8.3 Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is the process by which cells synthesise organic molecules (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight
What does photosynthesis require?
This process requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, some bacteria)
Where does photosynthesis occur within plants?
In plants, photosynthesis occurs within a specialised organelle called the chloroplast
What are the two (v. general) steps of photosynthesis?
light dependent and light independent reactions
What is the general role of the light dependent reactions?
The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
What is the general role of the light independent reactions?
The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)
- What is the role of the light dependent reactions?
The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH)
Where do the light dependent reactions take place?
These reactions occur within specialised membrane discs within the chloroplast called thylakoids
What are the 3 main steps of the light dependent reactions?
Excitation of photosystems by light energy
Production of ATP via an electron transport chain
Reduction of NADP+ and the photolysis of water
- What are photosystems?
light d
Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane
2.What are photosystems classed according to?
LD
Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm)
- What happens when a photosystem absorbs light energy?
LD
When a photosystem absorbs light energy (chlorophyll within it), delocalised electrons within the pigments become energised or ‘excited’
- What is done with the excited electrons?
LD
These excited electrons are transferred to carrier molecules within the thylakoid membrane
- Where are the excited electrons from photosystem II transferred to?
LD
Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane
- WHat happens as the electrons pass through the chain?
LD
As the electrons are passed through the chain they lose energy, which is used to translocate H+ ions into the thylakoid
- What does the buildup of protons cause?
LD
This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force
- How do the H+ ions return to the stroma?
LD
The H+ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis)
- What does ATP Synthase do?
LD
ATP synthase uses the passage of H+ ions to catalyse the synthesis of ATP (from ADP + Pi)
- What is ATP synthesis in the light dependent reaction known as?
This process is called photophosphorylation – as light provided the initial energy source for ATP production
- Where are the de-energised electrons moved to?
LD
The newly de-energised electrons from Photosystem II are taken up by Photosystem I
- Once the electrons are at photosystem I, what are they used for?
LD
Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+
- What does the reduction of NADP+ created?
LD
This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions
- What replaces the lost electrons from photosystem I?
LD
The electrons lost from Photosystem I are replaced by de-energised electrons from Photosystem II
- What replaces the lost electrons from photosystem II?
LD
The electrons lost from Photosystem II are replaced by electrons released from water via photolysis
- What is water split into and how?LD
Water is split by light energy into H+ ions (used in chemiosmosis) and oxygen (released as a by-product)
- Where are the products of the light dependent reactions used?
The products of the light dependent reactions (ATP and NADPH) are used in the light independent reactions
Why is it called photophosphorylation?
Photophosphorylation may be either a cyclic process or a non-cyclic process
- What does cyclic phosphorylation involve?
Cyclic photophosphorylation involves the use of only one photosystem (PS I) and does not involve the reduction of NADP+
- What happens when light is absorbed by photosystem 1?
Cyclic P
When light is absorbed by Photosystem I, the excited electron may enter into an electron transport chain to produce ATP
- WHat happens to the de-energised electrons?
Cyclic P
Following this, the de-energised electron returns to the photosystem, restoring its electron supply (hence: cyclic)
- What are the 2 differences of cyclic phosphorylation?
As the electron returns to the photosystem, NADP+ is not reduced and water is not needed to replenish the electron supply
- How many photosystems does non-cyclic phosphorylation involve?
Non-cyclic photophosphorylation involves two photosystems (PS I and PS II) and does involve the reduction of NADP+
- What happens when light is absorbed?
NCP
When light is absorbed by Photosystem II, the excited electrons enter into an electron transport chain to produce ATP
- What does photoactivation of photosystem I do?
NCP
Concurrently, photoactivation of Photosystem I results in the release of electrons which reduce NADP+ (forms NADPH)
- What reaction is in NCP and not in cyclic P?
The photolysis of water releases electrons which replace those lost by Photosystem II (PS I electrons replaced by PS II)
When is cyclic phosophorylation used?
Cyclic photophosphorylation can be used to produce a steady supply of ATP in the presence of sunlight
Can ATP be stored?
However, ATP is a highly reactive molecule and hence cannot be readily stored within the cell
What does NCP produce?
Non-cyclic photophosphorylation produces NADPH in addition to ATP (this requires the presence of water)
What is needed to synthesise organic molecules in light independent?
Both NADPH and ATP are required to produce organic molecules via the light independent reactions
What is the main advantage of NCP?
Hence, only non-cyclic photophosphorylation allows for the synthesis of organic molecules and long term energy storage
What is the purpose of the light independent reactions?
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules
Where do the light independent reactions occur?
The light independent reactions occur in the fluid-filled space of the chloroplast called the stroma
What are the light independent reactions collectively known as?
The light independent reactions are collectively known as the Calvin cycle
What are the 3 main steps of Calvin cycle?
Carboxylation of ribulose bisphosphate
Reduction of glycerate-3-phosphate
Regeneration of ribulose bisphosphate
- What compound does Calvin begin with?
The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
- WHat happens to ribulose biphosphate?
calvin
An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP
- What happens to the carboxylated RuBP?
calvin
The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP)
- How many RuBP are involved in one cycle?
