Transport in Transport in AngiospermsAngiospermsTopic 9.2Topic 9.2

Transpiration 9.2.5-Transpiration 9.2.5-9.2.109.2.10

The loss of water vapor from leaves occurs The loss of water vapor from leaves occurs through stomata.through stomata.

Stomata are surrounded by 2 guard cells which Stomata are surrounded by 2 guard cells which open and close open and close

As water exits the leaves, it is replaced by As water exits the leaves, it is replaced by water entering via the rootwater entering via the root

Energy from the sun drives this process: Energy from the sun drives this process: transpiration-adhesion-cohesion-tension transpiration-adhesion-cohesion-tension theory.theory.

The process of The process of transpirationtranspiration

Heat is produced when light strikes a leaf.Heat is produced when light strikes a leaf. Water in the spongy mesophyll tissue Water in the spongy mesophyll tissue

enters the vapour phase.enters the vapour phase. Water evaporates through the stomatal pore Water evaporates through the stomatal pore

down a humidity gradient.down a humidity gradient. The evaporation of water draws (pulls) more The evaporation of water draws (pulls) more

water by mass flow into the spongy water by mass flow into the spongy mesophyll space.mesophyll space.

Water molecules are held together cohesion Water molecules are held together cohesion due to hydrogen bonds between water due to hydrogen bonds between water molecules.molecules.

In turn this draws water from the end In turn this draws water from the end of the xylem by the same cohesion.of the xylem by the same cohesion.

Water is therefore drawn up the stem Water is therefore drawn up the stem by cohesion between water molecules by cohesion between water molecules and adhesion to the xylem vessel and adhesion to the xylem vessel walls.walls.

This transpiration 'pull or tension' This transpiration 'pull or tension' extends all the way down the xylem to extends all the way down the xylem to the rootthe 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 COThe CO22 dilemma! dilemma!

Plants can only acquire carbon dioxide Plants can only acquire carbon dioxide when their stomata are open. when their stomata are open. Remember CORemember CO22 is necessary for carbon is necessary for carbon

fixation in photosynthesis.fixation in photosynthesis. Guard cells regulate the opening and Guard cells regulate the opening and

closing of the stomataclosing of the stomata

Guard cellsGuard cells

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

When the cells take in water the outer part When the cells take in water the outer part bulges which opens the stomata. When they bulges which opens the stomata. When they lose water they sag and collapse over the lose water they sag and collapse over the stomatal opening.stomatal opening.

Potassium ions are actively pumped into the Potassium ions are actively pumped into the guard cells causing water to enter via osmosis, guard cells causing water to enter via osmosis, thus opening the stomata.thus opening the stomata.

K ion pumps are stimulated by light from the K ion pumps are stimulated by light from the blue part of the spectrum.blue part of the spectrum.

As the ions diffuse out so does water, As the ions diffuse out so does water, collapsing the guard cells.collapsing the guard cells.

The hormone abscisic acid causes K ions The hormone abscisic acid causes K ions to diffuse from the guard cells rapidly. to diffuse from the guard cells rapidly. This hormone is made by the roots in This hormone is made by the roots in times of droughttimes 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

C4 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 C3 plant

Roots 9.2.1 – 9.2.3Roots 9.2.1 – 9.2.3

Recall that root epidermal cells have Recall that root epidermal cells have extensions called root hairs to increase extensions called root hairs to increase the surface area for water and mineral the surface area for water and mineral absorption.absorption.

As a root pushes through the soil it is As a root pushes through the soil it is protected by a root cap. protected by a root cap.

•Zone of cell division – closest to tip, cells undifferentiated, mitosis is occurring

•Zone of elongation – cells are enlarging, G1 of cell cycle

•Zone of maturation – cells are differentiating and beginning to function. This is where root hairs begin to be noticed.

Water movementWater movement

Water must travel through the root to the Water must travel through the root to the vascular cylinder which is in the center of vascular cylinder which is in the center of the root.the root.

The vascular cylinder is surrounded by The vascular cylinder is surrounded by endodermis and pericycleendodermis and pericycle

Pericycle consists of cells that can Pericycle consists of cells that can produce lateral rootsproduce lateral roots

Endodermis is a cylinder 1 cell thick that Endodermis is a cylinder 1 cell thick that forms a selective barrier which regulates forms a selective barrier which regulates passage of substances from soil into VCpassage of substances from soil into VC

Each endodermal cell has a barrier called Each endodermal cell has a barrier called the Casperian strip. This strip is made of the Casperian strip. This strip is made of suberin, a waxy, impermeable layersuberin, a waxy, impermeable layer

The impermeable suberin ensures that all The impermeable suberin ensures that all water and minerals entering the plant water and minerals entering the plant must cross a semipermeable membrane.must cross a semipermeable membrane.

