Photosynthesis Flashcards
Chloroplasts
organelles in plant cells where photosynthesis occurs
Each chloroplast is surrounded by a
double-membrane envelope
Each of the envelope membranes is a
a phospholipid bilayer
stroma
is the fluid that fills the chloroplasts and surrounds thylakoids
What is found inside stroma
-a separate system of membranes (thylakoid, grana and stroma lamella)
-(70S) ribosomes
-a loop of DNA
-starch grains
-dissolved CO2, sugars, enzymes and other molecules
site of light-dependent stage of photosynthesis
membrane system inside stroma (thylakoid membrane)
chlorophyll
a green pigment that absorbs energy from light, used in photosynthesis
Two stages of photosynthesis
-the light-dependent stage, which takes place in the thylakoids
-the light-independent stage, which takes place in the stroma
light-dependent stage
the first series of reactions that takes place in photosynthesis, it requires energy absorbed from light
light-independent stage
the last series of reactions that place in photosynthesis, it doesn’t require light but does need the substances that are produced in the light-dependent stage
The membrane system consists of a series of flattened fluid-filled sacs known as
thyalkoids
thylakoid membranes
the membranes inside a chloroplast that enclose fluid-filled sacs
thylakoids stack up to form structures known as
grana (singular – granum)
Grana are connected by membranous channels called
stroma lamellae
stroma lamellae
ensures the stacks of sacs (granum) are connected but distanced from each other
The membranes of the grana create a large surface area to
increase the number of light-dependent reactions that can occur
Function of loop of DNA
codes for some of the chloroplast proteins (other chloroplast proteins are coded for by the DNA in the plant cell nucleus)
Function of (70S) ribosomes
the proteins coded for by the loop DNA of a chloroplast produced by the 70S ribosomes
Function of starch grains
Sugars formed during photosynthesis are stored as starch inside starch grains
NADP
Nicotinamide adenine dinucleotide phosphate, a coenzyme that transfers hydrogen from one substance to another, in the reactions of photosynthesis
photolysis
chemical process by which molecules are broken down into smaller units through the absorption of light.
in this case it’s splitting a water molecule into oxygen, hydrogen ions (H+) and electron using energy from light
Photophosphorylation
the process of utilizing light energy from photosynthesis to convert ADP to ATP
During the light-dependent stage of photosynthesis
1)Light energy is used to breakdown water (photolysis) to produce hydrogen ions, electrons and oxygen in the thylakoid lumen
2)A proton gradient is formed due to the photolysis of water resulting in a high concentration of hydrogen ions in the thylakoid lumen
3)Electrons travel through an electron transport chain of proteins within the membrane
4)Reduced NADP (NADPH) is produced when hydrogen ions in the stroma and electrons from the electron transport chain combine with the carrier molecule NADP
5)ATP is produced (from ADP and Pi by ATP synthase during a process called photophosphorylation)
6)Photophosphorylation uses the proton (H+) gradient generated by the photolysis of water
7)Energy from ATP and hydrogen from reduced NADP are passed from the light-dependent stage to the light-independent stage of photosynthesis
purpose of the light-dependent reactions
produce ATP and reduced NADP, which are then used to complete the process of photosynthesis through the light-independent reactions.
thylakoid spaces
-the spaces inside thylakoids
-fluid filled sacs enclosed by thylakoid membranes
thylakoid membranes contain
pigments, enzymes and electron carriers required for the light-dependent reactions
photosystems
the arrangement of pigments into light-harvesting clusters
In a photosystem, different pigment molecules are arranged in
-funnel-like structures
-each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre
different photosynthetic pigments
absorb different wavelengths of light
two groups of pigments
Chlorophylls
-Chlorophyll a and b
Carotenoids
-Carotene and xanthophyll
Colour of chlorophyll a
Yellow-green
Colour of chlorophyll b
Blue-green
Colour of carotene
Orange
Colour of xanthophyll
Yellow
Chlorophylls absorb wavelengths in the
-the blue-violet and red regions of the light spectrum
-reflect green light, causing plants to appear green
Carotenoids absorb wavelengths of light mainly in the
blue-violet region of the spectrum
absorption spectrum
a graph that shows the absorbance of different wavelengths of light by a particular pigment
action spectrum
a graph that shows the rate of photosynthesis at different wavelengths of light
At what wavelengths spectrum of light is rate of photosynthesis highest and why ?
