plant physiology module 6

solute transport

active vs passive transport

- Passive Transport: the movement of molecules occurs spontaneously, down a gradient of free energy or chemical potential (downhill)

- Active Transport: the movement of molecules against, or up a gradient of chemical potential. It is not spontaneous, and requires that work be done by using cellular energy

what is chemical potential?

- Is defined as the sum of the concentration, electrical, and hydrostatic potentials

- A sum of all the forces that may act on a molecule to drive net transport

Membrane permeability:

The extent to which a membrane permits the movement of a substance. It can be expressed in terms of a diffusion coefficient for the solute through the membrane

graphed out development of a diffusion potential and a charge separation between two compartments:

diffuse until equalib reached

membrane permeability can impact what is able to diffuse

Membrane Transport Processes: transporter proteins (3)

• which can go against electrochemical gradient?

Transport proteins on the biological membranes

- Channels

- Carriers

• Passive transport

• Active transport (Pumps - carriers that carry out primary active transport)

- Pumps

Typical values for the permeability, P, of a biological membrane to various substances

charged ions<non polar gasses

we need K+ more than Cl- thus, more perm

Ion concentrations in the cytosol and the vacuole

Controlled by both passive and active transport processes

solid arrow=active transport

dotted arrow=passive transport

What are channel proteins?

- Transmembrane proteins that function as selective pores, through which molecules or ions can diffuse across the membrane

- Is passive

- The specificity of transport primarily depends on pore size and electric charge

- Is mainly limited to ions and water

- Channel transporters enhance diffusion across membranes

What are carrier proteins?

- It is not necessary to have pores that extend completely across the membrane

- The substance being transported by a carrier is bound to a specific site on the carrier protein

- Be highly specific due to binding requirement

- Specialize for specific ions or organic metabolites

- Binding causes a conformational change in the carrier, which exposes the substance to the solution on the other side.

- The rate of transport much slower than through a channel

primary active transport: definition, E, what is transported

- Is coupled directly to a source of energy, such as ATP hydrolysis, an oxidation-reduction reaction, or the absorption of light by the carrier protein

- Carry out by pumps (membrane proteins)

- H+, Ca2+, or large organic molecules

ion pumps: electrogenic transport and electroneutral transport

- Electrogenic transport: ion transport involving the net movement of charge across the membrane.

Na+/K + - ATPase

plasma memb H+-ATPase

- Electroneutral transport: ion transport involving no net movement of charge across the membrane.

H +/K + - ATPase

Hypothetical steps in the transport of a cation against its chemical gradient

able to pump against gradient bc of ATP hydrolysis providing E input

makes outside of cell more acidic, making its cell wall softer and allowing expansion

Secondary Active Transport

• symport vs antiport

- Uses stored energy,

- The energy driving the transport is provided by the proton motive force rather than directly by ATP hydrolysis

- Symport (Symporter): the two substances move in the same direction through the membrane

- Antiport (Antiporter): a coupled transport in which the energetically downhill movement of protons drives the active (energetically uphill) transport of a solute in the opposite direction.

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

symport and antiport brings molec in against its gradient without use of ATP

saturation kinetics (Vmax means what)

Carrier transport often shows saturation kinetics (Vmax)

Vmax cannot be exceeded

When the substrate-binding site on the carrier is all occupied, Vmax is approached

Vmax is an indicator of the number of molecules of the specific transport protein in the membrane

Overview of the various transport processes on the plasma membrane and tonoplast

• aquaporins

• cation transporters (2)

• what compound is common for needing transporter in plants

- Genes encoding many transporters have been identified

- Transporters exist for diverse nitrogen-containing compounds

- Cation transporters are diverse

• Cation channels: 56 genes in Arabidopsis

• Cation carriers: a variety of ion carriers exist in plant cells

- Anion transporters have also been identified in plants

- Aquaporins might transport uncharged solutes

What Is Photosynthesis?

Means literally “synthesis using light”

- Organisms (Plants & some Bacteria) use solar energy to synthesize carbohydrates and generate oxygen from carbon dioxide and water