Topic 8.2 (adapted from Stephen Taylor)

Photosynthesis

Photosynthesis defined…

• Process used by plants, algae and some bacteria to produce organic compounds

The Chloroplast

• Chloroplast: Site of photosynthesis in eukaryotic cells.
• Thylakoids: Disk shaped membranes containing photosynthetic pigments. Site of light dependent reactions.
• Grana: Stacks of thylakoids.
• Stroma: Fluid filled space surrounding grana. Site of light independent reactions.

Image source unknown. Question from the IBO.

2 Main Stages of Photosynthesis

Light Dependent

• Input:
• Light Energy
• Water
• Outcome:
• ATP
• H+ ions

Light Independent

• Input:
• ATP
• H+ ions
• Output:
• O₂
• Glucose

Properties of Light

• Sunlight is a form of electromagnetic radiation, transmitted as waves or photons
• Sunlight is a mixture of all colors of light. Each color has a different wavelength, and thus a different amount of energy

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• Frequency: # of waves per unit of time

high frequency= high energy

• Wavelength: distance between the 2 wave crests

low frequency = long wavelength

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Photosynthetic Pigments

• Photosynthetic plant pigments have the capability to absorb certain wavelengths (colors) of light and change the light energy to chemical energy
• The pigments are found in chloroplasts on the thylakoids

4 plant pigments

• Chlorophyll a: light to medium green, main photosynthetic pigment
• Chlorophyll b: blue-green, accessory pigment
• Carotene: orange, accessory pigment
• Xanthophyll: Yellow, accessory pigment
• Accessory pigments absorb other colors of light (green) that chlorophyll a can’t

• Photosystems: groups of accessory pigments surrounding a central chlorophyll a
• Collect light energy and send energized electrons to the central chlorophyll a to be used in the light dependent reactions

Spectra used by Photosynthesis

• Action:
• Range of wavelengths of light which can be used in the light dependent reactions
• Absorption:
• Range of wavelengths of light that are absorbed by chlorophyll

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Two major steps (reactions) of Photosynthesis

• Light Dependent Reactions (LDR) – needs light to occur. Makes ATP, NADPH, and O₂
• Light Independent Reactions (LIR) – may occur in light or dark. Converts CO₂ to sugars

• The ATP & NADPH accumulate and diffuse to the stroma where they are used to make sugar.
• ATP is a short-term energy carrier and main energy source for all cells
• NADPH is a short-term energy carrier and also carries electrons & H⁺ ions from the breakdown of water

• Photosystem I (PSI) – part of the LDR where chlorophyll is energized a 2^nd time to make 2 NADPH. The reaction center of PSI uses light energy of 700 nm
• Photosystem II (PSII) – part of the LDR where photolysis occurs and chemiosmosis makes ATP via photophosphorylation. The reaction center of PSII uses light energy of 680 nm

Light Dependent Reactions of Photosynthesis

• All the reactions of photosynthesis that are directly dependent upon light
• Occur in the part of the cell known as the thylakoids (Stacks of thylakoids are known as grana.).
• Many chlorophyll molecules are found embedded into the membranes of the thylakoids

• The purpose of the light reactions is to convert light energy into chemical energy in the form of ATP & NADPH.
• ATP & NADPH are energy carriers. They carry energy to the 2nd part of photosynthesis, known as the dark reactions.
• The light dependent reactions can be split into the following specific steps:
• Light absorption & splitting of water
• Production of ATP
• Movement of electrons through electron acceptors to power a hydrogen pump
• Re-energizing electrons so they can produce NADPH

Two main parts of the LDR

• Cyclic Photophosphorylation – excited electrons are cycled through a series of electron acceptors to make ATP
• Noncyclic Photophosphorylation –
• Light energizes chlorophyll, which splits H₂O (photolysis). The O₂ is released to air
• More light energizes another chlorophyll which releases energy to make 2 ATP
• A different wavelength of light energizes another chlorophyll, which then releases energy to make 2 NADPH

• Photophosphorylation – Use of light energy in LDR to make ATP
• Chemiosmosis – Use of H⁺ gradient to create ATP during LDR.. Hydrogen pump transports H⁺ into thylakoid. H⁺ also made by photolysus. H⁺ diffuses through thylakoid membrane via protein channel.
• ATP Synthase – takes energy from the flow of protons and gives it to ADP and P to make ATP

• Cyclic photophosphorylation is part of Photosystem I. It is perhaps the earliest way cells formed ATP.
• In modern plants, it occurs when so much NADPH is made that no NADP+ is available to pick up the energized electron & H+ (NADP+ is the final electron acceptor). This potentially could shut down PS I. Instead, the excited electrons are shunted through the cyclic electron transport chain allowing it to keep using the excited electrons while creating ATP.

