Lecture notes BIOS5030 Cell Biology (BIOS5030) on The Membrane (movement)
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Course
BIOS5030 Cell Biology (BIOS5030)
Institution
The University Of Kent (UKC)
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Topic 2: The Membrane
à 2 lecture scheduled at 12pm today due to missed lecture on Wednesday 25 th October.
nd
Covering:
à What is the membrane and why are we interested in it?
à Membrane organisation and lipid composition and function.
Textbooks
Alberts. Molecular Biology of the cell 6th edition. Chapter 10 & 11
The membrane controls what goes in and out of the cells. The lipids don’t allow everything to
pass through, we have gates to control this movement. If the cell didn’t have a membrane,
there wouldn’t be cell as you wouldn’t be able to concentrate the proteins. The plasma
membrane forms the boundary around the cell and maintains the differences between the
cytosol and environment.
Membranes are used for:
- Energy production – they have a hydrophobic core which allow you to create
gradients, this is important for the generation of ATP via ATOP synthase which
powers the transport of small molecules. These transmit electrical signals in nerve
and muscle cells. Neurotransmission is dependent on membranes being
impermeable to sodium and potassium movement.
Membranes are the first port of call:
- Signaling
- Receptors
- Mechanotransduction: membrane proteins can sense the environment which
influences changes within the cell through the cytoskeleton.
Key components of the immune system in the presentation and processing of antigens.
General Membrane structure:
The composition of membranes is different depending on the membrane we are looking at
i.e. mitochondria membrane is used to generate ATP. Biological membranes all have a
common structure, called the fluid mosaic structure. Membranes form a thin 5nm sheet,
which are:
o flexible
1
, o Semi permeable
o Self-repairing
o Form spontaneously in aqueous environments.
The membrane consists of 2 major macromolecules which are held together via
noncovalent interactions – hydrophobic and electrostatic:
o Proteins
o Lipids
If we have covalent bonds, we have a rigid structure which would not be able to adapt to
different environmental ques.
The biolayer forms due to chemical properties of phospholipids driving membrane formation
which hold it together. Phosphoric lipids are amphipathic and composed of:
- Hydrophilic headgroup
- Hydrophobic alkyl tail -the composition of the fatty acid tail changes its chemical
properties. Making it fluid at different temperatures.
Paper: Singer & Nicholson. Thermodynamic considerations. The fluid mosaic model.
To get the most structure in the membrane you should
expose it to an aqueous environment. This is based on
the idea that the membrane is a fluid environment.
Combination of headgroup and alkyl chain makes for very
complex mixtures. Major lipids in eukaryotic membranes,
have different sizes, structures, and composition of fatty
acid tails:
1. Phosphatidylethanolamine (PE)
2. Phosphatidylserine (PS)
3. Phosphatidylcholines (PC)
4. Sphingomyelin – these are chemically different, there are 60 different human
sphingolipids characterised, however we are still unsure about their influence on the
membrane. These are prominent in neurons. They are called sphingolipids as they
are mysterious like the sphynx in Egypt. You will have come across the
glycosphingolipids defining blood groups.
Sphingomyelin is found in the plasma membrane and is a major component of the myelin
sheath around the axons, insulating the cell and allowing rapid transmission of electrical
signals.
2 diseases caused by enzymes in the sphingomyelin are:
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