Download Cell Membrane and Transport

Cell Membrane and Transport
Maintaining homeostasis and
providing nutrients to cells
Cell Membrane Structure
The cell membrane is composed of
lipids and proteins.
 The lipids are arranged in a bilayer.
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The bilayer is a “barrier” that is
impermeable to most molecules.
The proteins are embedded in the
bilayer.

Specific molecules can be helped across
the membrane by these proteins.
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What are membranes?
Membranes cover the surface of every cell,
and also surround most organelles within
cells. They have a number of
functions, such as:
 keeping all cellular components
inside the cell
 allowing selected molecules to move in and out of the cell
 isolating organelles from the rest of the cytoplasm,
allowing cellular processes to occur separately.
 a site for biochemical reactions
 allowing a cell to change shape.
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Membranes: timeline of discovery
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Evidence for the Fluid Mosaic Model
It was discovered (using images from a TEM) that the
phospholipid “heads” face the exterior/interior of a cell and
the phospholipid “tails” are in the middle.
intracellular space (blue)
This picture shows the cell
membranes of two adjacent cells.
1st cell
membrane
1 light layer =
phospholipid tails
2 dark layers:
phospholipid heads
2nd cell membrane
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Evidence from freeze-fracturing
In 1966, biologist Daniel Branton used freeze-fracturing to
split cell membranes between the two lipid layers, revealing
a 3D view of the surface texture.
This revealed a
smooth surface
with small bumps
sticking out.
These were later
identified as
proteins.
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E-face:
looking up at
outer layer of
membrane
P-face:
looking down
on inner layer
of membrane
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The fluid mosaic model
The freeze-fracture images of cell membranes were further
evidence against the Davson–Danielli model.
They led to the
development of
the fluid mosaic
model, proposed
by Jonathan
Singer and Garth
Nicholson in 1972.
E-face
P-face
protein
This model suggested that proteins are found
embedded within, not outside, the phospholipid bilayer.
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Exploring the fluid mosaic model
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The chemical properties of lipids determines the
bilayer nature of the cell membrane.
Cell membranes contain special
lipids known as phospholipids

Phospholipids: Composed of

two fatty acid chains attached to a glycerol
molecule and phosphate group.
Phosphate Group
Glycerol
Fatty Acid Chains
Phospholipids, continued:

Have a hydrophilic “head” that
“loves” water (both are polar)


Hydrophilic heads form outside of
layer so they can touch water inside
and outside cell
Have a hydrophobic “tail”
(hydrocarbon chain) that “fears”
water (is non-polar)

Hydrophobic tails face interior of
bilayer so they can avoid water
Phospholipid Bilayer
Role of the Cell Membrane

The cell membrane is described as
“selectively permeable”


How does this feature relate to the
job/function of the cell membrane?
Cell membrane acts as a “guard”
Allows nutrients into cell
 Allows for removal of wastes and release of
substances made by the cell that are
needed by other cells.

Factors that affect Passive
Transport:

Whether a molecule can move through
the membrane depends on:
1) the size of the molecule
2) the type of molecule (polar or nonpolar,
charged, etc.)

Molecules move by one of the following methods:
diffusion, facilitated diffusion, or osmosis.
The direction of movement (in/out) depends on
the concetration gradient.

Concentration Gradient
Direction of movement:
Out of the blood, into the lungs
Concentration Gradient = a difference in
concentration of a substance in one area
compared to another.
All 3 types are passive transport

Passive Transport: movement of
substances without any energy input by
a cell.
1.
Diffusion: molecules move straight through the
membrane.
Facilitated diffusion: molecules or ions move
through protein channels embedded in the
membrane.
Osmosis: water molecules move through the
membrane (mostly through protein channels).
2.
3.
Methods of Transport
Direction of transport

In passive transport, the net movement
is always “down the concentration
gradient”.
Molecules move from an area of higher
concentration to an area of lower
concentration.
 It’s “passive” because it doesn’t require
cellular energy.

Net Movement
Think of dye or sugar molecules in water. (Water particles NOT
shown.) Dye or sugar molecules will DIFFUSE through the water.
What happens in the alveoli?
Example: Transport of Oxygen
HIGH
Concentration gradient for O2
LOW
Net movement

If the membrane is
permeable to both
water and solutes,
both will diffuse to
reach equilibrium.

Often, the membrane
is NOT permeable to
the solute(s). In this
case osmosis occurs;
water diffuses (high to
low) to balance the
concentration on both
sides. (egg lab)
HIGH
LOW
Example: Osmosis
Predicting osmosis
Osmosis in action
Example: Transport of Glucose
HIGH
Net movement
LOW
Facilitated Diffusion:
 Integral
Proteins can “facilitate”
or assist in transporting a
substance by:
as a channel or tunnel
 2. acting as a carrier or transporter

1. acting
Diffusion through a carrier protein
Active Transport
Cells move molecules from an area of
LOW concentration to an area of HIGH
concentration
 Molecules move AGAINST the
concentration gradient
 Requires the cell to use energy in the
form of ATP

Animation
What is active transport?
Substances can move passively in and out of cells by
diffusion until the concentration on both sides of the cell
membrane reaches an equilibrium.
Substances can continue to move in and out of a cell using
a process called active transport.
During active transport, protein
carriers in the cell membrane ‘pick
up’ particles and move them against
the concentration gradient.
As the name suggests, active transport
requires energy from the cell, which is
made available by respiration (ATP).
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What is active transport?
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Active transport in plants
Plants need to absorb mineral elements such as nitrogen,
phosphorus and potassium from the soil for healthy growth.
When the concentration of minerals in soil is lower than
inside the plant, active transport is used to absorb the
minerals against the concentration gradient.
What would happen if the
plant relied on diffusion to
absorb minerals?
minerals
The cells would become
drained of minerals because
they would travel down the
concentration gradient.
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Example: Sodium-Potassium
Pump
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Three Na+ ions in the cytoplasm bind to carrier protein
Shape of protein is changed, allowing the three Na+
out of cell
Two K+ ions outside of cell bind to protein
Shape of the protein is changed
The two K+ are allowed into the cytoplasm
Similar to facilitated diffusion:
Animation 1
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Different from facilitated diffusion:
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Uses a carrier protein,
Animation 2
requires energy.
Overall: Na+ (sodium) becomes concentrated on the
outside of a cell.

Important in the proper functioning of neurons and the
kidneys.
Other types of active transport
Not all active transport moves molecules
from a low concentration to a high
concentration
 Active transport used in two other
situations:

Moving very large molecules through
membrane
 Moving large quantities of smaller
molecules through membrane

Other examples of active transport:

Endocytosis: Process in which cells
ingest fluids, macromolecules, and
large particles that are outside the cell
Animation 1
Animation 2
Other examples of active transport:

Exocytosis: how cells release large
molecules (proteins) or get rid of
large amounts of wastes