photosynthesis Flashcards
what is photosynthesis?
the process where energy from light is used to make glucose from H2O and CO2 in a series of reactions (the light energy is converted to chemical energy in the form of glucose)
what is the overall equation for photosynthesis?
6CO2 + 6H20 -> C6H12O6 + 6O2
what is a metabolic pathway?
a series of small reactions controlled by enzymes, e.g. respiration and photosynthesis
what is phosphorylation?
adding phosphate to a molecule, e.g. ADP is phosphorylated to ATP
what is photophosphorylation?
adding phosphate to a molecule using light
what is photolysis?
the splitting of a molecule using light energy
what is photoionisation?
when light energy ‘excites’ electrons in an atom or molecule, giving them more energy and causing them to be released. The release of electrons causes the atom or molecule to become a positively charged ion
what is hydrolysis?
the splitting of a molecule using water
what is a redox reaction?
reactions that involve oxidation and reduction
what is reduction?
gain of electrons, loss of oxygen or gain of hydrogen
what is oxidation?
loss of electrons, gain of oxygen or loss of hydrogen
what is the coenzyme involved in photosynthesis?
NADP
what is the role of NADP?
transfers hydrogen from one molecule to another can reduce or oxidise a molecule
where does photosynthesis occur?
in the chloroplasts
what are chloroplasts?
flattened organelle surrounded by a double membrane found in plant cells
what are thylakoids?
fluid filled sacs stacked up in the chlorophyll into structures called grana linked together by thylakoid membrane called lamellae
what are photosynthetic pigments?
coloured substances that absorb the light energy needed for photosynthesis, found in the thylakoid membranes attached to proteins e.g. chlorophyll a, chlorophyll b, and carotene
what is a photosystem?
the protein and pigment
what are the two photosystems?
photosystem II and photosystem I
what wavelength of light does photosystem I best absorb?
700 nm
what wavelength of light does photosystem II best absorb?
680 nm
what is the stroma?
a gel like substance that is contained within the inner membrane of the chloroplast and surround the thylakoid, contains enzymes, sugars, and organic acids, store carbohydrates produced in photosynthesis that aren’t used straight away as starch grains
what are the two stages of photosynthesis?
the light dependent reaction and the light independent reaction
what is the light dependent reaction?
requires light energy, takes place in the thylakoid membranes, light energy is absorbed by chlorophyll in the photosystems, light energy excites the electrons in the chlorophyll leading to their eventual release from the molecule, the chlorophyll has been photo ionised, some energy released from electrons is used to phosphorylate ADP to ATP, and some is used to reduce NADP to form reduced NADP, ATP transfers energy and reduced NADP transfers hydrogen to the LIDR, during this process H2O is oxidised to O2
what is the light independent reaction?
also called the calvin cycle, does not require light energy but relies on the products of the LDR, takes place in the stroma of the chloroplasts, ATP and reduced NADP supply the energy and hydrogen to make simple sugars from CO2
what is the energy resulting from the photoionization of chlorophyll used for in the LDR?
making ATP from ADP and Pi in photophosphorylation, making reduced NADP from NADP, splitting water into protons (H+ ions), electrons, and oxygen in photolysis
what are the stages of non cyclic photophosphorylation?
1) light energy excites electrons in the chlorophyll
2) photolysis of water produces protons (H+ ions), electrons, and
O2
3) energy from the excited electrons makes ATP
4) reduced NADP is generated
how are the photosystems linked in the thylakoid membrane?
by electron carriers
what are electron carriers?
proteins that transfer electrons
what is the electron transfer chain?
a chain of proteins (electron carriers) through which the excited electrons flow
what is the first stage of non cyclic photophosphorylation?
light energy is absorbed by PSII, it excites electrons in the chlorophyll, the electrons move to a higher energy level (they have more energy), high energy electrons are released from the electron transport chain to PSI (reducing each electron carrier as they move, have a higher affinity to the next electron carrier)
what is the second stage of non cyclic photophosphorylation?
as the excited electrons from chlorophyll leave PSII to move down the electron transport chain, they must be replaced so light energy splits water (photolysis) into protons (H+ ions), electrons, and oxygen
what is the equation for the photolysis of water?
