Transport across cell membrane

Question 1

What is the relationship between all eukaryotic cells cell membrane?

Answer 1

The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same.

Question 2

What are the different structures found in the membrane? (6)

Answer 2

• Phospholipids
• Carrier proteins
• Channel proteins
• Cholesterol
• Glycolipids
• Glycoproteins

Question 3

What is the main molecule which forms the bi layer?

Answer 3

Phospholipid

Question 4

Describe the structure of a phospholipid (2)

Answer 4

1) Hydrophilic head > of both phospholipid layers point to outside of cell surface membrane > attracted by water

2) Hydrophobic tails > point to centre of cell membrane > repelled by water on both sides

Question 5

What are the functions of the phospholipids? (3)

Answer 5

1) Allow lipid squabble substances to enter and leave the cell

2) Prevent water squabble substances entering and leaving the cell

3) Make the membrane flexible and self sealing

Question 6

What are the 2 main ways proteins are embedded in the cell membrane? (2)

Answer 6

1) Surface of the bi layer > give mechanical support to the membrane > or in conjunction with glycolipids act as cell receptors for molecules such as hormones

2) Completely span the phospholipid bilayer > carrier and channel proteins

Question 7

Describe the role of carrier proteins (2)

Answer 7

1) Bind to ions or molecules like glucose and amino acids

2) Change shape to move molecules across the membrane

Question 8

What are the functions of proteins in the cell membrane? (6)

Answer 8

1) Provide structural support

2) Act as channels transporting water soluble substances across the membrane

3) Allow active transport across membrane through carrier proteins

4) Form cell surface receptors for identifying cells

5) Help cells adhere together

6) Acts as receptors > EG for hormones

Question 9

Describe the structure of channel proteins relates to its function (2)

Answer 9

1) Form water filled tubes

2) Allows for water soluble ions to diffuse across the membrane

Question 10

Where in the phospholipid bilayer is cholesterol found?

Answer 10

Within the bilayer

Question 11

Describe and explain the functions of cholesterol (3)

Answer 11

1) Reduce lateral movement of other molecules > phospholipids > achieved by cholesterol pulling together the fatty acid tails of phospholipid molecules > without making membrane too rigid (ADDS STRENGTH)

2) Make the membrane less fluid at high temperatures

3) Prevents leakage of water and dissolved ions from the cell > cholesterol molecule very hydrophobic

Question 12

Describe the structure of glycolipids (1)

Answer 12

EXTENDS from the phospholipid bilayer into watery environment outside the cell > acts as a cell surface receptor for specific chemicals

1) Carbohydrate covalently bonded with a lipid

Question 13

Describe the functions of glycolipids (3)

Answer 13

1) Act as recognition sites

2) Helps maintain the stability of the membrane

3) Helps cells to attach to one another and form tissues

Question 14

Describe the structure of glycoproteins (1)

Answer 14

1) Carbohydrate chains attached to many extrinsic proteins > on outer surface of membrane

Question 15

Describe the functions of glycoproteins (3)

Answer 15

1) Acts as recognitions sites > receptors > hormones and neurotransmitters

2) Helps cells to attach to one another and form tissues

3) Allows cells to recognise one another > EG lymphocytes can recognise an organisms own cells

Question 16

Describe the factors that limit the movement of substances in the membrane (4)

Answer 16

1) Not soluble in lipids > cannot pass through the phospholipid layer

2) Too large to pass through the channels in the membrane

3) Of the same charge as the charge on the protein channels >even if they are small enough to pass through, they arc repelled

4) Electrically charged (polar) >
have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.

Question 17

Describe why the cell surface membrane is referred to as a fluid mosaic model (2)

Answer 17

1) Fluid > individual phospholipid molecules can move relative to one another > gives the membrane a flexible structure that is constantly changing in shape

2) Mosaic > proteins that are embedded in the phospholipid bilayer vary in shape

Question 18

What is diffusion?

Answer 18

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 19

Describe simple diffusion

Answer 19

• Passive transport > energy comes from the natural, inbuilt motion of particles >rather than from some external source such as ATP

Question 20

What are membranes around and within cells known as?

