Topic 4: Membranes Flashcards
What are lipids?
- Water-insoluble (hydrophobic) molecules made of hydrocarbons
What are biological lipids?
- Triglycerides (triacylglycerols): energy storage
- Phospholipids: component of cell membrane
- Sterols: component of cell membrane
What are fatty acids?
- Hydrocarbons with a carboxyl group at one end
- Building blocks of triglycerides and phospholipids
How do fatty acids vary?
- # of carbons in hydrocarbon/chain length
- Presence and number of carbon-to-carbon double bonds
What is a saturated fatty acid?
Has no carbon-carbon double bond, straight, and tightly compact
What is an unsaturated fatty acid?
Has one or more carbon double bonds (cis double cond), creating bends; preventing a tightly compact structure
What is an unnatural unsaturated fatty acid?
Has one or more carbon double bonds (cis double cond), but does not create bends
What is a Triglyceride?
- An energy storage molecule
- 3 fatty acid “tails” bound to a glycerol anchor
What is a Phospholipid?
Head group:
- Organic molecules
- Phosphate
- Glycerol
Attached to 2 fatty acid tails
What is an amphipathic molecule and give an example
- Contains a hydrophobic end and hydrophilic head
e.g.
Phospholipid
What do phospholipids spontaneously form in water?
- Bilayers
- Head groups facing out with tails pointing toward each other
How is the lipid bilayer fluid?
- Most phospholipid molecules are independent (not attached, free to move along the plane of the membrane)
How do you increase membrane fluidity?
- Increase temperature, unsaturated fatty acids (few van der Waals interactions, shorter chain length of fatty acid
What regulates membrane fluidity?
Sterols
How do sterols regulate membrane fluidity?
Animal cells insert cholesterol into the bilayer which
- Prevent excess viscosity by stopping the phospholipids from packing too tightly together
- Prevent excess fluidity by filling gaps between phospholipids
How does membrane fluidity affect the permeability of the membrane?
- Fluid membranes are leaky: more solutes can pass across the bilayer quickly
- Viscous membranes are better barriers: fewer solutes can cross the membrane more slowly
Lipid bilayers have selective permeability, name the least diffusible to most diffusible molecules
- Ions (least)
- Large uncharged polar molecules
- Small uncharged polar molecules
- Non-polar molecules (most)
How can proteins span a membrane?
When a protein spans a membrane, it can form a channel that allows specific molecules, like ions or water, to flow through the membrane, facilitating their movement in and out of the cell.
e.g.
Aquapourin forms a channel to allow water through
What is diffusion?
- Tendency of dissolved molecules to evenly distribute themselves in a solution
- High-concentration to low-concentration
- Equilibrium will eventually be reached (concentration gradient is eliminated)
Diffusion works across membranes if…
The solute is free to pass through the bilayer
What is osmosis?
Diffusion of water
When does osmosis occur?
If the solute cannot move, then the water will move from low solute concentration to high solute concentration
What is Tonicity?
The ability of a solution to affect the shape and movement of water in and out of cells, based on the concentration of solutes in the solution compared to the inside of the cell
What are hypotonic conditions?
- Lower concentration of solutes outside the cell, and higher concentration of solutes inside the cell
- Water flows into the cell
What are hypertonic conditions?
- Higher concentration of solutes outside the cell, and lower concentration of solutes inside the cell
- Water flows outside the cell
What are Isotonic conditions
- The same amount of concentration
- Equilibrium
What is Simple Diffusion Passive Transport?
- Doesn’t require any input of energy
- Small hydrophobic and small polar solutes diffuse directly across the phospholipid bilayer
What is the direction of movement for Simple Diffusion Passive Transport?
- The direction of movement is with the concentration gradient
- Powered by PE in the gradient
A larger concentration gradient means
The faster rate of diffusion
What is Facilitated Diffusion Passive Transport?
Proteins shaped like tunnels will cross the membrane and allow the diffusion of large polar molecules or ions down their gradient
- Substrate specific
- No energy required
- Rate depends on the concentration gradient
- Direction of transport is reversible
What is channel protein-facilitated transport?
Forming channels in the membrane that allow specific molecules or ions to pass through freely along their concentration gradient.
What is carrier protein-facilitated transport?
Binding to a specific substance, changing shape, and then releasing the substance on the other side of the membrane.
Simple diffusion vs Facilitated diffusion
SD: As concentration difference across membrane increases, so does the rate of transport
FD: Approaches a maximum rate when all transporters are occupied
To establish a concentration gradient…
Cells must move solutes away from equilibrium, requiring an energy source
What is Primary (1°) Active Transport?
- Requires energy (ATP), to power transport
- Protein “pumps” that are highly specific and cross the membrane
- Move solute against its concentration gradient from low to high concentration
What is Primary (1°) Active Transport often used for?
Used by cells to generate chemical gradients and electrochemical gradients
What is Second (2°) Active Transport Pumps?
- Highly specific protein pumps that move solutes against the concentration gradient
- Powered by the energy released as a different solute moves with the concentration gradient
2 kinds: - Symporters
- Antiporters
What is a symporter?
- A part of Secondary Active Transport
- Both solutes move in the same physical direction
What is an antiporter?
- A part of Secondary Active Transport
- The solutes move in opposite physical directions
