Structure, Function and the Movement of Materials The Cell Membrane Structure, Function and the Movement of Materials

Structure of the Cell Membrane The cell membrane is a bi-layered membrane composed mainly of phospholipids.

Fluid-mosaic membrane model This model is used to describe the appearance of the cell membrane. It states: Phospholipids form a bilayer There are other molecules located throughout this bilayer (such as cholesterol) The membrane has a fluid consistency which allows molecules to move sideways within the bilayer

How is homeostasis achieved in the cell? The cell membrane is selectively permeable - certain substances can pass into the cell.

How do particles move across the cell membrane? Diffusion Osmosis Facilitated Diffusion Active Transport - Endocytosis, Pinocytosis, Phagocytosis, and Exocytosis

1. Diffusion A method of passive transport where small molecules pass through the cell membrane This random constant movement of molecules in a liquid is called diffusion.

Molecules from a region of high to low concentration. The difference between these regions is called the concentration gradient

Ex. oxygen and carbon dioxide As oxygen gets used in the cell, more oxygen diffuses across the membrane. Carbon dioxide that is generated inside the cell, leaves the cell through diffusion.

2. Osmosis The diffusion of water across a cell membrane. Water molecules (solvent) move freely across the membrane, from an area of HIGH concentration to an area of LOW concentration

Three osmotic conditions are: Isotonic Condition: This occurs when the concentration of water inside the cell equals the concentration outside the cell.  There is an equal amount of water entering as there is leaving the cell and no change to the cell. Hypotonic Condition: When the concentration of water outside the cell is greater than that inside the cell.  More water will move into the cell causing it to swell.  Animal cells may burst and plant cells will become rigid.

Hypertonic Condition: When the concentration of water is greater inside the cell than outside.  More water will migrate out of the cell than enter it.  In both animal and plant cells the cytoplasm shrinks.

Dilute solution Concentrate solution Water Semi permeable membrane

Osmosis in animal and plant cells

3. Facilitated Diffusion? The passive movement of a substance in or out of the cell, through a special protein gate. Some substances, like the glucose molecule, are too big to freely move across the cell membrane.

Are selective and will only allow certain molecules to cross the membrane.

How facilitated diffusion works: NO ENERGY is being used. How facilitated diffusion works: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

What is Active Transport? The process of moving material across a cell membrane AGAINST the concentration gradient.

This process is important for several reasons: 1. It is necessary to maintain nutrients inside the cell for the growth and maintenance 2. All waste products from organelles must be removed completely from inside the cell because they are toxic.

This process requires energy, called ATP. Substances are transported from an area of LOW concentration to an area of HIGH concentration. This process requires energy, called ATP. (It is like pushing an object up a hill) Approximately 40% of our energy is used for active transport at rest. http://www.youtube.com/watch?v=STzOiRqzzL4

Some examples of these specialized cells are: Kidney cells pump glucose and amino acids out of urine and back into the blood. Intestinal cells pump in nutrients from the gut. Root tip cells pump in nutrients from the soil. Gill cells in fish pump out sodium ions.

To transport substances in and out of the cell, against the concentration gradient, a specialized CARRIER PROTEIN is used, which is a type of protein. An example of a protein pump is the SODIUM/POTASSIUM PUMP. It pumps SODIUM out of the cell and POTASSIUM into the cell.

This pump is working constantly for several reasons: To store NUTRIENTS it needs for maintenance and growth. 2. To create an electrical potential across the cell membrane to allow NERVE CELLS and MUSCLES to work

How sodium/potassium pump works: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Review Watch this video to review how diffusion, osmosis and active transport works http://www.abpischools.org.uk/page/modules/homeostasis_kidneys/kidneys3.cfm?coSiteNavigation_allTopic=1

What is Bulk Membrane Transport? Sometimes molecules are too large to fit through protein gates. Therefore, these molecules have to pass through the lipid bilayer of the cell membrane. In order to do this, a vesicle is used to form a “bubble” around the molecule so it can pass through the membrane

This vesicle is often made from the cell membrane

Types of Bulk Membrane Transport Endocytosis (think ENter) Pinocytosis, Phagocytosis, Receptor Mediated Endocytosis Exocytosis (think Exit)

Endocytosis The process where the cell membrane folds into itself and pinches off forming a vesicle.

Pinocytosis: (Cell Drinking) This process involves the cell taking in of small droplets of extracellular fluid along with dissolved materials. The cell membrane encircles the droplets and pinches off inside the cell once it is inside.

Phagocytosis: (Cell Eating) Same process of the cell membrane encircling the particles and then pinching off inside the cell. The particles are much bigger and more specific, such as worn out cell fragments or bacteria. Specialized cells within the body or single celled organisms carry out this process.

Receptor-assisted Endocytosis Large, specific molecules enter the cell using this method Example: Cholesterol is a fat that all cells need, but it cannot pass through the membrane without help.

How does cholesterol get inside the cell? Each droplet of blood has a tag, which is a protein marker that will match a protein receptor on the cell membrane. The tag binds to the receptor and this stimulates endocytosis, bringing the cholesterol into the cell. The vesicle empties its contents into the cell and returns to the cell membrane.

Review of ENdocytosis

Exocytosis This is the reverse process of endocytosis. Vesicles inside the cell move to the cell membrane where they empty their contents outside the cell. In this way vesicle material is returned to the cell membrane.

Endocytosis vs. Exocytosis – Review Videos http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis http://bcs.whfreeman.com/thelifewire8e/content/cat_040/0504003.html