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PHOTOSYNTHESIS - PowerPoint PPT Presentation

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PHOTOSYNTHESIS. Introduction. Cells, Matter, and Energy. ALL cells need energy and matter for growth and reproduction. Some organisms (like plants) obtain their energy directly from the Sun. Other organisms must consume food to obtain energy. Photosynthesis.

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Cells matter and energy
Cells, Matter, and Energy

  • ALL cells need energy and matter for growth and reproduction.

  • Some organisms (like plants) obtain their energy directly from the Sun.

  • Other organisms must consume food to obtain energy.


  • Autotrophs: self feeders, organisms capable of making their own food

  • Photoautotrophs: use photosynthesis = makes organic compounds (glucose) from light. Converts sun energy into chemical energy usable by cells.

  • Chemoautotrophs: use chemosynthesis = makes organic compounds using energy from the oxidation of inorganic chemicals, such as sulfur released from deep hydrothermal vents.

Thinking about energy
Thinking About Energy

  • Energy = Capacity to do work

  • Potential Energy = Stored energy (the energy must be released for it to do any work e.g. apple hanging by a stem)

  • Kinetic Energy = The energy of motion (apple falling to the ground)

  • Chemical Energy = Energy stored in the bonds of molecules. Type of potential energy. Once the chemical bonds are broken, the atoms have extra kinetic energy. The atoms can move, do work, make things happen!

Essential energy transformations
Essential Energy Transformations

  • Photosynthesis = light energy from the Sun is used to transform carbon dioxide and water into energy-rich food molecules.

    CO2 + H2O C6H12O6 +O2

Light Energy

Carbon Dioxide




Essential energy transformations1
Essential Energy Transformations

2. Cellular Respiration = all of the chemical reactions needed to break down (metabolize) carbohydrates and other molecules to transfer chemical energy to ATP.

C6H12O6 + 6O2 6H2O + 6CO2 + ATP

Light Energy

Energy Storage Molecule

Carbon Dioxide




What is photosynthesis
What is Photosynthesis?

  • Involves over 100 chemical reactions.

  • The overall process happens in two main stages:

    • 1. PHOTO stage: light dependent

    • 2. SYNTHESIS stage: light independent

Photo stage
Photo stage

  • Splits water and produces ATP.

  • Photosystem reactions need light energy.

  • Stores chemical energy in the bonds of glucose.

  • Synthesis reactions need chemical energy (ATP) and H+ from photo stage.

Synthesis stage

Photosynthesis overall1
Photosynthesis Overall

  • Carbon dioxide and water plus light energy are the raw materials of photosynthesis.

  • Enzymes and chlorophyll are accessories that are needed to make photosynthesis occur

What is light
What is light?

  • Visible and Invisible radiation from the Sun and other sources of radiant energy.

    • Radiowaves, microwaves, x-rays, etc

  • Visible radiation is usually

    simply called LIGHT.

Electromagnetic spectrum
Electromagnetic Spectrum

  • All forms of electromagnetic radiation travel at 300 000 000 m/s

  • Different frequency of light results in different wavelengths, which are perceived as different colours.

  • The highest frequency of light is violet and the lowest frequency is seen as red.

  • A combination of all of the frequencies is interpreted as White light.

Photon model of light
Photon Model of Light

  • Light travels through space in the form of individual energy “packets” called photons.

  • The amount of energy in a photon depends on the frequency of light. The higher the frequency the more energy the photon is able to deliver.

    • More energy in a photon of violet than in red.

The chemistry of pigments
The Chemistry of Pigments

  • To use the energy of light for photosynthesis, a plant must absorb photons of light.

  • Molecules that absorb light are called Pigments.

  • Most plant leaves contain chlorophyll pigments which give leaves their green colour.

  • Absorption is only one of three possible outcomes when light strikes a surface.

    • The other two are reflection and transmission


  • Photosynthesis takes place in chloroplasts

  • Chloroplasts contain light absorbing pigment molecules (chlorophyll a & b)

  • Chlorophyll absorbs red, violet, and shades of blue.

  • The chlorophyll passes the energy onto other molecules that can be used by the synthesis reactions.

Chloroplast structure
Chloroplast Structure

  • Very small – 40 chloroplasts in 1mm.

  • Very powerful - perform hundreds of reactions in just 1 second.

  • Has a double membrane.

Chloroplast structure1
Chloroplast Structure

  • Folded THYLAKOID membranes form stacks known as GRANA. The folding increases the surface area for reactions to occur.

  • Inside the thylakoid is a space called the LUMEN

Chloroplast structure2
Chloroplast Structure

  • In and around the grana is a watery substance called STROMA

  • The chloroplast also contains lots of ENZYMES.

