!!!Cell structure - Transport Across Cell Membranes

Question 1

Give 3 functions of the phospholipid bilayer in the cell surface membrane

Answer 1

• to allow lipid soluble substances to enter and leave the cell
• to prevent water soluble substances enter and leave the cell
• to make the membrane flexible and self sealing

Question 2

What do proteins that occur only in the surface of the phospholipid bilayer do?

Answer 2

• give mechanical support to the membrane

- act in conjunction with glycolipids as cell receptors for molecules such as hormones

Question 3

What are the two type of proteins that span the phospholipid bilayer from one side to the other and what do they each do?

Answer 3

• protein channels which form water filled tubes to allow water soluble ions to diffuse across the membrane
• carrier proteins which bind to ions or molecules like glucose or amino acids, then change shape in order to move these molecules across the membrane

Question 4

Give 6 functions of proteins in the cell surface membrane

Answer 4

• to provide structural support
• to 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
• act as receptors e.g. for hormones

Question 5

What are the functions of cholesterol in the cell surface membrane?

Answer 5

• to reduce lateral movement of other molecules including phospholipids
• to make the membrane less fluid at high temperatures
• to prevent leakage of water and dissolved ions from cells

Question 6

How does the cholesterol in the cell surface membrane prevent loss of water and dissolved ions from the cell?

Answer 6

As it’s very hydrophobic

Question 7

What are glycolipids made of and how are they positioned in the phospholipid bilayer?

Answer 7

• They are made 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 a cell surface receptors for specific chemicals

Question 8

Give 3 functions of the glycolipids in the cell surface membrane

Answer 8

• act as recognition sites
• help maintain the stability of the membrane
• help cells to attach to each other and so form tissues

Question 9

What are the 3 functions of glycoproteins in the cell surface membrane?

Answer 9

• to act as recognition sites
• to help cells to attach to each other and so form tissues
• to allow cells to recognise one another e.g. lymphocytes can recognise an organism’s own cells

Question 10

What are the 4 reasons that molecules do not generally diffuse freely across the cell surface membrane?

Answer 10

• they are not soluble in lipids and so can’t pass the phospholipid bilayer
• they are too large to pass through the channels in the membrane
• they are of the same charge as the charge of the protein channels so even if they are small enough to pass through, they are repelled
• they are polar and so have difficulty passing though the non polar hydrophobic tails in the phospholipid bilayer

Question 11

Give 5 functions of membranes within cells

Answer 11

• control the entry and exit of materials in discrete organelles like mitochondria and chloroplasts
• they separate organelles from the cytoplasm so specific metabolic reactions can take place within them
• they provide an internal transport system e.g. rough er
• they isolate enzymes that may damage the cell e.g. lysosomes
• provide surfaces on which reactions can occur e.g. protein synthesis using ribosomes on rough er

Question 12

What is the fluid mosaic model? What does the fluid and mosaic part refer to?

Answer 12

The arrangement of all the molecules that are combined into the structure of the cell surface membrane

• fluid as the phospholipid molecules can individually move relative to one another giving the membrane a flexible structure
• mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern

Question 13

What is the definition of diffusion?

Answer 13

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 14

How does facilitated diffusion work?

Answer 14

By allowing the movement of ions and polar molecules that cannot simply diffuse down the concentration gradient by using protein channels and carried proteins in the cell surface membrane

Question 15

How do protein channels work?

Answer 15

By forming water filled hydrophilic channels across the membrane which allow specific water soluble ions to pass through by the ion binding with the protein causing the protein to change shape in a way that closes to one side of the membrane and opens to the other

Question 16

How do protein channels have control over the entry and exit of ions?

Answer 16

Because the channels are selective as they only open in the presence of a specific ion and if that ion is not there, then the channel remains closed.

Question 17

How do carrier proteins work?

Answer 17

When a molecule like glucose which is specific to the protein is present, it binds with the protein causing it to change shape in a way the molecule is released to the inside of the membrane

Question 18

What do molecules use to move down the concentration gradient in diffusion?

Answer 18

Their own kinetic energy only (there is no external source of energy used)

Question 19

What is simple diffusion?

Answer 19

When non polar molecules like oxygen, co2 and small fatty acids pass directly through a membrane from an area of high concentration to low concentration down the concentration gradient

Question 20

What is the definition of active transport?

Answer 20

The movement of molecules or ions into or out of a cell from a region of lower concentration to higher concentration using ATP and carrier proteins

Question 21

What are the two ways ATP is used in active transport?

Answer 21

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

Question 22

In what 4 ways does Active transport differ from passive forms of transport?

