CP Biology
2014-2015
UNIT 3B: Cell Membrane & Transport
Introduction to Cell Transport
Name ______ ____________
What types of materials need to be transported into or out of living cells?
IN: OUT:
Transport is the name of the life process in which materials are exchanged between and organism and its environment .
Transport can be as simple as moving materials between a one celled organism and its environment or as complex as the transport systems in higher organisms such as humans.
Whether it is simple or complex, the final point of transfer is at the cell membrane .
All cells live at least partly in touch with water-based
Example of transport in unicellular and multicellular organisms:
Blood Vessel
("aqueous") solutions. In order for substances to move into or out of a cell, they must cross the cell membrane between the water-based solution on the outside of the cell (its
"environment") and the water-based solution on the inside of the cell (="cytosol" or "cell solution").
Notes, summary, questions:
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(Textbook 7.2 pg. 204)
All cells contain some type of boundary that separates the inside of the cell from the outside environment.
Cell Wall
Some cells have rigid supportive structure called a cell wall surrounding the outside of the cell.
Recall: what polysaccharide makes up cell walls?
_______________________________________
Cell walls are porous and DO NOT regulate what goes in and out of a cell.
Cell Membrane
ALL cells have a cell membrane.
 Made of a double layered sheet called a phospholipid bilayer
 It is a flexible structure and forms a boundary between the cell and its environment.
 The cell membrane regulates what enters and leaves the cell and also protects and supports the cell
Cell membranes have this typical layered structure because of the chemical properties of the lipids that make up the membrane.
Notes, summary, questions:
The Properties of Lipids
What does hydrophilic mean?
_________________
What does hydrophobic mean?
_________________
Phospholipids have two main portions:
1) a hydrophilic head
2) Two hydrophobic fatty acid tails
Contains a polar (charged) phosphate group
Non-polar (not charged) oily regions of the molecule
Use the words attracted or not attracted to fill in the blanks below.
The hydrophilic heads are _________________ to water.
The hydrophobic tails are _________________ to water.
When these lipids are mixed with water, their hydrophobic fatty acid "tails" cluster together while their hydrophilic
"heads" are attracted to water.
A lipid bilayer is the result. (see picture on next page)
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The Fluid Mosaic Model
Protein molecules are embedded in the phospholipid bilayer of most cell membranes.
Because the protein molecules can move around and
"float" among the lipids ( fluid) , and because so many different kinds of molecules make up the cell membrane
(mosaic) , scientists describe the membrane as a “Fluid
Mosaic” , i.e.
What are all these different molecules doing?
Proteins a) Some proteins form channels and pumps to help move materials across the cell. b)
Some proteins
1) act as enzymes
2) can attach to the cell’s cytoskeleton
(internal support structure of the cell) helping cells to respond to the environment in order to move or change shape.
Carbohydrates act like chemical identification signs
(markers, allowing individual cells to recognize themselves (also to recognize itself from invaders).
The Phospholipid Bilayer
Which component of the phospholipids are hydrophobic?
_________________________________________
How are they oreinted in the bilayer?
_________________________________________
Which component of the phospholipids are hydrophilic?
__________________________________________
How are they oriented in the bilayer?
__________________________________________
Circle a membrane protein in the diagram above.
Put a box around a carbohydrate attached to the bilayer in the diagram above.
Notes, summary, questions:
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Selective Permeability
Some materials are allowed to enter and leave a cell, and some are not.
Since a cell needs to constantly exchange materials with its environment, many substances can cross biological membranes.
However, some substances are
 too large or
 too strong of a charge to pass across the phospholipid bilayer of the cell membrane.
Most biological membranes are selectively permeable , which means some substances can pass across the membrane, while others cannot pass through
Selectively permeable membranes can also be referred to as semipermeable membranes.
Notes, summary, questions:
What does it mean if we say that a membrane is impermeable to a substance?
_______________
Which of the following materials would move easily through the cell membrane?
 water
______________________________________________
 small molecules like O
2
and CO
2
_____________________________________________
 charged particles
 any other particles that the cell must have to survive
_____________________________________________
______________________________________________
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I.
PASSIVE TRANSPORT
One of the most important functions of the cell membrane is to keep the cell's internal conditions relatively stable .
In order to maintain this condition of homeostasis , the cell membrane must control the transport of materials into or out of the cell (in other words, from one side of the membrane to the other).
Passive Transport : the movement of materials across a cell membrane without using cellular energy.
1) Diffusion
2)
Facilitated Diffusion
3) Osmosis
All matter contains a certain amount of heat. This heat causes molecules to tend to spread out into the available space. In any solution, particles are moving constantly because of the kinetic energy of the particles.
Every living cell exists in a liquid environment; therefore we can look at the movement of molecules between the solution inside the cell and the solution outside the cell.