Calvin
A single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP
- What is glycerate-3-phosphate converted into?
calvin
Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP
- How are hydrogen atoms transferred to the compound and what provides energy?
calvin
Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy
- How many cycles are needed to form sufficient GP’s?
calvin
Each GP requires one NADPH and one ATP to form a triose phosphate – so a single cycle requires six of each molecule
- How many TP molecules are used to form half a sugar molecule?
calvin
Of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule
- How many cycles are need to produce a single glucose?
Calvin
Hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch
- What happens to the remaining TP? Calvin
The remaining five TP molecules are recombined to regenerate stocks of RuBP (5 × 3C = 3 × 5C)
- What does the regeneration of RuBP require?
Calvin
The regeneration of RuBP requires energy derived from the hydrolysis of ATP
Who is the calvin cycle named after?
The light independent reactions are also collectively known as the Calvin cycle – named after American chemist Melvin Calvin
What did calvin do?
Calvin mapped the complete conversion of carbon within a plant during the process of photosynthesis
What experiment is calvin said to have carried out?
Calvin’s elucidation of photosynthetic carbon compounds is commonly classed the ‘lollipop experiment’
This is due to the fact that the apparatus he utilised was thought to resemble an upside-down lollipop
- What was added to the “lollipop” apparatus?
lollipop experiment
Radioactive carbon-14 is added to a ‘lollipop’ apparatus containing green algae (Chlorella)
- What is allowed to reach the apparatus?
Lollipop
Light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
- What is done with the algae?
lollipop
After different periods of time, the algae is killed by running it into a solution of heated alcohol (stops cell metabolism)
- How were the dead a;gae samples analysed?
lollipop
Dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
- How were the carbon samples on the chromatogram identified?
lollipop
Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X-ray film exposure)
- What were the findings of the lollipop experiment?
By comparing different periods of light exposure, the order by which carbon compounds are generated was determined
- What did calvin use the findings of the lollipop experiment for?
Calvin used this information to propose a sequence of events known as the Calvin cycle (light independent reactions)
What is the general outline of the calvin cycle?
Ribulose bisphosphate (RuBP) is carboxylated by carbon dioxide (CO2) to form a hexose biphosphate compound
The hexose biphosphate compound immediately breaks down into molecules of glycerate-3-phosphate (GP)
The GP is converted by ATP and NADPH into molecules of triose phosphate (TP)
TP can be used to form organic molecules or can be recombined by ATP to reform stocks of RuBP
What are chloroplasts?
Chloroplasts are the ’solar energy plants’ of a cell – they convert light energy into chemical energy
What can chloroplasts convert light energy into?
This chemical energy may be either ATP (light dependent) or organic compounds (light independent)
What type of tissue possesses chloroplasts?
Only photosynthetic tissue possess chloroplasts (e.g. is present in leaves but not roots of plants)
What are the 5 main features of chloroplasts?
thylakoids
grana
photosystems
stroma
lamellae
How are thylakoids adapted to the function of the chloroplast?
flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation
How are grana adapted to the function of the chloroplast?
thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane
How are photosystems adapted to the function of the chloroplast?
pigments organised into photosystems in thylakoid membrane to maximise light absorption
How is the stroma adapted to the function of the chloroplast?
central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur
How are lamellae adapted to the function of the chloroplast?
connects and separates thylakoid stacks (grana), maximising photosynthetic efficiency
What are the typical features that a diagram of the chloroplast should have?
Usually round in appearance with a double membrane exterior
Flattened discs (thylakoids) arranged into stacks (grana), connected by lamellae
Internal lumen of thylakoids is very small (allows for a more rapid generation of a proton motive force)
Ribosomes and chloroplast DNA are usually not visible at standard resolutions and magnifications
Starch granules may be visible and will appear as dark spots within the chloroplast
How is energy from plants transferred to animals?
Animals then consume these organic compounds as food and release the stored energy via cell respiration
What is the electromagnetic spectrum?
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation
What electromagnetic radiation does the sun emit?
The Sun emits its peak power in the visible region of this spectrum (white light ~ 400 – 700 nm)
What are colours?
Colours are different wavelengths of white light and range from red (~700 nm) to violet (~400 nm)
What are the colours of the visible spectrum?
The colours of the visible spectrum are (from longest to shortest wavelength):
Red Orange Yellow Green Blue Indigo Violet (Mnemonic: Roy G. Biv)
Where does chlorophyll absorb the most light?
Chlorophyll absorbs light most strongly in the blue portion of the visible spectrum, followed by the red portion
Where does chlorophyll absorb the least light?
Chlorophyll reflects light most strongly in the green portion of the visible spectrum (hence the green colour of leaves)
What is the role fo pigments?
Pigments absorb light as a source of energy for photosynthesis
What is the absorption spectrum?
The absorption spectrum indicates the wavelengths of light absorbed by each pigment (e.g. chlorophyll)
What is the action spectrum?
