Transport in Angiosperms Topic 9.2

Transpiration  The loss of water vapor from leaves occurs through stomata.  Stomata are surrounded by 2 guard cells which open and close  As water exits the leaves, it is replaced by water entering via the root  Energy from the sun drives this process: transpiration-adhesion-cohesion-tension theory.

The process of transpiration  Heat is produced when light strikes a leaf.  Water in the spongy mesophyll tissue enters the vapour phase.  Water evaporates through the stomatal pore down a humidity gradient.  The evaporation of water draws (pulls) more water by mass flow into the spongy mesophyll space.  Water molecules are held together cohesion due to hydrogen bonds between water molecules.

 In turn this draws water from the end of the xylem by the same cohesion.  Water is therefore drawn up the stem by cohesion between water molecules and adhesion to the xylem vessel walls.  This transpiration 'pull or tension' extends all the way down the xylem to the root

The xylem   2 major cell types   Tracheids – dead at maturity. Have tapered ends that connect to other tracheids via pits   Vessel elements – dead at maturity with lignified (woody) cell walls. These lie end to end like straws. These are most efficient at moving water.

  Ancient plants had only tracheids   Most modern plants have only vessel elements

The CO 2 dilemma!  Plants can only acquire carbon dioxide when their stomata are open. Remember CO 2 is necessary for carbon fixation in photosynthesis.  Guard cells regulate the opening and closing of the stomata

Guard cells  The walls of the cells are of uneven thickness (inner wall thicker than outer)

 When the cells take in water the outer part bulges which opens the stomata. When they lose water they sag and collapse over the stomatal opening.  Potassium ions are actively pumped into the guard cells causing water to enter via osmosis, thus opening the stomata.  K ion pumps are stimulated by light from the blue part of the spectrum.

 As the ions diffuse out so does water, collapsing the guard cells.  The hormone abscisic acid causes K ions to diffuse from the guard cells rapidly. This hormone is made by the roots in times of drought

To study   Table page 250 – how environmental factors affect transpiration   Table page 253 – details of the transpiration-cohesion-tension theory

Xerophytes adaptations   Plants adapted to dry (arid) climates   Small thick leaves   Stomata in pits   Waxy cuticle   Fewer stomata   Hairs on leaves   Loss of leaves in dry months   Water storage in stems

Alternate photosynthetic processes   Both processes developed to conserve water   CAM – crassulacean acid metabolism. Carbon dioxide is fixed at night and incorporated into organic acids. It is released during the day for photosynthesis   C 4 photosynthesis – Carbon dioxide is incorporated into a 4-C compound then moved to the interior of the leaf. More Carbon dioxide can be fixed than in a C 3 plant

Roots –  Recall that root epidermal cells have extensions called root hairs to increase the surface area for water and mineral absorption.  As a root pushes through the soil it is protected by a root cap.

Zone of cell division – closest to tip, cells undifferentiated, mitosis is occurring Zone of elongation – cells are enlarging, G 1 of cell cycle Zone of maturation – cells are differentiating and beginning to function. This is where root hairs begin to be noticed.

Water movement  Water must travel through the root to the vascular cylinder which is in the center of the root.  The vascular cylinder is surrounded by endodermis and pericycle

 Pericycle consists of cells that can produce lateral roots

 Endodermis is a cylinder 1 cell thick that forms a selective barrier which regulates passage of substances from soil into VC  Each endodermal cell has a barrier called the Casperian strip. This strip is made of suberin, a waxy, impermeable layer  The impermeable suberin ensures that all water and minerals entering the plant must cross a semipermeable membrane.

Passage of water and minerals into root  Root hairs absorb soil solution (water and minerals)  The soil solution can travel 2 routes  Apoplastic route  Symplastic route

Apoplastic route  Water and minerals travel between cell walls through the cortex region  Some solution enters cells some does not  When the solution reaches the endodermis, the Casperian strip forces the solution into a cell so that it has to pass through a cell membrane

Symplastic route  Soil solution passes through cells (and their membranes) on their way to the stele (vascular cylinder)

 Once past the endodermis, water and minerals enter the xylem vessels to be transported throughout the plant.  Is the solution (now called xylem sap) pushed or pulled through a plant?  Push due to root pressure  Pull by tension generated by transpiration

Movement of ions through plants  Minerals dissolved in water may enter by diffusion if concentration inside the root is lower than concentration outside root  Fungal hyphae called mycorrhiza increase the root surface area for water and mineral absorption (mutualism)

 If mineral concentration inside the plant is higher than in the soil, active transport is required. This is also necessary if the ion cannot cross the phospholipid bilayer.  Active transport requires a transport protein  K + ions move through proteins called potassium channels

Proton pump  ATP provides energy to pump H + ions out of cell  This makes the inside of the cell more negative than the outside  The hydrogen ion gradient causes a voltage difference called membrane potential. (-120 mv)  Membrane potential is a form of potential energy that can be used to absorb mineral ions.

 Hydrogen ions may displace cations attached to the soil, freeing them so they may enter root  Hydrogen ions may combine with anions and drag the anion into the root via cotransport

9.2.4 Plant support  Cellulose cell walls – walls may thicken for additional support  Lignified cell - A complex polymer, the chief noncarbohydrate constituent of wood, that binds to cellulose fibers and hardens and strengthens the cell.  Collenchyma, vascular bundles, sclerenchyma, provide flexible support

Turgor pressure  How do osmosis, the plant cell vacuole, and the plant cell wall work together to provide turgor pressure?  What happens if turgor pressure is lost?

Movement of Sugars AKA Translocation  Sugars and organic molecules are transported through sieve tube members and their companion cells  Place where molecules originate is called the source. Could be a leaf or a storage organ  Place where molecules are going is called the sink. Could be where growth is occurring or a fruit or tuber

  Companion cells load sucrose (soluble and not metabolically active) into sieve tubes.   Water follows via osmosis causing positive pressure in sieve tube. This makes phloem sap flow.   Companion cells unload sucrose at sink (requires ATP as it is against concentration gradient.   Sugars may be converted to starch in sink

  Water used in translocation is recycled by xylem

  1. Source produces organic molecules   2. Glucose from photosynthesis produced   3.Glucose converted to sucrose for transport   4. Companion cell actively loads the sucrose   5. Water follows from xylem by osmosis   6. Sap volume and pressure increased to give Mass flow   7. Unload the organic molecules by the companion cell   8. Sucrose stored as the insoluble and unreactive starch   9. Water that is released is picked up by the xylem   10. water recycles as part of transpiration to re supply the sucrose loading   Source: