Tonicity & transport

Water potential

• Represented by Ψ

• All the forces that act on an aqueous fluid that determine the direction the water of the fluid will move (from high potential to low potential)

Osmosis

• Water moving across a semipermeable membrane to equalize the solute concentration on both sides

• Pure water has the highest energy

• The higher the [solute] the lower free energy of the system

Isotonic environment

• [solute] in the environment = [solute] in the cell

• Good for cells without a wall (can maintain cell volume)

• Bad for cells with a wall (can’t keep pressure on the wall, lose turgidity)

• Eg: Blood plasma & blood cells

Hypotonic environment

• [solute] in environment < [solute] in cell

• Good for cells with a wall (maintain internal pressure)

• Bad for cells with no wall (they burst)

Lysed

A cell bursts

Turdigity

Internal pressure

Hypertonic environment

• [solute] in environment > [solute] in cell

• Animal cells shrivel

• Walled cells become plasmolysed

Plasmolysed

Membrane pulls away from the wall. Shrivelling in plants

Simple diffusion

• Passive transport

• Small non-polar substances

• Water can diffuse through membrane but not efficiently

Transport via channel proteins

• Facilitated diffusion of ions & water without energy

• Ion channels regulated through gates (certain voltage of ligand activated)

Aquaporins

• Water channels

• Channel proteins

Passive transporters

• Not really “passive”, the energy to operate the transporter comes from the transporteé

• Transport monomers

• Constantly change shape

GLUT1

• Human glucose transporter

• Passive transporter

• Brings glucose in

Active transporters

• Change shape at every phosphorylation / dephosphorylation

• Transport ions, monomers, dimers required energy

Primary cotransporters

• Moves ions against both concentration gradients

• Eg: Na+K+ pump

Secondary cotransporters

• Use the favourable concentration gradient of one molecule to move another

• Eg: SGLT-1 symporter, TST2 antiporter.

Antiport

Opposite physical direction

Symport

Same physical direction

• SGLT1

• Symporter

• Secondary cotransporter

• Active transport

• Bring glucose in

Tag-teaming cotransporters

• Eg: Na+Ca++ exchanger

• Uses active transport to maintain concentration gradient of molecule A, while moving molecule B against its gradient so molecule C can be moved against its concentration gradient using energy from molecule A

Exocytosis

Puts waste into vesicles, fuses it with the membrane, then pulls membrane and exposes its contents

Endocytosis

Cargo approaches cell membrane, creates a vesicle at the membrane, taken into the cell

Phagocytosis

• For LARGE LARGE stuff. Membrane extends around the object to engulf it.

• Contained in a vacuole

• Neutrophils do this

Pinocytosis

• “Sniffing” what is in the outside environment

• Non specific form of endocytosis

Receptor-mediated endocytosis (RME)

• Receptor proteins on membrane bind to ligands to trigger reuptake

• Specific form of endocytosis

• Big example : RBC development. Fe++ put on their receptors

• Eg : CD4 (in cold virus), CDR5 (in HIV), CD77 (from E.Coli)