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Photosynthesis. Energy for Life Processes All organisms use energy to carry out the functions necessary to life Autotrophs are organisms that manufacture.

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Presentation on theme: "Photosynthesis. Energy for Life Processes All organisms use energy to carry out the functions necessary to life Autotrophs are organisms that manufacture."— Presentation transcript:

1 Photosynthesis

2 Energy for Life Processes All organisms use energy to carry out the functions necessary to life Autotrophs are organisms that manufacture their own food from inorganic substances & energy Heterotrophs are organisms that cannot manufacture their own organic compounds from inorganic substances –Must consume autotrophs or other heterotrophs –Obtain energy indirectly

3 Autotrophs & Photosynthesis Most autotrophs use photosynthesis to covert light energy from the sun into chemical energy –Plants, but also some algae & bacteria, are photosynthetic organisms –All life ultimately depends on autotrophs & the process of photosynthesis

4 What is Photosynthesis? Conversion of light energy from sun into usable chemical energy Chemical equation of photosynthesis –6 H 2 O + 6 CO 2 + light energy  C 6 H 12 O O 2 –Reads: six molecules of water and 6 molecules of carbon dioxide in the presence of light are converted into one molecule of glucose and 6 molecules of oxygen

5 Biochemical Pathways Photosynthesis is part of a biochemical pathway with cellular respiration –Biochemical pathway is where the product of one reaction is used in the next reaction

6 Light Reactions Begins with the absorption of light in chloroplasts –Structure of chloroplasts Surrounded by pair of membranes Inside inner membrane is another system of membranes—thylakoids, which are arranged as flattened sacs –Layered thylakoids form stacks called grana –Stroma is the solution surrounding the thylakoids

7 Light & Pigments Light from sun appears white, but is actually made of variety of colors –Visible spectrum represents the colors white light separates into—ROY G BIV –Different colors of spectrum are different wavelengths Pigment is a compound that absorbs light –Color that is not absorbed, is reflected

8 Chloroplast Pigments Pigments found in the thylakoid membranes –Most important are chlorophylls Two common types are chlorophyll a & b –Chlorophyll a absorbs more red light than chlorophyll b –Chlorophyll b absorbs more blue light than chlorophyll a –Neither chlorophyll a nor b absorb much green light—reason why plants are green—color green is reflected

9 Chlorophyll a & Accessory Pigments Chlorophyll a is directly involved in light reactions of photosynthesis Accessory pigments assist chlorophyll a in capturing more light energy because they absorb colors of light that are not absorbed by chlorophyll a –Chlorophyll b –Yellow, orange, & brown carotenoids

10 Electron Transport Photosystem represents a cluster of pigment molecules (chlorophylls & carotenoids) found in thylakoid membrane –2 Types of photosystems Photosystem I and photosystem II –Similar in pigments they contain but differ in their roles in light reactions

11 Stage 1: Light Reaction Light reactions begin with absorption of light by chlorophyll a & accessory pigments in thylakoids –By absorbing light, molecules acquire some energy –Energy passed quickly to other pigment molecules until reaches specific pair of chlorophyll a molecules

12 5 Step Process of Light Reaction 1. Light excites electrons in chlorophyll a molecules of photosystem II 2. Excited electrons move to a primary electron acceptor 3. Electrons are transferred along series of molecules called electron transport chain –As electrons pass from molecule to molecule, lose most of their energy

13 5 Step Process of Light Reaction 4. At same time light was absorbed by photosystem II, light is also absorbed by photosystem I. –Electrons move from pair of chlorophyll a molecules in photosystem I to another primary electron acceptor. – Electrons lost by chlorophyll a molecules are replaced by electrons that have passed through electron transport chain from photosystem II

14 5 Step Process of Light Reaction 5. Electrons from photosystem I are transferred long 2 nd electron transport chain. –Chain brings electrons to side of thylakoid membrane that faces the stroma. –There, NADP + combines with H + to form NADPH

