The cell-surface membrane
Chapter 4
Section 4.1
Phospholipids recap:
Describe the structure of a phospholipid.
Similar in structure to a lipid, but
one of the fatty acid molecules is
replaced with a phosphate
molecule
Describe the properties of a phospholipid.
Phosphate molecules are hydrophilic and attract water.
Fatty acids molecules are hydrophobic.
A phospholipid is a polar molecule (it has 2 ends that behave differently). In an
aqueous environment they position themselves so that the hydrophilic end is close
to water and the hydrophobic end is as far away from water as possible.
The cell surface membrane
Learning Objective:
In order to be successful in this lesson you must be able to:
relate the structure of the membrane to
its role around/inside cells.
The Cell Surface Membrane
Describe the
arrangement
of proteins,
glycoproteins,
glycolipids,
phosphilipds
and
cholesterol in
the fluid
mosaic model
of
membranes
explain the
roles/importance
of the constituent
parts of the
membrane
PROGRESS
relate the structure
of the membrane to
its role
around/inside cells.
Cells have many plasma membranes:
plasma membrane
tonoplast
outer mitochondrial membrane
inner mitochondrial membrane
outer chloroplast membrane
nuclear envelope
Plasma membranes allow cellular compartments
to have different conditions
pH 4.8
Contains digestive
enzymes, optimum
pH 4.5 - 4.8
lysosome
Membrane acts as
a barrier
pH 7.2
cytosol
Plasma membranes are flexible and able to break
and fuse easily
Neutrophil engulfing
anthrax bacteria.
Cover credit:
Micrograph by Volker
Brinkmann, PLoS Pathogens
Vol. 1(3) Nov. 2005.
5 μm
The cell surface membrane
All membranes within cells have the same basic structure and
are known as plasma membranes.
The cell surface membrane is the name given to the plasma
membrane that surrounds the cell.
Functions of cell surface
membranes
Question: Explain why phospholipids form a
bilayer in plasma membranes (4).
• Phospholipids have a polar phosphate group which are
hydrophilic and will face the aqueous solutions
I will ask this question
after
• The fatty acid tails are hydrophobic
and will
move away from an
the next 3 slides
aqueous environment
• As both tissue fluid and cytoplasm is aqueous
• phospholipids form two
layers
with
the hydrophobic tails facing
Click
to reveal
answers
inward
• and hydrophilic heads outwards interacting with the aqueous
environment
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Describe the arrangement of proteins, glycoproteins, glycolipids, phosphilipds and
cholesterol in the fluid mosaic model of membranes
Membranes are mainly made of phospholipids
phosphate group
hydrophilic
head
phosphoester bond
glycerol
ester bond
fatty acid
hydrophobic
tail
Describe the arrangement of proteins, glycoproteins, glycolipids, phosphilipds and
cholesterol in the fluid mosaic model of membranes
The hydrophilic heads are attracted by water and the
hydrophobic heads are repelled by it.
Hydrophobic (water-hating) tail
air
aqueous solution
Hydrophilic (water-loving) head
Phospholipids form
micelles when
submerged in water
Describe the arrangement of proteins, glycoproteins, glycolipids, phosphilipds and
cholesterol in the fluid mosaic model of membranes
In 1925 Gorter and Grendel proposed that the unit
membrane is formed from a phospholipid bilayer
Extracellular space (aqueous)
Phosphate heads
face aqueous
solution
phospholipid
bilayer
Cytosoplasm (aqueous)
Hydrophobic tails
face inwards
Question: Explain why phospholipids form a
bilayer in plasma membranes (4).
• Phospholipids have a polar phosphate group which are
hydrophilic and will face the aqueous solutions
• The fatty acid tails are hydrophobic and will move away from an
aqueous environment
• As both tissue fluid and cytoplasm is aqueous
• phospholipids form two
layers
with
the hydrophobic tails facing
Click
to reveal
answers
inward
• and hydrophilic heads outwards interacting with the aqueous
environment
Click here to hide answers
explain the roles/importance of the constituent parts of the membrane
Function of the phospholipid bilayer
• To allow lipid soluble substances to enter and leave the cell
• Forms a barrier to prevent water-soluble substances entering
and leaving the cell
• To make the membrane flexible and self sealing so that it can
form vesicles or fuse with other membranes.
Describe the arrangement of proteins, glycoproteins, glycolipids, phosphilipds and
cholesterol in the fluid mosaic model of membranes
Initial studies showed that the plasma membrane had
layers:
Scientists also found that protein were present in membranes so
Davson-Danielli proposed in 1935 the following model for
membrane structure:
Protein
Phospholipid
bilayer
The development and use of electron microscopes
showed that the Davson-Danielli model was incorrect
Linked to Cells unit
In the early 1970s Singer and Nicholson used techniques such as
freeze-etching to confirm the lipid bilayer.
They also showed that the proteins were not just layered on the
top but were evenly distributed throughout the protein in a mosaic
pattern.
