photosynthesis ib topic 3.8 (page 83). what is photosynthesis? photosynthesis is the production of...
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PhotosynthesisIB topic 3.8 (page 83)
What is photosynthesis? Photosynthesis is the production
of organic compounds (carbohydrates, lipids, and proteins) using light energy and simple inorganic substances such as carbon dioxide and water
It is an example of energy conversion◦Light energy is converted into
chemical energy
First organisms Prokaryotes were the first
organisms to perform photosynthesis◦About 3500 million years ago
They were joined billions of years later by algae and plants
One consequence of photosynthesis is the rise in oxygen concentration of the atmosphere, which began about 2300 million years ago
Why is this important? Photosynthetic organisms produce foods
that begin food chains We count on the Sun as a constant
energy source for warmth and foodSunlight must be converted into a useful
form of chemical energy for non-photosynthetic organisms
The most common chemical energy produced from photosynthesis is glucose◦Most common molecule for fuel in cell
respiration (remember?)
Pigments and light absorption
The vast majority of plants appear green to our eyes
Zoom into a leaf cell … the only structures in a leaf that are actually green are the chloroplasts
Plants contain a variety of pigments in chloroplasts
The photosynthetic pigment that dominates in most plant species is the molecule chlorophyll
Light absorption (cont.)Light absorption in
photosynthesis involves chemical substances called pigments
A white or transparent substance does not absorb visible light
Pigments are substances that do absorb light and therefore appear colored to us
Pigments Pigments that absorb all colors
appear black (b/c they emit no light)
Pigments that, for example, absorb all colors but blue appear blue to us (b/c this part of the sunlight is not absorbed)
Instead, the blue light is reflected and can pass into our eye
Pigments and light absorption The 1st stage in photosynthesis is
absorption of sunlight Visible light is a form of
electromagnetic radiationSunlight is a mixture of different
wavelengths of visible light◦Which we see at different colors◦We call this the visible portion of the
spectrum Blue, green, and red
The electromagnetic spectrum
Absorption spectrum of several plant pigments
Visible light spectrum Substances can do one of two
things when they are struck by a particular wavelength (color) of light: ◦Absorb that wavelength (energy is
being absorbed/used) ◦Reflect that wavelength (energy is
not being absorbed/used; you will see that color)
Let’s Check In – Turn and TalkYou are walking outside with a friend
who is wearing a red and white shirt. Explain why the shirt appears red and white
Sunlight is mixture of visible lightWhen sunlight hits the red pigments in the
shirt, the blue and green wavelengths of light are absorbed
The red wavelengths are reflected, thus, our eyes see red
White all wavelengths of light are reflected; eyes and brain interpret this mixture as white
How does this apply to photosynthesis? Chlorophyll is a green pigmentChlorophyll reflects green light and
must absorb other wavelength of visible light
When a plant is hit by sunlight, the red and blue wavelengths of light are absorbed by chlorophyll and used for photosynthesis
Almost all of the energy of the green wavelengths is reflected, not absorbed◦Do not try to grow plants in only green light
Fun Fact Chlorophyll is not the only pigment type
found in green plantsMost green plants also contain other
pigments in smaller quantities that reflect colors like red and yellow
We see this in foliage – turning of the leaves in the fall◦End of growing season, ◦Preparation for cold weather◦These colors were always there in the leaf, but
were hidden by the dominant color of chlorophyll until late in the season
Stages of photosynthesis Photosynthesis produces sugar
molecules as food source for the plant
Sugars (glucose) are held together by covalent bonds
Requires energy to create those covalent bonds◦Sun is the source of that energy
Two stages of photosynthesis1st stage:
◦Set of reactions that “trap” light energy and convert it to ATP (chemical energy)
2nd stage: ◦Set of reactions in which ATP is used
to help bond carbon dioxide and water molecules together to create a sugar (glucose)
1st stage – a deeper look Called light-dependent reactions Chlorophyll (and other pigments) absorb
light energy and convert to ATPLight energy is also used to accomplish a
reaction called photolysis of water◦ Water molecule is split into hydrogen and oxygen
The oxygen that is split away is released from the plant (waste product)◦ Good for us & millions of other organisms that
need oxygen for aerobic respiration Useful products for the plant formed during
this stage are ATP and hydrogen
2nd stage – a deeper look Called light independent reactions ATP and hydrogen are used as forms
of chemical energy to convert carbon dioxide and water into useful organic molecules for the plant
Remember? Carbon dioxide is one of the few inorganic molecules that contain carbon
Glucose (6C) is organic ◦Product of photosynthesis
2nd stage – a deeper look (cont.)
