PHOTOSYNTHESIS

Biology ATAR Unit 2Text: Chapter 9Pages 226-229

Learning outcomesExplain/describe/understand light dependent

and light independent reactions in terms of:1. The sites at which they occur2. Requirements3. Products4. Factors affecting the rate of

photosynthesis 5. The importance of ATP and ADP cycles

for cell functioning

Keywords Chloroplasts

Stroma Grana Thylakoids Chlorophyll

Calvin cycle C3 photosynthesis

Limiting factors CO2 Light intensity Temperature O2 Nutrients

6CO2 + 6H2O + light energy C6H12O6 + 6O2

Wavelength (nm)

10–5 nm

Increasing energy

Visible light

650nm

10–3 nm 1 nm 103 nm 106 nm 1 m 103 m

380400 500 600 700 750

Radiowaves

Micro-wavesInfraredX-rays UVGamma

rays

Chlorophyll light absorption

Chloroplasts Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily

in the leaves where the chloroplasts are foundA chloroplast contains: Membrane – a double membrane Stroma – a fluid Chlorophyll – the green pigment that captures

light for photosynthesis Thylakoids – contain chlorophyll Grana – stacks of thylakoids

Structure of a chloroplast

Structure of a chloroplast

Steps of PhotosynthesisStep 1: THE LIGHT DEPENDENT STAGETakes place in the grana Light hits the grana of the chloroplasts and

causes the chlorophyll to vibrate. Then:1. Water splits into O2 and H+

2. ADP is converted into ATP (light energy transformed into chemical

energy)

The O2 is released into the atmosphere

A mechanical analogy of the light reactions

NADPH

Photosystem II

e–

Millmakes

ATP

Phot

onPhotosystem I

ATP

e–e–

e–

e–

e–

e–

Phot

on

Steps of PhotosynthesisStep 2: THE LIGHT INDEPENDENT

STAGEThe Calvin cycle or C3 photosynthesisTakes place in the stroma CO2 that has diffused into the chloroplasts

from the atmosphere is joined to the H+ (from the light dependent stage) to form glucose (C6H12O6)

This is a complex pathway The first step in the pathway forms 3-

phosphoglyceric acid, a 3 carbon compound, hence referred to as C3 photosynthesis

H2O

ADPP

LIGHTREACTIONS

(in thylakoids)

Light

Chloroplast

NADPH

ATP

O2

CALVINCYCLE

(in stroma)

Sugar

CO2

NADP+

Animations Calvin cycle http://highered.mheducation.com/sites/9

834092339/student_view0/chapter39/calvin_cycle.html

Factors Affecting PhotosynthesisCO2 concentration There are local variations to CO2 levels in the air

that occur in different habitats and at different times of the day. An increase in CO2 will increase the rate of photosynthesis, until another factor limits it.

Light intensity The rate of photosynthesis increases as light

intensity increases until another factor limits it. Only about 1% of the light that hits the plant is captured and converted to chemical energy.

Factors Affecting PhotosynthesisTemperature Photosynthesis relies on enzymes, and enzymes are

temperature sensitive. The rate of photosynthesis increases with temperature, the optimum being between 20-40ᵒC, depending on the plant species

High concentration of O2 O2 inhibits photosynthesis by competing with CO2 for the active

site of enzymes. Photosynthesis increases if the amount of O2 is decreased.

Nutrients Chlorophyll contains nitrogen and magnesium. If the soil is

deficient in these nutrients, the plant cannot make enough chlorophyll and the leaves appear yellow.

Photosynthesis: a review The sugar produced by plants during

photosynthesis provides the starting materials to make structural components such as cellulose

50% of this sugar goes toward cellular respiration (plantsrespire!)

Photosynthesis: a review Most plants make considerably more

food each day than they need They stockpile this sugar as starch,

storing it in roots and tubers. Energy is stored in fruit as fructose.

Plants not only produce fuel for themselves, but ultimately provide food for virtually all other organisms (heterotrophs)