Answer 20

Plasma membranes

Question 21

Describe facilitated diffusion

Answer 21

• Passive process
• Relies only on the inbuilt motion (kinetic energy) or diffusing molecules
• No ATP
• Occurs down a concentration gradient
• Occurs at specific points down the plasma membrane > protein molecules > carrier and channel proteins

Question 22

Describe the process of facilitated diffusion within a channel protein (2)

Answer 22

1) Open in the presence of a specific ion > if ion not present channel remains closed > control over entry and exit of ions

2) Ions bind with protein > causing it to change shape > closes it to one side of the membrane and opens it to the other side

Question 23

Describe the process of facilitated diffusion within a carrier protein (2)

Answer 23

1) Presence of molecule such as glucose present > is specific to protein present > binds with proteins

2) Causes change in shape > molecule is released to side of the membrane

NO external energy needed

Question 24

What is osmosis?

Answer 24

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 25

How can water molecules pass through cell surface membranes?

Answer 25

They are permeable

Question 26

What is a solute?

Answer 26

Any substance that is dissolved in a solvent

• The solute and the solvent together form a solution

Question 27

What is water potential measured in?

Answer 27

Psi - pressure created by water molecules

Question 28

What can affect water potential? (1)

Answer 28

1) The addition of a solute to pure water will lower its water potential

• water will move by osmosis from a region of higher (less negative) water potential (e.g., -20 kPa) to one of lower (more negative) water potential (e.g., -30 kPa).

Question 29

Effect on plant cell if external solution is higher WP?

Answer 29

Net movement of water > enters cell

Protoplast > swells

Cell condition > turgid

Question 30

Effect on plant cell if external solution is lower WP?

Answer 30

Net movement of water > leaves cell

Protoplast > shrinks

Cell condition > plasmolysed

Question 31

What is active transport?

Answer 31

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 32

What is ATP used for in active transport? (2)

Answer 32

1) Directly move molecules

2) Individually move molecules using a concentration gradient which has already been set up by (direct) active transport.

Question 33

How does active transport differ from passive transports? (4)

Answer 33

1) Metabolic energy in the form of ATP is needed

2) Substances are moved against a concentration gradient, that is from a lower to a higher concentration

3) Carrier protein molecules which act as ‘pumps’ are involved

4) Process is very selective, with specific substances being transported

Question 34

Describe the process of active transport of a singular molecule (4)

Answer 34

1) The molecule or ion binds to receptor sites on the carrier protein
- (carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it)

2) On the inside of the cell/organelle, 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

3) The molecule or ion is then released to the other side of the membrane

4) 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

5) Phosphate molecule recombines with ADP to form ATP during respiration

Question 35

What is the movement of molecules in the sodium potassium pump?

Answer 35

• Multiple molecules or ions being moved in the same direction
• Molecule or ion is moved into a cell/organelle at the same time as a different one is being removed from it

Question 36

What is the process of the sodium potassium pump?

Answer 36

• Sodium ions are actively removed from the cell/organelle
• While potassium ions are actively taken in from the surroundings

(process is essential to a number of imporant processes in the organism)

Question 37

What is the function of the micro villi?

Answer 37

Provide more surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place

Question 38

What two mechanisms increase the rate of movement across membranes?

Answer 38

• Microvilli
• Increase the number of protein channels and carrier proteins in any given area of membrane

Question 39

How is the rate of movement by facilitated diffusion across epithelial cell-surface membranes increased?

Answer 39

• Carbohydrates and proteins are being digested continuously > normally a greater concentration of glucose and amino
  acids within the ileum than in the blood
• Concentration gradient down > which glucose moves by facilitated diffusion from inside the ileum into the blood
• Blood
  is constantly being circulated by the heart > glucose absorbed into
  it is continuously being removed by the cells as they use it up during respiration
• Maintains concentration gradient between the inside of the ileum and the blood

Question 40

What is the role of active transport in absorption of glucose into the ileum?

Answer 40

Ensure all glucose and amino acids is absorbed into the blood

Question 41

Describe the process of co-transport between the ileum and capillary (4)

Answer 41

1) Sodium ions are actively transported out of epithelial cells, by the sodium potassium pump, into the blood

2) 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 chis concentration gradient > sodium ions diffuse in through this second carrier (co-transport protein) > they carry either amino acid or glucose molecules into cell with them

4) The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier