Lecture 15: Cellular membrane processes Flashcards
How thick is the cell membrane?
8 nm
What is the model for the cell membrane called?
Fluid mosaic model
What is the composition of the cell membrane?
50% lipid and 50% protein held together by hydrogen bonds
What are the 3 types of lipid molecules found in cellular membrane?
Glycolipid
Phospholipid
Cholesterol
What is a phospholipid?
A phosphate head and two fatty acid tails 0 amphipathic
Makes up 75% of lipids
What does amphipathic mean?
Contains both a polar and non-polar region
What does membrane fluidity mean?
The lipids and proteins are able to move around within the plane of the membrane leaflet - on a side of phospholipids however cannot move between phospholipid layers
What are the determinants of fluidity?
Lipid tail length
Cholesterol
Double bonds in fatty acid tails
How does lipid tail length affect fluidity?
Longer lipid tail lengths decrease fluidity
How does cholesterol level affect fluidity?
More cholesterol molecules within the cell membrane make it less fluid
How does number of double bonds affect fluidity?
More double bonds in the fatty acids of the phospholipids increase fluidity
What are integral proteins?
Proteins in the cell membrane which extend completely across the membrane
What are peripheral proteins?
Attached to either inner or outer surface of cell membrane and are easily removed
Example is proteins that bind to the cytoskeleton of the cell determining the shape of the membrane
What are features of integral membrane proteins?
Amphipathic - have hydrophobic and hydrophilic regions
The hydrophobic regions usually coil up into helices making up the inner part of the protein
What are the functions of membrane proteins?
Receptor proteins Cell signaling Linkers Enzymes Ion channels Transporter proteins
What is a channel protein?
A transport protein which allows specific substsances to move through a water-filled pore.
What is a transporter protein?
A transport protein which moves specific substances across membrane by changing shape
What does selective permeability mean?
Allows some substances to pass and blocks other substances
What molecules can pass the phospholipid bilayer (with no proteins involved)?
- Non-polar uncharged molecules (oxygen, nitrogen)
- Lipid soluble molecules (steroids, fatty acids)
- Small uncharged polar molecules (water, urea, CO2)
What molecules cannot pass the phospholipid bilayer without transport proteins?
- Large uncharged polar molecules (glucose, amino acids)
- Ions (sodium, potassium, chlorine, calcium)
What is diffusion?
Movement of substances across a selectively permeable membrane down a concentration gradient
What is equilibrium?
The state in which there is no diffusion occurring as the concentration is equal to each other
Why does diffusion occur?
Due to particle’s having kinetic energy - more molecules move away from an area of high concentration to an area of low concentration
What are the factors that affect the rate of diffusion?
Surface area Distance of diffusion Size of molecule Temperature Steepness of concentration gradient
How does the properties of diffusion limit the size of a cell? - what is the limit?
Longer diffusion distances mean slower rates of diffusion. Hence, if a cell is too large, normal cellular processes cannot occur therefore cells are capped at 20 micrometers
What are the two types of gradients that exist across a membrane?
Concentration gradient
Electrical gradient
What is an electrical gradient?
The difference in charge across the membrane due to the presence of ions
How is an electrochemical gradient across a membrane maintained?
Due to the selective permeable nature of membranes some molecules cannot pass through maintaining the electrical gradient
What is osmosis?
Net movement of water through a selectively permeable membrane from an area of high water potential to an area of lower water potential
What determines the permeability of a membrane to water?
The permeability of lipid bilayer + presence of aquaporins (family of proteins that allow osmosis of water)
Pw= Pd + Pf
Can the permeability of a membrane to water be affected by temperature?
Yes
What is osmotic pressure?
The pressure applied to a solution to prevent the inward flow of water across a semi-permeable membrane
What are the ion gradients across the membrane? Describe the charges between the cytoplasm and extracellular fluid
HIgh Na+ and Cl- outside
High K+ inside
Cells use 30% of resting energy to maintain this gradient