4H2O -> 4H+ + 4e- + O2
what is a proton?
a hydrogen ion
what is the third stage of non cyclic photophosphorylation?
excited electrons lose energy as they move down the electron transport chain, this energy is used to transport protons into the thylakoid so that it has a higher concentration of protons than the stroma, forming a proton gradient across the thylakoid membrane, so they move down their concentration gradient via the enzyme ATP synthase embedded in the thylakoid membrane, the energy from this causes ADP and Pi to combine and form ATP
what is the chemiosmotic theory?
the theory that describes the process of electrons flowing down the ETC and creating a proton gradient across the membrane to drive ATP synthesis called chemiosmosis
how is NADPH generated in non cyclic photophosphorylation?
light energy is absorbed by PSI which excites the electrons again, to meet the energy of the final acceptor NADP, electrons are transferred to NADP along with a proton from the stroma to form reduced NADP
what is cyclic photophosphorylation?
only uses PSI, electrons from chlorophyll aren’t passed onto NADP, but passed back to PSI by electron carriers, electrons are recycled and can repeatedly flow through PSI, doesn’t produce reduced NADP or O2 and only produces small amounts of ATP
what is the light independent reaction also known as?
the Calvin cycle
where does the calvin cycle take place?
in the stroma
what does the Calvin cycle produce?
triose phosphate from CO2 and ribulose biphosphate (a 5-carbon compound)
what is triose phosphate used for?
to make glucose and other useful organic substances
what does the Calvin cycle need to keep it going?
ATP and H+ ions
what does it mean that the reactions in the LIDR are linked in a cycle?
the starting compound ribulose biphosphate (RuBP) is regenerated
what happens in the first stage of the Calvin cycle?
CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast, and is combined with ribulose biphosphate (RuBP), a 5-carbon compound, in a reaction catalysed by the enzyme rubisco. This gives an unstable 6-carbon compound which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP)
what happens in the second stage of the Calvin cycle?
the hydrolysis of ATP (from the LDR) provides energy to turn the 3-carbon compound, GP, into a different 3-carbon compound called triose phosphate (TP) in a reduction reaction, this reaction requires H+ ions from reduced NADP which recycles to NADP, some triose phosphate is then converted into useful organic compounds (e.g. glucose) and some continues in the Calvin cycle to regenerate RuBP
what happens in the third stage of the calvin cycle?
five out of every six molecules of TP produced in the cycle aren’t used to make hexose sugars but regenerate RuBP, regenerating RuBP uses the rest of the ATP produced by the LDR
how are carbohydrates made in the Calvin cycle?
hexose sugars (e.g. glucose) are made by joining two triose phosphate molecules together and larger carbohydrates (e.g. sucrose, starch and cellulose) are made by joining hexose sugars together in different ways
how are lipids made from the Calvin cycle?
made using glycerol which is synthesised from triose phosphate and fatty acids which are synthesised from glycerate 3-phosphate
how many turns of the Calvin cycle are needed to make one hexose sugar?
six times
why does the Calvin cycle need to turn six times to make one hexose sugar?
three turns of the cycle produces six molecules of TP because two molecules of TP are made for every one molecule of CO2 used, five out of these six are used to regenerate RuBP, so three turns only produces one TP to produce a hexose sugar,, hexose sugars have six carbons, so two TP molecules are used to make one hexose sugar, six turns of the cycle need 18 ATP and 12 reduced NADP from the LDR
what are the optimum conditions for photosynthesis?
high light intensity of a certain wavelength, temperature around 25 degrees Celsius, and carbon dioxide at 0.4%
why is high light intensity at a certain wavelength important for photosynthesis?
light is needed to provide the energy for the light dependent reaction, the higher the intensity of the light, the more energy it provides, only certain wavelengths of light are used for photosynthesis, the photosynthetic pigments chlorophyll a, chlorophyll b and carotene only absorb the red and blue light in sunlight, green light is reflected
why is a temperature of around 25 degrees Celsius important for photosynthesis?
involves enzymes (ATP synthase, rubisco), below 10 degrees Celsius enzymes become inactive, above 45 degrees Celsius they may start to denature, at high temperatures the stomata close and less CO2 enters the leaf
why is carbon dioxide at 0.4% important for photosynthesis?
makes up 0.04% of gases in the atmosphere increasing this to 0.4% gives a higher rate of photosynthesis but any higher the stomata start to close
what are the three main stages of the Calvin cycle?
carbon fixation, reduction, regeneration
what is carbon fixation?
CO2 combined with RuBP to make 2 GPs (glycerol 3-phosphate)
what is reduction (in the Calvin cycle)?
ATP is reduced NADP are used to reduce GP to TP (triose phosphate)
what is regeneration (in the Calvin cycle)?
RuBP is regenerated
how many ATP and rNADP are required to turn the Calvin cycle 6 times to produce glucose?
18 ATP, 12 rNADP
which organic substances are made from TP?
carbohydrates, lipids, amino acids
what colour light does PSI absorb?
red light
what colour light does PSII absorb?
orangey light
