Photosynthesis
Chapter 6
Miss Colabelli
Biology CPA
Obtaining Energy
 Autotroph: energy (glucose) from sunlight.
 6CO2 + 6H2O + sunlight  C6H12O6 + 6O2
 Heterotroph: energy from food
Why is Photosynthesis important?
 Makes organic molecules
out of inorganic materials
 It begins all food chains &
webs
 All life is supported by this
process
 It also makes oxygen gas!
Photosynthesis-starts ecological food webs!
Photo-synthesis
= "putting together with light."
 Plants use sunlight to
turn water and carbon
dioxide into glucose
 Plants use glucose as
food
 Autotrophs make
glucose and
heterotrophs are
consumers of it
Overview of Photosynthesis
 Light Reactions
 Sunlight is absorbed and converted to
chemical energy
 Chemical energy is temporarily stored
in plants as ATP and NADPH
 Calvin Cycle
 CO2, ATP, and NADPH make organic
compounds
6CO2 + 6H2O
C6H12O6 + 6O2
Photosynthesis
Carbon dioxide + water
sunlight
glucose + oxygen
absorbed by chlorophyll
6CO2 + 6H2O + energy  C6H12O6 + 6O2
Plants
in
Action
Capturing Light Energy
 During light reactions, the plant’s chloroplasts
absorb the sunlight
Elodea
Same plant that we used for transport. The green circular
organelles are the chloroplasts!
Elodea using
photosynthesis
What gas do
you think is in
the bubbles
that the plant
made?
Why are plants green?
 The light we see from the sun is white light, but its way more
than white!
 Colors that we see are reflected from the object
Pigment in Plants
 Why do we see green?
 Green color from white light reflected NOT absorbed by
the chlorophyll in the chloroplast
Visible light is only
a small part of the
electromagnetic
spectrum (all forms
of light).
Converting Light Energy to
Chemical Energy
 Absorbing light energy to make chemical energy
 Pigments: Absorb different colors of white light
(ROY G BIV)
 Main pigment: Chlorophyll a
 Accessory pigments: Chlorophyll b and Carotenoids
 These pigments absorb all wavelengths (light)
EXCEPT green!
http://www.youtube.com/watch?v=ljPU1nDVq-0
http://www.youtube.com/watch?v=-yrZpTHBEss
Photosystem II
Light  Energy Reactions
 Photosystems
 Each photosystem is light dependent
 Photosystem II is occurs first
 Occurs on the membrane of the thylakoid sacs in the
chloroplasts
1. Water is split and electrons are passed to a protein
2. Once light is absorbed the electrons are excited and
move to the following protein in the membrane
3. The primary electron acceptor donates the electron
to the electron transport chain
Light  Energy Reactions
 Photosystem I occurs next
4. The electrons from the electron transport chain
are transferred to the primary acceptor protein
once light is absorbed
5. Primary electron acceptor donates to another
electron transport chain (towards the stroma)
6. The electrons are donated once last time to a
molecule NADP+ (low energy) and this become
NADPH (higher energy)
http://www.dnatube.com/video/2899/Photosynthesis-101-presented-by-Dr-Undergrad
How do you get an e- from
water?
 There is an enzyme that can break water molecules
into 3 products
 One of the final products is electrons
2H2O
4H+ + 4e- + O2
 For every 2 molecules of water, you get 4 electrons and
oxygen gas
 This is the oxygen we breathe!
Making ATP!
Facilitated
Diffusion!!
 The thylakoid has its membrane
and proteins throughout
 H+ ions are higher concentration in the cell
 ATP synthase: protein in membrane that transfers
H+ ions via facilitated diffusion
 This process give the needed energy to combine
ADP + extra P in the cell to make ATP
http://www.youtube.com
/watch?v=AUPugYBkN
JQ&list=PLB3AF0B8D
290D071D
http://www.stolaf.edu/peop
le/giannini/flashanimat/met
abolism/photosynthesis.swf
Calvin
 Named after American
biochemist Melvin Calvin
 Most commonly used pathway by most
plants
 Calvin cycle is used by plants that are called
C3 because of the 3-Carbon molecules that
are made
Calvin Cycle
 Light-independent reaction
(Dark Reaction)
 Does not require light
 Calvin Cycle




Occurs in stroma of chloroplast
Requires CO2
Uses ATP and NADPH as fuel to run
Makes glucose sugar from CO2 and
Hydrogen
Calvin Cycle
 Uses products from the light reactions + CO2
to make sugars and other compounds
 What are the products of the light reactions?
 Where does the CO2 come from?
Step 1
 CO2 is diffused into the stroma of the
chloroplast
 A 5-Carbon molecule named RuBP combines
to the CO2
 The Enzyme that catalyzes this reaction is rubisco
 This becomes a 6-Carbon molecule that is very
unstable
 Split to become two 3-Carbon molecules called 3phosphoglycerate (3-PGA)
6-Carbon Sugar  3-PGA + 3-PGA
Step 2
 3-PGA is still unstable
 3-PGA  glyceraldehyde 3-phosphate (G3P)
 For this to occur, each 3-PGA molecule gets
a phosphate from ATP and a proton from
NADPH
 Once the molecule receives the P and proton
it converts into G3P
3-PGA + P + H  G3P
Where did the P and H come
from?
*ATP ADP + P
*NADPH  NADP+ + H
Step 3
 One G3P molecule leaves the Calvin cycle
 This will be used to make a carbohydrate later
Step 4
 The other G3P molecule gets converted BACK to
RuBP due to an addition of another phosphate from
ATP
 G3P + P  RuBP
 This RuBP goes back to the Calvin cycle to be fixed
again
Stoma
This opening how plants exchange gases!
 Stoma Open
 CO2 can
increase
 O2 will
decrease and
leave cells
 Stoma Closed
 CO2 decrease
 O2 increases
C4 Pathway
 Plants that use this are called C4 plants and
have stomata closed during hot part of day
 Enzyme fixes CO2 to a 4-carbon compound
when CO2 is low and O2 is high
 Corn, sugar cane, & crab grass
 Usually tropical climates
CAM Pathway
 Water conserving pathway
 Hot dry climates
 Stoma closed during day & open at night
 Opposite of ordinary plants
 Pineapples & cacti
CAM Pathway
 During the day
 Stoma are closed
 CO2 is released from compounds and
enters Calvin cycle
 During the night
 Stoma are open
 Take in CO2 and fix into carbon
compounds
PHOTOSYNTHESIS
 What affects photosynthesis?
 Light intensity: as light increases, rate of photosynthesis
increases until it reaches a certain point
PHOTOSYNTHESIS
 Carbon Dioxide: As CO2 increases, rate of photosynthesis
increases until it reaches a certain point
PHOTOSYNTHESIS
 Temperature:
 Temperature Low = Rate of photosynthesis low
 Temperature Increases = Rate of photosynthesis increases
 If temperature is too hot, rate drops