Chapter 6: Photosynthesis

Section 6-1: Capturing the Energy in Light

Plants are energy producers Like animals, plants need energy to live

unlike animals, plants don’t need to eat food to make that energy

Plants make both FOOD & ENERGY animals are consumers plants are producers

Energy needs of life All life needs a constant input of energy

Heterotrophs (Animals) get their energy from “eating others”

eat food = other organisms = organic molecules extract energy through respiration

Autotrophs (Plants) produce their own energy (from “self”) convert energy of sunlight build organic molecules (CHO) from CO2 capture energy & synthesize sugars

through photosynthesis, then extract through it respiration

consumers

producers

Critical thinking: Discuss the validity of the following

statement: “All living things are directly or indirectly

dependent on photosynthesis.” almost organisms derive energy from

autotrophs, heterotrophs eat organisms that get energy from plants.

How does ATP transfer energy?

PO–

O–

O–O P

O–

O–

O–O P

O–

O–

O–O 7.3

energy+PO–

O–

O–O

ATP ADP releases energy

Fuel other reactions

ADPATP

How do plants make energy & food? Plants use the energy from the sun

to “charge up” ATP to make sugars

glucose, sucrose, cellulose, starch, & moresun

ATP

sugars

H2O

Building plants from sunlight & air Photosynthesis

2 separate processes ENERGY obtaining reactions

collect sun energy use it to make ATP

SUGAR building reactions take the ATP energy collect CO2 from air &

H2O from ground use all to build sugars

ATP

sun

sugars

+

carbon dioxideCO2

sugarsC6H12O6

CO2

waterH2O+

Parts of a leaf capturing light energy

Chloroplast – has a double membrane Thylakoids – flattened sac membrane Grana (granum- singular) – stacks of thylakoids Stroma – a solution that surrounds the grana

sun

Light https://www.youtube.com/watch?v=iLDbW_XvxHQ

Light from the sun appears white, but it is a combination of all the colors (ROY G BIV) or the visible spectrum

Light is measured in wavelengths. Smaller wavelengths (violet)

have more energy. Larger wavelengths (red) have

less energy.

Pigments Objects can reflect, transmit, or absorb light. Pigments are molecules that absorb light. Most

pigments absorb some colors more than others. The color a pigment appears to the eye is

actually the light being reflected Ex. A red shirt appears red

because it absorbs all the colors of light except red, which bounces back to the eye.

PIgmentsChlorophyll a(most abundant)

Chlorophyll b(accessory)

Carotenoid(accessory pigment)

Absorbs Violet – blue regionabsorbs 430nm (blue) and 662nm (red)

Red –Blue regionabsorbs 453nm to 642 nm wavelength

Absorbs between 450nm – 550nm

Reflects Green / yellow yellow Red, orange or yellow

Solubility Fat soluble Highly soluble(composed of carbonyl group)

Fat soluble

Function Absorbs light Absorbs light and Increase the range of light absorbs

Protects plants from free radicals formed from UV rays

Light: absorption spectra Photosynthesis gets energy by absorbing

wavelengths of light.

Why areplants green?

Photosystem Group of clusters of pigments and

enzymes inside the thylakoid

TYPES OF PHOTOSYSTEMS Photosystem 1 Photosystem 2*have similar pigments (chlorophyll a and

50% chlorophyll b) but different roles in light reaction

What do plants need to grow? The “factory” for making

energy & sugars chloroplast

Fuels sunlight carbon dioxide water

The Helpers enzymes H2O

sugars

ATP

Make ATP!Make sugar!

I can do it all…And no one

even notices!

enzymes

CO2

sun

Overview of light reaction:

Location: Thylakoid

source of energy: sunlight

Products: ATP, NADPH and Oxygen

STEPSANIMATION: https://www.youtube.com/watch?v=iLDbW_XvxHQ

STEP1 Light enters the thylakoid going to the photosystem

II Absorbed light energy excites electrons

Excited electrons moves higher to chlorophyll a

Excited electrons leaves chlorophyll reaction center

Step II Excited electrons move to primary electron

acceptor

Water Splitting Water splitting enzyme

splits water molecules into protons (H+), electrons and oxygen

Oxygen is released into the air

4 Electrons replaced the lost electrons in photosystem II

4 Protons are left in the thylakoid

Step 3 Electrons in the primary electron acceptor is transferred

to electron transport chain Energy from excited electrons are used to push protons

(4H+) photosystem I

Step 4 Light enters photosystem I and excites the electrons Excited electrons move higher to chlorophyll reaction

center and leaves the reaction center going to primary electron acceptor

Electrons moves to a different transport chain bringing the electrons to the side of thylakoid membrane facing the stroma

Step 5 Electrons combines with a proton causing NADP to be

reduced to NADPH (NADP + H = NADPH)

Chemiosmosis Moving electrons in the

transport chain builds a concentration gradient creating a potential energy

The concentration gradient powers up ATP synthase to make ATP (ADP+P = ATP) - Phosphorylation

ATP and NADPH are brought too the stroma for dark reaction phase.

