SBI 4UI - Photosynthesis

photosynthesis

- converts light energy in sunlight to chemical froms of energy that can be used in biological processes
- all the food we wat is a product of photosynthesis

photosynthesis equation

6CO2 + 6H2O + light energy \------\> C6H12O6 + 6O2

chlorophyll

- absorbs light energy to begin photosynthesis
- several types
- named after what colour they reflect

chlorophyll is composed of...

- porphyrin ring
- long hydrocarbon tail

chlorophyll a

- primary pigment
- absorbs red and blue light,
- reflects blue and green
- contains a -CH3 at R

chlorophyll b

- absorbs blue, green and orange
- reflects yellow-green
- contains -COH at R

porphyrin ring

- magnesium ion surrounded by hydrocarbon ring with alternating single & double bonds
- contains delocalized electrons

delocalized electrons

- absorb light energy and begin photosynthesis

phytol chain

- hydrocarbon tail that anchors chlorophyll molecule to a cell membrane

cyanobacteria

- largest group of photosynthetic prokaryotes
- first organisms to use photosynthesis
- likely allowed life to evolve
- contain chlorophyll a to carry out photosynthesis
- contains phycobilin

phycobilin

- chlorophyll b photosynthetic pigments

photosynthesis in algae, protists, plants

- chloroplasts give leaves and stems the green colour
- leaves are the main photosynthetic organs

vascular bundle

transports:
- water
- minerals
- carbohydrates

upper epidermis

allows light to pass to mesophyll

mesophyll

- space where chloroplasts are abundant
- location where most of photosynthesis occurs

cuticle

- waxy and water resistant coating that covers the plant to stop cells from loosing too much water

guard cells

create openings called stromata

transpiration

- evaporation of water from leaves
- controlled by guard cells
- produces a cooling effect - like sweating
- creates a "transpiration pull"

transpiration pull

- when water evaporates from the leaves of a plant, water is pulled up to replace what was lost
- helps move: water, minerals, and other substances upward

weather that promote transpiration:

- sunny
- dry
- warm
- windy

chloroplast

- photosynthesis factories of plants and algae

stroma

protein-rich semi-liquid in the interior of chloroplasts

thylakoid

- membrane bound flattened sacs that stack to form granum
- chlorophyll embedded in the membranes

granum

stacks of thylakoid's

lamellae

\-----

chromoplasts

- make and store pigments other than chlorophyll
- plastids
- contain red & orange pigments

chromoplast pigments can be seen in...

- leaves in the fall
- when fruits ripen
- flowers

amyloplasts

- unpigmented plastids
- store starch in stems, tubers, and seeds

tubers

- enlarged structure used as storage organs for nutrients
- ex. potatoes

stromata

- controls transpiration
- guard cells control size of stromata

opened guard cell/stromata

- K+ ions diffuse into guard cells, water also moves in and guard cells swell opening the stromata
- usually occurs in the daytime

closed guard cell/stromata

- K+ ions diffuse out of guard cells, water also moves out and guard cells become flaccid and stromata closes
- usually happens at night

size of guard cell changes...

when water moves by osmosis

direction of osmosis follows...

diffusion of potassium ions across the guard cell membrane

stage 1 of photosynthesis

- light is captured
- chlorophyll is required
- occurs in thylakoid membranes
- energized by light

stage 2 of photosynthesis

- uses captured light energy to make ATP and reduce NADP to NADPH
- requires chlorophyll
- occurs in thylakoid membranes
- energized by light

stage 3 of photosynthesis

- uses free energy of ATP and the recuing power of NADPH to make glucose and oxygen
- occurs in the stroma
- endergonic
- isn't activated by light

NADP

- nicotinamide adenine dinucleotide phosphate
- energy shuffling enzyme
- reduced by hydrogen atoms to NADPH

wave length model of light

- light is a form of radiation
- some are visible and some are invisible

highest frequency & smallest wavelength

violet

lowest frequency & biggest wavelength

red

when all frequencies are present

white light is seen

below red...

infrared

above violet...

ultraviolet

as we go above ultraviolet light...

there is more energy and it can becomes more dangerous

reflected light

- light that is thrown back or bounced off an object
- colour that we see

transmitted light

- light passing through the object
- ex. glass

absorbed light

- every colour except the one that we see

pigments

molecules that can absorb specific wavelengths of light

plants appear green by...

