Energy and molecules: photosynthesis and respiration
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Photosynthesis: reaction and role
Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy absorbed by chlorophyll in chloroplasts. The overall chemical equation is 6CO2 + 6H2O → C6H12O6 + 6O2 and the reaction is endothermic because it stores light energy in chemical bonds. Photosynthesising organisms form the primary energy store for food chains and maintain atmospheric oxygen and carbon dioxide levels, so almost all higher life depends on their activity .
Uses of glucose produced by photosynthesis
Glucose provides immediate fuel for cellular respiration and supplies carbon for biosynthesis. Excess glucose undergoes conversion into insoluble starch for storage, into fats and oils for long-term energy reserves, into cellulose for cell walls and growth, and into amino acids (with nitrate ions) to make proteins. Energy stored in plant biomass transfers along food chains when consumers eat producers, with only a small fraction passed to the next trophic level .
Limiting factors for photosynthesis
Rate of photosynthesis depends on light intensity, carbon dioxide concentration, temperature and chlorophyll availability. Light provides the energy for bond rearrangement; low temperature reduces molecular collisions and reaction rates; low carbon dioxide reduces reactant availability; and low chlorophyll (often from magnesium deficiency) reduces light absorption. Any one factor in short supply becomes the limiting factor and reduces overall rate .
Cellular respiration and organic fuels
Aerobic respiration occurs in mitochondria and breaks down glucose in the presence of oxygen to produce carbon dioxide, water and energy (C6H12O6 + 6O2 → 6CO2 + 6H2O). Organic compounds other than glucose, such as fats and proteins, also act as fuels: fatty acids yield large amounts of energy after enzymatic breakdown, and amino acids can feed respiration after deamination. Respiration supplies energy that powers movement, growth, synthesis and other life processes in all cells .
Molecular structure determines biological function
Proteins fold into precise three-dimensional shapes that create active sites for enzyme catalysis and binding surfaces for other functions. A change in primary structure alters folding and can change or stop function. Organelles and tissues show structural adaptation: chloroplast membranes contain pigment systems for light capture, xylem cells form hollow, lignified tubes for one-way water transport, and phloem cells transport dissolved sugars in living sieve tubes. Structure-to-function relationships enable efficient energy capture, transfer and use at molecular and tissue levels .
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