Photosynthesis Flashcards
What is a chloroplast?
An organelle found in plants and algae that is the site of photosynthesis.
Describe the structure of a chloroplast.
Double membrane (chloroplast envelope)
· Grana - stacks of flattened disks (thylakoids) that contain photosystems, electron transport chain, ATP synthase
Grana connected by intergranal lamellae
* Stroma - fluid-filled matrix containing enzymes
Where are chloroplasts located in an angiosperm leaf?
Mainly found in the mesophyll layers.
How are chloroplasts adapted for
photosynthesis?
-Thylakoids give a large surface area for light-independent reactions
-Photosynthetic pigments arranged into photosystems to maximise light absorption
-Stroma directly surrounds grana - products of photosynthesis diffuse directly into the stroma.
-Contain their own DNA and ribosomes
-Inner chloroplast membrane less permeable than outer enabling control over the movement of substances
How are angiosperm leaves adapted for photosynthesis?
Large surface area to maximise light absorption
Thin to reduce the diffusion distance for CO2
Upper epidermis is transparent, allowing light to strike mesophyll layers
Palisade cells are densely packed and contain many chloroplasts
Air spaces reduce the diffusion distance for CO2
The vein network transports water and minerals to the leaf and takes sugars away
Stomata allow CO2 to diffuse into the leaf
What is a transducer? Why are chloroplasts described as transducers?
Something that converts one type of energy into another. Chloroplasts transduce light energy into the chemical energy of ATP
What is a photosynthetic pigment?
A molecule present in chloroplasts that absorbs certain wavelengths of light.
Give some examples of photosynthetic
pigments.
Chlorophylls a and b
Beta carotene
Xanthophylls
What is the purpose of chromatography?
To separate different products from a mixture.
What is photosynthesis?
A complex metabolic pathway that synthesises organic molecules in the presence of light. Overall:
6CO2+6H2O→ C6H12O6+602
Name the two stages of photosynthesis.
Light-dependent stage
Light-independent stage
What is a Photosystem?
Protein complex consisting of an antenna complex and reaction centre
Involved in the absorption of light and the transfer of electrons in photosynthesis. Two types; Photosystems I and II.
How do Photosystems I and II differ?
They absorb different wavelengths of
light.
Describe the process of light harvesting
The antenna complex absorbs light energy of varying wavelengths and transfers it to the reaction centre
Energy absorbed by two chlorophyll a molecules which emit ‘excited’ electrons
What is an absorption spectra?
A graph showing the wavelengths of light absorbed by a pigment.
What is an action spectra?
A graph of the rate of photosynthesis against each wavelength of light absorbed by a pigment.
Describe the light-dependent stage of
photosynthesis.
- First stage of photosynthesis
- Takes place in the thylakoids of the chloroplast
- Uses light energy to produce ATP, reduced NADP and
State the sources of electrons for the electron transport chain.
Cyclic and non-cyclic photophosphorylation.
Define cyclic photophosphorylation.
The formation of ATP involving Photosystem I only.
Outline cyclic photophosphorylation.
Involves Photosystem I only
* Excited electrons enter the electron transport chain to produce ATP and then return to Photosystem I
* No reduction of NADP and no water required to replace lost electrons
What is the purpose of cyclic photophosphorylation?
Produces additional ATP to meet surplus energy demands of the cell.
Define non-cyclic photophosphorylation.
The formation of ATP and reduced NADP involving both Photosystems I and II
Outline non-cyclic photophosphorylation.
Involves Photosystem I and II
Excited electrons enter the electron transport chain to produce ATP
NADP acts as a final electron acceptor and is reduced
* Water is photolysed to compensate for electrons lost from Photosystem II
What is the purpose of non-cyclic photophosphorylation?
Produces ATP and reduced NADP for
the Calvin cycle.
How does chemiosmosis produce ATP in the light-dependent stage?
Protons flow down their concentration gradient from the thylakoid space into the stroma via ATP synthase
ATP synthase phosphorylates ADP to form ATP as
protons flow through it
Describe photolysis.
The splitting of a molecule of water in the presence of light that occurs during the light-dependent stage of photosynthesis. This produces protons, electrons and
oxygen:
H2O→ 2H+ + 2e+1⁄20,
What happens to the products of photolysis?
H+ - used in proton pumping and to reduce NADP
e-replaces electrons lost from chlorophyll a in PSII
O2 - by-product, used for respiration or diffuses out of the leaf as waste gas
Explain how the electron transfer chain results in the production of reduced
NADP.
NADP acts as a final electron acceptor, and is subsequently reduced.
Describe the light-independent stage of photosynthesis.
- Second stage of photosynthesis
Does not require light energy and takes place in the stroma - Uses carbon dioxide and the products of the light-dependent stage to build organic molecules
What is the light-independent stage also known as?
The Calvin Cycle
Name the three main stages of the
Calvin cycle.
- Carbon fixation
- Reduction
- Regeneration
What happens during carbon fixation of
the Calvin cycle?
Reaction between CO, and ribulose bisphosphate (RuBP) catalysed by enzyme RuBisCo
Forms unstable 6C intermediate that breaks down into two molecules of glycerate 3-phosphate (GP)
What happens during reduction of the Calvin cycle?
2x GP are reduced to 2x triose phosphate (TP)
Requires 2x reduced NADP and 2× ATP formed during the light-dependent reaction
Forms 2x NADP and 2x ADP that enter the
light-dependent reaction
What happens during regeneration of the Calvin cycle?
- After 1C leaves the cycle, the 5C compound RuP forms
- RuBP is regenerated from RuP using 1× ATP
Forms 1× ADP
How are nutrients produced as a result of photosynthesis?
- Formation of amino acids from GP (requires nitrates and sulfates)
- TP molecules used to produce sugars e.g. glucose, fructose, sucrose
What is a limiting factor?
A variable that limits the rate of a
particular reaction.
Name the factors that limit the rate of photosynthesis. What stage do they limit?
- Light intensity - light-dependent stage
Light wavelength - absorption by chlorophyll - CO2 levels - light-independent stage
- Temperature - enzyme-controlled reactions pH - enzyme-controlled reactions
Describe the role of nitrogen in plant
metabolism.
Synthesis of amino acids, nucleotides and chlorophyll.
What does nitrogen deficiency in plants cause?
- Stunted growth
- Chlorosis (yellowing of the leaves)
Describe the role of magnesium in plant
metabolism.
Synthesis of chlorophyll.
What does magnesium deficiency in plants cause?
- Chlorosis (yellowing of the leaves)
