TOPIC 2B-CELL MEMBRANES

Question 1

A) What are all cells and many organelles in eukaryotic cells surrounded by?

Answer 1

-membranes

Question 2

A) Describe how “cell surface membranes” that surround cells work

B) How can substances move across the cell surface membrane?

Answer 2

A)-are barrier between cell and its environment–>control which substances able to enter/leave a cell
-are partially permeable-some molecules able to diffuse through but others not B)-via diffusion/osmosis/active transport.

Question 3

A) What is the function of the membranes around organelles?

B) How does a membranes “partially permeable” feature work?

Answer 3

A)-divide cell into different compartments–> act as a barrier between organelle and cytoplasm

B)-control what substances able to enter/leave organelle.

Question 4

A) What is the basic structure of all cell membranes?

B) Briefly explain the model suggested in 1972 to describe the arrangement of molecules in the membrane

C) What is the involvement of cholesterol molecules in this phospholipid bilayer?

Answer 4

A)-mostly same–> all composed of lipids (mainly phospholipids)/proteins/carbohydrates (attached to proteins or lipids).
B)-phospholipid molecules form double layer (bilayer) -bilayer fluid as phospholipids constantly moving.
C)-are present within bilayer

Question 5

A) Describe the presence of proteins in the phospholipid bilayer

B) Outline how receptor proteins on cell-surface membrane work

Answer 5

A)-proteins scattered through bilayer like mosaic pattern -these include channel/carrier proteins that let large molecules/ions pass through membrane
B)-they allow cell to detect chemicals released from other cells
–> the chemicals signal cell to respond in some way
–>e.g: hormone insulin binds to receptor proteins on liver cells
–>tells cells to absorb glucose.

Question 6

A) Describe the movement of proteins in the bilayer

B) Define “glycoproteins”

C) What are “glycolipids”?

Answer 6

A)-some able to move sideways through bilayer but others fixed in one position
B)-some proteins have a polysaccharide (carbohydrate) chain attached
C)-these are some lipids that also have a polysaccharide chain attached.

Question 7

A)Explain the key structural features of phospholipids

B) Outline how a phospholipid molecule is arranged in the bilayer

C) How do phospholipids act as a barrier to dissolved substances?

Answer 7

A)-phospholipid molecules have “head” + a “tail”
-head is hydrophilic–> attracts water
-tail hydrophobic–> repels water
B)-molecules automatically arrange themselves into a bilayer–> head faces out towards water either side of membrane
C)-centre of bilayer hydrophobic so memebrane not allow water-soluble substances (like ions) through.

Question 8

A) What is “cholesterol”?

B) Where is “cholesterol” found?

Answer 8

A)-type of lipid

B)-present in all cell membranes (except bacterial cell membranes)

Question 9

A) Describe the function of cholesterol molecules in the cell memebrane

Answer 9

A)-the fit between the phospholipids–> they bind to hydrophobic tails of the phospholipids–> causing them to pack more closer together–> restricts movement of phospholipids making membrane less fluid BUT more rigid

Question 10

A) Outline examples of cholesterol working in animal cells

Answer 10

A)-helps maintain animal cell shape (that dont have cell walls)–> this particularly important for cells not supported by other cells (e.g: red blood cells that float free in blood).

Question 11

A)What is the permeability of cell membranes affected by?

B)Briefly outline how the “beetroot” experiment can used to investigate things that affect permeability

Answer 11

A)-different conditions like temperature and solvent concentration
B)-beetroot cells contain coloured pigment that leaks out
–>higher membrane permeability–>more pigment leaks.

Question 12

A)State and describe the full method that you could use to investigate how temperature affects beetroot membrane permeability

Answer 12

1-scalpel–>cut 5 equal size beetroot pieces (using cutting board)–> rinse pieces for pigment removal from cutting
2-add 5 pieces to separate 5cm3 of water test tubes –> use measuring cylinder to measure/pipette water
3-each test tube in different temp water bath (e.g: 10, 20, 30, 40, 50 degrees C) for same time (stopwatch)
4-remove beetroot pieces to leave just coloured liquid
5-use colorimeter–> machine that passes light through liquid and measures how much light absorbed
–>higher absorbance–> more pigment released–>higher permeability of membrane
6)-can connect colorimeter to computer and use software to collect data + draw graph results.