Section Review

1. Explain why autotrophs and heterotrophs depend on photosynthesis to obtain the energy they need for life processes.

Autotrophs needs sunlight to manufacture their own food while heterotrophs eat autotrophs and other organisms to obtain energy

Section Review

2. Describe the role of chlorophylls in biochemical pathway of photosynthesis.

Chlorophyll an and b absorbs light energy that excites electrons making theses electrons. The energy of these excited electrons are used to form ATP and NADPH

Section Review

3. List the three substances produced when water molecules are broken down during the light reaction.

Hydrogen (proton), electrons and oxygen

Section Review

4. Explain why the splitting of water is important to the continuation of the light reactions

Provides the electrons needed to replace those that are replaced by chlorophyll a during the light reactions.

Section Review

5. Name the product of the process known as chemiosmosis.

ATP

Section Review

6. Thinking about the main role of pigments in photosynthesis, explain how the pigments in colored objects such as clothes differ from plants.Photosynthetic pigments do not only absorb light but are also associated with enzymes and membranes to harness ligt energy. The pigments in clothing simply absorbs and reflect light

Section Review

7. The molecule that precedes the electron transport chains of photosystem I and II is an electron acceptor. What is the original molecule that is the electron donor for both of these systems?

water

Section Review

8. Explain how the light reactions would be affected if there were no concentration gradient of protons across the thylakoid membraneWithout the concentration gradient, movement of H+ and ATP production will not be produced.

CALVIN CYCLEDark reaction phase

• A series of enzyme assisted-chemical reactions that produces 3 carbon sugar.

• C3 plants are plants that undergoes Calvin cycle

• This process happens in the stroma

Dr. Melvin Calvin American Biochemist for

whom the Calvin cycle is named

Used radioactive carbon(C14) to discover the path carbon takes in the process of photosynthesis

Receives the 1961 Nobel Prize in Chemistry

Died January 8, 1997 (85 years old)

Carbon Fixation Carbon dioxide (CO2) will be “fixed” before it

enters the Calvin cycle CO2 is bonded into organic compounds,

resulting into a 3 carbon molecule. That is why, plants undergoing the Calvin

cycle pathway are called C3 plants because they start with 3 carbon molecules and produce 3 carbon sugar

Calvin Cycle Steps

Step 1 CO2 enters the plant and goes inside the stroma

Step 1: carbon Fixation CO2 combines with 5 carbon molecules forming RuBP (Ribulose

biphosphate) which is a very unstable 6 carbon molecule

Step 1 6 carbon molecule (RuBP) splits into 3 carbon

molecule 3-PGA (3-phospoglycyrate)

Step 2: G3P production 3-PGA receives a phosphate from ATP molecule then receives a

Proton (Hydrogen) and released G3P (glyceraldehyde 3-phosphate)

Step 3: Carbohydrate /sugar production one G3P molecule leaves Calvin Cycle and used to

make carbohydrates /sugar

Step 4: Back to RuBP Remaining G3P receives a phosphate from ATP resulting

into RuBP again. RuBP enters the Calvin cycle to repeat the entire process

Table summary of Calvin CycleThe Calvin cycle needs Carbon Dioxide to work.

Starting materials needed

Carbon Acceptor

EnzymeFor carbon

fixation

ATP (ADP +P)

used

NADPH(NADP+H)

used

Product

Step 1 3 Carbon dioxide

3 RuBP Rubisco none none six 3-PGA

Step 2 Six 3-PGA none none 6 6 Six G3P

Step 3 Six G3P none none none none 1/2 glucose

Step 4 5G3P none none 3 none 3 RuBP

The table above represents 3 turns to make ½ glucose molecule. Then another 3 turns is needed to make 1 whole glucose molecule. Thus, a total of 18 ATP and 12 NADPH is used to make 1 glucose.

Alternative Pathway: C4 pathway– On hot dry climate, stomata of plants are

partially closed to reduce water loss– Stomata is the passage way for CO2 – Stomata is the outlet of water.

Alternative Pathway: C4 pathway– There is an enzyme that can fix CO2 into 4carbon compounds

even when the CO2 level is low and the O2 level is high (since light reaction is not affected so production of Oxygen is continuous).

C4 pathway steps

– CO2 will be fixed by en enzyme to form 4 carbon compounds.

