2.4 Membranes Objectives
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Topic 2: Cells
2.4 Membranes
Orange book: pg.
Green book: pg.
2.4.1 Draw and label a diagram to show the structure of membranes (pg. 26,
2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids
help to maintain the structure of cell membranes
2.4.3 List the functions of membrane proteins
2.4.4 Define diffusion and osmosis
2.4.5 Explain passive transport across membranes by simple diffusion and
facilitated diffusion
2.4.6 Explain the role of protein pumps and ATP in active transport across
membranes
2.4.7 Explain how vesicles are used to transport materials within a cell
between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane
2.4.8 Describe how the fluidity of the membrane allows it to change shape,
break and re-form during endocytosis and exocytosis.
2.4.1 Membrane Structure
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2.4.1 Draw and label a diagram to show the structure of membranes
Orange book: pg. 26
Green book: pg.
To do:
 Complete the colouring-in handout “The Fluid Mosaic Model”
 Colour and annotate the handout “The Cell Membrane”
View the diagrams below illustrating membrane structure.
  Use your textbook, the diagrams below, diagrams/ pictures from the
internet and your handouts to familiarize yourself with the cell membrane.
In your green exercise books draw a clear labeled diagram of the structure of
the cell membrane. The most important labels for you to identify and remember
are:





Phospholipids  hydrophilic head and hydrophobic (fatty acid) tail
Peripheral protein
Integral (channel) protein
Glycolipid
Glycoprotein

Cholesterol
Plasma membranes of two adjacent cells (electron micrograph).
Molecular structure of the plasma membrane
2.4.2 Phospholipids
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2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids
help to maintain the structure of cell membranes
Orange book: pg. 26
Green book: pg.
To do:
 Complete the colouring-in handout “Evidence of Membrane Ultrastructure”
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
In your green exercise books (using diagrams to illustrate) describe the
structure and properties of phospholipids.
Membrane Structure
The cell membrane (or plasma membrane) surrounds all living cells, and is the
cell's most important organelle. The plasma membrane:
 defines the boundaries of the cell
 governs its interactions with other cells
 and controls how substances can move in and out of the cell.
The membranes that surround the nucleus and other organelles are almost
identical to the cell membrane. Membranes are composed of phospholipids,
proteins and carbohydrates arranged in a fluid mosaic structure, as shown in
this diagram.
The phospholipids form a thin, flexible sheet, while the proteins "float" in the
phospholipid sheet like icebergs, and the carbohydrates extend out from the
proteins.
Phospholipid Structure
Each phospholipid is composed of a 3-carbon compound called glycerol. Two of
the glycerol carbons have fatty acids attached. The third carbon is attached to
a highly polar organic alcohol that includes a bond to a phosphate group. On the
other hand, because the organic alcohol with phosphate is highly polar, it is
soluble in water. This structure means that membranes have two distinct areas
when it comes to polarity and water solubility. Once area is water soluble and
polar, and is referred to as hydrophilic (water loving). This is the
phosphorylated alcohol side. The other area is not water soluble and is nonpolar. It is referred to as hydrophobic (water fearing).
Phospholipids and the Cell Membrane
The phospholipids are arranged in a bilayer, with their polar, hydrophilic
phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid
tails facing each other in the middle of the bilayer. This hydrophobic layer acts
as a barrier to all but the smallest molecules, effectively isolating the two sides
of the membrane. Different kinds of membranes can contain phospholipids with
different fatty acids, affecting the strength and flexibility of the membrane,
and animal cell membranes also contain cholesterol linking the fatty acids
together and so stabilising and strengthening the membrane. Because the fatty
acid ‘tails’ do not strongly attract one another, the membrane tends to be fluid
or flexible. This allows animal cells to have a variable shape and also allows the
process of endocytosis.
2.4.3 Membrane Proteins
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2.4.3 List the functions of membrane proteins
Orange book: pg. 26
Green book: pg.
