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Chapter 6: Photosynthesis How Plants Make Food and the Importance of Captured Energy.

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Presentation on theme: "Chapter 6: Photosynthesis How Plants Make Food and the Importance of Captured Energy."— Presentation transcript:

1 Chapter 6: Photosynthesis How Plants Make Food and the Importance of Captured Energy

2 Historical Perspective Life started on this planet about 3 ½ billion years ago, but without photosynthesis it would not have thrived. Cyanobacteria (formerly called blue-green algae) were the first simple organisms to master photosynthesis. Later, these evolved into the chloroplasts of plants. Photosynthesis profoundly changed the planet. The carbon-dioxide and ammonia rich atmosphere slowly changed to an oxygen /nitrogen atmosphere, poisoning many of the original organisms, but providing a new, more efficient way of releasing energy for the survivors. The oxygen also produced a thin layer of ozone (O 3 ) above the atmosphere, that protected us from solar radiation, and paved the way for organisms to live on land.

3 Plants use these things to make food: –Water (H 2 O) –Carbon Dioxide (CO 2 ) –Energy (from sunlight) –Chlorophyll (a green pigment that acts as an organic catalyst: C 55 H 72 O 5 N 4 Mg) Overall reaction: 6 CO H 2 O + light → C 6 H 12 O Glucose (a simple sugar)

4 Chloroplast Part of a plant cell where photosynthesis takes place. Contains: –Outer Membrane –Inner Membrane –Stroma (fluid) –Thylakoids –Chlorophyll Plant cells (stained) See Diagram on Page 112

5 Thylakoid A Thylakoid is a small, flat disk or sac inside a chloroplast. Several thylakoids stacked together make a granum (pl. grana) The surface of thylakoids is where most of the LIGHT reactions in photosynthesis take place.

6 Electron Micrograph of Thylakoid

7 Stroma The fluid inside the chloroplast, but outside of the thylakoids. The reactions of the Calvin Cycle occur in the stroma.

8 Pigment A pigment is a coloured compound Pigments capture certain colours of light The most important pigments in plant cells are: –Chlorophyll-a (green primary pigment) –Chlorophyll-b (blue-green accessory pigment) –Carotenoids (yellow, orange and brown accessory pigments)

9 ATP Adenosine triphospate Important energy-releasing compound Breaks down into ADP and phosphate

10 NADP, NADPH Nicotinamide Adenine Dinucleotide Phosphate This chemical compound transfers hydrogen ions (protons) from place to place When its not carrying the hydrogen its called NADP, When it is carrying hydrogen it is NADPH

11 RuBP Ribulose biphosphate A simple sugar (monosaccharide) Has only 5 carbon atoms (glucose has 6) Used in photosynthesis

12 PGA, PGAL Phospho-glyceric acid, Phosphoglyceraldehyde Simple organic compounds They have only 3 carbon atoms They can be used to make sugars (like glucose)

13

14 Photosynthesis Photosynthesis is the process by which plants convert the energy of sunlight into chemical energy in organic compounds Carbon dioxide and water are “fixed”, or joined together to make simple sugars (eg. Glucose) This is a fancy way of saying that plants make their own food

15 Diagram Overview of Photosynthesis Photosynthesis Light Reactions Calvin Cycle Sunlight is absorbed Electrons are transported Water molecules are split Oxygen is released Hydrogen is retained as NADPH Carbon dioxide taken in CO 2 is joined to other molecules. PGAL is made. Consists of 1 2 The process by which plants make their own food… Water Sunlight CO 2 input oxygen glucose (food) output is called where Goes to next step where makes Glucose (food)

16 Main Steps in Photosynthesis  The Light Reactions  The parts of photosynthesis that require sunlight and water. Oxygen is produced  The Calvin Cycle (A.K.A. the Dark Reactions)  The parts of photosynthesis that can take place with or without sunlight. Carbon dioxide is fixed or converted into organic compounds.

17 The Light Reactions  Absorption of Light by Chlorophyll  Light energy excites a chlorophyll-a electron  Electron Transport  Excited electrons pass from molecule to molecule through the thylakoid membrane.  Water molecules are split:  2 H 2 0 → 4 H e - + O 2 oxygen is released  Chemiosmosis -- H + is used to make:  NADPH (NADP + H + → NADPH)  ATP(ADP + P → ATP)

18 The Calvin Cycle (formerly called The Dark Reactions)  CO 2 is bonded to RuBP (a 5-Carbon sugar)  This splits into 2 PGA (a 3-Carbon acid)  The 2 PGAs are converted to PGALs  ATP is used up (it gives up its phosphate)  NADPH is used (it gives its hydrogens)  Most of the PGAL is converted into RuBP to restart the cycle, but…  Some PGAL is used to make organic compounds (like glucose)