In addition they found that the membrane was fluid and had
considerable sideways movement of molecules within it.
Hence they proposed the Fluid-Mosaic Model for Plasma
Membrane Structure.
Describe the arrangement of proteins,
explain the roles/importance of the
glycoproteins, glycolipids, phosphilipds and
constituent parts of the membrane
cholesterol in the fluid mosaic model of membranes
The fluid mosaic model of the plasma membrane:
The proteins can move freely through the lipid bilayer.
In which 2 ways are proteins arranged within the membrane? What is the function of
each type ?
Describe the arrangement of proteins, glycoproteins, glycolipids, phosphilipds and
cholesterol in the fluid mosaic model of membranes
The membrane contains many types of protein:
carbohydrate chain
Glycoprotein: For cell
recognition so cells group
together to form tissues
protein
extrinsic protein
Carrier
protein
intrinsic protein
channel protein
hydrophilic channel
explain the roles/importance of the constituent parts of the membrane
Proteins
Proteins are interspersed throughout the cell surface membrane.
They are embedded in the phospholipid bilayer in two main ways:
Some proteins occur on the surface of the phospholipid bilayer and never
completely cross it,
•
Function of these is to give mechanical support or in conjunction with
glycolipids to act as cell receptors for molecules such as hormone.
Other proteins span the phospholipid bilayer from one side to the other.
•
Some are protein channels which form water filled tubes to allow water
soluble ions to diffuse across
•
Others are carrier proteins that bind to ions or molecules like glucose
and amino acids, then change shape to move these molecules across the
membrane.
explain the roles/importance of the constituent parts of the membrane
The functions of proteins in the membranes
• Provide structural support
• Act as channels transporting water-soluble substances across the
membrane
• Allows active transport across the membrane through carrier proteins
• Forms cell-surface receptors for identifying the cells
• Helps cells adhere together to form tissues
• Acts as receptors for other molecules such as hormones
Cholesterol, Glycolpids and Glycoproteins
Complete your table to describe the arrangement of glycolipids,
glycoproteins and cholesterol
Cholesterol
Cholesterol molecules are also found within the phospholipid
bilayer of the cell surface membrane adding strength to the
membrane.
They are very hydrophobic and therefore play an important role in
preventing the loss of water and dissolved ions from the cell.
They also pull together the fatty acid tails of the phospholipid
molecules, limiting their movement and that of other
molecules, but without making the membranes as a whole to
rigid.
Question: Describe the function of cholesterol
molecules in the cell surface membrane(4)
• They add strength to the membranes,
• Reduce lateral movement of other molecules including the
phospholipids
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• Make the membranes Click
lesstofluid
at high temperatures
• Prevent leakage of water and dissolved ions from the cell.
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Glycolipids
Glycolipids are made of a
carbohydrate covalently
bonded with a lipid.
The carbohydrate portion
extends into the watery
environment outside the
cell where it acts as a
cell-surface receptor for
specific chemicals.
Question: What is the function of glycolipids in the
cell surface membrane? (3).
Glycolipids
• Acts as recognition sites
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• Helps maintain the stability
of the membrane
• Helps cells attach to one another to form tissues
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Glycoproteins
Carbohydrate chains are attached to many extrinsic proteins on
the outer surface of the cell membrane.
These act as cell-surface receptors, more specifically for
hormones and neurotransmitters
Question: Describe the structure and function of
the glycoprotein (4)
• Consists of an extrinsic protein located on the outer surface of
the cell membrane with added carbohydrate chains
• Used for cell recognition, for example, lymphocytes can
recognise an organisms own cells,
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answers
• Used as receptors for Click
hormones
neurotransmitters
• Helps cells to attach to one another to form tissues
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Permeability of the cell-surface membrane
The cell surface membrane controls the movement of substances
into and out of the cell. In general most molecules do not
diffuse freely across it. Why?
https://www.youtube.com/watch?v=5CXXuo7CgOc
Question: Why can some substances not diffuse
freely across the cell surface membrane? (4)
• Some substances are not soluble in lipids
• Some are too large to pass through the protein channels
• Some are of the same charge as the protein in the membrane
and are therefore repelled
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• Some are electrically charged (polar) and have difficulty
passing through the non-polar hydrophobic tails in the
phosolipid bilayer.
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The fluid mosaic model of the plasma
membrane:
The proteins can move freely through the lipid bilayer.
Question: Explain why the model for membrane
structure is known as the fluid mosaic model (3).
• The phospholipid molecules can move freely laterally and
makes the membrane fluid.
• The proteins are distributed throughout the membrane un
evenly
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• Both together form a mosaic pattern.
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Transport of molecules across the cell
membrane:
http://www.sumanasinc.com/webcontent/animations/content/diffu
sion.html
http://highered.mheducation.com/sites/0072495855/student_view
0/chapter2/animation__how_facilitated_diffusion_works.html