To do:
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
 Visit the following website:
 Construction of the Cell Membrane
This is a short animation taking you through the basics of the cell membrane
structure. It illustrates some of the proteins found in the cell membrane
http://www.wisc-online.com/objects/index_tj.asp?objID=AP1101
Visit the following website:
Membrane Structure – Tutorial
This is an in-depth site on membrane structure – there are questions to
complete as you go through the site. Some of it can be left out as it is too
detailed, this is indicated below.
http://www.bio.davidson.edu/people/macampbell/111/membswf/membranes.swf
The summary will be completed in class from the board into your green exercise
books
Proteins
Proteins are a major component of cellular membranes and create extreme
diversity in membrane function. Proteins of various types are embedded in the
fluid matrix of the phospholipid bilayer. This creates the mosaic effect
referred to in the fluid mosaic model. There are usually two major types of
protein. Proteins which span from one side of the phospholipid bilayer to the
other are called integral proteins, whilst those that sit on one of the surfaces
are peripheral proteins. They can slide around the membrane very quickly and
collide with each other, but can never flip from one side to the other. Proteins
comprise about 50% of the mass of membranes, and are responsible for most of
the membrane's properties.
Integral Proteins
Integral proteins have both hydrophobic and hydrophilic regions in the same
protein. The hydrophobic region, with non-polar amino acids, is in the mide-
section of the phospholipid membrane, holding the protein in place. Their
hydrophilic region is exposed to the water solutions on either side of the
membrane.
Peripheral Proteins
Peripheral proteins do not protrude into the middle hydrophobic region, but
remain bound to the surface of the membrane. Often these peripheral proteins
are anchored to an integral protein.
Membrane Protein Functions
It is the membrane proteins that impart different functions to different
membranes. There are many different proteins but hey tend to have six general
functions:
 Receptors e.g. hormone binding sites





Enzymes e.g. metabolic pathways and contact digestion
Cell adhesion e.g. tight junctions between cells of intestine
Cell-to-cell communication and identity e.g. glycoproteins to recognize ‘self’
Channels e.g. for passive transport
Active transport e.g. sodium/ potassium ATPase
Receptors - proteins on the outside surface of cell membranes can act as
receptors by having a specific binding site where hormones or other chemicals
can bind. This binding causes a change in the shape of the protein which then
triggers other events in the cell. They may also be involved in cell signalling and
cell recognition.
Enzymes – enzymes embedded in the plasma membranes of cells carry out the
final stages of starch and protein digestion in the small intestine. They are
involved in breaking down hormones and neurotransmitters once their job is
done. Often the enzymes are grouped so that a sequence of metabolic reaction
(metabolic pathway) may occur.
Cell Adhesion – provided by proteins when they hook together in various ways to
provide permanent or temporary connections. These connections, referred to as
junctions, may include gap junctions or tight junctions.
Cell communication and identity markers – usually a glycoprotein to allow a cell
be recognized as ‘self’ and not ‘foreign’. It allows the body to recognize those
cells that belong and those cells that do not.
Channel proteins - proteins that span the membrane are usually involved in
transporting substances across the membrane. Materials will move from an area
of high concentration to an area of low concentration
Active Transport – proteins shuttle a substance from one side of the
membrane to another by changing shape. This process requires energy in the
form of ATP. It does not require a difference in concentration to occur.
Membrane Structure – Tutorial
This is an in-depth site on membrane structure – there are questions to
complete as you go through the site. Some of it can be left out as it is too
detailed, this is indicated below.
http://www.bio.davidson.edu/people/macampbell/111/membswf/membranes.swf
Remember - Certain aspects of this tutorial are very detailed. You only need
to pay particular attention to obtaining the correct answers to the following
questions. When you answer – read the information provided and then translate
into your own words as best as possible.
1. Fluid Mosaic Model
What are biological membranes mainly composed of?
How are membrane lipids organised?
How are the proteins arranged and what is their function?
Why are membranes described as being fluid?
Pop Quiz
……………………………………… are the primary determinants of membrane structure,
while …………………………………… carry out membrane function.
* Remember to click on the figure to simulate membrane fluidity.
2. Internal Membranes
Which type of cells contain membrane bound organelles?
List some organelles which are surrounded by membranes.
What is the function of the cell membrane?
What is the significance of having cellular structures with their own
membranes?
3. Membrane Functions
Describe each of the following membrane functions.
Signaling
Barrier
Transport
Localisation
Communication
4. Three Classes of Membrane Lipids
What are the general functions of lipids in living organism?
Name the three membrane lipids.
Describe the structure of phospholipid.
What is a glycolipid composed of?
What is the meaning of hydrophobic and which part of the phospholipid molecule
is hydrophobic?
What is the meaning of hydrophilic and which part of the phospholipid molecules
is hydrophilic?
