Transport across membranes

Question 1

Cell-surface membrane

Answer 1

plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and environment
* contols the movement of substances in and out of cell
* 7-10 nm
* arranged in fluid mosaic
composed of:
1. Phospholipid bilayer
2. Proteins
3. Cholesterol
4. Glycolipids
5. Glycoproteins

Question 2

Phospholipid bilayer

Answer 2

stucture of the cell membrane
* Hydrophillic heads- point to outside and attract water
* Hydrophobic tails- point to centre of membrane, repel water
* allows lipid-soluble substances to enter + leave cell
* prevents water soluble substances from enterinng + leaving
* makes membrane flexible

Question 3

Fluid mosaic model

Answer 3

Fluid- phospholid molecules are fluids and can moved around eachother, makes membrane flexible
Mosaic- pattern produced by scattered protein molecules embedded in the bilayer that vary in shape, size and pattern

Question 4

Proteins

Answer 4

embedded into the phospholipid bilayer
* can span across the bilayer from one
* channel proteins- form water-filled tubes to allow water-soluble ions to diffuse across the membrane
* carrier proteins- bind to ions or molecules like glucose, then change shape in order to move these molecules acroos the membrane (for active transport)
* provide structural support
* form cell-surface receptors e.g. for hormones

Question 5

Cholesterol

Answer 5

hydrophobic molecules that occur within the phospholipid bilayer, regulate fluidity
* adds strength to membranes
* prevents water loss and dissolved ions
* reduces lateral movement of other molecules
* makes membrane less fluid at high temperatures

Question 6

Glycolipids

Answer 6

carbohydrate covalently bonded with a lipid
* carbohydrate extends from the phospholipid bilayer into the watery environment outside
* acts as recognition sites
* helps maintain stability of membrane
* helps cell attach to one another to form tissues

Question 7

Glycoproteins

Answer 7

carbohydrate chains attached to many extrinsic proteins on the outer surface of membrane
* act as cell surface receptors/ recognition sites
* helps cells attach to one another– tissues
* allows cells to recognise one another

Question 8

Extrinsic proteins

Question 9

Intrinsic proteins

Question 10

What molucules cant pass through the phospholipid bilayer?

Answer 10

1. molecules not soluble in lipids- cannot pass through the phospholipid bilayer
2. molecules too large- cannot pass through channels in the membrane
3. molcules of the same charge as channel proteins- are repelled
4. electrically charged (polar) molecules- have difficulty passing through the non-polar hydrophobic tails

Question 11

Simple diffusion

Answer 11

net movement of molecules/ions from a region high concentration to low concentration, down the concentration gradient
* passive process
* only small (fit through spaces between bilayer) or non-polar (have no charge + soluble) can diffuse across membrane e.g carbon dioxide and oxygen

Question 12

Facilitated diffusion

Answer 12

• Charged ions (e.g sodium ions) and larger molecules (e.g glucose) cannot pass through the phospholipid bilayer due to the hydrophobic fatty-acid tails in phospholipid membrane
• movement of these molecules is facillitated by transmembrane channels + proteins that span membrane
• passive process

Question 13

Channel proteins

Answer 13

water-filled hydophilic pores that form channels across the membrane to allow charged ions through membrane
* allow specific water-soluble ions through
* selctive, only allow specific ions through
* opens in presence of specific ion, or remains closed
*

Question 14

Carrier proteins

Answer 14

proteins that move large molecules across membranes
* specific molecules bind to protein and fits through
* Binding causes it to change shape to allow molecule to inside of membrane

Question 15

Factors affecting diffusion

Answer 15

1. Concentration gradient- the steeper the concentration gradient, the greater the rate of diffusion
2. Temperature- the higher the temperature the higher the kinetic energy for molecules to move around, increases rate of diffusion
3. Surface area- the grater the surface area, more molecules/ions can cross it at any moment
4. Nature of molecules/ions- large, polar molecules diffuse slowly

Question 16

Osmosis

Answer 16

movement of water molecules from an area of high water potential to an area of low water potential, across a partially permemable membrane

Question 17

Solute

Answer 17

any substance dissolved in a solvent e.g water
* solvent and solute form a solution