Question 13

A) Explain what happens to membrane permeability for TEMPERATURES BELOW 0 DEGREES C

Answer 13

A)-phospholipids not have much energy–>cant move much

• ->they packed close together + membrane rigid
• BUT channel/carrier proteins in membrane deform–> increases membrane permeability
• ice crystals may form + pierce membrane–> making it highly permeable when it thaws.

Question 14

B) Describe and explain what happens to membrane permeability for TEMPERATURES BETWEEN 0-45 DEGREES C

Answer 14

B)-phospholipids able to move around + not packed as tight together–> membrane partially permeable
-as temp increases phospholipids move more as have more energy–> increases membrane permeability.

Question 15

C) What happens to membrane permeability at TEMPERATURES ABOVE 45 DEGREES C?

Answer 15

C)-phospholipid bilayer starts to melt (break down) + membrane more permeable

• water inside cell expands–>puts pressure on membrane
• channel + carrier proteins deform–> cant control what in/out of cell–> increases membrane permeability.

Question 16

A) Describe and explain diffusion

Answer 16

A)-net movement of particles (molecules or ions) from area of high to low concentration
-molecules diffuse both ways–> but net movement to area of low concentration–> this continues until particles evenly distributed throughout liquid/gas.

Question 17

A) Define the term “concentration gradient”

B) What type of process is diffusion?

C) What is simple diffusion?

Answer 17

A)-path from area of high conc to lower conc–> particles diffuse down conc gradient

B)-passive process–> no energy needed for it

C)-when molecules diffuse directly through a cell membrane.

Question 18

A) What minimum condition is needed for particles to diffuse across cell membranes?

B) EXAMPLE: Why can oxygen and CO2 easily diffuse cell membranes?

Answer 18

A)-that they are able to move freely through membrane

B)-this as these molecules are small–> so can pass through spaces between phospholipids
–> they are also non-polar so makes them soluble in lipids so they can dissolve in hydrophobic bilayer

Question 19

A) At what rate would LARGER MOLECULES and CHARGED PARTICLES diffuse through the phospholipid bilayer?

Answer 19

A)-LARGER MOLECULES–> like amino acids/glucose diffuse v. slowly as too big
-CHARGED PARTICLES–> like ions/polar (partially charged molecules) diffuse slowly too as are water soluble and centre of bilayer hydrophobic.

Question 20

A) Define the term “facilitated diffusion”

B) Describe the concentration gradient and they type of process facilitated diffusion is

Answer 20

A)-to speed up things large/charged particles diffuse through carrier proteins/channel proteins in membrane instead.
B)-facilitated diffusion moves particles down a conc gradient from high to low conc
-its also a passive process–> doesn’t use energy.

Question 21

A) What is the function of “carrier proteins”?

B) How do carrier proteins work?

Answer 21

A)-move large molecules across membranes down conc gradient
-different carrier proteins facilitate diffusion of different molecules
B)-first large molecules attaches to carrier protein in membrane
–> next protein shape changes
–> this releases molecule opposite side of membrane.

Question 22

A) Briefly outline how “channel proteins” work

Answer 22

A) -channel proteins form pores in membrane for charged particles to diffuse through

• -> down conc gradient
• different channel proteins facilitate diffusion of different charged particles.

Question 23

A) How does CONCENTRATION GRADIENT affect simple diffusion?

Answer 23

A) -higher it is–>faster diffusion rate

• as diffusion takes place difference in conc between 2 sides of membrane decreases until reaches an equilibrium (i.e: conc on both sides equal)
• -> means diffusion slows over time

Question 24

B) Explain how the THICKNESS OF EXCHANGE SURFACE affects simple diffusion

C) Outline and explain how SURFACE AREA affects simple diffusion

Answer 24

B) -thinner the exchange surface (i.e: shorter distance particles have to travel)–>faster diffusion rate
C)-larger surface area (e.g: of cell-surface membrane) the faster the diffusion rate.

