Photosynthesis Flashcards
What is the endosymbiont theory
Theory that mitochondria and chloroplasts were once aerobic bacteria that were ingested by early eukaryotic cells
Describe the light dependent stage
Photosynthetic pigments trap light energy
Found on membranes in chloroplasts
Describe the light - independent stage
Enzyme driven reactions
In stromatolite off chloroplasts
What is cyclic phosphorylation
- A photon of light hits PSI and 2e- becomes excited
- The 2e- are passed along a chain of electron carriers
- Small amounts of ATP are made by phosphorylation
- Electrons passed back to PSI
Describe chemiosmosis
The process which the movement of hydrogen ions across a biological membrane generates ATP
How is ATP made in phosphorylation
Energy released from 2e- as they move through ETC is sued to pump H+ across the thylakoid membrane from the stroma into the thylakoid
Creating an electrochemical gradient and H+ diffuse through ATP synthase into the stroma making ATP from ADP and Pi
Describe photolysis in non-cyclic photophosphorylation
An enzyme associated with PSII splits water in the presence of light
H2O —> 2H+ + 2e- + 1/2O2
What is the primary pigment that absorbs light in chloroplasts
Chlorophyll a
What are the two stages of photosynthesis
Light-dependent stage - energy from sunlight is absorbed and used to form ATP. Hydrogen from water is used to reduce coenzyme NADP to NADPH
Light-independent stage - hydrogen from NADPH and carbon dioxide is used to build organic molecule, such as glucose. ATP supplies the required energy
Describe non-cyclic phosphorylation
When the light absorbed in photosystem II excites electrons to be released from PSII and are passed to an electron transport chain. ATP is produced via chemiosmosis
The electrons lost from PSII are replaced from water molecules broken down using energy from the sun.
Excited electrons are released from the reaction centre at PSI, passed to another electron transport chain, and ATP is again produced by chemiosmosis.
The electrons lost from this reaction centre are replaced by electrons that have just travelled along the first electron transport chain being released from PSII
Electrons leaving the electron transport chain following PSI are accepted, along with a hydrogen ion, by the coenzyme NADP forming NADPH
NADPH provides the hydrogen or reducing power in the production of organic molecules, such as glucose, in the light independent stage which follows.
Describe photolysis
When water molecules are split into hydrogen ions, electrons and oxygen molecules using energy from the sun
The electrons released replace electrons lost from the reaction of PSII.
This is why water, along with light and carbon dioxide, is a raw material of photosynthesis
Summarise the photolysis reaction
H20 —> 2H+ + 2e- + 1/2 O2
Describe cyclic phosphorylation
The electrons leaving the transport chain after PSI can be returned to PSI instead of being used to form NADPH leading to cyclic phosphorylation
This means PSI can still lead to the production of ATP without any electrons being supplied from PSII. Reduced NADP is not produced when this happens
Describe the Calvin cycle
Carbon dioxide enters the intercellular spaces within the spongy mesophyll of leaves by diffusion from atmosphere through stomata .
It diffuses into cells and into the stroma of of chloroplasts where it is combined with a five-carbon molecule RuBP. The carbon dioxide is fixed - incorporated into an organic molecule
The enzyme RuBisCO catalyses the reaction and an unstable six-carbon intermediate is produced. RuBisCO is the key enzyme in photosynthesis. It is very inefficient as competitively inhibited by oxygen - so lots is required so carry out photosynthesis successfully
6-carbon intermediate immediately breaks down, forming two 3-carbon glycerate 3-phosphate molecules (GP)
Each GP molecule is converted to another 3-carbon molecule, triose phosphate (TP), using a hydrogen atom from NADPH and energy supplied by ATP, from the light dependent stage
What processes within photosynthesis are light dependent and which are light-independent
Light dependent = non-cyclic phosphorylation, photolysis and cyclic phosphorylation
Light independent = carlvin cycle, regeneration of RuBP
Summarise the Calvin cycle in 3 steps
- fixation - carbon dioxide is fixed in the first step
- reduction - GP is reduced to TP by the addition of hydrogen from NADPH using energy supplied by ATP
- regeneration - RuBP is regenerated from the recycled TP
Describe the process of the regeneration of RuBP
For one glucose molecule to be produced, six carbon dioxide molecules have to enter the Calvin cycle, resulting in six full turns of the cycle. This will result in the production of 12 TP molecules, two of which will be removed to make the glucose molecule.
