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 –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 bio- molecules (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