What are autotrophs?
– make their own food through the process of
photosynthesis (produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment)
– sustain themselves, and do not usually consume organic molecules derived from other organisms
– known as producers of the biosphere
– almost all plants are autotrophs (specifically photoautotrophs)
What are heterotrophs?
- unable to make their own food, they live on compounds produced by other organisms
- the biosphere’s consumers
- occurs when an animal eats plants or other animals or
- decomposers: consume the remains of dead organisms by decomposing and feeding on organic litter such as carcasses, feces, and fallen leaves (fungi and prokaryotes)
What is the mesophyll?
– Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf
What is the stomata?
– Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata
What is the stroma?
– the dense fluid within the chloroplasts surrounding the thylakoid membrane and containing ribosomes and DNA; involved in the synthesis of organic molecules from carbon dioxide and water
What are thylakoids?
- Suspended within the stroma is a third membrane system, made up of flattened sacs called thylakoids which segregates the stroma from the thylakoid space inside these sacs
- Thylakoids often exist in stacks called grana that are interconnected; their membranes contain molecular “machinery” used to convert light energy to chemical energy
- Chlorophyll, the green pigment that gives leaves their colour, resides in the thylakoid membranes of the chloroplast; It is the light energy absorbed by chlorophyll that drives the synthesis of organic molecules in the chloroplast
Where is the O2 given off from plants derived from?
- the prevailing hypothesis predicted that the O2 released during photosynthesis came from CO2
- This idea was challenged in the 1930s by C. B. van Niel of Stanford University who was investigating photosynthesis in bacteria that make their carbohydrate from CO2 but do not release O2
- One group of bacteria used hydrogen sulphide (H2S ) rather than water for photosynthesis, forming yellow globules of sulphur as a waste product; Van Niel reasoned that the bacteria split H2S and used the hydrogen atoms to make sugar
- Thus, van Niel hypothesized that plants split H2O as a source of electrons from hydrogen atoms, releasing O2 as a by-product
How is photosynthesis a redox reaction?
- CO2 becomes reduced to sugar as electrons along with hydrogen ions (protons) from water are added to it
- Water molecules are oxidized when they lose electrons along with hydrogen ions
- Because the electrons increase in potential energy as they move from water to sugar, this process requires energy (endergonic)
- light is used to boost the energy of the electrons so that energy can be stored in the atomic bonds of the carbohydrates
What is the light reaction of photosynthesis?
- Water is split, providing a source of electrons and protons (hydrogen ions, H+ ) and giving off O2 as a by-product
- Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ which reduces NADP+ to NADPH by adding a pair of electrons along with a H+
- The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation
- occurs in the thylakoids
H20 –> ATP + NADPH
– these products go to the Calvin cycle
What is the Calvin cycle?
- The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast (carbon fixation)
- The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons (provided by NADPH from the light reaction)
- To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions
- occurs in the stroma
What is a wavelength?
– wavelength: the distance between the crests of electromagnetic waves
What is the electromagnetic spectrum?
– the entire spectrum of electromagnetic radiation, ranging in wavelength from less than a nanometre to more than a kilometre
What is visible light?
- Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation
- The segment of the electromagnetic spectrum most important to life is the narrow band from about 380 nm to 750 nm in wavelength (visible light)
- Although the sun radiates the full spectrum of electromagnetic energy, the atmosphere acts like a selective window, allowing visible light to pass through while screening out a substantial fraction of other radiation
What are photons?
- a quantum, or discrete quantity, of light energy that behaves as if it were a particle
- The amount of energy is inversely related to the wavelength of the light: the shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light
What are pigments?
- When light meets matter, it may be reflected, transmitted, or absorbed
- pigments: substances that absorb visible light
- Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear; If a pigment is illuminated with white light, the colour we see is the colour most reflected or transmitted by the pigment
- We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light
What is a spectrophotometer?
- an instrument that measures the proportions of light of different wavelengths absorbed and transmitted by a pigment
- A graph plotting a pigment’s light absorption versus wavelength is called an absorption spectrum
What are the three types of pigments in a chloroplast?
- chlorophyll a: a photosynthetic pigment that participates directly in light reactions, which convert solar energy to chemical energy.
- chlorophyll b: works in conjunction with chlorophyll a by transferring energy to it. Also works with another pigment called cartenoids
- cartenoids: hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light. May broaden the spectrum of colours that can drive photosynthesis, however a more important function of at least some carotenoids seems to be photoprotection: These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous to the cell
Which type of light works best for photosynthesis?
- The spectrum of chlorophyll a suggests that violet-blue and orange-red light work best for photosynthesis, since they are absorbed, while green is the least effective colour
- This is confirmed by an action spectrum, which profiles the relative effectiveness of different wavelengths of radiation in driving the process; illuminating chloroplasts with light of different colours and then plotting wavelength against some measure of photosynthetic rate, such as CO2CO2 consumption or O2O2 release.
