Download Cell membranes - Brian Whitworth

Cells
Topic 6 Membranes, Cell
structure and Transport
Learning Objectives
After studying this topic you should be able to:
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 CEB Chapter 4, pages 60 and 61, 69-70
and Chapter 5, pages 74-87
 Mastering Biology, Chapters 4 and 5
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Membrane Structure
Cell Membrane
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Every cell is encircled by a membrane and most
cells contain an extensive intracellular membrane
system.
Membranes fence off the cell's interior from its
surroundings.
Membranes let in water, certain ions and
substrates and they excrete waste substances.
Without a membrane the cell contents would
diffuse into the surroundings, information
containing molecules would be lost and many
metabolic pathways would cease to work.
Identify (in drawings or micrographs) and describe the
structure and function of the cellular membrane.
Describe the differences between the following pairs of
terms: diffusion versus osmosis, passive transport versus active
transport, hypertonic versus hypotonic, endocytosis versus
exocytosis, phagocytosis versus pinocytosis.
Identify (in drawings or micrographs) and describe the
structure and function of the various components of
the cytoskeleton.
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The cell is highly organized with many
functional units or organelles inside. Most
of these units are limited by one or more
membranes.
To perform the functions of an organelle,
the membrane is specialized in that it
contains specific proteins and lipid
components that enable it to perform its
unique roles.
In essence membranes are essential for
the integrity and function of the cell.
The cell would die!
The Cell
Membrane
Cell membranes
What is their structure?
Cell Structure and Function
1
Cells
We don’t know
currently
The fluid mosaic model
Lipids arranged in bi-layer with
proteins embedded or
associated with them.
There are a number of hypotheses and
we will consider the one which is
currently accepted
The fluid mosaic model - more
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This proposes that the cell membrane is made up of 2
main layers – lipids and proteins.
The phospolipids form themselves into a bi-layer with
the water seeking ends facing out and the water hating
ends facing in.
The proteins are embedded in this layer but can move
around or flip over.
Special carrier molecules take important elements, like
ions, at the cell membrane, using energy supplied by the
cell and use the proteins that are embedded in the lipid
layer.
Fluid Mosaic Model
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The
membrance
is a complex
3d
Thecell
cell membrane
is a complex
3 d structure.
circular structure
Fluid Mosaic Model
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Cell Structure and Function
Sometimes the elements bind to the proteins, which flip
over, thus transporting the element into the cell.
Some proteins form a ‘pore’ through which the element
can pass from the outside to the inside of the cell
membrane.
The movement of the phospholipid and protein
components through the plasma membrane permits the
membrane to change shape.
This flexibility is crucial to many different types of cells.
In animal cells, cholesterol also contributes to
the fluidity of the plasma membrane.
Cholesterol is a small lipid molecule that nestles
among the hydrophobic tails of the
phospholipids in the interior of the membrane.
It prevents phospholipid molecules from
packing together too tightly and making the
membrane rigid.
2
Cells
Structure
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Composition of the cell membrane
Fluid-like composition…like soap bubbles
Composed of:
Lipids in a bi-layer – what is this?
 Proteins embedded in lipid layer (called transmembrane proteins)
 Proteins floating within the lipid sea (called
integral proteins)
 Proteins associated outside the lipid bi-layer
(peripheral proteins).
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The fluid mosaic model of a cell membrane
Peripheral Proteins
Phospholipids
Integral Proteins
Cell
Membrane
Transmembrane proteins
Membrane Lipids
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Composed largely of phospholipids
Phospholipids composed of glycerol and two fatty
acids + PO4 group
Phospholipids are polar molecules - they have a
charge.
Phospholipid Molecule
Model
phosphate (hydrophilic) – like water
glycerol
fatty acids (hydrophobic) – hate water
Membrane Lipids
The fluid mosaic model
form a Bi-layer
Outside layer
Inside Layer
Cell Structure and Function
3
Cells
What does the membrane
do? = Function
Transport Across The
Cell Membrane
Membrane Permeability
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Biological membranes are physical barriers, but
which allow small uncharged molecules to
pass…
lipid soluble molecules pass through
Big molecules and charged ones do NOT pass
through
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allows for different conditions between
inside and outside of cell
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subdivides cell into compartments with
different internal conditions
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allows release of substances from cell via
vesicle fusion with outer membrane:
Membrane Permeability
1.) lipid soluble solutes go through faster
2.) smaller molecules go faster
3) uncharged & weakly charged go faster
4) Channels or pores may also exist in the
membrane to allow transport
1
2
Its about concentration
The concentration of the solution, with
respect to other solutions is important
Types of solutions
Isotonic --- when both solutions have the same
concentration of dissolved substances
Hypertonic --- a solution with a higher
concentration of dissolved substances(solutes)
Hypotonic --- a solution with a lower
concentration of dissolved substances(solutes)
Cell Structure and Function
4
Cells
Passive Transport
Two types of transport
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Involves concentration gradients ONLY.
