Biology 3.2 - photosynthesis

What is photosynthesis?

the process in green plants by which water and carbon dioxide combine using light energy to form glucose and oxygen

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

What type of reactions does photosynthesis consist of?

• light dependant reactions

• light independent reactions

what are the two types of light dependant reactions?

• non-cyclic photophosphorylation

• cyclic photophosphorylation

Where do the light dependent reactions occur?

in the thylakoid membrane

• happen in presence of light

What happens in the light dependent reactions?

• light energy is harvested and used to produce atp and a reduced hydrogen carrier (NADP → reduced NADP)

• water is split forming an oxygen

where do the light independent reactions occur?

in the stroma

• do not require light

what happens in the light independent reactions (calvin cycle)

• use the atp and reduced NADP from light dependant reactions

• glucose is produced from carbon dioxide

A chloroplast is a transducer, what is a transducer?

a transducer changes energy from one form to other

• chloroplasts change energy in photons of light into chemical energy

diagram of chloroplast

What was englemann’s experiment?

he was trying to discover which wavelengths of light are absorbed most by plants and used in photosynthesis

Describe and explain the results of englemanns experiment?

• bacteria clustered in red and blue region of the spectrum because they are the wavelengths of light that are absorbed the most

• There will be a higher rate of light dependant stage in those wavelengths and more photolysis of water to release the oxygen

• So the bacteria clustered there because they need the oxygen for aerobic respiration

• very few bacteria in green region because very little green light is absorbed/it is reflected

Describe a suitable control for englemanns experiment and explain why

use white light only

• bacteria would distribute themselves evenly along the tube

• shows that it is the different wavelengths of light causing the distribution of bacteria seen in the investigation

What is a pigment?

a coloured substance that absorbs specific wavelengths of light

Where does the colour of the substance in a pigment come from?

comes from the light/wavelengths reflected, not absorbed

what are the two main classes of photosynthetic pigments that act as transducers in flowering plants?

chlorophylls

• chlorophyll a = blue/green colour and is primary pigment

• chlorophyll b = yellow/green (absorb red and blue/violet light and reflect green)

• chlorophyll c + b are accessory pigments

carotenoids

what are carotenoids?

• carotenes + xanthophylls

• accessory pigments

• help absorb a range of wavelengths of light not absorbed by chlorophyll a and pass the energy to chlorophyll a

• are orange or yellow in colour

• absorb blue/violet light

how would a lack of magnesium result in poor growth in a plant?

• leads to a lack of chlorophyll so it cannot absorb as much light energy

• so less light dependent stage would occur and it wouldn’t produce as much ATP and NADPH for the light independent stage

• the rate of photosynthesis will decrease so there will be less glucose produced

• there is less glucose for respiration meaning less ATP for protein synthesis or cell division

What graph is this?

absorption spectrum of photosynthetic pigments

what does the absorption spectrum show?

how much light energy a particular pigment absorbs at each wavelength

why is there less absorption in green light on the absorption spectrum?

it is reflected

what is the advantage of having multiple photosynthetic pigments?

• can absorb a wider range of wavelengths of light

• more light dependent stage

• more ATP + reduced NADP formed for light independent stage

• rate of photosynthesis higher

what graph is this?

action spectrum of photosynthetic pigments

what does the action spectrum show?

the rate of photosynthesis at different wavelengths of light

(the amount of carbohydrate synthesised by plants exposed to different wavelengths of light)

what is the correlation between the absorption spectrum and the action spectrum?

• close correlation

• suggests that the photosynthetic pigments are responsible for absorbing the light energy in photosynthesis

where are the photosystems located?

in the plane of the thylakoid membrane of the chloroplasts

what is each photosystem made up of?

• antenna complex

• reaction centre

what does an antenna complex contain?

• clusters of up to 400 photosynthetic pigment molecules

• chlorophylls and carotenoids are anchored into the phospholipids of the thylakoid membrane and held together by protein molecules

• allows a range of wavelengths of light to be absorbed

what does a reaction centre contain?

• two molecules of the primary pigment, chlorophyll a

• when chlorophyll a molecules absorb light, their excitation allows each one to emit an electron

diagram of photosystem

what are the two types of reaction centre?

• photosystem I (PSI)

• photosystem II (PSII)

describe photosystem I

• arranged around a molecule of chlorophyll a

• absorption peak of 700nm

• reaction centre is called P700

• richer in chlorophyll than b

describe photosystem II

• arranged around a molecule of chlorophyll a

• absorption peak of 680 nm

• reaction centre is called P680

• richer in chlorophyll b than a

How can you separate the different pigments in chlorophyll?

using paper chromatography

how can you identify the separate pigments after using paper chromatography?

• qualitatively - by colour

• quantitatively - by calculating the retention factor (R_f) using a particular solvent

How do you calculate the R_f value?

distance moved by pigment from original position/distance moved by solvent front

what do the light dependent stages of photosynthesis produce?

• atp from adp + Pi by photophosphorylation - provides the chemical from light energy to synthesise energy - rich glucose

• reduced NADP/NADPH by addition of 2H’s to NADP - provides the reducing power to synthesise glucose from carbon dioxide

• oxygen - produced from the splitting of water molecules by photolysis - oxygen diffuses out of the chloroplast and photosynthetic cells and out of the leaf through the stomata