Transport across membranes Flashcards
Define diffusion
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed
Explain what is meant by a passive process?
-No metabolic energy is required -Relies only on the inbuilt motion of the diffusing molecules ( kinetic energy)
Describe factors that affect diffusion into a cell across a membrane
The concentration gradient
Size/ charge of the particles
Temperature
Surface area of the membrane
Carrier proteins or channels
- Distance of the diffusion pathway
Why dont all molecules pass through a membrane at the same rate?
Because the centre of the membrane is hydrophobic due to the fatty acid tails ( non polar) meaning that hydrophobic molecules diffuse quickly across the membrane
Hydrophilic substances such as ions or polar molecules cannot diffuse across the membrane as easy, with an exception to water as this is small
What is facilitated diffusion?
Where hydrophilic molecules have to pass through the membrane via protein molecules
They can then cross the membrane without having to interact with the
hydrophobic centre of the phospholipid bilayer
How do carrier proteins facilitate diffusion?
Carrier proteins are intrinsic, and when a polar or charged molecule such as glucose that is specific to the protein is present, it binds to the protein causing it to change shape so molecule is released to the other side of the membrane
How do channel proteins faciliate diffusion
These proteins form water filled hydrophilic channels across the membrane and allow water soluble ions to pass through, these channels are selective, each opening in the presence of a specific ion, if a particular ion is not present , the channel will remain closed which controls the entry and exit of materials. The ions bind with the protein so that it changes shape causing it to close on one side of the membrane and opens to the other side
Define osmosis
The passage of water from a region with a higher water potential to a region where it has a lower water potential through a partially permeable membrane
What is the water potential of pure water?
Zero meaning any other substance has a water potential of less than zero
What is water potential?
Pressure created by water molecules, the more negative the value the lower the water potential
How does the water potential of a cells external solution influence the state of a cell
If water potential of the cells external solution is higher( less negative) the water will move by osmosis into the cell causing it to swell and burst,turgid if the water potential is equal there will be no movement of water, however if it is less the water will leave the cell and the cell will shrink (plasmolysed)
What is active transport?
Movement of molecules or ions in or out of a cell from a region of lower concentration to a higher concentration using ATP or carrier proteins
What is ATP used for in active transport?
Directly move molecules, individually move molecules using a concentration gradient which has already been set up by direct active transport
Why is active transport not a passive process
Metabolic energy in the form of ATP is needed
Substances are moved against a conc gradient
Carrier proteins are involved and the process is extremely selective with specific substances being transported
Describe the process of simple active transport
The carrier of proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it, the molecule or ion will then bind to receptor sites on the carrier protein.
Inside the cell, ATP will bind to the protein causing it to split into ADP and a phosphate molecule meaning the protein changes shape and opens on the other side of the membrane, releasing the molecule or the ion to the other side of the membrane
The phosphate molecule is then released from the protein molecule causing the protein to revert to its original shape for the process to repeat
What is the sodium potassium pump?
Using ATP, the sodium potassium pump actively transports sodium ions out of the epithelial cells and into the bloodstream, and at the same time it actively transports potassium ions into the epithelial cell, this now means there is an extremely low concentration of sodium ions in the cell
How does active transport occur in the illeum when not all glucose molecules can be absorbed into the cells?
Due to the sodium potassium pump, there is a low conc of sodium ions inside the cell as these have been activley trasnported out of the cell by the pump into the blood, however there is a much higher conc in the lumen of the ileum meaning there is a concentration gradient for sodium ions between the lumen of the ileum and the interior of the epithelial cells. The sodium ions will now diffuse along the concentration gradient through a co transport protein in the cell surface membrane, and as they do this they bring amino acid molecules or glucose molecules with them, meaning they are being transported against their concentration gradient, so they are being activly transported, the energy from this is being indirectly provided from the conc gradient of the sodium ion
How are the epithelial cells adapted for increasing rate of movement across membranes?
Cells lining the illeum possess microvilli
This provides a larger surface area for insertion of carrier proteins and channels
More rapid facilitated diffusion
Is facilitated diffusion passive or non passive and why?
Passive process
Relies on the inbuilt kinetic energy of moving particles
No external input such as ATP
Occurs down a concentration gradient
Compare and contrast active transport and facilitated diffusion
Facilitated diffusion involves carrier and channel proteins whereas active transport involves just carrier proteins
Faciliated diffusion occurs dowjn a conc gradient whereas active transport occurs against a conc gradient
How is the movement of substances across the cell membrane determined by the structure of the cell membrane?
- Non polar hydrophobic molecules can easily move through cell membrane due to hydrophobic fatty acid molecules
- Polar hydrophilic molecules cannot as they are repelled by hydrophobic fatty acid tails
-Carrier proteins and protein channels are needed to allow for rapid diffusion by facilitated diffusion and they also need this for active transport - Number of carrier proteins and protein channels also affects the rate at which molecules are moved across the membrane
-Charge of channel proteins also determines which molecules diffuse across, protiens are specific
