11: Photosynthesis Flashcards
Where is the site of photosynthesis?
Chloroplasts within the leaf
What are the structures in the chloroplast?
Grana - stacks of thylakoid membranes
Stroma - fluid-filled matrix
Thylakoid membranes - contain photosynthetic pigments
Intergranal lamella - tubular extensions that connect grana
Chloroplast envelope - double plasma membrnae
What are the rough dimensions of a chloroplast?
2-10μm long
1μm diameter
How is the structure of the leaf adapted to photosynthesis?
Large SA:V to absorb most sunlight
Leaves arrange so minimal overlapping and shadows
Thin - short diffusion pathway & more light absorbed
Transparent cuticle/epidermis - most light onto chlorophyll underneath
Many stomata - short diffusion pathway to outside
Air spaces - rapid diffusion to cells in gas phase
Mesophyll cells - densely packed and contain many chloroplasts
What is the overall equation of photosynthesis?
6CO2 + 6H2O -> C6H12O6 + 6O2
What are the three main stages of the photosynthesis?
Capturing of light energy
Light-dependent reaction (LDR)
Light-independent reaction(LIR)
What does the stroma contain?
Enzymes
Starch grains
DNA
70S ribosomes
Why is DNA and ribosomes in the chloroplast more efficient?
Proteins needed in photosynthesis are made in the organelle so they don’t have to be retrieved from the cytoplasm
Define oxidation
Loss of electrons could gain oxygen
Define reduction
Gains electrons could lose oxygen
What is a photosynthetic pigment?
Different chemicals which absorb different λ of light
What are the dominant photosynthetic pigments?
Chlorophyll a (dominant pigment)- absorbs best in red & blue, least in green
What is a photosystem?
Clusters of different photosynthetic pigments
Allows for absorption of as many different λ of light possible
What is photosystem (PS) I and II?
Photosystem I - inter-granal lamellae (p700)
Photosystem II -thylakoid membranes (p680)
What are primary and accessory pigments?
Primary - chlorophyll a absorbs most of the light
Accessory - carotene etc. which absorbs other λ of light
What are the three main stages of the light-dependent reaction?
Photophosphorylation
Photolysis
Reduction of NADP to reduced NADP
What occurs in photophosphorylation?
Light absorbed by chlorophyll a (PSII)
Electrons promoted to higher energy level then removed
Electrons passed along electron carriers to PSI
Energy released as transferred allowing for the reaction: ADP + Pi -> ATP
What is the precise mechanism by which ATP is produced?
Chemiosmotic theory
Describe chemiosmotic theory
Protons pumped from stroma into thylakoid using proton pumps in the membrane
Energy for this from photolysis of water, also provides protons
Proton conc. increases in thylakoid, meaning conc gradient from thylakoid to stroma
Pass membrane to stroma through ATP synthase channel proteins - changing structure of the enzyme which catalyses ADP + Pi -> ATP
What is an alternative name for ATP synthase channel proteins?
Stalked granules - named so as they form pores/granules on the membrane surface
Describe the photolysis of water
Light causes break down of water to: protons, electrons and oxygen
Occurs in the thylakoid space
Why is photolysis necessary?
Provides electrons to replace those lost in chlorophyll
Protons for the production of ATP and reduced NADP
Oxygen is just a by-product or can be used for respiration
What is the equation for the photolysis of water?
2H2O -> 4H+ + 4e- + O2
How is NADP reduced?
Protons from photolysis of water are taken up by NADP This reduces NADP to NADPH
What is NADP and its function?
Electron carrier - takes them from chlorophyll
Reduced to NADPH which is used in the light-independent stage
What is the site of the light-dependent reaction?
Thylakoid membrane in the chloroplast
How are the chloroplasts adapted for the LDR?
Thylakoid membrane provides large SA:V for attachment of: chlorophyll, electron carriers and enzymes
Proteins in grana hold PS allowing for max absorption
Granal membranes have ATP synthase channels allowing ATP production
Contain DNA and ribosomes so proteins can be quickly made on-site
What is DCPIP?
Blue indicator
Turns colourless when oxidised (gains electrons) from the LDR
How is the stroma adapted to the LIR?
Fluid contains enzymes for the LIR
Fluid surrounds grana allowing products from LDR to diffuse easily to stroma
Contains DNA & enzymes so it can make proteins
What is the calvin cycle?
The various stages of the light-independent reaction
What are the products of LDR?
ATP and NADPH
What are the abbreviations in the Calvin cycle?
Ribulose biphosphate (RuBP) a 5-carbon compound
Glycerate 3-phosphate (GP) a 3-carbon compound
Triose phosphate (TP)
Ribulose bisphosphate carboxylase enzyme (rubisco)
What are the stages in the Calvin cycle?
CO2 + RuBP -> 2xGP (uses rubisco enzyme)
NADPH reduces 2xGP to 2xTP using energy from ATP
NADPH->NADP
2xTP converted to organic substances (starch, cellulose, glucose etc.) or used to regenerate RuBP using ATP from LDR
How is energy obtained from ATP?
Energy released when ATP -> ADP + Pi
Define a limiting factor
Something which limits the rate at which a process can take place
Rate of process at any time not affected by all factors, only the one which is least favourable
Describe the effect of complete darkness on photosynthesis?
Rate of photosynthesis would be 0
Changes in temp or CO2 has no effect
Define the law of limiting factors
At any given moment, the rate of physiological process is limited by the factor that is at its least favourable value
How do you know that light is the limiting factor?
As light intensity increases, rate of photosynthesis increases
How can CO2 concentration be increased in the glasshouse?
Paraffin heaters which produce CO2
Define a photosynthometer
An instrument that measures the amount of photosynthesis a plant is undergoing
What is the apparatus set up of a photosynthometer?
Test tube in a beaker of water (used as a water bath) containing pondweed and KHCO3
Capillary tube connects test tube to a syringe
Scale (mm) along the capillary tube
Set up in a dark room next to light source
Why is KHCO3 used in a photosynthometer?
Provides a source of CO2 to the plant
How is a photosynthometer used?
Capillary tube is used to collect O2 produced in photosynthesis in a set period of time
Syringe draws O2 bubble to scale, which can be used to work out the volume if diameter of tube is known
What is the lollipop reaction for LIR?
Algae in nutrient media has air and CO2, and radioactive 14C in hydrocarbonate added
Vessel is lit and valve removes samples at set intervals
Samples placed into hot methanol to kill algae
The 14C content in solution at different times shows the different chemicals being formed in the order
Why does hot methanol used in the lollipop experiment?
Denatures the enzymes in the algae, preventing the reaction from continuing
Describe what occurs during the light dependent reaction (this is from a mark scheme - LEARN IT)
Chlorophyll absorbs light energy
Excites electrons / electrons removed (from chlorophyll)
Electrons move along carriers/electron transport chain releasing energy
Energy used to join ADP and Pi to form ATP
Photolysis of water produces protons, electrons and oxygen
NADP reduced by electrons /electrons and protons /
hydrogen
What do plants need energy for?
DNA replication
Cell division
Protein synthesis
Active transport
