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 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: Areas of equal concentration

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

Hypoto

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Hypert

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Isoto

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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