Download Other types of transport

12/21/2009
Protein channels
Other types of transport
Cells function similarly in all living
organisms.
Larger molecules like sugars, starches,
and proteins sometimes diffuse through
protein channels.
• Because the molecules move from greater
to lesser concentration through the
channels, this process also does not require
energy.
• Diffusion and osmosis
do not require energy
from the cell.
• This is because the
molecules move with
a concentration
difference (from
higher to lower).
Active transport
• Sometimes a cell needs to move molecules
against a concentration difference (from
lower to higher concentration).
• Active transport is a process that allows
molecules to move across the cell
membrane from lower to higher
concentrations.
Active transport requires energy.
Other types of active transport
• Protein molecules act as “pumps” to move
the molecules across the cell membrane.
• Your nerve cells have lots of protein pumps
to move ions across the cell membrane.
• This is how signals travel through your
nervous system.
• A cell can take in larger
particles of food by
“engulfing” them.
• The cell membrane forms a
pocket around the particle.
• Once inside the cell, the
pocket breaks loose from
the cell membrane.
• It forms a vacuole within
the cytoplasm.
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Cells also send material out of the cell in
the same way.
• When this happens, a vacuole fuses with
the cell membrane and the contents are
forced outside of the cell.
• Both of these processes are types of active
transport because they require energy
Cells need a large surface area
• Everything the cell needs to take in or has
to get rid of has to go through the cell
membrane.
• Therefore the cell membrane needs to have
a large surface area in relation to the
volume of the cell.
Why are cells so small?
• One characteristic of cells is that they are
very small. Why?
• The answer has to do with the cell
membrane.
The surface area-to-volume ratio is the
area of the cell’s outer surface in
relationship to its volume.
• Volume is the space inside an
object. (cm3)
– So the volume of a cube with 2 cm
sides is 2 X 2 X 2 = 8
• Surface is the outer part of an
object. (cm2).
– A cube has 6 sides, so its surface area
is 2(2x2) + 2(2x2) + 2(2x2) = 24
As a cell gets bigger, so does its surface
area.
• However, the volume of a cell increases at a
faster rate than the surface area of its cell
membrane.
• If a cell gets too large, its cell membrane
will not have enough openings to meet the
demands of its volume.
• This limits the size of cells.
One way to increase surface area is to
make the cell long and thin or skinny and
flat.
• The nerve cells in your body are very
long and thin.
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