Chapter 36 Transport in Plants

Plants require: CO2 Light Sugar H2O O2 Minerals Macronutrients (besides C, H, O) Ca, K, Mg, N, P, S Micronutrients B, Cl, Cu, Fe, Mn, Mo, Ni, Zn Oxygen is taken in by roots, and is a by-product of PSN. Fig. 36.2

Here we explore how these substances are transported within plants… but first we need some more background… Fig. 36.2

Background… Cell wall helps maintain a cell’s shape, but the plasma membrane regulates the traffic of molecules into and out of a cell Fig. 36.8

Plasmodesmata provide cytosolic connections among cells Background… Plasmodesmata provide cytosolic connections among cells Cytosol = cytoplasm minus organelles Fig. 36.8

Background… Vacuoles often account for 90% of a plant cell’s volume, but they are never shared by adjacent cells Fig. 36.8

Background… Substances can move from cell to cell via the symplastic, apoplastic, or transmembrane routes Fig. 36.8

Solutes tend to diffuse down concentration gradients Background… Solutes tend to diffuse down concentration gradients Passive transport is diffusion across a membrane Passive transport is generally slow, unless solutes travel through transport proteins in the membrane Some transport proteins are selective channels Some selective channels are “gated” – environmental stimuli open or close them

E.g., proton pumps are very common active transport proteins Background… Active transport requires energy to move solutes up their concentration or charge gradient E.g., proton pumps are very common active transport proteins Fig. 36.3 Proton pumps create membrane potentials; potential energy can be used to perform cellular work

The K+ ions pass through transport proteins Background… The membrane potential provides the energy to uptake some minerals, e.g., K+ ions The K+ ions pass through transport proteins Fig. 36.4

The coupled ions pass through [co-]transport proteins Background… The membrane potential provides the energy for cotransport of ions up their concentration gradients, as H+ ions move down theirs Fig. 36.4 The coupled ions pass through [co-]transport proteins

Background… The membrane potential provides the energy for cotransport of some neutral molecules (e.g., sugar) up their concentration gradients, as H+ ions move down theirs Fig. 36.4

Osmosis is diffusion of water across a membrane Background… Osmosis is diffusion of water across a membrane To predict the movement of water across a membrane alone (e.g., an animal cell), it is sufficient to know whether the inside solute concentration is < or > the outside solute concentration To predict the movement of water across a membrane plus cell wall (e.g., a plant cell), we must know both the solute concentration difference and the pressure difference

Ψ (psi)  measured in MPa (megapascals; 1 MPa ≈ 10 atmospheres) Background… Water potential is a combined measure of solute concentration and pressure Ψ (psi)  measured in MPa (megapascals; 1 MPa ≈ 10 atmospheres) Pure water in an open container: Ψ = 0 MPa Solutes reduce the value of Ψ Pressure increases the value of Ψ Negative pressure (tension) decreases the value of Ψ

Ψ = ΨP + ΨS ΨP can be positive, 0, or negative ΨS is always  0 Background… Water potential = pressure potential + solute (osmotic) potential Ψ = ΨP + ΨS ΨP can be positive, 0, or negative ΨS is always  0 Water will always move across a membrane from higher to lower Ψ

Let’s consider 4 examples Background… Let’s consider 4 examples Add solutes alone to one side. Fig. 36.5

Let’s consider 4 examples Background… Let’s consider 4 examples Add solutes and pressure simultaneously; the two in correct amounts cancel each other, since solute concentration is inversely proportional to water potential and pressure is directly proportional to water potential. Fig. 36.5

Let’s consider 4 examples Background… Let’s consider 4 examples Add solutes and even more pressure… Fig. 36.5

Let’s consider 4 examples Background… Let’s consider 4 examples Add solutes to one side, but even more negative pressure to the other; in this case the negative pressure on one side overrides what would otherwise be a tendency for water to move towards the solutes. Fig. 36.5

Background… Let’s consider real cells Flaccid cell: ΨP = 0 Fig. 36.6

Background… Let’s consider real cells Flaccid cell: ΨP = 0 Fig. 36.6

ΨP = 0 Background… Let’s consider real cells Flaccid cell: If a cell loses water to the environment, it plasmolyzes Fig. 36.6

Background… Let’s consider real cells Flaccid cell: ΨP = 0 Fig. 36.6

Background… Let’s consider real cells Flaccid cell: ΨP = 0 Fig. 36.6

ΨP = 0 Background… Let’s consider real cells Flaccid cell: If a cell gains water from the environment, it becomes turgid Turgor = pressure that keeps cell membrane pressed against cell wall Fig. 36.6

ΨP = 0 Background… Let’s consider real cells Flaccid cell: If a cell gains water from the environment, it becomes turgid Aquaporins are transport proteins that form channels for water Fig. 36.6

Armed with this background… How do roots absorb water and minerals?

