotide phosphate – reduced to NADPH in the light dependent reaction of photosynthesis

 

[q] NADPH

[a] Formed from NADP+ and is a reducing agent in the reactions of the Calvin cycle.

 

[q] Rubisco

[a] An enzyme used in the Calvin Cycle (LIR) to fix carbon dioxide by adding it to RuBP

 

[q] Redox reactions (OIL RIG)

[a] Oxidation is losing electrons.

Reduction is gaining electrons.

 

[q] Photophosphorylation

[a] ADP gains extra phosphate, becomes ATP (A-double-phosphate to A-triple-phosphate)

 

[q] Thylakoid space

[a] Very small volume.

A steep proton gradient builds up after relatively few photons of light have been absorbed.

 

[q] Calvin Cycle

[a] Makes sugar from CO2 by using ATP and NADPH.

Makes 3 Carbon Sugar (TP).

3 cycles to make 1 G3P.

6 cycles to make glucose.

 

[q] Carbon Fixation (Phase 1 of calvin cycle)

[a] CO2 attaches to a 5 carbon sugar (RuBP) by enzyme rubisco.

Becomes 6 carbon (unstable). Splits in half to become CCCP

 

[q] Reduction (Phase 2 of calvin cycle)

[a] Each CCCP receives a phosphate from ATP (PCCCP).

NADPH donates e- to P reduction (CCCP). Creates TP

 

[q] Regeneration (Phase 3 of calvin cycle)

[a] TP meets with 4 other TP.

Uses ATP to ADP to create PCCCCCP RuBP.

Ready for CO2 to begin cycle again.

 

[q] 18 ATP, 12 NADPH

[a] To make 1 glucose (how many ATP and NADPH)

 

[q] C1.3.1—What is photosynthesis?

What transformations of energy take place?

Using what reactants?

[a] Photosynthesis transforms light energy into chemical energy;

Chlorophyll absorbs light energy;

enabling the synthesis of glucose and oxygen;

from water and carbon dioxide;

this is the way carbon compounds are created;

providing chemical energy for life processes in ecosystems;

 

[q] C1.3.2—How is carbon dioxide converted into glucose?

What else is required?

[a] Using hydrogen from splitting water molecules;

driven by light energy;

The equation is: Carbon Dioxide + Water → Glucose + Oxygen.

 

[q] C1.3.3—How is oxygen produced from photosynthesis?

By what groups of organisms?

[a] Oxygen in photosynthesis is a by-product from splitting water molecules;

This process occurs in plants, algae, and cyanobacteria

 

[q] C1.3.4—How can we separate and identify the light-absorbing pigments used in photosynthesis?

[a] Using chromatography which separates and identifies photosynthetic pigments;

Pigments are run using a solvent;

For example, propanone;

Pigments are separated by solubility;

and move different distances through the paper;

Chromatogram is produced;

either by thin-layer or paper chromatography;

 

[q] C1.3.4—What is an Rf value?

How can it be calculated?

[a] Measure the distance from the origin to the pigment;

Measure the distance to the solvent front using a ruler;

Divide the distance to pigment by the distance to the solvent front;

This gives an Rf value

which can be compared to known pigment samples;

 

[q] C1.3.5—What is visible light composed of?

[a] Visible light has a range of wavelengths;

these wavelengths are associated with colors;

with violet having the shortest wavelength; 400nm;

and red has the longest wavelength; 700nm

 

[q] C1.3.5— What wavelengths of light are absorbed by photosynthetic pigments?

What happens as a result of the absorption of light energy?

[a] Photosynthetic pigments, like chlorophyll, absorb specific wavelengths of light;

This absorption leads to the excitation of electrons and is crucial for the transformation of light energy into chemical energy in photosynthesis;

chlorophyll absorbs mainly red and blue light, reflecting green light;

 

[q] C1.3.6—What is an action spectrum?

How does the action spectrum appear for chlorophyll?

[a] An action spectrum is a graph showing the rate of photosynthesis when using different wavelengths of light;

Most activity happens in the blue and red wavelengths not in the green middle section);

 

[q] C1.3.6— What is an absorption spectrum?

How does the absorption spectrum appear four chlorophylls?

