Plasma membranes Flashcards
What is a Plasma membrane
Basically the cell surface mmebrane of all cells and organelles
Describe the fluid-mosaic model of membrane structure
● Molecules free to move laterally in phospholipid bilayer
● Many components - phospholipids, proteins,
glycoproteins and glycolipids
Describe the arrangement of the components of a cell membrane (7)
● Phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards
● Proteins
○ Intrinsic / integral proteins span bilayer eg. channel and carrier proteins
○ Extrinsic / peripheral proteins on surface of membrane
● Glycolipids (lipids with polysaccharide chains attached) found on exterior surface
● Glycoproteins (proteins with polysaccharide chains attached) found on exterior surface
● Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails
How does the phospholipid bilayer affect entry and exit of substances in and out of the cell? (4)
- It allows the movement of lipid soluble and non charged molecules
- Prevents the movement of Water soluble/Polar molecules
- Membrane is partially permeable
- Large molecules do not move through;small molcecules do
Explain the role of cholesterol (sometimes present) in cell membranes
● Restricts movement of other molecules making up membrane
● So decreases fluidity (and permeability) / increases rigidity
What is the importance of cholesterol
- Makes the membrane less fluid at high temps
- Prevents water and dissolved ions leaking out of the cell
What is the difference between Peripheral/extrinsic and Integral/intrinsic proteins?
- Peripheral/extrinsic do not extend completely across the membrane
- Integral/Intrinsic spans across from one side of the bilayer to the other
What is the function of the peripheral/extrinsic proteins
- To provide mechanical support
- They connect to proteins or lipids to make glycoproteins and glycolipids—> act as receptors,cell recognition,
What is the function of integral/intrinsic proteins
Transport molecules across the membrane through carrier and channel proteins
What is the function of glycoproteins/lipids
Cell recognition ;act as receptors
What do channel proteins do
Form tubes that fill with water to enable water soluble ions to diffuse
What do carrier proteins do
Bind with larger molecules, such as glucose and amino acids and change shape to transport them to the other side of the membrane
What are the factors allowing substances to pass through the membrane (simple diffusion)
Lipid soluble substances e.g. some hormones: estrogen
Very small molecules e.g. O2 CO2 H2O
What are the factors that do not allow molecules to pass through the membrane in Simple diffusion
Water soluble (polar) substances
Sodium ions
Large molecules e.g. glucose
Explain the limitations imposed by the nature of the phospholipid bilayer
● Restricts movement of water soluble (polar) & larger substances eg. Na
+/ glucose
● Due to hydrophobic fatty acid tails in interior of bilayer
Explain the arrangement of phospholipids in a cell membrane (3)
● Bilayer, with water present on either side
● Hydrophobic fatty acid tails repelled from water so point away from water / to interior
● Hydrophilic phosphate heads attracted to water so point to water
What features of a particle would allow it to diffuse directly through the phospholipid bilayer
Small and non polar (uncharged)
What structure in the membrane has the function ‘to allow the facillitated diffusion of large or charged particles’
Channel proteins
Suggest how cell membranes are adapted for other functions
● Phospholipid bilayer is fluid → membrane can bend for vesicle formation / phagocytosis
● Glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition
