Ask A Scientist© Biology Archive

Author:      chrissy
Text:        I have had trouble understanding how photosynthesis and cellular 
respiration works.

Response #:  1 of 1
Author:      wizkid
Text:        Photosynthesis is the process by which autotrophs (organisms such 
as plants which are capable of making their own food) manufacture glucose.  
Autotrophs start out with water and carbon dioxide to produce glucose, oxygen, 
and some water.  Photosynthesis occurs in two phases:  1) the light reaction, 
and 2) the dark reaction.  During the light reaction, light energy splits 
water molecules into hydrogen atoms and oxygen atoms.  The hydrogen atoms 
combines with a chemical called nicotinamide-adenine dinucleotide phosphate 
(NADP) to form NADPH2, which is used later in the dark reaction.  The oxygen 
atoms bind to each other and molecular oxygen (O2) is released into the 
atmosphere.  Also, the light energy is used to produce the energy molecule 
called adenine triphosphate (ATP), which is also used in the dark reaction.  
So, the main products of the light reaction are NADPH2, ATP, and O2.  During 
the dark reaction, enzymes use the energy from NADPH2 and ATP (formed in the 
light reaction) to convert carbon dioxide (CO2), obtained from the atmosphere, 
into glucose and water.  The glucose and water can be stored for future use by 
the plant.  The dark reaction is also called the "Calvin Cycle," after the 
scientist who first described it.  Cellular respiration is the process by 
which organisms break down glucose for energy.  The big picture is that cells 
use glucose and oxygen to form carbon dioxide, water, and ATP (the energy 
molecule).  There are two forms of cellular respiration:  1) aerobic (using 
oxygen) and 2) anaerobic (not using oxygen).  In aerobic respiration, 1 
molecule of glucose undergoes three processes called 1) glycolysis, 2) the 
Krebs cycle, and 3) the electron transport chain.  Oxygen is needed at the end 
of the processes to "pull" electrons through.  The energy stored in the 
glucose is used to produce 36 molecules of ATP.  ATP can then be used by cells 
for energy.  Anaerobic respiration only involves glycolysis, which does not 
need oxygen.  However, only 2 ATP are formed.  This is only a very broad 
picture of photosynthesis and respiration, and if you have more specific 
questions, I will try to answer them.  The things to remember are that 
photosynthesis uses carbon dioxide, water, and light to make glucose and 
oxygen, while respiration uses glucose and oxygen to make carbon dioxide, 
water, and ATP.