[a] An absorption spectrum is a graph showing the percentage of light absorbed at different wavelengths by a pigment or a group of pigments;

 

[q] C1.3.6— What are the similarities and differences between action and absorption spectrum?

[a] Both show the impact of different wavelengths of light;

The absorption spectrum shows wavelengths of light absorbed by photosynthetic pigments; whereas the action spectrum indicates the effectiveness of different wavelengths in causing photosynthesis to occur;

 

[q] C1.3.7—What factors can limit the rate of photosynthesis?

[a] A limiting factor is a factor that can decrease the rate of photosynthesis below its optimum level;

Temperature, carbon dioxide, light intensity;

 

[q] C1.3.7—How do temperature, carbon dioxide and light intensity affect the rate of photosynthesis? Detailed explanation

[a] Light intensity: rate of photosynthesis increases as light intensity increases

photosynthetic rate levels-off at high light levels

as other factors are then limiting;

Carbon dioxide: photosynthetic rate increases as CO2 concentration increases; 

up to a maximum when rate levels-off

Temperature: rate of photosynthesis increases with increase in temperature; 

due to more kinetic energy and more successful collisions; up to maximum level;

high temperatures reduce the rate of photosynthesis; due to denaturing of enzymes;

 

[q] C1.3.8—How can carbon dioxide enrichment experiments in greenhouses be used to predict future rates of photosynthesis and plant growth?

[a] Concept: In these experiments, plants are grown in controlled greenhouse environments where the CO2 concentration is artificially increased.

Example: A typical experiment might involve growing a crop like wheat or soybeans in a greenhouse with CO2 levels elevated to, say, 700 parts per million (ppm), compared to the current atmospheric level of around 400 ppm.

Observations: Researchers can observe changes in the rate of photosynthesis, plant growth patterns, yield quantity, and quality;

 

[q] C1.3.8—What are Free Air Carbon-dioxide Experiments (FACE)?

How can they be used to predict future rates of photosynthesis and plant growth?

[a] Free Air Carbon-dioxide Experiments (FACE) involve having a network of pipes that give out extra carbon dioxide; so they are open to the air;

however, plants within the enriched area receive much higher levels of carbon dioxide;

Researchers can observe changes in the rate of photosynthesis, plant growth patterns, yield quantity, and quality;

 

[q] AHL ONLY – C1.3.9—What are photosystems and how do they generate and emit excited electrons?

[a] Photosystems are molecular arrays of pigment molecules;

they located in membranes of cyanobacteria and within chloroplasts; in eukaryotes;

They consist of chlorophyll and accessory pigments;

with a special chlorophyll at the reaction center;

When light is absorbed, the reaction center emits an excited electron

which carries energy required for further steps;

 

[q] AHL ONLY – C1.3.10—What are the advantages of structured arrays of different types of pigment molecules in a photosystem?

[a] The structured array of different pigments in a photosystem is crucial because a single pigment molecule, like chlorophyll, cannot perform photosynthesis independently;

This array allows for efficient light absorption;

and energy transfer within the photosystem;

 

[q] AHL ONLY – C1.3.11—How and where is oxygen generated by photosynthesis?

(Light dependent reactions)

[a] Light dependent reactions:

photolysis of water occurs;

In photosystem II;

generating oxygenprotons, and electrons;

The oxygen is released as a waste product;

while the protons and electrons are used in other steps of photosynthesis;

 

[q] AHL ONLY – C1.3.12—How and where is ATP produced in photosynthesis?

(Light dependent reactions)

[a] Light dependent reactions:

ATP production occurs in the thylakoids; in chloroplasts;

through using the energy from a proton gradient;

which is generated when excited electrons carrying energy from photosystems;

are used to pump protons into the thylakoid space;

protons diffuse through ATP synthase;

with the kinetic energy turning it, creating ATP from ADP + Pi (inorganic phosphate);

Excited electrons come from either photosystem I in cyclic photophosphorylation;

or photosystem II in non-cyclic photophosphorylation;

 

[q] AHL ONLY – C1.3.13— What is reduced NADP?

Where is it formed?

