Transcript Concentration gradient

Cells and Their
Environment
Cell Membrane
Phospholipids
Lipid
Bilayer
Fluid Mosaic
Integral Proteins
Phospholipid
Transportation

How do substances move into and out of
cells?

Passive Transport and Active Transport
Passive Transport

Movement across the cell membrane that
does not require energy
Some Important Terms
Concentration gradient – a difference in
the concentration of a substance across a
space
 Equilibrium – concentration of a substance
is equal throughout space

Diffusion

Movement of substance from area of high
concentration to low concentration (down
its concentration gradient)–resulting in
equilibrium
Example of Diffusion
Small and nonpolar molecules diffusing
across the cell membrane down their
concentration gradient
 This is the simplest type of passive
transport

Osmosis
The diffusion
of water

Movement of a
substance down
its concentration
gradient
A simple rule to remember is:
Salt is a solute, when it is concentrated inside or outside the cell,
it will draw the water in its direction. This is also why you get
thirsty after eating something salty.
3 Possibilities for the direction of
water movement in a cell
1. Water moves out
2. Water moves in
3. No net water movement
The word "HYPER" means more, in this case there are more solute
(salt) molecules outside the cell, which causes the water to be sucked in
that direction.
In plant cells, the central vacuole loses water and the cells shrink,
causing wilting.
In animal cells, the cells also shrink.
In both cases, the cell may die.
Water Moves Out
– Hypertonic
Solution
The word "HYPO" means less, in this case there are less solute (salt)
molecules outside the cell, since salt sucks, water will move into the
cell.
The cell will gain water and grow larger. In plant cells, the central
vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps
the plant from bursting
In animal cells, the cell may be in danger of bursting, organelles called
CONTRACTILE VACUOLES will pump water out of the cell to prevent
this.
Water Moves In –
Hypotonic Solution
If the concentration of solute (salt) is equal on both sides, the
water will move back in forth but it won't have any result on the
overall amount of water on either side.
"ISO" means the same
Water moves in &
out at the same
rate!
Extra
This is why it is dangerous to drink sea water its a myth that drinking sea water will cause you
to go insane, but people marooned at sea will
speed up dehydration (and death) by drinking
sea water.
 This is also why "salting fields" was a common
tactic during war, it would kill the crops in the
field, thus causing food shortages.

Transport Proteins

Provide polar passageways through which
ions and polar molecules can move across
the cell membrane

Each channel is specific for a substance –
selectivity – the cell has the Control!
How can ions get into a cell?
Even though we have added a third
party….

This is still a form of passive transport

Why?

There is no use of energy by the cell
required because the ions are moving
down their concentration gradient
Facilitated Diffusion
Passive transport….why?
Active Transport

How do substances enter a cell when they
are working against their concentration
gradient?

Active transport - Requires the cell to use
energy from ATP
Carrier Proteins as “Pumps”
Why do we like the Na-K Pump?
1.
Na ions continuously diffuse into the cell
membrane through ion channels (why is
this a problem) and the pump prevents
these ions from accumulating
2.
Maintains the concentration gradient of
Na ions and K ions across the cell
membrane
What about the large molecules
(proteins and polysaccharides)?

Endocytosis – the movement of a
substance into a cell by a vesicle
Endocytosis - terms

Phagocytosis – cell eating
– Membrane sinks in and captures solid
particles for transport into the cell
– Examples:
 Solid particles often include: bacteria, cell
debris, or food

Pinocytosis – cell drinking
– Cell brings in a liquid
Endocytosis 
comments
Phagocytosis and pinocytosis are not
selective
– Membrane sinks inward and captures
whatever particles/fluid present.
– Vesicle forms and merges with the Golgi
body…
Receptor Mediated Endocytosis

Receptor Mediated Endocytosis is a
highly specific form of endocytosis.
– Receptor proteins on the outside of the cell
bind specific substances and bring them into
the cell by endocytosis
Receptor Mediated Endocytosis
1. Receptor proteins on PM bind specific
substances (vitamins, hormones..)
2. Membrane sinks in and forms a pit
– Called a coated pit
3. Pit pinches closed to form a vesicle around
bound substances

Cytoskeleton aids in pulling in the membrane and
vesicle formation
Example
Plasma membrane
Coat protein
Coated
vesicle
Coated
pit
Specific
molecule
Coated
pit
Material bound
to receptor proteins
How to move large substances out
of the cell?

Exocytosis – the movement of a substance
by a vesicle to the outside of a cell (used
to export proteins modified by the Golgi)
How cells communicate
Release signal molecules that carry
information to nearby cells and throughout
the body
 Ex. hormones

What else is in the cell membrane?


Receptor Proteins - proteins in the cell
membrane that bind these signal
molecules and then “read the message”
Marker Proteins – proteins that identify
cell type.
Cells