The Cell in Its Environment
Key Concepts
• How do most small molecules cross
the cell membrane?
• Why is osmosis important to cells?
• What is the difference between
passive transport and active
transport?
Key Terms
• Selectively
permeable
• Diffusion
• Osmosis
• Passive transport
• Active transport
• Some substances can pass through
the membrane while others
cannot.
• Molecules move from an area of
higher concentration to an area of
lower concentration.
• The diffusion of water molecules
through a selectively permeable
membrane.
• The movement of dissolved
materials through a cell membrane
without using cellular energy.
• The movement of materials
through a cell membrane using
cellular energy.
Introduction
• Cells have structures that protect their contents
from the world outside.
• All cells are surrounded by a cell membrane that
separates the cell from the outside environment.
• The cell membrane is selectively permeable,
which lets some things enter and leave the cell.
–
–
–
–
–
Oxygen
Carbon Dioxide
Water
Food molecules
Waste products
Diffusion
What is Diffusion?
• The process by which
molecules move from
an area of higher
concentration to an
area of lower
concentration.
• The main method by
which small molecules
move across the cell
membrane.
When Does Diffusion Stop?
When equilibrium is reached!
Animation of Diffusion
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/anima
tion__how_diffusion_works.html
• How does diffusion play a role in the
following situations?
– smelling cookies baking in your oven
– making a pitcher of lemonade
– adding chemicals to a pool
• Can you think of any other examples of
diffusion?
Osmosis
What is Osmosis?
• The diffusion of water
molecules through a
selectively permeable
membrane.
• Many cellular processes
depend on osmosis
because cells cannot
function properly
without adequate water.
How is Osmosis
Related to Diffusion?
• Molecules tend to move
from an area of higher
concentration to an area of
lower concentration.
• Water molecules move by
diffusion from an area
where they are highly
concentrated through the
cell membrane to an area
where they are less
concentrated.
Effects of Osmosis on Cells
Animation of Osmosis
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/anima
tion__how_osmosis_works.html
• Osmosis has a number of life-preserving functions.
– assists plants in receiving water
– helps in the preservation of fruit and meat
– used in kidney dialysis
• Osmosis can be reversed to
remove salt and other
impurities from water.
Passive & Active Transport
What is Passive Transport?
• The movement of
materials through
a cell membrane
without using
cellular energy.
• Diffusion and
osmosis are
examples of
passive transport.
Animation of
Facilitated Diffusion
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/anima
tion__how_facilitated_diffusion_works.html
What is Active Transport?
• The movement of
materials through a
cell membrane
using the cell’s
energy.
– The movement of a
substance in the
opposite direction
than they naturally
move by diffusion.
• Minerals
• Some sugars
• Most amino acids
Methods of Active Transport
• Transport Proteins
• “pick up” molecules
• Examples of
substances that are
carried . . .
– Calcium
– Potassium
– Sodium
• Engulfing
• Cell membrane
surrounds and
engulfs, or encloses, a
particle.
• A vacuole is formed
around the engulfed
particle.
• Energy is required by
the cell to perform this
function.
Animation of Active Transport
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/anima
tion__how_the_sodium_potassium_pump_works.html
“Transport” is like
Riding a Bike?
• Passive Transport
• Riding a bike down a
hill . . .
• Active Transport
• Pedaling a bike up a
hill . . .
Why Are Cells Small?
• Cells become less efficient as they
grow.
• The smaller they are, the easier it is
for them to do their jobs.
• The smaller they are, the easier it is
for substances to be moved in and
out.
Cells . . .
. . . More than meets the eye!!