Transport across cell membranes

Question 1

What is the function of the cell surface membrane?

Answer 1

• allows different conditions to be managed inside and outside
• controls movement of substances

Question 2

What are the phospholipids function within a phospholipid bilayer?

Answer 2

• allow lipid soluble sustances to enter and exit
• prevent water soluble substances from entering and exiting
• make the membrane flexible and self sealing

Question 3

What does the phospholipid bilayer look like?

Answer 3

Hydrophilic heads point to the outside of the surface membrane - attracted by water.
Hydrophobic tails point into the centre of the membrane - repelled by water.

Question 4

What are the function of the proteins embedded into the phospholipid surface?

Answer 4

To give mechanical support to the membrane or in conjunction with glycoproteins - as cell receptors for molecules e.g. hormones.

Question 5

What are the function of the proteins that span across the phospholipid bilayer, and what are the 2 names?

Answer 5

Channel proteins - form water filled tubes to allow water soluble ions to diffuse across membrane.
Carrier proteins - bind to ions or molecules like glucose/ amino acids, and change shape to move them across membrane.

Question 6

What are the functions of the proteins in the membrane?

Answer 6

• to provide structural support
• to act as channels to transport water soluble substances across
• allow active transport through carrier proteins
• help cells adhere together
• act as receptors - e.g. for hormones

Question 7

What are the main functions of cholesterol?

Answer 7

• reduce movement of other molecules e.g. phospholipids
• make the membrane less fluid at high temps.
• prevent water loss and leakage of dissolved ions.

Question 8

What are the main functions of glycolipids?

Answer 8

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

Question 9

What are the main functions of glycoproteins and where does it bind to?

Answer 9

Binds to the extrinsic proteins on the outer surface of cell membrane.

• act as recognition sites
• helps cells attach to one another and form tissues
• allows cells to recognise each other.

Question 10

What are the functions of membranes within cells?

Answer 10

• control entry/ exit of materials
• separate organelles from cytoplasm
• provide an internal transoirt system
• isolate enzymes that can cause damage
• provide surfaces for reactions to occur

Question 11

What are the reasons a lot of molecules can’t diffuse across the cell-surface membrane?

Answer 11

• not soluble in lipids
• too large to pass through the channels
• of the same charge of the channel proteins so are repelled
• polar (charged), can’t pass through the non-polar hydrophobic tails

Question 12

What the meaning behind the words “fluid” and “mosaic” model?

Answer 12

fluid - phospholipids can move
mosaic - proteins vary in size, shape and pattern like mosaic tiles.

Question 13

What is the defintion of diffusion?

Answer 13

The net movement of molecules or ions from an area of high concentration to an area of low concentration.

Question 14

What’s the definition of facilitated diffusion?

Answer 14

A passive process that moves molecules or ions across a cell membrane with the help of specialised proteins (carrier and channel proteins)

Question 15

How do channel proteins work?

Answer 15

• hydrophilic water filled channels across membrane
• allow water soluble ions across the membrane

Question 16

How do carrier proteins work?

Answer 16

• when a molecule specific to the protein is present, it binds with the protein.
• changes shape and releases it to the other side

Question 17

What is the definition of osmosis?

Answer 17

The movement of water from a higher water potential to a lower water potential through a partially permeable membrane

Question 18

Describe the brief process of osmosis.

Answer 18

• solution on left has lower concentration of solute than the right.
• water and solute molecules are in random motion - kinetic energy
• partially permeable membrane only allows water molecules through
• water molecules diffuse from left (higher w.p.) to right (lower w.p.) down a w.p. gradient
• when the no. of water molecules on either side are equal, equilibrium reached - no NET movement of water

Question 19

What is the definition of active transport?

Answer 19

The movement of molecules or ions into or out of a cell from a lower concentration to a higher concentration using ATP and carrier proteins.

Question 20

What is ATP used for in active transport?

Answer 20

• to directly move molecules
• individually move molecules using a concentration gradient from active transport ( co-transport)

Question 21

How is active transport different from passive forms?

Answer 21

• energy (ATP) is needed
• against a concentration gradient from low to high
• carrier proteins act as ‘pumps’
• the process is very selective (specific substances)

Question 22

Outline the general process of active transport of a single molecule.

Answer 22

• carrier protein span the membrane and bind to the molecule/ ion
• molecule/ ion binds to receptor site on carrier protein
• ATP binds to the protein causing it to split to ADP and a phosphate molecule
• the protein changes shape and opens the opposite side
• molecule/ ion is released to other side
• phosphate molecule released
• protein changes to original shape
• phosphate molecule recombines with ADP to form ATP during respiration

Question 23

What are the rate of diffusion factors?

Answer 23

• concentration gradient
• dissusion distance
• surface area
• number of proteins ( carrier/ channel) to diffuse through

Question 24

What is the defintion of an isotonic solution?

Answer 24

A solution which has the same water potential as the inside of the cells

Question 25

What is the defintion of an hypotonic solution?

Answer 25

Solution that has a higher water potential than the cell contents

Question 26

What is the defintion of an hypertonic solution?

Answer 26

A solution that has a lower water potential than the cell contents