Photosynthesis IB 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 food Sunlight 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 radiation Sunlight 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 Talk You 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 light When 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 pigment Chlorophyll 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 plants Most 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 photosynthesis 1st 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 ATP Light 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 intensity Carbon dioxide concentration Temperature 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 zero As 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

Recap Photosynthetic 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