Photosynthesis

Objectives – State that photosynthesis involves the conversion of light energy into chemical energy – State that light from the sun is composed of a range of wavelengths (colors) – State that chlorophyll is the main photosyn- thetic pigment – Outline the differences in absorption of red, blue, and green light by chlorophyll.

Metabolism review Plants and other autotrophs are the producers of the biosphere. Photoautotrophs use light as the energy source. Chemoautotrophs harvest energy from oxidizing inorganic substances, including sulfur and ammonia. Uniquely bacterial. Heterotrophs get organic compounds from other organisms. These organisms are consumers and decomposers. Almost all heterotrophs depend on photoauto- trophs for food and for oxygen, a byproduct of photosynthesis.

Metabolism review Photosynthesis and respiration recycle compounds required for all life on Earth. The energy for this comes from the sun. Animals need plants, and vice versa.

Definition of photosynthesis Photosynthesis is the conversion of light energy to chemical energy that is stored in glucose or other organic compounds. Occurs in plants algae cyanobacteria which began creating our oxygen-rich atmosphere 2 billion years ago as a waste product.

Definition of photosynthesis During photosynthesis water and carbon dioxide are converted into glucose and oxygen. Sunlight provides the energy for the reaction. The chemical equation (you must remember) : 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 light energy

Energy of sunlight Sunlight is composed of a range of wavelengths. Distance between crests of waves is called the wavelength. Wavelengths of electro- magnetic radiation range from 1 km (radio waves). We see a small fraction of the sun’s energy.

Energy of sunlight The range of electromagnetic radiation is the electromagnetic spectrum. The most important segment for life is between nm: visible light.

Energy of sunlight Although a wave, many of light’s properties are those of a discrete particle, the photon, with fixed quantities of energy. Energy in a photon is inversely related to its wavelength. The atmosphere screens out most wavelengths, letting only visible light pass in sig. quantity. Consider UV light: → a smaller wavelength produces more energy than visible light, caus- ing skin cancer.

Light absorption by chlorophyll A spectrophotometer measures the ability of pigments to absorb various wavelengths of light. Green light is not absorbed Blue (& red) light is absorbed.

Light absorption by chlorophyll An absorption spectrum plots a pigment's light absorption versus wavelength. Photosynthesis performs work with the wave- lengths of light that are absorbed. Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green. There are several pigments in leaves. Carotenoids are the yellow/red pigments.

Pigments in leaves

Chlorophyll captures sunlight When light meets matter, it may be reflected, transmitted, or absorbed. Chlorophyll is a pigment that absorbs red & blue light, while transmitting and reflecting green light.

Chlorophyll captures sunlight Chlorophyll is the main photosynthetic pigment. It contains magnesium. It captures solar energy.

Sunlight converted to chemical energy Light energy is used to produce ATP (adenosine triphosphate). The energy in the electron from chlorophyll (knock- ed off by a photon from the sun) is used to add a PO 4 - group to ADP. When energy is needed by a cell, the PO 4 - group is removed, and the energy is released. Release energy → ← Insert energy

Photosynthesis Chloroplast

Objectives C.4.1 – Draw & label a diagram showing the structure of a chloroplast as seen in electron micrographs – State that light energy is used to produce ATP and to split water molecules (photolysis) to form oxygen and hydrogen – State that ATP and hydrogen (derived from the photolysis of water) are used to fix CO 2 to make organic molecules – Explain that the rate of photosynthesis can be measured directly by production of oxygen or uptake of CO 2, or indirectly by an increase in biomass – Outline effects of temperature, light intensity, & CO 2 concentration on rate of photosynthesis

Structure of a chloroplast In electron micrographs the stacks of thylakoids (called grana) are obvious.

Structure of a chloroplast Reminder: the theory of endosymbiosis says that chloro- plasts are derived from what were once free-living bacteria. Both have similar DNA and ribosomes, and similar size. Chloroplasts have a double membrane. Stroma is cytoplasmic.

Chloroplast structure vs. function Chloroplasts have : 1) Large surface area of thylakoid membrane for light absorption and enzymatic energy capture. 2) Compartmentalization within thylakoids for accumulation of protons. 3) Space for enzymes of the Calvin cycle in the stroma.

Photosynthetic reactions Photosynthesis consists of light-dependent and light-independent reactions. 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 light energy

The light-dependent reactions Light energy is used to produce ATP and to split water molecules (photolysis) to form oxygen and hydrogen. The light-dependent reactions: Light is required. 1 st, electrons & hydrogen are moved from water to NADP + (nicotinamide adenine dinucleotide phosphate) → NADPH (carries the energy-rich electrons to the Calvin cycle). 2 nd, ATP is produced by photophosphorylation.

The light-dependent reactions Light energy is used to produce ATP ( adenosine triphosphate ). The energy traveled from the sun, to the plant, to be eaten by the animal. Solar energy is used to add a PO 4 - group to ADP. When energy is needed by a cell, the PO 4 - group is removed, and the energy is released. Release energy → ← Insert energy

The light-dependent reactions When chlorophyll absorbs a photon, one of its electrons is elevated to an orbital with more potential energy. An electron from Mg in the porphyrin ring is excited.

The light-dependent reactions Normally the excited electron would immediately drop down to a lower energy state and give up its energy as fluorescence. However, the energy is captured and slowly released to make ATP. Fluorescence Both photosystems harvest light (see photons).

Photosynthesis fixes CO 2 ATP and hydrogen (from the photolysis of water) are used to fix, or bind, CO 2 to make organic molecules. The light-independent reactions: Occur with or without sunlight. Use the energy in the ATP and the NADPH from the light reactions. Combine six CO 2 molecules to make a 6-carbon sugar (glucose). 24H + + 6CO 2 → C 6 H 12 O 6 + 6H 2 0 (partial equation) (NADPH is oxidized to NADP + ; CO 2 is reduced)

The rate of photosynthesis The rate of photosynthesis can be measured directly by the production of oxygen or uptake of CO 2, or indirectly by an increase in biomass. Conversion of light energy into plant biomass More sunlight should produce more biomass. Weigh plant material

The rate of photosynthesis The rate of photosynthesis can be measured directly by the production of oxygen or uptake of CO 2, or indirectly by an increase in biomass.

Variables affect photosynthesis Temperature, light intensity, and CO 2 concen- tration affect the rate of photosynthesis. Why does photos- synthesis decrease as the temperature rises beyond a certain point?

Variables affect photosynthesis Temperature, light intensity, and CO 2 concen- tration affect the rate of photosynthesis. Chlorophyll (an enzyme) becomes saturated with CO 2 (the substrate) at some point.

Variables affect photosynthesis Temperature, light intensity, and CO 2 concen- tration affect the rate of photosynthesis. There are only so many chlorophyll molecules in a leaf; at some point all are being used. What is happening here?