Chapter 8 Section 2 - Photosynthesis

Photosynthesis

Light energy is trapped and converted into chemical energy during photosynthesis.

Overview of Photosynthesis Purpose of Photosynthesis

To convert light energy into an energy that organisms can use.

Cellular Energy

Overview of Photosynthesis Purpose of Photosynthesis

Plants make sugars through photosynthesis, an energy that all life on earth can use.

Protists such as algae and certain bacteria can use photosynthesis as well.

Without photosynthetic organisms, there would be no life on earth!

Cellular Energy

Photosynthesis The Reactants of Photosynthesis are:

Light – from the sun Carbon Dioxide (CO2) – from the air Water (H2O) – from the roots

The Products of Photosynthesis are: Glucose (C6H12O6) - sugar Oxygen (O2)

Cellular Energy

Overview of Photosynthesis

Photosynthesis occurs in two phases:

Cellular Energy

Light-dependent reactions (Light reactions)

Light-independent reactions (sometimes called dark reactions or Calvin cycle)

Overview of Photosynthesis

Occurs in the chloroplasts of plant cells.

Cellular EnergyPhotosynthesisPhase One: Light Reactions

Takes place in the thylakoids.

Chloroplasts Chloroplasts are the

organelles that capture light energy. Disk-shaped

Cellular EnergyPhotosynthesisPhase One: Light Reactions

Chloroplasts 2 compartments:

Thylakoid: membranes that contain stacks called grana; where light reactions take place.

Photosystems I and photosystems II contain light-absorbing pigments and protein complexes important in the light reactions.

Stroma: Fluid-filled space outside grana. Where light-independent reactions take place.

Cellular EnergyPhotosynthesisPhase One: Light Reactions

Pigments• Light absorbing molecules found in thylakoid

membranes.

• Different pigments absorb different wavelengths of light and reflect others.

• Chlorophylls are the major light-absorbing pigments in plants.

• Chlorophylls absorb most in blue-violet region of visible light spectrum and reflect light in green spectrum.

Chloroplast Pigments

• Chloroplasts contain several pigments:

– Chlorophyll a – Chlorophyll b – Carotenoids

Cellular Energy

PhotosynthesisBasics of the Light Reactions

The absorption of light is the first step of photosynthesis.

When light strikes a thylakoid in a chloroplast, energy is transferred to and excites electrons in chlorophyll.

https://www.youtube.com/watch?v=YeD9idmcX0w

Cellular Energy

PhotosynthesisBasics of the Light Reactions

Energized electrons are used to break down (split) water molecules.

o Oxygen is released through openings on the underside of the leaf called stomata.

o Activated electrons are passed along a series of molecules along the thylakoid membrane, the electron-transport chain. This generates energy in the form of NADPH.

o Energy produced from protons (H+) is used to synthesize ATP.

Section 2

Cellular Energy

PhotosynthesisBasics of the Light Reactions

Energy stored is stored temporarily in energy carrying molecules, ATP and NADPH that will be used in the Calvin cycle (light-independent reaction).

https://www.youtube.com/watch?v=hj_WKgnL6MI

https://www.youtube.com/watch?v=joZ1EsA5_NY

Review: ATP CYCLE• Occurs continuously in cells • About 10 million new ATP molecules are made in every

cell every second!!!

ADP

1. Stored EnergyEnergy stored in chemical bonds.

2. Releasing EnergyEnergy released by breaking bonds – used to power cells

3. Energy DepletedADP has less chemical energy than ATP

4. Making ATP

Energy released by other chemical reactions and processes can be used to bond a phosphate to ADP to make ATP

+ P

A series of enzyme-assisted reactions in which chemical energy stored in ATP and NADPH is used to synthesize carbohydrates such as glucose.

Phase Two: The Calvin Cycle (Light-Independent Reactions)

Does not require light. Occurs in the stroma of chloroplast. Requires CO2.

Uses ATP and NADPH as fuel to run. Makes glucose sugar from CO2 and hydrogen.

Plants use sugars formed during the Calvin cycle as a source of energy and to form starch, which stores energy, and cellulose for structure of cell wall.

Phase Two: The Calvin Cycle (Light-Independent Reactions)

Chloroplast

Light

Stack ofthylakoids ADP

+ P

NADP

Stroma

Lightreactions

Calvincycle

Sugar used for

Cellular respiration Cellulose

Starch

Other organic compounds

https://www.youtube.com/watch?v=E_XQR800AgM

Cellular EnergyPhotosynthesis

Factors That Affect The Rate Of Photosynthesis

Light intensity If you increase the amount of light, then

photosynthesis will increase up to a point.

Temperature

CO2 concentrations Water

Cellular Energy

Alternative Pathways

Many plants in extreme climates have alternative pathways for photosynthesis.

C4 plants In hot, dry environments where light

energy is plentiful but water is scarce.

SugarcaneCornSome grasses

Cellular EnergyAlternative Pathways

CAM plants

Water conserving plants.

Open their stomata at night.

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