PHOTOSYNTHESIS

Using light to make food

PHOTOSYNTHESIS• Autotrophs are the producers of the biosphere

– Plants are autotrophs• Producing their own food and sustaining themselves

without eating other organisms• What are we? • We are heterotrophs

– Plants, algae, and some bacteria are photoautotrophs

• Producers of food consumed by virtually all organisms

Figure 7.1A–D

• PLANTS USE ENERGY FROM THE SUN TO TRANSFORM CO2 AND WATER INTO ENERGY RICH CARBOHYDRATES(glucose).

• Plants are AUTOTROPHS :make their own food

• Plants supply all the energy (food) for life on earth and the oxygen that most

organisms need

How old is photosynthesis? How important is it?

• Evidence of photosynthesis exists in rocks 3.5 billion years old.

• Photosynthesis is the largest biochemical process on earth.

Importance?

• Photosynthesis supplies OXYGEN to earth atmosphere and FOOD to all organisms

Photosynthesis is the process by which certain organisms use light energy to make sugar and oxygen gas from carbon dioxide and water.

Lightenergy

PHOTOSYNTHESIS

6 CO2 6+ H2O

Carbon dioxide Water

C6H12O6 6+ O2

Glucose Oxygen gas

Where in the plant does photosynthesis occur?

In the CHLOROPLAST. The leaf is a food factory. The light trapping pigments are inside the chloroplasts embedded in the thylakoid membranes.

• STOMATA or STOMA are openings on the leaf surface through which CO2 and O2 as well as water vapor go in and out of leaves.

Where does it occur?

Photosynthesis occurs in chloroplasts• Occurs primarily in the leaves, in the chloroplasts, which

contain stroma, and stacks of thylakoids called grana

Figure 7.2

Leaf Cross Section

Leaf

Mesophyll Cell

Mesophyll

VeinStoma

CO2O2

Chloroplast

Chloroplast

Grana Stroma

TE

M 9

,750

Stroma

Granum Thylakoid Thylakoidspace

Outermembrane

Innermembrane

Intermembranespace

LM 2

,600

ENERGY-CONVERTING ORGANELLES

Chloroplasts conver t solar energy to chemical energy (in sugars).

. Found in plants and some protists. This is where PHOTOSYNTHESIS takes place

TE

M 9

,750

Chloroplast

Stroma

Intermembranespace

Inner and outermembranes

Granum

Sites to check:• http://www.mhhe.com/biosci/esp/2001_gbio/

folder_structure/ce/m6/s1/index.htm

• http://www.tvdsb.on.ca/westmin/science/Biology12/Metabolic%20Processes/stoma.htm

• http://www.tvdsb.on.ca/westmin/science/Biology12/Metabolic%20Processes/chloro.htm

• http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m6/s3/index.htm

ENERGY-CONVERTING ORGANELLES

Chloroplasts conver t solar energy to chemical energy (in sugars).

. Found in plants and some protists. This is where PHOTOSYNTHESIS takes place

TE

M 9

,750

Chloroplast

Stroma

Intermembranespace

Inner and outermembranes

Granum

Thylakoids• Thylakoids are membranes that look like

green flattened sacs stacked upon each other.

• Granum is a stack of these sacs

• The STROMA is the fluid between the membranes.

Plants produce O2 gas by splitting water

– The O2 liberated by photosynthesis

• Is made from the oxygen in water

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

Labeled

Experiment 1

Experiment 2

6 CO2 12 H2O

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

Notlabeled

C6H12O6 6 H2O 6 O2

+

+

+

+

+

+

Figure 7.3A–C

•Photosynthesis is a redox process, as is cellular respiration

– In photosynthesis

• H2O is oxidized and CO2 is reduced

Figure 7.4A, B Reduction

Oxidation

6 O2 6 H2O

Reduction

Oxidation

6 O26 CO2 6 H2O C6H12O6

C6H12O6 6 CO2

PHOTONS AND PIGMENTS

• PHOTONS are packets of light energy

• Light: electromagnetic energy waves that travel in waves of different lengths

• PIGMENTS are molecules that absorb light energy (photons)

• CHLOROPHYLL is the main pigment in plants. Absorbs the blue and red portions of the visible light spectrum and scatters or reflects back the green wave length. This is why plants look green.

