Photosynthesis

Photosynthesis – An Overview Living things get energy from food

Food is broken down and energy is stored as

ATP Heterotrophs have to consume other

organisms for energy

Autotrophs are able to make their own food – Sugar (C6H12O6)/ glucose

How do Autotrophs Make Their Own Food? Through photosynthesis – the process

of using energy from the sun to convert Water and Carbon Dioxide into Oxygen and Sugar.

Equation:

6H2O + 6CO2 6O2 + C6H12O6

An organism’s metabolism is part of Earth’s carbon cycle

Photosynthesis takes in Carbon as CO2 and transfers that carbon to glucose

Glucose is a carbohydrate

Light and Pigment Plants also need light and chlorophyll to undergo

photosynthesis.

Plants absorb the sun’s light with chlorophyll. There is chlorophyll a and chlorophyll b.

Sunlight is a mixture of colors (ROYGBIV)

Light and Pigment cont. The color we see with our eyes is the result of

colors being reflected and absorbed.

A red flower appears red because all of the colors from the spectrum are being absorbed, EXCEPT red. Red is the color being reflected.

Both chlorophylls absorb the red and blue spectrums easily. They DO NOT ABSORB green light, they reflect green light. (See the previous color spectrum chart)

Stop & Think 1. Why are light & chlorophyll needed for photosynthesis?

2. Why are plants green?

3. How well would a plant grow under pure yellow light?

4. Write the equation for photosynthesis. Identify

reactants and products.

5. Use the graph below to figure…

In which regions of the spectrum will chlorophyll a absorb the most

light?

In which regions of the spectrum will chlorophyll b absorb the most

light? Chlorophyll b

Chlorophyll a

Cellular Energy

Cells use a form of chemical energy

called Adenosine Triphosphate (ATP)

Cells store & use ATP to fuel necessary

metabolic reactions

Such as maintaining internal chemical

conditions (homeostasis)

10 MILLION molecules of ATP are

consumed & regenerated per second

per cell!

Adenosine Triphosphate (ATP)

Nucleotide

Sugar 3 Energy Rich Phosphate

Bonds

ATP-Energy Currency ATP (adenosine triphosphate) -nucleotide with

two extra energy-storing phosphate groups

Energy is released when the bonds that hold the phosphate groups together are broken

The removal of a phosphate group from ATP produces adenosine diphosphate -ADP:

H2O + ATP ADP + P + energy

Energy is stored when a phosphate is added

to ADP to become ATP (reverse the equation)

ATP – ADP Cycle:

The Reactions of Photosynthesis Photosynthesis occurs inside the

chloroplast of plant cells.

Two main stages of Photosynthesis: 1. The Light Reactions

a) Need Light to occur

b) Takes place in the Thylakoid Membranes

2. The Calvin Cycle a) Does NOT need light to occur

b) Takes place in the Stroma of the chloroplast

Organization of the Chloroplast

Electron Transport Chains

Electrons get “excited” when they

absorb energy from light (photons)

The energy gets transported down a

chain to make new molecules

There are two electron transport chains

(ETC) in photosysthesis

Provides energy to make ATP

Provides energy to make NADPH

Carrier Molecules A red hot coal gets hot in a campfire. If

it had to be transported to another place, you would not use your hands! You could use a pan or bucket – a carrier – to transport it.

High-energy electrons gain a great deal of energy from sunlight Needs a special carrier molecule to transport

it.

Carrier Molecules

NADP+ is the electron carrier involved in photosynthesis. It accepts and holds 2 electrons (e-) along with Hydrogen ions (H+) to become NADPH.

NADPH can now carry the electrons to other areas where the cells need help building molecules.

The Light Reactions

1. Light strikes the chlorophyll in the chloroplasts of plant cells.

2. Electrons absorb the light, get excited,

and gain energy. 3. The high-energy electrons are passed

along the electron transport chain.

4. Chlorophyll is now missing electrons.

Light Reactions cont. 5. To produce new electrons, water

molecules split in the process of photolysis, leaving H+ ions and an Oxygen atom. The Oxygen is released as a gas into the atmosphere.

6. Energy from the electrons is available to transport the H+ ions.

(The hydrogen ions and electrons are looking for a place to go)

Light Reactions cont.

7. NADP+ picks up the electrons and hydrogen ions and becomes NADPH. The NADPH will be used in the Calvin Cycle.

8. The release of the hydrogen ions from photolysis creates ATP. The ATP will be used in the Calvin Cycle.

Calvin Cycle 1. CO2 enters the cycle from the atmosphere.

It combines with a 5-Carbon molecule in the process of carbon fixation.

2. Carbon fixation results in the formation of an unstable 6 carbon molecule that breaks down into two 3-carbon molecules.

3. The cell used the ATP and NADPH from the light reactions to convert the 3-carbon molecules into higher energy forms.

Calvin Cycle

4. The carbon molecules are then used to form various sugar (C6H12O6) molecules.

5. The extra carbon molecules, ADP and NADP+ are reintroduced to the light reactions to begin the next cycle (Back to the Light Reactions)

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

ATP

NADPH

O2

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

ATP

NADPH

O2

Calvin Cycle

CO2

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

ATP

NADPH

O2

Calvin Cycle

CO2

[CH2O]

(sugar)

Grand Totals

During the Light Reactions: Enters = H2O Leaves = O2

During the Calvin Cycle: Enters = CO2 Leaves = C6H12O6

_____________________________

Total Equation: 6CO2 + 6H20 6O2 + C6H12O6

Stop & Think

1. Summarize the light dependent reactions.

2. Summarize the events of the Calvin

Cycle.

3. What is the function of NADP+?

4. Why must the light reactions take place

before the Calvin Cycle can occur?

Factors that Affect the Rate of Photosynthesis

1. Amount of water: a shortage will slow or even stop rate of

photosynthesis desert plants have waxy coatings to reduce water

loss

2. Temperature: Functions best between 0ºC & 35ºC Above or below this will damage enzymes & slow the

rate of photosynthesis

Factors that Affect the Rate of Photosynthesis

3. Intensity of Light: Greater light intensity will increase the

rate of photosynthesis

will eventually reach its maximum rate

varies from plant to plant

4. Amount of Carbon Dioxide: More CO2 increases rate of photosynthesis

Will reach its maximum rate, or saturation

point

Stop & Think

A) When light intensity is below 200 photons/m2/s, do sun plants or shade plants have a higher rate of photosynthesis?

B) Does the relationship in question 1 change when light intensity increases above 400 photons/m2/s? Explain.

C) The average light intensity in the Sonoran Desert is about 400 photons/m2/s. According to the graph, what would be the approximate rate of photosynthesis for sun plants that grow in this environment?

D) Suppose you transplant a sun plant to a shaded forest floor that receives about 100 photons/m2/s. Do you think this plant will grow and thrive? Why or why not? How does the graph help you answer this question?