Chapter 4 Transport Across Cell Membrane

Question 1

Why are membranes described as a fluid mosaic model

Answer 1

Fluid: as the individual phospholipid molecules can move relative to one another giving the membrane a flexible structure that is constantly changing shape
Mosaic: the proteins that are embedded in the phospholipid bilayer vary in shape, size, pattern

Question 2

How is the cell membrane created

Answer 2

All molecules arranged within the phospholipid bilayer create the partially permeable membrane that is the cell surface membrane

Question 3

Components of membrane / components in phospholipid bilayer

Answer 3

Components of the membrane

• phospholipid bilayer
• cholesterol
• peripheral proteins
• integral proteins (protein channels and protein carriers)

Found in phospholipid bilayer:

• cholesterol
• peripheral proteins
• integral proteins (protein channels and protein carriers)

Question 4

How does the phospholipid bilayer contribute to the cell membrane

Answer 4

• It aligns as a bilayer (2 layers) due to the hydrophilic heads being attracted to water and the hydrophobic tails being repelled by water , so hydrophilic head will face outside of the cell and the tail will face inside of the cell
• allows lipid soluble substances to enter and leave the cell
• prevents water soluble substances entering and leaving the cell
• makes the membrane flexible and self sealing

Question 5

Why are the phospholipid heads hydrophilic and the tails hydrophobic

Answer 5

• hydrophilic head - the head contains a phosphate group which has a negative charge and a glycerol molecule causing the head to have an overall negative charge and so is attracted to water which makes it hydrophilic
• hydrophobic- as the tails contain fatty acid chains so do not have a charge so will repel water

Question 6

How does cholesterol contribute to the cell membrane

Answer 6

• it restricts the lateral movement of other molecules in the membrane
• this makes it useful as it makes the membrane less fluid at high temperatures which is important as if membrane was too fluid then the gaps between phospholipids will become too large and water and dissolved ions will leak out of the cell, causing the cell to shrivel or burst

Question 7

How does peripheral proteins contribute to the cell membrane and where are they found

Answer 7

• they are found in the surface of the bilayer and never extend across it
• By either providing mechanical support or are connected to proteins or lipids to make glycoproteins and glycolipids which provides the function of cell recognition

Question 8

How does integral proteins contribute to the cell membrane and where are they found

Answer 8

• they completely spam the bilayer from one side to the other
• They are protein carriers or channel proteins which is involved in the transport of molecule across the membrane

Question 9

What do protein channels do

Answer 9

They form tubes that fill with water to enable polar molecules to diffuse into the cell

Question 10

What do proteins carriers do

Answer 10

• they bind to ions causing the carrier protein to change shape and transport the molecule to the other side

Question 11

Why is the cell membrane partially permeable

Answer 11

Due to the components in the phospholipid bilayer only lipid soluble substances and very small molecules can diffuse into the cell

Question 12

What molecules cannot pass through the membrane through diffusion

Answer 12

• water soluble (polar) substances
• large molecules
• ions

Question 13

Function of glycoproteins

Answer 13

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

Question 14

Structure of glycolipids

Answer 14

They are made up of a carbohydrate covalently bonded with a lipid, the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as the cell surface receptor for specific chemicals

Question 15

Function of glycolipids

Answer 15

• act as cell recognition sites
• helps cells attach to one another to form tissues
• allows cells to recognise one another

Question 16

Key types of transport

Answer 16

• simple diffusion
• facilitated diffusion
• active transport
• osmosis

Question 17

What is simple diffusion

Answer 17

The net movement of molecules or ions from a region of higher concentration to a region of lower concentration until equilibrium is reached

Question 18

Key points of simple diffusion

Answer 18

• for the molecules to move they have kinetic energy which enables them to constantly be in motion
• ## the molecules are lipid soluble and small which enables them to diffuse across the membrane

Question 19

How does facilitated diffusion differ from simple diffusion

Answer 19

• as proteins are used to transport molecules
• this means that it can transport ions and polar molecules which cannot simply diffuse across the membrane using protein channels and carrier proteins

Question 20

How are protein channels involved in facilitated diffusion

Answer 20

• they are filled with water it enables polar molecules to pass through the membrane which cannot pass through the phospholipid bilayer
• gated protein channels are only open in the presence of certain ions when they bind to a certain protein
• continuous protein channels allow ions to transport continuously into the cell through facilitated diffusion as they are always open

Question 21

What is osmosis

Answer 21

The movement of water from an area of higher water potential to an area of a lower water potential across a partially permeable membrane

Question 22

What is water potential

Answer 22

The pressure created by water molecules and is measured in kPa

Question 23

Hypertonic hypotonic and isotonic

Answer 23

Hypotonic - when the solute concentration outside the cell is lower than the solute concentration inside the cell
Hypertonic- when the solute concentration outside the cell is higher than the solute concentration inside the cell
Isotonic- when there is no net movement of water molecules across the membrane.

