Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Photosynthesis • • All energy on earth comes from the sun. We depend on: 1. Plants 2. Algae (underwater plants) 3. Cyanobacteria (photosynthetic bacteria-unicellular) – To provide this energy to us! – These organisms obtain energy directly from the sun and are called autotrophs – We are heterotrophs-we obtain energy by eating other organisms A little bit about Cyanobacteria • The endosymbiotic theory proposes that an ancestor of cyanobacteria was engulfed by an ancestor of today’s eukaryotic cell and gave rise to plant cells • Algal and plant cells contain chloroplasts (primarily in the mesophyll and guard cells) Leaf Structure and Photosynthesis The leaf is perfectly designed for maximum function • Ground tissue: function = photosynthesis palisade and spongy mesophyll have many Chloroplasts and air spaces enable efficient gas exchange. • Vascular tissue: function = support and transport » xylem and phloem • Dermal tissue separates the inside of the leaf from the air outside the plant. • Epidermal cells • waxy cuticle • Guard cells for stomata Structure of a leaf Photosynthesis primarily occurs in chloroplasts of leaves Lilac (Syringa) • Leaves are thin to maximize surface area exposed to sunlight. • It also limits the distance that gases need to travel to the chloroplasts Plant structure • Obtaining raw materials – sunlight • leaves = solar collectors – CO2 • stomates = gas exchange – H2O • uptake from roots – nutrients • N, P, K, S, Mg, Fe… • uptake from roots stomate transpiration gas exchange Plant Structure: Leaves • Stomata – Stomata = pores in epidermis used for gas exchange • CO2 into cell for photosynthesis • H2O out of leaf by evaporation to facilitate transpiration (process by which water pulled up plant) – Guard cells = epidermal cells that open and close stomata • Stomata open when guard cells swell with water • Stomata close when guard cells collapse together (shrivel) Stomates Stomates • Stomata are open in the daytime and closed at night • Light activated proton pumps in guard cell membranes cause K+ to move from neighbouring epidermal cells into guard cells-this results in H2O moving into guard cells (by osmosis) causing them to swell. • Increased turgor pressure causes them to open. • As the concentration of sucrose in guard cells decreases in the evening, water moves out of the cells and stomata close. Stomata typical of dicots Potato (Solanum) Stomata typical of monocots Maize (Zea) Scanning electron microscope images leaves Chloroplasts cross section of leaf absorb sunlight & CO2 CO2 chloroplasts in plant cell chloroplast chloroplasts contain chlorophyll make energy & sugar chloroplast H+ Plant structure ATP + + H+ H H+ + H H + H+ H+ H+ + H H thylakoid • Chloroplasts – double membrane – stroma outer membrane inner membrane • fluid-filled interior – thylakoid sacs stroma – grana stacks (sing. granum) • Thylakoid membrane contains – chlorophyll molecules – electron transport chain – ATP synthase • H+ gradient built up within thylakoid sac thylakoid granum Chloroplasts • Have 3 membranes • The third membrane is called the thylakoid. • The thylakoid is folded and looks like stacks of coins called granum • The stroma is the space surrounding the grana Chloroplasts • Chlorophyll molecules are embedded in the thylakoid membrane • Act like a light “antenna” • These molecules can absorb sunlight energy. Image from Biology 11: College Preparation. Pg 73. Nelson, Toronto. 2003. Chlorophyll • There are several types of chlorophyll • Chlorophyll a (blue-green) and chlorophyll b (yellow-green) • Both contain a porphyrin ring attached to a long H-C tail or phytol tail (hydrophobic to anchor into membrane) • Porphyrin rings are also found in the cytochromes of ETC Chlorophyll • The ring has a Mg atom in the centre and alternating single/double bonds • Electrons in the porphyrin ring absorb light energy and begin photosynthesis • Chlorophyll a differs in that it has a methyl group (-CH3) at position –R while chlorophyll b contains an aldehyde group (-CHO) • Chlorophyll a is the primary light-absorbing pigment. Overview Overall Reaction What is the equation for photosynthesis? Similar to what? carbon dioxide + water glucose (simple sugar) + oxygen CO2 (g) + H2O (l) + light energy [CH2O]+ O2(g) 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O How are they different? 3.1 Homework • P.145 • Q 1-7 3.2 Light energy and photosynthetic pigments Overview • There are 3 distinct stages to photosynthesis; 1. Capturing light energy 2. Using this energy to make ATP and reduced NADP+ (energy shuttling coenzyme-similar to NAD+) 3. Using the free energy of ATP and the reducing power of NADPH + H+ to synthesize organic compounds such as glucose, from CO2 Light (dependent)Reactions (Stages 1 and 2) • Happen ONLY in sunlight on the thylakoid membrane 1. Light is absorbed by chlorophyll molecules 2. The energy generates molecules of ATP Light Independent ReactionsCalvin Cycle-Carbon Fixation (formerly the “dark reactions”) • Happen in sunlight, and in the dark. – Hence “independent of light” 1. ATP generated by sunlight drives the Calvin Cycle. 2. Monosaccharides (eg. glucose) are manufactured in the cycle. 3. Monosaccharides are used to “build” polysaccharides (eg. Starch). Summary Image from: Biology 11: College Preparation. Pg 74. Nelson, Toronto. 2003. Photosynthesis • Light reactions – light-dependent reactions – energy conversion reactions • convert solar energy to chemical energy • ATP & NADPH • Calvin cycle It’s not the Dark Reactions! – light-independent reactions – sugar building reactions • uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 Absorption of Light by Chlorophyll Pigments of photosynthesis • Chlorophylls & other pigments – embedded in thylakoid membrane – arranged in a “photosystem” • collection of molecules – structure-function relationship A Look at Light • The spectrum of color V I B G Y O R Light: absorption spectra • Photosynthesis gets energy by absorbing wavelengths of light – chlorophyll a • absorbs best in red & blue wavelengths & least in green – accessory pigments with different structures absorb light of different wavelengths • chlorophyll b, carotenoids, xanthophylls Why are plants green? What wavelengths of light do you think plants use the least in photosynthesis? •Chlorophyll A and B absorb light mostly in the red and blue regions of the spectrum •Carotene and xanthophyll absorb light from other regions and pass the energy to chlorophyll Light absorption by chlorophyll Photosynthetic pigments are arranged as “photosystems” Photosystems of photosynthesis • 2 photosystems in thylakoid membrane – collections of chlorophyll molecules – act as light-gathering molecules – Photosystem II reaction • chlorophyll a • P680 = absorbs 680nm wavelength red light center – Photosystem I • chlorophyll b • P700 = absorbs 700nm wavelength red light antenna pigments Measuring light absorption • A spectrometer is used to measure the amount of absorption at each wavelength • An action spectrum shows the rate of photosynthesis at different light intensities Brown seaweeds • subjected to low light intensities • Chlorophyll C instead of chlorophyll B • fucoxanthin -very wide absorption spectrum -absorbs light in parts of the spectrum where chlorophyll is less efficient Red seaweeds • Have chlorophyll D • Phycoerythrin Green seaweeds • Found in shallow water • Do not have fucoxanthin • Does not have phycoerythrin Accessory Pigments • Chlorophyll is not the only lightabsorbing molecule in chloroplasts • Accessory pigments like orange carotenoids (beta-carotene) and yellow xanthophylls help absorb other light energy which may damage chlorophyll Changing colours • During spring and summer leaves appear green because of the high concentration of chlorophyll • As temperatures cool, chlorophyll is broken down and we see the oranges and yellows and even reds (anthocyanin) Practice Questions • P.154 • Q 1-3, 6-8