Biology ATAR Unit 2
Text: Chapter 9
Pages 226-229
Explain/describe/understand light
dependent and light independent
reactions in terms of:
1. The sites at which they occur
2. Requirements
3. Products
4. Factors affecting the rate of
photosynthesis
5. The importance of ATP and ADP cycles
for cell functioning
 Chloroplasts
 Stroma
 Grana
 Thylakoids
 Chlorophyll
 Calvin
cycle
 C3 photosynthesis
Limiting factors
 CO2
 Light intensity
 Temperature
 O2
 Nutrients
6CO2 + 6H2O + light energy
C6H12O6 + 6O2
Increasing energy
10–5 nm 10–3 nm
Gamma
X-rays
rays
1 nm
103 nm
UV
1m
106 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
380 400
600
500
Wavelength (nm)
700
650
nm
750
Chlorophyll light absorption
 Photosynthesis
occurs in chloroplasts
 In
most plants, photosynthesis occurs primarily in
the leaves where the chloroplasts are found
A 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
Step 1: THE LIGHT DEPENDENT STAGE
Takes 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
e
light reactions ATP
–
e–
e–
e–
e–
e–
NADPH
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Step 2: THE LIGHT INDEPENDENT STAGE
The Calvin cycle or C3 photosynthesis
Takes 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 3phosphoglyceric acid, a 3 carbon compound,
hence referred to as C3 photosynthesis
CO2
H2O
Chloroplast
Light
NADP+
ADP
P
LIGHT
REACTIONS
CALVIN
CYCLE
(in stroma)
(in thylakoids)
ATP
NADPH
O2
Sugar
Animations
 Calvin
cycle
 http://highered.mheducation.com/sites/9
834092339/student_view0/chapter39/calvi
n_cycle.html
Factors Affecting Photosynthesis
CO2 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 Photosynthesis
Temperature
 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 (plants
respire!)
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)