Transcript Lesson 2: Leaves - Halton District School Board

Unit 5: Plants

Site of photosynthesis
 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
 Gas exchange between the interior of the plant
and its environment
 Epidermis contains pores through which gases
can pass in and out.
 In photosynthesis, the plant uses CO2 and
releases O2 and H2O vapours through these
pores.

Storage
 The sugars (glucose) that are produced during
photosynthesis are stored in the leaves as well as
other parts of the plants

Protection from predators
 Ex: cacti leaves are reduced to sharp spines; some
leaves have surface hairs or toxins that prevent
herbivores from eating it

The epidermal cells are tightly packed in a
single layer and covered by a waxy coating
called a cuticle.

The cuticle
 prevents water loss
 provides a physical barrier against bacteria,
moulds and insects.

The Epidermis

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:



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palisade mesophyll (region directly under the
upper epidermis). The tightly packed, elongated
arrangement maximizes the amount of light the
plant can collect for photosynthesis
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.

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.

How is this problematic?

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.
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.

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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
 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

https://www.youtube.com/watch?v=eo5XndJ
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