Leave out the following two pages – they are too detailed:
5. Examples of Common Membrane Lipids
6. Examples of phosphoglycerides.
7. Self-assembly of a Lipid Bilayer
Describe the hydrophobic effect.
Explain why polar molecules are soluble in water and non-polar molecules are not.
8. Membrane Fluidity
Which two ways can membrane lipids move?
Explain why membrane fluidity can occur.
Leave out the following two pages – they are too detailed:
9. Fluidity-O-Meter
10.Membrane Asymmetry
11. Membrane Proteins
What is a peripheral membrane protein?
What is an integral membrane protein?
HW3 Membrane Questions
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2.4.1 Draw and label a diagram to show the structure of membranes
2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids
help to maintain the structure of cell membranes
2.4.3 List the functions of membrane proteins
??  
Some of the following questions may need a little bit of extra research as we
have not yet covered the section on Biological Molecules.
35 marks
Please type the answers in blue so it can be clearly seen.
1. The table below refers to components of the cell surface membrane (plasma
membrane) and to their roles in transporting substances across the membrane.
Complete the table by inserting an appropriate word or words in the empty
boxes. (6)
Component
Phospholipids
Subunits
Chemical bond
Role in
between
subunits
Transport
Fatty acids,
glycerol and
phosphates
Carbohydrate
side chain
Protein
Receptor
Peptide
2. The diagram below shows the structure of a molecule found in the cell
surface membrane.
a. Name the type of molecule shown in the diagram. (1)
b. Name A and B as labelled on the diagram. (2)
c. Region X is said to be hydrophobic.What is meant by the term
hydrophobic?(1)
d. Explain why the cell surface membrane is described as a fluid-mosaic. (2)
3. The diagram shows the structure of a phospholipid molecule,
a. Name the part of the molecule labelled Y. (1)
b. Describe how a phospholipid molecule differs in structure from a triglyceride
molecule. (1)
c. Chitin is a nitrogen-containing polysaccharide. Name one chemical element
present in a phospholipid which would not be present in chitin.(1)
4. An artificial membrane was made. It consisted only of a bilayer of
phospholipid molecules. In an investigation, the permeability of this artificial
membrane was compared with the permeability of a plasma membrane from a
cell. Explain why:
a. both membranes allowed lipid soluble molecules to pass through. (1)
b. only the plasma membrane allowed glucose to pass through.(2)
5. The following extract has been taken from a dictionary of biological terms.
cell membrane: a membrane found either on the outside of a cell or within it.
Cell membranes are extremely thin. They are only about 7 nm thick and so
cannot be seen with a light microscope. A transmission electron microscope
however shows a cell membrane consists of three lines forming a sandwich. The
two outer lines are dark in colour while there is a lighter one in between. As it is
impossible, even with an electron microscope, to see how the actual molecules
are arranged in a cell membrane, it is necessary to produce a model to explain
the membrane’s properties. The most accurate model of membrane structure
that has been developed is the fluid mosaic model and this can be used to
describe most of the properties of a cell membrane. Cell membranes play a very
important part in the biology of cells and they are particularly important in
regulating the movement of substances into and out of cells.
a. Describe the structure of a cell membrane. (5)
b. Describe two ways in which the appearance of a plant cell wall would differ
from a cell membrane when viewed with an electron microscope. (2)
c. Describe the part played by cell surface membranes in regulating the
movement of substances into and out of cells. (6)
d. Describe how the distribution of cell membranes in a prokaryotic cell such as
a bacterium differs from that in a cell from a plant leaf.(4)
2.4.4 Diffusion and Osmosis
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2.4.4 Define diffusion and osmosis
Orange book  pg. 27 & 31
Green book  pg. 23
To do:
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
Diffusion Websites:
Simple Diffusion Animation
http://www.wisc-online.com/objects/index_tj.asp?objID=AP1903
Diffusion Animation
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusio
n_works.html
Watch the Brainpop animations:
“Diffusion: Matter Likes to Mix”
“Passive Transport: Getting Stuff in and out of Cells, the easy way”.
