Photosynthesis Flashcards
Write a chemical equation for the overall reaction of photosynthesis
6 H2O + 6 CO2 - C6H12O6 + 6 O2
Outline the overall reaction of photosynthesis
- The production of fuel molecules (like glucose) by using high energy photons to split the strong bonds in water molecules and combining the hydrogen atoms with carbon dioxide.
Oxygen is released as a by-product of the photolysis of
water - It is split into two main stages: the light-dependent reactions and the light-independent reactions
What is a photosynthetic pigment?
A molecule present in chloroplasts that absorbs certain wavelengths of light
What is a photosystem?
A protein complex consisting of an antenna complex and a reaction centre.
It’s involved in the absorption of light and transfer of electrons in photosynthesis. Two types, PS1 and PS2
How to photosystems 1 and 2 differ?
They absorb different wavelengths of light
Describe the process of light harvesting in photosynthesis
Antenna complex absorbs light energy of varying wavelengths and transfers it to the reaction centre.
Energy is absorbed by two chlorophyll a molecules which emit ‘excited’ electrons.
What type of reaction forms ATP?
An endergonic reaction which requires energy from processes like respiration.
Draw the reversible chemical equation for the breakdown and synthesis of ATP
ATP ⇌ ADP + Pi
Describe the structure of ATP
ATP consists of adenosine with 3 phosphate groups bonded to it covalently.
Why is ATP called the universal energy currency?
It’s used as an energy source in all organisms.
Why is ATP only used as a short term energy store?
It is a very unstable molecule
What is the photophosphorylation of ATP?
The use of photons to provide the energy to carry out the unfavorable addition of a phosphate group onto ADP to from ATP
Describe the light-dependent stage of photosynthesis
First stage of photosynthesis. It takes place in the thylakoids of the chloroplast and uses light energy to produce ATP, reduced NADP and oxygen.
State the sources of electrons for the electron transport chain.
Cyclic and non-cyclic photophosphorylation.
Define cyclic photophosphorylation
The formation of ATP involving Photosystem 1 only
Outline cyclic photophosphorylation
Involves photosystem 1 only. Excited electrons enter the electron transport chain to produce ATP and then return to PS1. No reduction of NADP and no water required to replace lost electrons.
What is the purpose of cyclic photophosphorylation?
Produces additional ATP to meet surplus energy demands of the cell.
Define non-cyclic photophosphorylation
The formation of ATP and reduced NADP involving PS1 and PS2.
Outline non-cyclic photophosphorylation
-Involves PS1 and PS2.
-Excited electrons enter the electron transport chain to produce ATP.
-NADP acts as a final electron receptor and is reduced.
-Water is photolysed to compensate for electrons lost from PS2
What is the purpose of non-cyclic photophosphorylation?
Produces ATP and reduced NADP for the Calvin cycle.
How does chemiosmosis produce ATP in the light-dependent stage?
Which enzyme?
-Protons flow down their concentration gradient from the thylakoid space into the stroma via ATP synthase.
-ATP synthase phosphorylates ADP to form ATP as protons flow through it.
Describe photolysis
The splitting of a molecule of water in the presence of water that happens during the light-dependent stage of photosynthesis. This produces protons, electrons and oxygen.
What happens to the products of photolysis?
H+ used in proton pumping and to reduce NADP
e- replaces electrons lost from chlorophyll a in PS2
O2 by-product, used for respiration or diffuses out of the leaf as waste gas.
Explain how the electron transfer chain results in the production of reduced NADP
NADP acts as a final electron receptor and is subsequently reduced.
Describe the light-independent stage of photosynthesis
-Second stage of photosynthesis
-Does not require light energy and takes place in the storm
-Uses CO2 and the products of the light-dependent stage to build organic molecules
What is the light-independent stage also known as?
The Calvin cycle
Name the 3 main stages of the Calvin cycle
- Carbon fixation
- Reduction
- Regeneration
What happens during carbon fixation of the Calvin cycle?
-Reaction between CO2 and ribulose bisphosphate (RuBP) catalyzed by the enzyme RuBisCo
-Forms unstable 6C intermediate that breaks down into two molecules of glycerinate 3-phosphate (GP)
What happens during reduction of the Calvin cycle?
-2 GP are reduced to 2 triose phosphate (TP)
-Requires 2 reduced NADP and 2 ATP during the light-dependent reaction
-Forms 2 NADP and 2 ADP that enter the light-dependent reaction.
What happens during regeneration of the Calvin cycle?
-After 1C leaves the cycle, the 5C compound RuP forms.
-RuBP is regenerated from RuP using 1 ATP
-Forms 1 ADP
How are nutrients produced as a result of photosynthesis?
-Formation of amino acids from GP (requires nitrates and sulfates)
-TP molecules used to produce sugars like glucose, fructose and galactose.
What is an absorption spectrum?
A pattern of bands that occurs when a substance absorbs different wavelengths of light.
What is an action spectrum?
A graph of the rate of photosynthesis against each wavelength of light absorbed by pigment.
What is the purpose of chromatography?
To separate different products from a mixture.
How can pigments in chloroplasts be separated by chromatography?
Different pigments in chloroplasts have different
properties and so will move different amounts
through the chromatography medium, allowing separate pigments to be distinguished
State the equation used to calculate retention value (Rf)
Rf = distance travelled by component / distance travelled by solvent
What factors affect the rate of photosynthesis?
Temperature, light intensity and carbon dioxide concentration.
What is a limiting factor?
A variable that limits the rate of a particular reaction.
Explain how t° affects the rate of photosynthesis
Higher t° provide more KE for enzymes involved in photosynthesis, so the rate increases as t° rises. Optimum t° is 25°C and if it becomes too high, the enzymes denature and the rate of photosynthesis decreases.
Explain how light intensity affects the rate of photosynthesis
Rate of photosynthesis is directly proportional to light intensity. Light intensity increases, rate of photosynthesis increases.
Why does the rate of photosynthesis eventually plateau even if light intensity continues to increase?
Another factor (t° or CO2 concentration) becomes limiting.
Explain how carbon dioxide concentration affects the rate of photosynthesis
As carbon dioxide concentration increases, the rate of photosynthesis increases.
When does t° become a limiting factor?
When temperature drops too low on cold winter days.
When does light intensity become a limiting factor?
At night
When does carbon dioxide concentration become a limiting factor?
Atmospheric CO2 concentrations generally remain constant. CO2 concentration only becomes limiting when light intensity and t° are not limiting factors.
