Transcript Plant Structure and Function

Plant Structure and Function
That’s called physiology in bio-speak
The structure of a plant
• Roots, Stems and Leaves
• Plant tissue
– Dermal: outer covering of the plant
– Vascular: fluid conducting systems of the plant
– Ground: the rest of the plant (support and
photosynthesis!!)
• Plant Cells
– Thin walled cells: parenchyma
• Bulk of leaves, stems and roots; photosynthesis cells
– Thicker walled cells: collenchyma and sclerenchyma
• Ground tissue, support of the plant
– Fluid transport cells: xylem and phloem
Roots
• Collects water and nutrients from the soil
• Provides support of the part of the plant
below ground level
• Primary root grow first
• Secondary root grows after plant is
established
– Increases root surface area
Roots
• Outer covering is the epidermis
• Epidermal cells grow root hairs, thin walled
cell projections
• Why…. its about surface area again….
• Absorb nearly all water and nutrients
through the root hairs
Roots
• Cortex: just beneath the epidermis
– Responsible for moving water form the
epidermis to the vascular bundle
• Vascular bundle
– Center of the root is xylem and phloem
Roots
• Work by osmosis
• Normally there is a higher concentration of salts
inside the cells than outside, water flows into the
cells
• Root burn is when the opposite happens and water
flows outwards into the soil
• Nutrients are brought into the cells by active
transport (remember this)
• What molecule provides the energy for active
transport (ATP)
Roots
• Inner layer of the cortex is made of cells
called endodermis
• They secrete wax on one side to regulate the
flow of water into the vascular bundle
• Waxy layer is called the Casparian Strip
Stems
• Connect the roots to the leaves, hold the plant
upright towards the sun
• Surrounded by a layer of epidermal cells
• Vascular tissue in monocots is scattered in the
ground tissue
• In dicots vascular tissue are arranged in a ring,
ground tissue inside the ring is called the pith,
ground tissue outside the ring is called the cortex
Stems
• Stems get longer and thicker as the plant grows
• Cells responsible for generating new cells are
called cambium
• New cells are produced at the boundary of the
xylem and phloem by the vascular cambium
• Most of what we call wood is the years growth of
xylem
• As the phloem grows the older phloem can crack
open
• A cell layer called the cork cambium produces
bark to seal up the leaky phloem
Stems
• Phloem is located directly under the bark
• Girdling a tree cuts the phloem
• In temperate climate more xylem is
produced in the summer than winter
• Annual tree rings are snapshots of seasonal
xylem production
Leaves
• Main organ for photosynthesis
• Outer covering of epidermal cells, vascular
tissue, and ground tissue
• Attached to the stem by a petiole
Leaves
• Epidermis
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Cuticle protects against water loss and insect damage
Underside of the leaf has stomata and guard cells
When the stoma are open gas can diffuse in
Gas must dissolve in water to be used in photosynthesis
Stoma are opened by water swelling up the guard cell
Stoma are closed by guard cells shrinking due to lack of
water
Leaves
• Mesophyll
– Cells packed with chloroplasts
– Upper layers are packed closely together
– Underside layers are loosely packed with big spaces in
between, for gas penetration
• Vascular tissue found in veins (water moves by
osmosis here too)
– In monocots veins run parallel to each other
– In dicots the veins form a web in the leaf
Xylem Transport
• Water enters the roots by osmosis
• Water molecules are strongly attracted to
each other (cohesion; H-bonding)
• Water molecules are attracted to other biomolecules (adhesion; H-bonding)
• Result is capillary action: water can climb
inside thin tubes (xylem)
• This is force number one!
Xylem Transport
• Plants use water for photosynthesis
• Plants lose water by evaporation through the
stoma
• As plants use and lose water more water flows out
of the xylem in the leaf, into the leaf mesophyll to
be used or lost.
• This cycle is called transpiration
• Cohesion of water results in transpiration pull.
• This is force number two! And keeps the water
flowing up into the leaves
Phloem Transport
• Sap flows in both directions in the plant
• From the leaves to wherever sugar is needed
in the plant
• Active transport and osmosis work together
to move sugars in the phloem
• Mechanism is called the pressure-flow
hypothesis