Osmosis Websites
Osmosis Animation
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis
_works.html
Hypo or Hyper Tonic
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm
Brief introduction to osmosis and includes isotonic, hypertonic and hypotonic.
http://www.wisc-online.com/objects/index_tj.asp?objID=AP11003
Plasmolysis Illustrated
http://ccollege.hccs.edu/instru/Biology/AllStudyPages/Diffusion_Osmosis/Elod
eagif.swf
Summary in your green exercise books to consisting of definitions for:
Diffusion
Osmosis
Passive and Active Transport
There are two general means of cellular transport:
• Passive transport
• Active transport
Passive transport does not require energy (in the form of ATP), but active
transport does. Passive transport occurs in situations where there are areas of
different concentration of a particular substance. Movement of the substance
occurs from an area of high concentration to an area of lower concentration.
Movement is said to occur along a concentration gradient.
When active transport occurs, the substance is moved against a concentration
gradient, so energy expenditure must occur.
Simple Diffusion (lipid diffusion)
A few substances can diffuse directly through the lipid bilayer part of the
membrane. The only substances that can do this are lipid-soluble molecules
such as steroids, or very small molecules, such as H2O, O2 and CO2. For these
molecules the membrane is no barrier at all. Since lipid diffusion is a passive
process, no energy is involved and substances can only move down their
concentration gradient. Lipid diffusion cannot be controlled by the cell, in the
sense of being switched on or off.
Diffusion is the movement of molecules from a high concentration to a low
concentration down their concentration gradient.
Osmosis
Osmosis is the diffusion of water across a membrane. It is in fact just normal
lipid diffusion, but since water is so important and so abundant in cells, the
diffusion of water has its own name - osmosis. The contents of cells are
essentially solutions of numerous different solutes, and the more concentrated
the solution, the more solute molecules there are in a given volume, so the fewer
water molecules there are. Water molecules can diffuse freely across a
membrane, but always down their concentration gradient, so water therefore
diffuses from a dilute to a concentrated solution.
Osmosis is the diffusion of water from a high concentration of water
molecules to a lower concentration of water molecules, down their
concentration gradient through a semi-permeable membrane.
Cells and Osmosis
The concentration (or water potential) of the solution that surrounds a cell will
affect the state of the cell, due to osmosis. There are three possible
concentrations of solution to consider:
Isotonic solution
a solution of equal water potential (or
concentration) to a cell
Hypertonic solution
a solution of more negative water potential (more
concentrated) than a cell
Hypotonic solution
a solution of less negative water potential (less
concentrated) than a cell
The effects of these solutions on cells are shown in the diagram:
These are problems that living cells face all the time. For example:
Simple animal cells (protozoans) in fresh water habitats are surrounded by a
hypotonic solution and constantly need to expel water using contractile vacuoles
to prevent swelling and lysis.
Cells in marine environments are surrounded by a hypertonic solution, and must
actively pump ions into their cells to reduce their water potential and so reduce
water loss by osmosis.
Young non-woody plants rely on cell turgor for their support, and without enough
water they wilt. Plants take up water through their root hair cells by osmosis,
and must actively pump ions into their cells to keep them hypertonic compared
to the soil. This is particularly difficult for plants rooted in salt water.
 Interactive Activity
Osmosis Animation
2.4.5 Simple Diffusion
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2.4.5 Explain passive transport across membranes by simple diffusion and
facilitated diffusion
Orange book  pg. 28-30
Green book  pg. 23
To do:
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
Simple Diffusion Animation
http://www.wisc-online.com/objects/index_tj.asp?objID=AP1903
*If you check the above site at home it should load.
Diffusion Animation
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusio
n_works.html
Facilitated Diffusion Animation
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitat
ed_diffusion_works.html
Summarise in your green exercise books the similarities and differences
between simple and facilitated diffusion.
Simple diffusion (lipid diffusion)
A few substances can diffuse directly through the lipid bilayer part of the
membrane. The only substances that can do this are lipid-soluble molecules
such as steroids, or very small molecules, such as H2O, O2 and CO2. For these
molecules the membrane is no barrier at all. Since lipid diffusion is (obviously) a
passive diffusion process, no energy is involved and substances can only move
down their concentration gradient. Lipid diffusion cannot be controlled by the
cell, in the sense of being switched on or off.
Diffusion is the movement of molecules from a high concentration to a low
concentration down their concentration gradient.
Passive Transport (Facilitated Diffusion)
Passive transport is the transport of substances across a membrane by a transmembrane protein molecule. The transport proteins tend to be specific for one
molecule (a bit like enzymes), so substances can only cross a membrane if it
contains the appropriate protein. As the name suggests, this is a passive
diffusion process, so no energy is involved and substances can only move down
their concentration gradient. There are two kinds of transport protein:
Channel Proteins form a water-filled pore or channel in the membrane. This
allows charged substances (usually ions) to diffuse across membranes. Most
channels can be gated (opened or closed), allowing the cell to control the entry
and exit of ions.
