Chapter 4
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Chapter 4. Cells and Their Environment. I. Diffusion A. Random Motion and Concentration Passive transport is movement across the cell membrane that does not require energy A concentration gradient is a difference in the concentration of a substance between two areas of space

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Chapter 4

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Chapter 4

Chapter 4

Cells and Their Environment


Chapter 4

I. Diffusion

A. Random Motion and Concentration

  • Passive transport is movement across the cell membrane that does not require energy

  • A concentration gradient is a difference in the concentration of a substance between two areas of space

  • Equilibrium is a condition in which the concentration of a substance becomes equal throughout a space

Equilibrium


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B. Movement of Substances

  • If there is a concentration gradient between two areas or substances, one substance will move from an area of high concentration to an area of lower concentration

    - this is called diffusion and it will continue until equilibrium is reached


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  • Many substances in the cytoplasm and in the fluid outside cells, enter or leave cells by diffusing across the cell membrane when there is a concentration gradient of that substance between the inside and outside of the cell


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  • The cell membrane is selectively permeable

    a. substances that are either very small or nonpolar can diffuse across the cell membrane down their concentration gradient by passive transport

    b. the nonpolar interior of the lipid bilayer repels ions and most polar molecules so they are prevented from diffusing across the cell membrane


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II. Osmosis

  • Osmosis is the diffusion of water through a selectively permeable membrane down its concentration gradient

  • This movement of water helps to balance the concentration of ions or polar molecules so that they are equal inside and out of the cell


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  • 3 possible directions of water movement

    1. Water moves out = hypertonic solution

  • In a hypertonic solution, there is a higher concentration of dissolved particles outside of the cell than in the cytoplasm

  • The concentration of free water molecules is lower outside of the cell

  • Water moves out of the cell until equilibrium is reached, causing the cell to shrink


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2. Water moves in = hypotonic solution

  • In a hypotonic solution, there is a lower concentration of dissolved particles outside of the cell than in the cytoplasm

  • The concentration of free water molecules is higher outside of the cell

  • Water moves in, causing the cell to swell


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3. No net water movement = isotonic solution

  • In an isotonic solution, the concentration of dissolved particles outside of the cell is equal to the concentration inside of the cell

  • Water moves into and out of the cell at equal rates


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III. Crossing the Cell Membrane

  • Transport proteins called channels provide polar passageways that allow ions and polar molecules across the cell membrane

  • Transport proteins are selective, they only allow certain substances in

  • There are different transport proteins for different substances


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  • Diffusion Through Ion Channels

  • Ion channels are transport proteins with a polar pore that allows ions to pass through the cell membrane

  • Diffusion of ions through ion channels is a form of passive transport; energy is not needed because the ions move down their concentration gradient


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  • There are two types of ion channels

    1. Ion channels that are always open

    2. Ion channels that can be closed by ion channel gates

  • Ion channel gates may open or close in response to different kinds of stimuli

    a. stretching of the cell membrane

    b. a change in electrical charge

    c. the binding of specific molecules to the ion channel

http://www.youtube.com/watch?v=Du-BwT0Ul2M


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B. Electrical Charge and Ion Transport

  • The inside of a typical cell is negatively charged compared to the outside of the cell

  • Remember opposite charges attract, and like charges repel

  • Thus a more positively charged ion located outside the cell is more likely to diffuse into the negative cell

  • A more negatively charged ion located inside the cell is more likely to diffuse out of the cell


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C. Facilitated Diffusion

  • A type of passive transport that moves substances down their concentration gradient by using a carrier protein but without using the cell’s energy

  • Three steps to facilitated diffusion

    1. Specific molecules, such as amino acids and sugars, bind to the carrier protein

    2. The carrier protein changes shape to expose the molecule to the other side of the cell membrane

    3. The carrier protein releases the molecule and returns to its original shape

http://www.youtube.com/watch?v=PYP4JYL-KQk


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IV. Movement Against a Concentration Gradient

