Biology AH
Extra notes Side 1
Membranes
Phospholipids are important components of all cell membranes.
Phospholipid Molecule
Simplified Drawing
Phospholipids are made up of two fatty acid chains joined to a polar phosphate containing head. The
head is hydrophilic (water loving) and the tail is hydrophobic (water hating).
Cut out the phospholipids on the next page and do the following with them:

On a piece of scrap paper draw a line across the middle. This line represents the surface of
some water. Imagine that you picked up a bunch of phospholipid molecules and dropped
them onto water. Position the phospholipids on the paper as you think they would arrange
themselves.

Imagine that you now stirred the water vigorously and all the phospholipids are submerged.
Position the phospholipids on the paper as you think they would arrange themselves (This
structure is known as a micelle).

Imagine if you had an unlimited number of phospholipids and they had to enclose an object
that contained water and then submerged in water. Position on the paper how you think the
phospholipids arrange themselves?

Look at the diagram of a phospholipid bilayer on side 3. Construct one of these on your sheet
of paper and label all the components.
Mrs Dow
Biology AH
Mrs Dow
Extra notes Side 2
Biology AH
Extra notes Side 3
2.4.1 Draw and label a diagram to show the structure of membranes.
http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/orient2.swf
http://wps.prenhall.com/esm_freeman_biosci_1/0,6452,498450-,00.html
All cells have a membrane at their boundary that acts as a selective barrier, thereby regulating the
cells chemical composition. This membrane is known as the plasma membrane. Membranes are
also found inside eukaryotic cells (remember prokaryotic cells do not have membrane bound
organelles) as part of membrane bound organelles. The plasma membrane and membranes found
within a cell are only about 7-10nm in diameter. This makes them extremely difficult to see in detail.
Many models were proposed by scientists and the presently accepted model is known as the fluid
mosaic model.
Mrs Dow
Biology AH
Extra notes Side 4
http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/img/fig8-5.jpg
Mrs Dow
Biology AH
Extra notes Side 5
Using information from the diagram:

Name the three biochemical groups that make up membranes.

Name a cellular organelle that possesses a membrane and describe the function the membrane has
in that organelle.

Name three other organelles made up from a system of membranes.

State the purpose of cholesterol in the membrane.

Suggest why organisms living in Polar regions have a high proportion of cholesterol in their
membrane.
Use the symbol for a
phospholipid shown below to
draw a simplified labeled
diagram.
Phospholipid
Mrs Dow
Biology AH
Extra notes Side 6
2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help
to maintain the structure of cell membranes.
http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/bil.swf dynamic nature of phospholipids
It is the hydrophilic nature of the heads and the hydrophobic nature of the tails that keeps the structure
of the bilayer. There are no strong bonds holding the phospholipids together. Instead they can float
around freely as long as their heads point out and their tails point in.
A phospholipid bilayer is very fluid – objects can move around in it very easily.
The membranes that are found surrounding and within cells are made up of a phospholipid bilayer
with objects floating around within the phospholipids.
Objects that are found within the phospholipid bilayer include proteins and cholesterol molecules. It
is often likened to a sea with icebergs floating in it.
The model that is used to describe the structure of membranes is known as the fluid mosaic model.
Why do you think that it is given this name?
Thinking about Science: Models of Membrane Structure
This fluid mosaic model was developed in the 1970’s by Singer- Nicholson. According to the model
the phospholipids and proteins are like pieces of a mosaic that can rotate and move sideways. They
cannot move from one half of the bilayer to the other. This model explains most, but not all of the
research findings about membranes. These findings do not support the model:




Some of the lipids in the membrane are grouped into rafts
Some proteins are associated with specific areas of phospholipids in the membrane and are
not free to move to other areas of the membrane
Some membrane proteins are anchored to other membrane proteins and are therefore not free
to move
Some membrane proteins which act as receptors are arranged in specific patterns
Discussion
In groups discuss the following points:



In this case, the word model is used to mean a type of hypothesis. What is the advantage of a
model rather than a written hypothesis
What should a scientist do after a model or hypothesis has been developed?
How should we treat this model in the light of findings that do not fit in with the model?
Mrs Dow
Biology AH
Extra notes Side 7
2.4.3 List the functions of membrane proteins.

Hormone Binding Sites
Some proteins in the membrane will act as sites for hormones to bind called receptor sites.
Name 3 Hormones:
For each of the hormones above, name a cell that would probably have a protein that acts as a receptor
site for that hormone.

Enzymes
Some proteins will act as enzymes. If you recall from IGCSE enzymes are molecules that are made
from protein. Enzymes are biological catalysts.
These enzymes which are fixed into the membrane are known as immobilized enzymes. Examples
may include enzymes immobilized into the membranes lining the small intestine. Many organelles
that have lots of chemical reactions taking place inside them will also have enzymes immobilized into
their membranes.
Name two organelles that you think will have proteins in their membranes that are acting as enzymes.

Cell Adhesion
Some proteins act to form tight junctions between cells in a tissue. These proteins form part of the
extracellular matrix.

Channels for passive transport
These allow hydrophilic particles to diffuse into and out of the cells by a process called facilitated
diffusion (this will be covered on the next few pages)

Pumps for Active Transport
These use energy in the form of ATP to move substances into or out of a cell against a concentration
gradient (moving from low to high).
Mrs Dow
Biology AH
Extra notes Side 8
The diagram below shows various proteins in a membrane. Discuss in pairs what you think each
protein is doing and label these onto the diagram.
2.4.4 Define diffusion and osmosis.
Diffusion is the movement of a substance from an area where it is present in a high concentration to
an area where it is present in a lower concentration. This is a form of passive transport, as it does
not require energy. Moving from a high concentration to a low concentration is known as moving
down the concentration gradient.
T:\Year groups\12\B\Syllabus 2007\ diffusion[1].swf
Define the term passive:
Why does diffusion occur? Where does the energy come for this process?
Mrs Dow
Biology AH
Extra notes Side 9
List two factors that you think will affect the rate of diffusion for each one explain why it will affect
the rate of diffusion.
Factor

Why it affects the rate of diffusion
Osmosis
Osmosis is the passive movement of water molecules from a region of lower solute concentration
to a region of higher solute concentration across a partially permeable membrane.
(A partially permeable membrane describes all cell membranes; it will allow small molecules such as
water to freely pass through but not larger molecules such as glucose)
Solute is the term used to describe a substance, which is dissolved in a solvent.
Give three examples of solutes that would be dissolved in the solvent water:
____________________
____________________
____________________
Osmosis is not the movement of water molecules from a region of higher concentration to a region of
lower concentration.
Mrs Dow
Biology AH
Extra notes Side 10
Look at the diagram below:
Which side of the membrane has the most water molecules?
What do the solute molecules do to the water molecules?
Which side has the highest concentration of water molecules that are free to move?
In which direction will osmosis take place?
Mrs Dow
Biology AH
Extra notes Side 11
2.4.5 Explain passive transport across membranes by simple diffusion and facilitated
diffusion.
Simple diffusion across membranes involves particles passing between the phospholipids in the
membrane. It can only happen if the phospholipid bilayer is permeable to the particles. If you
remember the fatty acid tails of a phospholipid molecule are hydrophobic, therefore the centre of the
phospholipid bilayer will be hydrophobic.
Label the hydrophobic part of the phospholipid membrane in the diagram above.
Because the centre of the phospholipid membrane is hydrophobic ions with positive and negative
charges cannot easily pass through. Polar molecules with very small charges can pass through but at
low rates. Small particles can pass through more easily than larger molecules. The diagram above
shows the simple diffusion of oxygen through a phospholipid bilayer.
For a typical animal cell the concentration of oxygen outside the cell is high.