- reflecting green
- absorbing all colours except green

absorbtion spectrum

- a plot of the amount of light energy of various wavelengths that a substance absorbs
- produced using a spectrometer

action spectrum

a plot of the effectiveness of light energy of different wavelengths in driving a chemical process

accessory pigments

- go through the antenna complex
- protect the chlorophyll but absorbing wavelengths that may damage the chlorophyll

accessory pigments include...

- xanthophyll
- carotenoids
- anthocyanins

photosystems

- clusters of pigment molecules that absorb particular wavelengths if light & transfer their energy to ADP, Pi, and NADP eventually forming ATP and NADPH

light reactions in photosynthesis

- light reactions
- carbon fixation

two points about light & pigment molecules that are important in photosynthesis

1. the absorption of a photon molecule excites an electron moving it from a low to a higher energy state
2. difference between energy level of low and high state must be equal to the energy of the photon light that was absorbed

after light absorption, there are three possible outcomes for the excited electron within a pigment molecule

1. electron returns to its low state by emitting energy as either fluorescence or thermal energy
2. the electron returns to its low state of energy & the energy is transferred to a different electron in a neighboring pigment molecule - requires two molecules to be very close & aligned
3. the high-energy electron is accepted by the primary electron-acceptor - one of the most important processes in photosynthesis

primary electron acceptor

a molecule capable of accepting electrons and becoming reduced during photosynthesis

antenna complex

- cluster of light-absorbing pigments embedded in the thylakoid membrane able to capture and transfer energy to special chlorophyll a molecules in the reaction center

reaction centre

a complex of proteins and pigments that contains the primary electron acceptor

pigments molecules are...

- bound to a number of different protiens - they don't float freely
- organized into photosystems

light reactions

- non-cyclic
- convert solar energy to chemical energy
- divided into three parts

three parts of the light reactions

- photoexcitation
- electron transport
- chemiosmosis

photoexcitation

- absorption of a photon by an electron of chlorophyll
- electrons move from a ground state to an excited state which is then trapped by a electron acceptor (chlorophyll is oxidized acceptor is reduced)

photosystems

- are embedded in the membrane
- where an electron absorbs energy and becomes excited

photosystems contain

- chlorophyll
- accessory pigments
- antenna complex
- reaction center
- all the pigments that allow light to be turned into chemical energy

reaction center

where the excited electron comes from when a photon of light hits it

photosystem I contains

P700

photosystem II contains

P680

proteins around P700 and P680 is what...

causes them to absorb different wavelengths of light

cytochrome complex

generates a concentration gradients and provides a connection between the two photosystems

water-splitting complex/photolysis

- water is split apart by a Z-protein so electrons can replace lost ones in P680 and protons can work toward making a concentration gradient

step 1 of the light reactions

- light energy is absorbed by photosystem II, causing an electron to become excited and be transferred to the primary acceptor
- water-splitting complex occurs replacing the electrons lost and protons make a concetration gradient

step 2 of the light reactions

- the electron is then given to plastoquinone (PQ)
- PQ then donates the electron to the cytochrome complex, releasing protons into the lumen making a concetration gradient

step 3 of the light reactions

- electrons go from PQ to plastocyanin (PC) which shuttles them to photosystem I
- this is the electron transport chain in photosynthesis

step 4 of the light reactions

- chemiosmosis occurs which drives the synthesis of ATP being made

step 5 of the light reactions

- the electron is then transferred to P700
- the energy of that electron is then transferred again to an electron found in P700 and then electron before looses its energy & becomes apart of P700
- the electron that is high energy is then transferred to the primary electron acceptor

step 6 of the light reactions

- the primary electron acceptor transfers the electron to ferredoxin
- ferredoxin then moves the electron to NADP reductase
- another electron transport chain drives the processes of reducing NADP to NADPH + H