Carrier Proteins have a binding site for a specific solute and constantly flip
between two states so that the site is alternately open to opposite sides of the
membrane. The substance will bind on the side where it at a high concentration
and be released where it is at a low concentration.
2.4.6 Active Transport
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2.4.6 Explain the role of protein pumps and ATP in active transport across
membranes
Orange book  pg. 33
Green book  pg. 23
To do:
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
Active Transport Animation
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sod
ium_potassium_pump_works.html
Give a concise definition of active transport in your green exercise book.
Active Transport (or pumping)
Active transport is the pumping of substances across a membrane by a transmembrane protein pump molecule. The protein binds a molecule of the
substance to be transported on one side of the membrane, changes shape, and
releases it on the other side. The proteins are highly specific, so there is a
different protein pump for each molecule to be transported. The protein pumps
are also ATPase enzymes, since they catalye the splitting of ATP  ADP +
phosphate (Pi), and use the energy released to change shape and pump the
molecule. Pumping is therefore an active process, and is the only transport
mechanism that can transport substances up their concentration gradient.
The rate of diffusion of a substance across a membrane increases as its
concentration gradient increases, but whereas lipid diffusion shows a linear
relationship, facilitated diffusion has a curved relationship with a maximum
rate. This is due to the rate being limited by the number of transport proteins.
The rate of active transport also increases with concentration gradient, but
most importantly it has a high rate even when there is no concentration
difference across the membrane. Active transport stops if cellular respiration
stops, since there is no energy.
The sodium-potassium pump (Na+/K+ATPase) is a very common pump. At this
stage you do not need to learn the details of ion exchange, it is more to
demonstrate the use of energy to bring about a conformational change.
1. A specific protein binds to three intracellular sodium ions
2. Binding of sodium ions causes phosphorylation by ATP
3. The phosphorylation causes the protein to change its shape, thus expelling
sodium ions to the exterior
4. Two extracellular potassium ions bind to different regions of the protein and
this causes the release of the phosphate group
5. Loss of the phosphate group restores the protein’s original shape thus
causing the release fo the potassium ions into the intracellular space.
2.4.7 Endocytosis and Exocytosis
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2.4.7 Explain how vesicles are used to transport materials within a cell between
the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.
Orange book  pg. 34
Green book  pg. 24
To do:
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
Endocytosis and Exocytosis Animation
Quick and simple animation to illustrate endocytosis and exocytosis.
http://www.wisc-online.com/objects/index_tj.asp?objID=AP11203
Endocytosis and Exocytosis Animation
Animation with audio explanation.
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html#
Give a concise definition of endocytosis and exocytosis in your green exercise
book.
Vesicles
The processes described so far only apply to small molecules. Large molecules
(such as proteins, polysaccharides and nucleotides) and even whole cells are
moved in and out of cells by using membrane vesicles.
Endocytosis is the transport of materials into a cell. Materials are enclosed by
a fold of the cell membrane, which then pinches shut to form a closed vesicle.
Strictly speaking the material has not yet crossed the membrane, so it is usually
digested and the small product molecules are absorbed by the methods above.
When the materials and the vesicles are small (such as a protein molecule) the
process is known as pinocytosis (cell drinking), and if the materials are large
(such as a white blood cell ingesting a bacterial cell) the process is known as
phagocytosis (cell eating).
Exocytosis is the transport of materials out of a cell. It is the exact reverse
of endocytosis. Materials to be exported must first be enclosed in a membrane
vesicle. Hormones and digestive enzymes are secreted by exocytosis from the
secretory cells of the intestine and endocrine glands. Sometimes materials can
pass straight through cells without ever making contact with the cytoplasm by
being taken in by endocytosis at one end of a cell and passing out by exocytosis
at the other end. Exocytosis usually begins in the ribosomes of the RER and
progresses through a series of four steps until the produced substance is
secreted to the environment outside the cell.
1. Protein produced by the ribosome of the rough ER enters the lumen of the
ER
2. Protein exits the ER and enters the cis side or face of the Golgi apparatus; a
vesicle is involved.
