Download Across the Membrane

Across the
Membrane
Movement of cellular
materials in and out of
the cell.
Cell membranes help
organisms maintain
homeostasis by controlling
what substances may enter or
leave cells.
Passive vs. Active Transport:
• Movement of substances without energy input
by the cell is a passive transport process
.
• Movement of substances with energy input by
the cell (ATPs) is an active transport process.
Diffusion
Diffusion – (simplest type of passive
transport)
The random movement of molecules
from an area of higher concentration
to an area of lower concentration.
It is driven by the kinetic energy of
the molecules.
• Diffusion movement occurs with, (or down)
the concentration gradient.
• A concentration gradient is a difference
in concentration of molecules across a
space.
• Diffusion will eventually cause the concentration
of molecules to be the same throughout the
space.
• When the concentration of molecules are the
same throughout the space, equilibrium is
reached and the concentration gradient
disappears.
• When equilibrium is reached, there is no net
movement of molecules
Modern Biology (Holt)
Diffusion & the cell membrane
The ability of substances to diffuse across a cell
membrane depends upon the size and type of the
molecules, as well as the chemical nature of the
cell membrane.
Cell membranes are selectively-permeable in
that they allow only certain substances to pass.
[Molecules that dissolve in lipids (such as CO2 O2),
diffuse through the cell membrane. Small
molecules that aren’t soluble, move through
membrane pores (such as H2O)].
Osmosis
The process by which a solvent (such as water)
diffuses across a selectively-permeable
membrane (such as a cell membrane) is called
osmosis. Osmosis always occurs with the
concentration gradient, since it is a form of diffusion.
Modern Biology (Holt)
THREE DIFFERENT SOLUTIONS:
Isotonic, Hypotonic, & Hypertonic
Modern Biology (Holt)
Modern Biology (Holt)
Facilitated Diffusion
It is a type of passive transport in which
molecules that cannot diffuse rapidly
into the cell, pass through pores in the
cell membrane.
Carrier proteins help move molecules
across the membrane. They bind to the
molecule they carry. The carrier molecule
changes shape to shield the molecule from
the interior of the cell membrane.
Modern Biology (Holt)
DIFFUSION THROUGH ION
CHANNELS:
Ion channels provide small passageways across the
membrane for the purpose of ion diffusion.
Ions such as Na+ & Cl- , that are not soluble in lipids,
can move through specific ion channels (Na+ moves
through Na+ channels)
Gated Ion Channels
• Some ion channels are always open, while
others are called gated channels and may
open or close in response to stimuli such
as:
• stretching of the cell membrane
• chemicals in the cell cytoplasm
• chemicals in the cell’s external
environment
• electrical signals