chemiosmosis

- movement of H+ ions through ATP synthase complex to make ATP
- electrochemical gradient drives the photophosphorylation (free energy released from the protons going with the gradient drives the synthesis of ATP by trapping the energy in the bonds)

carbon fixation

- taking CO2 and putting it into bigger carbon molecules

Calvin cycle

- occurs in the stroma
- ATP & NADPH from light reaction are used to fix CO2 into carbohydrate molecules
- divided into three stages

three stages of the Calvin cycle

1. carbon fixation
2. reduction reactions
3. regeneration

carbon fixation (Calvin cycle)

- carbon dioxide reacts with RuBP (5 C) to produce 3-phosphogyerate (3 C) (PGA)

reduction (Calvin cycle)

- 3-phosphoglyerate gains another phosphate group by the hydrolysis of ATP producing 1,3-biophosphoglyerate (PGAP)
- the molecule is then reduced by NADPH+H, producing glyceraldhyde-3-phosphate (PGAL) and NADP

regeneration (Calvin cycle)

- some PGAL molecules are combined to make/regenerate RuBP to start the cycle all over again
- the other PGAL is released which can combine with another PGAL to produce glucose

rubisco

- the most important enzyme, because without it carbon dioxide cannot be fix and without fixing carbon dioxide, ecosystems can collapse

per Calvin cycle

- carbon fixation: three Co2 fixed, 6 PGA are formed
- reduction: 6 PGAP formed, uses 6 ATP and 6 NADPH, forms 6 PGAL
- regeneration: 5 PGAL are used to reform RuBP, 3 ATP are used, 1 PGAL moves on to make 1/2 a glucose

to make one glucose...

- carbon fixation: 6 CO2 used
- reduction: uses 12 ATP and 12 NADPH from photosystem I and II
- regeneration: uses 6 ATP, and 2 PGAL to make 1 glucose

proton gradient is made across the thylakoid membrane by three mechanisms:

1. protons move into thylakoid space by the reduction & oxidation of plastoquinone as it moves from photosystem II and the cytochrome complex
2. protons are also moved into the thylakoid as water molecules are split
3. the removal of one proton from the stroma for each NADPH molecule forms, makes a gradient as electrons are being removed from one side

the two photons of light (absorbed by the 2 photosystems) are required...

to span the energy difference between H2O and NADPH

similar processes of plants and animals

- use an ETC
- use ATP synthase to make ATP
- use and regenerate carrier molecules
- complementary carbon fixing (Calvin) and carbon releasing (Krebs)
- require a continuous supply of ATP

key organelle have similar features in plants & animals

- chloroplasts & mitochondria have an inner folded membrane that allow concentration gradients
- have their own set of DNA

C3 plants

- found in cool, moist environments
- called C3 because the first organic molecule of the Calvin cycle is a 3 carbon molecule
- ex. peas, wheat, rice

photorespiration

- occurs when rubisco binds with oxygen instead of carbon dioxide
- RuBP is oxidized instead of undergoing carboxylation to PGA
- RuBP + O2 \=\> glycolate (2C) + PGA (3C)
- at warmer temperatures, RuBP carboxylase bonds to O2 well so very little carbon fixation occurs

C4 plants

- hot dry environments
- unique anatomy - isolates bundle-sheath cells around the vascular bundle
- so tightly packed in the bundle-sheath cells that oxygen cannot compete with carbon dioxide (prevents photorespiration)

C4 fixation

- CO2 is fixed onto PEP forming oxaloacetate which is then converted into malate
- malate undergoes decarboxylation into pyruvate which converts to PEP

CAM plants

- hot dry arid environments
- Crassulacean Acid Metabolism
- separates reactions into times

CAM plants during the night

- stomata open, allowing CO2 to diffuse into the leaves
- CO2 is fixed into oxaloacetate then converted to malate which is stored in vacuoles until the day

CAM plants during the day

- stomata are closed to to preserve water
- malate is transported out of the vacuole and the CO2 enters the Calvin cycle