19 Calvin Cycle 6 carbon dioxide molecules join 6 RuBP molecules 6 unstable six-carbon sugars form They immediately split into 12 PGAs The PGAs transform into 12 PGALs (using up ATP + NADPH from light reactions) 10 PGALs are used to make 6 RuBPs 2 PGALs are used to make glucose RuBP CO 2 Unstable 6-C PGA PGAL START CYCLE AGAINEND RuBP GLUCOSE

20 Calvin Cycle 6 carbon dioxide molecules join 6 RuBP molecules 6 unstable six-carbon sugars form They immediately split into 12 PGAs The PGAs transform into 12 PGALs (using up ATP + NADPH from light reactions) 10 PGALs are used to make 6 RuBPs 2 PGALs are used to make glucose RuBP CO 2 Unstable 6-C PGA PGAL START CYCLE AGAINEND

21 Overall Equation for Photosynthesis:  6 CO H 2 O + light → C 6 H 12 O Which means that:  6 carbon dioxide molecules, plus  6 water molecules, with  enough sunlight, produces  One molecule of glucose.  6 molecules of oxygen. Important!

22 Other Pathways  The notes on photosynthesis describe the most common form… C3 photosynthesis. Plants that grow in hot, dry places use variations of photosynthesis.  C4 photosynthesis occurs in corn, sugar cane and crabgrass.  CAM photosynthesis occurs in cactus and pineapple.

23 Follow up Activities Draw the Calvin Cycle diagram (p. 117) in your notebook Draw the Calvin Cycle diagram (p. 117) in your notebook Skim pages in the textbook, looking particularly at the diagrams Skim pages in the textbook, looking particularly at the diagrams Carefully read the Summaries (page 121) Carefully read the Summaries (page 121)

24 Leaf structure

25 Plural: stomata Palisade Layer (lots of photosynthesis) Palisade Layer (lots of photosynthesis) Spongy Layer (some photosynthesis) Spongy Layer (some photosynthesis) Epidermis Layer (no photosynthesis) Epidermis Layer (no photosynthesis) AKA: vascular bundle

26 Assignment Page 122 # Page 122 #1 - 22

27 Photosynthesis The Light Reactions, showing photosystems The Light Reactions, showing photosystemsLight Reactions Light Reactions The Calvin Cycle. The Calvin Cycle.Calvin CycleCalvin Cycle

28 Answers Page 122 #1 to 22

29 1Chlorophyll is well named because chloro- means green and chlorophyll is a green pigment. 2Stroma is the liquid inside a chloroplast, stomata (s. stoma) are the small holes or pores on the underside of leaves.

30 3Carbon fixation means taking carbon dioxide from the air and incorporating it into more complex organic compounds, like sugar 4A biochemical pathway is a series of linked reactions, where the product of one reaction is used by the next reaction. 5Calvin cycle does not belong, because the others are all part of the light reactions.

31 6(a)O 2 is a product, the others are used up. 7(c)CO 2 is a reactant in the Calvin cycle. 8(b)accessory pigments absorb colours of light that chlorophyll a doesn’t. 9(d)C4 plants fix CO2 into 4-carbon compounds 10(c) Oxygen is produced when water is split.

32 11(d) Light reactions take place on the thylakoid membrane 12(a) During chemiosmosis ATP is formed 13(c) Carbon fixation occurs during the Calvin cycle, not during the light reactions. 14(b) most of the PGAL is recycled to make more RUBP and keep the cycle going. 15(b) most of the Calvin cycle occurs in the stroma

33 16.Photosystem II is the starting point where sunlight causes chlorophyll-a to lose electrons. These electrons move through a transport chain to Photosystem I, where more light is absorbed and more electrons are released. The electrons are eventually replaced by electrons from water molecules that have been split. 17.The Calvin cycle is a biochemical pathway, because each time a substance is formed as a product of one reaction, it is immediately used as a reactant in another reaction.

34 18.Most CAM plants are found in hot, dry climates, where plants need to keep their stomata closed during the day, and only open them at night. 19.ATP is synthesized during the step called chemiosmosis, when H + ions are diffusing through the membrane. They provide energy to put together the ATP

35 20 During the summer the leaves are rich in chlorophyll, which is bright green and masks most other pigments. In the fall the chlorophyll production stops and the chlorophyll begins to disappear, leaving behind other pigments (like yellow and orange carotenoids). Note: In some plants the decaying pigments may also produce bright red colours

36 21.The structure of a chloroplast is well suited to its function. The chloroplast contains many thylakoids that provide lots of membrane surface for reactions to occur on. These membranes also contain chlorophyll and other pigments to absorb different colours of light. The chloroplasts contain fluids (stroma) where other reactions can occur.

37 22.In the Calvin cycle (1) CO 2 combines with RuBP to form an unstable 6C sugar. This immediately breaks into two PGA molecules. The PGAL molecules are immediately changed into PGAL molecules (using up some NADH an ATP). Most of the PGAL is used to replace the RuBP, but some of it is changed into glucose.


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