Unit 3 Photosynthesis and Cellular Respiration

Energy and Life

Sec. 8-1

Autotrophs and Heterotrophs

• Energy for most life processes comes from the sun via plants performing photosynthesis.
• To live, organisms must release the energy stored in sugars and other compounds.
• Autotrophs are organisms that can make their own food, primarily through photosynthesis.

• Heterotrophs eat autotrophs.

Chemical Energy and ATP

• Products of photosynthesis release energy when bonds are broken between carbon and hydrogen atoms in molecules.

• Living cells cannot use the energy from this process directly. They use the energy to make ATP from ADP.

• Break bonds in sugars allow a phosphate to be added to ADP.
• Releasing the phosphate releases energy when needed.

 Using Biochemical Energy

• Energy in ATP is used to fuel many life processes within a cell.
  • Move substances across cell membranes
  • Synthesize proteins and nucleic acids
  • Response to chemical signals on cell membrane
  • Make lightJ

• Cells do not keep large amounts of ATP on hand because it is not good for storing energy over long periods of time.

Photosynthesis

Sec. 8-2 and 8-3

Photosynthesis Equation

• Anabolic reaction that absorbs energy and uses smaller molecules to make bigger ones holding the energy absorbed

• Carbon dioxide, water, light and pigments (primarily chlorophyll) are needed for photosynthesis.
• When light is absorbed, energy is transferred to electrons, raising their energy levels and thus storing energy.
• Photosynthesis takes place in chloroplasts.

• Chloroplast above, contains stacks of thylakoids known as grana.
• Proteins in thylakoid membranes arrange chlorophyll and other pigments into photosystems which are the light-collecting part of chloroplasts.
• When electrons absorb energy, special molecules called electron carriers move them along without losing much energy to other molecules which will eventually make ones the cell needs, like glucose. This is called the electron transport chain.
• Light-dependent reactions use water, ADP, NADP⁺ to produce oxygen, ATP and NADPH.
• Energy in ATP and NADPH is high but does not store well. Calvin cycle (or light independent reaction) uses ATP and NADPH to create the high energy molecules needed for storage, like glucose. Calvin cycle takes place in the stroma.

• Factors that affect the rate of photosynthesis: water, temperature, light intensity and duration, carbon dioxide concentration

Chemical Pathways

Sec. 9-1, 9-2

Cellular Respiration

• Catabolic reaction, releasing energy

• First step, glucose is broken down in the presence of oxygen in the cell cytoplasm, releases a small amount of ATP and the molecule pyruvate.
• In the presence of oxygen (aerobic), pyruvate is broken down further in the mitochondria via the Krebs cycle and electron transport chain. High energy electrons are passed by their carriers to convert ADP to ATP.
  • One molecule glucose turns to 36 ATP using 38% of the energy in the molecule. Rest is released as heat.
• In the absence of oxygen (anaerobic), pyruvate is broken down via 2 possible pathways:
  • Alcoholic done by yeast and other microbes producing  alcohol, carbon dioxide and NAD⁺   
  • Lactic acid done in muscles, some unicellular organisms (which humans use to make pickles, sauerkraut, yogurt for sour taste) producing lactic acid and NAD⁺

• Exercise uses small amount of ATP available with minutes. If pushing hard, next energy used is via lactic acid fermentation. If exercise is done through pacing, energy is used via stored glycogen (carbohydrate) in tissue for about 15 minutes, after that the body breaks down fats.