Movement of substances

Question 1

Where are plasma membranes found?

Answer 1

• Around the nucleus (nuclear envelope), chloroplast, mitochondria and around the cell

Question 2

Describe the functions of the plasma membrane

Answer 2

• To form a boundary between the cellular structure and the environment
• To allow different conditions to be established inside and outside the cell
• Maintain and control the movement of substance between environments

Question 3

How are phospholipids arranged in the cell membrane?

Answer 3

• The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane attracted by the water on both sides.
• The hydrophobic tails of both phospholipid layers point into the center of the cell membrane, repelled by the water on both sides

Question 4

What is the cell membrane composed of?

Answer 4

• It is composed of a transport protein, phospholipid, cholesterol, glycoproteins and glycolipids

Question 5

What are the functions of phospholipids in the membrane?

Answer 5

• 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 6

Describe the fluid mosaic model of membranes

Answer 6

Fluid -> phospholipid bilayer in which individual phospholipids can move -> membrane has flexible shape.
Mosaic -> extrinsic + intrinsic proteins of different sizes and shapes are embedded

Question 7

What is the role of cholesterol in membranes?

Answer 7

• Cholesterol->
  Are very hydrophobic so play important role in preventing loss of water and dissolved ions from the cell.
  They also pull together the fatty acid tails of the phospholipids limiting their movement and that of other molecules without making the membrane too rigid.
  Steroid molecule in some plasma membranes.

Question 8

What is the role of proteins in the cell membrane ?

Answer 8

• Extrinsic (only extend halfway through or set on top of the membrane)
  Mechanical support to the membrane
  Act as cell receptors in conjunction with glycolipids for molecules such as hormones
  Bind cells together
  Involved in cell signalling
• Intrinsic (extend all the way through membrane )
  Protein channels which form water filled tubes to allow water-soluble ions to diffuse across membranes
  Others are carrier proteins that bind to ions or molecules like glucose and amino acid, then change shape in order to move these molecules across the membrane

Question 9

What is the role of glycolipids?

Answer 9

• Made up of carbohydrates covalently bonded with a lipid
• The carbohydrate portion extends outside the cell where it acts as a cell surface receptor for specific chemicals.
• Act as recognition sites
• help maintain the stability of the membrane
• help cells to attach to one another and so form tissues

Question 10

What is the role of glycoproteins?

Answer 10

• Carbohydrates chains are attached to many extrinsic proteins on the outer surface of the cell membrane
• These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.
• They act as recognition sites
• Help cells to attach to one another and so form tissues
• allows cells to recognise one another

Question 11

Why can’t most particles diffuse freely across the cell surface membrane?

Answer 11

• not soluble in lipids and therefore cannot pass through phospholipid layer
• too large to pass through the channels in the membranes
• of the same charge as the protei channels repelled
• polar and therefore have difficulty passing through the non-polar hydrophobic tails.

Question 12

What is diffusion?

Answer 12

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

Question 13

What is true for all passive processes?

Answer 13

• all particles are constantly in motion due to the kinetic energy that they possess.
• this motion is random
• particles are constantly colliding with each other and other objects

Question 14

What is facilitated diffusion?

Answer 14

Facilitated diffusion is the transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration with the help of a transport molecule

Question 15

What are protein channels?

Answer 15

• form water-filled hydrophilic channels across the membrane
• They are selective and only open in the presence of a specific ion.
• ions bind with the protein in a way that causes it to change shape.

Question 16

What are carrier proteins?

Answer 16

• unlike channel proteins with a fixed shape, they change shape.
• causes the binding site of the carrier protein to be open to one side of the membrane first and the. Open on the other side when it switches shape.

Question 17

What affects the rate of diffusion?

Answer 17

• Temperature -> more kinetic energy, more movement, higher rate of diffusion
• Concentration gradient -> the higher the difference in concentration between regions, the faster diffusion takes place
• Surface area -> the more area exposed for diffusion to happen on, the more likely it is particles will collide with it hence the higher rate of diffusion
• Distance -> the further a particle has to travel, the lower the rate of diffusion as this takes more time.

Question 18

What is osmosis?

Answer 18

the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.

Question 19

What is water potential?

Answer 19

• Water potential is the pressure created by water molecules
• It is measured in units of pressure usually kiloPascals (kPa)
• Pure water at 25 degrees celsius and 100kPa has a water potential of zero

Question 20

How could you find the water potential of cells of tissues?

Answer 20

• Place them in a series dilution and where there is no net gain or loss of water from the ells or tissues, the water potential inside the cells or tissues must be the same as that of the external solution.

Question 21

What will happen to red blood cells in solutions of different water potentials?

Answer 21

• If a red blood cell is placed in a solution with a higher water potential such as water, it will absorb water by osmosis because it has a lower water potential.
• Cell surface of the red blood cell is very thin and cannot stretch to that extent and will break, bursting the cell and releasing its contents.
• If a red blood cell is placed in a solution with a lower water potential, water will leave the cell by osmosis because it has a higher water potential.
• The cell will shrink and become shrivelled.

Question 22

What will happen to plant cells in solutions of higher water potentials?

Answer 22

• When cells are in a solution of higher water potential such as water, water will enter the cell by osmosis as the cell has a lower water potential.
• The protoplast will swell and will be pushed against the cell wall.
• Because the cell wall has limited expansion, a pressure builds up which resists the entry of more water.
• The cell becomes turgid.

Question 23

What will happen to plant cells in solutions of lower water potentials?

Answer 23

• Water will leave the cell by osmosis.
• The volume of the cell decreases and the protoplast stop pressing on the cell wall.
• Here the cell is at incipient plasmolysis.
• Further loss of water will cause the cell contents to shrink further and the protoplast to pull away from the cell wall.
• Here the cell is plasmolysed .

Question 24

What is active transport?

Answer 24

• the movement of molecules of 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 25

How does active transport differ from passive forms of transport?

Answer 25

• Metabolic energy in the form of ATP is needed.
• Substances are moved against a concentration gradient, that is from a lower to a higher concentration.
• Carrier protein molecules are involved.
• The process is selective

Question 26

Describe the transport of a single molecule or ion via active transport.

Answer 26

• The carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it.
• The molecule or ion binds to receptor sites on the carrier protein
• On the inside of the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule. As a result the protein molecule changes shape and opens to the opposite side of the membrane.
• The molecule or ion is then released to the other side of the membrane.
• The phosphate molecule is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated. The phosphate molecule then recombines with the ADP to form ATP during resperation.

Question 27

How does glucose enter the ilium epithelium?

Answer 27

• Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by a sodium-potassium pump.
• This creates a concentrations gradient as there is a higher concentration of sodium ions in the lumen than inside the cell
• This causes sodium to diffuse from the lumen into the epithelial cell, down their concentration gradient.
• They do this via the sodium-glucose co-transporter proteins ( facilitated diffusion)
• The co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.
• Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion.