Photosynthesis Chapter 8

Autotrophs vs. Heterotrophs make their own food include plants, some protists, and some bacteria CANNOT make their own food; must obtain energy from outside sources Includes animals, fungi, and some bacteria

Chemosynthesis Chemosynthesis is one way in which organisms can produce their own food by converting chemicals into useable nutrients. Example: Chemosynthetic bacteria living in the depths of the ocean use inorganic materials and heat from ocean vents to produce food.

Photosynthesis Photosynthesis is the process in which plants use sunlight, carbon dioxide, and water to produce oxygen gas and stored energy in the form of sugar. Photosynthesis occurs in chloroplasts, primarily those found in the leaves of a plant.

Chemosynthesis vs. Photosynthesis

Chloroplast Chloroplasts have two membranes The outer membrane is like a normal membrane surrounding other eukaryotic organelles The inner membrane is folded into specialized sacs called thylakoids.

Chloroplast Thylakoids are stacked on top of each other to form columns called grana (singular: granum) The space in the chloroplast is called the stroma.

Leaf Cross-Section Cuticle = waxy covering on top of the leaf Upper Epidermis = top layer of cells on the leaf

Leaf Cross-Section Palisade layer = is just under the upper epidermis; primary location of photosynthesis Spongy Mesophyll = below the palisade layer; has rounded cells and air space for gas exchange

Leaf Cross-Section Veins = tube-like tissues that deliver water from the roots to the top of the plant and carry sugars from the leaves to the lower parts of the plant.

Leaf Cross-Section Lower Epidermis = bottom layer of the leaf Stomata / Stomata = specialized cells on the underside of the leaf; used for gas exchanged

Stomata Plants “breath in” carbon dioxide and release oxygen through openings on the underside of leaves called stomata / stomates Guard cells form the “lips” of the mouth while the stomate is the opening.

Stomata (cont.) Stomata open when they need carbon dioxide. Plants lose water through evapo-transpiration when stomates are open, so they close if they lose too much water.

Parts of Photosynthesis Light Dependent Reactions Light Independent Reactions (Calvin Cycle)

Light Reactions / Light Dependent Occurs in thylakoid membrane Sunlight absorbed by a green pigment called chlorophyll within thylakoids

Light Reactions (cont.) Absorbed light energy causes proteins within the thylakoids to split water molecules into oxygen gas and hydrogen ions; oxygen is released. Ions are charged atoms because they have unequal numbers of protons (+) and electrons (-) Hydrogen normally only has one proton and one electron

Light Reactions (cont.) Electrons move through PSII, electron transport chain, and PSI. PSII and PSI = photosystems I & II; contain pigments and give energy to ions The electron transport chain move the charged electrons from PS II to PS I

Light Reactions (cont.) As the electrons move through the thylakoid membrane, the protons build up a positive charge outside of the cell. Protons are forced back into the membrane because of diffusion, more specifically, chemiosmosis They move through a special protein channel called ATP synthetase, which causes ADP+P to become ATP

Light Reactions (cont.) At the end of the light reactions, the electrons and protons are captured by an electron carrier called NADP+ to become NADPH

Light Reactions

Light Reactions Summary STARTS WITH: Water Sunlight NADP+ ADP+P ENDS WITH: Oxygen gas (O2) NADPH ATP

Calvin Cycle / Light Independent Takes place in the stroma of chloroplast. An enzyme called rubisco attaches a carbon dioxide molecule to a 5-carbon sugar called RuBP.

Calvin Cycle / Light Independent (cont) This molecule is unstable and immediately breaks into two 3-carbon sugars called PGA. This part of the Calvin Cycle is called “carbon fixation.”

Calvin Cycle / Light Independent (cont) Using ATP and NADPH from the light reactions, each molecule of PGA is converted to PGAL. ATP becomes ADP+P while NADPH becomes NADP+. Both of these molecules are recycled and sent back to the light reactions.

Calvin Cycle / Light Independent (cont) PGAL is converted to 4-, 5-, and 6-carbon sugars throughout the cycle. After three turns of the cycle, five molecules of RuBP are produced that can later be recycled in the Calvin Cycle. One molecule of sugar is made. Usually plants produce and store sucrose, but books generally state glucose is stored.

Calvin Cycle

Calvin Cycle Summary STARTS WITH: ENDS WITH: Carbon dioxide Rubisco RuBP ATP NADPH ENDS WITH: Sugar (sucrose or glucose usually) RuBP (more made for future Calvin Cycles) ADP + P (sent to light reactions) NADP+ (sent to light reactions)

Photosynthesis as a Chemical Equation

Factors Influencing Rates of Photosynthesis Carbon Dioxide Levels Light Intensity As CO2 increases, rate increases As light intensity increases, rate increases BUT then plateaus

Factors Influencing Rates of Photosynthesis Temperature Oxygen Levels As temperature increases, rate increases BUT then plateaus As O2 increases, rate decreases