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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
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