Photosynthesis (A-level only) Flashcards
Photosynthesis: Light-dependent reaction
The PS2 molecule absorbs light this excites electrons in the chlorophyll to a higher energy level this is called photoionisation
Higher energy electrons leave the chlorophyll and go into the thylakoids membrane along a series of carrier proteins called the electron transfer chain
Electrons are transferred from one carrier protein to the next in a series of redox reactions until they reach the PS1 molecule
During each transfer of electrons the redox reactions release energy
Energy pumps hydrogen ions aginst a concentration across the thylakoid membrane through the carrier proteins of the ETC this happens via active transport
Hydrogen ions diffused back across the membrane through a protein channel formed by ATP synthase this process is called chemosmosis
The movement of protons through ATP synthase causes the molecules shape to change this allows the enzymes to catalyse a reaction between ADP and PI forming ATP and ATP this is called phosphorylation
In the thylakoid water is split in the presence of light this is called photolysis
this produces a proton that it is transferred to the coenzyme called NADP
Electrons are also produced and are transferred to the PS2 molecule to replace electrons are lost in photoionisation
Oxygen is produced and diffuses out
In the PS2 molecule light is absorbed causing photoionisation
Electrons from the PS1 molecule and hydrogen are used to reduce NADP
ATP and reduced NADP move into the stroma
Photoionisation
When light is absorbed by a molecule and electrons become excited to a higher energy level
Photolysis
The splitting of a molecule using light energy
Phosphorylation
The addition of a phosphate group to a molecule
Chemiosmosis
The chemosmosis is the machanisim of ATP generation when energy is stored in form of proton concentration across the membrane
Light-independant reaction
Carbon dioxide diffuses into the stoma and reacts with RUBP this process is called Carbon fixation
The reaction is catalysed by the enzyme RuBisCO
This produces an unstable six carbon molecule which splits to give 2 molecules of GP
Reduced NADP loses hydrogen and GP gains this hydrogen to form TP this reforms NADP and is known as a redox reaction
ATP is hydrolysed to ADP and PI to release energy for the redox reaction
TP is used to regenerate are RUBP using the phosphate group from this hydrolysis of ATP
Some of the TP is used to make organic molecules like glucose
Glucose can be joined to produce starch which is stored in the stroma
This cycle needs to occur six times to generate enough TP to produce one glucose molecule and regenerate all six RUBP molecules
Limiting factors
Light intensity, carbon dioxide concentration and temperature
How does light intensity affect photosynthesis?
Increased light intensity will increase the rate of photosynthesis until it reaches a saturation point
How does carbon dioxide concentration affect photosynthesis?
CO2 is a substrate for an enzyme controlled reaction, so as CO2 concentration increases, so does photosynthesis rate, but once all the enzyme active sites are used up, the reaction cannot go faster.
A minimum CO2 concentration is needed for photosynthesis to begin
How does temperature affect photosynthesis?
As the temperature rises, the rate of photosynthesis increases as the reaction speeds up.
However, photosynthesis is controlled by enzymes.
Most enzymes are destroyed at high temperatures (40-50 C) so if this occurs the rate of photosynthesis will fall
How do you agricultural practices in greenhouses maximise photosynthesis rates?
Add carbon dioxide in the air by burning something
Light come past the glass and lamps can be used at night
Glasshouses trap heat and heaters and cooling systems can be used to reach optimum temperature
How do you agricultural practices in open fields maximise photosynthesis rates?
Competing weeds are removed by spraying herbicides
Crop sensitive to temperature are covered with fleas/polythene
