3.2.3 Transport across cell membranes Flashcards
what is the name of the model of a plasma membrane structure?
fluid-mosaic model
the model of the plasma membrane is ‘fluid’ because…
the phospholipids are constantly moving
the model of the plasma membrane is ‘mosaic’ because…
proteins are scattered/dotted through the bilayer
which six structures make up a plasma membrane?
- phospholipid bilayer
- integral proteins
- peripheral proteins
- glycoproteins
- glycolipids
- cholesterol
why do the phospholipids arrange in a bilayer?
the hydrophilic heads are attracted to water and the hydrophobic tails are repelled by water
what is the function of the phospholipid bilayer?
creates a partially permeable membrane so only allows small, non-polar molecules to diffuse through
what is the characteristic of an integral protein?
they span across from one side of the bilayer to the other
what are the functions of integral proteins?
are carrier proteins or channel proteins involved in the transport of molecules across the membrane
what is the characteristic of a peripheral protein?
they do not extend completely across the membrane
what are the functions of peripheral proteins?
- provide mechanical support
- can act as receptors and allow the cell to detect chemical released from other cells so that they can responds
what is a glycoprotein?
a protein with a carbohydrate attached
what is a glycolipid?
a lipid with a carbohydrate attached
what are the functions of glycoproteins / glyolipids?
- can act as antigens to facilitate cellular recognition
- play an important role is cell signalling and communication
- help with cell adhesion (make connections which allow cells to connect to one another to form tissues)
what is the function of cholesterol?
- restricts the lateral movement of other molecules in the membrane which makes the membrane less flexible
- this is useful as it makes the membrane less fluid at high temperatures and prevents water and dissolved ions leaking out
what is diffusion?
the net movement of molecules / ions from an area of high concentration to an area of low concentration
molecules can diffuse simply if they are…
lipid soluble and very small
molecules cannot diffuse simply if they are…
water soluble (polar) and large
what are the four factors affecting simple diffusion?
- concentration gradient
- membrane thickness
- surface area
- temperature
how and why does the concentration gradient affect simple diffusion?
- the greater the concentration difference the faster the rate of diffusion
- diffusion slows down over time as equilibrium is reached so concentration gradient must be maintained
how and why does the membrane thickness affect simple diffusion?
- the thinner the membrane the faster the rate of diffusion
- thin membranes reduce the distance particles have to travel
how and why does the surface area affect simple diffusion?
- the larger the surface area the greater the rate of diffusion
- increasing the SA (e.g., by folding) means more particles can be exchanged in the same amount of time
how and why does the temperature affect simple diffusion?
- the higher the temperature the faster the rate of diffusion
- at higher temperatures, particles have more kinetic energy so they will be able to move around faster
what is facilitated diffusion?
hydrophilic substances move down a concentration gradient across the cell membrane via integral membrane proteins
what are the two types of integral proteins involved in facilitated diffusion?
- channel proteins
- carrier proteins
what is a channel protein?
a protein with a central pore which is lined with hydrophilic amino acids and water
how do channel proteins facilitate diffusion?
hydrophilic substances can pass through the channel from one side of the membrane to the other
what are two features of channel proteins?
- channel proteins are selective for the chemical that can pass through
- some types of channel proteins only open in response to a certain trigger whereas others are always open
how do carrier proteins facilitate diffusion?
- carrier proteins have a binding site for a specific molecule
- when the molecule binds it causes the tertiary structure of the carrier protein to change shape
- this change in shape enables the molecule to be released to the other side of the membrane
what are the two factors affecting the rate of facilitated diffusion?
- concentration gradient
- number of channel/carrier proteins
how does the concentration gradient affect the rate of facilitated diffusion?
- the greater the concentration gradient the faster the rate of facilitated diffusion
- this is true until all the proteins are in use or once equilibrium is reached
how does the number of channel/carrier proteins affect the rate of facilitated diffusion?
- the greater the number of channel/carrier proteins the faster the rate of facilitated diffusion
- this is true until all the proteins are in use so it becomes a limiting factor
what is osmosis?
the movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane
what is water potential?
the pressure created by water molecules
what unit is water potential measured in?
kilopascals (kPa)
what is the water potential of pure water?
