Cells (transport across cell membranes)

Question 1

describe the fluid mosaic model of the membranes

Answer 1

-states that membranes are composed of a phospholipid bilayer in which the phospholipids, as amphipathic molecules, have a hydrophilic head and a hydrophobic tail
-the membrane is fluid as the individual phospholipids can move side to side within the layer, meaning the membrane has a flexible shape
-the membrane is described as mosaic because intrinsic and extrinsic proteins of different shapes and sized are embedded

Question 2

explain three features of a plasma membrane which make it adapted to its function

Answer 2

-phospholipid bilayer acts as a barrier to prevent polar substances from entering the cell
-membrane contains cholesterol which connects phospholipids and reduces fluidity to make the bilayer more stable by restricting the movement of the phospholipids
-membrane contains glycolipids which act as cell signalling receptors, and allows drugs, hormones and antibodies to bind to the cell

Question 3

explain the role of extrinsic proteins

Answer 3

-these proteins are partly embedded in the bilayer
-glycoproteins allow cell signalling to occur as they have binding sites/receptors which can receive messages such as hormones
-individuals cells have their own glycoproteins called antigens which are recognisable to the immune system.

Question 4

explain the role of intrinsic proteins.

Answer 4

-these proteins span the width of the bilayer
-channel proteins allow the movement of too large or hydrophilic molecules to pass through the bilayer by facilitated diffusion
-carrier proteins allow selected molecules through by facilitated diffusion or active transport

Question 5

describe simple diffusion.

Answer 5

-it’s a passive process as it requires no energy from ATP hydrolysis
-it is the net movement small, lipid soluble molecules directly though the bilayer from an area of high concentration to an area of low concentration until they reach equilibrium

Question 6

what factors influence simple diffusion

Answer 6

-thinner exchange surfaces give a faster rate of diffusion because the particles don’t have to travel as far
-larger surface area gives a faster rate of diffusion
-the higher the concentration gradient the faster the rate of diffusion

Question 7

describe facilitated diffusion.

Answer 7

-large, polar molecules are transported down their concentration gradient via specific channel or carrier proteins with complementary binding sites.
-it’s a passive process as it requires no energy from ATP hydrolysis -channel proteins are hydrophilic channels which bind to a specific ion in order for one side of the protein to open and the other closes
-carrier proteins have a binding site which specific molecules bind to causing a conformational change to occur which releases the molecule on the other side of the membrane.

Question 8

describe active transport through carrier proteins.

Answer 8

-ion binds to receptors on its specific carrier protein
-ATP binds to the side of the protein on the inside of the cell
-ATP is broken down into ADP and a phosphate molecule causing the carrier protein to change shape
-carrier protein releases the ion on the other side of the membrane
-the phosphate molecule and ADP are released from the protein causing the carrier protein to return to its original configuration

Question 9

what three factors affect the rate of active transport

Answer 9

-speed of carrier proteins
-number of carrier proteins present
-rate of respiration in the cells
if it has a slower rate there is less ATP produced
if there is less ATP there is slower active transport

Question 10

how are cells adapted to maximise the rate of transport across the membrane?

Answer 10

-contain many carrier/channel proteins for faster facilitated diffusion on active transport
-have a folded membrane or microvilli which increases the surface area
-large number of mitochondria so to synthesise more ATP in aerobic respiration and release energy for active transport

Question 11

define osmosis

Answer 11

the net movement of water molecules from an area of high water potential to an area of lower water potential until dynamic equilibrium is reached, across a selectively permeable barrier

Question 12

define an isotonic solution

Answer 12

the same water potential as inside of the cell
means there is no net movement of water particles into the cell

Question 13

define hypotonic solution

Answer 13

water potential is higher than inside the cells
means the water moves into the cell by osmosis
in a plant cell this causes the cell to become turgid as the protoplast swells
in an animal cell this cause the animal cell to undergo lysis

Question 14

define hypertonic solution

Answer 14

water potential is lower than inside the cell
means the water moves out of the cell by osmosis
in a plant cell this causes the cell to become plasmolysed as the protoplast to shrink
in an animal cell this causes the cell to become crenated

Question 15

what three factors affect membrane permeability

Answer 15

-high temperatures denature membrane proteins and they provide phospholipid molecules with more kinetic energy
-pH changes the tertiary structure of membrane proteins
-use of solvent may dissolve membrane

Question 16

what is co transport?

Answer 16

the movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient. symport proteins transport substances in the same direction. antiport proteins transport substances in opposite directions

Question 17

explain co transport in the absorption of amino acids and glucose in the small intestine

Answer 17

-sodium ions are actively transported out of the epithelial cells and into the bloodstream which means the sodium ion concentration is lower in the epithelial cells that into the lumen of the small intestine
-sodium ions bind to carrier protein to move into the epithelium and glucose and amino acids from the lumen also bind to the carrier protein so they are co transported by facilitated diffusion

Question 18

explain the link between active transport and a large number of mitochondria

Answer 18

-mitochondria are the site of aerobic respiration so where ATP is produced
-ATP is required for active transport to move substances against their concentration gradient

Question 19

explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen ions does not

Answer 19

-chloride ions are polar
-cannot cross the lipid bilayer
-chloride ions are transported by facilitated diffusion
-oxygen is non-polar
-oxygen can diffuse across the lipid bilayer

Question 20

Describe how you can investigate the permeability of a cell membrane in the lab using beetroot

Answer 20

-cut five equal sized pieces of beetroot with a scalpel
-rinse to remove pigment on the surface after cutting
-add each piece to a test tube with 5 cm cubed of water
-place each in a different temperature water bath for the same amount of time
-remove the beetroot so only coloured liquid is left
-determine the absorption of the coloured liquid left using a colorimeter
-high absorbance means more pigment is released
-plot on a graph

Question 21

how does temperature affect membrane permeability?

Answer 21

-below 0 degrees the phospholipids do not have much energy so cannot move much. this means the membrane is rigid except for channel proteins and carrier proteins which deform to increase permeability
-from 0 to 45 degrees the phospholipids are able to move around so the membrane is only partially permeable, but permeability gradually increases with temperature
-over 45 degrees the phospholipid bilayer begins to melt, making the membrane more permeable. the channel proteins and carrier proteins deform increasing permeability