(Light dependent reactions)

[a] Light dependent reactions:

NADP is reduced; by photosystem I; in the stroma;

to reduced NADP;

by accepting two electrons from photosystem I;

and a hydrogen ion from the stroma;

[q] AHL ONLY – C1.3.14—What is chemiosmosis

(Light dependent reactions)

[a] Light dependent reactions:

Chemiosmosis is the production of ATP using a gradient of protons

and energy from electron transport;

Protons diffuse down their concentration gradient;

from the thylakoid space;

through ATP synthase;

producing ATP;

 

[q] AHL ONLY – C1.3.14—Where do the light-dependent reactions take place?

[a] The light-dependent reactions take place in the thylakoids;

which contain:

Photosystem II;

ATP synthase;

a chain of electron carriers;

Photosystem I;

leading to photolysis of water;

synthesis of ATP by chemiosmosis;

and reduction of NADP;

 

[q] AHL ONLY – C1.3.15—What is rubisco?

What is its role in carbon fixation?

What cycle is involved?

(Light-independent reactions)

[a] Take place in the stroma of chloroplasts;

Rubisco is the most abundant enzyme on Earth;

it catalyzes the first major step of the Calvin cycle;

the fixation of carbon dioxide; to RuBP;

The Calvin cycle then produces glycerate 3-phosphate;

Rubisco operates slowly and inefficiently in low CO2 concentrations;

so high concentrations of Rubisco are needed in the chloroplast stroma are needed;

 

[q] AHL ONLY – C1.3.16—What is Triose phosphate?

How is it formed?

What is required? (Light-independent reactions)

[a] (Light-independent reactions)

In the Calvin cycle, glycerate-3-phosphate (GP);

formed from CO2 fixation;

is converted into triose phosphate (TP);

This conversion requires energy from ATP;

and reducing power from Reduced NADP respectively;

Triose phosphate is a crucial intermediate;

leading to the synthesis of glucose;

 

[q] AHL ONLY – C1.3.17—How is RuBP regenerated in the Calvin cycle?

What is required? (Light-independent reactions)

[a] The regeneration of RuBP;

in the Calvin cycle is critical for it to continue;

Regeneration involves converting five out of six molecules of triose phosphate;

back into three molecules of RuBP;

using ATP;

if there is no ATP, then GP accumulates;

[q] AHL ONLY – C1.3.18—How are other carbon compounds produced using products of the Calvin cycle and mineral nutrients?

(Light-independent reactions)

[a] The Calvin cycle fixes carbon producing intermediates that serve as precursors for the synthesis of various organic compounds;

like carbohydrates, amino acids, and others;

These compounds are synthesized through metabolic pathways that often trace back to an intermediate of the Calvin cycle;

demonstrating the cycle’s central role in the biosynthesis of essential organic molecules.

[q] AHL ONLY – C1.3.19—How do the light-independent reactions depend on the light dependent?

[a] The light-dependent reactions generate ATP and NADPH;

which are essential for the light-independent reactions (Calvin cycle);

A lack of light reduces the light-dependent reactions;

reducing ATP and Reduced NADP supply;

thereby reducing or stopping the Calvin cycle;

if there is no ATP or reduced NADP, then GP accumulates;

Triose Phosphate cannot be made; 

so Glucose can’t be made;

Not enough Carbon dioxide affects the light-dependent reactions;

as NADP+ cannot be remade by the Calvin cycle;

so reduction by photosystem II to reduced NADPH stops; 

[q] Previous IB essay on how light-dependent reactions depend on light-independent reactions

[a] Reduced NADP (NADPH) is produced in the light-dependent reactions;


through reduction of NADP+ using electrons which are photoactivated; by photosystem II; ATP is produced;


using the proton gradient created when protons are pumped into thylakoid space by electron carriers; by photosphorylation;


as light-dependent reactions produce ATP and NADPH, and they are used up and would run out without light;


if there is no ATP and reduced NADP,


Glycerate-3-phosphate cannot be reduced to triose phosphate;


RuBP is therefore not regenerated, (as triose phosphate is used to regenerate RuBP);


and as ATP required for RuBP regeneration from triose phosphate;


carbon dioxide fixation therefore stops;


Glycerate-3-phosphate accumulates;


and there would be no RuBP to add to carbon dioxide;


stomata also close in the dark;


carbon dioxide is therefore not absorbed;

[q] C1.3.1—Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis 

This energy transformation supplies most of the chemical energy needed for life processes in ecosystems.

