Transcript Cell Membrane and Transport

Cell Membrane Structure
and Function
Cell Membrane
• 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.
The cell would die!
Membrane Structure
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
Membrane
Cell membranes
What is their structure?
We don’t know currently
There are a number of hypotheses and
we will consider the one which is
currently accepted........
Structure - The Cell Membrane
The fluid mosaic model of membrane
structure
• The membrane is a mosaic (mixture) of
different protein molecules floating in a
bilayer (double layer) of phospholipids
• Each phospholipid has a hydrophilic (waterloving) head & hydrophobic (water-hating)
tails
• Because of this feature of phospholipids, the
lipid bilayers assemble themselves
spontaneously
The fluid mosaic model
Lipids arranged in bi-layer with
proteins embedded or associated
with them.
Phospholipid Molecule
Model
Draw This
phosphate (hydrophilic) – like water
glycerol
fatty acids (hydrophobic) – hate wate
Membrane Lipids
form a Bi-layer
Outside layer
Inside Layer
The fluid mosaic model
• This proposes that the cell membrane is made
up of 2 main layers – lipids and proteins.
• The lipids form themselves into a bi-layer with
the water seeking ends (hydrophilic) facing out
& the water hating ends (hydrophobic) facing in.
• The proteins are embedded in this layer but can
move around or flip over.
• Special carrier molecules take in 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.
Membrane 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 is known as fluidity.
• This flexibility is crucial to many different
types of cells.
The
cellmembrane
membrance
is a3complex
The cell
is a complex
d structure.
3d circular structure
Structure
• Fluid-like composition…like soap
bubbles
• Composed of:
– Lipids in a bi-layer – what is this?
– Proteins embedded in lipid layer (called
trans-membrane proteins)
– Proteins floating within the lipid sea (called
integral proteins)
– Proteins associated outside the lipid bilayer (peripheral proteins).
The fluid mosaic model
Composition of the cell
membrane
Peripheral Proteins
Phospholipids
Integral Proteins
Cell
Membrane
Trans – membrane proteins
Phospholipids
• Make up 75% of cell surface membrane
• Amphipathic – hydrophilic head (waterloving and polar) and hydrophobic tail (waterhating and non-polar)
• Phospholipid bilayer forms spontaneously
in aqueous environment eg. extra-cellular
fluid and cytosol
• Selectively permeable – small molecules
and lipid soluble molecules pass through
easily. Hydrophilic substances cannot diffuse
and rely on membrane pore/channels to pass
Cholesterol
• A steroid which makes up 20% of animal membranes
(but rarely found in plants) (in humans cholesterol is
present in almost same proportion as phospholipids)
• Also amphipathic – fit between phospholipids
• Important because it makes cell membrane more rigid
and prevents membrane being too fluid & breaking up
(37degrees = relatively high temp) = mechanical
stability
• Hydrophobic regions prevent ions & polar molecules
passing through – especially important in neurons
Glycolipids
• 5% of membrane lipids have short
carbohydrate chains attached
• Occur on external surface of membrane
Proteins
• Variety of functions:
– Transport proteins for ions and polar
molecules
– Enzymes: eg ATPase in mitochondrial
membrane, chloroplast membrane, intestinal
wall cells (for hydrolysis of disaccharides)
Glycoproteins
• Most proteins in plasma membrane have
short carbohydrate molecules attached
• H-bond with water to help stabilise
membrane structure
• May act as receptor for hormones,
neurotransmitters
• Antigens = glycoproteins that help cells
recognise each other. Each cell has it’s
own antigen
Proteins - Glycoproteins
• transmembrane proteins span the entire
membrane & are usually glycoproteins:
• Four main functions
– Act as channels: Na+/K+ Pump to maintain ion
concentrations either side of the membrane
– Transporters: some proteins identify & attach to
specific substances eg. nutrients,
neurotransmitters
– Receptors: recognise & bind to target molecules
such as hormones
– Enzymes eg. ATPase
Functions of membranes within
cells (organelle membranes)
• Intracellular membranes have a structure
very similar to that of cell surface
membranes
• However, proportions of molecular
components differs considerably eg.
chloroplast membrane contains very little
carbohydrate
• Almost all cellular process involves
intracellular membranes:
Transport across the cell
membrane
• Passive transport does NOT require
energy
– Diffusion – small uncharged molecules
– Osmosis - water
– Facilitated diffusion - glucose
• Active transport REQUIRES ENERGY
– Ion pumps
– Endocytosis
– Exocytosis
Active and Passive Transport
Summary
What does the membrane do?
• allows for different conditions
between inside and outside of cell
• subdivides cell into compartments
with different internal conditions
• allows release of substances from
cell via vesicle fusion with outer
membrane:
Membrane Permeability
• Biological membranes are physical
barriers, but which allow small uncharged
molecules to pass…
• They are described as semi-permeable
Because;
• Lipid soluble molecules and small
molecules pass through
• Big molecules and charged ones do NOT
pass through
Membrane Semi-permeability
1) lipid soluble solutes go
through faster
1) smaller molecules go faster
1) uncharged & weakly charged
go faster
2) Channels or pores may also
exist in membrane to allow
transport of larger molecules
1
2
Its about concentration
The concentration of the solution,
with respect to other solutions is
important
Isotonic --- when both solutions have the same
concentration of dissolved substances
Hypertonic --- a solution with a higher
concentration of dissolved substances
Hypotonic --- a solution with a lower
concentration of dissolved substances
Animal cells
Plant cells
Two types of transport
Passive and Active
Passive Transport
•
•
Involves concentration gradients ONLY.
NO CELL ENERGY is used—this is why
it is called “passive”
Passive Transport
3 types
• Diffusion- simple movement from regions of
high concentration to low concentration.
