B4: Plant nutrition
Adaptation
Function
supported by stem and leaf stalk
to expose as much of the leaf as possible to the sunlight and air
large surface area
to expose as large an area as possible to the sunlight and air
thin
to allow sunlight to penetrate to all cells; to allow CO2 to diffuse in and O2 to diffuse out as quickly as possible
stomata in lower epidermis
to allow CO2 to diffuse in and O2 to diffuse out
air spaces in spongy mesophyll
to allow CO2 and O2 to diffuse to and from all cells
no chloroplasts in epidermal cells
to allow sunlight to penetrate to the mesophyll layer
chloroplasts containing chlorophyll present in the mesophyll layer
to absorb energy from sunlight, so that CO2 will combine with H2O
palisade cells arranged end on
to keep as few cell walls as possible between sunlight and the chloroplasts
chloroplasts inside palisade cells often arranged broadside on
to expose as much chlorophyll as possible to sunlight
chlorophyll arranged on flat membranes inside the chloroplasts
to expose as much chlorophyll as possible to sunlight
xylem vessels within short distance of every mesophyll cell
to supply water to the cells in the leaf, some of which will be used in photosynthesis
phloem tubes within short distance of every mesophyll cell to take away sucrose and other organic products of photosynthesis Table B4.01 Adaptations of leaves for photosynthesis.
B4.04 Uses of glucose One of the first carbohydrates to be made in photosynthesis is glucose. There are several things that may then happen to it (Figure B4.06).
Used for energy Energy may be released from glucose in the leaf. All cells need energy, which they obtain by the process of respiration (Chapter B8). Some of the glucose which a leaf makes will be broken down by respiration, to release energy.
Stored as starch
Sucrose is used for fruit growth or stored in fruits.
Sucrose is used for shoot growth. glucose sucrose starch
Sucrose is used for root growth or stored in roots as starch.
Sucrose is transported in phloem tubes.
Glucose may be turned into starch and stored in the leaf. Glucose is a simple sugar (Section B3.02). It is soluble in water, and quite a reactive substance. It is not, therefore, a very good storage molecule. First, being reactive, it might get involved in chemical reactions where it is not wanted. Secondly, it would dissolve in the water in and around Figure B4.06 The products of photosynthesis. Original material © Cambridge University Press 2017
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