4.Transport across cell membranes Flashcards
What are all membranes around and within all cells called
Plasma membranes
What is the cell-surface membrane
Plasma membrane that surrounds cells, forming the boundary between the cytoplasm and outside the cell. It controls the movement of substances in and out the cell
Why do phospholipids form a bi-layer
- polar hydrophilic heads of the two phospholipid layers point to the outside of the cell-surface membrane as it is attracted to water on both sides
- non-polar hydrophobic tails of the two phospholipid layers point into the centre of the cell membrane as they are repelled by water
What is the function of the phospholipids in the membrane
- allow lipid soluble substances to leave and enter the cell
- prevent water-soluble substances to leave and enter the cell
Name two ways proteins are found on the bi-layer
- found on the surface and provide mechanical support or (in conjunction with glycolipids) as cell receptors
- other proteins span the bi-layer completely. Some are carrier proteins some are protein channels.
What are the functions of the proteins embedded in the bi-layer
- provide structural support
- act as channels transporting water-soluble substances across the membrane
- allow active transport across the membrane through carrier proteins
- form cell surface receptors for identifying cells
- act as receptors (eg. for hormones)
Where are cholesterol molecules found
Within the phospholipid bilayer of the cell surface membrane.
What are the function of cholesterol molecules
- reduces lateral movement of other molecules including phospholipids
- make the membranes less fluid at higher temperatures
- prevent leakage of water and dissolved ions from cell, as they are very hydrophobic
What are the function of glycolipids on cell-surface membranes
- act as recognition sites
- help maintain the stability of the membrane
- help cells attach to one and another and so form tissues
What are the functions of glycoproteins on the cell-surface membrane
- act as recognition sites
- help cells attach to one and another and so form tissues
- allows cells to attach to one another (eg lymphocytes )
Why can most molecules not freely diffuse across the cell-surface membrane
- not soluble in lipids/ non-polar and therefore cannot pass through the phospholipid layer
- too large to pass across channels in the membrane
- are the same charge as the protein channels so are repelled
What model is the cell surface membrane described as
Fluid-mosaic model
Explain why the cell-surface membrane is described as fluid-mosaic
- fluid as the phospholipid molecules can move within the membrane meaning the membrane is flexible and constantly changing shape
- mosaic as there are proteins embedded in the phospholipid
Name all the ways movement across a membrane can occur by
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
- co-transport
Define diffusion
The net movement of molecules or ions from a high concentration to a low concentration
What are some molecules that can pass through the bilayer via passive diffusion
Small non-polar molecules such as O2 and CO2
What is passive diffusion
Diffusion that occurs due to the kinetic energy of the constantly moving molecules and does not require ATP or proteins
What is facilitated diffusion
Passive diffusion of molecules through either protein channels or carrier proteins
What are protein channels
- proteins that form hydrophilic channels across the membrane which allow specific water-soluble ions to pass through.
- the channels are selective each only opening in the presence of a specific ion.
How do protein channels work
The ion binds with the protein causing it to change shape in a way that closes it on one side and opens it on the other
How to carrier proteins work
A molecule specific to the protein (eg. glucose) binds to the carrier protein causing it to change shape which causes it to be released on the other side of the membrane.
Define osmosis
The movement of water molecules from a region of high to low water potential through a selectively permeable membrane.
Explain osmosis
- a solution has a low concentration of solute molecules on one side and a high on the other than are in random motion due to kinetic energy
- the selectively permeable plasma membrane only allows water molecules across it
- water molecules move down the water potential gradient
- dynamic equilibrium is established where there is no net movement of water
What will happen if a cell is placed in a higher water potential relative to inside the cell
Water will move inside the cell down the concentration gradient causing the cell to burst
What will happen if a cell is placed in a lower water potential relative to inside the cell
Water will move inside the cell down the concentration gradient causing the cell to shrivel
What is the water potential of blood plasma
The same as inside red blood cells to stop the movement of water in or out the red blood cell via osmosis which could cause it to burst or shrivel
Define active transport
The movement of molecules or ions from an area of lower to higher concentration using ATP and carrier proteins
What is ATP used for in active transport
- directly move molecules
- indirectly move molecules by using ATP to create a conc. gradient for molecules to passively diffuse down
Name the differences in active transport and passive diffusion
- ATP use
- up or down concentration gradient
- carrier proteins
Name two ways to increase the rate of movement across membranes
- increased surface area (eg through micro villi)
- increased amount of protein channels per unit area
Describe the direct active transports of a single molecule
- molecule specific to the carrier protein binds to a receptor site on it
- on the inside of the cell ATP binds to the protein which causes the carrier protein to change shape
- molecule is released on the other side of the membrane which causes the protein to revert back to its original shape
Describe two ways rate of movement across membranes can be increased
- increase surface area of membrane (eg. microvilli do this in the gut).
- increase density of carrier proteins and protein channels
Why is active transport important in digestion
Passive diffusion only results in equal concentrations either side of the intestinal epithelium, To absorb more molecules than this energy must be used in active transport.
Name the mechanism by which glucose and amino acid molecules are absorbed from the small intestine
Co-transport
Describe the steps involved in co-transport
- sodium ions are actively transported out of epithelial cells into the blood (by the sodium-potassium ion pump) using a carrier protein (K+ is transported in)
- this maintains a higher concentration of sodium ions in the lumen than inside the epithelial cells
- sodium ions therefore diffuse via facilitated diffusion down this concentration gradient from the lumen into the epithelial cells (via a carrier protein).
- as the Na+ ions diffuse they carry either an amino acid or a glucose molecule into the cell with them
- the glucose or the amino acid diffuse into the blood stream via facilitated diffusion
Describe the steps involved in co-transport
- sodium ions are actively transported out of epithelial cells into the blood (by the sodium-potassium ion pump) using a carrier protein (K+ is transported in)
- this maintains a higher concentration of sodium ions in the lumen than inside the epithelial cells
- sodium ions therefore diffusion passively down this concentration gradient from the lumen into the epithelial cells (via a carrier protein).
- as the Na+ ions diffuse they carry either an amino acid or a glucose molecule into the cell with them
- g
What is co-transport an example of
Indirect active transport