How do roots absorb water and minerals? Solutes pass into roots from the dilute soil solution Note: See the early part of the lecture for the details of how active transport creates membrane potentials that fuel the uptake of ions. Fig. 36.9

How do roots absorb water and minerals? Symplastic route: Active transport occurs through proton pumps, that set up membrane potentials, that drive the uptake of mineral ions Fig. 36.9

How do roots absorb water and minerals? Apoplastic route: Some water and dissolved minerals passively diffuse into cell walls Fig. 36.9

How do roots absorb water and minerals? Solutes diffuse through the cells (or cell walls) of the epidermis and cortex (the innermost layer of which is the endodermis) Fig. 36.9

How do roots absorb water and minerals? At the endodermis, only the symplastic route is accessible, owing to the Casparian strip Chapt. 35 observed that the endodermis regulates the passage of substances into the vascular stele Fig. 36.9

How do roots absorb water and minerals? The final layer of live cells actively transports solutes into their cell walls Solutes then diffuse into xylem vessels to be transported upward Fig. 36.9

How do roots absorb water and minerals? The final layer of live cells actively transports solutes into their cell walls The final layer may be an endodermal cell… Fig. 36.9

How do roots absorb water and minerals? The final layer of live cells actively transports solutes into their cell walls. … or a cell of the pericycle (outermost layer of stele) Fig. 36.9

How do roots absorb water and minerals? Note: In this figure the pericycle is drawn as a continuous layer of cells

How do roots absorb water and minerals? Mycorrhizal mutualism (fungus + roots) Fungus helps plant obtain water and minerals (e.g., P) Plant feeds sugars to the fungus No fungus With fungus

How do roots absorb water and minerals? Some species (including many legumes) have root nodules that house N-fixing bacteria Bacteria convert N2 to NH4+ (ammonium), providing plant with fixed N Plant feeds sugars to the bacteria

How does xylem transport xylem sap?

In some species of trees the process moves water and nutrients How does xylem transport xylem sap? In some species of trees the process moves water and nutrients > 100 m upwards! Fig. 36.13

How does xylem transport xylem sap? Nearly all of the energy to drive the process comes from the sun Evapor-ation at the top pulls water up from the bottom Unbroken chains of water molecules (held together by cohesive H-bonds) fill xylem vessels Fig. 36.13

The evaporation of water out of leaves is called transpiration How does xylem transport xylem sap? The evaporation of water out of leaves is called transpiration Fig. 36.12

Water vapor escapes through stomata How does xylem transport xylem sap? Water vapor escapes through stomata Fig. 36.12 Fig. 36.15

Transpiration creates a water pressure gradient How does xylem transport xylem sap? Transpiration creates a water pressure gradient Fig. 36.13

How does xylem transport xylem sap? Transpiration creates a water pressure gradient Water flows upward through xylem vessels by bulk flow down the pressure gradient Lower Ψ at the top is the tension that pulls water up from the bottom Fig. 36.13

The Transpiration-Cohesion-Tension Mechanism How does xylem transport xylem sap? The Transpiration-Cohesion-Tension Mechanism Fig. 36.13

How do plants regulate the transport of xylem sap?

K+ is actively transported into and out of guard cells How do plants regulate the transport of xylem sap? Stomata K+ is actively transported into and out of guard cells

How do plants regulate the transport of xylem sap? Stomata When [K+] is high, the amount of H20 is high, and guard cells open stomata

How do plants regulate the transport of xylem sap? Stomata When [K+] is low, the amount of H20 is low, and guard cells close stomata

Light stimulates the uptake of K+ by guard cells, opening stomata How do plants regulate the transport of xylem sap? Stomata Light stimulates the uptake of K+ by guard cells, opening stomata

Low [CO2] stimulates the uptake of K+ by guard cells, opening stomata How do plants regulate the transport of xylem sap? Stomata Low [CO2] stimulates the uptake of K+ by guard cells, opening stomata

How do plants regulate the transport of xylem sap? Stomata Low H2O availability inhibits the uptake of K+ by guard cells, closing stomata

How does phloem transport phloem sap?

How does phloem transport phloem sap? Sugars manufactured in leaves diffuse to phloem companion cells Fig. 36.17

How does phloem transport phloem sap? Companion cells actively transport sugars into sieve-tube members (elements) Fig. 36.17

How does phloem transport phloem sap? Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory: 1. At sources, sugars are actively transported into phloem Fig. 36.18

How does phloem transport phloem sap? Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory: 2. Water follows by osmosis from source cells and xylem; this creates high pressure Fig. 36.18

How does phloem transport phloem sap? Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory: 3. At the sink, sugars diffuse out of the phloem and water follows by osmosis; this creates low pressure Fig. 36.18

How does phloem transport phloem sap? Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory: Sugar solution flows from high to low pressure Fig. 36.18

How does phloem transport phloem sap? Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory: 4. Water may be taken up by the transpiration stream in the xylem Fig. 36.18

How does phloem transport phloem sap? Pressure-Flow Theory

Not all herbivores chew leaves… Some exploit sap E.g., aphids tap sieve-tube elements for phloem sap