1. © SSER Ltd.

3. THE MAJOR PHOTOSYNTHETIC ORGAN OF GREEN PLANTS
Leaf Structure
THE MAJOR PHOTOSYNTHETIC ORGAN OF GREEN PLANTS
This typical dicotyledonous
sycamore leaf is the major
photosynthetic organ
for the sycamore tree
Dicotyledonous leaves are
structurally adapted for
their photosynthetic role

4. THE DICOTYLEDON LEAF The palisade layer of cells located close
A section through the leaf
from upper to lower surface
reveals a characteristic
arrangement of tissues
The palisade
layer of cells
located close
to the upper
epidermis are
the principal
photosynthetic
cells of the leaf
These palisade
cells are packed
with chloroplasts
chloroplasts are the
site of photosynthesis

5. STRUCTURE OF THE DICOTYLEDON LEAF
waxy cuticle
upper epidermis
Layer of palisade
mesophyll cells – main photosynthetic layer
xylem vessel
Spongy mesophyll layer;
layer of irregular shaped
cells with numerous air
spaces
substomatal
air space
lower epidermis
stoma
guard cell
thin cuticle

6. STRUCTURE OF THE DICOTYLEDON LEAF
UPPER EPIDERMIS
PALISADE
CELLS
STOMA
AND GUARD CELLS
SPONGY
MESOPHYLL
CELLS
XYLEM
INTERCELLULAR
AIR SPACES
PHLOEM
MAGNIFY
PARENCHYMA
LOWER EPIDERMIS
TS lamina and midrib of a privet leaf (Ligustrum) – a typical dicotyledon
STRUCTURE OF THE DICOTYLEDON LEAF

7. STRUCTURE OF THE DICOTYLEDON LEAF
UPPER EPIDERMIS
PALISADE
CELLS
LEAF VEIN
INTERCELLULAR
AIR SPACES
LOWER EPIDERMIS
SPONGY
MESOPHYLL
CELLS
GUARD
CELL
STOMA

8. ADAPTATIONS OF THE DICOTYLEDON LEAF FOR PHOTOSYNTHESIS
The colourless flattened cells of the epidermis and
the colourless protective waxy cuticle readily allow light
to pass through the leaf surface to the photosynthetic tissue
The closely packed palisade
cells with their numerous
chloroplasts maximise light
absorption for photosynthesis
Numerous transport tissues
permeate the leaf structure
allowing water to be efficiently delivered to the photosynthetic cells
The extensive network of air
spaces in the spongy mesophyll
layer provides for an easy
passage of gases to and from
the palisade cells and an
efficient gas exchange system
via the stomata
Stomata are the sites of gas exchange and their opening and closing is controlled by specialised epidermal cells called guard cells – such regulation allows for efficient gas exchange while, at the same time, reducing water losses through transpiration

9. According to Fick’s Law:
ADAPTATIONS OF THE DICOTYLEDON LEAF FOR PHOTOSYNTHESIS
The leaf is very thin and presents a large surface area for maximising light absorption and gas exchange
According to Fick’s Law:
Rate of diffusion =
surface area x difference in concentration
thickness of the diffusion barrier
The large surface area presented by dicotyledon leaves and the large differences in concentration for oxygen and carbon dioxide between the leaf interior and
the external environment, produce a high value for the top line of this equation
The thinness of the leaf, providing short diffusion paths for gases from the environment to the photosynthesising cells of the palisade layer, produces a low value for the bottom line of the equation
The consequences of these leaf features allow for a rapid rate of diffusion of CO2 gas to the photosynthesising cells – a feature essential for efficient photosynthesis

10. MORE ABOUT PALISADE CELLS
Palisade cells are well
suited to their role as the
principal photosynthesising
cells of the leaf
A PALISADE CELL
vacuole containing
cell sap
nucleus
cell wall
cell surface
membrane
chloroplasts
The column-shaped
nature of the cells allows
for close packing of the
palisade layer close to the
surface of the leaf, and
each cell possesses
numerous chloroplasts
The chloroplasts are able to move freely around the cytoplasm
The chloroplasts have
the ability to orientate themselves in positions that maximise light
absorption in dim light
and reduce potential damage when light intensities are very high

11. MORE ABOUT PALISADE CELLS
IN DIM LIGHT
IN INTENSE LIGHT
In dim light, chloroplasts tend to
aggregate at the top surface of the
cell and to orientate themselves so
as to display a large proportion
of their surface area to the
incoming light rays
In intense light, chloroplasts
aggregate at the lower end
of the cell and orientate themselves
in a vertical position. This behaviour
reduces the chances of damage to
the chloroplasts through bleaching

12. CLOSED STOMA OPEN STOMA STOMATA AND GUARD CELLS
Stomata are the sites of gas exchange between the leaf and the environment
Stomata are the pores that allow for exchange of gases and their size is regulated by specialised epidermal cells called guard cells
CLOSED STOMA
OPEN STOMA
epidermal
cells
guard cells
containing
chloroplasts
Numerous stomata are found in the lower epidermis of leaves although they may be present, in fewer numbers, in the upper epidermis
The large number of stomata, together with the regulation of their size by guard cells, allows for efficient gas exchange at the interface between leaf and environment; this facilitates rapid uptake of CO2 for use by the photosynthetic cells during daylight hours

13. thick, inelastic inner wall of guard cell stomatal pore
thin, elastic outer wall of guard cell
epidermal cell

15. The leaves of many typical sun and shade plants display adaptations that enable them to thrive in their respective environments
thick cuticle to reduce transpiration losses in the more intense sunlight
thin cuticle
more than
one palisade
layer with
smaller
chloroplasts
single palisade
layer with larger chloroplasts
to maximise
light absorption
thicker leaf;
greater distance
between upper
and lower
epidermis
thinner leaf;
shorter distance
between upper
and lower
epidermis

16. Autotrophs manufacture their own food using simple inorganic sources and, in doing so, provide the resources on which heterotrophs depend
Approximately 600 billion tonnes of carbon dioxide is converted into organic food by the autotrophs each year, with 400 billion tonnes of oxygen being released into the environment
All life is centred around the activities of the autotrophs, with photosynthesis being the central process for powering
our existence