Question 24

How to tell the concentration of solutes using water potential

Answer 24

The more solutes dissolved in water the more negative the water potential
Pure water has the water potential of 0 kPa

Question 25

What would happen if an animal cell was placed in a hypotonic, and , hypertonic solution

Answer 25

Hypotonic- water will move in the cell by osmosis and as animal don’t have a cell wall the pressure will cause the cell to burst
Hypertonic - animal cells will shrivel due to large volumes of water leaving the cell by osmosis

Question 26

What is active transport

Answer 26

The movement of molecules and ions from an area of lower concentration to an area of higher concentration, against the concentration gradient using ATP and carrier proteins

Question 27

Role of carrier proteins in active transport

Answer 27

They act as a pump to move substances across the membrane which is very selective as only certain molecules can bind to carrier proteins to be pumped

Question 28

How is ATP and carrier proteins used in active transport (process of active transport)

Answer 28

1) certain molecules that are complementary to the receptor site can bind to the receptor site on carrier proteins
2) ATP will bind to the carrier protein on the inside of the membrane and is hydrolysed into ADP and Pi
3) This causes the protein to change shape and open towards the inside of the membrane
4) this causes the molecule to be released to the other side of the membrane
5) The Pi molecule (inorganic phosphate) is then released from the protein and results the protein reverting to its original shape

Question 29

Type of active transport

Answer 29

• co transport

Question 30

What happens in the co transport of glucose and sodium ions in the ileum

Answer 30

1) the sodium ions are actively transported out of the epithelial cell (layer of cells that cover the wall of the ileum) against their conc gradient by the sodium potassium pump into the blood, causing the potassium ions to move into the epithelial cell, maintaining the electrochemical gradient, through active transport
2) this reduces the sodium ion concentration in the epithelial cell
3) the sodium ions from the lumen diffusion down their concentration gradient into the epithelial cell by a co transporter protein through active transport
4)the co transporter protein also enables the glucose or amino acids to attach to their complementary receptor site on the protein at the same time and be transported into the epithelial cell against their conc gradient
5) the glucose/amino acids pass into the blood plasma by facilliated diffusion

Question 31

Why is the conc of glucose in the blood lower than in epithelial cells

Answer 31

As the blood is continuously flowing so carries away the absorbed glucose to cells that need it for respiration which maintains a conc gradient between ileum and blood

Question 32

how is the rate of movement increased across membranes in the co transport of glucose and sodium ions

Answer 32

• the epithelial cells on the ileum contain microvilli which increase the surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport take place, so there will be an increase in carrier proteins increases the rate of movement
• As the blood is continuously flowing so carries away the absorbed glucose to cells that need it for respiration which maintains a conc gradient between ileum and blood so the rate of movement by facilitated diffusion is increased

Question 33

what is ATP used for in active transport

Answer 33

• individually move molecules using a conc gradient that has been set up by active transport, which is known as co transport

Question 34

how does active transport differ from passive transport

Answer 34

• ATP is required
• substances are moved against the conc gradient
• carrier proteins are involved
• the process is very selective

Question 35

what is passive transport and an example

Answer 35

• the movement of molecules where the energy comes from the natural kinetic energy of the particles rather from an external source
  examples: diffusion (S or F) and osmosis

Question 36

what happens when plant cells are placed in an
hypertonic solution

Answer 36

cell contents will shrink and the protoplast (cell membrane, nucleus and cytoplasm) will pull away from the cell causing the cell to be plasmolysed

Question 37

what happens when plant cells are placed in an
hypotonic solution

Answer 37

water moving in the plant causes the protoplast (cell membrane, nucleus and cytoplasm) to swell and press on the cell wall and as the cell wall is only capable of limited expansion a pressure builds up that prevents the entry of more water causing the cell to be turgid

Question 38

what happens when plant cells are placed in an
isotonic solution

Answer 38

the volume of the cell will decrease and the protoplast (cell membrane, nucleus and cytoplasm) will no longer touch the cell wall causing the cell to reach incident plasmolysis

Question 39

How are carrier proteins used in facilitated diffusion

Answer 39

Carrier proteins allow small polar molecules to be transported across the membrane by facilitated diffusion.
Once the polar substance is attached to the carrier protein it causes the protein to change shape and transport the molecule across the membrane