Cell membranes

Question 1

What are cell membranes

Answer 1

-Barrier between the cell and its environment
-Partially permeable

Question 2

Ways substance can move across the cell membranes?

Answer 2

-Diffusion
-Osmosis
-Active transport

Question 3

Describe the fluid mosaic structure of cell membrane?

Answer 3

-Phospholipid molecules form a double layer (bilayer)
-Bilayer is fluid because phospholipids are constantly moving
-Cholesterol molecules are present within the bilayer
-Proteins are scattered through the bilayer, like tiles in a mosaic. (includes channel proteins and carrier proteins)
-Receptor proteins allow the cell to detect chemicals released from other cells. The chemicals signal to the cell to respond in some way
-Some proteins have a polysaccharide chain attached called glycoproteins.
-Some lipids have a polysaccharide chain attached called glycolipids.

Question 4

Role of the phospholipids in the cell membrane.

Answer 4

-Phospholipids molecules have a head and a tail
-Head is hydrophilic and tail is hydrophobic
-Molecules arrange themselves into a bilayer
-Centre of the bilayer is hydrophobic so the membrane doesn’t allow water soluble substances though it. acts as a barrier

Question 5

Role of the cholesterol in the cell membrane.

Answer 5

-Is a type of lipid
-Present in all cell membranes
-Fit between the phospholipids, bind to the hydrophobic tails, causing them to pack closely, restricts the movement of the phospholipids, making the membrane less fluid and more rigid
-Helps to maintain the shape of the animal cells, important for the cells that aren’t supported by other cells

Question 6

Membrane permeability changes with ……..

Answer 6

Temperature and solvent concentration

Question 7

How the membrane permeability changes at 0 Celsius degrees

Answer 7

-Phospholipids don’t have much energy so they can’t move much
-Phospholipids are packed closely and the membrane is rigid
-Channel proteins and carrier proteins in the membrane deform, increasing the permeability.
-Ice crystals may form and pierce the membrane making it highly permeable

Question 8

How the membrane permeability changes at between 0 and 45 Celsius degrees

Answer 8

-Phospholipids can move and aren’t packed closely.
-Membrane is partially permeable
-As temp increases, phospholipids may move more, this increases the permeability

Question 9

How the membrane permeability changes above 45 Celsius degrees

Answer 9

-Phospholipid bilayer starts to melt, more permeable
-Water inside the cell expends, putting pressure on the membrane
-Channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell, increases the permeability

Question 10

Practical- Permeability of the cell membrane can be investigated in the lab using beetroot

Answer 10

-Beetroot cells contain a coloured pigment that leaks out, the higher the permeability, the more pigment leaks out the cell.

1)Cut 5 equal pieces of beetroot by using a scalpel-rinse the pieces to remove any pigment released during cutting
2)Add the pieces into 5 test tubes containing 5 cm3 of water.
3)Place each test tube into water bath at different temperatures for the same length of time
4)Remove the pieces of beetroot, leaving just the coloured liquid
5)Use colorimeter( machine that passes light through the liquid and measures how much of that light is absorbed, higher absorbance, more pigment released so higher permeability)
6)Connect the colorimeter to computer and collect the date to draw a graph

Question 11

What is diffusion?

Answer 11

-passive process, no energy needed
-From an area of high conc to an area of low conc
-Molecules will diffuse both ways but the net movement will be to the area of lower conc , continues until the particles are evenly distributed
-Particles diffuse down a conc gradient
-When molecules diffuse directly through a cell membrane its known as simple diffusion

Question 12

What is facilitated diffusion and what does it use?

Answer 12

Carrier proteins and protein channels
-some larger molecules diffuse slow through the phospholipid bilayer because they are big
-Charged particles, ions and polar molecules, diffuse slowly because they are water soluble and the centre of the bilayer is hydrophobic
-Carrier proteins and protein channels speed things up
-Moves down the conc gradient, higher conc to a lower conc
-Passive process

Question 13

How do carrier proteins play a role in diffusion

Answer 13

-Move large molecules across the membranes
-Different carrier proteins facilitate the diffusion of different molecules
1)Large molecule attaches to a carrier protein in the membrane
2)Protein changes shape
3)Releases the molecule on the opposite side of the membrane

Question 14

How do channel proteins play a role in diffusion

Answer 14

-form pores in the membrane for charged particles to diffuse through
-Different channel proteins facilitate the diffusion of different charged particles.

