Membranes & Movements Across Them.

Topic 1.3, 1.4

Cell membranes surround us..

Cell membranes enclose the cell and most of the organelles.

They have the same structure.

The basic unit of construction

The phospholipid

What do they look like?

Model 1: Danielli –Davson

Describe what you see…

Singer – Nicholson Fluid Mosaic Model

Model 2: Singer-Nicolson

Describe what you see…

Fluid mosaic model: Phospholipid bilayer

Phosphate head

Hydro?

Two lipid tails

Hydro?

Integral proteins (pass through the membrane)

Peripheral proteins (embedded on the outside)

Glycoproteins (combination of protein and carbohydrate)

Glycolipids (combination of protein and lipid)

Protein functions

Hormone Binding Site: On surface. Allows one type of hormone to bind. Triggers signal to inside of cell.

Enzymes: either catalyse reactions inside or outside the cell, depending on location of active site.

Electron Carriers: arranged in chains so electrons can be passed along.

Cell adhesion

Channels for Passive Transport: allow a specific substance to pass through middle of protein.

Pumps for Active Transport: release energy from ATP to move specific substances across the membrane.

Transport across membranes

Passive, active

and (active) by vesicles

Passive

Does not require cellular energy

Active

Requires cellular energy

Diffusion

Diffusion is Passive (no energy required)

Net movements with (along) concentration gradient

Particles from high conc low conc

Hypertonic hypotonic

Simple diffusion: some particles (small un-charged like Oxygen)

Facilitated diffusion: channel proteins

Facilitated vs. Active transport

• Active transport happens against the concentration gradient, and requires energy in the form of ATP

• Facilitated diffusion happens down the concentration gradients, and requires no energy.

• Both occur through integral protein channels!

Osmosis

Passive movement of water

Net movements

From low solute conc high solute conc

Hypotonic hypertonic

High water potential Low water

potential

Solute: that which is dissolved in a solvent (water)

Active transport

Moves substances against the concentration gradient

Protein pumps for particular substances

Uses energy from ATP (adenosine triphosphate)

Transport by vesicles (both made possible by fluidity of

membrane) Endocytosis

Membrane pulls in

Vesicle forms

Droplet taken in

‘Cell drinking’

Exocytosis Protein from ribosome

to rER

Vesicle buds off rER; go to Golgi Apparatus

GA buds off

Vesicle moves to membrane

Fuses

Protein expelled

ExocytosisVesicle moves to membrane, and fuses with the plasma membrane, expelling the contents eg. A digestive enzyme

Endocytosis

Phagocytosis

The same as Pinocytosis but a solid particle is ingested, instead of a liquid.

White blood cells (Phagocytes) ingest pathogens

Sodium – Potassium Pump

Membrane of a neurone

Sodium ions are pumped out, potassium ions pumped in. The channel proteins are closed

The membrane is poised for action

As soon as the channels open, the ions will flood in by: what process?

-Facilitated diffusion

Nerve transmission

(Sodium floods back in)

Organ transplants:Mind the osmolarity!

Organs which are being used for medical procedures eg. Heart transplants, need to be bathed in an isotonic solution.

Why?Image credit www.biology4bdp.weekly.com