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Photosynthesis. Introduction. Autotrophs : any organism that can make their own food. Photoautotrophs : use light energy Chemoautotrophs change inorganic chemicals such as hydrogen sulfate into chemical energy. Important structures.

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Presentation Transcript
  • Autotrophs: any organism that can make their own food.
    • Photoautotrophs : use light energy
    • Chemoautotrophs change inorganic chemicals such as hydrogen sulfate into chemical energy
important structures
Important structures
  • Chloroplast: Site of photosynthesis in eukaryotic cells.
  • Thylakoids: Disk shaped membranes containing photosynthetic pigments. Site of light dependent reactions.
  • Grana: Stacks of thylakoids.
  • Stroma: Fluid filled space surrounding grana. Site of light independent reactions.
properties of light
Properties of Light
  • White light from the sun is composed of a range of wavelengths.
  • Chlorophyll is the main pigment that absorbs sunlight.
  • Chlorophyll absorbs blue light and to a lesser extent, red light.
  • It reflects green light hence its intense green color.
what do chlorophyll and fireworks have in common
What do chlorophyll and fireworks have in common?
  • Fireworks are made up of various metal salts that go through oxidation and reduction reactions, producing great heat that causes electrons to move from their usual level around the nucleus (ground state) up to a higher position further away from the nucleus (excited state). The energy from heat has been imparted to the electron(s), which now has more potential energy. These excited electrons move almost instantaneously back down to their ground state, releasing their stored potential energy in the form of light. The greater the electrons movement away from the nucleus to the excited state, the more potential energy it has and the more energy it will release.
  • It should be pointed out that chlorophyll absorbs light energy, specifically its electrons, and this energy is used to pump protons instead of producing light. This proton gradient in turn is used to produce ATP, which along with other energized electrons are transferred to the Calvin cycle.
Suppose a large meteor hit the earth. How could smoke and soot in the atmosphere wipe out life far beyond the area of direct impact?
  • Because any particles in the atmosphere can block the light from the sun and reduce the excitation of electrons in chlorophyll molecules, photosynthesis depends on a relatively clean atmosphere. Any reduction in the available sunlight can have serious effects on plants. Scientists believe that if a large meteor hit the earth—as one did when the dinosaurs were wiped out 65 million years ago—smoke, soot, and dust in the atmosphere could block sunlight to such an extent that plants in the region, or even possibly all of the plants on earth, could not conduct photosynthesis at high enough levels to survive. And when plants die off, all of the animals and other species that rely on them for energy die as well. As dire as it sounds, all life on earth is completely dependent on the continued excitation of electrons by sunlight.


two outer membranes

thylakoid membrane system

Organelle of photosynthesis in plants and algae


thylakoid compartment


a A look inside the leaf

b One of the photosynthetic cells inside leaf

leaf’s upper epidermis

photosynthetic cell in leaf

leaf vein

Leaf Structure

leaf’s lower epidermis

photosynthesis equation
Photosynthesis Equation


6H2O + 6CO2

6O2 + C6H12O6


carbon dioxide



  • Two stages:
    • light-dependent reactions require light to work
    • light-independent reactions do not require light
two steps in photosynthesis
Two Steps in Photosynthesis





Where the two stages of photosynthesis occur inside the chloroplast


light-dependent reactions

light-independent reactions



photosynthesis light dependent
Photosynthesis: Light-Dependent
  • Photosystems - pigments surrounding a central chlorophyll a molecule....the reaction center
  • Each pigment absorbs a different wavelength of light & transfers its energy to the reaction center which in turns energizes an electron.
  • Only the chlorophyll that is the reaction center can give the energized electrons to the electron acceptors! The energized electron is then used elsewhere to make ATP or NADPH
two potential fates of excited electrons
Two Potential Fates of Excited Electrons

Electron returns to resting, unexcited state.

Excited electrons are passed to other atoms.

the passing of electrons in their excited state
The Passing of Electrons in Their Excited State

Chief way energy moves through cells

Molecules that gain electrons always carry greater energy than before receiving them

