Cells and Their Environment B-2-5 Explain how active, passive, and facilitated transport serve to maintain the homeostasis of the cell.

Cell Membrane  the gatekeeper  Monitor what enters and exits the cell  Maintains homeostasis  Composed of phospholipids

 The heads of the PHOSPHOLIPIDS are composed of glycerol and a phosphate group and like to dissolve in water.  The heads of the PHOSPHOLIPIDS are composed of glycerol and a phosphate group and like to dissolve in water.  The tails of the PHOSPHOLIPIDS are mostly fatty acids made up of long carbon and hydrogen chains.  The tails of the PHOSPHOLIPIDS are mostly fatty acids made up of long carbon and hydrogen chains.

How does this work? Polarity  Hydrophillic “water loving”  phosphate group has a negative charge, and an affinity for water.  PO 4 is attracted to H 2 O  Hydrophobic “water hating”  Insoluble in water  Fatty acids repel H 2 O

Embedded Proteins  Marker – cells to recognize their cell type  Receptor – enables the cell to detect and respond to its surroundings.  Transport - aid in moving substances into and out of the cell  Enzymes – assist with chemical reactions

Let’s Review  Homeostasis is the ability of a cell to maintain a stable, constant, internal environment despite changes that occur outside of the cell membrane.  Cell membrane monitors “what enters and exits the cell”.

How is homeostasis maintained?  Passive transport is the movement of materials across the cell membrane without the use of energy.  Uses a concentration gradient  Examples: Diffusion, Osmosis, and Facilitated Diffusion

What is a gradient?  Concentration gradient - a difference between concentrations in a space.  The natural movement of molecules is from an area of high amount of material to an area of low amount.

Diffusion  Diffusion - the process by which molecules spread from areas of high concentration, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM 

Osmosis  Osmosis - the diffusion of water (across a membrane)  Water will move in the direction where there is a high concentration of solute (and hence a lower concentration of water).  Selectively Permeable - membranes that allow some things through, the cell membrane is selectively permeable, water and oxygen move freely across the cell's membrane, by diffusion

Osmosis 

Watch This  Osmosis Animation   Watch this animation of water molecules moving across a selectively permeable membrane. Water molecules are the small blue shapes, and the solute is the green.  What's happening? The solute (green blobs) is more concentrated on the right side, which pulls the water molecules toward that side. The green blobs would move to the left to spread out evenly, but the membrane won't let those pass.

Salt Sucks!  A simple rule to remember is: SALT SUCKS  Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty. 

Type of Solutions Hypotonic - IsotonicHypertonic Osmosis

Facilitated Diffusion  Sometimes, large molecules cannot cross the plasma membrane, and are "helped" across by carrier proteins - this process is called facilitated diffusion. facilitated diffusion animation facilitated diffusion animation

Active Transport  Active Transport - this type of transport requires that the cell use energy, because substances are moving against the concentration gradient.  Moving from an area of low concentration to an area of high concentration

Sodium Potassium Pump  Animations Animations 

Other Examples  Endocytosis - cell takes in large particles by engulfing them  Exocytosis - cell gets rid of particles, opposite of endocytosis 

What do you know? 