[a]  Photosynthesis is the conversion of light energy into chemical energy, which is essential to life on earth.
Not only does it produce energy-rich carbohydrates but oxygen as well, both of which have been crucial to the establishment of complex ecosystems.

[q] C1.3.2—Conversion of carbon dioxide to glucose in photosynthesis using hydrogen obtained by splitting water 

Students should be able to write a simple word equation for photosynthesis, with glucose as the product.

[a]

Photosynthesis is composed of light-dependent and light-independent reactions.
Light dependent reactions occur in the thylakoids of chloroplasts where light energy is used to split water in a process called photolysis (“photo” meaning light and “lysis” meaning breaking down).

This assists in converting light energy into chemical energy.
Light independent reactions occur in the stroma of chloroplasts where the chemical energy generated from the light dependent stages is used to fuel the production of glucose from carbon dioxide.

[q] C1.3.3—Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria

Students should know the simple word equation for photosynthesis. They should know that the oxygen produced by photosynthesis comes from the splitting of water. 

[a] In plants, algae, and cyanobacteria, oxygen is a by-product of photosynthesis which results from the photolysis of water.

[q] C1.3.5—Absorption of specific wavelengths of light by photosynthetic pigments

Include excitation of electrons within a pigment molecule, transformation of light energy to chemical energy and the reason that only some wavelengths are absorbed. Students should be familiar with absorption spectra. Include both wavelengths and colors of light in the horizontal axis of absorption spectra.

[a] A photon is a type of electromagnetic radiation that can be seen by humans as visible light.

The higher the energy of a photon, the shorter its wavelength, and vice versa.

When electrons within a pigment molecule like chlorophyll absorb a photon, they become excited/filled with energy and aid in the transformation of light energy to chemical energy.

The amount of energy/wavelength absorbed depends on the chemical identity of the pigment molecule.

[q] C1.3.4—Separation and identification of photosynthetic pigments by chromatography

Application of skills: Students should be able to calculate Rf values from the results of chromatographic separation of photosynthetic pigments and identify them by color and by values.
Thin-layer chromatography or paper chromatography can be used.

[a]

[q] C1.3.6—Similarities and differences of absorption and action spectra 

Application of skills: Students should be able to determine rates of photosynthesis from data for oxygen production and carbon dioxide consumption for varying wavelengths.

They should also be able to plot this data to make an action spectrum. 

[a] Absorption spectra are graphs depicting the wavelengths absorbed by a chemical substance, like chlorophyll.

The x-axis represents the wavelengths in nanometers and the y-axis shows the amount of light absorbed from 0-100% or arbitrary units (au).

Action spectra are graphs depicting rate of photosynthesis (y-axis) plotted against wavelengths of visible light (x-axis).

[q] C1.3.7—Techniques for varying concentrations of carbon dioxide, light intensity or temperature experimentally to investigate the effects of limiting factors on the rate of photosynthesis 

Application of skills: Students should be able to suggest hypotheses for the effects of these limiting factors and to test these through experimentation.

[a] Carbon dioxide: increasing CO2 concentration increases the rate of photosynthesis until another limiting factor becomes short in supply. CO2 levels can be varied through several methods:
• Using aquatic photoautotrophs and dissolving different amounts of sodium bicarbonate
• In a sealed chamber, CO2 injectors can vary gas concentrations with a sensor to help in monitoring
Light intensity: increasing light intensity increases the rate of photosynthesis until another limiting factor becomes short in supply.
• Adjusting distance of photoautotroph from light source
• Adjusting light source intensity then measuring light intensity using a lux meter
Temperature: increasing temperature increases rate of photosynthesis up until the optimum temperature is reached in which any further increase causes denaturation of enzymes like rubisco, decreasing the rate of photosynthesis.
• Using a water bath and monitoring using thermometer
• Changing temperature of the atmosphere through air conditioning or enclosed chamber

[q] C1.3.8—Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth 

Include enclosed greenhouse experiments and free-air carbon dioxide enrichment experiments (FACE).

[a] With the rise in CO2 levels since the industrial revolution, it is important to also understand how this might not only affect photoautotrophs but the ecosystems in which they exist in as well.

