Photosynthesis

Energy is the ability to do work

• Living things depend on energy.• Organisms that make their own food =

autotrophs• Plants and some other organisms are able to

use light energy from the sun to make food• Organisms that cannot make their own food =

heterotrophs

Chemical Energy

• Adenosine triphosphate (ATP) – chemical compound used to store and release energy

ATP

• ATP consists of adenine (A), ribose (a 5-carbon sugar, remember RNA?), and three phosphate groups.

• The 3 phosphate groups hold high energy bonds

ADP

• ADP is adenosine diphosphate – meaning it has two phosphate groups instead of three

• Cells can add a phosphate group to ADP to make ATP to store energy. – ATP is like a full battery. ADP is like a battery that

needs recharging

Releasing energy

• The energy stored in ATP is used by breaking the chemical bonds between the second and third phosphate groups.

Why ATP is important• Powers cell membrane sodium-potassium

pump – vital for nervous system connections• Powers movement of organelles• Powers synthesis of proteins and nucleic acids• Powers responses to chemical signals– Firefly light (chemical = luciferin) is powered by

ATP in a process called bioluminescence

Firefly luciferin

Photosynthesis

Photosynthesis – plants use the energy of sunlight to convert water and carbon dioxide into high energy carbohydrates (sugar and starch) and oxygen (as waste)

Investigating Photosynthesis

• Where does the mass of a tree come from?• Trees start as seedlings and grow into trees– Metasequoia (Dawn Redwood)

Jan van Helmont’s Experiment

• 1643 – Belgian physician – “Where does mass of tree come from?”

• Watered plant for five years• Seedling grew into tree – over 75 kg (~200 lb)• Mass of soil didn’t change over five years• Concluded mass was from water• Water = “hydrate” part of carbohydrate, what

about “carbo-”?

Joseph Priestley’s Experiment

• 1771 – Joseph Priestley, English minister • Lit candle, placed jar over candle, watched

candle go out.• Concluded oxygen was needed for fire• Placed sprig of mint plant in jar• After a few days, the candle could be relighted

and would stay lit for a while• Concluded plants produce oxygen

Jan Ingenhousz’ Experiment

• 1779 – Ingenhousz, Dutch scientist• Found that plants only produce oxygen when

exposed to light• Concludes plants need sunlight to produce

oxygen

The Photosynthesis Equation

• Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen

Carbon dioxide + Water + Light Sugar + Oxygen6CO2 + H2O + Light C6H12O6 + O2

Light and Pigments

• In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll, which is a molecule in chloroplasts, an organelle in plant cells.

Light and Pigments

• Energy from the sun travels to Earth in the form of light.

• Light is of various wavelengths.

• Your eyes see the wavelengths as different colors.

Light and Pigments

• Plants gather the sun’s energy with light absorbing molecules called pigments.

• Main pigment is chlorophyll – two types chlorophyll a and chlorophyll b.

Light and Pigments

• Chlorophyll readily absorbs light in the red and blue-violet regions of the spectrum.

• Chlorophyll does not absorb light well in the green region – thus the leaves of plants appear green.

• Because light is energy, the plants are absorbing the energy from that light.

Inside a Chloroplast

• Photosynthesis takes place inside chloroplasts• Contain saclike photosynthetic membranes

called thylakoids.• Thylakoids are arranged in stacks called grana.

Inside a Chloroplast

• Chlorophyll and other pigments in the thylakoid membrane are organized into photosystems.

• The region outside the thylakoid membrane is called the stroma.

• Two reactions in photosynthesis:– Light-dependent: inside thylakoid membrane– Calvin cycle (Light-independent/dark): in stroma

Chloroplast

Electron Carriers• When sunlight strikes a leaf, it ‘excites’

electrons, meaning that they gain energy.• Plant cells use electron-carriers to move these

high energy electrons (think of hot coals) – process called electron transport.

Electron Carriers

• NADP+ (Nicotinamide adenine dinucleotide phosphate) is an electron carrier.

• NADP+ accepts and holds 2 high-energy electrons along with a hydrogen ion (H+)

• When it is holding these, it is converted to NADPH. Now it can carry these electrons to chemical reactions elsewhere in cell

Light-Dependent Reactions• Light-dependent reactions require light.• This is why plants need light to grow.• The light-dependent reactions of plant cells

produce oxygen as a gas and convert ADP and NADP+ into the energy carriers ATP and NADPH

Light-Dependent Reactions

• Step 1: Pigments in photosystem II (discovered second, hence name) absorb light– High energy electrons passed on to electron transport chain

(ETC)– Thylakoids break up water molecules to use 2 H+ ions and

oxygen is released – source of O2 we breathe

• Step 2: Electrons move through ETC from photosystem II to photosystem I– H+ ions moved from stroma to inner thylakoid

• Step 3: Pigments in photosystem I use energy from light to energize electrons. NADP+ picks these up and H+ ions to become NAPH

Light-Dependent Reactions (Cont.)

• Step 4: As electron pass from chlorophyll to NAP+, H+ ions are pumped across membrane, providing energy to make ATP.

• Step 5: An enzyme, ATP Synthase, helps H+ ions cross the membrane– ATP Synthase binds ADP and a phosphate group to

produce ATP

Confused yet?

The Calvin Cycle (Light-Independent Reactions)

• The ATP and NADPH formed by the light-dependent reactions have chemical energy but not enough to sustain plant.

• During the Calvin Cycle, plants use that energy to make energy that can be stored = high-energy sugars

Calvin Cycle (Light-Independent Reactions)

• Step 1: 6 Carbon dioxide molecules enter cycle from atmosphere. Combine with six 5-carbon molecules = 12 3-carbon molecules

• Step 2: 12 3-carbon molecules converted into high-energy forms. Energy to do this comes from ATP and NAPH produced earlier

• Step 3: Two 3-carbon molecules removed from cycles to produce sugars for metabolism and growth in the form of sugars, lipids, and proteins.

• Step 4: Remaining carbon molecules are recycled for later Calvin Cycles.

Factors affecting Photosynthesis

• Water– Plant adaptation –

plants in dry conditions have a waxy coating on leaves to prevent water loss

• Temperature• Intensity of Light – Maximum rate– Conifers in Winter may

only occasionally carry out photosynthesis