PLANT PHYSIOLOGY

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PLANT PHYSIOLOGY

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Nutrient uptake

Solute transport

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Overview

1. Ion transport in roots

2. Passive and active transport

3. Membrane transport processes

4. Phloem transport

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1. Ion transport in roots

1.1. Solutes move through both apoplast and symplast 1.2. Ions cross both symplast and apoplast

1.3. Xylem parenchima cells participate in xylem loading

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Pathways for water and solute uptake by the root

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Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 106.

Diagram showing electrochemical potentials of K⁺ and Cl^– across a maize root

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Plasmodesmata connect the cytoplasms of neighbouring cells facilitating cell-to-cell communication and solute transport

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2. Passive and active transport

2.1. Concentration gradients and electrical-potential

gradients are integrated by the electrochemical potential 2.2. Movement of solutes across membranes down their free-energy gradient is called passive transport

mechanisms

2.3. Movement of solutes against their free-energy is known as active transport and requires energy input

2.4. The Nernst equation distinguishes between active and passive transport

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Relationship between chemical potential and transport (passive, active) processes

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Development of a diffusion potential and a charge separation between two compartments separated by a membrane

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Diagram of a pair of microelectrodes used to measure membrane potentials across cell membranes

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Diagram of the whole-cell and membrane patch configuration

Source: Taiz L., Zeiger E. (2010): Plant Physiology. Web material, http://5e.plantphys.net

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Cyanide (CN^-) blocks ATP production that leads to the collapse of the membrane potential

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3. Membrane transport processes

3.1. Biological membranes contain specialized proteins that facilitate solute transport

3.2. Channels enhance diffusion across membranes 3.3. Carriers bind and transport specific substances

3.4. Primary active transport, called pumps, requires direct energy source

3.5. Secondary active transport uses stored energy

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Three classes of membrane transport proteins:

channels, carriers, and pumps

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Models of K⁺ channels in plants: (A) top view, and (B) side view of a channel

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Carrier transport often shows enzyme kinetics, including saturation

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Hypothetical model of secondary active transport

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Two examples of secondary active transport coupled to a primary proton gradient

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3. Membrane transport processes

3.6. Cations are transported by both cation channels and cation carriers

3.7. Anions are transported in the direction of passive efflux

3.8. Aquaporins forms water channels in membranes

3.9. Plasma membrane H⁺-ATPases are important for the regulation of cytoplasmic pH and for the control of cell turgor

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Aquaporin activity is regulated by phosphorilation as well as by pH, calcium concentration, etc.

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Hypothetical steps in the transport of a proton against its chemical gradient by H⁺-ATPase

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Ion concentrations in the cytosol and the vacuole are controlled by passive (dashed arrows) and active (solid arrows) transport

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Overview of the various transport proteins in the plasma membrane and tonoplast of plant cell

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4. Phloem transport

4.1. Pathways of translocation

4.2. Materials translocated in the phloem

4.3. The pressure-flow model, a passive mechanism for phloem transport

4.4. Photosynthate distribution: allocation and partitioning

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Transverse section of a 3-year-old stem of an ash (Fraxinus excelsior) tree

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Schematic drawings of mature sieve elements:

(A) external view, (B) longitudinal section

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ATP-dependent sucrose transport in sieve-element loading

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Pressure-flow model of translocation in the phloem

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A physical model of the pressure-flow hypothesis for translocation in the phloem

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A simplified scheme for starch and sucrose synthesis during the day

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Summary

The biological movement of molecules and ions from one location to another is known as transport. Plants

exchange solutes and water with their environment and among their tissues and organs. Both local and long- distance transport processes in plants are controlled largely by cellular membranes. Active and passive membrane transports are distinguished. Membranes contain specialized proteins - channels, carriers, and pumps - that facilitate solute transport.

Translocation in the phloem is the movement of the

products of photosynthesis from mature leaves to areas

of growth and storage. It also transmits chemical signals

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PLANT PHYSIOLOGY

Nutrient uptake

Solute transport

Overview

1. Ion transport in roots

2. Passive and active transport

3. Membrane transport processes

4. Phloem transport

1. Ion transport in roots

2. Passive and active transport

3. Membrane transport processes

3. Membrane transport processes

4. Phloem transport

Summary

The biological movement of molecules and ions from one location to another is known as transport. Plants

Translocation in the phloem is the movement of the

products of photosynthesis from mature leaves to areas

of growth and storage. It also transmits chemical signals

Questions

• What is meant by the term "passive transport" and

"active transport"?

• Both membrane channels and carriers show changes in protein conformation. What is the role of such

conformation changes (a) in channels, and (b) in carriers?

• In the transport of an ion from the soil solution to the xylem, what is the minimum number of times it must cross a cell membrane?

• Describe the pressure-flow model of translocation in the

phloem.

THANK YOU FOR YOUR ATTENTION

Next lecture:

The light reactions of the photosynthesis Photosynthesis inhibiting herbicides

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