PHOTOSYNTHESIS 6.1

  The main form of energy from the sun is in the form of electromagnetic radiation   Visible radiation (white light) used for photosynthesis  Remember : ROY G. BIV?

The electromagnetic spectrum

 A Red Object absorbs the blue and green wavelengths and reflects the red wavelengths

Why are plants green?

  pigment a compound that absorbs light   different pigments absorb different wavelengths of white light.   chlorophyll is a pigment that absorbs red & blue light (photons) so green is reflected or transmitted.  Chlorophyll is located in the thylakoid membranes So, Plants are green because the green wavelength is reflected, not absorbed.

2 types of chlorophyll  Chlorophyll a – involved in light reactions  Chlorophyll b – assists in capturing light energy – accessory pigment  Carotenoids – accessory pigments – captures more light energy  Red, orange & yellow

The electromagnetic wavelengths and the wavelengths that are absorbed by the chlorophyll

During the fall, what causes the leaves to change colors?

Fall Colors  In addition to the chlorophyll pigments, there are other pigments present  During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments  Carotenoids are pigments that are either red, orange, or yellow

Photosynthesis is -  conversion of light energy into chemical energy that is stored in organic compounds (carbohydrates > glucose)  Used by autotrophs such as:  Plants  Algae  Some bacteria (prokaryotes)

  glucose - energy-rich chemical produced through photosynthesis   C 6 H 12 O 6  Biochemical pathway – series of reactions where the product of one reaction is consumed in the next  E.g. photosynthesis product is glucose which is used in cellular respiration to make ATP (carbohydrate)

Remember Redox Reaction reduction/oxidation The transfer of one or more electrons from one reactant to another Two types: 1.Oxidation is the loss of e - 2.Reduction is the gain of e -

Oxidation Reaction The loss of electrons from a substance or the gain of oxygen. glucose 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 Oxidation Carbon dioxide Water Oxygen

Reduction Reaction The gain of electrons to a substance or the loss of oxygen. glucose 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 Reduction

Photosynthesis equation Light energy 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Chlorophyll Reactants: Carbon dioxide and water : glucose and oxygen which is a byproduct Products: glucose and oxygen which is a byproduct

Where does photosynthesis take place?

Plants  Mainly occurs in the leaves: a.stoma - pores b.mesophyll cells Stoma Mesophyll Cell Chloroplast

Mesophyll Cell of Leaf Cell Wall Nucleus Chloroplast Central Vacuole Photosynthesis occurs in these cells!

Stomata (stoma) Pores in a plant’s cuticle through which water vapor and gases (CO 2 & O 2 ) are exchanged between the plant and the atmosphere. Guard Cell Carbon Dioxide (CO 2 ) Oxygen (O 2 ) Found on the underside of leaves Stoma

Chloroplast Organelle where photosynthesis takes place. Granum Thylakoid Stroma Outer Membrane Inner Membrane Thylakoid stacks are connected together

Parts  chloroplasts – dbl membrane organelle that absorbs light energy  Thylakoids – flattened sacs contain pigment - chlorophyll  Grana (pl: granum) – layered thylakoids (like pancakes)  Stroma – solution around thylakoids  Stomata – pore on underside of leaf where O 2 is released and CO 2 enters  Stroma : chloroplast :: cytosol : cytoplasm

Thylakoid Thylakoid Membrane Thylakoid Space Granum Grana make up the inner membrane

What do cells use for energy?

Energy for Life on Earth  Sunlight is the ULTIMATE energy for all life on Earth  Plants store energy in the chemical bonds of sugars  Chemical energy is released as ATP during cellular respiration

Structure of ATP  ATP stands for adenosine triphosphate  It is composed of the nitrogen base ADENINE, the pentose (5C) sugar RIBOSE, and three PHOSPHATE groups  The LAST phosphate group is bonded with a HIGH ENERGY chemical bond  This bond can be BROKEN to release ENERGY for CELLS to use

Removing a Phosphate from ATP Breaking the LAST PHOSPHATE bond from ATP, will ---  Release ENERGY for cells to use  Form ADP (adenosine diphosphate)  Produce a FREE PHOSPHATE GROUP

High Energy Phosphate Bond

FREE PHOSPHATE can be re-attached to ADP reforming ATP Process called Phosphorylation

Phosphorylation

Photosynthesis 1. Light Reaction - Produces energy from solar power (photons) in the form of ATP and NADPH. 2. Calvin Cycle  Also called Carbon Fixation or Carbon Cycle, Uses energy (ATP and NADPH) from light reaction to make sugar (glucose). SUN

 3 stages of photosynthesis-

Stages:  STAGE 1 - LIGHT REACTIONS - energy from sun is used to split water into H+ an O2  STAGE 2 – energy is converted to chemical energy & stored in ATP & NADPH in stroma  STAGE 3 - CALVIN CYCLE where carbon is fixed into glucose

