transport across cell membranes- chapter 4

Question 1

why are phospholipids important components for the cell-surface membranes

Answer 1

• hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane attached by water on both sides
• hydrophobic tails of both phospholipid layers point to the centre of the cell membrane, repelled by water on both sides

Question 2

what are 3 functions of phospholipids

Answer 2

1. allow lipid-soluble substance to enter and leave the cell
2. prevent water soluble substances entering and leaving the cell
3. make the membrane flexible and self sealing

Question 3

what are the two main ways that proteins are embedded in the phospholipid belayer

Answer 3

some proteins occur in the surface on bilayer and never extend completely across it, act either to give mechanical support to membrane or in conjugation with glycolipids, as cell receptors for molecules such as hormones

others completely span the bilayer from one side to the other. some protein channels which from water filled tubes to allow water soluble ions to diffuse across the membrane. other carrier proteins that bind to ions or molecules like glucose and amino acids then change shape in order to move the molecules across the membrane

Question 4

what are 6 functions of proteins in the membrane

Answer 4

1.provide structural support
2. act as channels transporting water soluble substances across the membrane
3. allow active transport across the membrane through carrier proteins
4. form cell surface receptors for identifying cells
5. helps cells adhere together
6. act as receptors, for example proteins

Question 5

what are 3 functions of cholesterol within the phospholipid bilayer membrane and why

Answer 5

1. reduce lateral movement of other molecules including phospholipids as they pull together the fatty acid tails+add strength
2. make the membrane less fluid at high temperatures
3. prevent leakage of water and dissolved ions from the cell as they are very hydrophobic

Question 6

what are the functions of glycolipids in the phospholipid bilayer membrane and why

Answer 6

they extend from the bilayer into the watery environment outside the cells so:

1. act as recognition sites
2. help maintain the stability of the membrane
3. helps cells to attach to one another and so form tissues

Question 7

what are the functions of glycoproteins in the phospholipid bilayer membrane and why

Answer 7

the carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane

1. act as recognition sites specifically for hormones and neurotransmitters
2. helps cells attach to one another and so form tissues
3. allows cells to recognise one another

Question 8

why do most molecules not diffuse freely across the cell surface membrane

Answer 8

• not soluble in lipids and therefore cannot pass through the phospholipid bilayer
• too large to pass through the channels in the membrane
• of the same charge as the charge on the protein channels and so even if they are small enough to pass through they are repelled
• electrically charged (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the bilayer

Question 9

explain the fluid mosaic model arrangement

Answer 9

the way in which all the various molecules are combined into the structure of the cell surface membrane

fluid= the individual phospholipid molecules can move relative to one another, this gives the membrane a flexible structure that is constantly changing shape

mosaic= the proteins that are embedded in the bilayer vary in shape, size and pattern in the same was the stones do in a mosaic

Question 10

explain simple diffusion (passive)

Answer 10

passive means that the energy comes from the natural, inhibit motion of particles, rather than from some external source such as ATP

Question 11

definition of diffusion

Answer 11

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

Question 12

why is the diffusion random

Answer 12

• all particles are constantly in motion due to kinetic energy that they process
• there’s no set pattern to the way particles move around
• particles are constantly bouncing off one another as well as off other objects

Question 13

explain facilitated diffusion

Answer 13

• Plasma membranes are not readily permeable to molecules. Only small, non-polar molecules like oxygen can diffuse across them easily.
• Charged ions and polar molecules do not diffuse easily because of the hydrophobic nature of the fatty-acid tails of the phospholipids in the membrane.
• The movement of these molecules is made easier (facilitated) by transmembrane channels and carriers that span the membrane. The process is called facilitated diffusion.
• It is a passive process, relying only on the inbuilt energy of diffusing molecules.
• Occurs down a concentration gradient but only occurs at specific points on the plasma membrane where there are protein channels and carrier proteins.

Question 14

explain the protein channels

Answer 14

• Form water-filled hydrophilic channels across the membrane.
• Allow specific water-soluble ions to pass through.
• Channels are selective, each opening in the presence of a specific ion.
• If the particular ion is not present, the channel remains closed. This gives control over the entry and exit of ions.
• The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.

