Photosynthesis

Chapter 8

Autotrophs vs. Heterotrophs

Autotrophs • make their own food

• include plants, some protists, and some bacteria

Heterotrophs • 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

1. Light Dependent Reactions 2. 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.

• 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)

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:– 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