Transport across cell membrane

Question 1

What are the three functions of phospholipids in the membrane?

Answer 1

• allow lipid-soluble substances to enter and leave
• prevent water-soluble substances entering and leaving the cell
• make the membrane flexible and self-sealing

Question 2

What are two types of proteins in the cell membrane?

Answer 2

protein channels and carrier proteins

Question 3

What are the six functions of the proteins in the membrane?

Answer 3

• 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

Question 4

What are two features of cholesterol and what are the three functions of cholesterol?

Answer 4

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

Question 5

What are three functions of glycolipids?

Answer 5

• act as recognition sites
• help maintain the stability of the membrane
• help cells to attach to one another and so form tissues

Question 6

What are three functions of glycoproteins?

Answer 6

• 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

Question 7

What are four reasons for most molecules not being able to freely diffuse across the cell-surface membrane?

Answer 7

• 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

Question 8

Explain why the fluid-mosaic model of the cell-surface membrane is called that?

Answer 8

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

Question 9

What is the definition of diffusion?

Answer 9

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.

Question 10

What is facilitated diffusion?

Answer 10

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.

Question 11

What are the two types of proteins that work in facilitated diffusion?

Answer 11

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.

Question 12

What is the definition of osmosis?

Answer 12

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.

Question 13

What is a solute?

Answer 13

It is any substance that is dissolved in a solvent.

Question 14

Under standard conditions of 25 degrees and 100kPa, what water potential is water said to have?

Answer 14

zero

Question 15

What happens if the w.p of external solution is higher (less negative) than the cell solution?

Answer 15

Water moves in

Cell swells and bursts

Question 16

What happens if the w.p of external solution is equal to the cell solution?

Answer 16

Water neither moves in or out

No change in the cell

Question 17

What happens if the w.p of external solution is lower (more negative) than the cell solution?

Answer 17

Water leaves the cell

The cell shrinks

Question 18

definition of active transport?

Answer 18

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

Question 19

what is ATP used for in active transport?

Answer 19

directly move molecules

for co-transport

Question 20

what are four ways in which it differs from passive forms of transport?

Answer 20

• 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

Question 21

explain active transport through the sodium-potassium pump

Answer 21

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

Question 22

what are two mechanisms to increase transport across membranes?

Answer 22

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

Question 23

what is the role of diffusion in absorption?

Answer 23

a concentration gradient is maintained and the rate of movement by f.d across epithelial cell-surface membranes is increased

Question 24

what is the role of active transport in absorption?

Answer 24

allows for co-transport to occur

Question 25

explain co-transport in the sodium-potassium pump (Na+ and G)

Answer 25

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

Question 26

what is endocytosis?

Answer 26

the inward transport of large molecules through the cell-surface membrane

Question 27

what is exocytosis?

Answer 27

the outward bulk transport of materials through the cell-surface membrane