Unit Two “Energy Acquisition” “Photosynthesis”

The Sun and Plants  What makes life possible on Earth is the presence of a medium sized star, the Sun  Also, just as important is the fact the Earth is at a near perfect distance from the sun: “not too hot, and not too cold”  However, animals like us humans still could not survive for any length of time without plants to convert light energy into biomass, i.e. food for us!

The Sun and Plants  Plants are called producers/autotrophs because they, in essence, make their own food by using sunlight and other chemical compounds found in nature (note: some algae and bacteria also make their own food)  The food they make enables them to grow (addition of biomass)….. and they do not ever consume other living organisms in the process  Because of this food generating ability, they are placed at the bottom of the food chain….all other species depend on them for survival

Photosynthesis  The process by which plants generate food (Glucose) for their cells is called “Photosynthesis”  There are 3 main stages of Photosynthesis: A) Capturing energy from sunlight B) Using that energy to make ATP and NADPH (Light Dependent Reactions) C) Using ATP and NADPH to make Carbohydrates (Calvin Cycle) Carbohydrates (Calvin Cycle)

Chemical Reaction for Photosynthesis  carbon dioxide + water + light energy ---> glucose + oxygen + water  CO 2 + H 2 O + light energy ---> C 6 H 12 O 6 + O 2 + H 2 O

Photosynthetic Structures  The Chloroplast is the light sequestering organelle, and is also the site of all three stages of Photosynthesis  The Chloroplast is composed of internal membranes that are organized into flattened sacs called “Thylakoids”  These Thylakoids are stacked on top of each other in columns called “Grana”  A fluid called the “Stroma” surrounds the stacks of Thylakoids

Photosynthetic Structures  The light sequestering pigment, Chlorophyll, is located within the Thylakoids in groups called “Photosystems”  The Photosystem is the starting point of Photosynthesis  The Chlorophyll captures “Photons”, tiny packets of energy from sunlight, which they subsequently transfer to proteins that assist in the generation of ATP and NADPH

Sunlight’s Energy  It is very fortunate the Chlorophyll in the Chloroplasts of plants is perfectly suited to the energy levels found in visible light that comes from the sun  Further, there are many energy sources like radio waves, gamma rays, microwaves, but each either has too little energy or too much energy to be compatible with plants energy harnessing system

The Electromagnetic Spectrum

Chlorophyll Absorption  The leaves of plants are usually green due to the fact Chlorophyll absorbs light in the violet, blue, and red range of the spectrum  Chlorophyll reflects the green portion of the visible spectrum, and therefore the leaves take on a characteristic green appearance  “Carotenoids” are a group of accessory pigments that capture violet to blue-green light and reflect yellow, orange, & red

Photosynthesis: Stage 1  A) Capturing energy from sunlight –A photon of light in the appropriate wavelength is sequestered by a chlorophyll molecule and this energy is passed to another chlorophyll molecule

Photosynthesis: Stage 2 (exergonic)  B) Using energy to make ATP and NADPH (Light Dependent Reactions) 1. the energy from excited electrons taken from water is used in the “Electron Transport System” to pump Hydrogen Ions (protons) across the Thylakoid membrane 2. Many Hydrogen Ions (protons) accumulate in the Thylakoid serving as the energy source to construct ATP molecules from ADP molecules

Photosynthesis: Stage 2 (exergonic) 3. The electron from water that had its energy used in the Electron Transport System is now directed to another photosystem to be reenergized by the absorption of another photon of light 4. This electron enters another Electron Transport System and is transferred to NADP+ 5. NADP+ bonds with a Hydrogen Ion (proton) to form NADPH, which is important in stage 3 production of carbohydrates

Light Dependent Reactions

Photosynthesis: Stage 3 (endergonic) C) Using ATP and NADPH to make Carbohydrates (Calvin Cycle) Carbohydrates (Calvin Cycle) 1. ATP generated in Stage 2 (Light Dependent Reactions) is used as the fuel for the Calvin Cycle 2. NADPH generated in Stage 2 provides the Hydrogens needed to build Glucose 3. Numerous proteins acting as enzymes drive the production of Glucose

Photosynthesis: Stage 3 (endergonic) 4. The Calvin Cycle has to go to completion six times in order to produce one Glucose molecule: each cycle completion adds just one Carbon atom from CO 2 to make the Glucose molecule, which has 6 Carbon atoms 5. To keep both Stage 2 and Stage 3 functioning, used ADP and NADP+ from the Calvin Cycle is returned to the Electron Transport System in the photosystems

Calvin Cycle

Photorespiration  During extreme environmental conditions (hot and dry), plants close their “Stomata” to reduce water loss  Once their stomata are closed they no longer absorb Carbon Dioxide, and therefore the Oxygen that is usually released as a byproduct of photosynthesis, is incorporated into the Calvin Cycle causing it to “short circuit”  Some plants use C 4 Photosynthesis (use of different cells and chemical reactions) during hot, dry days in order to continue to photosynthesize

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