Transport: Cell Membrane Structure and Function · 2018-03-09 · •The cell membrane selects what...
Transcript of Transport: Cell Membrane Structure and Function · 2018-03-09 · •The cell membrane selects what...
Transport: Cell Membrane
Structure and Function
Biology 12
Chapter 4
FLUID-MOSAIC MODEL OF
MEMBRANE STRUCTURE
• The cell membrane (plasma membrane)
is made of two layers of phospholipid
molecules (bilayer) which give it a fluid
consistency. It has proteins scattered
through it like a mosaic.
• The phospholipid bilayer gives the cell
membrane its structure. Each
phospholipid has a polar (charged) head
and 2 non-polar tails. The polar heads
are hydrophilic and the non-polar tails
are hydrophobic. When surrounded by
water (e.g. tissue fluid or cytoplasm) the
heads face the water and the tails face
away from the water.
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Outside
Inside
plasma membrane
glycolipid
glycoprotein
integral protein
cholesterol
peripheral protein
filaments of cytoskeleton
hydrophobic
tails
hydrophilic
headsphospholipid
bilayer
carbohydrate
chain
MOLECULES IN THE PLASMA
MEMBRANE• Glycolipids have a structure similar to
phospholipids, except that the hydrophilic
head is a carbohydrate chain. (Cell to
cell recognition)
• Cholesterol helps to keep the cell
membrane fluid at low temperatures and
reduces the permeability of the
membrane.
Proteins
• Peripheral proteins are on the surface of
the membrane.
• Glycoproteins have a carbohydrate
chain attached (cell recognition)
• Integral proteins extend right through the
cell membrane.
4.1 Plasma Membrane
Structure and Function
• Functions of Proteins
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1. Channel Protein
Allows a particular
molecule or ion to
cross the plasma
membrane freely .
2. Carrier Protein
Selectively interacts
with a specific
molecule or ion so
that it can cross the
plasma membrane.
e.g. Sodium Potassium
pump
b.a.
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c.
Cell Recognition
Protein:
With
Carbohydrate
chain.
e.g. Rejection of
heart after
transplant.
d.e.
Enzymatic Protein:
Catalyzes a specific
Reaction.
Receptor Protein:
Shaped in such a way
that a specific
molecule can bind to
it.
e.g. hormones,
Neurotransmitters.
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Channel Protein Carrier Protein
.
b. c.
Cell Recognition
Protein
d. e.
Enzymatic Protein
.
Receptor Protein
.
a.
4.2 SELECTIVELY PERMEABLE
• The cell membrane selects what it will let
in and out of the cell. It lets different
things through in different ways, but not
everything can get through.
• Mechanisms of transport include diffusion
and osmosis, protein assisted
transport, and endocytosis and
exocytosis (transport by vesicle).
DIFFUSION
• Diffusion is the net movement of a substance
from a region of higher concentration to a
region of lower concentration. It requires no
energy.
The rate of diffusion is affected by:
1. Concentration gradient – a solution consists
of two parts: solvent (often water) and solute
(particles dissolved in solvent). The difference in
solute concentration between two areas causes
diffusion. The greater the difference, the faster
the diffusion. E.g. if there is more oxygen outside
the cell than inside, it will diffuse to the inside.
2. Temperature – increasing temp causes the
particles to move faster, therefore increasing the rate
of diffusion
3. Ionic/molecular size – smaller substances will
diffuse more rapidly because they have fewer
collisions with other substances. Large molecules do
not diffuse through the membrane.
4. Shape of ion/molecule – may prevent it from
diffusing rapidly
5. Lipid Solubility- Lipid soluble molecules can move
through the lipid bilayer. Generally these molecules
are other lipids. Steroid hormones like testosterone
and estrogen are examples of such molecules. This
easy access to cells explains the powerful and wide
ranging effects of such hormones
6. Charge (+/-) Ions or molecules with a charge
cannot pass through the lipid bilayer by diffusion.
Water is not able to diffuse directly through the
phospholipid molecules, so instead it diffuses
through channels made of proteins called
aquaporins. Na+ and K+ also travel through
protein channels.
OSMOSIS
• Osmosis is the diffusion of water across a
differentially permeable membrane. Tonicity is
the strength of a solution in relationship to osmosis.
Tonicity can be described in three ways:
• ISOTONIC In isotonic solutions, the
concentration is the same on both sides of the
membrane, therefore there is no net gain or loss
of water (0.9%NaCl is isotonic to red blood cells)
• HYPOTONIC refers to a lower solute
concentration on one side of the cell. If a cell is
placed in a hypotonic solution, then the cell will
gain water and may burst. For example, any
concentration of salt solution less than 0.9% is
hypotonic to red blood cells. This may result in
hemolysis (disruption of red blood cells).
• HYPERTONIC refers to a higher solute
concentration. If a cell is placed in a hypertonic
solution, it will lose water. A salt concentration
greater than 0.9% causes shrinking of red
blood cells (crenation)
• The shrinking of cytoplasm due to osmosis is
called plasmolysis.
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Plant cells
chloroplast
nucleus
nucleus
6.6 µm 6.6 µm 6.6 µm
25 µm 40 µm25 µm
plasma
membrane
In an isotonic solution, there is no net
movement of water .
In a hypotonic solution, water enters the cell,
which may burst (lysis).