• Non-cyclic Photophosphorylation produces ATP using the energy from excited electrons provided by PSII
• As electrons pass through the non-cyclic pathway, they do not return to the original photosystem (this does not create a cycle)

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• Photolysis provides H+ ions to replace those lost in the photosystems.
• The excited electrons provide energy for a proton pump to actively transport additional H+ into the thylakoid.
• The high concentration of H+ diffuse past ATP synthase as they pass out of the membrane to the lower H+ concentration.
• The energy created as H+ passes the ATP synthase forms ATP.
• (Diffusion of H+ ions from high to low concentration through ATP synthase to form ATP is called chemiosmosis.)

• The electrons provided by non-cyclic photophosphorylation are boosted a 2nd time to a higher energy level by photosystem I to then create 2 NADPH.

LIR of Photosynthesis

• All reactions of photosynthesis not directly dependent upon light are known as the light independent reactions
• They occur whether there is light present or not
• They only depend on the presence of the energy carriers ATP and NADPH made during the LDR

• The LIR occur in the part of the chloroplast known as the stroma
• The purpose of the light independent reactions is to take the energy from ATP and energized electrons and hydrogen ions from NADPH and add them to CO₂ to make glucose (sugar)
• The LIR reduce CO₂ by adding energized electrons and protons (H⁺) to it and removing one oxygen atom. This effectively converts the CO₂ to CH₂O

Measuring Rates of Photosynthesis

• The rate of photosynthesis may be measured by
• the quantity of carbon dioxide consumed per unit of time (direct)
• measuring the quantity of oxygen produced per unit of time (direct)
• The increase in biomass of a pre-determined area of plants per unit time (indirect)
• Rate determined by the rate-limiting step – rate/step that is occurring the slowest

Factors Affecting Photosynthetic Rate

• Light Intensity
• Carbon Dioxide Concentration
• Temperature

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• Each factor affects a different rate-limiting step

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How Light Intensity Affects Photosynthetic Rate

• As light intensity increases, the photosynthetic rate increases until a point is reached where the rate begins to level off.
• At low light intensity, photosynthesis occurs slowly because only a small quantity of ATP and NADPH is created by the light dependent reactions.
• As light intensity increases, more ATP and NADPH are created, thus increasing the photosynthetic rate.
• At high light intensity, photosynthetic rate levels out, not due to light intensity but due to other limiting factors, including competition between oxygen and carbon dioxide for the active site on RUBP carboxylase.

At low light intensity, rate of photosynthesis is proportional to light intensity.

plateau

At low light intensity, rate of photosynthesis

is proportional to light intensity.

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How Carbon Dioxide Concentration Affects Photosynthetic Rate

• As carbon dioxide concentration increases, the rate of photosynthesis increases.
• At high concentrations, the rate of photosynthesis begins to level out due to factors not related to carbon dioxide concentration. One reason might be that some of the enzymes of photosynthesis are working at their maximum rate.
• In general, carbon dioxide is found in low concentration in the atmosphere, and so atmospheric carbon dioxide levels may be a major limiting factor on photosynthesis when at low levels.

plateau

CO₂ is a substrate in an enyme-catalysed light-dependent reaction.

CO2 is a substrate

in an enzyme-catalysed

light-dependent reaction.

At low CO2 concentration,

rate is positively correlated

with concentration

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How Temperature Affects Photosynthetic Rate

• As temperature increases above freezing, the rate of photosynthesis increases. This occurs because molecules are moving more quickly and there is a greater chance of a collision resulting in a chemical reaction.
• At some point, a temperature is reached that is an optimum temperature. The photosynthetic reaction rate is at its quickest rate at this point.
• Above that temperature, the enzymes begin to denature (as in RUBP carboxylase), slowing the rate of photosynthesis until a temperature is reached where photosynthesis does not occur at all.

Increased temp. gives increased energy and increased rate of photosynthesis

Optimum temperature

Above the optimum temp., enzymes are denatured and rate drops steeply.

Which enzymes are used in respiration?

Above the optimum temp.,

enzymes are denatured

and rate drops steeply.

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Increased temp. gives

increased energy

and increased

rate of photosynthesis

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Optimum temperature