TRANSPORT ACROSS MEMBRANES

Question 1

what is a plasma membrane ?

Answer 1

The plasma membrane defines the borders of cells and most organelles. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell/organelle, while other materials cannot move freely.

Question 2

why is the plasma membrane referred to as a ‘fluid mosaic model’

Answer 2

due to the movement and mixture of phospholipids , glycoproteins , glycolipids and proteins it is made of that are scattered across the membrane.

Question 3

what is the phospholipids link the the plasma membrane ?

Answer 3

phospholipids align as a bilayer creating a partially permeable membrane , cell surface membrane.

Question 4

when cholesterol is sometimes found in plasma membranes, what is its function ?

Answer 4

they restrict the movement of molecules and make membranes less fluid at high temperatures and prevents water and dissolved ions from leaving the cell.

Question 5

what types of proteins are embedded across the cell ?

Answer 5

. peripheral proteins : extrinsic proteins
. integral proteins : intrinsic proteins

Question 6

what do peripheral proteins do ?

Answer 6

they provide mechanical support and are connected to proteins and lipids forming glycoproteins and glycolipids . However the function of Peripheral proteins is cell recognition and they act as receptors.

Question 7

what do channel proteins do ?

Answer 7

they form tubes filled with water to enable water soluble ions to diffuse through.

Question 8

what do integral proteins do ?

Answer 8

they are channel proteins or carrier proteins involved in the transport of molecules across the membrane.

Question 9

what do carrier proteins do ?

Answer 9

they bind to larger molecules such as glucose or amino acids and change shape to transport them across the membrane.

Question 10

what molecules can pas through the partially permeable membrane ?

Answer 10

lipid soluble substances or small molecules like C02 , H20 and O2.

Question 11

what are the 4 key types of transport ?

Answer 11

. simple diffusion
. faciliated diffusion
. active transport/co-transport
. osmosis

Question 12

What molecules cannot pass through a partially permeable membrane ?

Answer 12

water soluble pr polar substances (SODIUM IONS) and large molecules ( glucose or amino acids)

Question 13

what is the process of simple diffusion ?

Answer 13

net movement of molecules from a high area of conc to a low area of conc across a conc gradient until equilibrium is reached.

Question 14

how do molecules move in simple diffusion ?

Answer 14

They move as they have energy due to kinetic energy that they posses which enables them and other molecules to constantly move in fluids.

Question 15

what happens if the molecules are not lipid soluble and small ?

Answer 15

facilliated diffusion takes place instead of simple diffusion.

Question 16

why does facilliated diffusion take place ?

Answer 16

it is a passive process but differes from simple diffusion as proteins are used to transport molecules as the ions and polar molecules cannot simply diffuse across the membrane so facilitated diffusion takes place using protein carrier and protein chanels.

Question 17

what is the process of osmosis ?

Answer 17

Osmosis is the net movement of water from and area of high water potential to an area of low water potential across a partially permeable membrane.

Question 17

explain the process of faciliated diffusion

Answer 17

protein channels form tubes filled with water so water soluble ions can diffuse and pass through the membrane. this is selective as chanel proteins only open in the presence of certain ions. Carrier proteins will bind to the molecule such as glucose and they will change shape so that the molecule can be released through the other side of the membrane.

Question 18

what is water potential ?

Answer 18

it is the pressure created due to water molecules. Pure water has a water potential of 0 , however when solutes are dissolved in it it becomes negative so more the negative a water potential is the more the solutes dissolved in it.

Question 19

what is an isotonic solution ?

Answer 19

it is when the water potential is the same in the solution and the cell within the solution.

Question 20

what is a hypotonic solution ?

Answer 20

this is when the water potential of a solution is more positive than the cell .

Question 21

what is a hypertonic solution ?

Answer 21

when the water potential is more negative in the solution than the cell

Question 22

what is active transport ?

Answer 22

it is the net movement of substance from a lower conc to a higher conc using metabolic energy and a carrier protein.

Question 23

explain the process of active transport.

Answer 23

active transport occurs through the carrier protein within the cell membrane and the molecule binds to a receptor complementary in shape to the carrier protein. ATP binds to the carrier protein and it is hydrolysed into ADP and PI which releases energy. This causes the carrier protein to change shape and release the molecule on the other side of the membrane. the phosphate ion is released and the protein gets its original shape back.

Question 24

explain the adaptations of cells that may give them rapid transport across internal and external membranes

Answer 24

. an increase in surface area
. an increase in number of carrier/channel proteins

Question 25

WHY does co- transport of glucose and sodium ions in the Ileum occur ?

Answer 25

to absorb glucose from the lumen to the gut you need a high concentration of glucose in the lumen compared to the epithelial cells , however epithelial cells have a higher concentration of glucose and thats why co-transport and active transport takes place.

Question 26

explain the process of co- transport and active transport that takes place in the Ileum

Answer 26

sodium ions are actively transported out the epithelial cells into the blood. This reduces the concentration of sodium ions in the epithelial cells. The sodium ions then diffuse down a conc gradient into the epithelial cells. The protein the sodium ions is diffusing through is a co - transporter, so the molecule , either glucose or amino acid , attach to the co - transporter and are transported into the epithelial cell against a concentration gradient. When the glucose is in the epithelial cell it can then move by faciliated diffusion from the epithelial cells into the blood.