2.1.5 Biological Membranes Flashcards
What is the role of membranes?
- Partially permeable barrier between cell/environment, between organelles/cytoplasm and within organelles
- Site of chemical reactions
- Sit of cell communication (signalling)
What is the fluid mosaic model?
The mixture and movement of the phospholipids, proteins, glycoproteins and glycolipids the membrane is made of
What is the role of the phospholipds?
- Align as a bilayer due to hydrophobic tails and hydrophilic heads
- Heads are attracted to water, so face outwards, and tails are repelled by water, so face inwards
What is the role of cholesterol?
- Hydroxyl group aligns with the phosphate heads and remaining portion tucks into fatty acid tail part of membrane
- Cholesterol is a lipoprotein
- It maintains the fluidity of the cell membrane
What is the role of glycolipids and glycoproteins?
- They are lipids and proteins on the cell membrane that have carbohydrate chains protruding
- Form hydrogen bonds with any surrounding water molecules to stabilise the membrane structure
- Used as receptor molecules to bind with hormones/neurotransmitters
What is the role of proteins?
- Can be extrinsic or intrinsic
- Extrinsic proteins provide mechanical support or make glycoproteins
- Function is cell recognition as receptors
- Intrinsic proteins are carrier or channel proteins
How does temperature affect the membrane?
- As temperatures increase, the kinetic energy of the phospholipids increases
- This makes them move faster so the bilayer is more fluid which increases permeability
- High temperatures may denature carrier and channel proteins
What happens to the membrane at different temperatures?
Below 0:
- Membrane is rigid as packed closely together
- Channel and carrier proteins may denature
- Ice crystals may form and pierce the membrane
- Increases permeability
0-45:
- Phospholipids can move freely
- Partially permeable
- As temperature increases, the permeability increases
Above 45:
- Phospholipids start to melt
- Water inside cell expands putting more pressure on membrane
- Channel and carrier proteins denature
- Increases permeability
How do solvents affect the membrane?
- Non-polar solvents can insert themselves into the bilayer
- This pushes phospholipids out of their orderly placement and increases movement
- The disruption of the membrane structure increases fluidity and increases permeability
- Solvents may denature proteins
How do molecules move across the membrane by simple and facilitated diffusion?
Diffusion = the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached
- Small or lipid soluble molecules move via simple diffusion
- Ions and polar molecules move via facilitated diffusion
- Facilitated diffusion uses channel proteins and carrier proteins
- Carrier proteins change shape to allow a specific type of molecule through
- Channel proteins from pores in the membrane
What is Fick’s law?
Rate of diffusion is proportional to (surface area x concentration difference)/length of diffusion path
What is osmosis?
The movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane
What is water potential?
The pressure exerted by water molecules that are free to move in a system (kPa)
- Pure water has a water potential of 0
- Lower water potential is negative
What happens when a cell is in a hypertonic solution?
Hypertonic = water potential is more negative
- Water moves out of an animal cell and it shrinks/crenates
- Plant cell loses water and goes flaccid as vacuole becomes flaccid and cytoplasm stops pushing against cell wall
- Known as plasmolysis
What happens when a cell is in a hypotonic solution?
Hypotonic = water potential is more positive
- Water moves into an animal cells so it expands
- It may burst, known as lysis
- Water enters a plant cell and fills vacuole so plasma membrane pushes against the cell wall
- Plant cell will be turgid
