Using Light to Make Food CHAPTER 7 PHOTOSYNTHESIS: Using Light to Make Food

Photosynthesis – process by which plants use light to make food molecules from carbon dioxide and water

Autotrophs – plants that make their own food (also known as producers) Photosynthetic autotrophs—organisms that use light as a source of energy to make food

Chloroplasts All green parts of a plant have chloroplasts (leaves are major sites of photosynthesis) Green color of plants is due to green pigment w/in chloroplasts called chlorophyll Chloroplasts mainly found in mesophyll cells – green tissue in interior of leaf

Consists of double membrane surrounding a thick fluid (stroma) – sugars are made from CO2 here --CO2 enters leaf and O2 exits by way of tiny pores known as stomata

Thylakoids (green sacs) are suspended in stroma – light energy is captured here Stack of these = Grana Thylakoid

Redox Reactions 6CO2 + 12H2O C6H12O6 + 6H2O +6O2 -Photosynthesis STORES energy ENDERGONIC -Carbon dioxide is reduced to glucose GAINING ELECTRONS -Water is oxidized to oxygen LOSING ELECTRONS -Water is first consumed then produced

-Plants make sugar from carbon and oxygen in CO2 and from some hydrogen in H2O -O2 is released, coming from the H2O, not from CO2

- Cellular respiration harvests energy by _______ the sugar & _______ O2 to H2O (respiration is energy-releasing, going from potential to kinetic by traveling down energy levels) - Photosynthesis goes uphill (gaining potential and traveling up energy levels) oxidizing reducing

The Light Reactions Convert light energy to chemical energy & O2 gas is waste product Occur in thylakoid membranes Stored in ATP & NADPH No sugar produced

Visible Light Sunlight is a type of energy called radiation (also known as electromagnetic energy) Travels in rhythmic waves Light reactions of photosynthesis only use certain wavelengths/colors of Visible Light

Light also behaves as discrete packets of energy called photons A photon is a fixed quantity of light energy (shorter the wavelength, the greater the energy)

A photon of violet light packs almost twice as much energy as a photon of red light Light may be reflected, transmitted, or absorbed (pigments are substances that absorb light)

Pigments involved in photosynthesis are chlorophyll a and b and carotenoids The chlorophylls are green and carotenoids are shades of yellow and orange

Photosystems Pigments are clustered in thylakoid membranes Chlorophyll a molecule & a primary electron acceptor make up the reaction center of the pigment assembly Reaction center & other pigments function collectively as a light-gathering antenna that absorbs photons

Energy is passed from molecule to molecule until it reaches the reaction center Combination of the antenna molecules, the reaction center, and the primary electron acceptor make up the photosystem This is the light-harvesting unit of the chloroplast’s thylakoid membrane

Two Photosystems Photosystem I is called P700 because the light it absorbs best is red light w/a wavelength of 700 nm Photosystem II is called P680 because the light it absorbs best is orange shade of red light w/a wavelength of 680 nm

ATP, NADPH, and O2 First event in light reactions is the absorption of light energy Second event is the excitation of electrons by light energy Third event is formation of ATP & NADPH using energy made available by the cascade of energized electrons down electron transport chains

Transport chains are similar to the one that functions in cellular respiration Consist of a series of electron-carrier molecules arranged in a membrane (the thylakoid of the chloroplast)

Chemiosmosis ATP is synthesized by chemiosmosis Electron transport chains associated w/the chloroplast’s photosystems are arranged in thylakoid membranes Electron transport chain in the chloroplast drives the transport of H+ through thylakoid membrane

Flow of H+ back through the membrane is harnessed by ATP synthase to make ATP In photosynthesis this is called photophosphorylation H+ ions, along w/electrons from the electron transport chain, join w/NADP+ to form NADPH

The Dark Reaction (Calvin Cycle) Cyclic series of reactions that assemble sugar molecules using CO2 and energy-containing products of light reaction

Carbon Fixation (making sugar) Calvin cycle is a sugar factory in the chloroplast (occurs in stroma) CO2 from air & ATP & NADPH from light reactions are reactants of this process

Carbon Fixation (making sugar) Using carbon from CO2, energy from ATP & high-energy electrons from NADPH, the Calvin cycle makes an energy rich sugar Called glyceraldehyde-3 phosphate (G3P) A plant uses this to make glucose or other organic molecules

Review Sugar molecules that a plant produces are its food supply – they are expended during cellular respiration Plants use sugars as building blocks for other organic compounds, including cellulose Plants are the ultimate source of food for all other organisms

Adaptation for Saving Water Most plants are C3 plants, which take carbon directly from CO2 in the air & use it in the Calvin cycle to build a 3-carbon molecule

Stomata in leaf surface usually close when the weather is hot & dry Minimizes water loss But CO2 and O2 are not exchanged as normal Calvin cycle is diverted to an inefficient process called photorespiration

Some plants have special adaptations that enable them to save water & avoid photorespiration ex: corn, sugarcane Special cells in C4 plants incorporate CO2 into a 4-carbon compound It’s broken down to release CO2 (this initiates the Calvin cycle)

Rice, Soybean, many tree species Corn, sorghum, sugarcane Characteristic C3 Plants C4 Plants Origin Temperate Tropical Examples Rice, Soybean, many tree species Corn, sorghum, sugarcane Carbon dioxide fixation 3 carbon molecule 4 carbon molecule Site of photosynthetic cycle Mesophyll cells Bundle sheath cells Carbon dioxide concentration Regulated by diffusion Elevated high concentrations Stomatal behavior Open for longer periods Open for shorter periods Water use efficiency* Not very efficient Very efficient Climatic adaptation Mostly cooler, moderate climate Mostly warmer, drier climate Carbon dioxide saturation High Low Light saturation Photorespiration *The ratio of carbon dioxide fixed to water used per unit area of the leaf.

CAM – Crassulacean Acid Metabolism These plants are also adapted to hot, dry climates ex: cacti, pineapples, succulents (aloe) Close stomata during day to prevent water loss & open at night Carbon compounds stored in vacuoles at night These compounds are broken down to release CO2 for photosynthesis during the day

Global Warming In atmosphere, CO2 retains heat from sun that would otherwise radiate back into space Burning of fossil fuels & wood releases excess CO2, which may be causing global warming Greenhouse Effect

Forest Replacement Replace w/younger growth of trees Increases photosynthesis, which reduces CO2, but burning at faster rate Older trees also remove CO2, but at slower rate