Biology Honors Name

(Ref: Section 7.3 in textbook)

Cell Transport Notes – Part I

This unit deals with how molecules the cell needs such as oxygen and nutrients pass through the cell membrane into the cell, and how wastes the cell needs to get rid of pass out of the cell. These substances move across a selectively permeable membrane. The membrane is considered selectively permeable because it allows only certain substances to pass across it.

The size of the molecules and other factors (such as polarity and charge) determine whether a substance can

(1) move across the lipid bilayer, (2) will have to move through protein channels or (3) will enter and leave the cell by endocytosis and exocytosis. The net direction of movement of the molecules is often related to their concentration and the existence of a concentration gradient.

The Concept of Concentration

Concentration: The amount of a substance in a defined area. In order to determine the concentration of a solution, you must consider the amount of solute AND solvent. The concentration of a solution is often expressed as a percent.

Concentration Gradient: Condition that is formed when there is more of something (molecules) in one place than another. (Different concentrations of a particle in a given area.)

Remember, in living things water is always the solvent (does the dissolving), and the solute (gets dissolved) can be any polar or ionic compound, such as glucose, amino acids, ions, etc.

Example:

(“1% glucose solution”) (“2% glucose solution”) (“10% glucose solution”)

If you were to put 10 grams of glucose (or food coloring, etc.) into each of the three beakers of water shown above, which one would have the highest concentration of glucose? Which one would have the lowest concentration of glucose?

The diagram below shows a model cell which contains 0.1g of glucose placed in a container filled with a solution made up of water and 10g of glucose (=black dots). Notice the size of the cell relative to the size of the container. Even though there are more glucose molecules present in the container, the concentration of glucose in the environment is lower than that of the cell.

Remember, molecules are in contrast, random motion due to kinetic (thermal or heat) energy.

Cell transport may be PASSIVE or ACTIVE.

Passive Transport:

Includes diffusion, osmosis, and facilitated diffusion

1. Requires NO additional energy other than the kinetic energy of the molecules (No ATP is needed).

2. Molecules move WITH (down) their [ ] gradient (from low solute [ ] to high solute [ ] )

Active Transport:

Includes molecular active transport (ex. the sodium-potassium pump) and bulk active transport (ex. endocytosis and exocytosis)

1. Requires energy from ATP

2. Molecules or solutes may be moved AGAINST (up) their [ ] gradient (from low solute [ ] to high solute [ ] ) using a specific membrane protein that acts as a “pump” for the molecule being transported

3. May involve large changes to the plasma membrane

Fill in the following chart comparing some of the different types of transport discussed so far in this chapter. Place a “check” in the box if the statement at the top of the chart applies to the process at the left.

Carrier molecule needed? / ATP needed? / Low to high concentration / High to low concentration / Movement against/up its conc. gradient / Movement down/with its conc. gradient
Molecular Active Transport
Bulk Active Transport
Facilitated Diffusion
Simple Diffusion &
Osmosis

PASSIVE TRANSPORT

I. Diffusion

Diffusion: The net movement of molecules from an area of high concentration to an area of lower concentration until the molecules are equally distributed and reach a dynamic equilibrium.

Once there is an equal distribution of molecules throughout the same area do you think the movement of molecules will stop? Explain.

Dynamic equilibrium: When the molecules are equally distributed and there is no NET (overall) movement of molecules in any one direction. Remember, DYNAMIC means molecules are still moving.

In diffusion, it is said that molecules move DOWN (with) their concentration gradient, from high to low concentration.

What will happen to the concentration gradient shown above if diffusion occurs?

______

II. Osmosis: A special kind of diffusion

Osmosis: The diffusion of WATER across a semi or selectively permeable membrane. Net movement is from an area of high water concentration to lower water concentration.

The direction water molecules tend to move depends upon where the water molecules are more highly concentrated. If you were comparing two solutions and said that solution A had a higher water concentration than solution B, how would the solute concentrations of solutions A and B compare?

There are special terms used to compare two solutions which help determine whether there will be a net movement of water molecules by osmosis. These terms, however, refer to the concentration of solute, not the concentration of water, within a solution.

Isotonic Solution (iso=same): A solution that has an equal solute concentration (equal water concentration) when compared to another solution

Hypotonic Solution (hypo=less): A solution that has a lower solute concentration (higher water concentration) when compared to another solution

Hypertonic Solution (hyper=more): A solution that has a higher solute concentration (lower water concentration) when compared to another solution

Example:

Solution A has a solute concentration of 4% and solution B has a solute concentration of 8%, when compared to each other in terms of solute concentrations which one would be hypotonic?______

Which solution has a higher water concentration?______

Refer to the diagrams shown:

Assuming that a selectively permeable membrane-separates side 1 from side 2, and it is permeable to water but impermeable to the solute (the small black dots), in which direction will the water move by osmosis? Explain.

If the membrane were permeable to both the solute and the water, what would happen? Explain.

_

Animal Cells and Osmosis

All cells are surrounded by a selectively permeable membrane which is permeable to water, and may or may not be permeable to certain solute molecules.

In the example below, red blood cells (rbc’s) are placed in one of three types of solutions. The cytoplasm of the cells contains many different solutes dissolved in water. In this example, the membrane is impermeable to solutes, but permeable to water. Look at the diagrams below and try to determine in which type of solution (hyper, hypo, or isotonic) the red blood cells were placed. Notice the shape of the red blood cell. Draw arrows to show the direction of water movement.

