4.Transport across cell membranes Flashcards
(37 cards)
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
