2.1.5 Biological membranes Flashcards
(38 cards)
What are the four main components of the cell surface membrane
- phospholipids
- cholesterol
- glycolipids and glycoproteins
- transport proteins
Describe the structure of phospholipids
phosphate heads are polar and hydrophilic, and the fatty acid tails are non-polar and hydrophobic
the non-polar tails prevent polar molecules from passing through - bilayer acts as a barrier to most water-soluble substances
also ensures water-soluble substances (sugars, amino acids and proteins) can’t leak out
How can phospholipids be chemically modified to act as signalling molecules
they can move through the bilayer to activate other molecules e.g. enzymes
they can be hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
What is the role of the cholesterol in the cell surface membrane
low temperatures - increases the fluidity and prevents it from becoming too rigid
high temperatures - cholesterol binds to the hydrophobic tails, causing them to pack closer together
- cholesterol increases the mechanical strength and stability of membranes (prevents them from bursting)
What is the role of glycolipids and glycoproteins in the cell surface membrane
both contain carbohydrate chains on the surface, which enables them to act as receptor molecules
some glycolipids and glycoproteins act as antigens for cell-to-cell recognition
Give the three main types of receptors that form from glycolipids/proteins
1) signalling receptors for hormones and neurotransmitters
2) receptors involved in endocytosis
3) receptors involved in cell adhesion and stabilisation
- carbohydrate chain can form hydrogen bonds with water molecules surrounding the cells
What is the role of transport proteins in the cell surface membrane
create hydrophilic channels to allow ions and polar molecules to travel through the membrane
each transport protein is specific to a particular ion or molecule
- allows the cell to control what substances enter or leave
Give the two types of transport protein
1) channel proteins
2) carrier proteins
How does temperature affect membrane permeability
as temperature increases, phospholipids become more fluid, which reduces the effectiveness of the cell membrane as a barrier to polar molecules, increasing permeability
at higher temperatures, diffusion will occur at a higher rate due to increased kinetic energy
- reversible change
at a certain temperature, transport proteins will begin to denature, which disrupts the membrane structure, preventing it from acting as an effective barrier
- irreversible change
How does solvent concentration affect membrane permeability
organic solvents dissolve the phospholipids in the membrane
this increases membrane permeability as the membrane loses its structure
What is diffusion
net movement of a substance from a region of high concentration to a region of low concentration
Give four factors which affect rate of diffusion
1) concentration gradient
2) temperature
3) surface area
4) properties of molecules or ions
How do properties of molecules or ions affect rate of diffusion
large molecules diffuse slower than smaller ones as they require more energy to move
non-polar molecules diffuse quicker than polar molecules as they are soluble in the phospholipid bilayer
What substances can diffuse naturally through the membrane (simple diffusion)
small nonpolar molecules
- steroid hormones (testosterone, oestrogen)
- lipid soluble vitamins
- respiratory gases
small polar molecules (very slow rate)
- water
- urea
What substances use facilitated diffusion to move into cells
large polar molecules e.g. glucose and amino acids
ion e.g. Na+ or Cl-
- occurs through channel proteins
Describe the structure and function of a channel protein
water-filled pores
most are gated, meaning that part of the channel protein must move in order to open or close the pore
- this allows the channel protein to control the exchange of molecules / ions
Describe the structure and function of carrier proteins
they have a specific shape to a specific molecule/ion
the specific molecule/ion must bind to the binding site in order to change the shape of the protein and move in/out of the cell
What is active transport
movement of molecules from a region of lower concentration to a region of higher concentration, requiring ATP released in respiration
- requires carrier proteins
Give four examples of the importance of active transport
1) reabsorption of useful ions into the blood after filtration of the kidneys
2) absorption of some products of digestion
3) loading of sugar from photosynthesising leaves into the phloem tissue
4) loading of inorganic ions from the soil into root hair cells
What is osmosis
movement of water molecules from a high water potential to a low water potential, across a partially permeable membrane
What is water potential
a measure of the quantity of water compared to solute, measured as pressure created by molecules in kPa
What is the water potential of pure water
0kPa
- the more negative the value, the lower the water potential
What happens to animal cells in hypertonic solutions
water will leave the cell by osmosis
the cell will shrink and shrivel up - crenation
What is a hypertonic solution
when the solution has a lower water potential than inside the cell