NO CELL ENERGY is used.
Passive and Active
Passive Transport
3 types
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Diffusion- simple movement from regions of
high concentration to low concentration.
Osmosis- diffusion of water across a semipermeable membrane.
Facilitated diffusion - protein transporters which
assist in diffusion.
Diffusion
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Movement generated by random motion of
particles.
Movement always from region of high
concentration to regions of low concentration.
Increased water pressure is caused by
water
moving to decrease a concentration gradient or
concentration difference between two areas.
Osmosis
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Cell Structure and Function
The movement of water across a semipermeable membrane.
Semi-permeable membrane – allows some
particles to cross the membrane, but not others.
Water moves from where water
concentration is high to where it is low in order
to equalize the concentration.
5
Cells
Osmosis
Cells in distilled water
What type of solution?
Hypotonic
Cells in a salt solution
What type of solution?
Hypertonic
Water in
Cells eventually
explode
Water out
Cells shrivel &
die
Think about it…
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Based on what you have learnt – why is it bad
to drink sea-water if you are thirsty (and lost at
sea)???
Transport Proteins
Facilitated Diffusion
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Transport proteins carry specific
molecules across the cell membrane
Movement is along a concentration
gradient (i.e. From higher to lower)
Each type of transport protein will carry only
one type of molecule.
This is how glucose is moved.
Cell Structure and Function
Move solutes faster across
membrane
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Highly specific to specific solutes
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Can be inhibited by drugs
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Also involved in ACTIVE
transport
6
Cells
Transport protein
Transport protein
Glucose
Concentration
gradient
Cell membrane
Glucose
Cell membrane
Concentration
gradient
Glucose binds to the
transport protein
The transport protein turns over and releases glucose
onto the inside of the cell, along the concentration gradient
Types of Protein Transporters:
Ion Channels
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Work by facilitated diffusion No E!
Deal with small molecules... ions
Open pores are “gated”- Can change shape.
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How? Do a diagram to show how you think this
might work. NOW
Important in cell communication
Transport protein
Concentration
gradient
Carrier molecule
Cell membrane
The carrier molecule binds to the transport protein, which
opens the pore allowing it to move through the cell membrane.
The pore closes once the carrier is inside the cell.
It is possible to stop the action of transport protein with drugs
which will block the pore.
Active Transport
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Cell Energy is used to move substances across
the cell membrane
The substances are moved against the
concentration gradient i.e. from where there is less
to where there is more.
Transport proteins
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Substances are moved molecule by
molecule.
It is similar to facilitated diffusion except that
cell energy (ATP) is used in the
process.
ATP = Adenosine Triphosphate
Cell Structure and Function
7
Cells
ACTIVE TRANSPORT
Salt ion
Transport protein
Concentration
gradient
Cell membrane
Energy is used
Transport
protein
Salt Ion
Concentration
gradient
Cell
membrane
Ion binds to the
transport protein
ENERGY
IS USED
The transport protein turns over and releases the ion
onto the inside of the cell, against the concentration gradient
Moving many large molecules at once
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Endocytosis
Transports macromolecules and large particles into the
cell.
Part of the membrane engulfs the particle and folds
inward to “bud off.”
The cell membrane envelopes the material
If material is liquid the process is called pinocytosis
‘cell drinking’
If material is solid the process is called phagocytosis
‘cell eating’
Exocytosis
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Material is packaged inside the cell and the
package fuses with the cell membrane while the
material goes out of the cell.
How Endocytosis works
Pseudopodia extend to engulf food
A food vacuole is formed
Pinocytosis works the same, but with no food, only liquid
How exocytosis works
Vacuole containing particles
is moved close
to the cell membrane
Fuses with the cell
membrane to expel
the particles
Cell Structure and Function
8
Cells
Sodium-Potassium Pump
Ion Channels
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Work fast: No conformational changes
needed
Not simple pores in membrane:
specific to different ions (Na, K, Ca...)
gates control opening
 Toxins, drugs may affect channels
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saxitoxin, tetrodotoxin
cystic fibrosis
Summary Questions
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What is the difference between diffusion and
osmosis?
What is the difference between active and
passive transport?
What is the difference between Hypotonic and
Hypertonic?
What is the difference between Endocytosis and
Exocytosis?
What is the difference between Phagocytosis and
Pinocytosis?
Key words – will be assessed!
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Diffusion
Osmosis
Selectively permeable
Passive Transport
Active Transport
Hypotonic
Hypertonic
Endocytosis
Exocytosis
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Phagocytosis
Pinocytosis
Homework
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Complete Bioflix study sheet: Membrane
transport (pge 35 study notes)
Complete Primary Functions of membrane
proteins table (pge 32 study notes)
Complete Topic 6 on Unit Assessment 1
Cell Structure and Function
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