Solute: a substance that is dissolved in a solution
Solvent: the dissolving substance in a solution
Two examples of solutions (solutes in a solvent):
1) particles in air (faster)
Examples: __________________
___________________________
___________________________
2) particles in liquid (slower)
Examples: ______________________________________
_______________________________________________
How does heating a liquid affect the movement of solutes and solvents?
Notes, summary, questions:
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As a result of molecules moving constantly, colliding with one another and spreading out randomly the particles tend to move from an area where they are more concentrated to an area where they are less concentrated.
Concentration: the amount of particles in a given area
(solution) in relation to other particles (often expressed as a percent). Usually the amount of solute per unit solvent.
Diffusion: the process by which particles move from an area of high concentration to an area of lower concentration.
Diffusion of a solute (dye) in water:
The process of diffusion drives the movement of many molecules which move across cell membranes.
Suppose a substance is present in unequal amounts on either side of a cell membrane. If the substance can cross the membrane, the particles will tend to move toward the area where it is less concentrated until it is evenly distributed .
Concentration Gradient : conditions in which the concentration of particles in two different areas are different. *NOTE each molecule has its own concentration gradient in any given solution.
Notes, summary, questions:
In your own words summarize what occurred when the dye was placed in the water.
_______________________________________________
_______________________________________________
_______________________________________________
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When a solute is first added to a solvent, the concentration gradient is high . After the solute spreads out, the concentration gradient is low (or non-existent) . In diffusion, molecules move "down" or " with" the concentration gradient, from higher concentration to lower concentration.
Once the concentration of the substance on both sides of the cell membrane is the same, equilibrium is reached.
Particles of the solution will continue to move across the membrane in almost equal numbers, so there is no further net change to the concentration of the solutions inside or outside the cell.
Dynamic Equilibrium: condition in which the concentration of solute particles in a given area is equal throughout the entire area .
NO CONCENTRATION GRADIENT REMAINS.
There is no “net” movement!
Notes, summary, questions:
What will happen to the concentration gradient over time as diffusion continues?_____________________________
________________________________________________
When dynamic equilibrium is reached, diffusion is equal in all directions. Do the molecules in the solution stop moving?
________________________________________
________________________________________________
________________________________________________
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Facilitated Diffusion
Molecules which pass most easily through the cell membrane tend to be small and not charged allowing them to dissolve easily in the membrane's lipid environment.
However, some substances seem to pass more quickly through the membrane than they should - as though they have a shortcut through the membrane.
Ex: water aquaporins, ions like Cl- and sugars like glucose
How does this happen? Proteins through special protein channels. in the cell membrane act as carriers or channels making it easy for certain molecules to cross.
Facilitated Diffusion: process in which molecules that cannot directly diffuse across the cell membrane pass
There are hundreds of examples of these special proteins which are very specific (like enzymes) and change shape in order to allow the passage of certain substances into or out of the cell. Although facilitated diffusion is fast and specific , it is still diffusion so it does NOT require any energy from the cell. Also the net movement will still tend to be with or along the concentration gradient (high--> low). ATP is not needed and it will continue until equilibrium is reached.
Ex: RBC's have special protein carriers that allow glucose to pass in/out of the cell
Notes, summary, questions:
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OSMOSIS .
Water has been added to the list of molecules that enter cells by facilitated diffusion, thanks to surprising new research.
Many cells have special water channel proteins called aquaporins by facilitated diffusion. This extremely important biological process is given a special name:
Osmosis :
- an example of facilitated diffusion
which allow water to pass right through them osmosis the diffusion of water across a selectively permeable membrane.
.
Osmosis deals ONLY with the diffusion of WATER.
Osmosis is similar to other types of diffusion but in the case
Why would water molecules normally have a hard time getting across the cell membrane?
_______________________________________________
_______________________________________________
_______________________________________________ of osmosis the water (solvent) will move and not the solute particles.
The water molecules will tend to move from an area of high concentration to an area of low concentration until equilibrium is reached.
Notes, summary, questions:
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PREDICTING THE DIRECTION OF OSMOSIS IN
CELLS
The direction of water movement into or out of a cell can have dire consequences on the survival of a cell. By knowing the concentrations of solute and solvent on the inside and outside of a cell, we can predict the direction of osmosis and the result on the cell.
Solutions on the outside of a cell (in its environment, in other words) can be described based on how they affect the cell:
HYPERTONIC = "ABOVE strength"
Cell Shrinksthe solution outside the cell has a higher concentration of solutes compared to the solution inside the cell. Water moves outside of the cell and the cell shrinks .
This type of environment is called a Hypertonic Solution.
HYPOTONIC = "BELOW strength"
Cell Swellsthe solution outside the cell has a lower concentration of solutes compared to the solution inside the cell. Water moves into the cell and the cell swells . This type of environment is called a Hypotonic Solution.