What happens when light is absorbed by a pigment?
– When a molecule absorbs a photon of light, one of the molecule’s electrons is elevated to an orbital where it has more potential energy; The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state (one orbital) and an excited state (high orbital), and this energy difference varies from one kind of molecule to another (which is why different pigments have a unique absorption spectrum)
– ground state: the electron is in its normal orbital
excited state: absorption of a photon boosts an electron to an orbital of higher energy
– Once absorption of a photon raises an electron to an excited state, the electron cannot stay there long. The excited state is unstable so the excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing their excess energy as heat.
– As excited electrons fall back to the ground state, photons are also given off, an afterglow called fluorescence
What is a photosystem?
Composed of two parts:
- A light-harvesting complex contains various pigment molecules (chlorophyll a/b and cartenoids) bound to proteins that function as a light- gathering antenna; When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule until it is passed to the pair of specialized chlorophyll a molecules
- A reaction center that contains a pair of specialized chlorophyll a molecules and a primary electron acceptor molecule; The pair of chlorophyll a molecules in the reaction-centre complex are special because their molecular environment enables them to use the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to the electron acceptor molecule, which then becomes reduced; As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; isolated chlorophyll fluoresces because there is no electron acceptor, so electrons of photoexcited chlorophyll drop right back to the ground state and the energy gets dissipated as light and heat
What are the two types of photosystems?
- Photosystem II: the reaction-centre chlorophyll a of photosystem II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum)
- Photosystem I: the reaction-centre chlorophyll a of photosystem II is known as P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum)
- These two pigments, P680 and P700, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the electron distribution in the two pigments and accounts for the slight differences in their light-absorbing properties
What is the linear electron flow?
1) A photon of light strikes one of the pigment molecules in a light-harvesting complex of PS II, boosting one of its electrons to a higher energy level. As the electron falls back to its ground state, the released energy is transferred to an electron in a nearby pigment molecule, causing its electron to be raised to an excited state. The process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-centre complex. It excites an electron in this pair of chlorophylls to a higher energy state.
– This electron is transferred from the excited P680 to the primary electron acceptor (becoming oxidized to P680+)
– An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons are supplied one by one to the P680+P680+ pair. The H+H+ are released into the thylakoid space. The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2.
– Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain; made up of the electron carrier plastoquinone(Pq), a cytochrome complex, and a protein called plastocyanin (Pc). Each component carries out redox reactions as electrons flow down the electron transport chain, releasing free energy that is used to pump protons (H+) into the thylakoid space, contributing to a proton gradient across the thylakoid membrane.
– The potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis
– At the same that that PSII is capturing light energy, PSI is doing the same thing; light energy is being transferred via light-harvesting complex pigments to the PS I reaction-centre complex, exciting an electron of the P700 pair of chlorophyll a; the photoexcited electron is then transferred to PS I’s primary electron acceptor (creating P700+); P700+ can now actas an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain fromPS II
– Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP)
– The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are required for its reduction to NADPH. Electrons in NADPH are in a higher energy level than water (where the electrons came from), so its electrons are more readily available for the reactions of the Calvin cycle than were those of water (This process also removes an H+ from the stroma)
– In summary, ATP provides chemical energy and NADPH provides reducing power for the Calvin cycle
The products of the light reactions are
– NADPH,
– ATP, and
– oxygen.
What is the Calvin cycle?
– To produce sugar, the necessary ingredients are
1) atmospheric CO2
2) ATP and NADPH generated by the light reactions.
– The Calvin cycle uses these to produce an energy-rich, three- carbon sugar called glyceraldehyde-3- phosphate (G3P).
– For the net synthesis of one molecule of G3P, the cycle must take place three times, fixing three molecules of CO2
Phase 1: Carbon fixation
– each CO2 molecule, one at a time, is attached to a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme that catalyzes this carboxylase-oxygenase, or rubisco
– the product a six-carbon intermediate that immediately splits in half, forming two molecules of 3-phosphoglycerate (3-PGA) because it is so energetically unstable that it
Phase 2: Reduction
– ATP reacts with 3-PGA resulting in transfer of phosphate and formation of 1,3 bisphosphoglycerate
– This can then oxidize NADPH and 1,3 bisphosphoglycerate becomes reduced, which also loses a phosphate group in the process, becoming glyceraldehyde 3-phosphate (G3P)
– for every three molecules of CO2 that enter the cycle, there are six molecules of G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate, because the rest are required to complete the cycle
Phase 3: Regeneration
– A series of chemical reactions uses energy from ATP to rearrange the atoms of 5 x G3P into 3 x RuBP
– RuBP can now receive CO2, and the cycle can continue
What are C3 plants?
–Most plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP
– Such plants are called C3 plants because
the first product of carbon fixation is a three- carbon compound, 3-PGA.
