Cell
Membrane
Chapter 3 and 4 – 9th Grade
Biology
Cell Membrane (Plasma Membrane)
• Outermost boundary of a cell
• Encloses the cell’s cytoplasm and
organelles from its surroundings
• Regulates what enters and leaves the
cell such as gases, nutrients, and
waste.
Cell Membrane Structure
• Fluid structure
(Not rigid like an
egg shell)
• The membrane
can choose what
enters and leaves
the cell
• Formed by a lipid
(fat) layer
The Phospholipid Layer
• The lipid layer is made up of phospholipids
• Phospholipid: a phosphate group with two fatty
acids attached
• Contains a polar “head” and two nonpolar “tails”
POLAR
HEAD
NON-POLAR TAILS
Phospholipids
Structure
• Polar “Head” is
Hydrophilic or
water loving
• Non-polar “Tails”
are Hydrophobic
or water fearing
Phospholipids
Phospholipid Bilayer
• Phospholipids form a bilayer to create the
cell membrane
The Phospholipid Bilayer
This bilayer is selectively permeable – it
determines what can enter and leave the cell
using proteins
Membrane Proteins
• Membrane proteins are imbedded in the bilayer
• Different types of proteins exist within the
membrane:
• Transport Proteins – Transports materials in and
out of the cell using channels
• Glycoproteins – Cell to cell communication
• Receptor Proteins – Helps cells communicate with
environment to form a reaction
• Enzymes – Helps with reactions within the cell
• Structural Proteins – Gives cell support and shape
• Marker Proteins – Advertises cell type
Proteins within the Membrane
Crossing the Cell Membrane
• Some substances like water can pass freely
through the phospholipid layer
• Other substances, like ions, need to use proteins to
pass through due to size and polarity
• Different methods may require or not require
energy
• Types of transport: PASSIVE and ACTIVE
Passive Transport
• This is the process of moving substances DOWN the
concentration gradient to reach equilibrium
• Uses NO energy
• Types: Diffusion, Facilitated Diffusion, and Osmosis
Passive Transport : Diffusion
• The process that requires no energy and involves
substances moving from an area of high
concentration to an area of low concentration.
• Diffuse means “to spread out”
• Results from the random movement of molecules
due to kinetic energy
Passive Transport : Facilitated
Diffusion
• Facilitated diffusion is a type of passive transport
• Carrier Proteins, a type of transport protein, uses
this method.
Facilitated Diffusion continued…
• Transport proteins are needed to allow specific
substances to pass into and out of the cell
• Use channels, which are polar passageways, for ions
and polar substances
Ion Channels
• Sodium (Na+), Potassium (K+), Calcium (Ca+2), and
Chloride (Cl-) are used for very important cell
functions
• Functions such as nerve impulses, heart and muscle
contraction
• These ions cannot freely pass through the bilayer, so
they need ion channels.
Ion Channels
• An ion channel may be always open, or only
open when stimulated
• This transport is passive, where no energy
from the cell is required to move materials.
Passive Transport : Osmosis
• Osmosis is the diffusion of water down its
concentration gradient through a semi-permeable
membrane
• Moving from low solute concentration to high
solute concentration (High water concentration to
low water concentration)
• Solute molecules (sugar, ions) cannot pass through
membrane so water must move across
Osmosis
• Hypertonic: Area with high concentration of solute
• Hypotonic: Area with low concentration of solute
Isotonic
Hypertonic
Hypotonic
• Isotonic: Areas of equal concentration
• http://www.youtube.com/watch?feature=fvwp&v=0c8ac
UE9Itw&NR=1
Active Transport
• This method involves moving substances AGAINST the
concentration gradient.
• Energy from the cell is required to move substances
Sodium – Potassium Pump
• Most important membrane pump in animal cells.
• Active transport pumps sodium ions out of the cell
and potassium ions into the cell.