Although there is a lot of research regarding the effects of CO2 on plants, most of these studies have been done in very controlled labs and greenhouses.

While this has allowed us to better understand the physiology of plants, it provides little insight into how these environmental factors influence ecosystems as a whole.
Free-Air Carbon Dioxide Enrichment (FACE) experiments enable the study of increased CO2 concentrations on plants and ecosystems under natural (free-air) conditions without confinement or enclosure (not within a greenhouse or lab, where plant competition and other ecological mechanisms are not considered).

This is done through installing CO2 blowers/injectors in certain areas of an ecosystem and relying on wind to disperse the gas in order to raise the levels of CO2 and observe its effects.

Some hypotheses around these experiments include:
• Increases in CO2 will increase crop yield (result: FACE experiments showed smaller increases in crop growth than expected)
• Different types of plants will respond more to increased CO2 than others (result: FACE experiments showed that trees were more responsive than grasses)
Since FACE experiments take years to complete, there is still a lot of data needed before predicting future rates of photosynthesis and plant/ecosystem growth with sufficient certainty.

[q] NOS: Hypotheses are provisional explanations that require repeated testing.

During scientific research, hypotheses can either be based on theories and then tested in an experiment or be based on evidence from an experiment already carried out.

Students can decide in this case whether to suggest hypotheses for the effects of limiting factors on photosynthesis before or after performing their experiments.

Students should be able to identify the dependent and independent variable in an experiment.

[a] An independent variable is the variable being manipulated/changed in an experiment so as to explore its effects; it is the ‘cause’.
A dependent variable is the variable being measured/observed in an experiment; it is the ‘effect’.
Hypotheses are provisional explanations that require repeated testing. During scientific research, hypotheses can either be based on theories and then tested in an experiment or be based on evidence from an experiment already carried out

[q] NOS: Finding methods for careful control of variables is part of experimental design.

This may be easier in the laboratory but some experiments can only be done in the field. Field experiments include those performed in natural ecosystems.

Students should be able to identify a controlled variable in an experiment.

[a] Controlled variables remain unchanged/fixed during an experiment to reach valid and precise conclusions. 

[q] C1.3.9—Photosystems as arrays of pigment molecules that can generate and emit excited electrons

Students should know that photosystems are always located in membranes and that they occur in cyanobacteria and in the chloroplasts of photosynthetic eukaryotes.

Photosystems should be described as molecular arrays of chlorophyll and accessory pigments with a special chlorophyll as the reaction center from which an excited electron is emitted.

[a] Photosystems are protein complexes embedded within thylakoidal membranes that aid in the conversion of light energy to chemical energy and are composed of two main components:
Antenna complex: consists of several light-harvesting protein complexes that contain accessory pigments (chlorophyll a, chlorophyll b, carotenoids).

These pigments protect the special chlorophylls in the reaction center from oxidation by absorbing sunlight and transfer this light energy between pigments within the antenna until reaching the reaction center.

Thus, the antenna complex acts as a funnel by harnessing and directing light energy to a site where it can be effectively used.
Reaction center: a protein complex that contains a special pair of chlorophyll molecules whose electrons, once excited by energy passed from accessory pigments, are immediately transferred to carriers in the Electron Transport Chain (ETC).
There are two types of photosystems (named according to the order in which they were discovered and not in the order of electron transfer) in thylakoids which have the same structure but differ on the basis of what they oxidize and reduce.
Photosystem II (PSII) is the first one involved in electron transfer which oxidizes water (photolysis) and reduces an electron-carrier molecule (plastoquinone, PQ).
Photosystem I (PSI) is the second photosystem involved in electron transfer which oxidizes an electron-carrier molecule (plastocyanin, PC) and reduces NADP+.

[q] C1.3.10—Advantages of the structured array of different types of pigment molecules in a photosystem

Students should appreciate that a single molecule of chlorophyll or any other pigment would not be able to perform any part of photosynthesis.

[a] A pigment is a molecule whose electrons selectively absorb some wavelengths and reflect others.

The reflected wavelengths that aren’t absorbed mix together to form the pigment’s color.

