2. Cell Membrane Structure and Function Flashcards
All membranes around and within cells (including those surrounding organelles) are called…
plasma membranes
What do we call the plasma membrane that surrounds cells?
Cell-surface membrane
What is the function of a cell surface membrane?
- To separate the internal and external environments of the cell.
- To control the movement of substances in and out of the cell.
Name the components of a cell surface membrane.
- Phospholipids
- Proteins (intrinsic and extrinsic)
- Glycoproteins
- Glycolipids
- Cholesterol
Phospholipids have polar and non-polar regions. A molecules that exhibits both of these properties is said to be…
Amphipathic
What is a polar molecule?
A molecule that does not have an evendistriibution of electrons making some regions more negative than others.
What is a non-polar molecule?
A moelucle that has an even distribution of electrons, meaning it does not have areas that are more negative than other areas.
Phospholipids are made form which 3 molecules?
Phosphate
Glycerol
Fatty acid ‘tails’x 2
Describe the phosphate head of a phospholipid.
- Polar
- Hydrophillic - water loving
Describe the fatty acid ‘tails’ of a phospholipid.
- Non-polar
- Hydrophobic - ‘water hating’
What are the functions of the phospholipids in the cell membrane?
- Allow lipid-soluble (non-polar) substances to enter/exit the cell.
- Prevent water soluble (polar) substances entering/exitingthe cell.
- Makes the membrane flexible and self-sealing.
Proteins in a cell membrane can either be __________ or _________.
Intrinsic or extrinsic
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer
Cholesterol molecule (green)
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer
2 x extrinsicproteins (carriers)
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer
Glycolipid molecules x 2
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer
Glycoproteins x 2
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer
Intrinsic proteins x 2 (channel and carrier)
What are the funcitons of proteins in the cell membrane?
- Provide structural support
- Act as channels to transport polar molecules (facillitated diffusion)
- Allow active transport (carrier proteins)
What is the function of cholesterol in cell-surface membranes?
- Reduces lateral movement of molecules that make up the membrane.
- Acts as a buffer against temp increases/decreases to maintain membrane fluidity.
- Prevents leakage of water and dissolved ions from the cell.
What is a glycolipid?
A carbohydrate (polysaccharide) covalently bonded to a lipid.
What are the functions of glycolipids?
- Act as cell recognition sites (ABO blood group antigens are glycolipids)
- Maintain stability of cell surface membrane.
- Help cells to attach to one another and form tissues.
What is a glycoprotein?
An extrinsic cell surface membrane prtoein with a carbohydrate covalently bonded to it.
What are the funcitons of glycoproteins in the cell surface membrane?
- Act as recognition sites (for hormones and neurotransmitters)
- Help cells to attach and form tissues.
Substances that can readily diffuse through the cell-surface membrane are small polar molecules and non polar molecules such as…
Gases like oxygen and CO2
- Water
- Lipid-based hormones (steroids) e.g.oestrogen
Substances that cannot readily diffuse through the cell-surface membrane are large andpolar molecules andsuch as…
- Glucose
- Ions
- Proteins
- Urea
In an investigation about diffusion and plasma membranes (beetroot discs) why would you ensure the temperature remained constant?
so that rate of diffusion remains constant
no change in fluidity of phosopholipids
no denaturation of membrane proteins
In an investigation about diffusion and plasma membranes (beetroot discs) why would you ensure the the beetroot discs where shaken?
To ensure all the discs exposed to water/ all discs fully submerged
maintain concentration/diffusion gradient
name the type of molecules that can diffuse through the membrane
lipid soluble
small
non-polar
during cell fractionation a detergent can be used - why?
cell mebranes made of phospholipids
detergent dissolves membranes
releasing cell contents
How does ultracentrifugation work to seprate cell organelles?
spin supernatent at low speed initially
then gradually increase the speed
separating organelles out based on their size/density
Membrane structure
•Made from lipids (mainly phospholipids), proteins and carbohydrates
It is about 7nm thick
It is described as a fluid mosaic structure.
Fluid?
Mosaic?
FLUID-
Individual phospholipid molecules move relative to each other- gives the membrane a flexible structure
MOSAIC- proteins are scattered in bilayer. They vary in shape, size and pattern- like the tiles in a mosaic
Label
What does cholesterol do?
Reduces lateral movement of molecules including phospholipidsMake membrane less fluid at high temperaturesPrevent leakage of water and dissolved ions from cellAdd strength to membrane
What does cholesterol do and what cell does it help?
Cholesterol also has hydrophobic regions so it creates a further barrier to polar substances moving through the membrane i.e. It restricts movement
Cholesterol helps to maintain the shape of red blood cells
Membranes contain proteins which means these…(6 pts)
Can move freely in bilayer and some are attached to the outside of cell or a structure in the cytoplasmMay act as enzymesAct as carriers to transport substances in and out of cellsAct as channel proteins to allow specific ions in and outAct as Receptor proteins for hormonesCan act as antigens and are important in cell recognition
Membrane Permeability - Below 0
The phospholipids don’t have much energy. They are packed closely together.
Channel and carrier proteins denature. This increases the permeability of the membrane. Ice crystals may form and pierce the membrane increasing permeability
Membrane Permeability - Between 0 and 45
The phospholipids can move around. The membrane is partially permeable. As the temperature increases the phospholipids gain kinetic energy and move more. This increases the permeability of the membrane
Membrane Permeability - Above 45
The phospholipid bilayer begins to melt. The membrane becomes more permeable. Proteins denature so they can’t control what enters and leaves the cell
Membrane Permeability - Above 45
The phospholipid bilayer begins to melt. The membrane becomes more permeable. Proteins denature so they can’t control what enters and leaves the cell
Membrane Permeability - Below 0
The phospholipids don’t have much energy. They are packed closely together.
Channel and carrier proteins denature. This increases the permeability of the membrane. Ice crystals may form and pierce the membrane increasing permeability
