photosynthesis Flashcards
what is photosynthesis
the process by which light energy from the suns transformed into chemical energy and used to synthesis large organic molecules from inorganic substances
define autotroph
an autotroph is an organism that makes its organic nutrients using an inorganic carbon source
define heterotroph
an organism that requires organic nutrients to supply it with a source of carbon.
What is light energy used for during ps
to produce complex organic molecules
explain how respiration in plants and animals depends upon the products of p/s
heterotrophs depend on autotrophs for the supply of organic molecules on which they feed
what are the two stages and where do they take palce
light dependent reactions- in the choloplast lamellae (thylakoid membranes)
light-independent reactions: in the chloroplast stroma
explain with the aid of diagrams and electron micrographs, how the strict of chloroplasts enables them to carry out their functions
- a a chloroplast is a eukaryoktic organelle
- it has a surrounding envelope of inner and outer phospholipid membranes
- a liquid matrix of stroma, where calvin cycle takes place- contains enzymes needed for LD reaction including rubsico
- a series of flattened, fluid-filled, membranous sac or thylakoids, which inlaces form stacks called grand connected by lamallae
- small (70s) ribosomes, as in bacteria
- DNA circles (loops)
- liquid droplets and starch grains
define photosynthetic pigment
a molecule that absorbs some colours of light but not others and transfers the light energy into chemical energy
EXPLAIN IMPORTANCE of p/s pigments in p/s
they absorb energy from light
in a chloroplast, the majority of pigments are chlorophyll a and chlorophyll b. types of chlorophyll absorb similar wavelength but chlorophyll a absorbs slightlyy longer ones than b
accessory pigs: carotenoids such as cerotene. these absorb a wide range of short wavelengths - inc more blue-green light than chlorophylls
they help by absorbing wavelengths of light that would otherwise not be used by the plant, they pass onside of this energy to chlorophyll. also help obtect cholorophyll from damage by very intense light.
where light dependent stage happen
thylakoid membranes
where light -independent stage happen
stroma
outline how light energy is converted into chemical energy (ATP and reduced NADP) in the light dependent stage (ref to cyclic and non cyclic photophospholylation)
LD in the thylakoids, light energy absorbed by chllorophyll . some energy used to make ATP, water molecules are split to produce hydrogen ions, electrons and oxygen. H+ and electrons are picked up by a coenzyme called NADP forming NADPH. the oxygen is a waste product and is excreted from the chloroplast.
cholorphyll molecules are clustered in photosystems in the thylakoid membranes.
energy is captured from photons of light that hit the photosystems and is funnelled down to a pair of molecules at the reaction centre of the photosystem complex.
two photosystems PSI and PSII both with a pair of molecules of chlorophyll a at the reaction reactions
photophosphorylation
phosphorylation using light.
production of ATP by combining a phosphate group with ADP using energy that originally came from light.
photo-phosphorylation happens when an electron is passed along a series of electron carries, forming an electron transport chain in the thylakoid membranes. the electron starts w=off with a lot of energy and it gradually loses some as it moves from one carrier to the next. the energy is used to cause a phosphate group to react with ADP.
cyclic photophosphorylation
involves only PSI not PSII resulting in the production of ATP but not NADPH
light is absorbed by PSI and the energy passed on to electrons in the chlorophyll a molecules in the reaction centre.
in each chlorophyll a molecule one of the electrons becomes so energetic that it leaves the chlorophyll molecules completely.
the electron is then passed along the chain of electron carriers. The energy from the electron is sued to make ATP. the electron, now having lost its extra energy, eventually returns to chlorophyll a in PSI.
non cyclic phosphorylation
both photosystems are needed.
results in the formation of ATP, NADPH and oxygen.
light is absorbed by both photosystems and electrons are emitted from both primary pigments (P700 and P680). electrons are absorbed by electron acceptors and passed along chains of electron carriers called the Z scheme (a way of summering what happens to electrons during the light-dependent stage reactions).
in the sequence of events
P700 in photosystem I absorbs electrons emmited by photosystem II
P680 in photosystem II absorbs electrons from the photolysis of water by enzymes in photosystem II
explain the role of water in the light deponent stage
water molecules are split to produce H+, electrons and oxygen. the H+ and electrons are picked up by coenzymes NADP forming NADPH- this is then used in the light independent stage.
how do the products of the light-dependent stage are used in the light independent stage (calvin stage) to produce triose phosphate
now ATP and NAPH from LD stage are used to produce carbohydrates from co2
in the stroma are rubisco- which catalyses the reaction between co2 and ribulose biphosphate (RuBP).
RuBP (5C) reaction with Co2 = 6C molecule- which immediately splits into 2 3C molecules- glycerate 3-phosphate (GP)
NADPH and ATP are used to provide energy and phosphate groups- which change GP into a 3C sugar called triose phosphate (TP)- the first carbohydrate of p/s
explain the role of carbon dioxide in the light-independent stage (calvin cycle)
co2 combines with RuBP in the presence of rubisco to form an unstable 6C compound, which quickly splits into 2 3C molecules of GP
what can TP be used to make?
carbohydrates, lipids and amino acids
two TP can combine to make a hexose phosphate molecule- from these glucose, fructose, sucrose, starch and cellulose can be formed
for AA production nitrogen needs to be added which plants obtain from the soil in the form of nitrate ions or ammonium ions
what happens to most of the TP
it’s recycled to RuBP
only about 1/6 of the TP is converted to hexose, while the rest is needed to regenerate RuBP- this required ATP
discuss co2 conc as a limiting factor in p/s
plants absorb co2 into their leaves by diffusion through the stomata. during daylight, co2 is used in the calvin cycle in the chloroplasts- so the conc of Co2 inside the lead is even lower than in the air outside, providing a diffusion gradient that keeps it moving into the leaf.
if co2 is limited supply then less GP is made and therefore less TP is made
lack of it means that there is less RubP to react with, so RuBP builds up.
the lower the rate of TP synthesis the less there is available to convert to RuBP or to other carbohydrates, aa, or lipids.
plant will prioritise the replacement of RubP, ensuring its level remain reasonably high.
if the low co2 conc continues over long period of time- there is little point maintaining high conc of RuBP as it doesn’t have much co2 to combine with. some species of plant appear to adapt to this situation by allowing the level of RuBP to fall.
discuss light intensity as a limiting factor of p/s
as LI increase as does the rate of p/s
eventually another factor is limiting so increasing light cannot increase the rate any further.
in light- LD stage supplies ATP and NADPH and reactions of CC working continuously.
light switched off-LD stage stops- so ATP and NADPH and CC also stop: because these substances are needed to fuel the conversion of GP into TP.
so no more GP being converted to TP causes a build up of GP.
cycle keeps running until most of the TP is used up, then halts.
discuss temperature as a limiting factor of photosynthesis
kinetic energy of molecules increases the higher the temp, molecules move faster and more refquently collide with each other, also collide with greater energy-
this effect is only seen in the light independent reactions, not the light dependent stage because this driving energy comes from light not kinetic energy of molecules.
if temp changes too much enzymes will denature and not carry out functions as efficiently
at higher temps- rubisco starts carrying out photorespiration, in which oxygen is combined with RubP- this wastes RuBP and reduces the rate of respiration.