-rate of photosynthesis is highest at the blue-violet and red regions of the light spectrum
-these are the wavelengths of light that plants can absorb
What is the strong correlation between the cumulative absorption spectra of all pigments and the action spectrum
-Both graphs have two main peaks – at the blue-violet region and the red region of the light spectrum
-Both graphs have a trough in the green-yellow region of the light spectrum
Chromatography
the experimental technique that is used to separate mixtures
How chromatography works
1)The mixture is dissolved in a fluid/solvent (called the mobile phase) and the dissolved mixture then passes through a static material (called the stationary phase)
2)Different components within the mixture travel through the material at different speeds
3)This causes the different components to separate
4)A retardation factor (Rf) can be calculated for each component of the mixture
Rf value =
distance travelled by component ÷ distance travelled by solvent
Two most common techniques for separating photosynthetic pigments
-Paper chromatography – the mixture of pigments is passed through paper (cellulose)
-Thin-layer chromatography – the mixture of pigments is passed through a thin layer of adsorbent (eg. silica gel), through which the mixture travels faster and separates more distinctly
each pigment will have a unique
Rf value
Rf value demonstrates
-how far a dissolved pigment travels through the stationary phase
-A smaller Rf value indicates the pigment is less soluble and larger in size
Rf values of carotenoids
-highest Rf values
-usually close to 1
Rf value of chlorophyll B
smallest Rf value
Rf value of chlorophyll A
an Rf value somewhere between those of carotenoids and chlorophyll B
The photophosphorylation of ADP to ATP can be
cyclic or non-cyclic
What decides if photophosphorylation is cyclic or non-cyclic
the pattern of electron flow in photosystem I or photosystem II or both
In cyclic photophosphorylation
only photosystem I is involved
In non-cyclic photophosphorylation
both photosystem I and photosystem II are involved
primary pigments known as
chlorophylls
accessory pigments known as
carotenoids
The majority of pigments in chloroplasts are
chlorophyll a and b
photosystems are made up of
large numbers pigment molecules and some protein molecules
The reaction centres of both photosystems contain two molecules of
chlorophyll a
what are the pigments that channel energy harvested from light to reaction centres
chlorophyll b, carotene and xanthophyll
the benefit of all pigments channeling light energy to the two molecules of chlorophyll a present in reaction centre
-increases the energy level of electrons in chlorophyll a molecule
-the high-energy electrons are than the reason why reactions light dependent stage can occur
photosynthetic pigments
coloured substances that absorb light of particular wavelengths, supplying energy to drive the reactions in the light-dependent stage of photosynthesis
reaction centre
-the part of a photosystem towards which energy from the light is funneled
-contains a pair of chlorophyll a molecules which absorb the energy and emit electrons
Photosystem II has a primary pigment that absorbs light at a wavelength of
680nm
Photosystem II has a primary pigment called
P680
Photosystem II is at the beginning of
the electron transport chain
Where does photolysis take place
Photosystem II
Photosystem I has a primary pigment that absorbs light at a wavelength of
700nm
Photosystem I has a primary pigment called
P700
Cyclic Photophosphorylation
1) Light is absorbed by photosystem I and passed to P700
2)An electron in P700 is excited to a higher energy level and is emitted from the chlorophyll molecule in a process known as photoactivation
3)This excited electron is captured by an electron acceptor in thylakoid membrane and transported via the electron transport chain before being passed back to the chlorophyll molecule in photosystem I (hence: cyclic)
4)As electrons pass through the electron transport chain they provide energy to actively transport protons (H+) from the stroma to the thylakoid lumen via a proton pump
5)A build-up of protons in the thylakoid lumen can then be used to drive the synthesis of ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis
the difference between the two forms of photophosphorylation
Cyclic photophosphorylation differs from non-cyclic photophosphorylation in two key ways:
-Cyclic photophosphorylation only involves photosystem I (whereas non-cyclic photophosphorylation involves photosystems I and II)
-Cyclic photophosphorylation does not produce reduced NADP (whereas non-cyclic photophosphorylation does)
Non-Cyclic Photophosphorylation: Photosystem II
1) Light is absorbed by photosystem II and passed to P680
2)An electron in P680 is excited to a higher energy level and is emitted from the chlorophyll molecule in a process known as photoactivation
3)This excited electron is passed down the electron transport chain, before being passed on to photosystem I
4)During this process, ATP is synthesised from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis
5)Photosystem II contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of water by light:
H2O → 2H+ + 2e- + ½O2
