Unit 3.2 - Photosynthesis uses light energy to synthesise organic molecules Flashcards
Location of photosynthesis
Chloroplasts
What do chloroplasts contain that absorb light energy?
Photosynthetic pigments
What do the photosynthetic pigments of chloroplasts do?
Absorb light energy at particular wavelengths of light
Photosynthetic pigments in chloroplasts
Xanthophylls
Carotene
Chlorophyll a
Chlorophyll b
Where are chloroplasts found?
In mesophyll tissues
Where predominantly in mesophyll tissues are chloroplasts found?
In the palisade mesophyll cells
Which other mesophyll tissue apart from the palisade mesophyll contains some chloroplasts?
Spongey mesophyll
Adaptations of leaves for photosynthesis
Large surface area
Thin
Air spaces in spongy mesophyll
Palisade cells are packed with chloroplasts and arranged with their long axes perpendicular to the surface
Cuticle and epidermis are transparent
Palisade mesophyll cells are elongated and densely arranged in layers
What does the large surface area of a leaf allow it to do?
Capture as much light as possible
Why has a leaf adapted to be thin for photosynthesis?
Allows light to penetrate through the leaf
Why do chloroplasts rotate and move within mesophyll cells?
Move into the best position for maximum absorption of light
What do the air spaces in the spongey mesophyll of leaves allow to happen?
Allows CO2 to diffuse to photosynthesising cells
Why are palisade cells packed with chloroplasts and arranged with their long axes perpendicular to the surface?
To capture as much light as possible
Why are the cuticle and epidermis of a leaf transparent?
So that light penetrates to the mesophyll
Why have leaves adapted so that palisade mesophyll cells are elongated and densely arranged in layers?
Can accommodate a large number and no cross walls to prevent light from penetrating through the cell = capture as much light as possible
What’s the main purpose of photosynthesis?
To synthesise carbohydrates by taking in simple inorganic compounds like CO2 and H2O and combining them to make complex carbohydrates
What’s the purpose of the starch granule in chloroplasts?
Glucose phosphorylised during photosynthesis is stored here as starch
Where are the photosynthetic pigments in chloroplasts?
Thylakoids
What are thylakoids?
Internal membranes of chloroplasts
How would thylakoids appear on an electron microscope photo?
Stained black
What’s a stack of thylakoids known as?
Granum
How are granum connected in chloroplasts?
By lamellae
What is the stroma of a chloroplast?
Fluid filled space
What provide evidence for the endosymbiotic theory of chloroplasts in chloroplasts? Explain
Circular DNA, like in prokaryotes
Theory states that Chloroplasts and mitochondria in eukaryotic cells were once free-living aerobic, prokaryotic organisms, that were then ingested by a large anaerobic, prokaryotic bacteria
What does the endosymbiotic theory suggest?
Theory states that Chloroplasts and mitochondria in eukaryotic cells were once free-living aerobic, prokaryotic organisms, that were then ingested by a large anaerobic, prokaryotic bacteria
Primary pigment in chloroplasts
Chlorophyll A
Accessory pigments in chloroplasts
Chlorophyll B
Carotenoids
What are the pigments chlorophyll B and carotenoids known as in chloroplasts?
Accessory pigments
What does a light absorption spectrum show?
The relative absorbance of light at different wavelengths by the photosynthetic pigments
Why does each photosynthetic pigment in chloroplasts have an absorption spectrum?
Different pigments absorb more light energy at certain wavelengths
What’s an absorbance spectrum?
A graph which shows how much light is absorbed by a pigment at different wavelengths of light
What does each photosynthetic pigment in chloroplast have on the light absorption spectrum?
Its peak absorbance at a different wavelength
Which colours does the absorbance of chlorophyll a peak at?
Red and blue
What does more light being absorbed by a photosynthetic pigment lead to?
Higher rate of photosynthesis
Why do chloroplasts contain multiple photosynthetic pigments?
Having more than one pigment increases the number of wavelengths of light that are absorbed by the leaf
Which photosynthetic pigment is involved in the reactions that lead to the synthesis of ATP in chloroplasts?
Chlorophyll A
Why do we perceive plants to be green?
