Transcript Membrane structure and transport across membranes

Who AM I?
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I work in a factory but I do not have
any manufacturing skills.
I’m not management material and so I
work in an entry-level position.
I’m nearly always assigned to the
warehouse where I’m responsible for
shipping and receiving.
I’m quite outgoing and so I act as the
factory’s first point of contact with the
outside world.
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Part of my job description is as
receptionist- and so all incoming and
outgoing phone calls come through me.
I’m also the one responsible for
checking ID cards and only admitting
those who are welcome. Who am ??
I am the cell membranea.k.a plasma membrane.
But don’t confuse me with
blood plasma………… OK!
My Vital Statistics
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I surround the cytoplasm of ALL living cells
I am ultra thin, less than 0.01 um (~8nm)
(too thin to seen with the magnification and
resolution of the light microscope)
Neuron 1
Plasma
membranes
Neuron 2
Viewed with TEM
•Proteins with various roles are scattered throughout me(like a mosaic).
STRUCTURE
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I’m made of double layer (bilayer) of
special lipids called phospholipids.
In animal cells, cholesterol molecules are
scattered amongst the phospholipids
within me. They help to keep me together
and regulate my fluidity
Proteins with various roles are scattered
throughout me (like a mosaic)
I am not rigid, but rather quite flexible.
Phospholipid Bilayer with
Protein and cholesterol
Outside of cell
e.g. Tissue fluid
Transport Cholesterol
protein
Phospholipid
molecule
Phospholipid
bilayer
Inside of cell
Fatty acid
tails(hydrop
hobic)
Phosphate
head(hydro
philic)
e.g. Cytosol
Fluid Mosaic Model Vs
Sandwich Model
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Current Model is the fluid mosaic model
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Fluid: embedded molecules can move sideways
within membrane
Mosaic: scattered appearance of embedded
protein and cholesterol
(contrast this with the sandwich model on
next slide)
Previous Model:Sandwich Model
It was proposed that the structure of the cell membrane was two layers of
protein(like bread of a sandwich) with a layer of phospholipid in between
(like sandwich filling)
NOT THE CORRECT MODEL
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INTERESTING FACT:
All membranes
contain proteins and lipid. However, the
proportion of each varies depending on
the membrane. For example
Type of membrane
% protein
% lipid
“Myelin” membrane
18
76
Mitochondrial inner membrane
76
24
Plasma membranes of human
red blood cells
44
43
FUNCTION
Holding cell contents:
 To contain the cell contents and
separate them from the outside
environment
 In multicellular organisms the
immediate environment of their cells is
the tissue fluid in which it they bathed.
Cells Capillaries and Lymphatic
system
Some Basic Background
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Internal environment
External environment
Extracellular Fluid
Intracellular Fluid
FUNCTION
To control what substances enter and
leave the cell:
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A semi (partially /differentially/ selectively)
permeable boundary.
It allows some substances to pass across it easily but
not others.
FUNCTION
Cell recognition:
 A substance called an ANTIGEN
(usually composed of protein
and carbohydrate
{glycoprotein}) plays a key roll
in cell recognition.
 An organism's immune system
uses the antigens on the cell
surface to recognize “SELF"
cells from “NON SELF” cells.
FUNCTION
Cell Signal reception:
 Receptor proteins enable
specific signalling
molecules (hormones) to
bind to them
 Binding leads to a cellular
response
INTERESTING FACTS
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Cell membrane of most bacterial cells
contain no cholesterol and would not
manage to be held together if not
surrounded by a cell wall.
The plasma membrane is too thin to be
resolved with a light microscope. The
boundary of the cell is obvious however,
and we label this as the cell membrane’s
location
FUNCTION
To control what substances enter and
leave the cell:
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A semi (partially /differentially/ selectively)
permeable boundary.
It allows some substances to pass across it easily but
not others.
FUNCTION
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The plasma membrane is fully permeable
to:
 Small polar molecules like water, urea,
ether, glycerol and alcohol
Non polar molecules (Carbon dioxide,
Oxygen, fat soluble vitamins and steroids)
(A cell does not control the passage of these
substances across the membrane. They cross
the membrane passively, down their
individual concentration gradient)
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continued
The plasma membrane is impermeable to:
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Large molecules (e.g. glucose, amino acids) and
Charged molecules (e.g. ions)
These can only pass across the membrane
through transport proteins, either passively or
actively.
