Cell MembraneChapter 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 BilayerThis 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: Areas of equal concentration

• http://www.youtube.com/watch?feature=fvwp&v=0c8acUE9Itw&NR=1

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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.

3 Sodium Ions out, 2 Potassium Ions in