Section 2 - Chapter 4 : Transport Across Cell Membranes

Question 1

What is the function of the cell membrane (plasma membrane)

Answer 1

• Surrounds cells and forms the boundary between the cell cytoplasm and the environment
• It allows different conditions to be established inside and outside the cell
• Controls the movement of substances in and out of the cell

Question 2

What is the structure of the Plasma Membrane

Answer 2

• Phospholipids
• Proteins
• Cholestrol
• Glycolipids
• Glycoproteins

Question 3

How are phospholipids structured in the cell-surface membrane

Answer 3

• Phsopholipids form a bilayer
• They are important components of the cell-membrane for:
  
  • The hydrophilic heads of the phospholipid layers point to the outside of the cell and attracted to water
  • The hydrophobic tails point to the centre of the cell membrane, repelled by water on both sides

Question 4

What is the function of the phospholipids in the membrane

Answer 4

• Allow lipid-soluble substances to enter and leave the cell
• Prevent water-soluble substances entering and leaving the cell
• Make the membrane flexible and self-sealing

Question 5

What are the 2 ways proteins are embedded in the phospholipid bilayer

Answer 5

Proteins are distributed throughout the cell membrane

• Some proteins occur on the surface of the bilayer and never extend completely across it
  
  • They act to give mechanical support to the membrane or in conjunction with glycolipids, cell receptors for molecules like hormones
• Other proteins completely span the phospholipid bilayer from 1 side to the other.
  
  • Some are protein channels - form water filled tubes to allow water soluble ions to diffuse across
  • Others are carrier proteins- bind to ions like glucose and amino acids, then change shape in order for the molecules to move across

Question 6

What are the functions of the proteins in the membrane

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

Question 7

What is cholestrol in the Membrane

Answer 7

• Cholestrol molecules occur within the phospholipid bilayer.
• Add strength to the membrane
• Chlolestrol is very hydrophobic - help prevent water loss and dissolved ions from the cell
• Also pulls together the fatty acid tails - limiting lateral movement of other molecules without being rigid
• Makes the membrane less fluid at higher temperatures

Question 8

What is the Glycolipids function and formation in the cell membrane

Answer 8

• Made of carbohydrates covalently bonded with lipids (Carbohydrate attatched to phospholipid)
• The functions are to:
  
  • Act as recognition sites
  • Help maintain stability of membrane
  • Help cells attatch to 1 another to form tissues

Question 9

What are the Glycoproteins functon and their formation

Answer 9

• Carbohydrate chains are attached to extrinsic proteins
• Glycoproteins also act as cell-surface receptors - more specifically for hormones and neurotransmitters
  
  • Act as recognition sites
  • Helps cells attach to form tissues
  • Allows cells to recognise 1 another

Question 10

Why do most molecules not freely diffuse across the membrane

Answer 10

• Not lipid soluble - cant pass phospholipid bilayer
• Too large to pass through channels in the membrane
• Of the same charge as the charge on the protein channels - are repelled
• Electrically charged (polar) and have difficulty passing through non-polar hydrophobic tails

Question 11

Why is the cell-surface membrane described as the fluid mosaic model

Answer 11

• Fluid : Because individual phospholipid molecules can move. Has many molecules that float along the lipids.This gives the membrane a flexible structure and constantly changing shape
• Mosaic : Because of proteins embedded in the phospholipid bilayer vary in shape,size,pattern

Question 12

What does ‘passive mean’ in terms of diffusion

Answer 12

• That the energy comes from the natural inbuilt motion of particles rather than from some external source such as ATP

Question 13

What are the key points that help when understanding diffusion

Answer 13

• All particles are constantly in motion due to the kinetic energy that they possess
• This motion is random, with no set pattern to the way the particles move around
• Particles are constantly bouncing off 1 another as well as other objects

Question 14

What is diffusion

Answer 14

• The net movement of molecules or ions from a region where they are more highly concentrated to an where their concentration is lower until evenly distributed

Question 15

What is facilitated diffusion

Answer 15

• Charged ions and polar molecules do not diffuse across the membrane
• Facilitated diffusion is a passive process - it only relies on the inbuilt motion (kinetic energy) of the diffusing molecules
• Occurs down a concentration gradient but differs as it occurs along specific points where there are transport proteins (channels and carriers)

Question 16

What are Protein Channels

Answer 16

• These proteins form water filled hydrophilic channels across the membrane.
• They allow specific water-soluble ions to pass to pass through
• The channels are selective - opening only to a specific ion (remains closed if not present) - control of entry and exit of ions
• The ions bind to the protein causing it to change shape. Closes on 1 side and opens on the other

Question 17

What are Carrier Proteins

Answer 17

• Span the plasma membrane
• When a molecule such as glucose that is specific to the protein is present, it binds with the protein
• Causes it to change shape so the molecule is released to the inside of the membrane

Question 18

What is Osmosis

Answer 18

• The passage of water from a region where it is higher water potential to a region where it has lower water potential through a selectively permeable membrane
• Permeable to water molecules and small molecules but not to larger molecules

Question 19

Give 2 similarities between diffusion and osmosis

Answer 19

• Movement down a gradient from high concentration to low concentration
• Passive/not active process

Question 20

What is a solute

What is a solvent

What is a solution

Answer 20

• A solute - is a substance dissolved in a solvent
• A solvent - a solution (water) that dissolves a solute
• A solution - a mixture of solute and solvent

Question 21

What is Water Potential

Answer 21

• Measured in kilopascals - represented by psi
• Pure water has a water potential of 0
• If you add a solute the water potential will lower - will be more negative
• Water Potential is the tendency of water molecules to move freely

Question 22

Use the example of red blood cells and what happens inside the cell

Answer 22

• Water potential of external solution is higher than the cell. Water enters the cell. It swells and bursts.
• Water potential of external solution is equal to cell. No change
• Water potential of external solution is lower than the cell. Water leaves cells. Shrinks

Question 23

What is the surrounding solution around is …..

1. higher (less negative)
2. is equal
3. lower (more negative)

Answer 23

1. Enters the cell, swells and bursts
2. Neither enters nor leaves (its in equilibrium)
3. Leaves the (shrinks)

Question 24

What happens if you add a solute to water

Answer 24

• Lowers water potential
• This is because water molecules are attracted to the solute- less free to move around (create a shell around solute)
• Water potential becomes more negative

Question 25

What is the equation for water potential

Answer 25

• Water Potential = Solute Potential + Pressure Potential
• Result of the combined effect of solute concentration and pressure
• Pressure increases the tendency of water molecules to move - it exerts a push through a membrane

Question 26

What is Active Transport

Answer 26

• 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 27

In Active Transport what is ATP used to do

Answer 27

• Directly move molecules
• Individually move molecules using a concentration which has been set up by active transport - known as co-transport

Question 28

How does Active Transport differ from passive forms of transport

Answer 28

• Metabolic energy in the form of ATP is required
• Moved against a concentration gradient low-high
• Carrier Proteins that act as ‘pumps’ are involved
• This process is very selective, with specific substances being transported

Question 29

How can Direct Transport of a single ion moved

Answer 29

• The molecule or ion binds to receptor sites on the carrier protein
• On the inside of the cell/organelle ATP binds to the protein causing it to split into ADP and a P (phosphate molecule)
  
  • This causes the protein to change shape and opens to the opposite side of the membrane
  • The molecule or ion is released to the otehr side
• The P is relased from the protein - it reverts to its original shape ready to be repeated
• The P molecule combines with the ADP to form ATP during respiration

Question 30

What are the differences and similarities between active transport and diffusion

Answer 30

• Both use carrier proteins
• Facilitated diffusion occurs down a concentration gradient while Active Transport occurs against a concentartion gradient
• Facilitated diffusion doesnt require metabolic activity (ATP). Active Transport metabolic energy is provided in the format of ATP

Question 31

What is the Sodium-Potassium pump and why is it used

Answer 31

• Sometimes more than 1 molecule or ion may be moved in the same direction or opposite direction at the same time and transport protein by active transport
• Sodium Potassium Pump - sodium ions are actively removed from the cell while the potassium is taken in from the surroundings

Question 32

How is the ileum adapted for gas exchange

Answer 32

• Large Surface area (microvilli and villi)
  
  • Higher rate of diffusion
• Carrier proteins (facilitated diffusion and active Transport)
• Rich Blood Supply (constantly moving) - ensures a concentration gradient
• Short pathway/thin
• Many mitochondrion

Question 33

How does co-transport of glucose or amino acids work

Answer 33

• Sodium ions are actively transported out of the epithelial cells, by the sodium potassium pump, into the blood
• This maintains a high concentration of sodium ions in the lumen of the intestine than inside epithelial cells
• Sodium ions diffuse into the epithelial cells down the concentration gradient through a different carrier protein (co-transport protein)
• As the sodium ions are diffusing they carry (G or AA) into the cell with them
• The G or AA pass into the blood by facilitated diffusion using another tyoe of carrier

Question 34

What is the function of cholestrol in the membrane

Answer 34

• Reduce lateral movement of other molecules including phospholipids
• Make the membrane less fluid at high temperatures
• Prevent leakage of water and dissolved ions from the cell.