Photosynthesis These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur (a) Plants bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). *Calvin cycle (c) Unicellular protist 10 m (e) Pruple sulfur bacteria 1.5 m 10.2 (b) Multicellular algae (d) Cyanobacteria 40 m
Overview: This process creates the nutrient source for the BIOSPHERE. Photosynthesis Is the process that converts solar energy into chemical energy Plants are photoautotrophs, using energy to make organic molecules from water and carbon dioxide. Photosynthesis also occurs in algae, some other protists, and some prokaryotes.
Major sites of photosynthesis - leaves
Sites of photosynthesis Organelle: Double membrane Thylakoids Grana/granum Stroma Lamella Chloroplast Mesophyll 5 µm Outer membrane Stroma Granum Thylakoid Thylakoid space Inner membrane Intermembrane space 1 µm
Guard cells: Stomata/Stoma
Stomata are created by pair of guard cells. Guard cells take in water by osmosis, become turgid, and swell Increases gap between cells stoma open Guard cells lose water, become flaccid, and shrink Decreases gap between cells stoma closed
Changes in turgor pressure due to reversible uptake of potassium ions - K + When stomata are open guard cells get K + from neighbouring epidermal cells. Increase solute conc. in guard cells osmosis follows Water in When stomata are closed - guard cells lose K +, diffuses out. Decrease solute conc. in guard cells osmosis follows Water out
Movement of K + occurs passively, in response to pumping of H + across membrane.
In general stomata are open during day, and closed at night. Why? Cues for stomatal opening? (How do plants know when its time?) Blue-light receptor in guard cells in plasma membrane stimulates activity of ATP-powered pumps Photosynthesis begins in guard cell chloroplasts making ATP available Depletion of CO 2 in air spaces within leaf Internal clock plant in dark continues roughly 24 hour cycling of stomatal opening circadian rhythm.
Evolutionary background: Land plants evolved from aquatic plants 425 million years ago, adapting to problem of dehydration. Early environment had different atmosphere than present. Trade off between prevention of excessive water loss, and photosynthesis If plants close stomata during the day, they reduce the amount of CO 2 available, and increase amount of O 2 in leaf air spaces.
Summarized as: Photosynthesis 6 CO 2 + 12 H 2 O + Light energy C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Chloroplasts split water into Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Reactants: 6 CO 2 12 H 2 O Products: C 6 H 12 O 6 6 H 2 O 6 O 2 Figure 10.4
Photosynthesis Preview Photosynthesis is broken into two processes The Light Reactions Occur in grana Split water, Release oxygen, Produce ATP, and Form NADPH The Calvin Cycle Occurs in the stroma Forms sugar from carbon dioxide, Using ATP for energy and NADPH for reducing power
Overview of Photosynthesis
Light Reactions Light reactions convert solar energy to the chemical energy of ATP and NADPH Light form of electromagnetic energy, travels in waves Wavelength The distance between the crests Determines the type of electromagnetic energy
The electromagnetic spectrum Is the entire range of electromagnetic energy, or radiation 10 5 nm 10 3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m Gamma rays X-rays UV Infrared Microwaves Radio waves Visible light 380 450 500 550 600 650 700 750 nm Figure 10.6 Shorter wavelength Higher energy Longer wavelength Lower energy
The visible light spectrum Include the colours of light we can see Includes the wavelengths that drive photosynthesis Pigments are substances that absorb visible light Reflect light, which include the colours we see Figure 10.7 Chloroplast Absorbed light Light Transmitted light Reflected Light Granum
The Spectrophotometer Machine that sends light through pigments and measures the fraction of light transmitted at each wavelength. The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths driving photosynthesis.
An absorption spectrum Is a graph plotting light absorption versus wavelength White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer 2 3 1 4 0 100 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 100 Figure 10.8 Blue light The low transmittance (high absorption) reading chlorophyll absorbs most blue light.
Absorption of light by chloroplast pigments The absorption spectra of three types of pigments in chloroplasts EXPERIMENT Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. RESULTS Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9
Rate of photosynthesis (measured by O 2 release) The action spectrum of a pigment Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis (b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.
The action spectrum for photosynthesis Was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga 400 500 600 700 (c) Engelmann s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. CONCLUSION Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.