In a hypertonic solution, water leaves the
cell, which shrivels (crenation).
In an isotonic solution, there is no
net movement of water.
In a hypotonic solution, the central vacuole
fills with water , turgor pressure develops, and
chloroplasts are seen next to the cell wall.
In a hypertonic solution, the central vacuole loses
water, the cytoplasm shrinks (plasmolysis), and
chloroplasts are seen in the center of the cell.
central
vacuole
cell
wall plasma
membrane
Animal cells
(all top): © David M. Phillips/Photo Researchers, Inc.; (bottom left, center): © Dwight Kuhn; (bottom right): © Ed Reschke/Peter Arnold
Do not copy this one
4.2 Permeability of the Cell
MembranePROTEIN ASSISTED TRANSPORT
• Many of the proteins that are scattered
throughout the membrane provide
channels for substances to pass through
the cell membrane. Some require no
energy and others require ATP.
1. FACILITATED TRANSPORT
Facilitated transport does not require energy.
The molecules diffuse with the concentration
gradient across the cell membrane by combining
with carrier proteins. This can occur as often as
100 times per second at one site.
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Inside
plasma
membranecarrier
protein
solute
Outside
2. ACTIVE TRANSPORT
• With active transport, the molecules move
against the concentration gradient, either in to or
out of the cell. The molecules or ions require a
carrier protein and an expenditure of energy
(ATP). Some examples of this are
• Iodine collects in the thyroid to produce thyroxin
• Sodium is withdrawn from urine by kidney cells
• The sodium potassium pump in neurons
• Cells that carry out a lot of active transport have
lots of mitochondria near the cell membrane to
produce the energy.
K+
K+
K+
K+
K+
K+K+
K+
K +
K+
K+
K+
K+
K+
K+
K+
K+K+
P
P
P
P
Inside
6. Change in shape results and
causes carrier to release 2 K+
inside the cell.
carrier
proteinOutsideK+
K+
K+
ADPATP
K+ K+
K+
3. Change in shape results and
causes carrier to release 3 Na+
outside the cell.
2. ATP is split, and phosphate
group attaches to carrier.
4. Carrier has a shape that
allows it to take up 2 K+.
5. Phosphate group is released
from carrier.
1. Carrier has a shape that allows
it to take up 3 Na+.
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ENDOCYTOSIS AND EXOCYTOSIS
• Endocytosis and exocytosis involve transport in
and out of the cell by vesicle. This requires
energy and is also considered as ‘active
transport’.
Endocytosis
• Endocytosis involves movement into the cell by
vesicle.
• Diagram:
• Endocytosis is often divided in to two
categories based on the size of the particle
ingested:
Phagocytosis (cell eating)
• This is common in macrophages, large white
blood cells in humans which phagocytize
bacteria and worn-out red blood cells. The
vesicle then fuses with a lysosome and the
contents are digested
Pinocytosis (cell-drinking)
• Vesicles form around liquid or very small
particles (in blood cells and in intestinal
wall)
• Receptor-mediated endocytosis is a type of
pinocytosis in which receptor proteins bind
to solutes in a ‘coated pit’, which becomes a
vesicle – this process is involved in bringing
substances into the cell, transfer of materials
between cells, and exchanges between
mother and fetus.
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paramecium
solute
solute
a. Phagocytosis
b. Pinocytosis
vacuole
coated vesicle
plasma membrane
coated pit
c. Receptor-mediated endocytosis
399.9 µm
vesicle
vacuole
forming
pseudopod
of amoeba
0.5 µm
vesicles
forming
coated
vesiclecoated
pit
receptor
protein
a(right): © Eric Grave/Phototake; b(right): © Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark Bretscher
Exocytosis
• In exocytosis, a vesicle fuses with the
plasma membrane and its contents are
secreted.
• Diagram:
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plasma membrane
Inside
Outside
secretory
vesicle
Passage of Molecules Into and
Out of the Cell
• Vesicles formed by Golgi apparatus secrete
cell products at cell membrane. (e.g. this is
the way that insulin leaves insulin-secreting
cells)
CELL SIZE
• Cells have to be small in order to function
effectively. The materials that it needs to use
and the wastes that it produces must pass
through its cell membrane.
• The surface area of the cell controls the
ability of the cell to get nutrients in and
wastes out. The volume of a cell controls the
amount of nutrients it needs.
Cell length SA V SA/V
• A 1mm 6mm 1mm 6/1 = 6
• B 2mm 24 mm 8mm 24/8 = 3
• C 4mm 96mm 64mm 96/64 = 1.5
• D 8mm 384mm 512mm 384/512 = 0.75
• As cells increase in size, the surface area/volume ratio
decreases.
• (squared function vs. cubed function)
• A small cell has more surface area per
unit of volume than a large cell.
• The surface area becomes the limiting
factor in the cell’s ability to survive. The
cell produces more wastes than it can get
rid of, or it can’t consume enough
nutrients for its increased volume.
Cell strategies to increase SA/V ratio
1. Size - stay small
2. Shape
- get flat – example: skin cells
- get long and thin – example: nerve cells
3. Add extensions – villi and microvilli in the
small intestine and the kidney
Assignment from Ch 4 p68-78
4.1 CYP p69
4.2 CYP p71
4.3 CYP p74, 76, 78
These are all found on your CYP Handout