Transport across membranes

Question 1

Define diffusion

Answer 1

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 2

Explain what is meant by a passive process?

Answer 2

No metabolic energy is required, relies only on the inbuilt motion of the diffusing molecules ( kinetic energy)

Question 3

Describe factors that affect diffusion into a cell across a membrane

Answer 3

The concentration gradient, size/ charge of the particles, the temperature or the surface area of the membrane, distance, channels/ carriers

Question 4

Why dont all molecules pass through a membrane at the same rate?

Answer 4

Because the centre of the membrane is hydrophobic due to the fatty acid tails ( non polar) meaning that hydrophobic molecules diffuse quickly across the membrane, however hydrophilic substances such as ions or polar molecules cannot diffuse across the membrane as easy, with an exeption to water as this is small

Question 5

What is facilitated diffusion?

Answer 5

Where hydrophilic molecules have to pass through the membrane via protein molecules, this means they can cross the membrane without having to interact with the
hydrophobic tails of the phospholipid bilayer

Question 6

How do carrier proteins facilitate diffusion?

Answer 6

Carrier proteins span the whole membrane, and when a polar or charged molecule such as glucose that is specific to the protein is present, it binds to the protein causing it to change shape so molecule is released to the other side of the membrane

Question 7

How do channel proteins faciliate diffusion

Answer 7

These proteins form water filled hydrophilic channels across the membrane and allow water soluble ions to pass through, these channels are selective, each opening in the presence of a specific ion, if a particular ion is not present , the channel will remain closed which controls the entry and exit of materials. The ions bind with the protein so that it changes shape causing it to close on one side of the membrane and opens to the other side

Question 8

Define osmosis

Answer 8

The passage of water from a region with a higher water potential to a region where it has a lower water potential through a partially permeable membrane

Question 9

What is the water potential of pure water?

Answer 9

Zero meaning any other substance has a water potential of less than zero

Question 10

What is water potential?

Answer 10

Pressure created by water molecules, the more negative the value the lower the water potential

Question 11

How does the water potential of a cells external solution influence the state of a cell

Answer 11

If water potential of the cells external solution is higher( less negative) the water will move by osmosis into the cell causing it to swell and burst,turgid if the water potential is equal there will be no movement of water, however if it is less the water will leave the cell and the cell will shrink (plasmolysed)

Question 12

What is active transport?

Answer 12

Movement of molecules or ions in or out of a cell from a region of lower concentration to a higher concentration using ATP or carrier proteins

Question 13

What is ATP used for in active transport?

Answer 13

Directly move molecules, individually move molecules using a concentration gradient which has already been set up by direct active transport

Question 14

Why is active transport not a passive process

Answer 14

Metabolic energy in the form of ATP is needed, substances are moved against a conc gradient, carrier proteins are involved and the process is extremely selective with specific substances being transported

Question 15

Describe the process of simple active transport

Answer 15

The carrier of proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it, the molecule or ion will then bind to receptor sites on the carrier protein. Inside the cell, ATP will bind to the protein causing it to split into ADP and a phosphate molecule meaning the protein changes shape and opens on the other side of the membrane, releasing the molecule or the ion to the other side of the membrane, the phosphate molecule is then released from the protein molecule causing the protein to revert to its original shape for the process to repeat

Question 16

What is the sodium potassium pump?

Answer 16

Using ATP, the sodium potassium pump actively transports sodium ions out of the epithelial cells and into the bloodstream, and at the same time it actively transports potassium ions into the epithelial cell, this now means there is an extremely low concentration of sodium ions in the cell

Question 17

How does active transport occur in the illeum when not all glucose molecules can be absorbed into the cells?

Answer 17

Due to the sodium potassium pump, there is a low conc of sodium ions inside the cell however there is a much higher conc in the lumen of the ileum meaning there is a concentration gradient for sodium ions between the lumen of the ileum and the interior of the epithelial cells. The sodium ions will now diffuse along the concentration gradient through a co transport protein in the cell surface membrane, and as they do this they bring amino acid molecules or glucose molecules with them, meaning they are being transported against their concentration gradient, so they are being activly transported, the energy from this is being indirectly provided from the conc gradient of the sodium ion

Question 18

How are the epithelial cells adapted for increasing rate of movement across membranes?

Answer 18

Cells lining the ileum posess microvilli , providing more surface area for the insertion of carrier proteins for which diffusion, active transport and faciliitated diffusion takes place

Question 19

Is facilitated diffusion passive or non passive and why?

Answer 19

Passive process, only relies on the inbuilt kinetic energy of moving particles, no external input such as ATP, occurs down a concentration gradient