There are several advantages of the structured array of different types of pigment molecules in a photosystem:
• The diversity of pigments broadens the range of wavelengths that can be absorbed by a photosystem, so collectively a greater amount of sunlight can be captured
• Some of the pigments help protect against photodamage by dissipating excess energy as heat
• The proximity of pigments to other ones in the antenna complex facilitates efficient and effective transfer of energy

[q] C1.3.11—Generation of oxygen by the photolysis of water in photosystem II

Emphasize that the protons and electrons generated by photolysis are used in photosynthesis but oxygen is a waste product.

The advent of oxygen generation by photolysis had immense consequences for living organisms and geological processes on Earth.

[a] When the special chlorophylls in PSII lose their two electrons to an electron carrier (plastoquinone), they become a strong oxidizing agent.

This causes the photolysis of water according to the following equation:

The protons produced contribute to the proton gradient during chemiosmosis.
The electrons produced compensate for those lost from the special chlorophylls, 2 for each molecule within the pair.
The oxygen generated is a waste product and leaves the plant through the stomata.

The production of oxygen billions of years ago had immense consequences for living organisms and geological processes on Earth.

Photoautotrophs first evolved inside oceans, so oxygen in water reacted with dissolved iron to produce iron oxide, which precipitated and formed banded iron formations (rocks).

Once iron and other elements in the ocean were oxidized, oxygen started to accumulate in the atmosphere, leading to the Great
Oxidation Event and the evolution of aerobic respiration.

[q] C1.3.12—ATP production by chemiosmosis in thylakoids

Include the proton gradient, ATP synthase, and proton pumping by the chain of electron carriers.
Students should know that electrons are sourced, either from photosystem I in cyclic photophosphorylation or from photosystem II in non-cyclic photophosphorylation, and then used in ATP production.

[a] Once electrons are transferred from the special chlorophylls to the electron-carrier molecules, they move through the ETC, and their energy is used by cytochrome complexes to pump protons into the thylakoid lumen.

Due to the small size of thylakoids, a steep proton gradient is quickly established between the stroma (low H+ concentration) and the thylakoid lumen (high H+ concentration).

This causes the protons to passively diffuse into the stroma through ATP synthase via chemiosmosis, which generates the ATP necessary to fuel the light-independent reactions later on.

The final electron acceptor in the ETC is PSI, which accepts the now low-energy electrons from plastocyanin into its reaction center.

Each of the electrons received are boosted to a high-energy state by light energy funneled through the antenna complex and then used to reduce NADP+ in the stroma.

This process is called noncyclic photophosphorylation, which produces around 1 ATP molecule per pair of electrons passed to NADP+.
Cyclic photophosphorylation occurs when the cell needs to produce more ATP molecules to power the light-independent reactions.

Instead of passing the re-energized electrons to NADP+, PSI passes them back to the cytochrome complexes in order to pump more protons into the thylakoid lumen, thereby increasing the proton gradient that drives ATP synthase to produce more ATP.

The cell has mechanisms to regulate how much light energy is converted into high-energy phosphate bonds (ATP) and how much into reducing power (NADPH).

[q] C1.3.13—Reduction of NADP by photosystem I

Students should appreciate that NADP is reduced by accepting two electrons that have come from photosystem I.

It also accepts a hydrogen ion that has come from the stroma.

The paired terms “NADP and reduced NADP” or “NADP+ and NADPH” should be paired consistently.

[a] NADP (nicotinamide adenine dinucleotide phosphate) is the same as a typical NAD molecule but with an added phosphate group.

NADP+ can undergo reduction to gain electrons and a hydrogen ion (NADPH), or oxidation to donate electrons and a proton.

NADP reductase is an enzyme embedded within the thylakoid membrane that catalyzes the reduction of NADP+ to NADPH.

Since a proton is used to reduce NADP+, this decreases the H+ concentration of the stroma which further increases the steepness of the proton gradient.

[q] C1.3.14—Thylakoids as systems for performing the light-dependent reactions of photosynthesis

Students should appreciate where photolysis of water, synthesis of ATP by chemiosmosis and reduction of NADP occur in a thylakoid. 

[a] Thylakoids are membrane-bound compartments within chloroplasts that perform the light-dependent reactions of photosynthesis.

A stack of disc-shaped thylakoids is called a granum (plural grana), which is connected to other grana via lamellae (unstacked stroma thylakoid).
PSII are mainly concentrated in grana and PSI in lamellae.

