12.6 Transport across Membranes Flashcards
Why is the fluid mosaic model referred to as ‘fluid’?
The phospholipid molecules form a double layer (bilayer) that is constantly moving around relative to one another, giving the membrane a fluid structure
Why is the fluid mosaic model referred to as ‘mosaic’?
The proteins are unevenly distributed throughout the membrane so it is a mosaic
Components within cell membrane
Aquaporins
Cholesterol
Channel proteins
Carrier proteins
Enzymes
Glycoproteins
Phospholipids
Receptor proteins
Phospholipid function
- Hydrophobic tails attracted towards eachother
- Hydrophillic heads point either inwards/outwards
= FORM PHOSPHOLIPID BILAYER - Allows lipid soluble, non-polar molecules to pass through by SIMPLE DIFFUSION
- Prevents passage of small, polar molecules, e.g. ions
Cholesterol function
- Decreases permeability and increases stability of the membrane
- More cholesterol = less fluidity of membrane
Channel proteins function
- Allow specific charged ions/ small molecules to move across membrane by FACILITATED DIFFUSION
e.g Na+ ions pass through Na+ channel proteins only
Carrier proteins function
- Allow transport of ions, polar and large molecules, e.g. glucose, amino acids by FACILITATED DIFFUSION and ACTIVE TRANSPORT
Receptor proteins function
- Other proteins act as specific receptors for complementary molecules
e.g. insulin may bind to insulin receptor proteins - Specific cells have specific receptors
Enzymes function
- Enzyme active site is specific and complementary to substrate, allowing them to bind and form ESCs
Glycoproteins function
- Composed of carbohydrates and protein
- On outer surface of membrane
- Important in cell recognition acting as antigens and immune cells detect shapes to identify if they are self or non-self cells
- Produced in golgi body
Aquaporins function
- Special channel protein specific to water
- Lots of aquaporins in cell= very permeable to water= carries out osmosis easily
Movement of substances in or out cells occurs by?
Simple diffusion
Facilitated diffusion
Osmosis
Active transport
Co-transport
Bulk transport (endo/exocytosis)
Diffusion definition
Net movement of molecules from an area of higher concentration to a lower concentration across a partially permeable membrane.
Ficks law
Rate of Diffusion= (Surface area x Concentration Gradient)/ Diffusion Distance
Factors affecting rate of diffusion ROD
Surface area
Temperature
Concentration gradient
Diffusion distance
How does temperature affect ROD
Increased KE
Faster ROD of molecules
How does surface area affect ROD
Larger SA, more space for molecules to pass through
Faster ROD
More proteins can be present
How does concentration gradient affect ROD
As concentration difference increase, ROD increase
How does diffusion distance affect ROD
Shorter the diffusion distance (fewer membranes to cross), faster the molecules will travel
Facilitated diffusion definition
Specific proteins help specific molecules to pass through phospholipid bilayer
Water soluble, charged, large molecules cant pass directly through bilayer, so need to pass through channel/carrier proteins.
Is FD a passive or active process?
Passive, NO ATP REQUIRED
when does facilitated diffusion plateau on a graph
When all the carrier proteins are saturated/ binding sites are full
Number of proteins is a limiting factor
Osmosis definition
Net movement of water molecules from a high water potential to a lower water potential through a partially permeable membrane
What happens when there is a higher WP outside and lower WP inside cell?
Water moves IN by osmosis
Animal cells- swelling and lysis occurs
Plant cells- swelling increasing mass (cellulose cell wall prevents lysis)
What happens when there is a lower WP outside and higher WP inside cell?
Water moves OUT by osmosis, mass is lost.
Animal cells- shrivel
Plant cell membrane pulls away from cell wall- plasmolysed
Isotonic
Equalised water potential inside and outside cell
Active transport definition
- Transport of molecules against their concentration gradient
- Low to high
- Only uses carrier proteins
- ATP required
Is active transport an active or passive process?
It is active as it requires ATP for a source of energy
Process of exocytosis
- Uses golgi vesicles to transport large quantities of molecules from inside the cell to outside the cell
- Used to move enzymes and glycoproteins from golgi apparatus to cell surface membrane to SECRETE PROTEINS
Is exocytosis an active or passive process?
Active
It requires ATP to move the vesicles to the cell surface membrane and the vesicle fuse with the membrane
Process of endocytosis
- Cell surface membrane is pulled inwards to create a vesicle
- Any molecules nearby are enclosed into the vesicle
Is endocytosis an active or passive process?
Active
Requires ATP to create the vesicle and move the vesicle
Explain the arrangement of phospholipids in a cell-surface membrane.
- Bilayer OR Water is present inside and outside a cell;
- Hydrophobic (fatty acid) tails point away/are repelled from water OR Hydrophilic (phosphate) heads point to/are in/are attracted to water;
Many different substances enter and leave a cell by crossing its cell surface membrane.
Describe how substances can cross a cell surface membrane.
1 (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;
2 Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
OR
Large / polar / water-soluble molecules go through proteins;
3 Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
4 Active transport is movement from low to high concentration / against concentration gradient;
5 Active transport / facilitated diffusion involves proteins / carriers;
6 Active transport requires energy / ATP;
7 Ref. to Na+ / glucose co-transport;
The movement of substances across cell membranes is affected by membrane structure.
Describe how.
- Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
- Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
- Carrier proteins allow active transport;
- Channel/carrier proteins allow facilitated diffusion/co-transport;
- Shape/charge of channel / carrier determines which substances move;
- Number of channels/carriers determines how much movement;
- Membrane surface area determines how much diffusion/movement;
- Cholesterol affects fluidity/rigidity/permeability;
