Photosynthesis Chapter 6

Section 1: Light Reactions All organisms use energy to carry out the functions of life Organisms are classified by how they get energy Autotrophs: use energy from sunlight Photosynthesis: convert light energy into chemical energy in the form of organic compounds (carbohydrates) Heterotrophs: get energy from food rather than sunlight

Light Reactions Cont. Almost all organisms ultimately depend upon autotrophs to obtain the energy necessary to carry out the processes of life

Overview of Photosynthesis

Overview of Photosynthesis Photosynthesis produces organic compounds from carbon dioxide and water Oxygen and some organic compounds are used by the cell during cellular respiration During Cellular respiration carbon dioxide and water are produced

Photosynthesis and Cellular Respiration The reactants of photosynthesis are the products of cellular respiration

Photosynthesis Can be broken down into two stages: Light reactions: light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH Calvin cycle: organic compounds are formed using carbon dioxide and chemical energy stored in ATP and NADPH

Capturing Light Energy Light reactions require light Light is absorbed in chloroplasts Each chloroplast contains Inner and outer membranes Thylakoids: membranes arranged in flattened sacs Grana: stacks of thylakoids Stroma: fluid inside inner membrane

The Chloroplast

Light and Pigments Light from the sun appears white, but it is actually made up of a variety of colors Light can be broken down into the visible spectrum

Light and Pigments Light can be reflected, transmitted, or absorbed by an object Pigments: compounds that absorb light Most pigments absorb some colors more than others Light that is reflected or transmitted has not been absorbed Green leaves: all colors absorbed besides green

Chloroplast Pigments Most important pigments are chlorophylls Several types of chlorophylls Chlorophyll a: absorbs red light Chlorophyll b: absorbs blue light Note: neither absorbs green light well

Chloroplast Pigments Mainly chlorophyll a in light reactions Chlorophyll b and carotenoids serve as accessory pigments Accessory pigments allow more light to be captured

Light Energy  Chemical Energy Light is absorbed and transformed into chemical energy Chemical energy is temporarily stored in ATP and NADPH Photosystems: clusters/groups of carotenoid and chlorophyll pigments There are two photosystems Photosystem I and Photosystem II

Photosystem I and II Photosystems have similar pigments, different roles in th light reactions

Light Reactions Light energy forces electrons to enter a higher energy level (excites them) in photosystem II 2. Electrons leave chlorophyll a molecules are accepted by the primary electron acceptor 3.Primary electron acceptor donates electrons to the electron transport chain (within the thylakoid membrane) -moving from molecule to molecule they lose energy

Light Reactions 4. Light is absorbed by photosystem I and II. Electrons from chlorophyll molecules in photosystem II replace electrons that leave chlorophyll molecules in photosytem I -this needs to happen in order for photosynthesis to continue!

Light Reactions 5. Primary electron acceptor of photosystem I donates electrons to a different ETC This ETC brings electrons to the thylakoid membrane Electrons combine with a proton and NADP+ This creates NADPH from NADP+

Light Reactions

Photosystems

Replacing Electrons In LR Electrons in PS II replace electrons in PS I Replacement electrons for PS II come from water Water is split into protons, electrons and oxygen 2H2O  4H+ + 4e- + O2

Photosynthesis Video https://www.youtube.com/watch?v=joZ1EsA5_NY

Making ATP in LR Chemiosmosis: synthesizes ATP during light reactions Relies on concentration gradient of protons across thylakoid membrane Concentration of protons is greater inside thylakoid than in the stroma (outside thylakoid) Potential energy from gradient harnessed by ATP synthase https://www.youtube.com/watch?v=3y1dO4nNaKY

Making ATP Continued ATP Synthase makes ATP by adding a phosphate group to ADP Energy for this reaction comes from movement of protons across thylakoid membrane Potential energy converted to chemical energy

Chapter 6 Sect. 2 The Calvin Cycle Series of enzyme-assisted chemical reactions that produces a 3 carbon sugar Carbon dioxide molecules are “fixed” into organic compounds (sugars) in a process known as carbon fixation 3 Carbon dioxide molecules are needed to make each organic compound

The Calvin Cycle

The Calvin Cycle

The Calvin Cycle 1. CO2 diffuses into the stroma from the surrounding cytosol An enzyme binds each CO2 with a 5 carbon molecule—ribulose bisphosphate (RuBP) The resulting 6 carbon molecule is very unstable and immediately splits into two 3 carbon molecules—3-phosphoglycerate (3-PGA)

The Calvin Cycle 2. 3-PGA is converted into glyceraldehyde 3-phosphate (G3P) in a two part process A. 3-PGA receives a phosphate croup from ATP B. Then compound receives a proton (H+) from NADPH and releases a phosphate group *The ADP and NADP+ are then used in the Light Rxns

The Calvin Cycle 3. One of the G3P molecules leaves the Calvin Cycle and is used to make carbohydrates where energy will be stored 4. Remaining G3P molecules are converted back into RuBP through addition of phosphate groups from ATP. The RuBP molecules enter the Calvin Cycle again

The Calvin Cycle

The Calvin Cycle The most common pathway for carbon fixation Plant species that fix carbon exclusively with the Calvin Cycle are called C3 plants because of the 3 carbon compound initially formed in this cycle

Alternative Pathways Plants that live in hot and dry conditions have evolved to fix carbon via alternative pathways CO2 enters leaves through stomata Plants living in hot and dry areas can lose a lot of water through these stomata Water loss can be reduced through closing or partially opening stomata

Alternative Pathways C4 Pathway Fixes carbon into four-carbon compounds Plants that use this pathway are called C4 plants C4 plants have partially closed stomata during the hot parts of the day Carbon compounds can still be produced with limited CO2