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• Photosynthesis


  The process in which green plants trap light energy and use it in the production of carbohydrates
• Photosynthesis


  1. Series of chemical reactions

  2. Most take place in a plant's chloroplasts
• Plants


  • Autotrophs (can make their own food)
  • The sun is the source of energy for almost all life on Earth
  • Light energy from the sun is captured by plants and other photosynthetic organisms and converted into chemical energy
• Photosynthesis


  1. Uses light energy to combine simple inorganic compounds - water and carbon dioxide - to produce carbon-containing organic molecules such as glucose which contain stored chemical energy

  2. Oxygen is released as a waste product

  3. The energy stored in molecules such as glucose provides a source of food for organisms that cannot photosynthesise (i.e. heterotrophs)
• Reactants for photosynthesis


  • Carbon dioxide from the environment, enters through the stomata
  • Water from the roots absorbed from the soil
• Products of photosynthesis


  • Oxygen: released from the leaf through the stomata
  • Organic compounds, such as glucose (C6H12O6 carbohydrate): Used by the plant for respiration
  • Some glucose is converted into sucrose and transported to other parts of the plant
  • Surplus of sugars is stored as starch by the plants
  • Cellulose (polysaccharide) can be made and used to build the cell wall
  • Energy stored in the organic compounds made by plants can be eaten by other organisms (other organic compounds such as lipids and amino acids are also made by photosynthesis, but we will only focus on glucose)
• Light energy


  • Travels in waves, and light of different colours have different wavelengths
  • White light (visible light) is made up of red, orange, yellow, green, blue, indigo and violet wavelengths (rainbow!)
  • The colour of an object is determined by the wavelength of the light that it reflects
  • Green leaves therefore reflect the green spectrum of light and absorbs all other wavelengths
• Absorption


  • Most of the light striking a leaf is absorbed by the photosynthetic pigments in the chloroplasts
  • Of the absorbed light, only 4% is used in photosynthesis, the rest is converted to heat energy and lost by radiation
• Reflection


  Light which cannot be absorbed (i.e. green wavelengths) are reflected
• Photosynthetic pigments


  • Found inside the chloroplasts in the leaf
  • The granum of the chloroplast is made up of bundles/stacks of thylakoids and it is here that the photosynthetic pigments can be found
  • There are 4 photosynthetic pigments: chlorophyll a, chlorophyll b, and the carotenoids - carotene and xanthophyll
  • These pigments absorb different wavelengths of light energy
• Absorption spectrum

  A graph that shows the range and extent to which each wavelength of light is absorbed by a particular pigment
• Action spectrum


  A graph showing the rate of photosynthesis at each wavelength of light in a plant
• Absorption spectrum (solid line) and action spectrum (dotted line)


  • Similar in shape, but not identical
  • Comparison of the 2 spectra helps to confirm that pigments other than chlorophyll a are used in photosynthesis
• Point X: the rate of photosynthesis remains high although the absorption by chlorophyll a is low
• Carotenoids


  Extend the range of wavelengths absorbed and pass the energy that they absorb to the chlorophyll pigments for photosynthesis
• • Chlorophyll a and b primarily absorb light in blue and red regions of spectrum
  • Carotenoids absorb light from other regions of the spectrum (i.e. green) and pass the energy on to chlorophyll
• • The total quantity of light absorbed is greater than if only one pigment was involved
  • This means that the plant can photosynthesise more, producing more glucose for processes such as respiration, growth etc.
• Photosynthesis - the two stages

  1. Light-dependent reactions such as photolysis

  2. Light-independent reactions i.e. Carbon fixation/Calvin Cycle
• Stage 1: Photolysis


  1. Location: Thylakoids

  2. Photochemical stage

  3. Light energy is used to split apart a water molecule in a process called photolysis

  4. Oxygen and hydrogen formed as by-products

  5. Light energy also absorbed by chlorophyll and some ATP is generated
• Stage 2: Carbon fixation stage (Calvin cycle)

  1. Location: Stroma

  2. Thermochemical stage

  3. It consists of several enzyme controlled chemical reactions which take the form of a cycle (Calvin Cycle)

  4. Conversion of inorganic carbon dioxide to organic molecules such as glucose is known as carbon fixation

  5. Glucose is formed

  6. ATP and hydrogen ions from the first stage are used with carbon dioxide to produce glucose

  7. ATP is needed in the enzymatic reactions taking place during the Calvin Cycle
• Granum (stack of thylakoids)


  • *NADP can combine with hydrogen (formed from stage 1) to make NADPH – brings it into the Calvin Cycle – no need to remember this at PreIB
• Photosynthesis stages


  • Photolysis (light-dependent), photochemical
  • Calvin cycle/Carbon fixation (light-independent), thermochemical
• Photosynthesis stage reactants and products

  • Photolysis: Reactants - Water
  • Photolysis: Products - Oxygen, hydrogen, ATP
  • Calvin cycle/Carbon fixation: Reactants - Carbon dioxide, hydrogen
  • Calvin cycle/Carbon fixation: Products - Glucose
• • Photolysis location: Granum (made up of thylakoids)
  • Calvin cycle/Carbon fixation location: Stroma
• Plants can make their own food it's ace. Here's how...
• Photosynthesis


  1. Produces Glucose

  2. Using Sunlight
• Photosynthesis


  The process that produces 'food' in plants. The 'food' it produces is glucose.
• Photosynthesis happens in the leaves of all green plants
• Chloroplasts


  Found in leaf cells and other green parts of a plant. Contain chlorophyll which absorbs sunlight and uses its energy to convert carbon dioxide and water into glucose. Oxygen is also produced.
• You need to learn the word and symbol equations for photosynthesis
• Photosynthesis equation


  • 6CO2 + 6H2O → C6H12O6 + 6O2
• Photosynthesis is an important process because it converts light energy to chemical energy, which is stored in the glucose. This chemical energy is released when glucose is broken down during respiration
• Parts of a typical leaf

  • Palisade mesophyll layer
  • Spongy mesophyll layer
  • Air space
  • Waxy cuticle
  • Upper epidermis
  • Chloroplast
  • Vascular bundle
  • Lower epidermis
  • Guard cell
  • Stoma
• Leaves


  • Broad, so there's a large surface area exposed to light
  • Most chloroplasts are found in the palisade layer near the top of the leaf to get the most light
  • Upper epidermis is transparent so light can pass through
  • Network of vascular bundles to deliver water, nutrients and take away glucose
  • Waxy cuticle to reduce water loss
• Leaves have adaptations for efficient gas exchange that also make photosynthesis more efficient, e.g. the lower surface is full of stomata that let CO2 diffuse directly into the leaf
• Limiting factor


  Something which stops photosynthesis from happening any faster
• Limiting factors for photosynthesis

  • Light intensity
  • CO2 concentration
  • Temperature
• Light intensity increases


  Rate of photosynthesis increases up to a point, then it flattens out as another factor becomes limiting
• CO2 concentration increases

  Rate of photosynthesis increases up to a point, then it flattens out as another factor becomes limiting
• Temperature increases


  Rate of photosynthesis increases up to around 45°C, then it rapidly decreases as enzymes are denatured