• Osmosis- specifically the diffusion of water
across a semi-permeable membrane.
• Facilitated diffusion - protein transporters
which assist in diffusion.
Diffusion
Diffusion is the passive movement of particles
from a high concentration of particles to a
lower concentration until they are spread out
evenly
Solution
Smell Particles & Air
Particles
Smell Particles diffused
evenly into the Air Particles
Diffusion
• 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.
Solutions
• A solution is made up of two parts: the
solute and the solvent.
• The solute dissolves in the solvent
• Before a gas can diffuse across a
membrane it must dissolve into a liquid.
Diffusion & Gas Exchange
• Animal cells use oxygen, so oxygen is less
concentrated inside the cell than outside. This causes
oxygen to diffuse into the cell
• Carbon dioxide is produced in an animal cell, so it is
more concentrated inside than outside – so it
diffuses out of the cell
Diffusion for Photosynthesis
• Diffusion of gases also happens in leaves:
Water + Carbon Dioxide → Oxygen and Glucose (& a
little water)
• For photosynthesis to happen Carbon
Dioxide has to get inside the leaves. It
diffuses in through stomata
• Water vapour and oxygen diffuse out of
the leaf at the same time
Diffusion & Photosynthesis
CO2
O2
Rate of Diffusion
•
•
•
•
The rate of diffusion depends on:
Size of the particles: smaller = faster
Temperature (eg. kinetic energy): hotter = faster
The concentration gradient: the higher it is =
faster the rate.
• State of the particles: gas > liquid > solid
• Distance - thickness of the exchange surface:
thinner = faster
• Surface area available: larger = faster
Facilitated Diffusion
• 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.
Facilitated Diffusion
Transport protein
Concentration
gradient
Glucose
Transport Proteins
• Move solutes faster
across membrane
• Highly specific to specific
solutes
• Can be inhibited by drugs
• Also involved in ACTIVE
transport
Transport protein
Concentration
gradient
Glucose
Cell membrane
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
• Work by facilitated diffusion No E!
• Deal with small molecules... ions
• Open pores are “gated”- Can change
shape.
– How? Do a diagram to show how you
think this might work.
• 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.
Osmosis
• Osmosis is a special type of diffusion.
• Osmosis is the diffusion of water.
Osmosis is the movement of
water molecules from a high
concentration of water to a low
concentration of water through
a partially-permeable membrane
Osmosis
Osmosis
Osmosis Experiment
1. Fill the partially
Glass tube
Level of
permeable membrane
sugar
with strong sugar
solution
Weak sugar
solution
solution
2. Tie it to a capillary
tube and stand it in a
weak sugar solution
3. Use your ideas about
Visking Tubing
osmosis to explain
with Strong
Sugar solution
why the liquid rises in
the tube
Osmosis and Cells - Notes
• Cells are surrounded by a cell membrane
that separates the contents of the cell from
the outside environment. The cell
membrane has tiny holes in it. This allows
small molecules to pass through, but not
large ones. The cell membrane is
partially-permeable.
• Osmosis occurs when two solutions are
separated by a partially permeable
membrane
Class Activity: Correctly Label
Weak Solution
Strong Solution
Solute
Molecule
Water Molecule
Which way will
the water
flow???
Partially Permeable
Membrane
Osmosis
Osmosis and Animal Cells
Hypotonic
solution
haemolysis
Hypertonic
solution
Osmosis & Plant Cells
Hypotonic
solution
Turgid Cell
Hypertonic
solution
Plasmolysed Cell
Plasmolysis in Elodea
Plasmolysis in Elodea:
http://www.mrphome.net/mrp
/Membrane_Transport.html
Active Transport
• 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
• 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
Transport protein
Concentration
gradient
Energy is used
Salt ion
Cell membrane
Ion binds to the
transport protein
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—Endocytosis
• 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
• If material is solid the process is called
phagocytosis
How Endocytosis works
Pseudopodia extend to engulf food
A food vacuole is formed
Pinocytosis works the same, but with no food, only liquid
Moving many large molecules
at once—Exocytosis
• Material is packaged inside the cell and
the package fuses with the cell membrane
while the material goes out of the cell.
How exocytosis works
Vacuole containing particles
is moved close
to the cell membrane
Fuses with the cell
membrane to expel
the particles
•Animation;
•YouTube - Endocytosis & Exocytosis
Exocytosis
the finer detail
Bulk Transport
Ion Channels
• 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
• saxitoxin, tetrodotoxin
• cystic fibrosis
Sodium-Potassium Pump
Na-K Pump Model: Part I
• 3 Na+ bind to inner region of protein
• Na+ binding triggers phosphorylation of
protein. ATP
ADP + Pi
• Phosphorylation causes conformation
change and Na+ binding site faces outside
• 3 Na+ released to outside
Na-K Active Pump: Part II
• 2 K+ ions on outside are able to bind
• K+ binding causes de-phosphorylation
and new conformation change
• 2K+ ions exposed to inside and
released
Cyclic process uses ATP energy to drive
Na & K ion transport against conc.
Gradient
Root Hair Cells and Active
Transport
• Root hair cells take in minerals using
active transport.
• If the concentration of minerals is higher in
the root hair than in the soil, then the cell
needs to use energy to actively transport
the minerals into the cell against the
concentration gradient.
Active Transport and Villi
• When there’s lots of nutrients in the gut
they diffuse naturally into the blood. But...
• Sometimes there are less nutrients in the
gut than in the blood, so the villi cells of
our gut use active transport to absorb
these nutrients against their concentration
gradient.
Villi and Glucose Uptake
Passive vs. Active Transport