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