1. plants and other autotrophs are the producers of the...
TRANSCRIPT
• Photosynthesis nourishes almost all of the living
world directly or indirectly.
• All organisms require organic compounds for energy and to make organic molecules from their carbon skeletons.
• Autotrophs produce organic molecules from CO2
and other inorganic raw materials obtained from the
environment. Heterotrophs (like us) can’t do this.
• Autotrophs are the producers of the biosphere.
1. Plants and other autotrophs are the
producers of the biosphere
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Using glucose as our target product, the equation
describing the net process of photosynthesis is:
• 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
• How is this similar to the general equation of respiration?
Chloroplasts split water molecules and
produce oxygen in photosynthesis
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Living is work.
• To perform their many
tasks, cells must bring in
energy from outside
sources.
• In most ecosystems, energy enters as sunlight.
• Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them.
Introduction
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.1
• Any green part of a plant has chloroplasts.
• However, the leaves are the major site of
photosynthesis for most plants.
• There are about half a million chloroplasts per square
millimeter of leaf surface.
• The color of a leaf comes from chlorophyll, the
green pigment in the chloroplasts.
2. Chloroplasts are the sites of
photosynthesis in plants
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Each chloroplast has two membranes around a
central fluid space, the stroma.
• In the stroma are
membranous sacs,
the thylakoids.
• Thylakoids may be stacked into columns called grana.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.2
• The light dependent reactions convert solar energy
to chemical energy.
• The light independent reactions, a.k.a. Calvin
cycle incorporates CO2 from the atmosphere into an
organic molecule and uses energy from the light
reactions to “fix” the new carbon piece into sugars.
This process is therefore called carbon fixation.
• Both processes happen inside the chloroplast.
Photosynthesis consists of two processes
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.4
• When light meets matter, it may be reflected,
transmitted, or absorbed.
• Different pigments absorb photons of different
wavelengths.
• A leaf looks green
because chlorophyll,
the dominant pigment,
absorbs red and blue
light, while transmitting
and reflecting green
light.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.6
• The light reaction can perform work only with
those wavelengths of light that are absorbed.
• In the thylakoid are several pigments that differ in
which colors of light they absorb best.
• Chlorophyll a, the
dominant pigment,
absorbs best in the
red and blue
wavelengths, and
least in the green.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.8a
• Collectively, these photosynthetic pigments
determine an overall action spectrum for
photosynthesis. Similar to the last graph?
• An action spectrum measures changes in some measure
of photosynthetic activity (for example, O2 release) as
the wavelength is varied.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.8b
• Only chlorophyll a participates directly
in the light reactions, but antenna
pigments absorb light and transfer
energy to chlorophyll a.
• Chlorophyll b, carotenoids and
xanthophylls can funnel the energy
from other wavelengths to
chlorophyll a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.9
• In the thylakoid membrane, chlorophyll is organized
along with proteins and smaller organic molecules
into photosystems.
• A photosystem acts like a light-gathering “antenna
complex” consisting of a few hundred chlorophyll a,
chlorophyll b,
and other accessory
pigments
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.11
• When any antenna molecule absorbs a photon,
it is transmitted from molecule to molecule
until it reaches a particular chlorophyll a, called
the reaction center, right in the middle of the
photsystem.
• The reaction center chlorophyll a becomes so
energized by the light that it loses 2 electrons
to a nearby molecule.
• This starts the light dependent reactions.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• There are two types of photosystems.
• Photosystem I
• Photosystem II
• These two photosystems work together to use light
energy to generate ATP and NADPH.
• The folding of the innermost membrane into
thylakoids allows for many photosystems to exist
here, enhancing the amount of light dependent
reactions that can happen. Does this sound
familiar? What theme is this an example of?
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Let’s watch the light dependent reactions
1. When photosystem II absorbs light, an excited electron is captured by the primary electron acceptor, leaving the reaction center chlorophyll awith an electron “gap”.
2. An enzyme extracts electrons from water and passes them to the chlorophyll a to fill the gap.
• This reaction (photolysis) splits water into two hydrogen ions and an oxygen atom, which combines with another to form O2.
• This is where the oxygen that all aerobically respiring organisms (like you and I) depend on comes from.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. Excited electrons from photosystem
II pass along an electron transport
chain of cytochrome molecules before
ending up at a photosystem I reaction
center.
4. As these electrons pass along the
transport chain, their energy is
harnessed to produce ATP, again by a
chemiosmotic process.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.12
5. At the bottom of this electron transport chain,
the electrons fill an electron “gap” in the PS I
chlorophyll a reaction center.
6. This gap was created when photons excite
electrons on the photosystem I complex.
• The excited electrons are captured by a second primary
electron acceptor which transmits them to a second,
shorter electron transport chain.
• Ultimately, these electrons don’t participate in
chemiosmosis, but instead are passed from the transport
chain to NADP+, creating NADPH.
