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Plant Transport Stem Support 1. Ground tissues: living cells that make up the bulk of stem / root of non-woody (herbaceous) plants Parenchyma support of herbaceous stem through turgidity sites of starch storage Collenchyma additional layers of cellulose thickening laid down unevenly [ Parenchyma : air spaces between living cells ; Collenchyma : thick cellulose wall at corners, thin cellulose elsewhere ] 2. Xylem tissue: cellulose hardening with lignin ( transport of water from root to shoot only, transpiration pull) cambium collen phloem epidermis xylem paren Vascular bundles pith Transport of water - Begin : elongated cells with cellulose walls & living contents connected End : dissolved away to become long, hollow tubes (mature) Because deposit of cellulose thickening to inside of lateral walls of vessel + hardening with lignin - Tough tissues & strengthens itself internally X collapse in itself (spirally thickened) - Centrally placed vascular tissue is contained by endodermis, layer of cells unique to the root At endodermis, waxy strip in radial walls blocks passage of water by apoplast route momentarily (CASPARIAN STRIP) water passes endodermis through osmosis At leaves, transpiration occurs ( evaporation of water vapour through stomata of green plant leaves & stems) - Water uptake (roots) - Transport : nutrients, oxygen, elaborated foods, plant growth substances 1. Mass flow (most significant) : through interconnecting free spaces between the cellulose fibres of plant cell walls. The pathway APOPLAST entirely avoids living contents of cells, hence including dead cells (e.g. xylem vessels) 2. Diffusion : through cytoplasm of cells and via the cytoplasmic connections between cells (plasmodesmata), a.k.a. SYMPLAST. (less significant because plant cells have organelles resistance) 3. Osmosis : from vacuole to vacuole of cells, driven by gradient in osmotic pressure ( recall definition for osmosis). A means for individual cells to absorb water. Roots absorb water from soil by osmosis whereas absorption of minerals is mainly achieved by active transport. The absorbed water and minerals are transported through xylem vessels. Process of absorption of water 1. Concentration of solute in root hair cells is higher than soil water water potential of root hair cell < soil water 2. Water enter root hair cells by osmosis 3. Water potential in root hair cells > adjacent cells, water movement through osmosis and so on (due to gradient in of water potential) 4. Water enters xylem vessels in Distribution of tissues in different parts of dicotyledonous plants Leaf Upper epidermis xylem Lower epidermis cambium phloem phloem Ste m epidermis pith phloem cortex xylem the middle of the root cambium epidermis Root phloem xylem Guard cells : - Contain chloroplasts that carry out photosynthesis in the presence of light. - Glucose is formed and thus lower the water potential of the cell - Water enters the guard cells from adjacent cells by osmosis, guard cells turgid - Inner wall is thicker than outer wall, cell stretches to outer side stoma opens At night, no photosynthesis, guard cells become flaccid stoma closes Xerophytes ( plants in dry envt. ) reduce water loss by evaporation - Numerous epidermal hair trap moistureRoot - Sunken stomata trap moisture - Rolling leaves enclosing stomata reduce contact between stomata and envt - Spiny leaves reduce surface area - Fleshy stems / leaves help store water - Shed leaves during dry seasons reduce rate of transpiration Ion uptake (ions in soil solution are at low conc.) Active transport: 1. Tranport of molecules against concentration gradient (from low to high) hoarding of valuable ions in cytoplasm 2. Selective process : (reflects needs of plants) 3. Involves pumps : globular protein carriers that traverse the lipid bilayer. Requre energy from reaction with ATP to change shape of such carriers ( pumping action) specific & “active” * Supply for ATP energy maintained through aerobic cell respiration by roots Adequate supply of ions 1. Mass flow of water: deliver fresh soil solution 2. Active uptake of ions through apoplast : maintains concentration gradient, ions diffuse outside apoplast adjacent to protein pumps 3. Mutual relationship with soil-inhabiting fungi: a) fungal hyphae: receive supply of sugar from plant root excessive sugar b) plant roots: root cell ions taken up when they were available in soil Uptake of minerals - Mineral uptake is achieved by active transport which requires energy because concentration of minerals in the cytoplasm of root hair cell is lower than that in the soil water (transport against concentration gradient) \ Transpiration pull / stream : ( losing water from leaf through stomata) - An evaporation of water in the form of water vapour from plant body surface into the atmosphere * Transpiration pull process ; transpiration stream column of water - A thin film of water covers each mesophyll cell Water evaporates from the film of water surrounding the mesophyll cell into the air spaces Higher water vapour concentration inside the leaf than the atmosphere, water vapour diffuse out through the stomata Water potential within the cell will decrease draw water from adjacent cells by osmosis Water is drawn from xylem vessels due to the water potential difference - As water if drawn from xylem vessels, a suction pressure will develop and this pressure will pull water up (transpiration stream) * A water potential difference set up between mesophyll cells and the xylem vessels due to continuous water loss resulted in a suction pressure. This helps to pull water up the xylem vessels from the root is known as transpiration pull. The process is responsible for transport of water and minerals. * Hydrostatic pressure is pressure exerted by a fluid at equilibrium due to force of gravity. Phloem: - - Solutes flow through phloem from region of high hydrostatic pressure ( in & around photosynthesizing cells & phloem sieve tubes nearby) to region of low hydrostatic pressure ( ground tissue cells of stems / root / phloem sieve tubes nearby) Source area: presence of sugar - Sink area: starch storage cells * presence of sugar raises osmotic pressure water flows in (vice versa) Xylem Phloem Contains dead cells arranged from end-to-end Living cells arranged end-to-end Hollow tubes (no cytoplasm) End of each cells is formed by a cross-wall of cellulose with holes, i.e. sieve plates cell walls of cells derive most strength from lignin Retain cytoplasm and extend through holes of sieve plates to form sieve tubes (NO nucleus) Provide support to plant Companion cells help regulate metabolic activites of sieve tube elements Transport water and minerals (one direction because of transpiration pull) Transport of organic substances ( Translocation) Factors affecting transpiration 1. Light intensity temperature diffusion & evaporation rate of transpiration 2. Temperature diffusion & evaporation rate of transpiration 3. Relative humidity diffusion rate rate of transpiration 4. Wind speed diffusion & evaporation rate of transpiration 5. Availability of water stomata closes, plants wilt rate of transpiration