6)As the excited electrons leave the primary pigment of photosystem II and are passed on to photosystem I, they are replaced by electrons from the photolysis of water
Non-Cyclic Photophosphorylation: Photosystem I
1)At the same time as photoactivation of electrons in photosystem II, electrons in photosystem I also undergo photoactivation
2)The excited electrons from photosystem I also pass along an electron transport chain
3)These electrons combine with hydrogen ions (produced by the photolysis of water) and the carrier molecule NADP to give reduced NADP:
2H+ + 2e- + NADP → reduced NADP
4)The reduced NADP (NADPH) then passes to the light-independent reactions to be used in the synthesis of carbohydrate
photoactivation
the emission of an electron from a molecule as a result of absorption of energy from light
oxygen-evolving complex
an enzyme found in photosystem II that catalyses the breakdown of water, using energy from light
Photophosphorylation & Chemiosmosis
1) energetic (excited) electrons are captured by an electron acceptor in a thylakoid membrane
2)These excited electrons are then passed along a chain of electron carriers known as the electron transport chain
3)The electron carriers are alternately reduced (as they gain an electron) and then oxidised (as they lose the electron by passing it to the next carrier)
4)The excited electrons gradually release their energy as they pass through the electron transport chain
5)The released energy is used to actively transport protons (H+ ions) across the thylakoid membrane. A ‘proton pump’ transports the protons across the thylakoid membrane, from the stroma (fluid inside chloroplast) to the thylakoid lumen (space within thylakoid)
6)This creates a proton gradient, with a high concentration of protons in the thylakoid lumen and a low concentration in the stroma
7)Protons then return to the stroma (moving down the proton concentration gradient) by facilitated diffusion through transmembrane ATP synthase enzymes in a process known as chemiosmosis
8)This process provides the energy needed to synthesise ATP by adding an inorganic phosphate group (Pi) to ADP (ADP + Pi → ATP)
9)The whole process is known as photophosphorylation as light provides the initial energy source for ATP synthesis
What gets passed down from the light-dependent stage to the light-independent stage of photosynthesis
Energy from ATP and hydrogen from reduced NADP
the light-independent reactions are known collectively as
Calvin cycle
Calvin cycle cannot continue indefinitely in darkness because
it requires inputs of ATP and reduced NADP from the light-dependent stage that will run out during darkness if they aren’t replenished using energy from light
three main steps within the Calvin cycle
1)Rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound
2)GP is reduced to triose phosphate (TP) in a reaction involving reduced NADP and ATP
3)RuBP is regenerated from TP in reactions that use ATP
Carbon fixation
1)Carbon dioxide combines with a five-carbon (5C) sugar known as ribulose bisphosphate (RuBP). An enzyme called rubisco (ribulose bisphosphate carboxylase) catalyses this reaction
2)The resulting six-carbon (6C) compound is unstable and splits in two giving two molecules of a three-carbon (3C) compound known as glycerate 3-phosphate (GP)
What do you mean by ‘fixed carbon’
the carbon has been removed from the external environment and has become part of the plant cell
Reduction of glycerate 3-phosphate
1)Energy from ATP and hydrogen from reduced NADP are used to reduce glycerate 3-phosphate (GP) to a phosphorylated three-carbon (3C) sugar known as triose phosphate (TP)
2)One-sixth of the triose phosphate (TP) molecules are used to produce useful organic molecules needed by the plant:
-Triose phosphates can condense to become hexose phosphates (6C), which can be used to produce starch, sucrose or cellulose
-Triose phosphates can be converted to glycerol and glycerate 3-phosphates to fatty acids, which join to form lipids for cell membranes
-Triose phosphates can be used in the production of amino acids for protein synthesis
Regeneration of ribulose bisphosphate
1)Five-sixths of the triose phosphate (TP) molecules are used to regenerate ribulose bisphosphate (RuBP). This process requires ATP
Glycerate 3-phosphate (GP) is used to produce
some ammino acids
Triose phosphate (TP) is used to produce
-Hexose phosphates (6C), which can be used to produce starch, sucrose or cellulose
-Lipids for cell membranes
-Amino acids for protein synthesis
Factors that plant need in order for photosynthesis to occur
-the presence of photosynthetic pigments
-a supply of carbon dioxide
-a supply of water
-light energy
-a suitable temperature
A shortage in any of the factors causes
photosynthesis to not occur at its maximum possible rate
Limiting factors of photosynthesis
light intensity
carbon dioxide concentration
temperature
The rate of photosynthesis increases as light intensity
increases
Why rate of photosynthesis increases as light intensity increases
-The greater the light intensity, the more energy supplied to the plant and therefore the faster the light-dependent stage of photosynthesis can occur
-This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which can then also occur at a greater rate
