A2 OCR Biology - Photosynthesis

Autotrophs - organisms that use light or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules

Herterotrophs - organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them

Oxygen first appeared in the atmosphere after being released from photosynthesis - photosynthesis appeared before respiration

Photosynthesis takes place in organelle called chloroplasts

• disc shape between 2-10 micrometers long
• surrounded my double membrane - envelope
• there is an intermembrane space, 10-20 nm wide, between inner and outer membrane
• outer membrane = permeable to small ions
• inner membrane less permeable and has transport proteins
• inner membrane folded in lamellae which are stacked up into grana
• between grana is the intergranal lamellae
• stroma is the fluid filled matrix, containingstarch grains, oil droplets, DNA and ribosomes
• grana = stacks of flattened membrane compartments called thylakoids

• inner membrane (containing transport proteins) controls the entry and exit of substances
• grana provide a large SA for photosynthetic pigments, electron carriers and ATP synthase
• photosynthetic pigments arranged into photosystems allow maximum aborption of light energy
• proteins embedded in grana hold photosystems in place
• stroma contains enzymes needed to catalyse reactions in the light independent stage
• grana are surrounded by the stroma so the products of the light dependent reaction can readily pass into stroma to take part in light independent reaction
• chloroplasts can make some proteins needed for photosynthesis using the DNA and ribosomes

• can absorb certain wavelengths of light and reflect others
• appear as the colour they are reflecting 

Chlorophylls:

• chlorophyll = mixture of pigments containing a long hydrocarbon chain and a porphyrin group (similar to haem)
• light hitting cholorphyll causes a pair of electrons to become excited
• two forms of chlorophyll a (primary pigment) = P680 and P700, both appear yellow-green
• each absorbs red light at a slightly different wavelength
• both found at centre of photosystems  - primary pigment reaction centre
• P680 - found in PSII and peak absorption is 680nm
• P700 - found in PSI and peak absorption is 700nm
• chlorophyll a also absorbs blue light, wavelength 450nm
• chloropyhll b (accessory pigment) absorbs wavelengths around 500 and 640nm - appear blue-green

• carotenoids reflect yellow-green light and absorb blue light
• do not contain porphyrin group and not directly involved in light dependent reaction
• absorb light wavelengths that are not well absorbed by chlorophylls and pass energy to the chlorophyll a at the base of the photosystem
• carotene (orange) and xanthophyll (yellow) are main carotenoid pigments.

• takes place in thylakoid membranes
• PSII contains enzyme that takes part in photolysis (splitting of water)
• Some oxygen used in respiration, most of it diffuses out through the stomata into the air