Question 25

A) In appropriate detail explain how microvilli increase the surface area for faster diffusion

Answer 25

A)-some cells (e.g: epithelial cells in small intestine) have microvilli–> projections formed by cell-surface membrane folding up on itself

• microvilli give cell largeer S.A–> in human cells microvilli can increase SA by about 600 times
• -> larger S.A means more particles can be exchanged in same time–>increasing diffusion rate.

Question 26

A) How does the CONCENTRATION GRADIENT affect FACILITATED DIFFUSION?

Answer 26

A)-higher conc gradient–> faster facilitated diffusion rate (up to a point)
–>as equilibrium reached facilitated diffusion rate will level off

Question 27

B) Explain how the N. OF CHANNEL/CARRIER PROTEINS affects the rate of facilitated diffusion

Answer 27

B)-once all proteins in membrane in use–>facilitated diffusion can’t occur faster even if conc gradient increased
–>so greater n. of channel/carrier proteins in cell membrane means faster facilitated diffusion rate.

Question 28

C) EXAMPLE: AQUAPORINS- What are AQUAPORINS and how they may be used in kidney cells?

D) Why can water diffuse directly through cell membranes even though its polar?

Answer 28

C)-are special channel proteins that allow facilitated diffusion of water through cell membranes
-some kidney cells adapted to have lots aquaporins
–aquaporins allow cells to reabsorb lots water otherwise excreted by body–> about 180 litres needs reabsorbing every day.
D)-as its relatively small.

Question 29

A) Describe OSMOSIS in detail

Answer 29

A)-diffusion of water molecules across partially permeable membrane from area of high water potential to low water potential
–>(i.e: high water conc to low water con).

Question 30

B) Define WATER POTENTIAL and compare the water potential of PURE WATER with OTHER SOLUTIONS

C) Outline the meaning of a solution being “isotonic”

Answer 30

B)-liklihood of water molecules to diffuse out of or into a solution
-pure water’s water potential highest
–>all other solutions have lower water potential than pure water.
C)-if 2 solutions have same water potential.

Question 31

A) How does the WATER POTENTIAL GRADIENT affect the rate of osmosis?

Answer 31

A)-higher water potential gradient–> faster osmosis rate
–>as osmosis takes place difference in water potential either side of membrane decreases so osmosis rate levels off over time

Question 32

B) Exactly how does the THICKNESS OF THE EXCHANGE SURFACE affect osmosis rate?

C) How does SURFACE AREA OF THE EXCHANGE SURFACE affect osmosis?

Answer 32

B) -thinner exchange surface–> faster osmosis rate

C) -larger S.A–> faster osmosis rate.

Question 33

A) How can you find the water potential of plant tissue?

Answer 33

A)-do simple experiment using potato cylinder

-but first need to make several different solutions of known conc to test cylinders in.

Question 34

B) PART 1: Show the method you would use to make 5 serial dilutions of sucrose solution, starting with an initial sucrose conc of 2M and diluting by factor of 2

Answer 34

B) 1-line 5 test tubes in a rack
2-add 10cm3 of initial 2M sucrose solution to first test tube + 5cm3 distilled water to other 4 test tubes
3-use pipette to draw 5cm3 solution from first tube and add to to distilled water in 2nd tube then mix fully
–>now have 10cm3 solution half as concentrated as solution in tube 1 (1M)
4-repeat process 3 more times–> create 0.5M/0.25M/0.125M solutions.

Question 35

A) Through the following example explain how you can make sucrose solutions of any con by finding the scale factor:

Make 15cm3 of 0.4 M sucrose solution

Answer 35

A) 1-start with solution of known conc-e.g: 1 M
2-find scale factor by dividing conc of this solution by conc of solutions want to make
–>=1 M/0.4 M=2.5
3-means solution want to make is 2.5 x weaker than one you have
–> to make solution 2.5 x weaker use 2.5 x less of it
–> so 15cm3/2.5=6cm3–> transfer this amount to clean test tube
4-top up test tube with distilled water to get volume want
–> in this case want to make 15cm3 so add 9cm distilled water to 6cm3 solution.