Plant Cells and Osmosis

Plants take water by osmosis. When a plant cell takes in water by osmosis, unlike an animal cell it does not burst. Explain why.

Turgor Pressure: Pressure exerted against the cell wall due to a larger amount of water that moves in by osmosis.

If enough turgor pressure builds within a plant cell, that pressure will cause the water to prevent any additional net movement of water into the plant cell. It’s as if the pressure exerted by the cell wall is pushing back against the inflow of water by osmosis. This is why a plant cell will only get so big – it will not grow indefinitely when placed in pure water, even though the inside will always be hypertonic to the outside.

Plasmolysis: The shrinking of a plant cell due to loss of water. This will occur if the solution outside the plant cell is hypertonic to the solution in the cell.

What would happen if you surround a plant with hypertonic solution? ______

Why would sprinkling salt on weeds kill them?_

Why do produce salespersons spray fresh water on the produce in a supermarket?

In the diagram below, plant cells are placed in one of three types of solutions. The cytoplasm of the cells contains many different solutes dissolved in water. In this example, the membrane is impermeable to solutes, but permeable to water. Notice the change f shape of the plant cells. Arrows are drawn to show the direction of water movement. Look at the diagrams below and try to determine in which type of solution (hyper, hypo, or isotonic) the plant cells were placed – write your answers on the blanks below each picture.

III. Facilitated Diffusion

You have already learned that diffusion and osmosis are forms of passive transport. Another type of passive transport is called facilitated diffusion. Facilitate means to make easier.

Facilitated diffusion:__Diffusion that is facilitated by a carrier protein that allows only specific solutes to move DOWN their concentration gradient, across a membrane.

Note: Because facilitated diffusion is a type of diffusion, molecules will move from an area of high to low concentration.

The types of molecules that will move across a membrane by facilitated diffusion include _ions and polar molecules that cannot pass the phobic bilayer

Note: The carrier molecule, or channel, in facilitated diffusion is often specific for the molecule it transports. For example, a different channel is needed for the transport of glucose than is needed for the transport of amino acids.

One possible way that carrier proteins can transport molecules is by changing positions in the membrane. In the example below, the carrier is able to “flip-flop” in the membrane by picking up molecules on one side, flipping around and then depositing them on the other side.

Note: In osmosis, water must pass across a semipermeable membrane from an area of high water concentration to an area of lower concentration therefore it is an example of diffusion. However, in living cells often the movement of water takes place through special proteins in the membrane called aquaporins so in those instances osmosis is an example of facilitated diffusion. (Either way, it’s passive transport.)

Active Transport

I. Molecular Active Transport

These proteins require ATP to pick up certain molecules and transport them against their gradient. This can for example enable a cell to maintain internal concentrations of small molecules that differ from concentrations of its environment (ie, animal cells maintain a higher [K+] and lower [Na+] than the environment by pumping 3 sodium ions out and 2 potassium ions into the cell).

Note: As in facilitated diffusion, the carriers (or pumps) that actively move molecules across the membrane are specific for the molecules they transport.

The Sodium- Potassium Pump: An Example of Molecular Active Transport

Sodium- Potassium Pump:

1. Carrier protein actively moves 3 Na+ out of the cell for every 2K+ it pumps in (Both AGAINST their concentration gradient.)

2. Establishes a difference in ionic charge across the membrane. More + ions are moved out than in. The outside of the membrane has a positive (+) charge compared or relative to the inside (-).

3. The concentration gradient formed is a type of potential energy. The ions “want” to move down their gradient (like H2O behind a dam). When a stimulus reaches the membrane, sodium channels spring open and the membrane charges are reversed.

II. Bulk Transport

Bulk Transport: A method of active transport in which where large or many molecules at a time can pass across the membrane using endocytosis/exocytosis

There are two major types of bulk transport: Endocytosis and Exocytosis. Some materials are too large to pass through the cell membrane and must enter/leave the cell in other ways.

Endocytosis: When a cell engulfs particles. Particles become encapsulated in a vesicle.

Once inside the cell, the vesicle that contains the engulfed particle (s) can fuse with a lysosome. The lysosomal enzymes can then break down the particles into monomers than can diffuse into the cytoplasm of the cell, where they can be used or disposed of.

Examples: Amoeba & Paramecium engulfing food; white blood cells engulfing bacteria

There are two types of endocytosis, based upon the size of the molecule that is being engulfed.

·  Pinocytosis-smaller dissolved particles and large molecules engulfed (liquid solution = cell 'drinking' ) ex. Proteins & other large molecules are taken in this way.

·  Phagocytosis large food particles and other large substances are engulfed (cell 'eating' )

What happens to the surface area of the plasma membrane as a result of endocytosis?

Exocytosis: when materials are secreted from cells - vesicles containing the substances (molecules) fuse with the plasma membrane and the molecules are released outside of the cell.

What happens to the surface area of the plasma membrane as a result of exocytosis?

Very large substances such as protein molecules and food particles usually enter cells by endocytosis and leave cells by exocytosis. They are usually too large to fit through membrane protein channels.

Note: These two processes (endocytosis and exocytosis) occurring in the same cell, allow for the plasma membrane to remain approximately the same size.

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