Download CELL MEMBRANES and TRANSPORT

CELL MEMBRANES
and
TRANSPORT
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The Cell
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Cell Membrane
All living things are surrounded by a
membrane.
 A cell membrane is also known as plasma
membrane.
 Controls exchange of materials such as
nutrients and waste between cells and their
environment.
 Has other important functions
for example to enable cells to receive
hormones.

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Cell Membrane
 Fluid
Mosaic Model - proteins embedded
in a phospholipid bilayer.
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Fluid mosaic model
FLUID- because individual phospholipids and proteins
can move around freely within the layer, like it’s a
liquid.
MOSAIC- because of the pattern produced by the
scattered protein molecules when the membrane is
viewed from above.
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Cell membranes are made of
PHOSPHOLIPIDS

HYDROPHILIC heads
(water liking)
-Attracted to the water

HYDROPHOBIC tails
(water fearing)
-repel water
A Phospholipid
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A Phospholipid Bilayer
Phospholipids can form:
BILAYERS
-2 layers of phospholipids
with hydrophobic tails
protected inside by the
hydrophilic heads.
The PHOSPHOLIPID
BILAYER is the basic
structure of membranes.
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 Bilayer:

2 layers of phospholipids
The heads stay on the outside and the tails
stay on the inside.
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Cell Membrane
 Cell



surface proteins
a. Channel proteins - transport food and
other molecules into the cell and transport
wastes out of the cells.
b. Receptor proteins - gather information
about the cell’s surroundings.
c. Cell surface markers - identify the type
of cell, important for cell recognition.
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Movement of selected molecules
across the cell membrane
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Cell Membrane
 Permeability


of the cell membrane
1. Semi permeable/selectively permeable only certain substances can pass across the
membrane.
2. Factors that determine whether a
molecule can pass through a membrane or
not:
• a. size
• b. type
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Transport Mechanisms

Transport Mechanisms - moving material in and
out of the cell

Concentration gradient - the difference in the
amount of a substance inside and outside of the
cell



1. Going “with the gradient”-moving from high to low
concentration
2. Going “against the gradient”-moving from low to
high concentration
3. Equilibrium exists when the concentration of
molecules is the same throughout a space (inside and
outside the cell)
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Transport Mechanisms
 Two
categories of transport based on
concentration gradient and the need for
energy

1. Passive transport does not require energy
• goes with the gradient.

2. Active transport requires energy
• goes against the gradient.
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Methods of Transport
Across Membranes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
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Methods of Transport
Across Membranes
Diffusion -passive transport - no
energy expended
2. Osmosis - Passive transport of water
across membrane
3. Facilitated Diffusion - Use of proteins to
carry molecules or ions across
4. Active Transport- requires energy to
transport molecules against a
concentration gradient – energy is
in the form of ATP
1.
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Types of Transport
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Passive Transport Mechanisms
Diffusion
Movement of molecules from an area of
high concentration to an area of low
concentration.
 Movement from one side of a membrane to
another
 Example: smoke across a room, food
coloring dropped into water, oxygen in
lungs

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Diffusion
Diffusion in action
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Passive Transport Mechanisms
Osmosis
 Diffusion


of water
Direction of osmosis-the direction of water
flow depends upon the concentration of solute
and solvent (water)
Tonicity-term used to compare 2 solutions
(usually the inside and the outside of the cell)
• a. Hypotonic - less solute (more water)
• b. Hypertonic - more solute (less water)
• c. Isotonic - equal amounts of solute and water
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Passive Transport Mechanisms
Osmosis


Water will flow from a hypotonic
solution to a hypertonic solution
Will water move in an isotonic
solution?
• Yes, but it won’t be noticeableequilibrium is established
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Osmosis
How Osmosis
works
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Solutions
 Solutions
are made of solute and a
solvent
 Solvent - the liquid into which the
solute is poured and dissolved.
 Solute - substance that is dissolved
or put into the solvent.

Salt and sucrose are solutes.
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Passive Transport Mechanisms

Turgor pressure/osmotic pressurepressure due to water in cell.
• Analogy: air pressure in a tire.


When a cell has high turgor pressure, it is
bigger and stiffer.
When a cell has low turgor pressure, it is
smaller and flimsy.
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Passive Transport Mechanisms
 Plasmolysis
- wilting of a cell due to loss
of turgor pressure
 Cytolysis
- bursting of a cell due to an
increase in turgor pressure.


How do plant cells avoid cytolysis? Cell walls
How do unicellular freshwater organisms
avoid bursting? Contractile vacuoles.
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Plant and Animal Cells put into
various solutions
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Passive Transport Mechanisms
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Passive Transport Mechanisms
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Passive Transport Mechanisms
 Facilitated
diffusion - transport of
specific molecules across a membrane
with the help of a channel protein

An example of a molecule that is often
transported in this manner is glucose
How Facilitated Diffusion Works
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Active Transport Mechanism
 Active
Transport Mechanism
(requires energy in the form of ATP)
 Ex.

Sodium-potassium pump - pumps sodium ions
out and forces potassium ions in.
• Important for conducting nerve impulses and
muscle contraction.

Proton pump-transport protons through
membranes of chloroplasts and
mitochondria
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Active Transport Mechanism
 Endocytosis
- process of bringing larger
molecules into the cell by engulfing
them.


1. Pinocytosis – liquids
2. Phagocytosis - solids
• amoebas eat this way
• white blood cell engulf bacteria
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