photosynthesis. energy is the ability to do work living things depend on energy. organisms that make...
TRANSCRIPT
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Photosynthesis
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Energy is the ability to do work
• Living things depend on energy.• Organisms that make their own food =
autotrophs• Plants and some other organisms are able to
use light energy from the sun to make food• Organisms that cannot make their own food =
heterotrophs
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Chemical Energy
• Adenosine triphosphate (ATP) – chemical compound used to store and release energy
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ATP
• ATP consists of adenine (A), ribose (a 5-carbon sugar, remember RNA?), and three phosphate groups.
• The 3 phosphate groups hold high energy bonds
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ADP
• ADP is adenosine diphosphate – meaning it has two phosphate groups instead of three
• Cells can add a phosphate group to ADP to make ATP to store energy. – ATP is like a full battery. ADP is like a battery that
needs recharging
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Releasing energy
• The energy stored in ATP is used by breaking the chemical bonds between the second and third phosphate groups.
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Why ATP is important• Powers cell membrane sodium-potassium
pump – vital for nervous system connections• Powers movement of organelles• Powers synthesis of proteins and nucleic acids• Powers responses to chemical signals– Firefly light (chemical = luciferin) is powered by
ATP in a process called bioluminescence
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Photosynthesis
Photosynthesis – plants use the energy of sunlight to convert water and carbon dioxide into high energy carbohydrates (sugar and starch) and oxygen (as waste)
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Investigating Photosynthesis
• Where does the mass of a tree come from?• Trees start as seedlings and grow into trees– Metasequoia (Dawn Redwood)
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Jan van Helmont’s Experiment
• 1643 – Belgian physician – “Where does mass of tree come from?”
• Watered plant for five years• Seedling grew into tree – over 75 kg (~200 lb)• Mass of soil didn’t change over five years• Concluded mass was from water• Water = “hydrate” part of carbohydrate, what
about “carbo-”?
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Joseph Priestley’s Experiment
• 1771 – Joseph Priestley, English minister • Lit candle, placed jar over candle, watched
candle go out.• Concluded oxygen was needed for fire• Placed sprig of mint plant in jar• After a few days, the candle could be relighted
and would stay lit for a while• Concluded plants produce oxygen
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Jan Ingenhousz’ Experiment
• 1779 – Ingenhousz, Dutch scientist• Found that plants only produce oxygen when
exposed to light• Concludes plants need sunlight to produce
oxygen
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The Photosynthesis Equation
• Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen
Carbon dioxide + Water + Light Sugar + Oxygen6CO2 + H2O + Light C6H12O6 + O2
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Light and Pigments
• In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll, which is a molecule in chloroplasts, an organelle in plant cells.
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Light and Pigments
• Energy from the sun travels to Earth in the form of light.
• Light is of various wavelengths.
• Your eyes see the wavelengths as different colors.
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Light and Pigments
• Plants gather the sun’s energy with light absorbing molecules called pigments.
• Main pigment is chlorophyll – two types chlorophyll a and chlorophyll b.
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Light and Pigments
• Chlorophyll readily absorbs light in the red and blue-violet regions of the spectrum.
• Chlorophyll does not absorb light well in the green region – thus the leaves of plants appear green.
• Because light is energy, the plants are absorbing the energy from that light.
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Inside a Chloroplast
• Photosynthesis takes place inside chloroplasts• Contain saclike photosynthetic membranes
called thylakoids.• Thylakoids are arranged in stacks called grana.
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Inside a Chloroplast
• Chlorophyll and other pigments in the thylakoid membrane are organized into photosystems.
• The region outside the thylakoid membrane is called the stroma.
• Two reactions in photosynthesis:– Light-dependent: inside thylakoid membrane– Calvin cycle (Light-independent/dark): in stroma
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Chloroplast
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Electron Carriers• When sunlight strikes a leaf, it ‘excites’
electrons, meaning that they gain energy.• Plant cells use electron-carriers to move these
high energy electrons (think of hot coals) – process called electron transport.
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Electron Carriers
• NADP+ (Nicotinamide adenine dinucleotide phosphate) is an electron carrier.
• NADP+ accepts and holds 2 high-energy electrons along with a hydrogen ion (H+)
• When it is holding these, it is converted to NADPH. Now it can carry these electrons to chemical reactions elsewhere in cell
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Light-Dependent Reactions• Light-dependent reactions require light.• This is why plants need light to grow.• The light-dependent reactions of plant cells
produce oxygen as a gas and convert ADP and NADP+ into the energy carriers ATP and NADPH
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Light-Dependent Reactions
• Step 1: Pigments in photosystem II (discovered second, hence name) absorb light– High energy electrons passed on to electron transport chain
(ETC)– Thylakoids break up water molecules to use 2 H+ ions and
oxygen is released – source of O2 we breathe
• Step 2: Electrons move through ETC from photosystem II to photosystem I– H+ ions moved from stroma to inner thylakoid
• Step 3: Pigments in photosystem I use energy from light to energize electrons. NADP+ picks these up and H+ ions to become NAPH
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Light-Dependent Reactions (Cont.)
• Step 4: As electron pass from chlorophyll to NAP+, H+ ions are pumped across membrane, providing energy to make ATP.
• Step 5: An enzyme, ATP Synthase, helps H+ ions cross the membrane– ATP Synthase binds ADP and a phosphate group to
produce ATP
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Confused yet?
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The Calvin Cycle (Light-Independent Reactions)
• The ATP and NADPH formed by the light-dependent reactions have chemical energy but not enough to sustain plant.
• During the Calvin Cycle, plants use that energy to make energy that can be stored = high-energy sugars
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Calvin Cycle (Light-Independent Reactions)
• Step 1: 6 Carbon dioxide molecules enter cycle from atmosphere. Combine with six 5-carbon molecules = 12 3-carbon molecules
• Step 2: 12 3-carbon molecules converted into high-energy forms. Energy to do this comes from ATP and NAPH produced earlier
• Step 3: Two 3-carbon molecules removed from cycles to produce sugars for metabolism and growth in the form of sugars, lipids, and proteins.
• Step 4: Remaining carbon molecules are recycled for later Calvin Cycles.
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Factors affecting Photosynthesis
• Water– Plant adaptation –
plants in dry conditions have a waxy coating on leaves to prevent water loss
• Temperature• Intensity of Light – Maximum rate– Conifers in Winter may
only occasionally carry out photosynthesis