PHOTOSYNTHESIS

All organisms need energy to drive life’s processes Ability to do work needed for all biological processes

Fundamental biological processes for making and using energy Photosynthesis: process by which plants convert radiant energy to chemical energy Respiration: process by which glucose molecules are broken down and stored energy is released Photosynthesis - plants make glucose Respiration – animals break down glucose

TYPES OF ORGANISMS BY ENERGY PRODUCTION Autotrophs organisms that produce organic molecules from inorganic substances (photosynthesis) - make own food - Photoautotrophs- use light energy to make food (plants) - Chemiautotrophs- oxidize inorganic chemicals to drive food making reactions (bacteria, fungi) Heterotrophs - organisms that obtain energy from other organisms (heterotrophs or autotrophs) - do not make own food

ENERGY PRODUCTION ATP: adenosine triphosphate molecule that stores useable energy composed of 3 parts adenine (N compound) ribose (5 C sugar) 3 phosphate groups

ATP/ADP CYCLE Structure of ATP and Conversion of ATP to ADP energy is stored in high energy bonds between phosphate groups bond must be broken to use energy ATPase: enzyme that breaks bonds between phosphate groups

A - (P~ P ~ P) ------------ A - ( P ~ P) + P + energy ATP  ADP A - (P~ P ~ P) ------------ A - ( P ~ P) + P + energy High energy molecule ATPase Adenosine diphospate mid energy molecule ADP  AMP A - (P ~ P) ----------- A – P + P + energy Mid energy molecule ATPase Adenosine monophosphate low energy molecule FORMATION OF ATP (reverse process) A – (P ~ P) + P + energy ------------- A - (P ~ P ~ P) ATPsynthase Phosphorylation: addition of phosphate group to ADP or AMP (pyrophosphate bond)

DISCOVERY OF PHOTOSYNTHESIS Jean Van Helmont (Dutch) grew tree from small seedling after 5 years was 75 kg (mass of soil unchanged) CONCL: change came from added CO2 Priestly (100 years later) English put candle in jar – went out put plant in jar, candle stayed lit CONCL: PLANT GAVE OFF O2 needed for burning Ingerhaousz (Dutch) did same experiment but showed O2 produced only when plant exposed to light CONCL: light necessary for plant to produce O2

6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2 PHOTOSYNTHESIS 6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2 process whereby autotrophs (plants) take in light energy and convert it to chemical energy (sugar) biochemical pathway- series of linked reactions where product of one reaction is consumed by the next reaction

Location of photosynthesis Chloroplast Thylakoid discs (photosysytem:200-300 thylakoids) Harvest sunlight Contains chlorophyll and accessory pigments Photosystem I and II are linked structurally and functionally Grana (stacks of thylakoid discs) location of light reactions Stroma (protein rich solution, outside grana) location of dark reactions

Light Energy and Pigments Comes from radiation (energy that travels in waves) from the sun wavelength- crest to crest sunlight- mixture of all visible wavelengths white light- all wavelengths reflected equally so looks white spectrum- all colors of white light

light is made of photons (particles which carry fixed amount of energy) when light strikes matter, some of its atoms absorb the photons energy is transferred to the atoms electrons and excites them to jump to next level - changed to light or heat

Pigment: substance that absorbs light in photosynthesis: absorbed light energy is used to make chemical bond energy wavelengths not absorbed are reflected (color we see) Absorption spectrum: colors (wavelengths) absorbed by a particular pigment

Photosynthetic pigments (located in thylakoids of chloroplasts) chlorophyll a - primary photosynthetic pigment - directly involved in converting light  chemical energy - hides other pigments chlorophyll b - accessory pigment - absorbs light and transfers energy to chlorophyll a carotenoids (orange, brown) xanthophylls (yellow) - accessory pigments - converts energy to chloro. a - seen in autumn when chloro. breaks down non photosynthetic parts of plant (flowers/fruits) - absorb different pigments so we see other colors

Overview of Stages of Photosynthesis 2 Stage Process light reactions (needs light) - occurs in thylakoid membranes 4 basic steps - sun’s energy is trapped by chlorophyll - electron transport - water is split and oxygen is released (O2 production) - ATP and NADPH are formed and released into stroma - purpose: to make ATP and NADPH (energy carrier molecule)