Label the outside and inside of the cell on the diagram
Draw an arrow to indicate in which direction the oxygen molecules will diffuse.
Use one word to describe the movement of the oxygen molecules.
Explain why there is an arrow showing the movement of oxygen out of the cell.
Mrs Dow
Biology AH
Extra notes Side 12
A diagram to summarise simple diffusion across the phospholipid bilayer:
Large particles and ions have a problem moving across the phospholipid bilayer. Because of their
charge or size they carry they cannot enter the uncharged region of the membrane where all the
phospholipid tails are. Instead they must diffuse through special channel proteins or be transported
across the membrane by carrier proteins.
Watch the animation about channel proteins and then draw and label a diagram to explain diffusion
using channel proteins.
T:\Year groups\12\B\Syllabus 2007\ channel[1].swf
The diameter and chemical properties of the channel ensure that only one type of particle can pass
through, for example potassium ions or sodium ions but not both.
Mrs Dow
Biology AH
Extra notes Side 13
Watch the animation about carrier proteins and then draw and label a diagram to explain diffusion
using carrier proteins.
T:\Year groups\12\B\Syllabus 2007\caryprot[1].swf
This differs from the charge lined channel proteins because there is a conformational change in the
shape of the carrier protein. The channel proteins found in the membrane have a binding site which is
designed to fit onto a specific molecule. Once they have combined with the appropriate molecule
they flick it across to the other side of the membrane.
Note using channel or carrier proteins are different from simple diffusion because they involve the use
of a protein found in the membrane. The word ‘facilitate’ means to help.
Cells can control which type of channel proteins and carrier proteins are found in their membranes as
well as the number of each type. In this way cells are able to control which substances can diffuse
into and out of cells. The rate of diffusion for a certain type of molecule into or out of a cell can be
speeded up by increasing the number of carrier or channel proteins that cell has in its membrane.
Compare the processes of simple and facilitated diffusion. (Note the word compare means that you
can include both similarities and differences)
Mrs Dow
Extra notes Side 14
Biology AH
2.4.6 Explain the role of protein pumps and ATP in active transport across
membranes.
Diffusion alone cannot supply the cell with all of its requirements. Some molecules are required by
the cell in higher concentrations than occur outside the cell. Others like sodium must be removed
from the cell in order that fluid balance is maintained.
A difference in concentration of a substance between two areas is known as a concentration gradient.
When substances move from a high to a low concentration they are said to have moved down the
concentration gradient. Therefore in diffusion substances move down the concentration gradient.
High concentration
Substances move down the
concentration gradient
Active transport involves moving substances across a membrane up a concentration gradient, i.e. from
a low to a high concentration.
This process requires energy and is therefore an active process.
Energy is supplied in the form of ATP. ATP (adenosine triphosphate) is the molecule that supplies
energy to cells. It is made up of adenosine (a base and sugar) and three phosphate groups. When
ATP releases one of its phosphate groups it also releases energy, it is this energy that is used for
transporting substance across the membrane against a concentration gradient.
+ energy
Adenosine triphosphate
Mrs Dow
Adenosine diphosphate
Extra notes Side 15
Biology AH
Specific proteins found in the membrane called pumps carry out active transport. These proteins
harness the energy from the breakdown of ATP and use it to pump substances from a low to a high
concentration.
These proteins bind with their specific substances on one side of the membrane only, the protein then
changes shape or moves within the membrane to transport the substance across the membrane.
Watch the animation about active transport and then draw and label a diagram to explain active
transport using protein pumps.
T:\Year groups\12\B\Syllabus 2007\atpase[1].swf
Questions on Active Transport
1.
How does active transport
a.
Resemble
b.
Differ from facilitated diffusion?
2. The data in the table below show the relative uptake of glucose and xylose (a 5 carbon sugar)
from living intestine and from intestine that had been poisoned with cyanide. Cyanide greatly
reduces the availability of ATP. Discuss these data.
Relative rate of uptake by intestine
3.
Sugar
Without cyanide
With cyanide
Glucose
100
28
xylose
18
18
What factors will affect the rate of active transport?
Mrs Dow
Extra notes Side 16
Biology AH
4.
Roots were cut off from barley plants and were used to investigate phosphate absorption.
Roots were placed in phosphate solutions and air was bubbled through. The phosphate
concentration was the same in each case, but the percentage of oxygen and nitrogen was
varied in the air bubbled through. The rate of phosphate absorption was measured. The table
below shows the results.
Oxygen / %
Nitrogen / %
Phosphate absorption
/µmolg-1h-1
0.1
99.9
0.07
0.3
99.7
0.15
0.9
99.1
0.27
2.1
97.1
0.32
21.0
79.0
0.33
a.
Describe the effect of reducing oxygen concentration below 21.0% on the rate of phosphate
absorption by roots. You should only use information from the table to answer this question.
b.
Explain the effect of reducing the oxygen percentage from 21.0 to 0.1 on phosphate
absorption.
Mrs Dow
Biology AH
Extra notes Side 17
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.
Recap on active transport and intro to endo and exo T:\Year groups\12\B\Syllabus 2007\actandendo1049_300k[1].asx