Question 18

Water potential

Answer 18

pressure created by water molecules; tendancy of water molecules in a system to move
* measured in kiloPascals (kPa)
* highest value = 0
* Pure water has a water potential of 0
* addition of a solute to pure water lowers its water potential (more concentrated)
* the more negative the water potential, the more solute dissolved in it

Question 19

Factors affecting osmosis

Answer 19

1. Water potential gradient- difference between the water potential of the solute and solvent. The higher the water potential gradient, the faster the
2. Surface area-
3. Thickness of exchange surface

Question 20

Osmosis in animal cells

Answer 20

if an animal cell is placed in a solution with a lower water potential, water will move out by osmosis- cell will shrink and wrinkle
e.g red blood cells
* red blood cells placed in pure water will absorb water because cell has a lower water potential
* animal cell normally live in a liquid which have the same water potential as cells e.g blood plasma

Question 21

Osmosis in plant cells

Answer 21

1. when plant is placed in pure water
   * water moves into plant by osmosis because of the plants lower/negative water potential
   * protoplasts swell and press on the cell wall
   * cell wall offers limited expansion and pressure builds so osmosis stops
   * cell is said to be turgid
2. when plant cell is placed in solution with a lower water potential
   * water leaves by osmosis
   * volume of cell decreases

Question 22

Protoplasts

Answer 22

everything inside a plant cell
* e.g. outer cell surface membrane, nucleus, cytoplasm, and inner vacuole membrane

Question 23

Incipient plasmolysis

Answer 23

the point at which the protoplast is just about to pull away from the cell wall and no longer presses on it

Question 24

Plasmolysis

Answer 24

water loss through osmosis causes cell contents to shrink and the protoplasts pull away from the cell wall

Question 25

Active transport

Answer 25

movement of molecules/ions in or out of a cell from a region of low concentration, to high concentration using carrier proteins and ATP
* carrier proteins are pumps
* carrier proteins have a specific tertiary structure

Question 26

What is ATP used for in active transport?

Answer 26

• provides metabollic energy for active transport to occur
• directly moves molecules
• individually move molecules using a concentration gradient already set up by active transport (co-transport)

Question 27

Decribe the process of active transport (single molcule)

Answer 27

1. Carrier proteins span membrane and bind to molecule/ion to be transported
2. Molecule binds to receptor site on carrier proteins
3. On the inside membrane, ATP binds to the protein and is hydrolysed into ADP and Pi (phosphate molecule)
4. Causes the protein molecule to change shape and open towards the inside of the membran
5. Molecule/ion is released into the other side of membrane
6. Pi molcule is released from protein, revering protien back to its original shape
   (Pi molecule recombines with ADP to form ATP)

Question 28

Sodium potassium pumps

Answer 28

sodium ions are actively removed from cell, while potassium ions are actively taken in from the surroundings
* Molecules bind to carrier proteim and ATP attaches to membrane protein on inside of cell
* Binding of Pi to protein causes protein to change shape so that access for molecules is open to inside of membrane but closed to outside

Question 29

Co-transport

Answer 29

allows 2 different substances to be transported, using a carrier protein
* molecule is transported against a concentration gradient when it’s driven with an ion which can diffuse down its concentration gradient, through a co-transport protein
* indirect form of active transport

Question 30

Co-transport of glucose and sodium ions in the iluem

Answer 30

To absorb glucose from lumen to gut, there must be higher concentration of glucose in lumen, then epithelial cell, but there is usually more glucose in epithelial cells so co-transport is needed:
1. Sodium ions are actively transported out of the epithelial into blood (ATP hydrolyses) by a carrier protein
2. Reduces sodium ion concentration in epithelial cell
3. Sodium ions can diffuse from the lumen down their concentration gradient into epithelial cell
4. Protein that sodium ions diffuse through is a co-transport protein, so glucose/amino acid attach and are transported into the epithelial cell, against the concentration gradient
5. Glucose moves by facilitated diffusion from the epithilial cell to blood

Question 31

How is the rate of movement increased in epithelial cells

Answer 31

1. Blood flow- maintains the concentration gradient of glucose between the epithelial cell and capillary
2. Microvilli- on epithelial cell increase the surface area for co-transport proteins
3. Proteins- increasing the number channel proteins and carrier proteins

Question 32

Bulk transport