PHOTOSYNTHESIS
OVERVIEW & 10.1
OVERVIEW
The Process That Feeds the Biosphere
• Life on Earth is solar powered.
• Photosynthesis: conversion of light energy to chemical energy.
• Nourishes all life
• Organisms acquire organic compounds and carbon skeletons
by one of 2 methods: autotrophic or heterotrophic nutrition.
Autotrophic Nutrition
• Autotrophs: “self-feeders” that sustain themselves without
eating anything derived from other organisms.
• Known as producers, because they are a source of organic
compounds for other nonautotrophic organisms.
• Almost all plants are autotrophs.
• More specifically, plants are considered photoautotrophs
which are organisms that use light as a source of energy to
synthesize organic substances.
Heterotrophic Nutrition
• Heterotrophs: organisms that are unable to make their own
food, so they obtain organic material by living on compounds
other organisms produce.
• Known as consumers, due to the fact that they consume other
organisms.
• Animals eat plants or other molecules.
• Heterotrophs are completely dependent on photoautotrophs
for food and oxygen.
10.1 Photosynthesis converts light energy to the
chemical energy of food
Quick outline:
I. Chloroplasts: The Sites of Photosynthesis in Plants
II. Tracking Atoms Through Photosynthesis: Scientific Inquiry
a. The Splitting of Water
b. Photosynthesis as a Redox Process
III. The Two Stages of Photosynthesis: A Preview
I. Chloroplasts: The Sites of Photosynthesis in
Plants
• Chloroplast: An organelle found only in plants.
• Found mainly in the cells of the mesophyll.
• Mesophyll: tissue in the interior leaf (30-40 chloroplasts in
each cell)
• Leaves are the major sites of photosynthesis and the color of
the leaves come from chlorophyll.
• Chlorophyll: green pigment located in the thylakoid
membranes within the chloroplast.
I. Chloroplasts: The Sites of Photosynthesis in
Plants
• Stomata: microscopic pores where carbon dioxide enters and
oxygen leaves the leaf.
• Stroma: fluid of the chloroplast surrounding the thylakoid
membrane; involved in the synthesis of organic molecules
from carbon dioxide and water`.
• Thylakoids: flattened membrane sacs inside the chloroplast,
used to convert light energy to chemical energy.
• Thylakoid space: interior of thylakoids.
• Grana: thylakoid sacs stacked in columns.
Page 183 in book
I. Chloroplasts: The Sites of Photosynthesis in
Plants
Main Point: In autotrophic eukaryotes,
photosynthesis occurs in chloroplasts,
organelles containing thylakoids. Stack of
thylakoids form grana.
II. Tracking Atoms Through Photosynthesis:
Scientific Inquiry
Overall photosynthetic equation known since 1800s:
In the presence of light, the green parts of plants
produce organic compounds and oxygen from carbon
dioxide and water.
II. Tracking Atoms Through Photosynthesis:
Scientific Inquiry
• Molecular formula:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
• Simplified by indicating the net consumption of water
6 CO2 + 6 H2O + Light energy  C6H12O6 + 6 O2
• Simplest possible form
CO2 + H2O  [CH2O] + O2
• NOTE: C6H12O6 represents glucose, but the actual product is a
three-carbon sugar. Glucose is used here to simplify.
• NOTE: [ ] indicate that CH 2O is not an actual sugar but
represents the general formula for a carbohydrate.
a. The Splitting of Water
• The chloroplast splits water into hydrogen and oxygen.
• Before scientists discovered that it was oxygen given off by
plants rather than carbon dioxide, they predicted
photosynthesis split carbon dioxide.
• The hypothesis predicted that the O 2 released in the synthesis
came from CO2.
CO2  C + O2
and then added water to the carbon
CO2  [CH2O]
a. The Splitting of Water
• The previous idea was challenged by C. B. van Niel of Stanford
University in the 1930s. By using sulfur bacteria, his findings
provided evidence that plants split water as a source of
electrons from hydrogen atoms, releasing oxygen as a byproduct.
Sulfur bacteria:
Plants:
General:
CO2 + 2 H2S  [CH2O] H2O + 2S
CO2 + 2 H2O  [CH2O] H2O + O2
CO2 + 2 H2X  [CH2O] H2O + 2X
a. The Splitting of Water
• 20 years later, van Niel’s work was solidified by using oxygen18 as a radioactive tracer to follow the fate of oxygen atoms
during photosynthesis.