The action spectrum indicates the overall rate of photosynthesis at each wavelength of light
What are the correlations between cumulative absorption spectra of all pigments and the action spectra? (2)
Both display two main peaks – a larger peak at the blue region (~450 nm) and a smaller peak at the red region (~670 nm)
Both display a trough in the green / yellow portion of the visible spectra (~550 nm)
Do photosynthetic organisms only rely on 1 pigment?
Photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many
These pigments include chlorophylls, xanthophyll and carotenes
What is chromatography?
Chromatography is an experimental technique by which mixtures can be separated
- What are the two phases?
chromatography
A mixture is dissolved in a fluid (called the mobile phase) and passed through a static material (called the stationary phase)
- What causes the mixtures to separate?
chromatography
The different components of the mixture travel at different speeds, causing them to separate
- What can be calculated from a chromatogram?
A retardation factor can then be calculated (Rf value = distance component travels ÷ distance solvent travels)
What are the two most common techniques for separating photosynthetic pigments?
paper chromatography
thin layer chromatography
What does paper chromatography involve?
uses paper (cellulose) as the stationary bed
What does thin layer chromatography involve?
uses a thin layer of adsorbent (e.g. silica gel) which runs faster and has better separation
What is the law of limiting factors?
The law of limiting factors states that when a chemical process depends on more than one essential condition being favourable, the rate of reaction will be limited by the factor that is nearest its minimum value
What is photosynthesis dependent on?
Temperature
Light intensity
Carbon dioxide concentration
Why is photosynthesis affected by temperature?
Photosynthesis is controlled by enzymes, which are sensitive to temperature fluctuations
Why does temperature initially cause photosynthesis to increase?
As temperature increases reaction rate will increase, as reactants have greater kinetic energy and more collisions result
Why does photosynthesis decrease at a certain temperature?
Above a certain temperature the rate of photosynthesis will decrease as essential enzymes begin to denature
What absorbs light and what is done with it?
Light is absorbed by chlorophyll, which convert the radiant energy into chemical energy (ATP)
Why does photosynthesis increase with light intensity?
As light intensity increases reaction rate will increase, as more chlorophyll are being photo-activated
Does photosynthetic rate increase linearly with light intensity?
At a certain light intensity photosynthetic rate will plateau, as all available chlorophyll are saturated with light
Will different wavelengths have different effects on photosynthesis?
Different wavelengths of light will have different effects on the rate of photosynthesis (e.g. green light is reflected)
What is the role of carbon dioxide?
Carbon dioxide is involved in the fixation of carbon atoms to form organic molecules
Why does photosynthesis increase with CO2 conc?
As carbon dioxide concentration increases reaction rate will increase, as more organic molecules are being produced
Does photosynthetic rate increase linearly with CO2 conc?
At a certain concentration of CO2 photosynthetic rate will plateau, as the enzymes responsible for carbon fixation are saturated
What two variables can be measured to measure rate of photosynthesis?
Photosynthesis can be measured directly via the uptake of CO2 or production of O2, or indirectly via a change in biomass
What may be a confounding variable when measuring photosynthesis?
It is important to recognise that these levels may be influenced by the relative amount of cell respiration occurring in the tissue
How can CO2 uptake be measured?
Measuring CO2 Uptake
Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water
Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water
Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity)
Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe)
Alternatively, carbon dioxide levels may be monitored via a data logger
How can O2 production be measured?
Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe
Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe
Alternatively, oxygen production could be measured by the time taken for submerged leaf discs to surface
Oxygen levels can also be measured with a data logger if the appropriate probe is available
How can biomass be measured to measure photosynthesis?
Glucose production can be indirectly measured by a change in the plant’s biomass (weight)
This requires the plant tissue to be completely dehydrated prior to weighing to ensure the change in biomass represents organic matter and not water content
An alternative method for measuring glucose production is to determine the change in starch levels (glucose is stored as starch)
Starch can be identified via iodine staining (turns starch solution purple) and quantitated using a colorimeter
What is the only significant source of photosynthesis?
Only one significant source of oxygen gas exists in the known universe – biological photosynthesis
What was done to oxygen before the development of photosynthetic organisms?
Before the evolution of photosynthetic organisms, any free oxygen produced was chemically captured and stored
When did photosynthetic organisms begin to appear?
Approximately 2.3 billion years ago, photosynthetic organisms began to saturate the environment with oxygen
This led to changes in the Earth’s atmosphere, oceans, rock deposition and biological life
How did o2 start accumulating in oceans?
Earth’s oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents)
When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide)
When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere
How did O2 start accumulating in the atmosphere?
For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free)
The current concentration of oxygen gas within the atmosphere is approximately 20%
How did o2 start accumulating in rocks?
Rock Deposition
The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs)
These deposits are not commonly found in oceanic sedimentary rock younger than 1.8 billion years old
This likely reflects the time when oxygen levels caused the near complete consumption of dissolved iron levels
As BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels
How did o2 start accumulating in bio life?
Biological Life
Free oxygen is toxic to obligate anaerobes and an increase in O2 levels may have wiped out many of these species
Conversely, rising O2 levels was a critical determinant to the evolution of aerobically respiring organisms