The light that’s reflected from the leaf is green, whilst the other wavelengths of light are absorbed by the leaf
Chlorophyll A does not absorb wavelengths of green and yellow (indicated by a very low degree of light absorption from about 500 to 600nm).
As the primary pigment, there;s more chlorophyll A than the other types of pigment
Which type of pigment is there the most of in chloroplasts?
Chlorophyll A
Why is there barely any light absorption in one part of the light absorption spectrum?
One theory is that when the plants were evolving, they evolved pigments that absorb light that was reflected by other living bacteria at the time, not absorbed by those too
Action spectrum
Show the rate of photosynthesis at different wavelengths of light
Which 2 types of spectrum show a very strong correlation when overlayed?
An action spectrum and an absorption spectrum
How can we guess that the wavelengths of light absorbed by photosynthetic pigments are actually used for photosynthesis?
If you overlay an action spectrum onto an absorption spectrum, the peaks show a very close correlation
What simple device can be used for measuring the rate of photosynthesis?
Photosynthetometer
What is a photosynthetometer used for?
Measuring the rate of photosynthesis
How do we set up and use a photosynthetomer?
Aquatic plant in a CO2 source, and a light source and a filter for different colours
Observe how much O2 is produced at different wavelengths of light by observing the bubbles produced
Who devised an experiment to show the action spectrum of photosynthesis?
Thomas Englemann
What did Englemann’s experiment determine?
Which wavelengths of light were used most for photosynthesis
How did Englemann complete his experiment?
Placed the photosynthetic filamental algae Spirogyra in a suspension of motile aerobic (oxygen seeking) bacteria
Used a prism to refract white light into its constituent rainbow colours
Under the microscope, he could observe where on the colour spectrum on the slide there were large numbers of bacteria where they had swam towards the spirogyra in the middle of the slide
What were the results of Englemann’s experiment and what did this mean?
Large numbers of bacteria in the red part of the spectrum (+few in the blue part)
This is where there is the highest rate of photosynthesis, therefore the bacteria moves there to obtain more oxygen
Why did some regions on the spectrum have less bacteria moving towards the filamental algae spirogyra during Englemann’s experiment?
These are the regions where the algae weren’t absorbing much light energy, therefore there was less photosynthesis, so less O2 produced, so less bacteria here
2 types of electron microscope
Transmission
Scanning
Transmission electron microscope
Electrons are fired through the sample
Scanning electron microscope
Looks at the surface of the thing that’s studied
Do any electron microscope photos have colour? Why?
No, as electrons all have the same wavelength
Where are photosystems found?
Embedded in the thylakoid membrane of chloroplasts
Where exactly are photosynthetic pigments located?
In photosystems embedded in the thylakoid membranes of chloroplasts
What’s the name for the simplified diagram of a photo system?
Antenna complex
What’s the purpose of a photosystem?
To capture visible light energy
What is a photosystem geared towards?
Channelling the energy that strikes the photosystem to the reaction centre
What does the primary pigment reaction centre of a photosystem always have?
A molecule of chlorophyll A
Where is a molecule of chlorophyll A always found in a photosystem?
In the primary pigment reaction centre
Accessory pigments in a photosystem
Xanthophylls
Carotenes
Chlorophyll b
Light harvesting
When photosynthetic pigments absorb light energy
Name for when photosynthetic pigments absorb light energy
Light harvesting
How is light harvesting achieved?
By antenna complexes within the thylakoid membranes of the chloroplasts
Why are there a range of pigments in photosystems?
Allow a range of wavelengths to be absorbed
Describe the process occurring inside a photosystem
Photons of light strike photosystem
Absorbed by accessory pigments
Transferred through the photosystem to the reaction centre
Energy is absorbed by chlorophyll A n the reaction centre
How many molecules of chlorophyll a does the reactions centre of a photosystem contain?
2
What happens when chlorophyll a molecules absorb light energy in the reaction centre of a photosystem?
They emit high energy electrons that got excited
What does the emitting of high energy electrons from the reaction centre of a photosystem lead to?
The light dependent stage of photosynthesis
What’s the purpose of the proteins associated with the antenna complex?
