7-3: Cell Boundaries
Biology 1
Introduction
• All cells are surrounded by a
thin, flexible barrier
– CELL MEMBRANE: thin, flexible
barrier around a cell; regulates what
enters and leaves the cell
• Plant cells are also supported
by a strong layer
– CELL WALL: strong supporting layer
around the cell membrane in plants,
algae, and some bacteria
Cell
Membrane
• Cell membrane regulates
what enters and leaves the
cell, also provides protection
and support
• Cell membranes are made
from double-layered sheets
– LIPID BILAYER: double-layered
sheet that forms the core of nearly
all cell membranes
Cell Wall
• Cell walls are present in
plants, algae, fungi and
prokaryotes
• Main function is to provide
support and protection
• Made of mostly carbs and
proteins
Diffusion
• Most important function of
cell membrane is to move
molecules from one side to
other
• DIFFUSION: process by which
molecules tend to move from
an area where they are more
concentrated to an area where
they are less concentrated
Diffusion
• Particles move back and forth
across cell membrane
• Particles continue moving until
they are equal everywhere
– EQUILIBRIUM: when the concentration
of a solute is the same throughout a
solution
• When perfume is sprayed in one
corner, then the whole room
smells
Osmosis
• The movement of water
through the membrane of a
cell
• OSMOSIS: diffusion of water
through a selectively
permeable membrane
Osmosis
• Water wants to move to where
there is LESS
• ISOTONIC: when the
concentration of two solutions is
the same
• HYPERTONIC: when comparing
two solutions, the solution with
the greater concentration of
solutes
• HYPOTONIC: when comparing
two solutions, the solution with
the lesser concentration of
solutes
Facilitated
Diffusion
• Some molecules (like sugar) pass
through cells very quickly
• Cell membranes have “helper”
proteins that allow certain
molecules to cross
– ONLY allow specific molecule (like
sugar) to pass through)
• FACILITATED DIFFUSION:
movement of specific molecules
across cell membranes through
protein channels
Great Pacific Media
Cell Membrane Passive Transport
http://www.youtube.com/watch?v=JShwXBWGMyY