Thus the cell can control or regulate which of
these substances pass across the membrane
and in what quantities
Transport across membranes
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Diffusion
Substance that easily cross the cell membrane by
diffusion include:
Non Polar molecules
Carbon dioxide, oxygen, DDT(a poison),
steroids and fat soluble vitamins
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Tiny polar molecules:
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water(osmosis), ether, chloroform, alcohol
Diffusion
High [ ]n
Concentration
gradient
Low [ ]n
Facilitated Diffusion
Some material need to cross the membrane
via membrane proteins(carrier proteins or
channel proteins)
This type of transport is passive;
movement is along a concentration
gradient, and involves specific proteins
which change shape to move molecules
across
Facilitated Diffusion via channel
proteins
High [ ]n
Concentration
gradient
Low [ ]n
Facilitated Diffusion via carrier
proteins
n
High
[
]
High [ ]n
Concentration
Concentration
gradient
gradient
n
Low
[
]
Low [ ]n
Material that Cross the cell
membrane facilitated diffusion
(via transport proteins)
Larger hydrophilic substances:
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Amino acids
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Glucose
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ADP
For example: ADP into mitochondria, glucose
into red blood cells
Ions
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Na+
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ClIn such a case there will be a net movement of
the substance down its concentration gradient
This process is PASSIVE: NO ENERGY
EXPENDED BY THE CELL
Protein
channel
Carrier
Protein
Active transport (requires
the input of energy (ATP))
Transport of IONS
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Uptake of ions or large uncharged molecules
against a concentration gradient
occurs via specific carrier proteins and
requires the input of energy (ATP), which
must be produced by the cell.
This allows for a stable intracellular
environment to be maintained even in
extreme environmental conditions .e.g.
bacteria that live in very salty environments.
Active Transport via Carrier
Proteins
Direction
of
transport
Required
ATP
(energy)
Low [ ]n
Here ions are moved
against their
Concentration
gradient
High [ ]n
Examples
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Sodium across membrane of a neuron
Uptake of mineral ions by root hair cells
uptake of glucose by intestinal cells
Low [ ]n
Concentration
gradient
High [ ]n
Example of active transport of
ions: Sodium Potassium Pump
From Krogh, “Biology: A Guide to the Natural World, 2nd edition
Bulk transport of cell products
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Proteins are made at the ribosomes
They are transported through the channels
of the Endoplasmic reticulum
If they are to be exported they are
transported to the golgi body
Here proteins are modified and
packaged into vesicles
Vesicles move to the cell membrane
where they are released via exocytosis
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BULK TRANSPORT (I)
Exocytosis
Movement of materials out of the cell by
fusion of vesicles with the plasma membrane
Requires cell to expend energy(ATP)
Example - export or removal of wastes in
single-celled organisms
 Example – cells exporting proteins
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Exocytosis
Exocytosis
Exocytosis in action
Exocytosis
Another example of bulk
transport
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In animal cells, and other cells without a
cell wall, if too much water enters the cell,
the cell membrane will rupture and the cell
will be killed.
Organisms living in fresh water
environments have mechanisms to remove
excess water and prevent bursting:
ie: contractile vacuole
Function of Contractile vacuole in
paramecium
BULK TRANSPORT (II)
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Endocytosis
Bringing of large materials into the cell by
infolding of the plasma membrane to
form vesicles.
Requires cell to expend energy(ATP)
Pinocytosis: liquid is bought into a cell in a
vesicle
Phagocytosis: solid matter is brought into a cell
in a vesicle
Extension: Receptor mediated endocytosis:
 Uptake of molecules bound to receptors
 example- uptake of hormones bound to
receptor proteins.
Pinocytosis (“drinking” cell)
0.5 m
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extra cellular fluid and materials suspended in it
(water and solutes) are enclosed in invaginating
vesicle
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Used in digestive tract
Phagocytosis (“eating” cell)
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Brings large materials into a cell by wrapping extensions of
the plasma membrane around the materials and fusing the
extension together to form a vesicle/food vacuole
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How the human immune system ingests whole bacteria
or one-celled creatures eat
pseudopodia “false feet” – plasma membrane
extensions
Receptor-Mediated Endocytosis
0.25 m
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More specific with receptor binding molecules,
bringing them in and concentrating them into a
coated pit which forms into a vesicle
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The way insulin gets into your cells.
Analysing information : A scientist carried out an experiment to determine the time it took for a cell
to manufacture proteins from amino acids. The scientist provided the cell with radioactively labelled
amino acids and then tracked them through the cell to establish the time at which protein synthesis
commenced. He monitored the cell 5 minutes, 20 minutes and 40 minutes after production started in
order to track the proteins from the site of synthesis to a point in the cell from which they were
discharged from the cell.
The scientist made an image of the cell at each of these times but forgot to mark each image with its
correct time. The images are given in figure 2.32. Radioactivity is indicated by the green spots.
a) Which cell corresponds to each of the particular times of viewing? List the correct order according
to time of viewing.
b) On what grounds did you make your decision?
FUNCTION
Cell recognition:
 A substance called an ANTIGEN
(usually composed of protein
and carbohydrate
{glycoprotein}) plays a key roll
in cell recognition.
 An organism's immune system
uses the antigens on the cell
surface to recognize “SELF"
cells from “NON SELF” cells.
FUNCTION
Cell Signal reception:
 Receptor proteins enable
specific signalling
molecules (hormones) to
bind to them
 Binding leads to a cellular
response
INTERESTING FACTS


Cell membrane of most bacterial cells
contain no cholesterol and would not
manage to be held together if not
surrounded by a cell wall.
The plasma membrane is too thin to be
resolved with a light microscope. The
boundary of the cell is obvious however,
and we label this as the cell membrane’s
location