Unit 5: Plants
Lesson 2: Leaves
LEAVES:
What are the functions of leaves?
 Site of photosynthesis
o Leaf cells absorb energy from sunlight through organelles called chloroplasts. When solar
energy is absorbed, the plant cell uses it to convert carbon dioxide and water into glucose
and oxygen
 Gas exchange
o Gas exchange between the interior of the plant and its environment is necessary. The
surface (epidermis) of most plant leaves contain may pores through which gases can pass in
and out. In photosynthesis, the plant uses carbon dioxide gas and releases oxygen gas and
water vapours through these pores. In cellular respiration, the plant uses oxygen and
produces carbon dioxide.
 Storage
o The sugars (glucose) that are produced during photosynthesis are stored in the leaves as
well as other parts of the plants
 Protection from predators
o Ex: cacti leaves are reduced to sharp spines; some leaves have surface hairs or toxins that
prevent herbivores from eating it
What is the structure of leaves?
The structure of a leaf helps it to carry out its functions.
External Structure:
The veins of the leaves contain vascular tissue. Recall
that vascular tissue allows for the transport of water
and nutrients and also support the plant body.
Internal Structure:
The epidermal cells are tightly packed in a single layer and covered by a waxy coating called a cuticle.
The cuticle prevents water loss and provides a physical barrier against bacteria, moulds and insects.
Epidermal cells do not contain chloroplasts, so they do not perform photosynthesis. Epidermal cells are
transparent, so light can pass through them to the cell within the leaf.
Chloroplasts are found mainly in the cells of the mesophyll, which means ‘middle leaf’. This ground tissue
is specialized for photosynthesis. There are two parts to the mesophyll:
1. palisade mesophyll (region directly under the upper epidermis). The tightly packed, elongated
arrangement maximizes the amount of light the plant can collect for photosynthesis
2. spongy mesophyll (region directly beneath the palisade). The loosely packed layer with lots of
air spaces allow for gas exchange between the mesophyll cells and the atmosphere through
stomata.
Stomata & Guard Cells
A stoma (plural: stomata) is an opening in the epidermis of a leaf, through which gases pass in and out.
Two kidney-shaped cells, called guard cells, control the opening and closing of a stoma. In terrestrial
plants, most of the stomata are in the lower epidermal layer, below the spongy mesophyll. When stomata
are open, carbon dioxide can enter the leaf and oxygen can escape. This helps the plant photosynthesize.
Ideally, plants would open their stomata whenever it was sunny. However, when stomata are open, water
vapour can also escape. A very thin leaf could dry out and die very quickly on a sunny or windy day.
Preventing too much water loss is therefore a major concern for many plants.
Guard cells are an adaptation to help the plant conserve water by altering their shape in response
to water levels in the leaf. If there is a good supply of water within the leaf, the guard cells expand and
bend apart -- opening the stomata. If there is a shortage of water, the guard cells become soft and
collapse -- closing the stomata. Guard cells also have a mechanism that responds to light levels. This lets
them close the stomata at night when carbon dioxide is not needed because there is not light for
photosynthesis.
In leaves, the vascular tissue (xylem and phloem) are arranged into veins that run through the spongy
mesophyll. Xylem carries water, mineral and nutrients upwards from the roots to the top of the plant.
Phloem carries food(sugars) from one part of a plant to another. It can move upwards or downwards.
Photosynthesis and Cellular Respiration
Photosynthesis
Light is used with Carbon dioxide from the air and
water from the root system to produce glucose
and oxygen. This occurs during the day.
6CO2 + 6H2O + Light Energy = C6H12O6 + 6O2
Cellular Respiration
The glucose is broken down (with oxygen from the air)
to create ATP with carbon dioxide and water as by
products.
C6H12O6+6O2 ----------> 6CO2+6H2O+36ATP
Questions:
1. What functions do leaves serve to the plant?
2. How do monocot and dicot leaves differ?
3. Describe the function of the following:
epidermis, cuticle, mesophyll, guard cells.
4. Explain how the organization of cells in the palisade mesophyll and spongy mesophyll in a typical
vascular plant help fulfill the leaf’s role?
5. Explain how the guard cells work to allow the stomata to open and close. Why does stomata open
and close?
The leaf is the primary
photosynthetic organ of the
plant. It consists of a
flattened portion, called the
blade, that is attached to the
plant by a structure called
the petiole. Sometimes
leaves are divided into two
or more sections called
leaflets. Leaves with a single
undivided blade are called
simple, those with two or
more leaflets are called
compound.
The outer surface of the leaf
has a thin waxy covering
called the cuticle (A), this
layer's primary function is to
prevent water loss within the
leaf. (Plants that leave
entirely within water do not
have a cuticle). Directly
underneath the cuticle is a
layer of cells called
the epidermis (B). The
vascular tissue, xylem and
phloem are found within the
veins of the leaf. Veins are
actually extensions that run
from to tips of the roots all
the way up to the edges of
the leaves. The outer layer
of the vein is made of cells
called bundle sheath
cells (E), and they create a
circle around the xylem and
the phloem. On the
picture, xylem is the upper
layer of cells (G) and is
shaded a little lighter than
the lower layer of cells phloem (H). Recall that
xylem transports water and
phloem transports sugar
(food).
Within the leaf, there is a layer of cells called the mesophyll. The word mesophyll is greek and means "middle" (meso) "leaf" (phyllon).
Mesophyll can then be divided into two layers, the palisade layer (D) and the spongy layer (F). Palisade cells are more column-like, and lie
just under the epidermis, the spongy cells are more loosely packed and lie between the palisade layer and the lower epidermis. The air spaces
between the spongy cells allow for gas exchange. Mesophyll cells (both palisade and spongy) are packed with chloroplasts, and this is where
photosynthesis actually occurs.
Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny holes within the epidermis called stomata.
Specialized cells, called guard cells (C) surround the stomata and are shaped like two cupped hands. Changes within water pressure cause
the stoma (singular of stomata) to open or close. If the guard cells are full of water, they swell up and bend away from each other which opens
the stoma. During dry times, the guard cells close.
Color the structures underlined above. Make sure that the entire picture is colored and that the color matches the words. For simplicity only part
of the picture is labeled.
Epidermis (yellow)
Guard cells (pink)
Spongy Mesophyll (light green)
Palisade Mesophyll (dark green)
Bundle Sheath(dark blue)
Phloem (purple)
Xylem (orange)