Membranes & Movements
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
What do they look like?
Model 1: Danielli –Davson
Describe what you see…
Singer – Nicholson Fluid Mosaic
Model 2: Singer-Nicolson
Describe what you see…
Fluid mosaic model:
Two lipid tails
Integral proteins (pass through the membrane)
Peripheral proteins (embedded on the outside)
Glycoproteins (combination of protein and
Glycolipids (combination of protein and lipid)
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
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
and (active) by vesicles
Diffusion is Passive (no
Net movements with
Particles from high conc
Simple diffusion: some
particles (small un-
charged like Oxygen)
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!
Passive movement of water
From low solute conc high solute conc
High water potential Low water
Solute: that which is dissolved in a solvent (water)
Moves substances against the
Protein pumps for particular substances
Uses energy from ATP (adenosine
Transport by vesicles
(both made possible by fluidity of
Membrane pulls in
Droplet taken in
Protein from ribosome
Vesicle buds off rER;
go to Golgi Apparatus
GA buds off
Vesicle moves to
Vesicle moves to membrane, and fuses with the
plasma membrane, expelling the contents eg. A
Membrane of a neurone
Sodium ions are pumped
out, potassium ions
pumped in. The channel
proteins are closed
The membrane is poised
As soon as the channels
open, the ions will flood
in by: what process?
Mind the osmolarity!
Organs which are
being used for
eg. Heart transplants,
need to be bathed in
an isotonic solution.
Why? Image credit www.biology4bdp.weekly.com