MBBS Biology


  • All organisms require a constant input of energy to do the work of life.
    • Energy cannot be recycled, so the story of life is a story of energy flow – its capture, transformation, use for work, and loss as heat.

  • Life runs on chemical energy.
    • Food is chemical energy stored in organic molecules.
    • Food provides both the energy to do life’s work and the carbon to build life’s bodies.
    • The carbon cycles between organisms and the environment, but the energy is “spent” and must be replaced.

  • Organisms obtain chemical energy in one of two ways.
    • Autotrophs make their own carbohydrate foods, transforming sunlight in photosynthesis or transferring chemical energy from inorganic molecules in chemosynthesis.
    • Heterotrophs consume organic molecules originally made by autotrophs.
    • All life depends absolutely upon autotrophs to make food molecules.

  • The process of photosynthesis produces more than 99% of all food for life, forming the foundation of most food chains.
    • Only three groups of organisms – plants, algae, and some bacteria – carry out the process of photosynthesis.

  • All organisms use similar energy-carrying molecules for food and to carry out life processes.
    • Glucose (C6H12O6,) is a nearly universal fuel delivered to cells, and the primary product of photosynthesis.
    •  ATP molecules store smaller amounts of energy and are used within cells to do work.
    • Chlorophyll and NADPH molecules hold energy temporarily during the process of photosynthesis.

  • The chemical equation below summarizes the many chemical reactions of photosynthesis.
    • The equation states that the reactants (carbondioxide, waterandlight) ,in the presence of chloroplasts, chlorophyll and enzymes, yield two products, glucose and oxygen gas.
    • Chlorophyll is a pigment that absorbs sunlight energy.
    • Chloroplasts are the organelles within plant and algal cells that organize enzymes and pigments so that the chemical reactions proceed efficiently.

  • In the process of photosynthesis, plants, algae, and blue green bacteria absorb sunlight energy and use it to change carbon dioxide and water into glucose and oxygen gas.
    • Glucose contains stored chemical energy and provides food for the organisms that produce it and for many heterotrophs.
    • Photosynthesized carbohydrates (represented here by glucose) make up the wood we burn and (over hundreds of millions of years) the coal, oil, and gas we now use as fossil fuels.
    • Most of the oxygen gas is waste for the organisms which produce it.
    • Both CO2 consumed and O2 produced affect the composition of earth’s atmosphere; before photosynthesis evolved, oxygen was not part of the atmosphere.

  • The single chemical equation below represents the overall process of photosynthesis as well as summarizes many individual chemical reactions that were  understood only after hundreds of years of scientific exploration.

  • Chloroplasts are the organelles where the process of photosynthesis takes place in plants and algae.
    • Chloroplasts resemble blue green bacteria, containing their own DNA and ribosomes.
    • The Endosymbiotic Theory holds that chloroplasts once were independent prokaryotic cells, but were engulfed by other larger prokaryotes, forming the first eukaryotic cells.
    • Chloroplasts are made of membranes, which enclose stacks of membrane sacs called thylakoids.
    • The membranes sequence pigments and electron carrier molecules for efficient photosynthesis.
    • Thylakoids create compartments, which allow concentration gradients to store energy.
    • Pigment molecules absorb specific wavelengths (colors) of light; chlorophyll is the primary pigment in photosynthesis.
    • Electron carrier molecules form electron transport chains, which transfer energy in small steps so that the energy can be stored or used for work.

  • Photosynthesis consists of two groups of chemical reactions: the Light Reactions and the Calvin Cycle.
  • Light Reactions transform energy from sunlight into chemical energy, and produce and release oxygen gas.
    • When light strikes pigment molecules, electrons absorb its energy and are excited.
    • Light also provides energy to split water molecules into electrons, hydrogen ions, and oxygen gas.
    • The oxygen gas is released as “waste”, but it is the source of the oxygen in Earth’s atmosphere.
    • Two pathways capture the energy from excited electrons as chemical energy stored in the bonds of molecules; both pathways involve electron transport chains.
      • One produces NADPH molecules, which stores energy and “hot hydrogen”.
      • A second pumps hydrogen ions into the thylakoids, forming an electrochemical gradient whose energy builds ATP molecules. This is “chemiosmosis”.

  • The Calvin Cycle uses the NADPH and ATP from the Light Reactions to “fix” carbon and produce glucose.   
    • Stomata underneath plant leaves allow gases (CO2, H2O, and O2) to enter and exit the leaf interior.
    • Carbondioxide enters the Calvin Cycle when an enzyme nicknamed “Rubisco” attaches it to a 5-carbon sugar. The unstable 6-carbon compound immediately breaks into two 3-carbon compounds, which continue the cycle.
    • Most plants fix CO2 directly with this pathway, so they are called C-3 plants.
    • Some plants have evolved preliminary fixation pathways, which help them conserve water in hot, dry habitats, but eventually the carbon enters the cycle along the“Rubisco” pathway.
      • C-4 plants such as corn use a 3-carbon carrier to compartmentalize initial carbon fixation in order to concentrate CO2 before sending it on to Rubisco.
      • CAM plants such as jade plants and some cacti open their stomata for preliminary CO2 fixation only at night.
    • In the Calvin Cycle,thefixedCO2 moves through a series of chemical reactions, gaining a small amount of energy (or “hot hydrogens”) from ATP or NADPH at each step.
    • Six turns of the cycle process 6 molecules of carbon dioxide and 12 “hot hydrogens” to produce a single molecule of glucose.
    • The cycle begins and ends with the same 5-carbon molecule, but the process stores chemical energy in food for nearly all life.
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