The endoplasmic reticulum is a network of flattened sacs that originate from and is continuous
with the outer membrane of the nucleus.
Proteins are synthesized on ribosomes that are attached to the surface of the rough endoplasmic
reticulum
Many proteins synthesized on the rER need to have additional groups added or will need to be
packaged so that they can be sent to a certain area within the cell or out of the cell. The Golgi
apparatus does all these jobs.
T:\Year groups\12\B\Syllabus 2007\cytosis[1].swf
The transporting of proteins from the rER to the Golgi apparatus and then either within or out of the
cell takes place in membrane bound sacs called vesicles
T:\Year groups\12\B\Syllabus 2007\vesiclebudding[1].swf
Cells have a mechanism for moving packets of materials into and out of cells. This process is
different from diffusion and active transport that are concerned with moving single ions or molecules
into or out of cells.
To move materials out of the cells such as proteins, a membrane bound vesicle that can fuse with the
cell surface membrane and empty their contents out of the cell surrounds the proteins.
To move materials into the cell the cell surface membrane invaginates to enclose the material and then
that part of the membrane buds off to form a vesicle.
Mrs Dow
Biology AH
Extra notes Side 18
The breaking and reforming of the cell surface membrane requires energy therefore endocytosis and
exocytosis are _______________ processes.
Mrs Dow
Biology AH
Extra notes Side 19
Question on 2.4.7 and 2.4.8
The diagram below shows part of an animal cell and is based on a series of electron micrographs.
a.
Name the structures labeled A, B, C, D and E.
b.
Label with the letter F, a structure where the protein contents of D are synthesized.
c.
What is the part played by E in the synthesis of the protein?
d.
Name the process illustrated in the diagram, which results in large particles entering the cell.
e.
This process is common in some types of white blood cell. Suggest one reason why this cell
activity is important to the body.
f.
Name one group of organisms which feed using the process illustrated in the diagram.
Mrs Dow
Mrs Dow
Type of particles
that are moved?
Specific or nonspecific?
Down or up the
concentration
gradient?
Force driving
the process
Type of
membrane
molecule
involved
Feature
Simple Diffusion
Facilitated
Diffusion
Osmosis
Active
Transport
Transport Methods Across Cell Membranes - Summary
Endo /
Exocytosis
Biology AH
Extra notes Side 20
Biology AH
Summary Diagram for Cell Transport Processes
Mrs Dow
Extra notes Side 21
Biology AH
Extra notes Side 22
Task – In and out of cells
This task will involve reorganizing facts about the movement of substances into and out of a cell.
You have to cut out the facts and then sort them out and rewrite them into a logical account. One of
the aims of this task is to help you to practice writing an essay, concentrating on its structure and
quality of English without having to worry too much about the content. The idea is not to find a
correct sequence and copy out the sentences word for word, but to produce your own individual piece
of writing. You may not understand all the statements below and will have to use textbooks and other
reference material.
Information relevant to diffusion, osmosis, facilitated diffusion and active transport.
Active transport requires energy from cell metabolism.
All animal cells have sodium-potassium pumps in the plasma membrane
Gases and other small uncharged molecules diffuse across the plasma membrane.
Ions diffuse down an electrochemical gradient as well as down a diffusion gradient.
Ions do not easily pass through the hydrophobic part of the membrane.
Molecules that are too large to diffuse through the phospholipid bilayer may diffuse through protein
channels
Passive transport is movement of substances down a concentration gradient.