• A significant result of shuffling of atoms during
photosynthesis is the extraction of hydrogen from water and
its incorporation into sugar.
Page 184 in book
b. Photosynthesis as a Redox Process
Recall Redox Reaction: chemical reaction involving the
transfer of one or more electrons from one reactant to
another; oxidation-reduction reaction.
Cellular Respiration
• Involve redox reactions.
• Energy is released from sugar
when electrons associated with
hydrogen are transported by
carriers to oxygen, forming
water as a by-product.
• Electrons lose potential as they
“fall” down the electron
transport chain.
•
•
•
•
Photosynthesis
Involve redox reactions.
Reverses the direction of the
electron flow.
Water is split reducing it to sugar.
Electrons increase in potential
caused by an energy boost from
light.
II. Tracking Atoms Through Photosynthesis:
Scientific Inquiry
Main Point: Chloroplasts split water into
hydrogen and oxygen, incorporating the
electrons of hydrogen into sugar molecules.
Photosynthesis is a redox process: H 2O is
oxidized and CO2 is reduced.
III. The Two Stages of Photosynthesis: A Preview
• Really 2 processes
1. Light reactions: The steps in photosynthesis that occur on
the thylakoid membranes of the chloroplast and convert
solar energy to the chemical energy of ATP and NADPH,
evolving oxygen in the process.
2. Calvin Cycle: Second of the 2 major stages in
photosynthesis, involving atmospheric CO 2 fixation and
reduction of the fixed carbon into carbohydrate.
III. The Two Stages of Photosynthesis: A Preview
1. Light reactions
• The photo part of photosynthesis.
• Light drives a transfer of electrons and hydrogen from
water to an acceptor called NADP+, which temporarily
stores the energy.
• Water is split and O2 is given off.
• NADPH is formed.
• ATP is generated.
• Photophosphorylation: process of generating ATP
using chemiosmosis to power the addition of a
phosphate group to ADP.
III. The Two Stages of Photosynthesis: A Preview
2. Calvin Cycle
• Named after Melvin Calvin.
• This is where sugar is produced with the help of NADPH
and ATP.
• Carbon fixation occurs; this is the incorporation of carbon
from CO2 into an organic compound by an autotrophic
organism.
• Sometimes referred to as the “dark-reactions” or “lightindependent reactions.”
Page 185 in book
III. The Two Stages of Photosynthesis: A Preview
Main Point: The light reactions in the grana
split water, releasing O 2, producing ATP, and
forming NADPH. The Calvin Cycle in the
stroma, forms sugar from CO 2, using ATP for
energy and NADPH for reducing power.
Optional Video
• http://www.youtube.com/watch?v=1gLa5EWn9OI
Citation of Diagrams in order of appearance
• All pictures other than diagrams from Clip Art
• http://www.google.com/imgres?q=chloroplast&start=18&num=10&
um=1&hl=en&rlz=1C1GPCK_enUS435US450&biw=1228&bih=598&tb
m=isch&tbnid=dtwhOtkPzEVDrM:&imgrefurl=http://www.biology.iu
pui.edu/biocourses/N100/2k4ch6photosynthnotes.html&docid=Nldz
wAeZg3rAdM&imgurl=http://www.biology.iupui.edu/biocourses/N1
00/images/ch9chloroplast.jpg&w=797&h=600&ei=VuWcTtvSI8iNsQ
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• http://www.google.com/imgres?q=tracking+atoms+through+photo
synthesis&um=1&hl=en&sa=N&rlz=1C1GPCK_enUS435US450&biw=1
228&bih=598&tbm=isch&tbnid=x9q0lRy7tWLblM:&imgrefurl=http:/
/bio1903.nicerweb.com/note01.html&docid=KOcWsrL1BHWJ1M&im
gurl=http://bio1903.nicerweb.com/doc/class/bio1903/Locked/media/
ch10/10_04TrackingAtoms.jpg&w=888&h=312&ei=ItWcTtLnLuGxsAL
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• http://www.google.com/imgres?q=chloroplasts&um=1&hl=en&rlz=1
C1GPCK_enUS435US450&biw=1228&bih=598&tbm=isch&tbnid=K4d
8SdEWeNX_DM:&imgrefurl=http://nanobiotechnews.com/structureof-thechloroplast.html&docid=ZSzcPiLfWzHB_M&imgurl=http://nanobiot
echnews.com/wpcontent/uploads/2011/04/chloroplast.gif&w=390&h=322&ei=bacTpWXEeKysALcovzwCQ&zoom=1