Transcript Introduction to Photosynthesis - OCC

Introduction to
Photosynthesis
Chapter 7
OCC BIO-114
Dave Werner
Essential Questions
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Why study Photosynthesis?
What will you gain from understanding
Photosynthesis?
How does Photosynthesis relate to
Cellular Respiration?
Animations
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Light & Dark Reactions
Podcast – Overview of Photosynthesis
Photosynthesis Rap
1.ALL LIFE REQUIRES ENERGY
2.Animals, fungi, and most protists obtain their energy by
consuming, directly or indirectly, organic food from their
environment (heterotrophs)
3.Some organisms (autotrophs) have the ability to
capture the energy of the sun to synthesize their own
organic food (green plants, algae)
4.THE ULTIMATE SOURCE OF ALL ENERGY ON EARTH IS
THE SUN
5.PHOTOSYNTHESIS is the link between life on earth and
the sun
6.It is a set of reactions which convert light energy from
the sun into chemical bond energy of glucose and ATP
Light and Pigments
The Nature of Sunlight
The Nature of Sunlight
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light is a form of energy known as
electromagnetic radiation
light travels in rhythmic waves which are
disturbances of electrical and magnetic
fields
The Nature of Sunlight
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the distance between crests of
electromagnetic waves is called the
wavelength
the entire range of radiation is known as
the electromagnetic spectrum
Light Energy (Fig.8.4)
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the narrow range from about 380 to 750nm in wavelength
is detectable by the human eye and is called visible light
the model of light as waves explains many of its
properties, but in certain respects it behaves as though it
consists of discrete particles
these particles called photons act like objects in that
each of them has a fixed quantity of energy
the amount of energy is inversely related to the
wavelength of light (shorter wavelengths have more
energy)
Why are plants green?
Photosynthetic Pigments
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as light meets matter, it may be
reflected, transmitted or absorbed
substances that absorb light are called
pigments
if a pigment is illuminated in white light,
the color we see is the color most
reflected or transmitted by the pigment
Light perception
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the major pigment in leaves,
chlorophyll, appears green because it
absorbs red and blue light while
transmitted and reflecting green
chlorophyll is actually a family of
pigments with similar chemical structures
Photoexcitation of Chlorophyll
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when energy is absorbed by a molecule
of pigment, one of the molecules
electrons is elevated to from its ground
state to a higher orbital around the
nucleus (excited state)
Photoexcitation of Chlorophyll
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the only photons absorbed are those whose
energy is exactly equal to the energy
difference between the ground state and an
excited state
the energy of the photon is converted to the
potential energy of an electron, making the
electron less stable
generally, when pigments absorb light, their
excited electrons drop back down to the
ground state very quickly releasing their
energy as heat and/or light (fluorescence)
Overview of Photosynthesis
Reactions
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Photosynthesis can be summarized with
this chemical equation:
• 6CO2 + 12H2O + LIGHT ENERGY/ENZYMES -> C6H12O6 + 6O2+ 6H2O
6CO2 + 12H2O + LIGHT ENERGY
-> C6H12O6 + 6O2 + 6H2O
The chemical change is the
reverse of cellular respiration
 The low energy inorganic
compounds (CO2 and water) are
converted into the high potential
organic molecule (glucose)
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The Chloroplasts: Sites of
Photosynthesis
The primary function of this
specialized organelle is to convert
light energy into ATP and NADPH
(nicotinamide adenine dinucleotide
phosphate)
 Chloroplasts are found mainly in the
cells of the mesophyll (about 50/cell),
the green tissue on the interior of the
leaf
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Leaf (Fig.8.1)
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Carbon dioxide enters the leaf, and oxygen
exits, by way of microscopic pores called
stomata
The double membrane of the chloroplast
regulates transport of materials in and out
Chloroplasts are filled with an aqueous solution
called the stroma which contains all the
necessary enzymes for photosynthesis
Chloroplast (Fig. 8.1)
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The conversion from light energy to ATP and
NADPH occurs in the thylakoid membranes
within the stroma
The thylakoid membranes contain all of the
pigments involved in the process including
chlorophyll (green pigment) and other
carotenoids
The thylakoids are organized into closely
packed stacks called grana
Choloroplast
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Within these thylakoids and grana, light
energy is converted into ATP and
NADPH – these are said to be LIGHTDEPENDENT REACTIONS
The reactions that actually convert CO2
to carbohydrate are LIGHTINDEPENDENT REACTIONS or DARK
REACTIONS
The Light Reactions - Video
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Must take place in the presence of light
Steps that convert solar energy to
chemical energy
Light absorbed by chlorophyll drives a
transfer of electrons from water to an
acceptor named NADP+ which
temporarily stores the energized
electrons
Light Reactions
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Water is split in the process and thus it is the light
reactions of photosynthesis that give off O2 as a byproduct
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The light reactions also generate ATP by powering
the addition of a phosphate group to ADP, a process
called photophosphorylation. This is done w/ the
help of ATP Synthase b/c H+ cannot diffuse through
the membrane.
