Transport across cell membrane Flashcards
What are the three functions of phospholipids in the membrane?
- allow lipid-soluble substances to enter and leave
- prevent water-soluble substances entering and leaving the cell
- make the membrane flexible and self-sealing
What are two types of proteins in the cell membrane?
protein channels and carrier proteins
What are the six functions of the proteins in the membrane?
- 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
- help cells adhere together
- act as receptors, eg for hormones
What are two features of cholesterol and what are the three functions of cholesterol?
they provide strength and are very hydrophobic
- reduce lateral movement of other molecules including phospholipids
- make the membrane less fluid at higher temperatures
- prevent leakage of water and dissolved ions from the cell
What are three functions of glycolipids?
- act as recognition sites
- help maintain the stability of the membrane
- help cells to attach to one another and so form tissues
What are three functions of glycoproteins?
- act as recognition sites
- help cells to attach to one another and so form tissues
- allows cells to recognise one another, for example, lymphocytes can recognise an organism’s own cells
What are four reasons for most molecules not being able to freely diffuse across the cell-surface membrane?
- not soluble in lipids and therefore cannot pass through the phospholipid layer
- too large to pass through the channels in the membrane
- of the same charge on the protein channels and so, even if they are small enough to pass through, they are repelled
- electrically charged (in other words are polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
Explain why the fluid-mosaic model of the cell-surface membrane is called that?
fluid- individual phospholiids can move relative to each other - gives membrane a flexible structure that is constantly changing
mosaic- the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern
What is the definition of 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.
What is facilitated diffusion?
When charged and polar molecules cannot easily diffuse through membranes, it is made easier by transmembrane channels and carriers that span the membrane.
It is a passive process.
What are the two types of proteins that work in facilitated diffusion?
Protein channels - allow specific water-soluble ions to pass through; they are selective and open in the presence of a specific ion.
Carrier proteins - when a molecule such as glucose that is specific to the protein is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane.
What is the definition of osmosis?
The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.
What is a solute?
It is any substance that is dissolved in a solvent.
Under standard conditions of 25 degrees and 100kPa, what water potential is water said to have?
zero
What happens if the w.p of external solution is higher (less negative) than the cell solution?
Water moves in
Cell swells and bursts
What happens if the w.p of external solution is equal to the cell solution?
Water neither moves in or out
No change in the cell
What happens if the w.p of external solution is lower (more negative) than the cell solution?
Water leaves the cell
The cell shrinks
definition of active transport?
the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
what is ATP used for in active transport?
directly move molecules
for co-transport
what are four ways in which it differs from passive forms of transport?
- metabolic energy in form of ATP needed
- substances moved against conc. gradient, from low to high
- carrier protein molecules act as pumps
- process is very selective, with specific substances transported
explain active transport through the sodium-potassium pump
outside cell- molecules bind to carrier protein and ATP attaches to the membrane protein on the inside of the cell
binding of phosphate ion to protein causes the protein to change shape so that access for the molecules is open to the inside of the membrane but closed to the outside
what are two mechanisms to increase transport across membranes?
epithelial cells lining the ileum have microvilli - provides a larger surface area
increase number of protein channels and carrier proteins in any given area of membrane
what is the role of diffusion in absorption?
a concentration gradient is maintained and the rate of movement by f.d across epithelial cell-surface membranes is increased
what is the role of active transport in absorption?
allows for co-transport to occur
explain co-transport in the sodium-potassium pump (Na+ and G)
Na+ a.t out of epithelial cells, occurs in carrier molecules
higher conc. of Na+ in lumen of intestine than inside epithelial cells
Na+ diffuse into epithelial cells down conc. gradient, glucose (or amino acids) come along with them
Glucose/a.a into blood plasma through f.d
SODIUM ION CONC. GRADIENT, NOT ATP, THAT POWERS MOVEMENT OF G/A.A INTO CELLS
what is endocytosis?
the inward transport of large molecules through the cell-surface membrane
what is exocytosis?
the outward bulk transport of materials through the cell-surface membrane