if light intensity continues to increase
The rate of photosynthesis will reach a plateau, at this point, light intensity is no longer a limiting factor of photosynthesis – another factor is limiting the rate of photosynthesis
The rate of photosynthesis increases as carbon dioxide concentration
increases
How carbon dioxide is a limiting factor
-It is required for the light-independent stage of photosynthesis, when CO2 is combined with the five-carbon compound ribulose bisphosphate (RuBP)
-This means the more carbon dioxide that is present, the faster this step of the Calvin cycle can occur and the faster the overall rate of photosynthesis
As temperature increases the rate of photosynthesis
-Increases
-However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature beyond which the enzymes begin to denature and the rate of reaction decreases
Why does temperature have no significant effect on the light-dependent reactions
these are driven by energy from light rather than the kinetic energy of the reacting molecules
Why the Calvin cycle is affected by temperature
the light-independent reactions are enzyme-controlled reactions (eg. rubisco catalyses the reaction between CO2 and the five-carbon compound ribulose bisphosphate)
How redox indicator works
when redox indicator (such as DCPIP or methylene blue) are present, the indicator takes up the electrons that are released by high-energy electrons from chlorophyll a molecules instead of electron acceptors in electron transport chain
Colour of oxidised DCPIP and methylene blue
blue
Colour of DCPIP and methylene blue after it gets reduced (gains electrons)
colourless (may appear green because of chlorophyll present in solution)
The rate at which the redox indicator changes colour from its oxidised state to its reduced state can be used as a measure
of the rate of photosynthesis
Method to carry out investigations using redox indicators: Step 1
-Leaves are crushed in a liquid known as an isolation medium
-This produces a concentrated leaf extract that contains a suspension of intact and functional chloroplasts
-The medium must have the same water potential as the leaf cells (so the chloroplasts don’t shrivel or burst) and contain a buffer (to keep the pH constant). It should also be ice-cold (to avoid damaging the chloroplasts and to maintain membrane structure)
Method to carry out investigations using redox indicators: Step 2
Small tubes are set up with different intensities, or different colours (wavelengths) of light shining of them
-If different intensities of light are used, they must all be of the same wavelength (same colour of light)
-If different wavelengths of light are used, they must all be of the same light intensity
Method to carry out investigations using redox indicators: Step 3
DCPIP of methylene blue indicator is added to each tube, as well as a small volume of the leaf extract
Method to carry out investigations using redox indicators: Step 4
-The time taken for the redox indicator to go colourless is recorded
-This is a measure of the rate of photosynthesis
Types of aquatic plant we can use to determine effect of limiting factors on rate of photosynthesis
Elodea or Cabomba (types of pondweed)
How to investigate effect of light intensity on rate of photosynthesis
change the distance (d) of a light source from the plant (light intensity is proportional to 1/d^2)
How to investigate effect of carbon dioxide concentration on rate of photosynthesis
add different quantities of sodium hydrogen carbonate (NaHCO3) to the water surrounding the plant, this dissolves to produce CO2
How to investigate effect of temperature (of the solution surrounding the plant) on rate of photosynthesis
place the boiling tube containing the submerged plant in water baths of different temperatures
Whilst changing one of these factors during the investigation ensure
the other two remain constant
Method to investigate effect of limiting factors on rate of photosynthesis
Step 1:
-Ensure the water is well aerated before use by bubbling air through it
-This will ensure oxygen gas given off by the plant during the investigation form bubbles and do not dissolve in the water
Step 2:
-Ensure the plant has been well illuminated before use
-This will ensure that the plant contains all the enzymes required for photosynthesis and that any changes of rate are due to the independent variable
Step 3:
-Set up the apparatus in a darkened room
-Ensure the pondweed is submerged in sodium hydrogen carbonate solution (1%) – this ensures the pondweed has a controlled supply of carbon dioxide (a reactant in photosynthesis)
Step 4:
-Cut the stem of the pondweed cleanly just before placing into the boiling tube
Step 5:
-Measure the volume of gas collected in the gas-syringe in a set period of time (eg. 5 minutes)
Step 6:
-Change the independent variable (ie. change the light intensity, carbon dioxide concentration or temperature depending on which limiting factor you are investigating) and repeat step 5
Step 7:
-Record the results in a table and plot a graph of volume of oxygen produced per minute against the distance from the lamp (if investigating light intensity), carbon dioxide concentration, or temperature
