Plant Structure and Function

1
Plant Structure and Function

• Thats called physiology in bio-speak

2
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

3
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

4
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

5
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

6
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)

7
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

8
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

9
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

10
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

11
Leaves

• Main organ for photosynthesis
• Outer covering of epidermal cells, vascular
  tissue, and ground tissue
• Attached to the stem by a petiole

12
Leaves

• Epidermis
• 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

13
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

14
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
  bio-molecules (adhesion H-bonding)
• Result is capillary action water can climb
  inside thin tubes (xylem)
• This is force number one!

15
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

16
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