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Unit 5 - Cell Membranes and Signalling

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Nick Budziszewski

on 10 December 2015

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Transcript of Unit 5 - Cell Membranes and Signalling

The Fluid Mosaic
Proteins floating in a lipid lake. HOW RELAXING!
Most of the membrane is made up of amphipathic phospholipids (hydrophyllic "heads" and hydrophobic "tails")
Membranes are fluid (like motor oil), and this fluidity can be controlled by two things:

Lipid composition
- Are there a lot of cholesterol molecules and/or saturated fatty acids? If so, the membrane will be more rigid than one containing less cholesterol or more unsaturated fatty acids

- Cold temperatures = less fluidity (remember that molecules move slower in colder temperatures). Lots of organisms can fix this problem by adding more unsaturated fatty acids to their membranes. It's like membrane antifreeze!
See pg 80 for more background info on this neat experiment!
Proteins are very common in the membrane (1 protein per 25 phospholipid in a typical cell).

The side chains of the protein determine where the protein is found - either embedded in the membrane (
) or stuck to the outer layers of the membrane (
Proteins are distributed asymmetrically in the membrane. In other words, proteins on one side of the membrane might not be the same as the proteins on the other side. This gives the inside and outside of the bilayer different...SUPER POWERS.

Even a protein that goes all the way through a membrane (
) may have different properties on each side.
Carbohydrates are located on the outside of the membrane and serve as recognition sites. They are bound covalently to lipids (
) or proteins (
How do things move through the membrane?
Active Transport vs. Passive Transport
- random movement, usually from high concentration to low concentration
- size of the molecule, temperature of the environment, and concentration gradient all determine how fast diffusion will occur
Simple Diffusion
- movement directly through the bilayer.

What properties should a molecule have in order to undergo simple diffusion?
Facilitated diffusion
- diffusion through a protein

Channel Protein

Carrier Protein
Examples of proteins used in diffusion - ion channels (can be gated), aquaporins
Movement of Water
Gated Channels
Carrier Proteins
Active Transport
- directional movement from low concentration to high
- movement against the concentration gradient
- requires they hydrolysis of ATP, either directly (primary A.T.) or indirectly (secondary A.T.)
Primary Active Transport
- direct use of hydrolysis of ATP to move things

Example: sodium-potassium (Na+ - K+) pump - two potassium ions come in, three sodium ions leave for every ATP used.
Vesicle Transport
Can we apply these concepts to larger organisms?
In general, maintaining homeostasis when it comes to the amount of stuff inside the body is controlled by the excretory system.
Major Functions
1. Regulating the amount of fluid in the body

2. Regulating the concentration of solutes (osmolarity)

3. Maintaining the levels of individual solutes (ions, glucose)

4. Eliminating toxic nitrogenous (nitrogen-based) wastes.
How can osmolarity be regulated?
Osmoconformers vs. Osmoregulators
Examples: marine invertebrates, cartilaginous fish
Osmoconformers maintain an isotonic solution with their environment. Can be dangerous if they are exposed to different conditions!
Osmoregulators actively regulate the osmolarity of their fluids. Can live in a variety of environments. Fresh water - conserve salts, get rid of water. Land - conserve both!
What types of ions are regulated?
Sodium, Chlorine, Potassium, Calcium, etc.
important for many cell processes (ion channels)
Hydrogen, Bicarbonate (HCO3-)
important to regulate pH
It's really important that organisms maintain isotonic solutions!
How are toxic, nitrogen-based wastes removed?
Ammonia vs. Urea vs. Uric Acid
Ammonia - easy to remove in water, but toxic if concentrated

Urea - Less toxic than ammonia, so requires less water. Requires more energy to expel than ammonia (it's a more complicated molecule)

Uric Acid - Very little water loss, but requires a lot of energy to create
Cell Signaling
Signal transduction pathways
allow cells to receive signals from the outside environment and respond to them.
Signal --> Receptor --> Response
Receptor proteins and enzymes are affected in a similar way.
Specificity - receptors and their ligands work like "locks and keys"
Allosteric regulation - many ligands cause a shape change in their receptor protein
Competitive inhibition - binding sites on receptors can be blocked by other molecules
Types of Receptors
Gated Ion Channels
Protein Kinase Receptor
G protein-linked Receptors
Signal Transduction Pathways:
Responding to Adrenaline/Epinepherine
Important terms:
second messenger, cascade, amplification
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