This means that 10 TP molecules are recycled to regenerate 6 RuBP molecules (used in 6 turns of the cycle)
What are the limiting factors affecting photosynthesis
Light intensity - light is needed as an energy source
Carbon dioxide concentration - carbon dioxide is needed as a carbon source - increasing carbon fixation in Calvin cycle
Temperature - affects the rate of enzyme controlled reactions
describe 2 ways grana are adapted to their function
- contain electron carriers / ETC
- large SA for light absorption
which part of the chloroplast is the sight of the metabolic pathway involving carbon dioxide
stroma
what ways will the structure of leaves in shade and sunlight differ
shaded leaf will have
larger SA
more grana (in chloroplasts)
more chloroplasts/ (palisade) mesophyll
outline the ways in which heterotrophic organisms are dependent on plants
- dependent on plants for food - acquire nutrients
- plants produce oxygen in photosynthesis that heterotrophs use in respiration
- autotrophs produce organic molecules during photosynthesis
name the primary pigment in the photosystems I and II
chlorophyll A
name an accessory pigment in photosystems I and II
chlorophyll B
xanthophylls
carotenoids
beta-carotene
why is it an advantage to plants having a range of accessory pigments in photosystems
able to absorb a range of wavelengths
Name the compound that is synthesised in the light-dependent stage as a result of generation of an electrical and pH gradient across the thylakoid membrane
ATP
In the Calvin cycle
Identify the enzyme that catalyses the fixation of carbon dioxide
Rubisco/ RuBP carboxylase
In the Calvin cycle
Identify the first stable product of carbon dioxide fixation
GP/ glycerate-3-phosphate
In the Calvin cycle
Identify the compound that is regenerated in the Calvin cycle so that more CO2 can be fixed
RuBP/ Ribulose biphosphate
Name two different polysaccharides that can be synthesised from the end products of the light-independent stage of photosynthesis
Starch / amylose/ amylopectin
And
Cellulose
Define autotroph
Can make organic molecules from inorganic molecules
Define heterotroph
Relies on/ needs to obtain organic molecules that have been made by other molecules
Fat acts as material for chloroplasts.
Suggest what this raw material might be used for in the chloroplasts
For membrane formation
Or
Phospholipid for membrane
Fatty acid/ pigments synthesis
Describe how light is harvested in the chloroplast membrane
- Primary and accessory pigments are in a photo system / antenna complex
- Photon absorbed by pigment
- Electron excited/ moves to a higher energy level and returned to pigment
- Photon passed from one pigment to another
- Photon passed to reaction centre / chlorophyll a
- Range of accessory pigments allow range of wavelengths to be absorbed
A herbicide acts by binding to proteins associated with photosystem II, blocking movement of electrons between carriers.
Explain the effect that the herbicide binding will have on photosynthesis
- Prevents non-cyclic phosphorylation
- No electrons available to form NADPH
- Idea that ATP production by cyclic-phosphorylation is not prevented
- No/less ATP and no NADPH available for Calvin cycle Name Identify/ light independent reaction/ conversion of GP to TP
Why might plants, treated wit herbicide, when illuminated under experimental conditions, be seen to fluoresce (emit light) and give off small quantities of heat
Idea that energy given off from high energy/ excited electron
Emitted by chlorophyll/ reaction centre
Explain why measuring carbon dioxide uptake may or may not give a better indication of photosynthetic activity than measuring oxygen production
Oxygen
Oxygen only produced in one (named) stage of photosynthesis
Oxygen produced might be used for respiration
Carbon dioxide
CO2 only used in one (named) stage of photosynthesis
CO2 produced during respiration might be used for: photosynthesis/ light independent reaction/ Calvin cycle
Both could be underestimate
Or
Represents net production (O2)
Or
Represents net use (CO2)
Name the products off the light-dependent stage of photosynthesis
Reduced NADP (NADPH)
ATP
Oxygen
Suggest how weed killer, that binds to electrons in PSI I, causes death of the plant
Prevents phosphorylation (cyclic and non-cyclic)
No/ less ATP/ NADPH for light independent stage/ Calvin cycle Name Identify
No (named) substrate made for respiration
Describe the structure of a chloroplast
Usually disc-shaped
Double membrane (envelope)
Thylakoids: flattened disc stacks to form Grana
Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
Storms: fluid filled matrix with high enzyme and substrate concentration and own loop of DNA
Where do the light-dependent and light-independent reactions occur in plants?
Dependent = thylakoids of chloroplasts
Independent = stroma of chloroplasts
What is the role of the photosynthetic pigment? \
Name 2 main groups
Role = embedded within thylakoid membrane. Absorb different wavelengths to maximise rate of photosynthesis
Group 1 = primary pigment: chlorophyll ( made of a and b) found in photosystems
Group 2 = accessory pigments: carotenoids (carotene and xanthophylls) found in light-harvesting systems
Name the processes in the light independent reaction
- photoionisation
- electron transport chain / ETC
- chemiosmosis
Non-cyclic only
- reduction of NADP
Photolysis of water
Explain the role of light in photoionisation
Chlorophyll molecules absorb energy from photons of light
This excites 2 electrons (raises them to higher energy level)
To be released from the chlorophyll
` suggest the role of DNA and ribosomes in chloroplasts
DNA coding for genes/ protein/ enzyme
Enzymes for production of chlorophyll/ pigment synthesis
Protein for electron acceptors / carriers
ATP synthase
Enzyme/ PSII for photolysis
Enzymes for Calvin cycle/ light independent reaction
During an experiment of rate of photosynthesis in aquatic plants,
Suggest why not all of the oxygen produced by the plant is collected
Some oxygen would:
Dissolve in water
Used in respiration/ oxidative phosphorylation
May escape the collection apparatus
Trapped in, a bubble attached to/ air spaces in the leaf
Name the products of the light dependent stage that are used in the Calvin cycle
ATP
NADPH
Discuss the fate of triose phosphate (TP) in the Calvin cycle
Regenerates/ produces RuBP
So cycle can continue for further CO2 fixation
Formation of sugar/ glucose/ hexose/ sucrose/ starch/ cellulose
Formation of fat/ triglyceride/ lipid/ fatty acids
10x TP for RuBP and 2xTP fr production
Or
Most TP used to produce RuBP and the rest for production
Name the pigment at the reaction centre of photosystems I and II
Chlorophyll A