Question 15

What factors do simple diffusion depends on?

Answer 15

-Conc of gradient- the higher it is, the faster the rate of diffusion, means diffusion slows down over time
-Thickness of the exchange surface- the thinner the exchange surface, the shorter the distance the particles have to travel, the faster the rate of diffusion
-Surface area- The larger, the faster the rate

Question 16

What factors do facilitated diffusion depends on?

Answer 16

-Conc gradient
-Number of channel or carrier proteins- once all the proteins in the membrane ae in use, facilitated diffusion can’t happen any faster

Question 17

What is osmosis?

Answer 17

-Diffusion of water molecules across a partially permeable membrane.
-Higher water potential to lower water potential
-Pure water has the highest water potential

Question 18

What is water potential?

Answer 18

-Potential of water molecules to diffuse out of or into a solution.

Question 19

What is isotonic?

Answer 19

If two solutions have the same water potential

Question 20

What is rate of osmosis depends on?

Answer 20

-Water potential gradient- the higher it is, the faster the rate of osmosis
-The thickness of the exchange surface-The thinner it is, the faster the rate
-The surface area of the exchange surface- the larger the surface area, the faster the rate

Question 21

Practical- Investigate water potential using serial dilutions.

Answer 21

-How to make 5 serial dilutions of a sucrose solution
1)line up 5 test tubes in a rack
2)add 10 cm3 of the initial 2M sucrose solution to the first test tube and 5 cm5 of distilled water to other four test tubes
3)Draw 5cm3 of the solution from the first test tube and add it to the distilled water in the second test tube and mix it (you have now 10 cm3 of solution thats half as concentrated as the first solution in the first test tube (1M)
4)Repeat the process to create solutions of 0.5M, 0,25M, 0.125M

Question 22

Practical- Use your solutions to find the water potential of plant tissue

Answer 22

1)Use a cork borer to cut the potatoes into identical sized chips
2)Divide them into groups of 3 and measure the mass of each group using a mass balance
3)Place one group into each of the sucrose solutions
4)Leave the chips in for at least 20 minutes
5)Remove the chips and pat dry them
6)Weight each group again and
7)Calculate the % change in mass for each group
8)Use the results to make a calibration curve, %change in mass against sucrose conc

-The potato chips will gain water and mass in solutions with higher water potential than the chips, and lose water in solutions with a lower water potential

Question 23

Main differences between the active transport and facilitated diffusion-

Answer 23

-Active transport usually moves solutes from a low conc to a high conc
-Active transport requires energy (ATP undergoes a hydrolysis reaction splitting ADP and P, this releases energy so that solutes can be transported)

Question 24

How do co-transporters work in active transport

Answer 24

-A type of carrier protein
-Bind two molecules at a time
-Conc gradient of one of the molecules is used to move the other molecule against its own conc gradient

Question 25

Factors affecting the rate of active transport-

Answer 25

-Speed of the individual carrier proteins
-Number of carrier proteins present
-The rate of respiration in the cell and the availability of ATP- if respiration is inhibited, active transport can’t take place.

Question 26

Where in body glucose is absorbed?

Answer 26

-in the small intestine)
-In the ileum the conc of glucose is too low for glucose to diffuse out in the blood. So glucose is absorbed from the lumen (middle) of the ileum by co-transport

Question 27

How does glucose is absorbed into the bloodstream?

Answer 27

-Glucose enters the ileum epithelium with sodium ions
-Sodium ions are actively transported out of the ileum epithelial cells into the blood, by the sodium potassium pump. This creates a conc gradient- higher conc of sodium ions in the lumen of the ileum than inside the cell.
-This causes sodium ions to diffuse from the lumen of ileum into the epithelial cell, down their conc gradient via the sodium glucose co-transporter proteins.
-The co transporter carries glucose into the cell with the sodium. As a result, the conc of glucose inside the cell increases
-Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel by facilitated diffusion.