A Lesson On Photosynthesis

This covers the biochemistry of photosynthesis, leaf structure and adaptations, guard cells and limiting factors. It is a useful medium to help learn the content of the photosynthesis chapter. (Some of the naming of structures may be different depending on what you have been told to learn i.e. the tonoplast is the vacuolar membrane).

Written from the OCR syllabus.

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The Structure of a Leaf
Leaf blade
(lamina)
Lateral vein
mid-rib of Leaf stalk
leaf with (petiole)
main vein
Cross-section of the Leaf…read more

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Adaptations of the Leaf for
Photosynthesis
· Waxy cuticle and upper epidermis prevent water evaporation out of the
leaf and is transparent to allow light through
· The palisade mesophyll cells are tall and closely packed to absorb
maximum light. They contain many chloroplasts. Most photosynthesis takes
place in the palisade cells.
· The spongy mesophyll also captures light and makes food like palisade
mesophyll. Spongy mesophyll cells have air spaces between them to allow
easy gas exchange.
· The vascular bundle veins: contain xylem (top part of vein) for water
transport and phloem (lower part of vein) to take away dissolved food
· Stomata (mostly on the underside of the leaf) allow gas exchange; since a
lot of water vapour can be lost through the stomata they only open for
photosynthesis in daylight using guard cells; at night they close to reduce
loss of water vapour…read more

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Palisade mesophyll and Chloroplasts
the site of Photosynthesis
Palisade Mesophyll Cell
Chloroplast…read more

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Light Dependant Stage
Photo-activation of chlorophyll · This is a photosystem. There
are 2 types photosystems in the
Accessory LD stage: photosystem I and
pigments photosystem II.
· In a photosystem there are
several hundred accessory
pigments such as chlorophyll b
and carotene.
These absorb light (energy) as
photons and pass them down to
the primary pigment (reaction
Primary pigments centre) which is made of
(reaction centre) chlorophyll a.
The absorbed light energy
excites electrons in chlorophyll.…read more

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Light Dependant Stage
Photophosphorylation
Photophosphorylation ­ the conversion of light into the chemical energy of ATP
ADP + Pi ATP
light adding a phosphate group
· Light is absorbed by a photosystem and passed down by accessory pigments to
the reaction centre.
· Here an electron is excited to a higher energy level and then emitted.
· The electron is now captured by an electron acceptor.
· The electron is then passed along a series of electron carriers.
· As it is being passed along it looses energy. This energy goes towards making
ATP.
There are two routes that electrons can now take:
1. The electron can be passed back to reaction centre: cyclic phosphorylation
(occurs in PS II ; P680)
2. Or the electron can be passed on and lost from original photosystem: non-cyclic
(occurs in PS I ; P700)…read more

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