3. As the protein moves through the Golgi apparatus, it is modified and exits on
the trans face inside a vesicle.
4. The vesicle with the modified protein inside moves to and fuses with the
plasma membrane – this results in the secretion of the contents from the
cell.
2.4.8 Membrane Fluidity
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2.4.8 Describe how the fluidity of the membrane allows it to change shape,
break and re-form during endocytosis and exocytosis.
Orange book  pg. 34
Green book  pg. 25
To do:
 Read the relevant sections in the green textbook.
 Read and highlight the key words in the text below.
In you green exercise books:
describe the structure of membranes which lead to fluidity.
Describe the role of cholesterol in membrane fluidity.
Dancing membrane animation 
http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane%20fluidity
.swf
Summary
Takes you through simple diffusion, facilitated diffusion and osmosis.
http://programs.northlandcollege.edu/biology/Biology1111/animations/transp
ort1.html
Membranes are fluid structures, rather like cooking oil, because most of the
membrane lipids and many of the membrane proteins easily rotate and move
sideways in their own half of the bilayer.
Membrane fluidity depends on both the number of double bonds in the fatty
acid tails of the lipids that make up the bilayer and on the amount of
cholesterol present. Each double bond puts a ‘kink’ in the fatty acid tail, which
increases membrane fluidity because it prevents lipid molecules from packing
tightly in the membrane. More cholesterol also increases membrane fluidity.
Due to the fluidity of membrane lipids, the lipid bilayer self-seals if it is torn or
punctured. When a needle is pushed through the cell membrane and pulled out,
the puncture site seals spontaneously, and the cell does not burst. This selfsealing also happens during exo- and endocytosis.
Summary of Membrane Transport
HW10 – Essay (25 marks)
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Title: Transport across the cell membrane (300 words).
Describe the role of simple diffusion, facilitated diffusion, osmosis, active
transport and endo/exocytosis in the transport of substances across the cell
membrane.
For each you should include:
A detailed description of the process
An example of the process.
You can use this space to plan your essay however the essay is to be hand
written and submitted for marking.
HW11 – 2.4 Revision Questions
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Revision Questions
* Answer in your green exercise books NOT on laptop
1. The diagram below shows part of a cell surface membrane.
a. Name the molecules labelled A and B. (2)
b. Cell surface membranes can be broken into small pieces. These small pieces
curl in on themselves to form membrane-bound spheres. These spheres are
filled with liquid and are known as vesicles.
An experiment was carried out into the movement of sodium ions across the
membranes of these vesicles. The vesicles were immersed in a solution of
sodium chloride. The concentration of sodium ions in the vesicles was measured
over a period of five minutes.
The procedure was then repeated with ATP added to the sodium chloride
solution.
During the experiment the temperature was kept constant at 23 °C. The results
are shown in the graph below.
i. Compare the uptake of sodium ions by the vesicles with and without ATP. (3)
ii. What do these results suggest about the mechanisms of transport of sodium
ions across these membranes? Explain your answer. (4)
c. State why the temperature was kept constant during this experiment. (1)
2. An experiment was carried out with cells of carrot tissue to determine the
effect of temperature on the absorption of potassium ions.
Slices of carrot tissue were immersed in a potassium chloride solution of known
concentration. The changes in concentration of potassium ions in the solution
were determined at intervals for 6 hours. From these measurements, the mass
of potassium ions taken in by the carrot cells was found. The experiment was
carried out at 2°C and 20°C. The solutions were aerated continuously.
The results are shown in the graph below. Absorption of potassium ions is given
as micrograms of potassium per gram of fresh mass of carrot tissue (μg g–1).
During the first hour, some of the potassium ions enter the cells by diffusion.
a. State two conditions which are necessary for a substance to enter a cell by
diffusion. (2)
b. Calculate the mean rate of absorption of potassium ions at 20°C, between 2
and 6 hours. Show your working. (3)
c. Compare the rates of absorption of potassium ions at 2°C and 20°C during
this experiment. (3)
d. Suggest an explanation for the differences in the rates of absorption of
potassium ions at the two temperatures. (3)
3. Read through the following passage on the cell surface membrane, then write
on the dotted lines the most appropriate word or words to complete the
passage. (5)
All cells are surrounded by a cell surface membrane. This is made up of
....................................................... arranged as a bilayer. Embedded in this bilayer
are ......................................................., some of which act as carriers. Some
molecules in the cell surface membrane have glycoside side chains made up of
........................................... sub-units. These side chains act as
................................................. and enable the cell to form vesicles around
extracellular substances, part of the process known as
.................................................
