Plant Transport

• How does water get from the roots of a tree to its top?

• Plants lack the muscle tissue and circulatory system found in animals, but still have to pump fluid throughout the plant’s body

Plant Transport

• Water first enters the roots and then moves to the xylem, the innermost vascular tissue

• Plants need water– As a starting product for photosynthesis– As a solvent to dissolve chemicals– For support– To ‘pay’ for water lost by transpiration

Plant Transport

• Heartwood is the xylem that has died; much darker

• Sapwood is the younger, outermost wood that has not yet become heartwood; conducts water from the roots to the leaves, and to store

Plant Transport

• Water movement (transport) occurs at three levels:– Cellular – Lateral transport (short-distance)– Whole plant (long-distance)

Plant Transport: Cellular level

• Diffusion – movement from an area of high concentration to an area of lower concentration

• Plays a major role in bulk water transport, but over short distances

• Although water diffuses through cell membranes, ions and organic compounds rely on membrane-bound (protein) transporters

• Some protein transporters form channels that allow molecules to diffuse (passive transport)

• Others require energy to move minerals and other nutrients against a concentration gradient (active transport)

Plant Transport: Cellular level

• Proton pumps hydrolyze ATP and use the release energy to pump hydrogen ions (H+) out of the cell; makes a proton gradient that is higher in H+ outside of the cell ( membrane potential)

• Makes the inside of a cell more negative than the outside, driving the transfer of positive ions (e.g., K+ ions)

Plant Transport: Cellular level

Plant transport: Cellular level

• Osmosis – passive transport– Transport of water (and its solutes) across a

semi-permeable membrane

• Unlike animal cells, plants have cell walls and this affects osmosis

• Water potential, Ψ (Greek letter psi), is used to predict which way water will move

• Water will move from solution with higher Ψ to a solution with lower Ψ

Water Potential, Ψ

• Water moves from higher Ψ to lower Ψ

• The addition of solutes lowers Ψ

• Increasing pressure raises Ψ

• In essence, Ψ measures the ability of soil water to move (into or out of the plant)

• Low osmotic concentration = high Ψ

• High osmotic concentration = low Ψ

Water Potential, Ψ

• If a single plant cell is placed into water, then the concentration of solutes inside the cell is greater than that of the external solution, and water will move into the cell by the process of osmosis

• The cell expands and presses against the cell wall, a condition known as turgid (swollen), due to the cell’s increased turgor pressure

Water Potential, Ψ

• Ψ is measured in units of pressure

• Pure water at standard temperature and pressure has a Ψ of zero

• The addition of solutes to water lowers its Ψ (makes it more negative), just as an increase in pressure makes it more positive

• Water will move from higher Ψ to lower Ψ

Water Potential, Ψ• Water will spontaneously flow

from a high potential to a low potential, like a ball rolling down a hill

• Ψ are usually negative

• Ψ is measured as pressure potential, ΨP and solute potential, ΨS

• The total potential energy of water in the cell = Ψ+P + ΨS

http://www.steve.gb.com/science/water_potential.html

• Pressure potential, refers to the turgor pressure resulting from pressure against the cell wall

• Water pressure also arises from an uneven distribution of a solute on either side of a membrane, which results in osmosis

• Solute potential, ΨS describes the smallest amount of pressure needed to stop osmosis

• Water flows from a solution with the less negative ΨS to the more negative ΨS

A watered plant regains its turgor

Water Potential, Ψ

• Water potential of the soil is negative, but not as negative as the cell, due to the high content of solutes

• Water moves from high (less negative) to low (more negative) water potential

http://www.steve.gb.com/science/water_potential.html

Plant Transport: Long distance

• Evaporation of water in a leaf creates a negative pressure (negative water potential) in the xylem, which literally pulls water up the stem from the roots

http://www.steve.gb.com/science/water_potential.html

Plant Transport: Long distance

• Water moves from the soil into the roots only if the soil’s water potential is greater

• It then moves along gradients of successively more negative water potentials in the stems, leaves and air

Water and Mineral Absorption

• Most of the water absorbed the plant comes in through root hairs – extensions of root epidermal cells

• Root hairs are almost always turgid, because their water potential is greater than that of the surrounding soil

• Collectively, have enormous surface area

• And don’t forget the mycorrhizae…

Root tips

Water and Mineral Absorption

• Minerals are absorbed at the root hair

• Minerals may either follow the cell walls or spaces in between them, or go directly through the plasma membrane of the cells

• They will, however, eventually reach the endodermis, where their entry is blocked by casparian strips – a waxy material that surrounds endodermal cells, before reaching the xylem

• Apoplast route – movement through cell walls and the spaces between cells

• Symplast route – cytoplasm continuum between cells

• Transmembrane route – membrane transport b/w cells and across membranes of vacuoles within the cells; provides the greatest control over which substances enter and leave

Water and Mineral Absorption

Casparian strip

• Transport into the endodermis is selective

• Passage through the cell walls blocked by casparian strips

• Substances must enter the cells of the endodermis in order to pass into xylem– Allows selectivity

Root hair

Xylem• Xylem sap brings minerals to leaves and

water to replace what is lost by transpiration

• Moves at rates of 15 meters/hour; travels vertically up distances of 100 meters in the tallest trees

• At night, when transpiration is low or absent, root pressure caused by the accumulation of ions in the roots, causes more water to enter the root hair cells by osmosis

Xylem

• Under certain circumstances, root pressure is so strong that water will ooze out of a cut plant stem for hours or even days– When root pressure is very high, it can force

water up to the leaves, where it may be lost (guttation)

– Guttation produces what is more commonly called dew on leaves

Phloem

• Carbohydrates manufactured in leaves and other green parts are distributed through the phloem to the rest of the plant– Translocation– Phloem sap consists primarily of sucrose

(30%), as well as hormones, amino acids, and minerals

– Phloem sap travels from sugar sources to sugar sinks (non-green parts, growing shoots and roots, and fruits)

Maple syrup is sap!• Sap in maple trees remains

frozen during the winter• Begins to flow again when

weather warms, and is triggered by cold nights and warmer days

• A hole is tapped into the tree allowing sap to drain

• Sugar maples have the greatest amount of sugar in the sap; produce 20 gallons of sap (=2 quarts of syrup)

Maple syrup is sap!

• The sap that produces maple syrup flows through the sapwood; the living portion of the xylem

Just in case you’re not starving yet…