4.Transport across cell membranes

Question 1

What are all membranes around and within all cells called

Answer 1

Plasma membranes

Question 2

What is the cell-surface membrane

Answer 2

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

Question 3

Why do phospholipids form a bi-layer

Answer 3

• 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

Question 4

What is the function of the phospholipids in the membrane

Answer 4

• allow lipid soluble substances to leave and enter the cell

- prevent water-soluble substances to leave and enter the cell

Question 5

Name two ways proteins are found on the bi-layer

Answer 5

• 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.

Question 6

What are the functions of the proteins embedded in the bi-layer

Answer 6

• 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)

Question 7

Where are cholesterol molecules found

Answer 7

Within the phospholipid bilayer of the cell surface membrane.

Question 8

What are the function of cholesterol molecules

Answer 8

• 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

Question 9

What are the function of glycolipids on cell-surface membranes

Answer 9

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

Question 10

What are the functions of glycoproteins on the cell-surface membrane

Answer 10

• act as recognition sites
• help cells attach to one and another and so form tissues
• allows cells to attach to one another (eg lymphocytes )

Question 11

Why can most molecules not freely diffuse across the cell-surface membrane

Answer 11

• 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

Question 12

What model is the cell surface membrane described as

Answer 12

Fluid-mosaic model

Question 13

Explain why the cell-surface membrane is described as fluid-mosaic

Answer 13

• 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

Question 14

Name all the ways movement across a membrane can occur by

Answer 14

• simple diffusion
• facilitated diffusion
• osmosis
• active transport
• co-transport

Question 15

Define diffusion

Answer 15

The net movement of molecules or ions from a high concentration to a low concentration

Question 16

What are some molecules that can pass through the bilayer via passive diffusion

Answer 16

Small non-polar molecules such as O2 and CO2

Question 17

What is passive diffusion

Answer 17

Diffusion that occurs due to the kinetic energy of the constantly moving molecules and does not require ATP or proteins

Question 18

What is facilitated diffusion

Answer 18

Passive diffusion of molecules through either protein channels or carrier proteins

Question 19

What are protein channels

Answer 19

• 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.

Question 20

How do protein channels work

Answer 20

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

Question 21

How to carrier proteins work

Answer 21

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.

Question 22

Define osmosis

Answer 22

The movement of water molecules from a region of high to low water potential through a selectively permeable membrane.

Question 23

Explain osmosis

Answer 23

• 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

Question 24

What will happen if a cell is placed in a higher water potential relative to inside the cell

Answer 24

Water will move inside the cell down the concentration gradient causing the cell to burst

Question 25

What will happen if a cell is placed in a lower water potential relative to inside the cell

Answer 25

Water will move inside the cell down the concentration gradient causing the cell to shrivel

Question 26

What is the water potential of blood plasma

Answer 26

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

Question 27

Define active transport

Answer 27

The movement of molecules or ions from an area of lower to higher concentration using ATP and carrier proteins

Question 28

What is ATP used for in active transport

Answer 28

• directly move molecules

- indirectly move molecules by using ATP to create a conc. gradient for molecules to passively diffuse down

Question 29

Name the differences in active transport and passive diffusion

Answer 29

• ATP use
• up or down concentration gradient
• carrier proteins

Question 30

Name two ways to increase the rate of movement across membranes

Answer 30

• increased surface area (eg through micro villi)

- increased amount of protein channels per unit area

Question 31

Describe the direct active transports of a single molecule

Answer 31

• 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

Question 32

Describe two ways rate of movement across membranes can be increased

Answer 32

• increase surface area of membrane (eg. microvilli do this in the gut).
• increase density of carrier proteins and protein channels

Question 33

Why is active transport important in digestion

Answer 33

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.

Question 34

Name the mechanism by which glucose and amino acid molecules are absorbed from the small intestine

Answer 34

Co-transport

Question 35

Describe the steps involved in co-transport

Answer 35

• 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

Question 36

Describe the steps involved in co-transport

Answer 36

• 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

Question 37

What is co-transport an example of

Answer 37

Indirect active transport