Can view this as passing of potential energy from molecule to molecule

photosynthesis light dependent2
Photosynthesis: Light-Dependent
  • As these pigments absorb photons from the sunlight that hits the leaves, electrons in the pigments become excited and then return to their resting state.
  • As the electrons return to their resting state, energy (but not the electrons) is transferred to neighboring pigment molecules.
  • This process continues until the transferred energy from many pigment molecules excites the electrons in a chlorophyll a molecule at the center of the photosystem.
photosynthesis light dependent3
Photosynthesis: Light-Dependent
  • This is where the electron journey begins.
    • The special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum.
  • Why must plants get water for photosynthesis to occur?
    • As electrons keep getting taken away from the special chlorophyll a molecule, the electrons must be replaced. The replacement electrons come from water.
photosynthesis light dependent5
Photosynthesis: Light-Dependent
  • Think of a pump pushing water into an elevated tank, creating a store of potential energy that can run out of the tank with great force and kinetic energy, which can be harnessed to do work, such as moving a large paddle wheel.
  • Similarly, the protons eventually rush out of the thylakoid sacs with great force—and that force is harnessed to build energy-storing ATP molecules, one of the two products of the “photo” portion of photosynthesis.
photosynthesis light dependent6
Photosynthesis: Light-Dependent
  • Product #1 of the “Photo” Portion of Photosynthesis: ATP
  • Product #2: NADPH
  • Product #3 (waste): O2
photosynthesis light independent
Photosynthesis: Light-inDependent
  • Also known as the Calvin Cycle
  • They occur whether or not light is present.
  • Occur in the stroma of the chloroplast.
  • The purpose of the reactions is to take the energy from ATP and energized ions from NADPH and add them to carbon dioxide to make glucose or sugar.
  • The reactions reduce carbon dioxide by adding energize electrons and protons to it and removing one oxygen atom. This effectively converts the carbon dioxide into CH2O.
factors that affect rates
Factors that Affect Rates
  • 3 factors can limit the speed of photosynthesis: light intensity, CO2 concentration, temperature
  • Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and CO2 .
factors that affect rates1
Factors that Affect Rates
  • Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air.
  • Even if there is plenty of light, a plant cannot photosynthesize if there is insufficient carbon dioxide.
factors that affect rates2
Factors that Affect Rates
  • If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesize if it gets too hot.
summary of photosynthesis
Summary of Photosynthesis








ADP + Pi










6 RuBP



phosphorylated glucose


end products (e.g., sucrose, starch, cellulose)

linked processes

Energy-storing pathway

Releases oxygen

Requires carbon dioxide

Aerobic Respiration

Energy-releasing pathway

Requires oxygen

Releases carbon dioxide

Linked Processes

Pores for gas exchange

how to get co 2 when stomata are shut
How to get CO2 when stomata are shut?
  • Primary sites for gas exchange in plants
    • CO2 for photosynthesis
    • O2 generated as a by-product in photosynthesis exits
    • water vapor evaporates
  • Closed stomata:
    • prevents water evaporation 
    • O2 cannot be released from the chloroplasts 
    • CO2 cannot enter them 
  • No carbon = no Calvin cycle
    • plant growth comes to a standstill; crops fail
evolutionary adaptations
Evolutionary Adaptations
  • Some plants are able to thrive in hot, dry conditions.
  • Evolutionary adaptations – along with recent agricultural (technological) advances – help battle world hunger
    • plants close their stomatato combat water loss through evaporation
c4 photosynthesis
C4 Photosynthesis

C4 plants produce an enzyme that is better at attracting carbon (in addition to RUBISCO) called PEP Carboxylase

allows CO2to be taken into the plant very quickly

"delivers" the CO2 directly to RUBISCO

Called C4 because the CO2is first incorporated into a 4-carbon compound

Stomata are open during the day

c4 photosynthesis1
C4 Photosynthesis

Adaptive Value:

photosynthesizes faster than C3 plants under high light intensity and high temperatures

better water use efficiency because PEP Carboxylase brings in CO2faster  doesn’t need to keep stomata open as much

C4 plants include several thousand species in at least 19 plant families, including corn & sugarcane

C4 photosynthesis also adds additional energy expense; outcompeted in mild climates by C3 plants.

cam photosynthesis
CAM Photosynthesis

CAM = CrassulaceanAcid Metabolism

after plant family in which it was first found (Crassulaceae)

because CO2is stored as an acid before use in photosynthesis

Stomata open at night (evaporation rates usually lower), closed during day

CO2 converted to acid, stored @ night


acid broken down

CO2 released to RUBISCO for photosynthesis

cam photosynthesis1
CAM Photosynthesis

Adaptive Value:

better water use efficiency than C3s under arid conditions

open stomata @ night when transpiration rates are lower

no sunlight, lower temperatures, lower wind speeds, etc.

May CAM-idleunder extremely arid conditions

leave stomata closed night and day

O2 given off in photosynthesis is used for respiration

CO2given off in respiration is used for photosynthesis

kind of like a perpetual energy machine, but plant cannot CAM-idle forever

allows plant to survive dry spells, recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells)


CAM Photosynthesis





evolutionary adaptations world hunger
Evolutionary Adaptations & World Hunger

C4 and CAM photosynthesis originally evolved as successful adaptations to hot/dry regions

Researchers are now using these adaptations to fight world hunger.

several genes that code for the C4 photosynthesis enzymes have been introduced from corn into rice

once in rice, genes increase rice plant’s ability to photosynthesize  higher growth rates and food yields