the working cell

THE WORKING CELLENERGY FROM SUNLIGHT

PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD• The Structure of Chloroplasts

– As learned previously, the chloroplast is the organelle where photosynthesis takes place.

– The chemical compound, chlorophyll is contained in the chloroplast.

– Everything in a plant that is green contains chloroplasts and chlorophyll (gives the plant its green color) and carries out photosynthesis.

IF IT’S GREEN, THERE’S PHOTOSYNTHESIS

CHLOROPLASTS

CHLOROPLAST

CHLOROPLAST & THYLAKOID

PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD

– The most chloroplasts are in the leaves where the major site for photosynthesis is located.

– Chloroplasts are concentrated in the cells of the mesophyll, the inner layer of tissue.

– Tiny pores found on the surface of the leaf are called stomata, through which CO₂ enters and O₂ exits.

– Veins carry the nutrients and water from the roots to the leaves.


– Organic molecules produced in the leaves are carried by the veins to other parts of the plant.

– The chloroplast is structured like the mitochondria: outer and inner membrane, and a thick fluid in the inner membrane called stroma.

– In the stroma are the thylakoids which are disk shaped sacs.• Membranes surround each thylakoid.

– Stacks of thylakoids are grana.


– Photosynthesis takes place on the thylakoid membranes or the stroma.

• Overview of Photosynthesis– In cellular respiration, electrons “fall”

from the glucose to the oxygen.– In photosynthesis, electrons from the

water are “boosted uphill” by the sun’s energy.

– These electrons, along with CO₂ and H⁺ ions, produce sugar molecules.

PHOTOSYNTHESIS

TWO STAGES OF PHOTOSYNTHESIS

PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD– There are two stages to photosynthesis: the light

reactions and the Calvin cycle.– The Light Reactions

• Light reactions convert the sun’s energy to chemical energy.

• This happens on the thylakoid membrane.• Chlorophyll captures the sun’s energy.• Then they remove the electrons from the water using

this energy from the sun.• The water is then broken down to oxygen and

hydrogen molecules.


• Oxygen is the waste product and leaves for the atmosphere through the stomata.

• The electrons from the water are used by the chloroplasts to make an energy rich molecule called NADPH.

– This is an electron carrier similar to NADH.

• The energy from the sun is also used to generate ATP.

• The net result is the conversion of the light energy to chemical energy stored in NADPH and ATP.

– The Calvin Cycle• Sugars are made from the atoms in CO₂ in the


• Calvin cycle, also using the hydrogen ions and the high energy electrons carried in the NADPH.

• The enzymes for this process are located outside the thylakoids and dissolved in the stroma.

• The “fuel” or energy required to make sugar comes from the ATP made by the light reactions.

• The Calvin cycle is also referred to as the “light-independent reactions.

– This is because it does not directly require light to begin.– It can’t, however, complete its cycle in the dark because it

requires inputs from the NADPH and ATP, produced in light.

PHOTOSYNTHESIS

REVIEW: Concept Check 8.1, PAGE 1621. Draw and label a simple diagram of a

chloroplast that includes the following structure: outer and inner membranes, stroma, thylakoids.

2. What are the reactants for photosynthesis? What are the products? What is the stimulus?

3. Name the two main stages of photosynthesis. How are the two stages related?

THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY

• Light Energy and Pigments– Sunlight is a form of electromagnetic energy

that travels in waves, like those on a beach.• A wavelength is the distance between two adjacent

waves.

– The electromagnetic spectrum shows us the ranges of electromagnetic energy from gamma rays to radio waves.

– That light which is visible is only a small fraction of the electromagnetic spectrum.• Visible light is from 400 nanometers (nm), violet, to

ELECTROMAGNETIC SPECTRUM


to about 700 nm, red.• The longer the wavelength the less the energy.• UV radiation can damage proteins and nucleic acids

as can all wavelengths shorter than visible light.

– Pigments and Color• Color is due to chemical compounds called

pigments.• If a material contains a pigment, and light shines on

it, three things happen to the different wavelengths:– They can be absorbed– They can be transmitted– They can be reflected

VISIBLE LIGHT STRIKING THE CHLOROPLAST


• Pigments in chloroplasts absorb blue-violet and red-orange light.

• Some of this absorbed light energy is converted to chemical energy.

• Chloroplasts pigments do not absorb green light well, so it passes through (transmitted) or bounces back (reflected).

• Leaves look green because the green light is not absorbed well.

– Identifying Chloroplast Pigments• Paper chromatography shows us the different

pigments in a green leaf.


• A leaf stain is placed on filter paper which is then placed in a cylinder containing a solvent.

• The pigments dissolve in the solvent and travel up the paper, separating out.

• Chlorophyll a, which absorbs blue-violet and red light and reflects green light, plays one of the major roles in photosynthesis.

• There are other helper pigments in chloroplasts: chlorophyll b (absorbs blue and orange and reflects yellow-green) and carotenoids (absorb blue-green and reflects yellow orange).

PAPER CHROMATOGRAPHY


• Harvesting Light Energy– When light strikes a leaf:

• In the thylakoid membrane, chlorophyll, along with other molecules, are arranged in clusters called photosystems.

– Contained therein are a few hundred pigment molecules, including chlorophyll a and b, and carotenoids.

– These are similar to a solar collecting panel, collecting all the light entering.

– When a pigment molecule absorbs light energy, the pigment’s electrons gains energy (from a low energy state to a high energy state, which is unstable).

