Photosynthesis

• Process of photosynthesis


  Plants use carbon dioxide and water in the presence of chlorophyll and sunlight to compose glucose and oxygen
• Photosynthesis


  The process where plants use carbon dioxide and water in the presence of chlorophyll and sunlight to compose glucose and oxygen
• Leaves


  • Most green parts of a plant can photosynthesize but the leaves are best adapted for this function
  • Leaves need to have a large surface area to absorb light
  • Leaves need chlorophyll to catch the light
  • Leaves need veins including xylem and phloem to supply water
  • Leaves have a system for carrying away the photosynthetic products to other parts of the plant
  • Leaves release oxygen and water vapour from the leaf cells
• Leaf structure


  Thin, flattened structures supported by petioles which support the leaf blades to be angled to receive the maximum amount of sunlight<|>Layers of cells with different functions inside the leaf
• Layers of a leaf

  • Epidermis
  • Mesophyll
  • Veins
• Epidermis


  Single layer of cells on the upper and lower surfaces of the leaf<|>Many plants have a thin waxy layer called the cuticle over the epidermis<|>In dicotyledonous plants, stomata occur only in the lower epidermis<|>In monocotyledonous plants, stomata are equally distributed on both surfaces of the leaf
• Mesophyll


  Tissue between the upper and lower epidermis<|>Consists of upper palisade mesophyll layer and lower spongy mesophyll layer<|>Palisade mesophyll cells are long and contain many chloroplasts<|>Spongy mesophyll cells vary in shape, are loosely arranged with many air spaces between them, and also contain chloroplasts
• Veins


  Major vein of the leaf is the midrib, continuation of petiole<|>Veins are branched off as a network throughout the leaf surface<|>Vascular bundles in the petiole, midrib and veins consist of xylem and phloem
• Elements in xylem


  • Vessels
  • Tracheids
  • Xylem parenchyma
  • Xylem fibres
• Elements in phloem


  • Sieve tubes
  • Companion cells
  • Phloem parenchyma
  • Phloem fibres
• Sieve tubes


  Elongated cells with perforated transverse walls (sieve plates)
• The cuticle (waxy layer) is present on both surfaces of a leaf and protects the leaf surface
• The epidermis helps to keep the leaf's shape and to reduce water loss
• The palisade mesophyll exposes many cells to light rays, where photosynthesis mainly takes place
• The spongy mesophyll allows diffusion of carbon dioxide and oxygen in and out of mesophyll cells, where photosynthesis also takes place
• The veins consist of vascular bundles (xylem and phloem) and support the leaf
• The lower epidermis protects inner tissues and reduces water loss, and is the site of gas exchange into and out of the leaf
• Chloroplast


  Membrane bound organelle containing chlorophyll a, b and other pigments<|>The most important function is to synthesize food by the process of photosynthesis
• Structure of chloroplast


  • Generally biconvex shaped
  • Different shapes in different plants like spheroid, filamentous, saucer, discoid or oval
  • Vesicular with colourless centre
• Parts of a chloroplast

  • Outer membrane
  • Intermembrane space
  • Inner membrane
  • Stroma
  • Thylakoid system
• Outer membrane


  Semi-porous and permeable to small molecules and ions
• Intermembrane space


  Thin space about 10-20 nm between the outer and inner membranes
• Inner membrane


  Forms a border to the stroma and regulates passage of materials in and out of the chloroplast
• Stroma


  Protein rich aqueous fluid within the inner membrane<|>Contains chloroplast DNA, ribosomes, thylakoid system, starch granules and many proteins<|>Site for the dark reaction of photosynthesis
• Thylakoid system


  Membranous sacs arranged in stacks known as grana, suspended in the stroma
• Semi-porous membrane


  Permeable to small molecules and ions which diffuse easily
• Intermembrane space


  Thin space about 10-20 nm between the outer and inner membranes of the chloroplast
• Inner membrane


  Forms a border to the stroma, regulates passage of materials in and out of the chloroplast
• Stroma


  Protein rich aqueous fluid within the inner membrane, site for the dark reaction of photosynthesis
• Thylakoid system


  Membranous sacs arranged in stacks known as grana, site for the light reaction of photosynthesis
• Chlorophyll


  Green pigment present on the membrane of thylakoid, reflects mostly the green portion of light
• Light and Dark Reactions of Photosynthesis

  1. Light reaction (light-dependent)

  2. Dark reaction (light-independent)
• Light reaction


  Light-dependent process including light absorption, photolysis, release of oxygen, and formation of ATP and NADPH
• Dark reaction


  Light-independent process where sugar molecules are formed using carbon dioxide, ATP and NADPH from light reaction
• Light reaction


  Produces ATP and NADPH
• Dark reaction


  Uses ATP and NADPH from light reaction to convert carbon dioxide into glucose
• Differences between light and dark reactions

  • Occurrence (presence/absence of light)
  • Phase (photochemical/biochemical)
  • Place (thylakoid membrane/stroma)
  • Ions (NADP utilizes H ions/NADPH combines with CO2)
  • Molecules (water split into H and O/CO2 converted to glucose)
  • End product (ATP and NADPH/glucose)
  • Photolysis (occurs/does not occur)
• Factors affecting photosynthesis


  • Light intensity
  • Temperature
  • Carbon dioxide concentration
• Light intensity increases


  Rate of photosynthesis increases until maximum
• Temperature increases up to 40°C


  Rate of photosynthesis increases