Light dependant reactions
Light Dependant Reactions

  • Light energy is used to split water molecules (photolysis) to form oxygen and hydrogen

    • Oxygen atoms (O2) are released into the atmosphere

    • Hydrogen atoms added to NADP to make NADPH+

Light dependant reactions1
Light Dependant Reactions

  • Oxygen molecules pass out the chloroplast membrane into the cell’s cytoplasm.

  • Most of the oxygen that is produced is waste product.

  • The plant’s own cells use some of the oxygen to carry out cellular respiration.

Photosystem light dependant reactions
Photosystem – Light Dependant Reactions

  • Chlorophyll pigments are packed into clusters called PHOTOSYSTEMS

  • Photosytems funnel absorbed energy to the REACTION CENTER

Photosystem light dependant reactions1
Photosystem – Light Dependant Reactions

  • Excited electrons are passed from the primary electron acceptor to ELECTRON TRANSPORT CHAINS

  • The electrons “fall” to a lower energy state, releasing energy that is harnessed to make ATP.


  • Adenosine triphosphate

  • One molecule of ATP contains three phosphate groups

  • When removing the third phosphate group, lots of energy given off

  • An EXCELLENT molecule for shuttling energy around within cells.


  • Nicotinamide adenine dinucleotide phosphate

  • NADPH is the reduced form of NADP+.

    • Reduction is the gain of electrons by a molecule, atom, or ion

Light independant reactions calvin cycle
Light Independant Reactions – Calvin Cycle

  • Does not require sunlight

  • Requires 18 ATP's, 12 NADPH's, and CO2 to produce glucose

  • Uses the products from the Light Reaction

  • Occurs in the STROMA of the chloroplast

  • Three phases of the Dark Reaction

    • Carbon Fixation

    • Reduction

    • Regeneration

Light independant reactions
Light Independant Reactions

  • Carbon fixation is a process which involves the conversion of carbon in a gas to carbon in solid compounds.

  • In order for carbon fixation to occur, energy in the form of ATP and hydrogen (from photolysis) are needed.

  • The carbon can be used to make organic compounds.

Phase 1 carbon fixation
Phase 1 - Carbon Fixation

The carbon of a CO2 molecule from the atmosphere is attached to a 5-carbon sugar called RuBP

This forms an unstable 6-carbon compound

The 6-carbon compound breaks down to form two 3-carbon molecules called PGAL(phosphoglyceraldehyde)

Think of PGAL as half a glucose

Phase 2 reduction
Phase 2 - Reduction

The 3 PGAL are converted to G3P using energy (ATP) and hydrogens from NADPH from the Light Reaction

For every 3 molecules of CO2 there are 6 molecules of G3P produced

Only 1 is net gain

What happens to the other 5?


Phase 3 regeneration
Phase 3 - Regeneration

Products need to be regenerated to keep the cycle going.

5 of the 6 G3P molecules are regenerated using ATP and producing 3 RuBP molecules which are then ready to receive new CO2 and continue the cycle

The one G3P molecule combined with another G3P molecule is used to make glucose, fructose, sucrose, starch and cellulose for the plant.

Rate of photosynthesis

Describes how much sugar a plant can produce over time

It describes how productive a plant is under various conditions

What things would control the rate of photosynthesis?

Rate of Photosynthesis

Rate of photosynthesis1

1. Light Intensity:

High Intensity Light causes the rate of photosynthesis to increase

The rate will increase until it reaches its saturation point

At the saturation point, the rate of photosynthesis remains constant

Rate of Photosynthesis

Rate of photosynthesis2

2. Temperature:

As temperature increases, so does the rate of photosynthesis

Enzymes function at an optimal temperature: If the temperature is too high or too low, enzymes will not function properly

Rate of photosynthesis will slow down or stop.

Rate of Photosynthesis

Rate of photosynthesis3

3. Water:

Water is one of the raw materials of photosynthesis

A shortage of water can slow or even stop photosynthesis

Water stress causes stomata to close, preventing CO2 from entering the leaf

Rate of Photosynthesis

Rate of photosynthesis4

4. Carbon Dioxide:

An increase in CO2 concentration causes the rate of photosynthesis to increase

More CO2 available means more sugar made in the light independent reaction

Rate of Photosynthesis

Plants adaptations
Plants Adaptations

In hot, dry environments plants maximize photosynthesis by limiting water loss.

Leaves of plants contain stomata which are tiny holes in the leaves that release by products and take in raw materials need for photosynthesis

Plants adaptations1
Plants Adaptations

Most plants will close their stomata to prevent water loss but this limits carbon dioxide intake

Some plants will only open the stomata during night

It is a fine balance between receiving the necessary supplies and preventing water loss.

Plants adaptations2
Plants Adaptations

If the CO2 concentration in the cell drops below 50 ppm, the cell begins to undergo PHOTORESPIRATION which results in the fixation of oxygen instead of carbon dioxide.

This is a very wasteful process as it produces a substance that is not useful to the cycle.