Prevent the light energy from escaping from the antenna complex
What are the 2 main stages of photosynthesis?
The light dependent stage
The light independent stage
Where does the light dependent stage of photosynthesis occur?
In the thylakoid membrane
2 types of reaction centre involved in the light dependent stage
Photosystem I
Photosystem II
Absorption peak of photosystem I and hence other name
700nm
P700
Absorption peak of photosystem II and hence other name
680nm
P680
Which two processes take place at the same time during the light dependent stage?
Non-cyclic photophosphorylation
Cyclic photophosphorylation
Which type of photophosphorylation in the light dependent stage involves 2 photosystems and what are they?
Non-cyclic photophosphorylation
PS I and PS II
Non-cyclic photophosphorylation
- At PS II, light energy comes in as a photon
- This is absorbed by PS II
- Energy is directed to the reaction centre
- The energy energises an electron at the reaction centre
- The high energy electron is emitted
- The electron from PS II is captured by an electron acceptor
- At the same time, light energy strikes PS I
- Electron is emitted from PS I
- This electron is captured by another electron acceptor
= 2 excited electrons absorbed by electron acceptors - Returning to PS II, the electron from the acceptor goes down an electron transport chain
- The energy from the electron is used for the phosphorylation of ADP to ATP to synthesise ATP via Chemiosmosis
- At the end of the electron transport chain, an electron is passed to PS I, as it’s lost an electron, and is absorbed by chlorophyll A
- The electron from PS I is captured by an electron acceptor
- The electron is combined with a proton to reduce NADP to NADPH
(Remember NAD is involved in respiration, NADP is involved in photosynthesis) - The products of non-cyclic phosphorylation are therefore ATP and reduced NADP, which go on to take part in the light independent stage of photosynthesis
- The electron that was transferred from PS II to PS I needs to be replaced
- This electron comes from water
- In a process called photolysis (also in the thylakoid), water is split using sunlight energy
- Gives out 2 electrons, 2 protons and 1/2 a molecule of O2 (an atom of O2)
H20 —> 2e- + 2H+ + 1/2O2 - Th electron is used to give an electron back to PS II
- The protons combine with the electrons from PS I to reduce NADP
- The oxygen is given off as O2 gas
Photolysis
Water is split using sunlight energy
Photolysis equation
H2O —> 2e- + 2H+ + 1/2O2
(Reverse of the reaction at the end of the electron transport chain)
Why is non-cyclic photophosphorylation called “non-cyclic”?
The electron does not return to its origin - PS II ends up an electron short, so photolysis must occur
Which photosystems does cyclic phosphorylation of the light dependent stage of photosynthesis use?
Just PS I
Explain cyclic photophosphorylation during the light dependent stage of photosynthesis
- Light entry strikes PS I
- PS I absorbs the photon of light (at 680nm)
- Light energy excites an electron to form a high energy electron
- The electron is captured by an electron acceptor
- The electron passes through the electron transport chain
- The electron’s energy is therefore used to pump protons from the stroma into the thylakoid space, creating an electrochemical gradient
- Protons return to the stroma through the ATP synthetase complex
- ADP is phosphorylated to ATP
- The electron (now at a lower energy state) is passed back to PS I
- The only product of cyclic phosphorylation is therefore just ATP
Why is cyclic photophosphorylation known as “cyclic”?
The electron originates from PS I and returns to PS I
Compare the products of cyclic photophosphorylation and non-cyclic photophosphorylation
Cyclic = ATP only
Non-cyclic = NADPH and ATP
Compare the photosystems of cyclic photophosphorylation and non-cyclic photophosphorylation
Cyclic = PS I only
Non-cyclic = PS I and PS II
Compare whether photolysis occurs during cyclic photophosphorylation and non-cyclic photophosphorylation
Cyclic = no photolysis
Non-cyclic = photolysis
Is oxygen produced during cyclic or non-cyclic phosphorylation?p
Non-cyclic
What is the point of photolysis?
Provides an electron for PS II
What occurs during photolysis?
Molecules of water absorb light energy and are split to form electrons, protons and oxygen
Where is the water for photolysis?
The thylakoid space
Where do the electrons produced during photolysis go?
