Photosynthesis
• ATP (adenosine
triphosphate)- basic energy
source of all cells, chemical
compound that cells use to
store and release energy
– Adenine, 5- carbon sugar
(ribose), and 3 phosphate
groups
• ADP (adenosine
diphosphate)-chemical
compound similar to ATP,
but has two phosphate
groups
– Key for storing energy
• How is the energy that is stored in ATP released?
– Chemical bond between second and third phosphate group is broken
• Because of the characteristics of ATP, it is an exceptional source of energy
• Organisms break down carbon –based molecules to produce ATP
– Carbohydrates
– Proteins
– Lipids
Sec 2- Photosynthesis: An Overview
• Photosynthesis- series of reactions that uses light
energy from sun to convert water and carbon dioxide
into sugars and oxygen
• What does photosynthesis require?
– Sunlight, water, CO₂ and chlorophyll-molecule in
chloroplast
• Plants gather the sun’s energy with light absorbing molecules
called pigments
• Chlorophyll- principal light absorbing pigment
– Chlorophyll a and chlorophyll b
– Absorbs light very well in certain regions of visible
spectrum, green light is reflected
Overview of Photosynthesis
• Photosynthesis takes
place in chloroplasts
– Grana (thylakoids)
– Stroma
• Thylakoids- sac-like
photosynthetic
membranes inside
chloroplasts
– Arrange in stacks known
as grana
Overview of Photosynthesis
• Light-dependent reactions capture energy from sunlight
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–
–
–
Takes place in thylakoids
Water and sunlight are needed
Chlorophyll absorbs energy
Energy is transferred along thylakoid membrane then to lightindependent reactions
– Oxygen is released
Overview of Photosynthesis
• Light-independent reactions (Calvin Cycle)- make sugars
– Take place in stroma
– Needs carbon dioxide from atmosphere
– Uses energy to build a sugar in a cycle of chemical reactions
Overview of Photosynthesis
• The equation for overall process is
– Proteins in thylakoids organize chlorophyll into
photosystems – 2 types of photosystems- light collecting
units of the chloroplast
– 2 types of reactions in photosystems- light dependent
reactions and light independent reactions= Calvin cycle
– Light dependent reactions-thylakoid membranes
– Calvin cycle= stroma, a region outside thylakoid
membranes
Electron Carriers
• Sunlight excites electrons in chlorophyll= great
deal of energy
• Electron carriers transport high energy electrons
from chlorophyll to other molecules
• NADPᶧ- electron carrier
• High energy electrons used to build molecules for
cell
Light Dependent Reactions
• Requires light
• Converts ADP and NADPᶧ into ATP and NADPH and produces
oxygen
• Steps of light dependent reaction
– Light is absorbed by electrons in photosystem II Are electrons ever
used up? Where do the electrons come from?
– High energy electrons move through electron transport chain to
photosystem I
– Photosystem I reenergizes electrons released. Electrons are used to
form NADPH
– Positively charged hydrogen ions fill up thylakoid membrane
– Hydrogen ions pass through ATP synthase, ADP is converted into ATP
Calvin Cycle
• Plants use energy that ATP and NADPH contain to build high
energy compounds
• Does not require light, known as light- independent reactions
• Steps of Calvin cycle
– CO₂ enters the cycle, 6 carbon dioxide molecules combined with six 5
carbon molecules to form twelve 3- carbon molecules
– ATP and NADPH are used to convert twelve carbon molecules into
higher energy forms
– Two 3 carbon molecules removed from cycle to form sugars, lipids,
amino acids
– 10 remaining 3- carbon molecules converted into six 5- carbon
molecules- used in next cycle
• Plants use sugars to build cellulose