Passage of water and Passage of water and minerals into rootminerals into root

Root hairs absorb soil solution (water and Root hairs absorb soil solution (water and minerals)minerals)

The soil solution can travel 2 routesThe soil solution can travel 2 routes Apoplastic routeApoplastic route Symplastic routeSymplastic route

Apoplastic routeApoplastic route

Water and minerals travel between cell Water and minerals travel between cell walls through the cortex regionwalls through the cortex region

Some solution enters cells some does Some solution enters cells some does notnot

When the solution reaches the When the solution reaches the endodermis, the Casperian strip forces endodermis, the Casperian strip forces the solution into a cell so that it has to the solution into a cell so that it has to pass through a cell membranepass through a cell membrane

Symplastic routeSymplastic route

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

Once past the endodermis, water and Once past the endodermis, water and minerals enter the xylem vessels to be minerals enter the xylem vessels to be transported throughout the plant.transported throughout the plant.

Is the solution (now called xylem sap) Is the solution (now called xylem sap) pushed or pulled through a plant?pushed or pulled through a plant? Push due to root pressurePush due to root pressure Pull by tension generated by transpiration Pull by tension generated by transpiration

Movement of ions Movement of ions through plantsthrough plants

Minerals dissolved in water may enter by Minerals dissolved in water may enter by diffusion if concentration inside the root is diffusion if concentration inside the root is lower than concentration outside rootlower than concentration outside root

Fungal hyphae called mycorrhiza Fungal hyphae called mycorrhiza increase the root surface area for water increase the root surface area for water and mineral absorption (mutualism)and mineral absorption (mutualism)

If mineral concentration inside the plant is If mineral concentration inside the plant is higher than in the soil, active transport is higher than in the soil, active transport is required. This is also necessary if the ion required. This is also necessary if the ion cannot cross the phospholipid bilayer.cannot cross the phospholipid bilayer.

Active transport requires a transport Active transport requires a transport proteinprotein

K K ++ ions move through proteins called ions move through proteins called potassium channelspotassium channels

Proton pumpProton pump ATP provides energy to pump HATP provides energy to pump H++ ions out of ions out of

cellcell This makes the inside of the cell more This makes the inside of the cell more

negative than the outsidenegative than the outside The hydrogen ion gradient causes a voltage The hydrogen ion gradient causes a voltage

difference called membrane potential. (-120 difference called membrane potential. (-120 mv)mv)

Membrane potential is a form of potential Membrane potential is a form of potential energy that can be used to absorb mineral energy that can be used to absorb mineral ions.ions.

Hydrogen ions may displace cations Hydrogen ions may displace cations attached to the soil, freeing them so they attached to the soil, freeing them so they may enter rootmay enter root

Hydrogen ions may combine with anions Hydrogen ions may combine with anions and drag the anion into the root via and drag the anion into the root via cotransportcotransport

9.2.4 Plant support9.2.4 Plant support

Cellulose cell walls – walls may thicken for Cellulose cell walls – walls may thicken for additional supportadditional support

Lignified cell - A complex polymer, the chief Lignified cell - A complex polymer, the chief noncarbohydrate constituent of wood, that noncarbohydrate constituent of wood, that binds to cellulose fibers and hardens and binds to cellulose fibers and hardens and strengthens the cell.strengthens the cell.

Collenchyma, vascular bundles, Collenchyma, vascular bundles, sclerenchyma, provide flexible supportsclerenchyma, provide flexible support

Turgor pressureTurgor pressure

How do osmosis, the plant cell vacuole, How do osmosis, the plant cell vacuole, and the plant cell wall work together to and the plant cell wall work together to provide turgor pressure?provide turgor pressure?

What happens if turgor pressure is lost?What happens if turgor pressure is lost?

9.2.11 Movement of 9.2.11 Movement of Sugars AKA TranslocationSugars AKA Translocation

Sugars and organic molecules are Sugars and organic molecules are transported through sieve tube members and transported through sieve tube members and their companion cellstheir companion cells

Place where molecules originate is called Place where molecules originate is called the source. Could be a leaf or a storage the source. Could be a leaf or a storage organorgan

Place where molecules are going is called Place where molecules are going is called the sink. Could be where growth is occurring the sink. Could be where growth is occurring or a fruit or tuberor 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:

http://www.click4biology.info/c4b/9/plant9.2.htm#11