Passed to PS II to replace those lost during non-cyclic phosphorylation
What are the protons produced during photolysis used for?
To reduce NADP
How does NADP become reduced?
Each NADP molecule picks up two protons and two electrons to become reduced
What happens to the oxygen produced during photolysis?
Diffuses out of the chloroplast as a waste product
Products of the light dependent stage of photosynthesis
ATP and reduced NADP
What are the products of the light dependent stage of photosynthesis essential for?
The light independent stage or Calvin cycle
Where does the light independent stage of photosynthesis occur?
In the stroma of the chloroplast
Where does the light independent stage occur
In the stroma of the chloroplast
Summarise what occurs dung the light independent stage
CO2 is fixed into carbohydrate
“Fixing” CO2
Takes CO2 from the air and combines it with a complex organic compound
Other word for the light independent stage
The Calvin cycle
What does the light independent stage make use f?
The products of the light dependent stage
Describe the light independent stage/Calvin cycle
1.) CO2 is taken up by 5C ribulose bisphosphate (RUBP)
2.) forms an unstable 6C compound —> unstable hexose (6C)
3.) unstable hexose instantly splits into 2 molecules of 3C glycerate-3-phosphate (GP)
4) this is all catalysed by the enzyme rubisco
5.) glycerine-3-phosphate then undergoes reduction. ATP and reduced NADP (from the light dependent stage, during non-cyclic phosphorylation) reduce GP to 3C triose phosphate (TP)
6.) some of the TP i converted into glucose, but most of the TP is converted into RUBP (using energy from ATP) - this regeneration of RUBP allows the light independent stage to continue
Which enzyme is involved in Calvin’s cycle?
Rubisco
What is the light independent stage dependent on and how can we prove this?
The light dependent stage as ATP and reduced NADP (the products of the light dependent stage) are used to reduce glycerate-3-phosphate, and later on ATP is used to convert TP to RUBP
What can the glucose generated during the Calvin cycle then be used for?
Amino acids, starch and lipids (molecules needed by plants)
How is glucose converted into amino acids?
Combined with nitrates
How is glucose converted into starch?
Built up and stored
How is glucose converted into lipids?
Chemically converted
What is the problem with rubisco?
Inefficient
What has to be done as rubisco is inefficient?
Lots of it needs to be produced
What are the names of the 4 stages of the Calvin cycle?
Carboxylation phase
Reduction phase
Synthesis phase
Regeneration phase
How many time does Calvin’s cycle have to occur for 1 molecule of glucose to be synthesised? Why?
6 times, as every time the cycle makes 1 complete turn, 1 carbon atom is added to the compounds made during the Calvin cycle
Describe Calvin’s experiment to discover the Calvin cycle
1.) used Chlorella algae (a photosynthetic protist)
2.) placed on a thin glass vessel —> the lollipop vessel
3.) algae as given plenty of light, CO2 and hydrogen carbonate (HCO3-) containing normal carbon (12C) —> this allowed the algae to photosynthesise
4.) at the start of the experiment, the carbon compounds were replaced with compounds containing radioactive carbon (14C)
5.) as the algae photosynthesised, it absorbed the 14C and fixed it through the Calvin cycle
6.) at 5 second time intervals, a sample of chlorella was placed into hot ethanol to top all enzymatic reactions (ethanol denatures the enzymes). - creates a “snapshot” of that moment in the cycle
7. )the carbon compounds were then separated by chromatography and the compounds containing 14C identified by autoradiography - this allows us to see where and when radioactivity appears and their sequence to find the order of the Calvin cycle
What was used during Calvin’s experiment?
Chlorella algae (a photosynthetic protist)
Vessel used during Calvin’s experiment?
Lollipop vessel
Radioactive carbon used during Calvin’s experiment
14C
What was chlorella placed in every 5 seconds during Calvin’s experiment and why?
The ethanol
Denatured the enzymes so stops all enzymatic reactions to create a “snapshot”
How was the chromatography completed during Calvin’s experiment?
The solvent would run through the chromatography paper and would then be turned so that the compounds separate
What was used to see the radioactivity on the chromatography paper used during Calvin’s experiment?