4. The table below refers to four membrane transport processes: diffusion,
facilitated diffusion, osmosis and active transport. If the statement is correct,
place a tick ( ) in the appropriate box and if the statement is incorrect, place a
cross (X) in the appropriate box. (4)
Process
Takes place against a
concentration
gradient
Diffusion
Facilitated
diffusion
Osmosis
Active transport
Requires energy in the
form of ATP
5. Experiments were carried out to investigate the uptake of mineral ions by
barley roots.
In the first investigation, isolated barley roots were immersed in an aerated
culture solution containing potassium ions (K+) and nitrate ions (NO3–). After
ten hours, the roots were removed and the concentrations of these ions in the
cell sap were determined. The results are shown in the table.
Ion
Concentration in
culture
Concentration in
cell
solution / mmol per
dm3
sap / mmol per
dm3
Potassium
7.98
97.8
Nitrate
7.29
38.1
a. Suggest why the culture solution was aerated. (2)
b. These results show that the concentration of potassium ions in the cell sap is
12.3 times greater than that in the culture solution. This is referred to as the
accumulation ratio. Calculate the accumulation ratio for nitrate ions. Show your
working. (2)
c. What do these results suggest about the mechanism for the uptake of
potassium and nitrate ions? Explain your answer. (2)
In a further experiment, the effect of temperature on the uptake of potassium
ions was investigated. Isolated barley roots were kept in aerated nutrient
solutions at a range of temperatures, and the concentrations of potassium ions
in the cell sap were measured after ten hours. The results are shown in the
table below.
Temperature / °C
Concentration of potassium
ions in cell sap /mmol per dm3
6
12
18
24
30
35
42
70
95
110
d. What effect does temperature have on the concentration of potassium ions
in the cell sap? (2)
e. Suggest an explanation for these results. (2)
6. The table shows some similarities and some differences between osmosis,
active transport and facilitated diffusion. Complete the table with a tick if the
feature applies or with a cross if it does not apply. (3)
Feature
Osmosis
Active
transport
Facilitated
diffusion
Requires energy
from ATP
Requires protein
carrier molecules
Can take place
against a
concentration
gradient
7. An investigation was carried out into the effects of temperature on the rate
of diffusion of chloride ions. Slices of carrot were placed in distilled water at
different temperatures. After 10 minutes the concentration of chloride ions in
the distilled water was measured. The results are shown in the graph.
a. Explain the increase in the rate of diffusion between 14°C and 40°C. (2)
b. Suggest what caused the sudden increase in the rate of diffusion at 40°C. (2)
8. The diagram represents part of an animal cell which has been put in distilled
water.
Use the diagram to:
a. explain why the water potential of the distilled water is higher than the
water potential of the cytoplasm of the cell. (2)
b. describe the property of the cell surface membrane which allows osmosis to
take place. (1)
c. Osmosis has been described as a special case of diffusion. Describe two ways
in which you would expect the movement of water into a cell by osmosis to be
similar to the diffusion of oxygen into a cell. (2)
9. The graph shows the expected and actual results of an experiment to
investigate the uptake of glucose by human red blood cells.
Curve B shows the result that would be expected if glucose enters the red
blood cells by simple diffusion.
Curve A shows the results obtained from the red blood cells. It shows that
these cells took up glucose by facilitated diffusion.
Explain the shape of the curve at glucose concentrations:
a. less than 2 mmol dm–3; (2)
b. greater than 5 mmol dm–3. (1)
10. A kidney consists of a large number of very small tubes called kidney
tubules. Some of the cells which line these tubules are able to absorb glucose.
The diagram shows how these cells absorb glucose from the contents of the
tubule and secrete it into the blood.
a. Glucose moves into the cell by facilitated diffusion. Osmosis also takes place
across the plasma membrane. Give two differences between facilitated
diffusion and osmosis. (2)
b. Explain the link between active transport and the presence of large numbers
of the organelles labelled A in this cell. (3)
c. Explain two ways, shown in the diagram, in which the structure and activities
of this cell ensure efficient absorption of glucose from the inside of the kidney
tubule. (2)
d. Explain what is meant by the following terms.
i. Osmosis (3)
ii. Facilitated diffusion (3)