Types of Solutions Outside the Cell that Affect the
Direction of Water Movement
Where is the concentration of water the highest?
________________________
Which direction does the water move?
________________________
Where is the concentration of water the highest?
________________________
Which direction does the water move?
________________________
Notes, summary, questions:
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ISOTONIC = "SAME strength"
Cell Stays the Samethe solution outside the cell has the same concentration of solutes compared to the solution inside the cell. Water moves equally in and out of the cell and the cell remains the same size . This type of environment is called an Isotonic Solution.
Solute and solvent concentrations can be expressed as percentages of the entire solution.
When added together, the solute and solvent concentrations must equal 100%. A solution with a 10 % solute concentration has a 90% solvent concentration.
What is the solvent concentration of a solution with a 3% concentration of solute? __________
What is the solvent concentration of a solution with a 15% concentration of glucose? __________
What is the solute concentration of a solution with 98% solvent? _________________________
What is the solute concentration of a solution with 75% water? _________________________
Given what you have learned about the movement of water , complete the following table:
Conditions
Solute concentration in the environment is equal to that in the cell
Solute concentration in the environment is greater than the cell
Solute concentration in the environment is less than the cell
Water will… Environment is…
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Ex: cell with 3% solute, 97% solvent
Beaker (environment) with 3% solute, 97% solvent a) ___________________________________ b) ___________________________________ c) ___________________________________
Ex: cell with 3% solute, 97% solvent
Beaker (environment) with 10% solute, 90% solvent a) __________________________________ b) __________________________________ c) __________________________________
Ex: cell with 3% solute, 97% solvent
Beaker (environment) with 1% solute, 99% solvent a) ___________________________________ b) ___________________________________ c) ___________________________________
Notes, summary, questions:
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Osmotic Pressure
Driven by differences in solute concentration, the net movement of water into or out of a cell produces a force known as osmotic pressure.
Because cells contain a variety of solutes (sugars, proteins, salts, etc.), they are almost always hypertonic fresh water; as a result, a typical cell exposed to fresh water will tend to swell up quickly from the entering water. This may in fact cause an animal cell to swell like an overinflated balloon.
Plant cells contain a central vacuole which stores excess water - shrinking and swelling as water enters or exits the cell.
Plant cells wouldn't generally burst thanks to their protective cell walls . In fact, most cells in large organisms are not in contact with fresh water on a regular basis - rather, they tend to be bathed in blood or other isotonic fluids which have solute concentrations approximately equal to themselves.
Cells which are plump and rigid in hypotonic environments are called turgid ; when a cell shrinks in a hypertonic environment this is called plasmolysis .
Given what you have learned about the movement of water, complete the following table:
Notes, summary, questions:
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Conditions
Environment
Solute concentration in the environment is equal to that in the cell:
Isotonic solution
Plant Cell (leaf cell)
Before After
Solute concentration in the environment is greater than the cell:
1.
Hypertonic
Solution
Solute concentration in the environment is less than the cell:
Hypotonic Solution
Animal cell (blood cell)
Before After
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II.
ACTIVE TRANSPORT
Active transport is any type of transport into or out of a cell that requires energy (ATP).
Active transport may involve the movement of molecules against a concentration difference, OR it may involve the movement of direction. large materials or clumps of materials into or out of a cell (requiring changes to the shape of the cell membrane).
A) Molecular Transport
In active transport, small molecules and ions are carried across the cell membrane by special proteins in the membrane that act like pumps and change their shape move molecules into or out of the cell.
to
The advantage of this type of transport (which requires a lot of a cell's daily energy!) is that it enables cells to concentrate substances in a particular location, even when diffusion might normally move the molecules in a different
Outside of cell
Low Solute concentration solute solute
Example:
__________________________________________
Inside of cell
High Solute concentration
Notes, summary, questions:
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B) Bulk Transport Endocytosis
The exact way in which large molecules or even clumps of material move across the cell membrane depends on the size and shape of the material being moved into or out of the cell. Due to the large changes made to the cell membrane, this is also a form of active transport and ATP is required .
Endocytosis : process of taking material into the cell by means of infoldings or pockets of cell membrane.
Ex: amoeba feeding Ex: WBC engulfing RBC
The "pocket" that is formed will break loose from the cell membrane and form a vesicle or vacuole within the cytoplasm. Some cells use this method to engulf large molecules, food, or other cells. Two types include:
Exocytosis
 Phagocytosis: type of endocytosis where solid (bulk) materials like foods are transported into a cell.
Ex: Amoeba engulfing food
 Pinocytosis: type of endocytosis where liquids (like solutions) are transported into a cell.
Exocytosis: the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell.
Example: Contractile vacuole in paramecium
Notes, summary, questions:
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