Calvin Cycle: - occurs after light reactions - can occur in light or dark 3 basic steps - carbon fixation to glucose - reduction of NADP to NADPH - regeneration of RuBP to start cycle over again

Photosystems Arrangements of chlorophyll and other pigments packed into thylakoids Absorb light energy Photosystem I: chlorophyll a absorbs at 700 nm (p680) Photosystem II: chlorophyll a absorbs at 680 nm (p700)

STEPS OF LIGHT REACTION 1. photosystem II absorbs light and excites electrons of chlorophyll a - molecules and electrons are forced to higher energy level ***purpose of photosystem II is to generate ATP and supply electrons to photosystem I *** 2. excited electrons leave chlorophyll a molecule (oxidation reaction)

primary electron acceptor sends electrons into ETC (series of molecules that transfers elec. thru a chain of molecules) - reduction reaction - chain uses energy of electrons to make ATP - water is split (photolysis) and O2 is released into atmosphere - electrons from water replace those lost in Photosystem II - pumps H ions (from splitting of water) to interior of grana - inside grana , there is a high concentration of H ions - chemiosmosis occurs (making of ATP) - H ions back move across grana membranes - ATPsynthase – enzyme: catalyzes reaction to make ATP from ADP

4. at end of electron transport chain electrons are passed to photosystem I - photosystem also absorbs light to excite electrons in chloro. A - electrons go thru separate electron transport chain in photosystem I to a different primary electron acceptor - purpose of photosystem I is to generate NADPH 5. NADP+ (nicotinamide adenine dinucleotide phosphate) - accepts electrons and H ions (reduces it to NADPH) - NADPH and ATP move into stroma

End products of light reactions 1. ATP and NADPH: needed to power dark reactions 2. O2: by product released into atmosphere

CALVIN CYCLE (DARK REACTIONS ) - light independent: can occur in light or darkness, always after light rxns - occurs in stroma - purpose: Carbon fixation to glucose molecule (from CO2 in atmosphere) - named after Melvin Calvin

Steps of Calvin Cycle (Dark Reactions) 1. Carbon fixation to glucose molecule - CO2 enters plant from atmosphere and binds with RuBP ribulose biphosphate (5 C sugar) - catalyzed by RuBP ribulose carboxylase oxygenase(rubisco) - forms unstable 6 C sugar - this splits into 2 PGA (phosphoglyceric acid)

Reduction of PGA to PGAL 2 steps - PGA binds with phosphate group from ATP - molecule reacts with H from NADPH and breaks phosphate bond - forms 2 PGAL (phosphoglyceraldehyde/G3P) (3 C molecule)

Regeneration of RuBP - Some PGAL used to make glucose - 2 PGAL  1 mol. C6H12O6 - most PGAL converted to RuBP to continue cycle (cyclical – continues over and over again) animation

End product of Calvin Cycle Glucose *6 turns of cycle needed to make 1 molecule of glucose* http://vcell.ndsu.nodak.edu/animations/photosynthesis/movie.htm

- Calvin cycle is most common pathway for carbon fixation - C3 plants: plants that fix C thru calvin cycle (because of 3 C PGA that is initially formed) - other plants fix C through alternative pathways and then release it into Calvin cycle - alternative pathways found in plants in dry hot climates - these plants use STOMATA (pores on undersurface of leaves) - major passageways thru which O2 and CO2 goes in and out - major passageways of water loss

Alternative Pathways of Carbon Fixation 1. C4 Pathway - during hottest part of day, stomata are partially closed - plants fix CO2 into four carbon compounds when CO2 is low (not at much C available due to consumption in Calvin cycle) - lose less water than C3 plants - corn, sugarcane, crabgrass

Alternative Pathways of Carbon Fixation, cont. 2. CAM Pathway - open stomata at night, closed during day - night: plants take in CO2 and fix into many compounds - day: CO2 is released from cpds and enters Calvin cycle - lose less water than C3 and C4 plants - cactus, pineapples

Alternative Pathways of Carbon Fixation, cont. 3. Special Cases Heterotrophic plants - parasitic - venus fly trap - perform photosynthesis but insects provide a source of nitrogen for making plant proteins

Factors Affecting Rate of Photosynthesis 1. light intensity - high intensity = high rate - saturation point: levels off after certain intensity because pigments can only absorb so much light 2. CO2 levels - same mechanism as light 3. temperature - higher temp = higher rate - if temp goes too high, rate slows down because enzymes may denature