0 kPa
what happens to the water potential of pure water when a solute is dissolved in it?
the water potential becomes negative
the more solute dissolved in the solution, the more ____ the water potential
negative
if solution A has a water potential of -50 kPa and solution B has a water potential of -250 kPa, which solution has a higher concentration of water?
solution A
if solution A has a water potential of -50 kPa and solution B has a water potential of -250 kPa, what would be the direction of osmosis?
water will move from A to B
what is an isotonic solution?
when the water potential is the same in the solution and the cell within the solution
what happens when a cell is placed in an isotonic solution?
there is no net (overall) movement of water
what is a hypotonic solution?
when the solution has a higher (more positive) water potential than the cell
what happens when an animal cell is placed in a hypotonic solution?
water will move into the cell by osmosis causing the cell to swell up and eventually burst
what happens when a plant cell is placed in a hypotonic solution?
water will move into the cell by osmosis causing the cell to become turgid (it will not burst due to the strength of the cell wall)
what is a hypertonic solution?
when the solution has a lower (more negative) water potential than the cell
what happens when an animal cell is placed in a hypertonic solution?
water will move out of the cell by osmosis so the cell will shrivel as it loses water from its cytoplasm
what happens when a plant cell is placed in a hypertonic solution?
- water will move out of the cell by osmosis
- cell will not shrink due to cell wall
- however, the cytoplasm will shrink and the cell membrane will pull away from the cell wall
- known as plasmolysis
what is active transport?
- the movement of molecules and ions from an area of lower concentration to an area of higher concentration (against a concentration gradient)
- using metabolic energy provided by ATP and carrier proteins
how are molecules actively transported using carrier proteins and ATP?
- molecule binds to the receptor on the carrier protein which is complementary in shape
- ATP binds to the carrier protein from the inside of the cell and is hydrolysed into ADP + Pi
- this causes the carrier protein to change shape and release the molecule to the other side
- the phosphate ion is then released and the carrier protein returns to its original shape
how does the co-transport of glucose and sodium ions in the ileum work?
- sodium ions are actively transported out of the epithelial cell into the blood by the sodium-potassium (Na+/K+) pump
- this reduces the sodium ion concentration in the epithelial cell, creating a concentration gradient
- sodium ions then move down their concentration gradient by facilitated diffusion from the lumen into the epithelial cell
- they do this using a co-transporter protein, so glucose also attaches and is transported into the epithelial cell against its concentration gradient
once in the epithelial cell, how is glucose transported into the blood?
facilitated diffusion
what are 5 features you would expect to find in a cell specialised for absorption?
- folded membrane/microvilli so large surface area for absorption
- large number of mitochondria so more ATP is made by respiration and to release energy for active transport
- large number of co-transport/channel/carrier proteins so fast rate for absorption
- membrane-bound digestive enzymes so maintains a concentration gradient for fast absorption
- constant blood flow maintains a concentration gradient
what is bulk transport?
- the transfer of a large quantity of material across a cell
- often molecules/particles which are too large to be transported by facilitated diffusion or active transport
- endocytosis and exocytosis
what is endocytosis?
large particles/molecules are transported into the cell
how does endocytosis work?
- cell membrane folds inwards to form a cavity around the particles (a process known as invagination)
- membrane completely encircles the particles to form a vesicle
- vesicle moves into the cell
what is exocytosis?
large particles/molecules are transported out of the cell
how does exocytosis work?
- vesicle containing proteins, for example, moves towards the cell membrane
- vesicle fuses with the cell membrane to secrete the protein
what do endocytosis and exocytosis require?
energy provided by ATP
what are the two factors affecting membrane permeability?
temperature and solvents
how does a temperature below 0°C affect the plasma membrane permeability?
- phospholipids are packed closely together and the
membrane is rigid - channel and carrier proteins can denature which
increases the permeability of the membrane - ice crystals can form and pierce the membrane, increasing permeability when it thaws
how does a temperature between 0 - 45°C affect the plasma membrane permeability?
- phospholipids move around freely and the membrane is partially permeable
- as temperature increases, kinetic energy increases so the membrane becomes more permeable
how does a temperature above 45°C affect the plasma membrane permeability?
- phospholipids start to melt increasing membrane permeability
- water inside the cell expands putting pressure on the membrane
- channel and carrier proteins in the membrane denature increasing permeability
how do solvents affect plasma membrane permeability?
- non-polar solvents such as alcohols or acetone insert themselves into the bilayer
- this pushes phospholipids out of their orderly placement and increases their movement
- disruption of the membrane structure increases its permeability
- solvent can also denature proteins by disrupting bonds