Since lamellae have more access to the stroma than grana, NADP+ reduction is easier in them compared to grana.

[q] C1.3.15—Carbon fixation by Rubisco

Students should know the names of the substrates RuBP and CO2 and the product glycerate 3- phosphate.

They should also know that Rubisco is the most abundant enzyme on Earth and that high concentrations of it are needed in the stroma of chloroplasts because it works relatively slowly and is not effective in low carbon dioxide concentrations.

[a] The Calvin Cycle constitutes the light-independent reactions of photosynthesis and involves 3 main steps:
1. Carbon fixation
2. Reduction
3. Regeneration
CO2 is a non-polar and small molecule that can easily diffuse through stomatal openings into plant cells.
This makes it a suitable provider of the carbon atoms needed as backbone for biomolecules like glucose.
In carbon fixation, the enzyme Rubisco catalyzes the reaction between CO2 molecule and ribulose bisphosphate (RuBP), a 5C compound with 2 phosphate groups, which produces 2 glycerate-3- phosphate (GP) molecules.
Rubisco is the most abundant enzyme on earth and found in all three domains of life.

It works relatively slowly compared to most enzymes, which according to evidence may be because of the reciprocal relationship between enzyme specificity and activity.

Ever since the great oxidation event, the enzyme had to evolve to precisely distinguish between CO2 and O2 molecules, which came at the cost of a reduced catalytic rate.

The enzyme is also not very effective at low CO2 concentrations, so high concentrations of
Rubisco are needed within the stroma of chloroplasts.

[q] C1.3.16—Synthesis of triose phosphate using reduced NADP and ATP

Students should know that glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP.

[a] In the reduction step, the 2 GP molecules are reduced into triose phosphate (TP), a 3C compound, by hydrolyzing 2 ATP molecules into ADP + Pi and oxidizing 2 NADPH molecules into NADP+.

[q] C1.3.17—Regeneration of RuBP in the Calvin cycle using ATP

Students are not required to know details of the individual reactions, but students should understand that five molecules of triose phosphate are converted to three molecules of RuBP, allowing the Calvin cycle to continue. If glucose is the product of photosynthesis, five-sixths of all the triose phosphate produced must be converted back to RuBP.

[a] In the regeneration step, and for every 3 Calvin cycles, 1 TP molecule is used to make glucose while the other 5 are used to regenerate RuBP to enable sustainable production.

3 ATP molecules are hydrolyzed into 3ADP + 3Pi in this step per 3 molecules of CO2 fixed.
Since 6 carbon atoms make up the structure of glucose, 6 CO2 molecules need to be fixed, so 6 Calvin Cycles need to be performed in order to produce 1 glucose molecule.

[q] C1.3.18—Synthesis of carbohydrates, amino acids and other carbon compounds using the products of the Calvin cycle and mineral nutrients

Students are not required to know details of metabolic pathways, but students should understand that all of the carbon in compounds in photosynthesizing organisms is fixed in the Calvin cycle and that carbon compounds other than glucose are made by metabolic pathways that can be traced back to an intermediate in the cycle. 

[a] All of the carbon in compounds in photosynthesizing organisms is fixed in the Calvin cycle. Other carbon compounds, like amino acids and lipids, are made by metabolic pathways that can be traced back to an intermediate in the cycle. For example, amino acids can be synthesized using TP and fatty acids using GP with the addition of phosphates and sulfurs from mineral nutrients like ammonium.

[q] C1.3.19—Interdependence of the light-dependent and light-independent reactions 

Students should understand how a lack of light stops light-dependent reactions and how a lack of CO2 prevents photosystem II from functioning.

[a] The light-dependent and independent reactions are interconnected and interdependent on each other.

If the light-dependent reactions do not occur, the Calvin cycle cannot proceed as it has no ATP and NADPH to fuel it, and if the Calvin cycle does not take place, then the light-dependent reactions will not have the biomolecules it needs to function.
Lack of CO2 prevents PSII from functioning, because there is no need to produce more ATP and NADPH if there is no carbon dioxide for the Calvin cycle to fix into carbohydrate.

 

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IB DP Biology HL C1.3 Photosynthesis Flashcards

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