Answer 22

• metabolic energy in the form of ATP is needed
• substances are moved against a concentration gradient that is from a lower to higher concentration
• carrier protein molecules which act as pumps are involved
• the process is very selective with specific substances being transported

Question 23

Describe the steps of direct active transport on a single molecule or ion

Answer 23

1. The carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it
2. The molecule or ion binds to receptor sites on the carrier protein
3. On the inside of the cell or organelle, the ATP binds to the protein causing it to split into ADP and a phosphate molecule causing the protein molecule to change shape and open to the opposite side of the membrane
4. The molecule or ion is then released to the other side of the membrane
5. The phosphate molecule is released from the protein causing the protein to revert back to its original shape ready for the process to be repeated and the phosphate molecule recombines with ADP to form ATP during respiration

Question 24

Compare and contrast active transport and facilitated diffusion

Answer 24

They both use carrier proteins however facilitated diffusion occurs down the concentration gradient while active transport occurs against the concentration gradient so facilitated diffusion does not require metabolic energy while active transport does which is provided in the form of ATP

Question 25

In active transport, give and describe what happens in an example of when a molecule or ion is moved into a cell/organelle at the same time as a different one is being removed from it

Answer 25

In a sodium potassium pump, sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surroundings

Question 26

What is the ileum?

Answer 26

Small intestine

Question 27

What are microvilli?

Answer 27

Finger like projections of the cell surface membrane of epithelial cells around 0.6 um in length

Question 28

How are microvilli useful?

Answer 28

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

Question 29

How can active transport across membranes be increased?

Answer 29

• by increasing the number of protein channels and proteins in any given area of membrane
• by providing more surface area for the insertion of carrier proteins

Question 30

Why is there a greater concentration of glucose and amino acids within the ileum than there is in the blood? What does this then mean?

Answer 30

As carbohydrates and proteins are being digested continuously meaning there is a downwards concentration gradient which glucose moves down by facilitated diffusion from the inside of the ileum to the blood

Question 31

How is the concentration gradient between the inside of the ileum and the blood maintained?

Answer 31

As the blood is being constantly circulated by the heart so the glucose absorbed into the blood is continuously being removed by the cells as they use it for respiration

Question 32

Why are glucose and amino acids also absorbed by active transport in the ileum and not just diffusion?

Answer 32

Because at best diffusion only results in the concentrations either side of the intestinal epithelium becoming equal so not all available glucose and amino acids are absorbed and some may pass out of the body

Question 33

State what the type of active transport that absorbs glucose and amino acids from the ileum by is called and why its called that

Answer 33

Co transport because glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium potassium pump

Question 34

What are the steps of the co transport of glucose molecules/amino acids from the ileum to the blood?

Answer 34

1. Sodium ions are actively transported out of the epithelial cells via the sodium potassium pump and into the blood taking place in a type of protein carrier molecule found in the cell surface membrane of 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

Question 35

How is the co transport of glucose molecules/amino acids from the ileum to the blood an indirect rather than direct form of active transport?

Answer 35

Because it’s the sodium ion concentration gradient rather than ATP directly that powers the movement of glucose and amino acids into the cells

Question 36

What is osmosis?

Answer 36

The passive diffusion of water from a region of high water potential to a region of lower water potential through a selectively permeable membrane

Question 37

What is water potential?

Answer 37

A measure of how free water molecules are to move

Question 38

What is the water potential of pure water?

Answer 38

0

Question 39

In a hypotonic solution, compared to the inside of the cell, state:

• the solute concentration outside the cell
• the water potential outside the cell
• net movement of water

Answer 39

• lower
• higher
• into the cell

Question 40

What is crenation?

Answer 40

When water leaves animal cells causing them to shrivel as a result of them being in a hypertonic solution

Question 41

In a isotonic solution, compared to the inside of the cell, state:

• the solute concentration outside the cell
• the water potential outside the cell
• net movement of water

Answer 41

• same
• same
• no net movement

Question 42

In a hypertonic solution, compared to the inside of the cell, state:

• the solute concentration outside the cell
• the water potential outside the cell
• net movement of water

Answer 42

• higher
• Lower
• out of the cell

Question 43

When plant cells are in a isotonic environment,

• What’s the water potential of the external solution compared to the the cell solution?
• what’s the net movement of water?
• what happens to the protoplast?
• what’s the condition of the cell?

Answer 43

• equal
• neither enters nor leaves
• no change
• incipient plasmolysis

Question 44

When plant cells are in a hypotonic environment,

• What’s the water potential of the external solution compared to the the cell solution?
• what’s the net movement of water?
• what happens to the protoplast?
• what’s the condition of the cell?

Answer 44

• Higher (so less negative)
• enters cell
• swells as the vacuole fills with water and turgor pressure develops and chloroplasts can be seen next to the cell wall
• turgid

Question 45

When plant cells are in a hypertonic environment,

• What’s the water potential of the external solution compared to the the cell solution?
• what’s the net movement of water?
• what happens to the protoplast?
• what’s the condition of the cell?

Answer 45

• lower so more negative
• leaves the cell
• shrinks and completely pulls away from the cell wall as the vacuole looses water and chloroplasts can be seen in the centre of the cell
• plasmolysed