PHOTOSYNTHESIS Honors Biology Ch. 6

Carbon Cycle Photosynthesis is a major part of the Carbon Cycle – the tracking of carbon through the Biosphere During photosynthesis plants and other autotrophs use carbon dioxide, solar energy and water to produce carbohydrates Autotrophs and Heterotrophs use oxygen to break down carbohydrates during cellular respiration releasing CO2 and H2O Decomposers release CO2 during the breakdown of organic compounds

AUTOTROPHS = organisms that manufacture their own food from inorganic substances and energy (plants, algae, bacteria) Photoautotrophs – use light Chemoautotrophs- use inorganic substances such as sulfur or ammonia as an energy source “producers” Chemical energy usually stored as carbohydrates Heterotrophs = organisms that cannot make their own energy and have to consume autotrophs or other heterotrophs Consumers Indirectly all life depends upon autotrophs Chemoautotroph = autotrophs that get their energy from chemicals

Biochemical Pathways Series of chemical reactions in which the products of one reaction are used up in the next reaction. Photosynthesis is a biochemical reaction using CO2 and H2O leading to the release of Oxygen O2

Cellular Respiration is another biochemical pathway in which both autotrophs and heterotrophs convert organic compounds and O2 into ATP, CO2 and H2O O2 + C6H12O6 → CO2 + H2O + ATP Chemosynthesis is a third biochemical pathway which uses heat energy in place of light energy. CO2 + H2SO4 + Heat → C6H12O6 +S

Cross section of a leaf Epidermis – Protective layer Mesophyll – Thick layer of leaf where most photosynthesis takes place Stomata – Cells which open and close to regulate gas exchange.

Anatomy of a Chloroplast Light reactions-the initial reactions in photosynthesis Chloroplast – organelle found in plants or algae surrounded by two membranes Thylakoid – Flattened membrane disk Grana – Stack of thylakoids within the chloroplast Stroma – solution that surrounds the thylakoids

CHLOROPLAST

Light Absorption in Chloroplasts Most Chloroplasts are in the Mesophyll

Visible Spectrum Visible spectrum - light is detectable by the human eye ROYGBIV (380-750 nm) Light travels in a series of waves, and can be measured in terms of wavelength. Different colors have different wavelengths. Light behaves as if it consists of particles called photons. Sun radiates the full spectrum of electromagnetic energy

Reflected (how you see things) Transmitted (passed through) Light can be Reflected (how you see things) Transmitted (passed through) Absorbed (changed from light energy to another form) Pigments – substances that absorb visible light. They absorb different wavelengths. An object is given color when it absorbs all other colors of light. * Each pigment has a characteristic absorption spectrum which can be determined by a spectrophotometer.

Absorption Spectrum Chlorophyll – found in thylakoid and used for photosynthesis chlorophyll a – absorbs less blue and more red than b chlorophyll b – absorbs more blue and less red

Chlorophyll b – Accessory pigment (green-yellow pigment _ PIGMENTS IN PLANTS Chlorophyll a molecules can participate directly in the light reaction (photosynthesis); accessory pigments help by transferring energy to chlorophyll a Chlorophyll b – Accessory pigment (green-yellow pigment _ Carotenoids – Other pigments found in thylakoids acting as accessory pigments (yellow, orange, and/or pink ) Accessory pigments allow plants to capture More energy from light Anthocyanin – Reds, purples and blues Xanthophyll's - yellows

Photosynthesis has 2 stages LIGHT REACTION (light dependent reaction): convert light energy to chemical bond energy in ATP and NADPH Occurs on the thylakoids NADP+  NADPH Oxygen is a byproduct Generates ATP CALVIN CYCLE (light independent reaction) – take carbon dioxide and REDUCE it to carbs/organic compounds Occurs in the stroma Carbon fixation Does not require light directly NADPH provides the reducing power ATP provides the chemical energy

Electron Transport Photosystem – a cluster of chlorophyll and carotenoids within a thylakoid membrane. Two types- Photosystem I and photosystem II.

Light Reaction Occur in the thylakoid membrane Both photosystems accessory pigments absorb light, acquiring energy. This energy is passed on until it reaches a specific pair of chlorophyll a molecules. 1. Light energy forces electrons to enter a higher level in the two chlorophyll a molecules of photosystem II.

Light Reactions 2.The excited electrons have enough energy to leave the chlorophyll a molecule and go to the primary electron acceptor. 3. The primary electron acceptor donates the electrons to the first molecule of a series known as the electron transport chain. The electrons lose energy as they are passed through this chain. The energy they loose is used to move protons into the thylakoid.

Light Reactions 4. At the same time photosystem I is also putting energy into electrons from a chlorophyll a molecule and passing them to another primary electron acceptor. These electrons are replaced by the electrons that have come through the electron chain from photosystem II. 5. The primary electron acceptor from photosystem I donates electrons to a different electron chain bringing them to The side of the thylakoid membrane that

faces the stroma. Where they combine with a proton (H+) and a NADP+ molecule. This reduces NADP+ to NADPH.

Restoring Photosystem II The electrons in photosystem II are replaced by the splitting of water into 02, 4e-, and 4H+. 2H20 = 4H+ + 4e- + 02 The protons are left in the thylakoid while 02 diffuses out of the plant.

Chemiosmosis Chemiosmosis- the synthesis of ATP. Relies on the concentration gradient of protons across the thylakoid membrane. High concentration inside thylakoid, low in the stroma. ATP synthase- a protein located in the thylakoid membrane that make ATP by adding a phosphate group to ADP. The energy for this reaction is supplied by the movement of protons through the protein.

NADPH and ATP supply the energy for the second set of reactions of photosynthesis.

Light Reactions p680 p680 Methuen.k12.ma.us