The Cell Cycle

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Diffusion across a membrane is passive transport. The pumping of solutes across a membrane is active transport and requires energy.

Most solutes pass through transport proteins embedded in the cell membrane.

Slide 9

The most important transport protein for active transport is the proton pump.

Proton pumps in plant cells create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work.

They contribute to a voltage known as a membrane potential.

Slide 10

Proton pumps provide energy for solute transport

CYTOPLASM

EXTRACELLULAR FLUID

ATP

H+

H+

H+

H+

H+

H+

H+

H+

H+

Proton pump generates mem- brane potential and gradient.

+

+

+

+

+

_

_

_

_

_

Slide 11

Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes.

The “coat-tail” effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells.

Slide 12

Solute transport in plant cells

CYTOPLASM

EXTRACELLULAR FLUID

K+

K+

K+

K+

K+

K+

K+

Transport protein

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_

_

_

+

_

+

+

+

+

(a) Membrane potential and cation uptake

NO3−

NO3−

NO3−

NO3−

NO3−

NO3−

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

(b) Cotransport of an anion with H+

H+

_

_

_

_

_

_

+

+

+

+

+

+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

_

_

_

_

_

_

+

+

+

+

+

+

(c) Cotransport of a neutral solute with H+

S

S

S

S

S

S

Slide 13

Cotransport - a transport protein couples the diffusion of one solute to the active transport of another.

NO3−

NO3−

NO3−

NO3−

NO3−

NO3−

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

_

_

_

_

_

_

+

+

+

+

+

+

Cotransport of an anion with H+

Slide 14

Diffusion of Water = Osmosis

To survive, plants must balance water uptake and loss.

Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure.