Chapter 3
Cell Membrane-Semi-permeable,
Cell Transport, Simple Diffusion,
Concentration Gradients
Slide 84-105
What Cells Need to Move Substances

Cells need certain things to function:




Proteins, carbohydrates, lipids-energy, building
blocks, enzymes, etc
Ions-messengers for brain, nerves, and muscles
O2, H20, CO2-ingredients for respiration and
photosynthesis
After cells use these things they create waste
products:

O2, CO2, H20, H202Lactic Acid, etc
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Movement Across the Plasma Membrane

A few molecules move freely


Water, Carbon dioxide, Ammonia, Oxygen
Carrier proteins transport some molecules
Proteins embedded in lipid bilayer
 Fluid mosaic model – describes fluid nature of
a lipid bilayer with proteins

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Semipermeable Membrane

A semipermeable membrane, also termed
a selectively permeable membrane, a
partially permeable membrane or a
differentially permeable membrane, is a
type of biological membrane that will allow
certain molecules or ions to pass through it
freely while blocking the passage of others.
by diffusion and occasionally specialized
"facilitated diffusion".
5
How do you build a semi-permeable cell
membrane?

Channels are made of proteins

proteins both “like” water & “like” lipids
bi-lipid
membrane
protein channels
in bi-lipid membrane
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Protein Channels

Proteins act as doors in the membrane

channels to move specific molecules through
cell membrane
HIGH
LOW
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Movement through the channel

Why do molecules move through
membrane if you give them a channel?
HIGH
?
LOW
?
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Illustration showing the principle behind a semi-permeable membrane. The
membrane's microscopic pores allow a water molecule (H2O) to pass, but the
larger glucose molecule (C6H12O6) is too large to pass through the pores.
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Selective Permeability of Membranes
https://www.youtube.com/watch?v=5jZXCDcM14g
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Passive Transport




For either form of transport to bring about a net
movement of materials into or out of a cell, a
concentration gradient must exist.
Substances diffuse down their concentration
gradient, the region along which the density of a
chemical substance increases or decreases
No work must be done to move substances down
the concentration gradient
The diffusion of a substance across a biological
membrane is passive transport because no energy
is expended by the cell to make it happen
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Concentration Gradient
A concentration gradient defines the
difference between the highest and lowest
concentrations of a solute within a given
medium.
 Through diffusion, compounds naturally
move from higher to lower concentrations,
meaning down their concentration
gradients.

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Concentration Gradient

Substances diffuse down their concentration
gradient, the region along which the density
of a chemical substance increases or
decreases

No work must be done to move substances
down the concentration gradient

The diffusion of a substance across a
biological membrane is passive transport
because no energy is expended by the cell to
make it happen
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Concentration Gradient
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Concentration gradients | Membranes and
transport | Biology | Khan Academy
https://www.youtube.com/watch?v=CNP-dCQ-Cmg
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Transport through Cytoplasmic Membrane

The cytoplasmic/plasma/cell membrane
 In Eukaryotes


Cytoplasmic membrane is a semipermeable membrane that
determines what goes in and out of the cell.
Substances may cross the cytoplasmic membrane of eukaryotic
cells by:


Some compounds are able to cross the plasma membrane strictly
through diffusion; others require diffusion and special protein channels;
still others require protein channels and the expenditure of cellular
energy.
Passive transport
Simple diffusion
Facilitated diffusion
Osmosis
Active transport
Endocytosis
Exocytosis






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Passive Transport

No energy spent by the cell! Substances move from high to
low concentration.

Diffusion



Movement of molecules or ions from a region of their higher
concentration to a region of their lower concentration.
A concentration gradient is all that is required for simple diffusion to
operate.
Osmosis

Diffusion of certain particles across a semi-permeable membrane.


Facilitated Diffusion


Diffusion of certain particles through a carrier protein in the cell
membrane.
Requires both a concentration gradient and a protein channel.


O2, CO2, H20
Glucose, amino acids, etc.-too big or are polar molecules
Can be in or out of the cell.
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Diffusion
(1:30)
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter5/how_diffusion_works.html
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Diffusion Through Cell Membrane

How is the concentration of a substance within a cell related to the
concentration of the same substance outside the cell? Many molecules
can pass freely across cellular membranes. Small molecules and nonpolar
molecules are the ones that can move most easily across the lipid bilayer
and enter the cell. Diffusion is the process that causes these molecules to
move in or out of the cell. Diffusion is driven by the random movements of
individual molecules in solution. If the concentration of a molecule is
greater outside a cell than inside the cell then, on average, more molecules
will randomly enter the cell, than leave the cell. Molecules that are capable
of diffusion across a cellular membrane will eventually reach equal
concentrations inside and outside of the cell. At this point the rate at which
individual molecules enter and leave will be equal.
http://lh5.ggpht.com/--YM0WjJW7PI/UBiB8IY02dI/AAAAAAAABRc/Wbhn_e53mEU/image_thumb%25255B8%25255D.png?imgmax=800
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Molecules move from high to low

Diffusion

move from HIGH to LOW concentration
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Diffusion

Move from HIGH to LOW concentration
passive transport
 no energy needed

diffusion
diffusion of water
osmosis
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Simple Diffusion

Move from HIGH to LOW
fat
inside cell
fat
fat
fat
fat
fat
Which way will
fat move?
LOW
HIGH
fat
outside cell
fat
fat
fat
fat
fat
fat
fat
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Simple Diffusion
https://www.youtube.com/watch?v=6yMPpDeNwqQ
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