Autoradiograms - use a photosynthetic film to see radioactivity
How can radioactivity be seen on an autoradiogram?
From a black patch
What was seen after 5 seconds on the autoradiograms during Calvin’s experiment?
More labelled glycerate-3-phosphate than any other compound - this indicates that glycerate-3-phosphate is the first product of carbon fixation
What was seen after 30 seconds on the autoradiograms during Calvin’s experiment?
A range of different labelled compounds occur, showing the intermediate and final products of the light independent reactions
Factors that affect the rate of photosynthesis
Light intensity
Carbon dioxide concentration
Temperature
Law of limiting factors
The rate of a metabolic process which relies on several factors is limited by the factor which is present at its least favourable level (lowest)
Why is the law of limiting factors true for photosynthesis?
Think - the rate of photosynthesis is affected by light intensity, CO2 concentration and temperature all at the same time, but is limited by the factor at its lowest level
At which concentrations of CO2 is the concentration of CO2 in the atmosphere limiting the rate of photosynthesis?
Between 0 and 0.1%
How do we know that the concentration of CO2 in the atmosphere is the limiting factor of photosynthesis att concentrations between 0 and 0.1%?
If the concentration of CO2 is increased, the rate of photosynthesis will increase, so it must be a limiting factor
What happens to the rate of photosynthesis when the concentration of CO2 in the atmosphere is raised beyond 0.1% and why?
The rate of photosynthesis doesn’t increase anymore, as another factor must be limiting the rate of photosynthesis
Why does the rate of photosynthesis decrease at lower CO2 levels?
CO2 is needed during the light independent stage (Calvin cycle)
Low concentration = lower rate of Calvin cycle (even if there’s sufficient light, ATP, NADPH e.t.c)
What is the atmospheric level of CO2?
0.04% (not optimal)
Could the rate of photosynthesis in our current atmosphere be higher? Explain
Yes, as the concentration of CO2 is at 0.04%, which isn’t optimal
How can farmers increase their yield by increasing the rate of photosynthesis for their crops?
Place more CO2 in greenhouses, as the 0.04% concentration in our atmosphere currently isn’t the optimal level
CO2 source example
Sodium hydrogen carbonate
How can we measure the effect of light intensity on the rate of photosynthesis?
Use the right set up and measure the oxygen production production at different light intensities
What’s the purpose of the beaker of water in the light intensity experiment?
To keep the temperature constant
Area of a light bulb
4pir^2
How is light intensity calculated?
Power
———
Area
Table headings for the light intensity experiment to measure the rate of photosynthesis
Distance between the pondweed and light source
Light intensity
Number of bubbles given off in 1 minute
Equipment for measuring the effect of light intensity on the rate of photosynthesis
Lamp (light source)
Ruler
Elodea (pondweed)
Beaker of water
Thermometer
Test tube containing a CO2 source (e.g - sodium hydrogen carbonate)
What happens to a plant as a light source is moved further away from it?
It receives less light energy
Why does a plant receive less light energy as the light source is moved further away from it?
The intensity of light is lower as it’s spread out over a larger surfed area (inverse square law)
How do we know if light intensity is the limiting factor of the rate of photosynthesis?
If you were to increase it, the rate of photosynthesis would still increase
What were used to act as heat filters during Calvin’s experiment?
Glass screens between the lamp and the lollipop apparatus
What were placed between the lamp and the lollipop apparatus during Calvin’s experiment and why?
Glass screens
Act as heat filters
At low temperatures, how does increasing the temperature affect the rate of photosynthesis?
The rate increases in more or less direct proportion
What happens to the rate of photosynthesis when it rises above a certain temperature?
The rate drops to zero
Why does the rate of photosynthesis drop to zero above a certain temperature?
The temperature affects the activity of the enzymes catalysing photosynthesis reactions, especially during the light independent stage.
Above a certain temperature, the enzymes will denature
Unit of light intensity + explanation
Foot - candles
(Distance unit, intensity unit)
Point on a CO2 exchange against light intensity graph where it flattens off
Light saturation point
Light compensation point
The light intensity at which the volume of gases used and produced by photosynthesis and respiration are equal
(Photosynthesis = respiration)
What do plant cells do as well as photosynthesise?