• NADPH will carry these high-energy electrons to the
Calvin cycle.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The Calvin cycle regenerates its starting material
after molecules enter and leave the cycle, just like in
the Krebs Cycle. Let’s watch
• CO2 enters the cycle and leaves as sugar.
• The cycle spends the energy of ATP and NADPH to
make the sugar.
• The actual sugar product of the Calvin cycle is not
glucose, but a three-carbon sugar, glyceraldehyde-3-
phosphate (G3P or PGAL)
4. The Calvin cycle uses ATP and NADPH to
convert CO2 to sugar: a closer look
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Each turn of the Calvin cycle fixes one carbon.
• For the net synthesis of one G3P molecule, the
cycle must take place three times, fixing three
molecules of CO2.
• To make one glucose molecules would require six
cycles and the fixation of six CO2 molecules.
• Since these reactions do not directly require light,
they are also called the light independent reactions.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The Calvin cycle has three phases.
• In the carbon fixation phase, each CO2 molecule is
attached to a five-carbon sugar, ribulose
biphosphate (RuBP).
• This is catalyzed by RuBP carboxylase or rubisco, the
most abundant protein in the world.
• The six-carbon intermediate splits in half to form two
molecules of 3-phosphoglycerate (PGA) per CO2.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.2
• If our goal was to produce one glucose net, we would start
with 6 CO2 (6C) and 6 RuBP (30C).
• After fixation and reduction we would have 12 molecules
of G3P (36C).
• Two of these 12 G3P (6C) combine to make 1 glucose.
(Remember in glycolysis this was reversed?)
• This molecule can exit the cycle to be used by the
plant cell.
• In the last phase, regeneration of the CO2 acceptor (RuBP),
the other 10 G3P molecules (30C) are rearranged to form
the 6 RuBP molecules (30C) we started with. In actuality,
there are millions of CO2 molecules involved.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.3
• In photosynthesis, the energy that enters the
chloroplasts as sunlight becomes stored as chemical
energy in
organic
compounds.
• Good summary
7:30
6. Photosynthesis is the biosphere’s
metabolic foundation: a review
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.20
• Sugar made in the chloroplasts supplies the entire
plant with chemical energy and carbon skeletons to
synthesize all the major organic molecules of cells.
• About 50% of the organic material is consumed as fuel
for cellular respiration in plant mitochondria.
• Carbohydrate in the form of the disaccharide sucrose
travels via the veins to nonphotosynthetic cells.
• There, it provides fuel for respiration and the raw
materials for anabolic pathways including synthesis of
proteins and lipids and building the extracellular
polysaccharide cellulose.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Plants also store excess sugar by synthesizing
starch.
• Some is stored as starch in chloroplasts or in storage
cells in roots, tubers, seeds, and fruits.
• Heterotrophs, including humans, may completely or
partially consume plants for fuel and raw materials.
• On a global scale, photosynthesis is the most
important process to the welfare of life on Earth.
• Each year photosynthesis synthesizes 160 billion metric
tons of carbohydrate per year.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Without Photosynthesis to grow plants:
• Cows
everywhere
would be
forced to starve
or jump.
Benchmark Clarifications
Students will explain how the products of photosynthesis are used
as reactants for cellular respiration and vice versa.
Students will explain how photosynthesis stores energy and
cellular respiration releases energy.
Students will identify the reactants, products and/or the basic
function of photosynthesis.
Students will identify the reactants, products and/or the basic
functions of aerobic & anaerobic cellular respiration.
Students will connect the role of ATP to energy transfers within
the cell.
Content Limits
Items will not require memorization of the stages,
specific events or intermediate molecules produced
during these processes.
Items do not require the balancing of equations.
Items will not assess plant structures.
Scenarios may include chemical equations.
Scenarios referring to adenosine triphosphate should use
the abbreviation ATP rather than the words adenosine
triphosphate.
Study the equation below. What product is
missing from the equation for photosynthesis?
Name the reactants and the products in the
equation.
sunlight
6CO2 + 6H2O → ____ + 6O2
Autotrophs, such as plants, use light to make
their own food. What happens to light
absorbed by a plant during photosynthesis?
A. It is converted to kinetic energy
B. It powers a reaction that produces carbon
dioxide and water
C. It is converted to chemical energy, which the
plant stores.
D. It powers a reaction that produces oxygen and
carbon dioxide.
Of the following factors, which will not have an
effect on the photosynthetic process?
a. Light intensity
b. Water availability
c. Nitrogen concentration
d. Temperature fluctuation
Gasses are a part of the photosynthesis process,
different phases. Which gas is removed from the
atmosphere during photosynthesis?
a. Hydrogen
b. Oxygen
c. Nitrogen
d. Carbon dioxide
A. Use few plants
B. Reduce the amount of water
C. Use a larger container
D. Move the light closer to the beaker
What are the reactants for photosynthesis and
how do they enter the plant?
There are reactants and products in the
photosynthesis process. What is the one
component in photosynthesis that is NOT
recycled and must be available to the plant?