Transport in Plants notes Flashcards
Describe the structure of Plant Roots
-Outer layer (stems and leaves)- EPIDERMIS
-Vascular Tissue- concentrated in a central STELE
-Stele- mainly composed of XYLEM tissue with a small amount of PHLOEM tissue
-Outside stele- single layer of cells- ENDODERMIS
-Layer of undifferentiated cells found between epidermis and endodermis- CORTEX- cells have small air spaces between cells and have starch grains
What is the main function of Xylem Tissue
Main cell involved in transport of water and inorganic ions
Describe the structure of Xylem cells
-DEAD tissue- chains of cells with no cell contents
-Made of cell walls whose END WALLS have broken- result in LONG HOLLOW TUBES- ideal for carrying water and mineral ions
-Within this, a SECONDARY WALL, impregnated with LIGNIN is formed inside the primary cellulose cell wall
-Lignin is impermeable to water
What is the function of Lignin
Provides great STRENGTH and prevents the vessels from collapsing when under pressure exerted by the transpiration stream ‘sucking’ water up the plant
-all vessel types provide structural support and waterproofing the tube
Describe the Lignin Patterns
-Protoxylem- first vessel to develop- lignin produces an ANNULAR or SPIRAL PATTERN
Function- allow vessels to elongate along with other tissues in the growth regions, such as the shoot
-Metaxylem- xylem vessels that are more mature- more extensive lignification- produce a RETICULATE or PITTED
Function: prevent further growth
Reticulate- thickened by interconnecting bars of lignin
Pitted- uniformly thickened, except at pores (pits) which allow rapid movement of water and ions out of the vessels to surrounding cells- require less pressure than living cells-no cell contents
What is the function of the Phloem tissue
They are the tissue responsible for the transport of sucrose (transport carbohydrate in plants), amino acids and other solutes are also present
-living
-conist fo sieve tube elements and companion cells
Describe the Sieve Tube Elements
-transporting cells
-lie end to end- form a continuous stack, called the sieve tube
-a small amount of cytoplasm at the edge of the sieve tube, containing ER and mitochondria, which line the inside of the cellulose wall
-The end walls where two sieve tube elements meet is called the SIEVE PLATE
-Sieve plate- contain numerous pores (perforations) which sugars pass
-sieve elements have MICROTUBULES- thought to help translocation of solutes
-next to each sieve tube element- Companion cell- Cytoplasm linked via PLASMODESMATA
-dense cytoplasm
-many mitochondria, due to high levels of metabolic activity
What is the role of Companion cells
Act as supporting cells, carrying out many metabolic activities for the highly specialised sieve tube elements
What is Transpiration
The evaporation of water from the mesophyll surface and the subsequent diffusion of water vapour through the stomata and into the atmosphere
How does water enter the root hair cell
Water and ions are taken up at the epidermis into the root hair cells which greatly increase the surface area
-Water moves from the soil (high water potential) to the vacuole (lower water potential) by osmosis
-the cell surface membrane is thin which allows for effective exchange surface
-the cell wall is fully permeable
Describe the apoplast pathway
-water moves along the cellulose microfibrils of the cell walls
most water goes via this way due to limited resistance to water movement
-the parallel arrangement of the microfibrils in the cellulose wall allows water to easily pass between the different layers, rather than through them
-this is further aided by the mesh-like arrangement of the walls
-as water moves through the wall, the cohesive properties of water (aided by hydrogen bonding) help pull the water column along
Describe the symplast pathway
-water moves via Osmosis through the cytoplasm of the cortex cells
-as the cell takes in water, the water potential becomes less negative and higher than the adjacent cell in the cortex- creates a water potential gradient
-water moves from the root hair cells, into the cortex cells by osmosis and across the cortex cells
How does the Casparian strip work
-apoplast pathway becomes blocked in the Endodermis layer
-the walls of the cells in the endodermis is impregnated with a waxy material called Suberin- which forms a wax around the cells, known as the Casparian strip
-Suberin is impermeable to water, therefore the Casparian Strip effectively stops water passing along the cell walls
-at this point, all water being transported by the apoplast pathway moves into the protoplast and joins water being transported by the symplast pathway
-This ensures WATER TRANSPORTED AT THIS POINT IS UDNER METABOLIC CONTROL
