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PHOTOSYNTHESISIntroduction
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 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
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!
The CARBON CYCLE
Essential Energy Transformations
1. Photosynthesis = light energy from the Sun is used to transform carbon dioxide and water into energy-rich food molecules.
CO2 + H2O C6H12O6 + O2Carbon
Dioxide Water
Light Energy
Glucose Oxygen
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 + 6CO2Carbon Dioxide
WaterGlucose Oxygen
ATPADP
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 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 Overall
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?
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 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 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
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
Chlorophyll 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 Very small – 40 chloroplasts in 1mm. Very powerful - perform hundreds of
reactions in just 1 second. Has a double membrane.
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 Structure In and around the grana is a watery
substance called STROMA The chloroplast also contains lots of
ENZYMES.
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 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
Chlorophyll pigments are packed into clusters called PHOTOSYSTEMS
Photosytems funnel absorbed energy to the REACTION CENTER
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.
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.
NADPH
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
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
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
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
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?
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.
Photosynthesis Overall
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
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
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
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
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
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 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 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.
