Transport Across Membranes Flashcards
Phospholipids
- Bilayer is present inside and outside a cell;
- Hydrophobic (fatty acid) tails point away from water
- Hydrophilic (phosphate) heads point to water
The phospholipid bilayer allows lipid-soluble (non-polar) molecules to pass through by simple diffusion but prevents the passage of small polar molecules
The fluid-mosaic model
- The position of the molecules within the membrane is fluid – they are able to move around within the membrane.
- Membrane is made up from a variety of different molecules arranged into a mosaic.
Cholesterol
Decreases permeability and increases the stability of the membrane.
MORE cholesterol = LESS fluidity of the membrane is
Different types of cells have different proportions of cholesterol
Channel proteins
Channel proteins are like pores within the membrane and only allow specific charged ions or small molecules to move across the membrane by facilitated diffusion . They can be open or closed
Proteins have specific tertiary structures, so they are specific and can only transport molecules that are complementary to the shape of the channel protein.
Carrier proteins
Carrier proteins aid the transport of ions/polar molecules and large molecules e.g. glucose and amino acids by facilitated diffusion and active transport.
Glycoproteins
Glycoproteins are composed of carbohydrate and protein, are on the outer surface of the membrane and are important in cell recognition, often acting as antigens.
The immune cells detect the specific shapes of glycoproteins to identify the cells as SELF or if they are NON-SELF (foreign).
Glycoproteins are produced in the golgi body
Aquaporins
special types of ‘channel proteins’ specific to water.
A cell that has a lot of aquaporins is said to be very permeable to water and will carry out osmosis easily.
Diffusion
Diffusion is the net movement of molecules from an area of higher concentration to a lower concentration across a partially permeable membrane
- passive process
- does not require energy
- Diffusion will stop when the molecules have reached equilibrium
- does not require a specific protein and only involves non-polar, small and lipid soluble molecules e.g. O2 , CO2
Fick’s Law
Rate of diffusion =(Surface Area x Concentration gradient) / (Diffusion distance)
Factors affecting rate of diffusion
Temperature:
Increased kinetic energy, therefore faster rate of diffusion of molecules
Surface area:
Larger surface area provides more “space” for molecules to pass through, therefore faster. This also allows for more proteins to be present. E.g. microvilli increase surface area of a cell.
Concentration gradient:
As concentration difference increases, rate of diffusion increases.
Diffusion distance:
the shorter the diffusion distance, the faster molecules will travel from one area to the next.
Facilitated Diffusion
to enter or exit the cell, water soluble molecules move through the membrane via channel proteins or carrier proteins
Facilitated diffusion levels off when all the carrier proteins are saturated/binding site full.
B plateaus at a lower rate as there are fewer specific carrier proteins for this molecule in the membrane.
Number of proteins becomes the limiting factor.
Osmosis
the net movement of water molecules from higher water potential (Ψ) to a solution with lower water potential (Ψ) through a selectively permeable membrane.
Higher Ψ than tissue / organelle: (lysis)
- Higher Ψ outside cell
- Lower Ψ inside cell
- Water moves IN by osmosis
Lower Ψ than tissue / organelle: (shrivelling)
- Higher Ψ inside cell
- Lower Ψ outside cell
- Water moves OUT by osmosis, mass is lost.
Isotonic:
- No net movement of water in or out of cells.
- Ψ inside cell = Ψ outside cell
Active Transport
transport molecules across the membrane against their concentration gradient, from low concentration to high concentration.
- It requires a specifically shaped carrier protein, with a complementary binding site, that only complementary molecule binds to.
- It requires a source of energy, supplied by ATP
Bulk transport: Exocytosis
uses (Golgi) vesicles to move very large quantitities of molecules from inside the cell to outside the cell.
Exocytosis is also used to move enzymes and glycoproteins from the Golgi apparatus to the cell surface membrane to secrete proteins.
ATP is required to move the vesicles to the cell surface membrane and the vesicle FUSE with the membrane.
Bulk transport: Endocytosis
The cell surface membrane is “pulled” inwards, to create a vesicle. Any molecules next to that part of the membrane are enclosed within the vesicle.
This process requires the breakdown of ATP into ADP and Pi
The movement of the vesicle in the cell also requires the breakdown of ATP into ADP and Pi