15 Restoring Photosystem II Photosynthesis would not occur if electrons from chlorophyll molecules in photosytem II did not replace electrons that leave chlorophyll molecules in photosystem I –Replacement of electrons is provided by water molecules Enzyme inside thylakoid splits water molecules into p +, e -, & oxygen –2 H 2 O  4 H e - + O 2 Oxygen is released out of the chloroplast & leaves the plant as a byproduct of photosynthesis

16 What is the difference between Photosystem I & II? Photosystem I provides electrons used to make NADPH from NADP + Photosystem II provides electrons to replace electrons lost by photosystem I & pumps protons into thylakoids

17 Light Reaction Animation ry/source/61a.swfhttp://www.biology4all.com/resources_libra ry/source/61a.swf

18 Chemiosmosis Process that occurs during light reactions, which is responsible for making ATP –Concentration of protons is higher inside thylakoid than in stroma –Relies on concentration gradient of protons –Gradient represents PE of protons

19 Chemiosmosis Cont’d –ATP synthase makes ATP by adding a phosphate group to ADP ATP synthase acts as both a carrier protein (allows protons to cross thylakoid membrane) & enzyme (speeds up reaction making of ATP from ADP) Therefore, ATP synthase converts PE of proton concentration gradient into chemical energy stored in ATP –ATP & NADPH, together, provide energy for second stage of photosynthesis

20 Stage 2: Calvin Cycle Uses energy stored in ATP & NADPH from light reactions to incorporate CO 2 into organic compounds –Carbon fixation Process occurs in stroma of chloroplast Sometimes referred to as dark reactions

21 Steps of Calvin Cycle Step 1 –CO 2 diffuses into stroma from surrounding cytosol –RuBP, 5-carbon carbohydrate, combines with CO 2 creating 6-carbon molecule –6 carbon molecule splits into PGA—2 three- carbon molecules

22 Steps of Calvin Cycle Step 2 –Each PGA molecule receives a phosphate group from ATP –The compound then receives proton from NADPH & releases a phosphate group to produce PGAL –Byproducts of these reactions create ADP, NADP + & phosphate to be used again in light reactions

23 Steps of Calvin Cycle Step 3 –Most PGAL is converted back into RuBP Allows the continuation of Calvin cycle –Some PGAL is used to make other organic compounds—carbohydrates, amino acids, & lipids

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25 How much ATP & NADPH are required to make 1 molecule of PGAL from CO 2 ? Takes three turns Calvin cycle to produce each molecule of PGAL Therefore, 3 turns of Calvin cycle uses 9 molecules of ATP & 6 molecules NADPH

26 Alternative Pathways Majority of plants are C 3 plants—fix carbon only through Calvin cycle Some plants need to adjust to excessive water loss –Water loss occurs through stomata Small pores located on undersurface of leaves Also passageway for CO 2 entering & O 2 leaving

27 Alternative Pathways C 4 pathway –Allow plants to fix CO 2 into four carbon compounds instead of three carbon compounds –During hottest part of day, stomata are partially closed –Lose ~ ½ water C 3 when producing same amount of carbohydrate –Corn, sugar cane, crabgrass

28 Alternative Pathways CAM Pathway –Open stomata at night & close them during day –Grow slower, but lose less water –Usually found in hot, dry climates –Examples: cacti, pineapples

29 Rate of Photosynthesis Affected by plant’s environment –Light intensity As light intensity increases, rate of photosynthesis initially increases, then plateaus Higher light intensity excites more molecules of chlorophyll –Carbon dioxide As CO 2 increases, rate of photosynthesis increases until it plateaus –Temperature Rate of photosynthesis increases as temp increases until reaches temperature threshold –Too hot—enzymes are ineffective & stomata close

30 Summary of Photosynthesis Light reactions: –Trap solar energy & make ATP –Electrons convert NADP + to NADPH –Oxygen is released into environment Calvin cycle: –Fix carbon dioxide into glucose & other organic molecules


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