Photosynthesis Flashcards
What is an autotroph?
An organism that synthesises its own nutrients
What is a photoautotroph?
An organism that uses light to synthesise nutrients via photosynthesis
What is the overall photosynthesis equation?
6CO2 + 6H2O = C6H12O6 + 6O2
remember the devils equation (666)
What is a photon?
A particle of light
What is carbon fixation?
The process where carbon dioxide is converted to sugars
What does Carbon fixation do in terms of the environment?
Helps to regulate CO2 levels in the atmosphere and oceans
What is it called when rates of photosynthesis and respiration are equal in a plant?
Compensation point
Describe the general structure of a chloroplast
Double membrane with an inter-membrane space
Stroma - a fluid matrix
Grana - Stacks of thylakoid membranes
Inter-granal lamellae - Connect grana together
What stage of photosynthesis takes place in the grana?
Light-dependent stage
Describe the structure of the thylakoids
A membrane that is folded into a disc-like sac
Why are the thylakoids folded?
Provides a large surface area which is important for:
Distribution of photosystems and photosynthetic pigments
Distribution of electron carriers and ATP synthase enzymes
Where do the products of the light-dependent stage go?
Diffuse out of thylakoids into stroma
What is the stroma made up of?
Enzymes needed to catalyse light independent stage
Starch grains
Oil droplets
Small ribosomes
Describe the structure of a Photosystem
A funnel shaped structure in the thylakoid membrane
Contains photosynthetic pigments which absorb specific wavelengths of light
A primary pigments reaction centre at the bottom which is either P700 or P680
What are the two types of Chlorophyll a in a photosystem?
P700 and P680
Why do photosystems contain a range of pigments?
So the plant can absorb a range of appropriate wavelengths
Which photosystem would you find P700 in?
PS1
Which photosystem would you find P680 in?
PS2
Name 2 accessory pigments?
Carotenoids
Xanthophylls
What are the 4 stages of the Light-dependent stage?
Light harvesting in Photosystems
Photolysis of water
Photophosphorylation
Formation of NADPH (reduced NADP)
What catalyses Photolysis?
An enzyme in PS2
Which Photosystem does photolysis occur in?
PS2
Why does photolysis occur?
To provide H+ ions for photophosphorylation
Provide e- to replace lost electrons in photophosphorylation
What is photophosphorylation?
Production of ATP from ADP and Pi
What are the two types of photophosphorylation?
Non-cyclic
Cyclic
Which photosystem (s) does non-cyclic photophosphorylation occur in?
PS1 and PS2
Which photosystem (s) does cyclic photophosphorylation occur in?
Only PS1
Which type of photophosphorylation produces more ATP?
Non-cyclic
Describe the process of Non-cyclic photophosphorylation
In PS2:
Photon strikes PS2 and energy is channeled to primary pigment
Energy excites a pair of electrons which escape the chlorophyll molecule
Electron carrier in thylakoid captures electrons
electrons are replaced by those from photolysis
Redox reactions pass e- down ETC in thylakoid releasing energy
This energy used to pump H+ into the inter-membrane space
Protons accumulate in the inter-membrane space forming an electrochemical gradient
Protons diffuse via channels associated with ATP synthase stimulating ATP production
As H+ passes through channel it is accepted by NADP to form NADPH
In PS1:
Photon excites an electron that is lost
protein-iron-sulphur complex called ferredoxin accepts the electrons and passes them to NADP in the Stroma
Describe the process of Cyclic photophosphorylation
Photon excites an electron in PS1 that is lost
Pass electron to ETC that then passes it back to PS1
ATP is produced as the electron moves down ETC
The cycle continues
Where does the light-independent stage occur?
Stroma
What are the 2 sources of CO2 for the light independent stage?
From the air, diffuses through the stomata of the leaf
From respiration, CO2 produced as a by-product
What is the Calvin Cycle?
The series of reactions where CO2 is converted to organic molecules
What is the role of RuBP (ribulose bisphosphate)?
A carbon dioxide acceptor
What is RuBisCO?
An enzyme that catalyses the reaction between CO2 and RuBP
Describe the stages of the Calvin Cycle
CO2 reacts with RuBP to form an unstable 6C compound (catalysed by RuBisCO)
6C compound breaks down into GP, the carbon has now been fixed
GP is reduced using H+ from NADPH from the LDR. This makes TP (triose phosphate). This uses ATP from the LDR
10/12 TP molecules are used to reform RuBP, the other 2 are the product.
Does the Calvin Cycle run when it is dark?
No, though it doesn’t directly use light energy is requires the products of the LDR which does use light energy
What are the uses of TP?
Synthesise carbohydrates such as Glucose, Sucrose, Starch and Cellulose
Synthesise Amino acids, Fatty acids and Glycerol
Reform RuBP
What are the main factors affecting photosynthesis?
Light intensity
Carbon Dioxide Concentration
Temperature
Water Stress
What is the limiting factor?
It is the factor that is preventing the rate of photosynthesis getting any faster
What happens to concentrations of GP, TP and RuBP when light intensity falls?
GP cant be reduced to TP
TP levels fall and GP accumulates
RuBP falls because there is no TP to regenerate it
What happens to concentrations of GP, TP and RuBP when CO2 concentration falls?
RuBP cant accept it
No GP formed
No TP formed
What happens to the rate of photosynthesis at different temperatures?
It is an enzyme catalysed reaction
Low temperatures it is slow
At optimum (30’C) it is at its fastest
Above 45’C enzymes become denatured and reaction slows
What happens to rate of photosynthesis when a plant is under water stress?
When plant is under water stress it closes stomata to reduce water loss
This also reduces gaseous exchange
Makes CO2 less available so photosynthesis greatly decreases