Small lipid soluble molecules such as steroid hormones diffuse through the phospholipid bilayer.
The change in the shape of the carrier protein causes the molecule to be taken through the protein to
the other side of the membrane.
The charge across the membrane affects the diffusion of ions.
The phospholipid bilayer is not very permeable to monosaccharides.
There are different proteins for different molecules or ions.
These methods transport individual molecules or ions into out of cells.
The sodium-potassium pump uses energy from ATP to transport potassium into and sodium out of the
cell.
To move a substance against a concentration gradient, a cell must use energy.
Carrier proteins change shape when the substance to be transported binds.
Carrier and channel proteins have a specific structure which gives the molecule a distinctive shape.
Carrier proteins have a specific binding site.
Proteins may form hydrophilic channels through the membrane.
Carrier and channel proteins span the membrane.
Water diffuses from a high solute to a low solute gradient.
Water is small enough to pass between the phospholipid molecules
The movement of water across a partially permeable membrane is known as osmosis
Mrs Dow
Biology AH
Extra notes Side 23
The sentences below give information relevant to endo and exocytosis
A bacterium is engulfed in a pocket of plasma membrane which then breaks off and forms a vacuole
with the bacterium inside.
A small pocket appears in the plasma membrane which is then pinched off to form a vesicle inside the
cell.
Endocytosis includes phagocytosis and pinocytosis
Large molecules such as proteins, are transported out of cells by exocytosis.
Macromolecules and larger particles, such as bacteria, are taken into the cell by endocytosis.
Pinocytosis is similar to phagocytosis, but pinocytosis takes in small droplets of the external solution,
forming vesicles.
The vesicle membrane fuses with the plasma membrane.
This is a means of moving substances in bulk into or out of the cell.
Vesicles containing proteins for export break away from the golgi apparatus.
Vesicles migrate to the plasma membrane.
Mrs Dow
Biology AH
Membranes
Phospholipids
Extra notes Side 24
IB Statements: 2.4.1 – 2.4.3, 2.4.8 Phospholipid Bilayer
Membranes consist of a _______________, with molecules
___________ in and _________________ to it.
___________________ pack together to avoid contact with water
___________________ orient towards water to form H-bonds
Membrane Stucture
Bilayer prevents passage of _________________ substances through
the membrane (e.g. ______________, _______________)
____________________ extend through the membrane to enable
these molecules to pass
Functions of Membrane Proteins
Membrane proteins have a variety of functions. These include





Fluidity of Membranes
Phospholipids in membrane are in a ____________ state.
Fluidity allows vesicles to be _______________ from membrane or
_______________ with it.
Mrs Dow
Extra notes Side 25
Biology AH
Transport Across Membranes
Passive Transport
IB Statements: 2.4.4 – 2.4.8
Osmosis
 Particles move ______________ conc. Gradient.
 Requires __________ energy
 Diffusion is the movement of particles from a region of
 Is the movement of ________________ from a region of
_____________________ to a region of __________________ across a
____________________________.
____________________ to a region of ______________ as the
result of ___________ motion of particles.
Active Transport
 Particles move ______________ conc. Gradient.
_________ Diffusion
____________ Diffusion
 __________ energy
1. Particle ______________ pump
Transport by Vesicles
2. Particle _____________ to _____________ site
3. Energy from _________ used to change _____________ of pump
4. Particle is ______________, then pump returns to original
_____________________
Mrs Dow