Transport across membranes Flashcards
Cell-surface membrane
plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and environment
* contols the movement of substances in and out of cell
* 7-10 nm
* arranged in fluid mosaic
composed of:
1. Phospholipid bilayer
2. Proteins
3. Cholesterol
4. Glycolipids
5. Glycoproteins
Phospholipid bilayer
stucture of the cell membrane
* Hydrophillic heads- point to outside and attract water
* Hydrophobic tails- point to centre of membrane, repel water
* allows lipid-soluble substances to enter + leave cell
* prevents water soluble substances from enterinng + leaving
* makes membrane flexible
Fluid mosaic model
Fluid- phospholid molecules are fluids and can moved around eachother, makes membrane flexible
Mosaic- pattern produced by scattered protein molecules embedded in the bilayer that vary in shape, size and pattern
Proteins
embedded into the phospholipid bilayer
* can span across the bilayer from one
* channel proteins- form water-filled tubes to allow water-soluble ions to diffuse across the membrane
* carrier proteins- bind to ions or molecules like glucose, then change shape in order to move these molecules acroos the membrane (for active transport)
* provide structural support
* form cell-surface receptors e.g. for hormones
Cholesterol
hydrophobic molecules that occur within the phospholipid bilayer, regulate fluidity
* adds strength to membranes
* prevents water loss and dissolved ions
* reduces lateral movement of other molecules
* makes membrane less fluid at high temperatures
Glycolipids
carbohydrate covalently bonded with a lipid
* carbohydrate extends from the phospholipid bilayer into the watery environment outside
* acts as recognition sites
* helps maintain stability of membrane
* helps cell attach to one another to form tissues
Glycoproteins
carbohydrate chains attached to many extrinsic proteins on the outer surface of membrane
* act as cell surface receptors/ recognition sites
* helps cells attach to one another– tissues
* allows cells to recognise one another
Extrinsic proteins
Intrinsic proteins
What molucules cant pass through the phospholipid bilayer?
- molecules not soluble in lipids- cannot pass through the phospholipid bilayer
- molecules too large- cannot pass through channels in the membrane
- molcules of the same charge as channel proteins- are repelled
- electrically charged (polar) molecules- have difficulty passing through the non-polar hydrophobic tails
Simple diffusion
net movement of molecules/ions from a region high concentration to low concentration, down the concentration gradient
* passive process
* only small (fit through spaces between bilayer) or non-polar (have no charge + soluble) can diffuse across membrane e.g carbon dioxide and oxygen
Facilitated diffusion
- Charged ions (e.g sodium ions) and larger molecules (e.g glucose) cannot pass through the phospholipid bilayer due to the hydrophobic fatty-acid tails in phospholipid membrane
- movement of these molecules is facillitated by transmembrane channels + proteins that span membrane
- passive process
Channel proteins
water-filled hydophilic pores that form channels across the membrane to allow charged ions through membrane
* allow specific water-soluble ions through
* selctive, only allow specific ions through
* opens in presence of specific ion, or remains closed
*
Carrier proteins
proteins that move large molecules across membranes
* specific molecules bind to protein and fits through
* Binding causes it to change shape to allow molecule to inside of membrane
Factors affecting diffusion
- Concentration gradient- the steeper the concentration gradient, the greater the rate of diffusion
- Temperature- the higher the temperature the higher the kinetic energy for molecules to move around, increases rate of diffusion
- Surface area- the grater the surface area, more molecules/ions can cross it at any moment
- Nature of molecules/ions- large, polar molecules diffuse slowly
Osmosis
movement of water molecules from an area of high water potential to an area of low water potential, across a partially permemable membrane
Solute
any substance dissolved in a solvent e.g water
* solvent and solute form a solution
Water potential
pressure created by water molecules; tendancy of water molecules in a system to move
* measured in kiloPascals (kPa)
* highest value = 0
* Pure water has a water potential of 0
* addition of a solute to pure water lowers its water potential (more concentrated)
* the more negative the water potential, the more solute dissolved in it
Factors affecting osmosis
- Water potential gradient- difference between the water potential of the solute and solvent. The higher the water potential gradient, the faster the
- Surface area-
- Thickness of exchange surface
Osmosis in animal cells
if an animal cell is placed in a solution with a lower water potential, water will move out by osmosis- cell will shrink and wrinkle
e.g red blood cells
* red blood cells placed in pure water will absorb water because cell has a lower water potential
* animal cell normally live in a liquid which have the same water potential as cells e.g blood plasma
Osmosis in plant cells
- when plant is placed in pure water
* water moves into plant by osmosis because of the plants lower/negative water potential
* protoplasts swell and press on the cell wall
* cell wall offers limited expansion and pressure builds so osmosis stops
* cell is said to be turgid - when plant cell is placed in solution with a lower water potential
* water leaves by osmosis
* volume of cell decreases
Protoplasts
everything inside a plant cell
* e.g. outer cell surface membrane, nucleus, cytoplasm, and inner vacuole membrane
Incipient plasmolysis
the point at which the protoplast is just about to pull away from the cell wall and no longer presses on it
Plasmolysis
water loss through osmosis causes cell contents to shrink and the protoplasts pull away from the cell wall
Active transport
movement of molecules/ions in or out of a cell from a region of low concentration, to high concentration using carrier proteins and ATP
* carrier proteins are pumps
* carrier proteins have a specific tertiary structure
What is ATP used for in active transport?
- provides metabollic energy for active transport to occur
- directly moves molecules
- individually move molecules using a concentration gradient already set up by active transport (co-transport)
Decribe the process of active transport (single molcule)
- Carrier proteins span membrane and bind to molecule/ion to be transported
- Molecule binds to receptor site on carrier proteins
- On the inside membrane, ATP binds to the protein and is hydrolysed into ADP and Pi (phosphate molecule)
- Causes the protein molecule to change shape and open towards the inside of the membran
- Molecule/ion is released into the other side of membrane
- Pi molcule is released from protein, revering protien back to its original shape
(Pi molecule recombines with ADP to form ATP)
Sodium potassium pumps
sodium ions are actively removed from cell, while potassium ions are actively taken in from the surroundings
* Molecules bind to carrier proteim and ATP attaches to membrane protein on inside of cell
* Binding of Pi to protein causes protein to change shape so that access for molecules is open to inside of membrane but closed to outside
Co-transport
allows 2 different substances to be transported, using a carrier protein
* molecule is transported against a concentration gradient when it’s driven with an ion which can diffuse down its concentration gradient, through a co-transport protein
* indirect form of active transport
Co-transport of glucose and sodium ions in the iluem
To absorb glucose from lumen to gut, there must be higher concentration of glucose in lumen, then epithelial cell, but there is usually more glucose in epithelial cells so co-transport is needed:
1. Sodium ions are actively transported out of the epithelial into blood (ATP hydrolyses) by a carrier protein
2. Reduces sodium ion concentration in epithelial cell
3. Sodium ions can diffuse from the lumen down their concentration gradient into epithelial cell
4. Protein that sodium ions diffuse through is a co-transport protein, so glucose/amino acid attach and are transported into the epithelial cell, against the concentration gradient
5. Glucose moves by facilitated diffusion from the epithilial cell to blood
How is the rate of movement increased in epithelial cells
- Blood flow- maintains the concentration gradient of glucose between the epithelial cell and capillary
- Microvilli- on epithelial cell increase the surface area for co-transport proteins
- Proteins- increasing the number channel proteins and carrier proteins
Bulk transport