Chlorophyll a Is the main photosynthetic pigment Chlorophyll b Is an accessory pigment H 3 C H H 3 C H CH 2 CH C C C C C C C CH 2 CH 3 C C C C CH 2 CH 2 C N N C H H H CH 3 Mg C C N N C O C C C C C C O H CH 3 CH 3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing head of molecule note magnesium atom at center C O O O CH 3 CH 2 Figure 10.10 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown
Accessory pigments Absorb different wavelengths of light and pass the energy to chlorophyll a When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable. Photon e Chlorophyll molecule Excited state Heat Photon (fluorescence) Ground state Figure 10.11 A
If an isolated solution of chlorophyll is illuminated It will fluoresce, giving off light and heat Figure 10.11 B
Thylakoid membrane A photosystem Is composed of a reaction centre surrounded by a number of lightharvesting complexes. Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary election acceptor e Figure 10.12 Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)
The light-harvesting complexes Consist of pigment molecules bound to particular proteins Funnel the energy of the photons of light to the reaction centre When a reaction centre chlorophyll molecule absorbs energy One of its electrons gets bumped up to a primary electron acceptor
The thylakoid membrane Is populated by two types of photosystems, I and II There are two paths electrons travel: Non-cyclic electron flow Is the primary pathway of energy transformation in the light reactions Route: Photosystem II to Photosystem I
Non-cyclic photophosphorylation H Produces 2 ONADPH, CO 2 ATP, and oxygen Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH O 2 [CH 2 O] (sugar) Light 1 2 H + + O 2 H 2 O 3 e e Primary acceptor e P680 2 Pq Cytochrome complex 5 4 PC Primary acceptor P700 Fd e e 7 NADP + reductase Light 8 NADP + + 2 H + NADPH + H + 6 ATP Figure 10.13 Photosystem II (PS II) Photosystem-I (PS I)
Cyclic Electron Flow Under certain conditions photoexcited electrons take an alternate path Only photosystem I is used, ONLY ATP is produced. Primary acceptor Fd Primary acceptor Fd Pq Cytochrome complex NADP + reductase NADP + NADPH Pc Figure 10.15 Photosystem II ATP Photosystem I
The light reactions and chemiosmosis: the organization of the thylakoid membrane LIGHT H 2 O CO 2 LIGHT REACTOR NADP + ADP ATP NADPH CALVIN CYCLE STROMA (Low H + concentration) O 2 Photosystem II 2 H + [CH 2 O] (sugar) Cytochrome complex Light Photosystem I Fd NADP + reductase 3 NADP + + 2H + H 2 O THYLAKOID SPACE (High H + concentration) 1 1 2 2 O 2 +2 H + Pq 2 H + Pc NADPH + H + To Calvin cycle Figure 10.17 STROMA (Low H + concentration) Thylakoid membrane ATP synthase ADP P H + ATP
Comparison between mitochondria and chloroplasts Generate ATP by : Source of energy : Mitochondria Chemiosmosis Electrons from glucose Chloroplast Chemiosmosis Energized electrons by the sun In both: ATP synthase: Spatial organization: Redox rxns of ETC generate a H+ gradient across a membrane Uses this proton-motive source to make ATP
Key Higher [H + ] Lower [H + ] Mitochondrion The spatial organization of chemiosmosis Differs in chloroplasts and mitochondria Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H + Diffusion Intermembrance space Thylakoid space Membrance Electron transport chain ATP Synthase Stroma Matrix ADP+ P H + ATP
Calvin Cycle The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar Is similar to the citric acid cycle Occurs in the stroma Has three phases Carbon fixation Reduction Regeneration of the CO 2 acceptor
Light H 2 O CO 2 LIGHT REACTION NADP + ADP ATP NADPH CALVIN CYCLE Input 3 CO 2 (Entering one at a time) O 2 [CH 2 O] (sugar) Rubisco 3 P P 3 P P Ribulose bisphosphate (RuBP) Short-lived intermediate Phase 1: Carbon fixation 6 3-Phosphoglycerate P 6 ADP 6 ATP 3 ATP 3 ADP CALVIN CYCLE 6 P 1,3-Bisphoglycerate P 6 NADPH 6 NADPH + 6 P The Calvin cycle Figure 10.18 Phase 3: Regeneration of the CO 2 acceptor (RuBP) 5 (G3P) P 1 G3P (a sugar) Output 6 Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction P P Glucose and other organic compounds
Cyclic Electron Flow Alternative cycle when ATP is deficient Photosystem I used but not II; produces ATP but no NADPH Why? The Calvin cycle consumes more ATP than NADPH. Cyclic photophosphorylation
What causes stomata to close during the day? Environmental stress, water deficiency High temperatures (hypothesis?)
In most plants, carbon fixation occurs via: Rubisco adds CO 2 to ribulose bisphosphate RuBP. These plants are called C 3 because: First organic product is 3-phosphoglycerate Examples: rice, wheat, soybeans On hot, dry days stomata close and less photosynthesis occurs, meaning: Starvation of Calvin cycle, reduction in sugar output. BUT it gets worse
Rubisco can accept O 2 in place of CO 2. (Enzyme specificity?) O 2 added to Calvin cycle instead of CO 2 causes a product that splits into a 2 carbon compound that is exported from the chloroplast. 2 carbon compound is broken down by mitochondria to CO 2. This is called photorespiration.
Photorespiration produces no ATP, and no food. Photorespiration decreases Calvin cycle output by reducing amount of carbons. Hypothesis: evolutionary baggage early atmosphere contained very little free oxygen. Photorespiration drains away as much as 50% of the carbon fixed by Calvin cycle no known benefit to plants.
Since environment causes stomata to close hot, dry, bright days. Some plants have alternate modes of carbon fixation that minimize photorespiration. These are C 4 plants and CAM plants.
Ex: Sugarcane, Corn, and Grass family
Ex: Succulents cacti, and pineapples Crassulacean acid metabolism How are C 4 plants and CAM plants Similar? Different?