Download Transport Across Membranes

Cell Membranes
Cell Membranes
• Cells are surrounded by a membrane: the
cell surface membrane or plasma membrane
• On an EM it appears as a double line
• The distance
across the
membrane is
7-8nm
The Fluid Mosaic Model
WHAT DOES A MEMBRANE
LOOK LIKE?
TRANSPORT ACROSS
MEMBRANES
There are four main methods by which
substances can move across a cell
membrane:
• 1. Diffusion
Passive Transport
Osmosis
• 2. Active Transport
• 3. Endocytosis and Exocytosis
1: Diffusion
• Diffusion is the movement of molecules
from a region where they are in high
concentration to a region of low
concentration.
• The difference in concentration is the
concentration gradient
molecule
membrane
• Diffusion is a PASSIVE process
• There is a net movement of molecules
down the concentration gradient until
equilibrium is reached
• Ions move by diffusion along
electrochemical gradients
Rate of diffusion
• The rate of diffusion depends on:
– Concentration gradient
– Temperature
– Size of molecule
– Lipid solubility
Extension: Look up Fick’s Law…
Facilitated DiffusionIntrinsic globular proteins
within the membrane function
as carriers for the transport
of certain molecules
concentration
concentration
gradient
gradient
Lipid
Bilayer
Carriers are specific for the molecules
that they transport them across the
membrane in the direction of the
concentration gradient at a faster rate
than occurs for simple passive diffusion
Glucose molecules enter body cells from the
bloodstream by facilitated diffusion
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
Lipid
Bilayer
concentration
concentration
gradient
gradient
LIMITING FACTORS AND FACILITATED DIFFUSION
The rate at which molecules like glucose enter cells by facilitated diffusion is
affected by factors other than the concentration gradient
Rate of
uptake
Rate of uptake reaches
a maximum at glucose
concentration A
Rate of uptake
increases as the
glucose
concentration
increases
(i.e. the steepness of
the concentration
gradient increases)
The availability of
carriers in the membrane
is the limiting factor
No further increase in
the uptake rate despite the
increasing glucose
concentration
When glucose
concentrations exceed a
certain value, there are
insufficient glucose carriers
within the membrane for
the rate of uptake to
increase any further
A
Increasing external concentration of glucose
Summary of Passive Transport
2: Osmosis
• Osmosis is the diffusion of water molecules from an area
of high concentration (of water) to an area of lower
concentration across a semi-permeable membrane
membrane
water
molecules
solute
molecules
net movement of water
dilute solution
concentrated solution
low concentration of solute
high concentration of solute
high concentration of water
low concentration of water
Water Potential
Osmosis can be quantified using water potential, so we can
calculate which way water will move, and how fast.
Water potential (ψ, the Greek letter psi, pronounced "sy") is
simply the effective concentration of water. It is measured in
units of pressure (Pa, or usually kPa)
water always "falls" from a high to a low water potential
100% pure water has ψ = 0, which is the highest possible
water potential, so all solutions have ψ < 0
you cannot get ψ > 0.
pure water
ψ

= 0 kPa
dilute solution
ψ = -200 kPa

water diffuses
from 0 to -200 kPa
concentrated solution
ψ
= -500 kPa
water diffuses
from -200 to -500 kPa
Cells and Osmosis.
The concentration of the solution that surrounds
a cell will affect the state of the cell, due to
osmosis. There are three possible
concentrations of solution to consider:
• Isotonic solution
a solution of equal
concentration to a cell
• Hypertonic solution a solution of higher (salt)
concentration than a cell
• Hypotonic solution a solution of lower (salt)
concentration than a cell
Hypotonic solution
Isotonic solution
Hypertonic solution
water enters, cell swells
and may burst (lysis).
no net movement of
water, cell normal size
water leaves, cell
shrinks and crenates
water enters, cell swells
a bit and becomes turgid.
no net movement of
water, cell normal size
water leaves, cytoplasm
shrinks and plasmolyses
Animal
Cell
Plant
Cell
3: Active Transport
• Pumping of molecules across cell
membrane using a protein pump
protein pump
active site
ATP ADP + Pi
UP a concentration Gradient
• Most Active Transport systems are driven
by metabolic energy derived from ATP
• Active Transport allows cells to uptake
necessary ions and molecules and remove
waste products
• These Mechanisms often referred to as
Pumps
• Most common is Sodium Pump
Na+ / K+ Pump
• Present in all animal cells, most abundant
and important of membrane pumps
3 Na+
+
outside cell
Na+K+
pump
inside cell
2 K+
ATP
ADP + Pi
-
• Sodium-Potassium Pump important in
controlling cell volume, reducing Na
thereby reducing water uptake by osmosis
• The accumulation of K ions are used in
cell metabolic processes eg protein
synthesis
• Na-K pump can be linked to active uptake
of organic molecules such as glucose and
amino acids
4: Endocytosis & Exocytosis
• Endocytosis
• Phagocytosis
Eg. Amoeba
White Blood Cells
(neutrophils & monocytes)
• Pinocytosis
Eg. Protozoans, white blood cells, cells in
embryos, liver and kidneys.