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THE LIGHT REACTIONS PRODUCE NO SUGAR
Electron Transport (Fig.8.2)
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chloroplasts and mitochondria generate ATP
by the same basic mechanism of
chemiosmosis
an electron transport chain (ETC) embedded
in the thylakoid membrane pumps protons
across the membrane as electrons are passed
through a series of carriers producing a protonmotive force (potential energy stored in the
proton gradient)
Restoring PSII (Fig. 8.13)
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2H2O → 4H + 4e- + O2
 Can you explain how PSII is restored?
Chemiosmosis
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Happens during Light Rxns.
Concentration gradient of protons across
thylakoid membrane.
Where do the protons come from?
ATP Synthase – protein that harnesses
energy.
Some Protons are used to make NADPH from
NADP+.
NADPH & ATP drive next set of rxns.
Energy Flow in Photosynthesis
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in its native environment of the thylakoid membrane,
chlorophyll is organized along with proteins,
pigments, and other kinds of smaller organic
molecules into photosystems
a photosystem has a light gathering "antenna
complex" consisting of a few hundred chlorophyll a,
chlorophyll b, and carotenoid molecules
all of the antenna molecules absorb photons of light
and the energy is transmitted from pigment molecule
to pigment molecule until it reaches the reaction
center
Photosystems II and I (Fig.8.13)
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there are two photosystems in the thylakoid
membranes, photosystem II and
photosystem I
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the chlorophyll a in both photosystems is
identical, it is their association with different
proteins that affects their light absorbing
properties
The Calvin Cycle (Dark
Reactions)
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The "Dark Reactions" include the biochemical,
enzyme-catalyzed reactions involved in the
synthesis of carbohydrate from carbon dioxide; these
are collectively know as the Calvin-Benson cycle
Light is not required directly for these reactions to occur
These reactions occur in the stroma
The Balance Sheet for
Photosynthesis
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A variety of organic compounds are created:
amino acids, lipids, and carbohydrates.
The Metabolic Fates of Glucose:
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About 50% of the glucose formed is used
immediately to meet the plants energy needs
Excess glucose can be converted to starch within
the stroma of the chloroplast or in specialized
storage cells of roots, tubers, seeds, and fruits
REMEMBER, plants actively metabolize glucose
(cellular respiration) and grow in the dark and in the
light
The glucose may be converted to sucrose (glucose
+ fructose) for transport (via the phloem cells) to the
non-photosynthetic leaves, roots, and stems
The glucose may be converted to CELLULOSE, to
build cell walls, especially in plant cells that are still
growing and maturing
Alternative Pathways
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(Fig.8.20)
Calvin Cycle=most common pathway for
carbon fixation.
Plants in harsh conditions fix carbon through
alternate pathways and then release it to
enter the Calvin cycle.
C3 Plants
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Most plants are C3 plants
C3 plants exclusively fix carbon through the
Calvin cycle b/c of 3-carbon PGA.
Build up of O2, which slows down sugar
production.
C4 Pathway - Video
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Corn, sugar cane, crabgrass.
Fix CO2 into 4-Carbon compounds = C4
pathway.
Partially close stomata during hottest
part of the day.
Lose about ½ as much water as C3
plants when producing same amount of
carbs.
CAM Pathway
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CAM plants have adapted to dry conditions by opening
their stomata during the night and closing them during the
day, opposite to how other plants behave
when the stomata are open CO2 is incorporated into a
variety of organic acids in a method of carbon fixation call
crassulacean acid metabolism (CAM)
the mesophyll cells of CAM plants store the organic acids
they make during the night in their vacuoles until morning
when the stomata close
CO2 is released from the acids during the day for
incorporation into the Calvin cycle
Lose less water than C3 & C4 plants.
Desert plants - Cactus
Rate of Photosynthesis
Now let's revisit the summary equation for
photosynthesis & note how each of the raw materials
end up in the chloroplasts so that the whole
photosynthesis deal can go down.
Now let's do the same, except pay attention to what
happens to the products of photosynthesis.