Cell Membrane � Outer membrane of

the cell to protect its contents

� Regulates the molecules that pass in and out of the cell (selectively permeable)

� Give the cell its shape

Cell Membrane

•  Outer membrane of the cell to protect its contents •  Regulates the molecules that pass in and out of the cell •  Give the cell its shape •  Maintains Homeostasis

Cell Membrane The cell

membrane is flexible and allows a unicellular organism to move

Cellular Homeostasis

�  Cells subject to constant changes

�  Must keep internal concentrations of substances like water, glucose, proteins, ions, & nutrients at a constant level, while eliminating waste

�  The cell does this through the cell membrane because it is selectively permeable

FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.

MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

Fluid Mosaic Model

This model is described as a phospholipid bilayer with proteins embedded in this double layer.

Structural Components:

1.  Phospholipids

2.  Integral/Channel proteins

3.  Receptor/Peripheral proteins

4.  Marker/Glyco proteins

5.  Carbohydrate chains

6.  Cytoskeleton

7.  Cholesterol

Phospholipids �  Each phospholipid has a “head” & “tails”

Head – phosphate group PO4

Tails - lipids

Proteins (peripheral and integral)

Membrane Components

Phospholipid Bilayer

Each phospholipid is an individual unit that is electrically attracted to another phospholipid.

These units “line up” beside each other to form a barrier. They are not connected! (Think bobbing for apples)

A double layer is formed, with the internal layer inverted (upside down)

Embedded Proteins

3 types:

1. Channel/Integral Proteins – act as the gateways in/out of the cell for transporting large food molecules, waste, & other molecules – each gate only allows passage of certain items. How can these proteins be selectively permeable?

2.  Receptor/Peripheral Proteins – communication link between the inside of the cell & the outside world

3.  Marker/Glyco protein – identifies the kind of cell & that the cell belongs to that organism – name tag that says “heart cell belonging to John”

The tag itself is a carbohydrate chain connected to the protein.

Why do your cells need name tags?

Diffusion

� The net movement of particles (liquid or gas) from an area of high concentration to an area of low concentration.

� No energy is required for these particles to move

Diffusion

An obnoxious individual with a lit cigar gets on an elevator on the 3rd floor with the cigar still burning. You are unfortunate to be in a tall building and he comments, “Hey we are both going to the 62nd floor!”

What are some of your own examples of diffusion?

Diffusion

Dynamic equilibrium

Diffusion of Liquids

Diffusion through a Membrane

Cell membrane

Solute moves DOWN concentration gradient

(HIGH to LOW)

Special name for Diffusion of Water: Osmosis

�  The movement of water across a selectively permeable membrane from from high concentration to low concentration.

�  Water moves down the concentration gradient

�  No energy (ATP) is required from the cell

Diffusion of H2O Across A Membrane/ OSMOSIS

HIGH CONCENTRATION to LOW CONCENTRATION

Dynamic Equilibrium

Osmosis Illustration

http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Environment surrounding a cell determines whether water will enter the cell or leave the cell.

Cells in Solutions

Osmosis in Red Blood Cells

Red Blood cell swelled & broke (lysed)

Remember:Diffusion does NOT require energy (ATP)

However, some types of movement across cell membrane do require energy (ATP):

Active Transport

-Requires energy (ATP) WHY???

-Moves materials from LOW to HIGH concentration

-AGAINST concentration gradient

EX. PUMPS, Endocytosis, Exocytosis

Active transport

v Examples: PUMPS Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.

v Called Na+-K+ Pump

Active Transport cont… Endocytosis – moving the big things in

Ex. Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc.

Active Transport cont… Pinocytosis (specific type of endocytosis)

Most common form of endocytosis. Takes in dissolved molecules as a vesicle.

Pinocytosis �  Cell forms an

invagination

�  Materials dissolve in water to be brought into cell

�  Called “Cell Drinking”

Example of Pinocytosis pinocytic vesicles forming mature transport vesicle

Transport across a capillary cell (blue).

Active transport cont… Phagocytosis (specific type of endocytosis)

Used to engulf large particles such as food, bacteria, etc. into vesicles

Called “Cell Eating”

Phagocytosis - Capture of a Yeast Cell (yellow) by Membrane Extensions of an Immune System Cell (blue)

Exocytosis The opposite of endocytosis is exocytosis. Large molecules that are

manufactured in the cell are released through the cell membrane.

Inside Cell Extracellular Environment

Moving the “Big Stuff” out

Molecules are moved out of the cell by vesicles that fuse with the plasma membrane.

Exocytosis- moving things out.

This is how many hormones are secreted and how nerve cells communicate with one another.

Moving the “Big Stuff”