Download Membrane Transport and Function

Membrane Transport and
Function
Pages 175-178, 195-200
Cell Wall
• The cell wall is a secondary boundary surrounding
certain cells.
• It lies outside the cell membrane and is porous
enough to allow water, oxygen, carbon dioxide,
and certain other substances to pass through.
• The cell wall is made of cellulose in plants, chitin
in fungi, and peptidoglycan in bacteria.
• The purpose of the cell wall is to provide
protection and support.
The Cell Membrane
• The cell membrane (or plasma membrane) is the major
barrier for all cells.
• It is composed of a phospholipid bilayer.
• A phospholipid is a lipid (fat) that has replaced one fatty
acid chain with a phosphate group.
• The phosphate “head” is hydrophilic and attracted to water.
The fatty acid “tails” are hydrophobic and are repelled by
water.
• The cell membrane is made of two sheets of phospholipids
that have formed a bilayer with the fatty acids towards the
middle of the bilayer and the phosphates towards the
exterior and interior of the cell.
http://aecbio11.wikia.com/wiki/Fi
le:Phospholipid_structure.jpg
http://bio1151.nicerweb.com/Loc
ked/media/ch07/bilayer.html
Further Membrane Structure
• The plasma membrane has proteins
embedded throughout it as well.
• These proteins aid the membrane in
transporting substances in and out of the cell.
• The membrane is also supported by
cholesterol molecules.
http://www.bio12.com/notes/ch3/ch3c.htm
Properties of the Plasma Membrane
• Because the plasma membrane is able to control what
passes through and lipids are hydrophobic, the cell
membrane is called semi-permeable (or selectively
permeable).
• Only certain substances are able to pass directly through
the plasma membrane. These are usually small nonpolar
molecules such as oxygen and carbon dioxide.
• Charged particles, polar molecules and large molecules
are unable to pass directly through the membrane and
require special transport proteins.
• Because the proteins are not bound to a specific location
in the membrane and are free to float around, the
membrane is described as a fluid-mosaic.
Movement of Substances
• Substances are always moving by diffusion.
• Diffusion is the random movement of molecules
from an area of high concentration to an area of
low concentration.
• Concentration is usually expressed as a molar
concentration (symbolized by a capital M) or as a
percentage.
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapt
er2/animation__how_diffusion_works.html
Diffusion Across the Membrane
• Many substances enter cells by diffusion.
• When the concentration of substances is the same on
both sides of the membrane, equilibrium is reached.
• However, substances still pass back and forth across the
membrane after equilibrium has been reached. Because
the concentration of substances on either side of the
membrane doesn’t change this is called dynamic
equilibrium.
• Osmosis is a special case of diffusion. It occurs when
water diffuses across a semi-permeable membrane.
Cell Behavior
• Cells will behave differently based on the concentration
of the solution they are placed in.
• An isotonic solution is one where the concentration of
substances outside the cell is the same as the
concentration of substances inside the cell.
• A hypertonic solution is one where the concentration
of substances outside the cell is higher than the
concentration of substances inside the cell.
• A hypotonic solution is one where the concentration of
substances outside the cell is lower than the
concentration of substances inside the cell.
• Water will diffuse across the cell membrane until
dynamic equilibrium is reached and the exterior
solution is isotonic.
• If cells are placed in isotonic solutions, water is
moving into the cell at the same rate water is
leaving the cell. The cell remains unchanged.
• If cells are placed in hypotonic solutions, water is
moving into the cells at a higher rate than water
is leaving the cell. The cell swells. Animal cells
can burst. Cell walls prevent plant cells from
bursting. The cells become turgid, and the plant
as a whole stands upright.
• If cells are placed in hypertonic solutions, water is
moving out of the cells at a higher rate than
water is entering the cells. The cells shrivel and
can die. In plant cells, the membrane pulls away
from the cell wall, and the plant as a whole wilts.
http://mcatdaily.blogspot.com/2010/05/difference-between-hypertonichypotonic.html
Material Acquisition
• Cells have two methods to obtain materials
from their environment.
• Passive transport is a method that does not
require energy. Substances move by diffusion
with the concentration gradient.
• Active transport is a method that does require
energy. Substances move against the
concentration gradient (low concentration to
high concentration).
Passive Transport
• Passive transport occurs anytime substances
pass through the membrane with the
concentration gradient.
• Some substances cannot pass directly through
the membrane. They travel through special
transport proteins. Because this occurs with
the concentration gradient, it is called
facilitated diffusion.
http://biology.about.com/od/cellularprocesses/ss/diffusion_2.htm
Active Transport
• Active transport requires energy to move
substances into a cell against the
concentration gradient.
• Active transport will always use some form of
transport protein.