Light Reaction (Electron Flow)  Occurs in the Thylakoid membranes  2 possible routes for electron flow: Use Photosystem I and Electron Transport Chain (ETC) and generate ATP only  OR use Photosystem II and Photosystem I with ETC and generate O 2, ATP and NADPH

Photosynthesis animation  k/j_explorations/ch09expl.htm k/j_explorations/ch09expl.htm k/j_explorations/ch09expl.htm

ELECTRON TRANSPORT - LIGHT REACTIONS in 5 steps

Photosystem I and II  Step 1 – light excites e- in photosystem II  Step 2 – e- move to primary e- acceptor Step 3 – e- move along electron transport chain (etc)  Step 4 – light excites e- in photosystem I  Step 5 – e- move along 2 nd (etc)  End – NADP+ combine H+ to make NADPH

Light reaction animation  s/Biology/Bio231/ltrxn.html s/Biology/Bio231/ltrxn.html s/Biology/Bio231/ltrxn.html

Electron transport chain song  Play the "Come On Down (The Electron Transport Chain)" song performed by Sam Reid.  Play the "Come On Down (The Electron Transport Chain)" song performed by Sam Reid. Come On Down (The Electron Transport Chain)Come On Down (The Electron Transport Chain)

Photolysis –photo-chemical splitting of water (restoring photosystem II)

Chemiosmosis – synthesis of ATP  Powers ATP synthesis  Takes place across the thylakoid membrane  Uses ETC and ATP synthase  H+ move down their concentration gradient forming ATP from ADP  Concentration of protons is greater in thylakoid than stroma

Chemiosmosis

The Calvin Cycle 6.2

Calvin Cycle -  Biochemical pathway in photosynthesis that produces organic compounds using ATP & NADPH  Carbon fixation – carbon atoms from CO 2 are bonded or ‘fixed’ into carbohydrates  occurs in stroma

Calvin Cycle  Carbon Fixation  C 3 plants (80% of plants on earth)  Occurs in the stroma  Uses ATP and NADPH from light reaction as energy  Uses CO 2  To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH.

Chloroplast Granum Thylakoid STROMA– where Calvin Cycle occurs Outer Membrane Inner Membrane

Calvin Cycle (C 3 fixation) 6CO 2 6C-C-C-C-C-C 6C-C-C 6C-C-C-C-C 12PGA RuBP 12G 3 P (unstable) 6NADPH 6ATP C-C-C-C-C-C Glucose (6C) (36C) (30C) (6C) 6C-C-C C3C3 glucose

Calvin Cycle Remember: C 3 = Calvin Cycle C3C3 Glucose

Step 1 -  CO 2 diffuses fr cytosol & combines with RuBP which splits into pair of PGA

Step 2 -  PGA gets phosphate gr fr ATP gets proton fr NADPH to become PGAL  Reaction produces: ADP, NADP+ & phosphate to be used again

Step 3 -  PGAL converts back to RuBP  Allows Calvin cycle to continue

Alternates:  C 3 plants – use Calvin cycle exclusively  Form 3-carbon compounds  C 4 pathway – evolved in hot, dry climate  Form 4-carbon compounds  Partially close stomata  E.g. Corn, sugar cane, crabgrass  CAM – open stomata at night, close in day  Grow slow, lose less water  E.g. cactus, pineapple

C 4 Plants  Hot, moist environments  15% of plants (grasses, corn, sugarcane)  Photosynthesis occurs in 2 places:  Light reaction - mesophyll cells  Calvin cycle - bundle sheath cells

C 4 Plants Mesophyll Cell CO 2 C-C-C PEP C-C-C-C Malate-4C sugar ATP Bundle Sheath Cell C-C-C Pyruvic Acid C-C-C-C CO 2 C3C3 Malate Transported glucose Vascular Tissue

CAM Plants  Hot, dry environments  5% of plants (cactus and ice plants)  Stomates closed during day  Stomates open during the night  Light reaction - occurs during the day  Calvin Cycle - occurs when CO 2 is present

CAM Plants Night (Stomates Open)Day (Stomates Closed) Vacuole C-C-C-C Malate C-C-C-C Malate C-C-C-C CO 2 C3C3 C-C-C Pyruvic acid ATP C-C-C PEP glucose

Rate of photosynthesis is effected by - light intensity, CO 2 or temperature  High intensity or high CO 2 = high rate  Growth graph levels off (plateau)  High temp = initial high rate but peaks  Rate drops when stomata closes

Recap  Photosynthesis converts light energy into chemical energy thru series of biochemical pathways  Electrons excite in photosystem II – move along ETC to photosystem I  electrons are replaced when water is split  oxygen is byproduct  ATP synthesized across thylakoid  Calvin cycle – carbon is fixed & sugar produced  3 turns produce PGAL (PGAL keeps cycle going  Other pathways – C 3, C 4, CAM

References 