Photosynthesis

Year 12 Biology U3 AOS 2

Photosynthesis

the process of using energy from the sun to convert water + CO2 into glucose and oxygen

• Organisms that can photosynthesise are autotrophs (plants/algae)

• Chlorophyll (green pigment) is needed and located in chloroplasts

Chemical Equation for Photosynthesis

6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O

Worded Equation for Photosynthesis

Carbon dioxide + water → Glucose + oxygen + water

Chloroplasts

a plastid in green plant cells, containing chlorophyll and the location in which photosynthesis takes place

Stroma

all the space in chloroplasts

Stomata

tiny pores on the surface of leaves

• OPENS to allow CO2 in the atmosphere to diffuse into the lead

• CLOSES to prevent water loss in dry conditions

Thylakoids

individual discs where chlorophyll is specifically located

Granum

one stack of thylakoids

Grana

multiple stacks of thylakoids

Light Dependent Reaction (LDR)

the initial stage of photosynthesis, occurring at the thylakoid membrane

• Purpose → to generate high energy coenzymes NADPH and ATP to power the second stage

INPUTS of LDR

• H2O (water)

• NADP+

• ADP + P

OUTPUTS of LDR

• O2 (oxygen)

• NADPH

• ATP

Light Independent Reaction (LIR)

the second stage of photosynthesis, occurring at the stroma

• Purpose → to use chemical energy (NADPH + ATP) produced from LDR to convert CO2 into glucose

INPUTS of LIR

• CO2

• NADPH

• ATP

OUTPUTS of LIR

• Glucose

• H2O

• NADP+

• ADP + Pi

Carbon Fixation

when inorganic CO2 is converted into organic molecules (Carbohydrates) which are of more use to cells

Rubisco

an enzyme that catalyses the 1st step of the light INdependent reaction (carbon fixation)

• BINDS to CO2 and ATTACHES it to a sugar

• FIXES CO2 into organic 3-PGA molecules → initiating Calvin Cycle

Disadvantage of Rubisco

• Has an AFFINITY to bind to CO2 and O2, meaning Rubisco is not specific to CO2 so it can bind O2 to active site

• SLOW

Photorespiration

an unwanted + wasteful process in plants

• Rubisco is more likely to pick up O2 at higher temps and when O2 concentration is higher than CO2

• C4 + CAM plants in higher temperature regions have ADAPTATIONS to reduce photorespiration

Features of C3 Plants (Standard Photosynthesis)

• NO adaptations to limit photorespiration → when the weather is hot + dry as Rubisco will pick up more O2

• CO2 is fixed DIRECTLY from the air

• Photosynthesis decreases

• Photorespiration increases

Features of C4 (Hot + Dry) Plants — Adapted Photosynthesis

• Involves TWO cells (mesophyll cell and bundle-sheath cell)

• CO2 is fixed in the mesophyll cell + Calvin Cycle in bundle-sheath cell → separated over space

Advantage of C4 Plants

• Mesophyll cells constantly pump CO2 into bundle-sheath, keeping CO2 concentration high + lowering chance of Rubisco picking up O2

• BUT costs additional ATP

• Photosynthesis increases

• Photorespiration decreases

Features of CAM (Desert) Plants — Adapted Photosynthesis

• CO2 is initially fixed when stomata are OPEN at NIGHT

• Released from storage (NOT cell) into the Calvin Cycle during DAY

• Stomata remains CLOSED during DAY to reduce water loss + keeping O2 concentration low → Rubisco attaches to CO2 more

• Photosynthesis increases