– 4 carbon compounds will be released to go to Calvin cycle

Alternative Pathway: CAM pathwayCAM (Crassulacean Acid Metabolism)–Somata opens at night so CO2 enters–carbon fixation happens at night and the Calvin cycle during the day–Plants undergoing CAM pathway grow slower

Jade Plant undergoes the CAM pathway

Alternative Pathway: CAM pathwaySTEPS–Carbon dioxide enters the stomata at night time–Enzyme fixed the carbon dioxide producing 4 carbon molecules–Released the 4 carbon molecules to the Calvin cycle at daytime

Jade Plant undergoes the CAM pathway

Photosynthesis summary Light reactions

produced ATP produced NADPH consumed H2O produced O2 as byproduct

Calvin cycle consumed CO2 produced G3P (sugar) regenerated ADP regenerated NADP NADPADP

SUMMARY TABLELight dependent Light independent

Raw materials Sunlight and Water Carbon Dioxide, ATP, NADPH

Products OxygenNADPHATP

GlucoseWater

Site Chloroplast (each thylakoid - granum)

Chloroplast (stroma)

Time of Day Daytime Daytime

Over-all Equation of photosynthesis

FACTORS THAT AFFECT PHOTOSYNTHESIS

Light intensity CO2 level TemperaturePhotosynthesis is affected by surrounding

environment

Light Intensity As light intensity

increases, the photosynthetic rate increases until a point is reached where the rate begins to level off.

At low light intensity, photosynthesis occurs slowly because only a small quantity of ATP and NADPH is created by the light dependent reactions.

Carbon dioxide level

As carbon dioxide concentration increases, the rate of photosynthesis increases.

At high concentrations, the rate of photosynthesis begins to level out due to factors not related to carbon dioxide concentration.

One reason might be that some of the enzymes of photosynthesis are working at their maximum rate.

Temperature

The light independent reactions of photosynthesis are dependent on temperature.

They are reactions catalyzed by enzymes.

As the enzymes approach their optimum temperatures the overall rate increases.

Rate of photosynthesis Three factors can limit the speed of

photosynthesis: light intensity, carbon dioxide concentration and temperature.

Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and carbon dioxide.

Increasing the light intensity will boost the speed of photosynthesis.

Review Questions

Q1) Name the part of the chloroplast where the Calvin cycle takes place.

The Calvin cycle occurs in the stroma of the chloroplast.

Q2) Describe what can happen to the three-carbon molecules made in the Calvin cycle.

The calvin cycle produces a compound called G3P. Most G3P are converted into RuBP to keep the Calvin cycle operating. However, some G3P

molecules are used to make other organic compounds, including amino acids, lipids, and

carbohydrates.

Q3) Distinguish between C3, C4, and CAM plants.

C3, plants are those organisms that can fix carbon into 3 carbon molecule. C4 are those that fix carbon

into a 4 carbon molecule because the stomata is partially closed. The CAM plants fix carbon at night

since stomata is closed at day time.

Q4) Explain why the light reactions and the Calvin cycle are dependent on

each other.

Light reaction produces the NADPH and ATP needed in Calvin cycle. Without Light reaction,

Calvin Cycle cannot occur

Q5) Explain why increased light intensity might not result in an increased rate of photosynthesis.

At some point, all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate stays level regardless of further increase in light intensity.

Q6) What would happen to photosynthesis if all of the three-carbon sugars produced in the Calvin cycle were used to make organic compounds?

The calvin cycle will stop operating, because if all G3P molecules were used to make sugars, no G3P

to be converted into RuBp.

Q7Explain how a global temperature increase could affect plants.

Below a certain temperature, the rate of photosynthesis increases as the temperature increases. Above that temperature, the rate of

photosynthesis decreases as temperature increases.

Q8) Explain how the world would be different if C4 plants and CAM plants had not evolved.

If C4 plants and CAM plants had not evolved, there would be no plant species that able to withstand in

hot and dry climates.

Seasons and PhotosynthesisSummer Autumn Winter

Rate of Photosynthesis

High low Very low

Photosynthesis happens in bulbs /tubers in stems

Some plants still use leaves needle trees, pine and broad leaf plants

Source of light high low Very low

Why leaves of trees fall off during autumn?

Shedding leaves helps trees to conserve water and energy.

It is a strategy to survive cold or dry weather.

Trees that lose all of their leaves for part of the year are known as deciduous trees. 

Ex. Maple and oak trees

Why leaves of plants changed colors during autumn and winter? These seasons don’t give plants

enough water and sunlight to photosynthesize

the trees will rest, and live off the food they stored during the summer.

They begin to shut down their food-making factories so the green chlorophyll disappears from the leaves, what will be left are the other accessory pigments with red or orange color.

What is the effect of herbicides on photosynthesis? The higher the amount of herbicides,

the lesser is the arte of photosynthesis Herbicides blocks electron transport

chain in photosystem 2.