  • Sometimes substances need to move against their concentration gradientsdepending on what the cell needs or does not need

    - substances have to move from an area of low concentration to an area of higher concentration


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  • Substances moving against their concentration gradients need energy to do so

  • Active transport proteins use the cell energy in the form of ATP to move substances against their concentration gradients

http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related


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  • Sodium-Potassium Pump (Na+/K+ Pump)

  • Actively transports both sodium and potassium against their concentration gradients by using ATP

    1. Transports 3 sodium ions, Na+, out of the cell where the concentration of sodium is usually higher

    2. Transports 2 potassium ions, K+, into the cell where the concentration of potassium is usually higher


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  • There are 4 steps to the Na+/K+ Pump

    1. 3 sodium ions, Na+, inside the cell bind to the pump. ATP gives up one of its phosphate groups which also binds to the pump

    2. The pump changes shape and releases the sodium ions outside the cell

    3. 2 potassium ions, K+, bind to the pump. The phosphate group releases from the pump

    4. The pump changes shape and releases the potassium ions into the cell

http://www.youtube.com/watch?v=SByeTZKAR1Q


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  • The pump is important for two main reasons

    1. It prevents too many sodium ions, Na+, from accumulating in the cell

    - sodium attracts water into the cell by osmosis which would cause the cell to burst

    2. It maintains the sodium and potassium ion gradients

    - the gradient helps transport other substances, such as glucose, across the cell membrane


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V. Movement in Vesicles

  • A vesicle forms when part of the cell membrane or Golgi apparatus surrounds the materials to be taken into the cell or released from the cell

  • Substances such as proteins and polysaccharides are too large to diffuse across the cell membrane or be transported through carrier proteins so they require vesicles to move across the cell membrane


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  • Endocytosis is the movement of a substance into a cell by a vesicle

    - the cell membrane forms a pouch around a substance, the pouch then closes up and pinches off to form a vesicle, bringing the substance into the cell


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  • Exocytosis is the movement of a substance outside of a cell

    - the Golgi apparatus forms a pouch around modified proteins within it, the pouch then closes up and fused with the cell membrane, releasing the substance outside of the cell

http://www.youtube.com/watch?v=4gLtk8Yc1Zc&feature=plcp


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VI. Membrane Receptor Proteins

  • Cells must be able to communicate with one another to coordinate your growth, metabolism, and other activities

  • Cells release signal molecules to nearby cells to communicate

    ex. Hormones are signal molecules that are made in one part of your body and carried in the bloodstream to other parts, where they carry out their job

http://www.youtube.com/watch?v=kIPYVV4aThM


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  • Receptor proteins on cells contain special sequences that bind to specific signal molecules outside of the cell so the cell can ignore other unimportant signal molecules


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  • Functions of Receptor Proteins

  • When a signal molecule binds to a receptor protein on a cell, the cell experiences a change by one of three ways

    1. Changes in permeability

  • A receptor protein may be bound to an ion channel. When a signal molecule binds to the receptor protein, the ion channel is forced open allowing specific ions to cross the cell membrane


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2. Second Messengers

  • A receptor protein may be bound to an intermediary protein that lays just inside the cell membrane

  • When a signal molecule binds to the receptor protein, the intermediary protein is activated and becomes a second messenger inside of the cell. The second messenger “forwards” the message from the signal molecule that remains outside of the cell

http://www.youtube.com/watch?v=4dUJ5GNpfrA


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3. Enzyme action

  • Receptor proteins may act like enzymes that speed up chemical processes inside of the cell when bound by signal molecules

  • Drugs act on these receptor proteins in two different ways

    1. By imitating specific signal molecules so they are always bound to receptor proteins making the reactions in your cells run constantly


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2. By blocking signal molecules from binding to the receptor proteins so the chemical reactions in your cells slow down

ex. Beta blockers are prescribed to patients with a rapid heartbeat


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