Equation: ◦6CO2 + 6H2O C6H12O6 + 6O2
Requires 6 molecules of carbon dioxide to form 1 glucose molecule
This conversion of an inorganic form of an element to an organic form is known as fixation
Fixation Therefore, photosynthesis can be
described as a series of reactions in which carbon dioxide and water are fixed into glucose, and oxygen is produced as a by-product
This fixation reaction requires energy
The energy to create the glucose comes directly from the ATP and hydrogen created in the 1st stage
Important to Note ◦Energy traced back to sunlight
◦Glucose is one of many possible organic molecules that can form from photosynthesis
Measuring the rate of photosynthesis Revisit equation:
◦6CO2 + 6H2O C6H12O6 + 6O2
How does this equation compare to what you know about cellular respiration? ◦Products and reactants are reversed
Measuring the rate (cont.)At any given time of year, a plant
has a fairly consistent rate of cell respiration ◦Consistent night and day AND
relatively low◦Plants do not have muscle and other
ATP demanding tissues like animals ◦They need ATP for various
biochemical processes, but not as much
Measuring the rate (cont.)The same consistency is not true
for the rate of photosynthesis Rate of photosynthesis depends
on environmental factors ◦Intensity of light, temperature
Measuring the rate (cont.)Daytime, especially on a warm, sunny
day = high rate of photosynthesis◦Rate of CO2 taken in and O2 released will
both be very high ◦The plant is also doing cell respiration
Correction should be made for both CO2 and O2 levels
At night, the rate of photosynthesis may drop to zero◦Giving off CO2 and taking in O2 just to
maintain relatively low rate of cell respiration
Measuring the rate (cont.)Measuring the rate of oxygen production or
carbon dioxide intake is considered a direct measurement◦ As long as correction is made for cell respiration
Another common approach: ◦ Keeping track of the change in biomass of
experimental plants◦ Mass of plants is considered to be an indirect
measurement◦ Increase or decrease in biomass may be traced
to a whole variety of factors besides photosynthetic rate.
◦ See Figure 3.26 in your text
Factors that affect rate of photosynthesis Light intensityCarbon dioxide concentrationTemperature
See page 87
Photosynthesis – the effect of light intensity At low light intensities the rate of
photosynthesis increases as light intensity is increased
But at high light intensities, further increases in light intensity have not effect
Several essential steps in photosynthesis are achieved using the light energy◦ATP is produced (from ADP + phosphate)◦Water molecules are split to release hydrogen
Needed later to make carbohydrate
Photosynthesis – the effect of CO2 concentration Although CO2 concentration has
been increasing over the past 100 years, it is still low enough to affect the rate of photosynthesis
At low CO2 levels, increases in concentration cause the rate of photosynthesis to rise
At high CO2 levels, further increases in concentration do not affect the rate of photosynthesis
Photosynthesis – the effects of temperature At low temps, the rate of
photosynthesis is very low or zeroAs temp increases the rate increases
until it reaches its maximum (optimum) Above this temperature, the rate
decreases and eventually becomes zero ◦Like enzymes, right?◦Not coincidental; fixation of carbon dioxide
it catalyzed by enzymes ◦ In most plants the optimum temperature is
between 25-35*C
RecapPhotosynthetic organisms produce
organic molecules (glucose) to begin food chain
Photosynthetic pigments (chlorophyll) are used to absorb light energy to being the process
Most plants are green because chlorophyll reflect the green wavelengths of light and absorbs the red and blue wavelengths
Recap (cont.)Photosynthesis occurs in 2
stages: light dependent and light independent
Light dependent◦Produce ATP and hydrogen; gives off
O2 Light independent
◦“fix” carbon dioxide and water into organic molecules (such as glucose) using ATP and hydrogen molecules produced during light dependent reactions
Recap (cont.)The rate of photosynthesis can be
measured directly by measuring either the production of oxygen or uptake of carbon dioxide
The rate of photosynthesis can be measured indirectly by measuring the biomass increase of a plant
Temperature, light intensity, and carbon dioxide can affect rate of photosynthesis