Question 36

B) PART 2: Outline how you can use the solutions created in PART 1 to find the water potential of potato cells

Answer 36

B)1-use cork borer to cut potatoes in to identical chips- 1cm3 diameter
2-divide chips in to groups of 3 them measure mass of each group via mass balance
3-place one group in to each sucrose solution
4-leave chips in solution for min 20 min (making sure all get same time in solution)
5-remove chips + pat dry with paper towel
6-weigh each group again + record results
7-calculate % change in mass of each group
8-use results to make calibration curve–> showing % change in mass against sucrose conc.

Question 37

A) What results should you expect from the experiment?

Answer 37

A)-potato chips will gain water (therefore mass too) in solutions with higher water potential than the chips AND lose water in solutions with lower water potential

Question 38

B) How may the water potential of the potato cells be determined?

Answer 38

B)-point where curve crosses x-axis (where % change of mass is 0) is point at which water potential of sucrose solution is same as water potential of potato cells
–>find conc at this point via looking up water potential for that conc of sucrose solution in e.g: textbook.

Question 39

A) Define active transport

Answer 39

A)-process using energy to move molecules + ions across membranes usually against a concentration gradient.

Question 40

B) Outline how carrier proteins are involved in active transport

Answer 40

B)-process similar to facilitated diffusion
–>molecule attaches to carrier protein–>protein shape changes and this moves molecule across membrane–> releasing it on other side.

Question 41

C) How does the CONCENTRATION GRADIENT vary between active transport and facilitated diffusion?

Answer 41

C)-active transport usually moves solutes from low to high conc
–> in facilitated diffusion they always move from high to low conc

Question 42

D) Outline how the requirement of energy varies between active transport and facilitated diffusion

Answer 42

D)- a. transport needs energy f. diffusion doesn’t -ATP is common energy source in cell–>produced by respiration

• ATP undergoes hydrolysis reaction–> splitting into ADP and Pi (inorganic phosphate)
• ->this releases energy so solutes can be transported.

Question 43

A) What are co-transporters and how do they work?

B) EXAMPLE: How does co-transport work with sodium ions and glucose?

Answer 43

A)-type of carrier protein
-they bind to 2 molecules at a time
-conc gradient of one of the molecules used to move other molecules against its own conc gradient.
B) -sodium ions move in to cell down their conc gradient–> this moves glucose into cell too–>against its conc gradient.

Question 44

A) How do the following factors affect the rate of active transport?

1-speed of individual carrier proteins
2-n. of carrier proteins present
3-rate of respiration in cell + ATP availability

Answer 44

A) 1-faster they work–> the faster active transport rate
2-more proteins there are–> faster active transport rate 3-if respiration inhibited–> active transport can’t take place.

Question 45

A) Why is glucose absorbed into the bloodstream from the small intestine via co-transport?

Answer 45

A)-in ileum (last part of small intestine) conc of glucose too low for glucose to diffuse out in to blood
–>so glucose absorbed from lumen (middle) of ileum via co-transport.

Question 46

B) Explain in full detail how this process glucose being absorbed in to the bloodstream in the small intestine via co-transport

Answer 46

B)1-Na+ ions actively transported out of ileum epithelial cells in to blood via Na-K pump–> creates conc gradient so now higher Na ion conc in lumen of ileum than in the cell
2-this causes Na ions to diffuse from lumen of ileum into epithelial cells down their conc gradient via Na-Glucose co-transporter proteins.
3-co-transporter carries glucose into cell with Na
–> as result glucose conc inside cell increases
4-glucose diffuses out of cell in to blood down its conc gradient via protein channel by facilitated diffusion.

Question 47

C) What does this co-transport of glucose in to the mammalian ileum show?

Answer 47

C)-shows same substance can be transported into/ out of cell in different ways
–>sometimes several methods of transport needed to move substance from A to B.