Respire
What do plant cells do as they respire?
Produce CO2
What do plants do with the CO2 they produce when respiring?
Use it for photosynthesis
What happens in terms of respiration and photosynthesis at very low light intensities?
The rate of photosynthesis is lower than the rate of respiration, therefore the plant gives out CO2
What happens above the light compensation point?
The rate of photosynthesis increases as the plant is using more CO2 than that produced
What happens to the photosynthetic rate AT the light compensation point?
The rate doesn’t increase - it becomes zero
Why is Mg2+ important?
It’s one of the inorganic ions needed by plants to survive, as it’s an essential component of chlorophyll
How is magnesium absorbed?
As Mg2+
Where is Mg2+ transported?
In the xylem
Function of Mg2+
Chlorophyll production
Activation of ATPase
What is magnesium needed by?
All tissues, especially the leaves
Which molecule does magnesium form a part of?
The chlorophyll molecule
What ones magnesium deficiency cause?
Chlorosis
Where does chlorosis begin?
Between the veins of older leaves
What happens during chlorosis?
Existing magnesium in the plant is mobilised and chlorophyll is broken down and transported to newly formed leaves
What does chlorosis cause?
The yellowing of old leaves
What is nitrogen essential for?
Photosynthesis and plant growth
How is nitrogen absorbed by plants?
By the roots as nitrate ions by active transport
What is nitrogen used for?
To synthesise amino acids, nucleic acids and nucleotides such as ATP
(Remember - plants synthesis their own complex organic molecules)
What different ways is nitrogen transported in a plant?
As nitrates in the xylem
As amino acids in the phloem
Where in the plant is nitrogen transported as nitrates?
In the xylem
Where in the plant is nitrogen transported as amino acids?
In the phloem
What is nitrogen transported as in the xylem?
Nitrates
What is nitrogen transported as in the phloem?
Amino acids
Symptoms of nitrogen deficiency
Reduced growth of all organs
Chlorosis (yellowing of leaves due to inadequate chlorophyll production)
Why does chlorosis cause the yellowing of leaves?
Inadequate chlorophyll production
Where does chlorosis first appear?
In older leaves
What do we use when extracting chloroplasts and why?
Ice cold solution to slow down any enzyme reactions
Which stage is the only stage of photosynthesis to involve CO2?
Light independent stage
Minerals taken up from soil
Nitrogen
Phosphate
Magnesium
What’s synthesised with a nitrogen source?
Amino acids
What’s synthesised with a phosphate source?
Phospholipids
What’s synthesised with both a nitrogen and a phosphate source?
Nucleotides
What’s synthesised with magnesium?
Chlorophyll
Why would we use a black paper cover in an experiment about plant minerals?
Exclude light
Prevent the growth of algae
Algae would absorb mineral ions from the solution
What does distilled water lack?
Minerals
2 Purposes of hot methanol in Calvin’s experiment
Kill algae and stop reactions at a precise time
Acts as a solvent to extract chemicals
Why do we need heat filters in Calvin’s experiment?
Light source produces heat
Increased rate
Denatured enzymes
When using chromatogram to compare with autoradiograms of known substances, what must be kept the same?
Solvent
Paper
Temperature
Why must the solvent be kept the same when comparing chromatogram with autoradiograms of known substances during Calvin’s experiment?
Different solubility of substances in different solvents
Why must the paper be kept the same when comparing chromatogram with autoradiograms of known substances during Calvin’s experiment?
Substances could travel different distances
Why must the temperature be kept the same when comparing chromatogram with autoradiograms of known substances during Calvin’s experiment?
Can affect solubility
Blue wavelengths of light
450nm
Red wavelengths of light
670nm - 680nm
Green wavelength of light
550nm
Which light has the most energy?
Blue light
What happens to the wavelength of light energy when its being passed form pigment to pungent in the antenna complex and why?
Its wavelength is being changed so that it’s the right wavelength to be absorbed by chlorophyll a
Enzymes involved in a Krebs cycle + explain
Decarboxylase enzymes for decarboxylation
Dehydrogenase enzymes for dehydrogenation