REACTION CENTER OF THE PHOTOSYSTEM

THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY– The electron then falls back to the ground state,

transferring energy to another molecule which, in turn, excites an electron in the receiving molecule, and so on.

– This happens to the energy until it reaches its destination, the reaction center of the photosystem.

– This consists of chlorophyll a next to another molecule called a primary electron acceptor.

– The primary electron acceptor traps the excited electron form the chlorophyll a molecule.

LIGHT REACTIONS

THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY– Other molecules in the thylakoid membrane use

the energy to make ATP and NADPH.• Chemical Products of the Light Reactions

– There are two systems involved in light reactions.

– The first traps the light energy and then transfers the excited electrons to an electron transport chain.

– This is the “water-splitting photosystem” because the electrons are replaced by splitting a molecule of water releasing oxygen and hydrogen ions.

LIGHT REACTIONS

THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY– The two photosystems are connected by an

electron transport chain which releases energy, which is used by the chloroplast to make ATP, similar to ATP production in cellular respiration.

– As in cellular respiration, the electron transport chain pumps hydrogen ions across a membrane (inner mitochondrial membrane in respiration and the thylakoid membrane in photosynthesis).• In respiration food is the source for electrons and in

photosynthesis light excited electrons from chlorophyll travel down the chain.

THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY– The second photosystem is the “NADPH-

producing photosystem.”– In this, NADPH is produced from transferring

the excited electrons and hydrogen ions to NADP⁺.

– Light reactions convert light energy to the chemical energy of ATP and NADPH.

– Sugar has yet to be produced and this is done in the Calvin cycle, utilizing ATP and NADPH produced by the light reactions.

LIGHT REACTIONS

REVIEW: CONCEPT CHECK 8.2, page 167

1. Explain why a leaf appears green.

2. Describe what happens when a molecule of chlorophyll a absorbs light.

3. Besides oxygen, what two molecules are produced by the light reactions?

4. Where in the chloroplast do the light reactions take place?

THE CALVIN CYCLE MAKES SUGAR FROM CARBON DIOXIDE

• A Trip Around the Calvin Cycle– Similar to the Krebs cycle, this is a regenerative

process with the starting material being a compound called RuBP, a sugar with five carbons.

– The chemical inputs are carbon dioxide and ATP and NADPH from the light reactions.

– In the Calvin cycle, the carbon comes from the CO₂, the energy from the ATP, and the high-energy electrons and hydrogen ions from the NADPH.

– The result is an energy laden sugar molecule.

CALVIN CYCLE

THE CALVIN CYCLE MAKES SUGAR FROM CARBON DIOXIDE

• This molecule is called G3P, not yet glucose, which the plant uses to make glucose and other organic molecules.

• Summary of Photosynthesis 6 CO₂ + 6 H₂O →→→ C₆H₁₂O₆ + 6 O₂– Light reactions in the thylakoid membranes

• Light energy to chemical energy = ATP + NADPH• Light reactions use water from the equation and release

oxygen– Calvin cycle, in the stroma, uses ATP and NADPH and

converts CO₂ to sugar.

SUMMARY OF PHOTOSYNTHESIS


1. What are the inputs and outputs of the Calvin cycle?

2. Which stage of photosynthesis uses each reactant from the overall photosynthesis equation? Which stage generates each product from the overall photosynthesis equation?

3. Why is the Calvin cycle called a cycle?

4. What molecule is the direct product of photosynthesis? How is that molecule then used by plant cells?

PHOTOSYNTHESIS HAS A GLOBAL IMPACT

• The Carbon Cycle– The carbon cycle is a process in which carbon

moves from inorganic to organic compounds and back.

– Using photosynthesis, the producers such as grass convert the inorganic carbon dioxide to organic compounds.

– The consumers, including Cape buffalo, eat the producers obtaining their organic compounds.

– These consumers are in turn eaten by lions, so eventually the CO₂ is returned to the atmosphere

CARBON CYCLE

THE LAST LAUGH!

PHOTOSYNTHESIS HAS A GLOBAL IMPACT by the producers and consumers– In the previous pictures, CO₂ in the atmosphere,

from the consumers, leads to photosynthesis by producers building organic compounds which leads to cellular respiration by the producers and consumers releasing CO₂..

– It is estimated that photosynthesis by plants produces 160 billion metric tons of organic material per year, or 25 stacks of biology books reaching from Earth to the sun.


• Photosynthesis and Global Climate– Carbon dioxide is the key element in the carbon

cycle– It is a reactant in photosynthesis by plants and a

product in cellular respiration.– Figuring all the CO₂ produced by organisms of

Earth, there is a very large amount produced in the atmosphere.

– CO₂ in the atmosphere aids in the greenhouse effect, keeping Earth’s temperature 10⁰C warmer than it otherwise would be.

GREENHOUSE EFFECT


• However, the amount of CO₂ in the atmosphere used to be 0.03 % (300 parts per million) of the atmosphere, it is now more than 360 parts per million.


1. Give an example of carbon moving from an inorganic compound to an organic compound in the carbon cycle. Give an example of the carbon moving from an organic to an inorganic compound.

2. How is carbon dioxide important to Earth’s climate?

the working cell

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light energy

chemical energy

suns energy

high energy electrons

energy rich molecule

light reactionslight

lightindependent reactions

thylakoid membranes