Chapter 11: PhotosynthesisChapter 11: Photosynthesis

Leaving Certificate BiologyLeaving Certificate Biology

Higher LevelHigher Level

PhotosynthesisPhotosynthesis

• Photosynthesis is the process of using sunlight energy and chlorophyll to produce glucose from carbon dioxide and water

Carbon dioxide + water

Chlorophyll

Sunlightglucose + oxygen

6CO2 + 6H2OChlorophyll

SunlightC6H12O6 + 6O2

PhotosynthesisPhotosynthesis

• Light: from the sun

• Carbon dioxide comes from either– Atmosphere (animals breathe out carbon

dioxide and burning of fuels creates carbon dioxide)

– Respiration within the leaf

• Water: from the ground through the roots

PhotosynthesisPhotosynthesis• General process:

– Sunlight is absorbed by chlorophyll (found in chloroplasts)

– This absorbed energy is used to split water into three different components:

• Hydrogen ions (also called protons) go into the pool of protons of the chloroplast

• Oxygen passes out of plant cell and leaf into the atmosphere where animals breathe it in

• Electrons are given to chlorophyll

– Chlorophyll that has energy because of sunlight transfers that energy to its electrons

• These electrons get combined with protons and CO2 to make glucose

Structure of chloroplastsStructure of chloroplasts• Chlorophyll absorbs all colours in white light

except green – that is why plants appear green

• Photosynthesis occurs in two stages:1. Light stage:

• Light-dependent – requires light• Occurs in thylakoid membranes of the chloroplast

PhotosynthesisPhotosynthesis

Granum (stacks of

thylakoids)

Chlorophyll molecules

Thylakoid membrane

Stroma

• Photosynthesis occurs in two stages:2. Dark stage:

• Light-independent – does not require light to occur• Occurs in the stroma of the chloroplast

PhotosynthesisPhotosynthesis

Granum (stacks of

thylakoids)

Chlorophyll molecules

Thylakoid membrane

Stroma

Light StageLight Stage• Sunlight photon

strikes a cluster of chlorophyll molecules called a photosystem

• The chlorophyll molecules transfer the energy to a reaction center chlorophyll (RCC)

Light StageLight Stage• The energy is absorbed by an electron which travels

to a higher orbit as a result• The RCC is now unstable and ordinarily the electron

would give out its excess energy in the form of fluorescence

• However, nature has evolved a way of slowing this process down so that the electron’s energy can be released slowly to make carbohydrate

• The energised electron is picked up by an electron acceptor and can take one of two paths:– Cyclic pathway (pathway 1)– Non-cyclic pathway (pathway 2)

Cyclic PathwayCyclic Pathway

Cyclic PathwayCyclic Pathway

• The energised electron from RCC is transferred along the electron transport chain

• It gives off it excess energy slowly to power the production of ATP

• When the electron has given off all of its energy it returns to the RCC

Non-Cyclic PathwayNon-Cyclic Pathway

Occurs in the thylakoid membrane

Non-Cyclic PathwayNon-Cyclic Pathway• Two energised electrons are captured by

NADP+ to become NADP–

• NADP+ + 2e– → NADP–

• NADP– attracts protons because of its negative charge

• NADP– + H+ → NADPH• NADPH is the electron and proton carrier

and transfers them to the dark stage (Calvin Cycle)

• The electrons are those from the RCC

Non-Cyclic PathwayNon-Cyclic Pathway• Due to the NADPH carrying the electrons

lost from RCC to the dark stage, the RCC is therefore electron-deficient (positively-charged: RCC+)

• The RCC+ splits two water molecules producing 4 electrons (4e–), 4 protons (4H+ ions) and oxygen (O2)

• RCC+ + 2H2O → RCC+ + 4e– + 4H+ + O2

Non-Cyclic PathwayNon-Cyclic Pathway

• The electron from water has too much energy to go back to the RCC and so is passed along the electron transport chain producing ATP in the process

• When the electron has lost all its energy in the electron transport chain it then goes back to the RCC

• The protons go into the proton pool within the chloroplast

• The oxygen does one of two things:– Leaves the leaf and diffuses out into the atmosphere– Is used by the cells of the leaf or other regions of the

plant for respiration

Fates of the Products of the Splitting of WaterFates of the Products of the Splitting of Water

Light Stage ProductsLight Stage Products

• The products of the light stage which go onto the dark stage are:– ATP– NADPH

• The by-product of the light stage which are either released into atmosphere or used in respiration is:– O2

Dark Stage (Calvin Cycle)Dark Stage (Calvin Cycle)

NADPH ATP

CO2

Carbohydrate(Glucose)

ADP & Pi

NADP+

Back to light stage

Back to light stage

STROMA

Dark Stage (Calvin Cycle)Dark Stage (Calvin Cycle)

• Carbon dioxide enters leaf through the stomata and enters leaf cells (ground tissue) and then enters the chloroplasts (specifically the stroma)

• NADPH and ATP leave the thylakoid membrane and enter the stroma

• Using energy supplied by ATP, carbon dioxide is reduced by the H+ ions and electrons from the NADPH

• The product of the dark stage is carbohydrate (glucose) which is then changed into starch and stored within the cell (vacuole) or transported through the phloem to another area of the plant where it may be needed (e.g. at the meristems) or stored (e.g. root tubers)

SUMMARYSUMMARY