Question 15

explain carrier proteins

Answer 15

• When a molecule such as glucose that is specific to the protein is present, it binds with the protein.
• This causes it to change shape in such a way that the molecule is released to the inside of the membrane.
• No external energy is needed.
• The molecules move from a region where they are highly concentrated to one of lower concentration, using only the kinetic energy of the molecules themselves.

Question 16

definition of osmosis

Answer 16

• 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.
• Cell-surface membranes and other plasma membranes such as those around organelles are selectively permeable.

Question 17

what is a solute

Answer 17

any substance that is dissolved in a solvent

Question 18

what forms a solution

Answer 18

a solute and solvent

Question 19

what is a water potential and what is the water potential of pure water

Answer 19

• Water potential is represented Ψ and is measured in units of pressure (kPa).
• Water potential is the pressure created by water molecules.
• Pure water is said to have a water potential of zero

Question 20

what are the effects of water potential and a solute

Answer 20

• The addition of a solute to pure water will lower its water potential.
• The water potential of a solution must always be less than zero.
• The more solute that is added (i.e. more concentrated a solution), the lower (more negative) its water potential.
• Water will move by osmosis from a region of higher (less negative) water potential to one of lower water potential

Question 21

what is the explanation of osmosis

Answer 21

• The solution on the left has a low concentration of solute molecules while the solution on the right has a high concentration of solute molecules.
• Both the solute and water molecules are in random motion due to their kinetic energy.
• The selectively permeable plasma membrane, however, only allows water molecules across it and not solute molecules.
• The water molecules diffuse from the left-hand side, which has the higher water potential, to the right-hand side, which has the lower water potential, that is, down a water potential gradient.
• At the point where the water potentials on either side of the plasma membrane are equal, a dynamic equilibrium is established and there is no net movement of water

Question 22

what is active transport

Answer 22

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 23

what is ATP used for in active transport

Answer 23

• directly move molecules
• individually move molecules using a concentration gradient which has already been set up by active transport- this is co transport

Question 24

what are the differences between active transport and passive transport

Answer 24

• Metabolic energy (ATP) is needed.
• Substances are moved against a concentration gradient.
• Carrier protein molecules which act as ‘pumps’ are involved.
• The process is very selective.

Question 25

explain direct active transport of a single molecule or ion

Answer 25

- The carrier proteins bind to the molecule of ion to be transported on one side of it. - The molecule 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 phosphate molecule. This causes the protein molecule to change 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. - The phosphate molecule then recombines with the ADP to form ATP during respiration.

Question 26

when is the sodium potassium pump used and explain

Answer 26

- Occasionally the molecule or ion is moved into the cell/organelle at the same time a different one is being removed e.g. sodium potassium pump. - In this the sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surrounding

Question 27

what increases the rate of movement across membranes

Answer 27

- The epithelial cells lining the ileum possess microvilli. These finger-like projections of the cell-surface membrane about 0.6um in length, collectively termed a ‘brush border’. - The microvilli provide more surface are for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place. - Another mechanism to increase transport across membranes is to increase the number of protein channels and carrier proteins in any given area of membrane

Question 28

what is the role of diffusion in absorption

Answer 28

- As carbohydrates and proteins are being digested continuously, there is normally a greater concentration of glucose and amino acids within the ileum than in the blood. -There is therefore a concentration gradient down which glucose moves by facilitated diffusion from inside the ileum into the blood. -This glucose is continuously being removed by the cells as they use it up during respiration. -This helps to maintain the concentration gradient between the inside of the ileum and the blood. -This means the rate of movement by facilitated diffusion across the epithelial cell-surface membranes is increased

Question 29

what is the role of active transport in absorption

Answer 29

Process: 1. Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell-surface membrane of the epithelial cells. 2. This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells. 3. Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them. 4. The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier. - Both sodium ions and glucose/amino acid molecules move into the cell, but while the sodium ions move down their concentration gradient, the glucose molecules move against their concentration gradient. - It is the sodium ion concentration gradient, rather than ATP directly, that powers the movement of glucose and amino acids into the cells. - This makes it an indirect rather than a direct form of active transport