• CAROTENOIDS reflect the reds and yellows

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

– wavelengths of visible light (red,blue), absorbed by pigments drive the light reactions of photosynthesis

Figure 7.6A, B

Increasing energy

10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m

Gammarays

X-rays UV Infrared Micro-waves

Radiowaves

Visible light

400 500 600 700 750

650nm

Wavelength (nm)

Transmittedlight

Absorbedlight

Reflectedlight

Light

Chloroplast

380

• Overview: Photosynthesis occurs in two stages linked by ATP and NADPH

– The complete process of photosynthesis consists of two linked sets of reactions

• The Light Dependent Reactions and• The Calvin Cycle (discovered by Calvin and

Benson)

Photosynthesis:

H2O + CO2 O2 + C6H12O6 + H2O water carbon light oxygen glucose water vapor

dioxide energy

STEPS OF PHOTOSYNTHESIS

• capture sun light

• use light energy to make ATP

• use ATP to make carbohydrate molecules from CO2(carbon dioxide) and H2O (water)

LIGHT DEPENDENT REACTIONS• is the first stage of photosynthesis.

• It takes place in the thylakoid membranes.

• Oxygen is released, Water molecules are split. Oxygen gas diffuses out

• ATP and electrons are carried to the next stage. Electrons are used to make ATP.

Light Dependent Reactions (In thylakoids of chloroplasts)

• This is where light energy is converted to a form that can be used by plants to build Carbon compounds (sugars)

• 1.The energy of sunlight hits the photosystems (cluster of pigment molecules) and excites chlorophyll (pigment) electrons to a higher energy state. Light energy is absorbed by the photosystems

• 2. Water molecules split, oxygen is released.• Light energy is converted to chemical energy

(ATP formation)• NADPH electron carrier take electrons from

chlorophyll to the Calvin Cycle.

– The light reactions• Convert light energy to chemical energy and produce O2

– The Calvin cycle assembles sugar molecules from CO2

• Using ATP and NADPH from the light reactions

Figure 7.5

Light

CO2H2OChloroplast

LIGHTREACTIONS(in thylakoids)

CALVINCYCLE

(in stroma)

NADP+

ADP+ P

ATP

NADPH

O Sugar

Electrons

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS

•ATP and NADPH power sugar synthesis in the Calvin cycle

– The Calvin cycle occurs in the chloroplast’s stroma• Consists of carbon fixation, reduction, release of G3P,

(glyceraldehyde 3-phosphate)and regeneration of RuBP

Figure 7.10A

InputCO2

ATPNADPH

CALVINCYCLE

G3POutput:

LIGHT INDEPENDENT REACTIONS-CALVIN CYCLE

• (In stroma of chloroplasts ) This is a cyclic pathway where the final products are also the first reactants on the cycle

• The Calvin Cycle uses the energy in ATP and NADPH to synthesize carbohydrates.

• Carbon enters the cycle as a molecule of CO2

LIGHT INDEPENDENT REACTION or CALVIN CYCLE

Takes place in the stroma of the chloroplasts.

• This is the “synthesis” part of photosynthesis. Making food (glucose) and trapping CO2 to incorporate carbon into living things.

CALVIN CYCLE- making glucose

• The energy from ATP and NADPH+ go into the bonds of a glucose molecule.

• Electrons ( hydrogens) from the carrier molecule are put together with CO2 to make glucose.

• An enzyme called RUBISCO fixes the CO2 ( from the air) by bringing together the CO2 and the sugar.

LIGHT INDEPENDENT REACTIONS-CALVIN CYCLE

• Fixes Carbon (from CO2) using the enzyme rubisco. This enzyme catalizes the binding of carbon from CO2 to RuBP ( ribulose bisphosphate). This is how all carbon enters the world of life.

• Put together sugars using ATP and NADPH as energy sources.

• The final product is a three carbon sugar called G3P (glyceraldehyde 3-phosphate)

• G3P is a sugar phosphate that can be modified to form glucose

PHOTOSYNTHESIS REVIEWED

Review: Photosynthesis uses light energy to make food molecules

Figure 7.11

Light

H2O CO2

NADP+

Photosystem II

Photosystem I

Electrontransportchains

ADPP+

RUBP

CALVINCYCLE

(in stroma)3-PGA

Stroma

G3PNADPH

ATP

O2

LIGHT REACTIONS CALVIN CYCLE

Sugars

Cellular respiration

Cellulose

Starch

Other organiccompounds

Thylakoidmembranes

Chloroplast

C3 PLANTS

• Plants that use CO2 directly from the air are called C3 plants. Soybeans, oats wheat and rice are C3 plants.

• In hot dry weather these plants close their stomata ( pores in underside of leaves) to reduce water loss. Since no CO2 can enter, the rate of photosynthesis is reduced and your crop productivity is poor.

In C3 plants a drop in CO2 and rise in O2 when stomata close on hot dry day divert the Calvin cycle to photorespiration

( Rubisco can bind oxygen in place of CO2 as CO2 becomes scarce. When Oxygen enters the Calvin cycle instead of CO2 so it cannot produce sugars )

C4 and CAM plants have special adaptations that save water

CAM PLANTS

• CAM PLANTS• In hot and very dry climates. (deserts) • Examples: pineapple, cactus, orchids all the

“succulent plants such as Aloe and Jade plants

• When stoma opens to get CO2 the water can get out. Survival depends on water retention.

• Adaptation: Stoma is closed during the day and open at night. Co2 is taken in at night and banked until the next day when it is given to the Calvin cycle to make carbohydrates.

• The CO2 taken in at night stays “banked” until the sun’s energy comes in the next day.

– CAM plants open their stomata at night• Making a four-carbon compound used as a CO2

source during the day

CO2

Figure 7.12 (right half) CAM plant

Day

CALVINCYCLE

3-C sugar

CO2

4-C compound

Night

Pineapple

CO2

C4 PLANTS• Have special adaptations to save water without shutting

down photosynthesis• In hot climates. • Examples: grasses , corn, sugarcane• In hot climates the stoma closes to keep water in. It also

keeps CO2 out and oxygen builds up inside the leaves. Since there is not enough CO2 the enzyme rubisco (that normally binds CO2) binds oxygen and the plants would not grow well.

• Adaptation : It continues to make sugars with closed stomata.

How? These plants use a different enzyme to bind CO2.This enzyme fix carbon twice to produce a 4 carbon compound which can then donate the C to the Calvin cycle

– C4 plants first fix CO2 into a four-carbon compound

• That provides CO2 to the Calvin cycle

Figure 7.12 (left half)

Sugarcane

C4 plant

CALVINCYCLE

3-C sugar

CO2

4-C compound

CO2Mesophyll cell

Bundle-sheath cell

ENVIRONMENTAL IMPACT OF PHOTOSYNTHESIS

• Food. Photosynthesis is the source of all the food that gives us energy ( from the sun)

• Oxygen production

• CO2 is the gas plants use to make sugars.

CO2 in the atmosphere retains heat from the sun that would otherwise radiate back into space

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE CONNECTION

•Photosynthesis moderates global warming– Greenhouses used to grow plants trap solar

radiation, raising the temperature inside

Figure 7.13A

Global warming and the Greenhouse effect

• Global warming is the slow and steady rise in Earth surface temperatures.

• Warming is caused by CO2 and other greenhouse gases.

• It is called the green house effect because CO2 traps heat and keeps it warm near the earth surface.

• When this occurs in moderation it is a good thing, otherwise the planet would be about 10 degrees colder all the time. The trouble is that we get overheating.

What are greenhouse gases?

• All the cars and industries that burn fossil fuel produce so much CO2 that we now have 30% more than ever before. This causes global warming.

• Some greenhouse gases are carbon dioxide CO2, methane CH4, water vapor and others

• http://www.net.org/globalwarming/sea_level/

Web sites to check:• http://www.fw.vt.edu/dendro/forestbiology/

photosynthesis.swf

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html

• http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/harvestinglight.swf