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Passive Transport: Diffusion and OsmosisSection 3.4

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Objectives

• SWBAT describe passive transport• SWBAT distinguish between osmosis,

diffusion, and facilitated transport.

• Main Idea 1: diffusion and osmosis are types of passive transport.

• Main Idea 2: some molecules diffuse through transport proteins.

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Vocabulary Section 3.4

• Osmosis • Diffusion • Passive transport • Concentration gradient • Isotonic• Hypertonic • Hypotonic • Solute

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Two Types of Transport• There are two ways of transporting materials

across a cell membrane. They are:– Passive Transport – the movement of molecules

across a cell membrane without energy input from the cell (the diffusion of molecules across a membrane).

– Active Transport – actively drives molecules across the cell membrane from a region of lower concentration to a region of higher concentration. Requires energy input.

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Diffusion• Diffusion - is the movement of

molecules in a fluid or gas from a region of higher concentration to a region of lower concentration.

• Concentration gradient – is the difference in the concentration of a substance from one location to another.

Lower concentration

Higher concentration

The egg swelled because water molecules moved from outside the egg, higher water concentration, to inside the egg where there was a lower concentration of water molecules.

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Diffusion• Diffusion results from the natural motion of

particles. Diffusion across a membrane from higher to lower concentrations is part of nature – it is a natural force.

• Concentration is defined as the number of molecules of a substance in a given volume.

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Diffusion

• In cells, diffusion is indispensable in moving substances across the cell membrane!– Small lipids (remember our hormone example –

aldosterone) and other nonpolar molecules (like carbon dioxide and oxygen) easily diffuse across the membrane.

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Oxygen Diffusion Example• Most cells continually consume oxygen. • Thus, oxygen concentration is almost always higher outside

the cell than inside it. • As a result, oxygen almost always is diffusing from outside the

cell, across the cell membrane, to the inside of the cell.• The cell expends no energy.

Oxygen high concentration

Lower oxygen concentrationOxygen

Diffuses

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Osmosis• Water molecules also diffuse across

semipermeable (selectively permeable) membranes from higher concentration to lower concentration, which we call osmosis.

• Osmosis – the diffusion of water molecules across a semipermeable membrane (a cell membrane in our case) from higher concentration of water molecules to lower concentration of water molecules.

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Osmosis – Our Naked Egg Example

In our naked egg experiment, we saw osmosis at work. • First, we saw water move

from the vinegar (where there was a higher concentration of water molecules) into the egg (where there was a lower concentration of water molecules. The egg expanded.

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Osmosis – Our Naked Egg Example

• Then, we put the egg in corn syrup. In the corn syrup there was a higher concentration of water molecules in the egg than the corn syrup.

• Therefore, the water flowed from the egg, higher concentration, to the corn syrup (lower concentration).

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Isotonic, Hypertonic, Hypotonic

• Isotonic – a solution is isotonic to a cell if it has the same concentration of dissolved particles, solutes, in the solution outside the cell as inside the cell. – Example: water molecules move into and out of the cell at

equal rates because the concentration of dissolved particles is the same outside the cell as inside the cell.

– This is a state of equilibrium (remember the word from biochemistry).

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Isotonic, Hypertonic, Hypotonic

• Hypertonic – a solution is hypertonic to a cell if it has a higher concentration of dissolved particles than inside the cell.– Example: if there is a higher concentration of dissolved

particles in the solution outside the cell than inside the cell, water will move out of the cell into the solution.

Think of our egg in corn syrup (the corn syrup was hypertonic to the cell. Water from in the cell flowed out of the cell into the corn syrup.

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When we say solutes, as in “a hypertonic solution has more solutes than a cell,” we are talking about the concentration of dissolved particles (for example, salts, sugars, etc.)

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Isotonic, Hypertonic, Hypotonic

• Hypotonic – the solution outside the cell has a lower concentration of dissolved particles than that inside the cell. – Example: water molecules will diffuse into the cell when the inside of

the cell has a higher concentration of dissolved particles and a lower concentration of water molecules than outside of the cell.

Water flowed from the vinegar, where there was a lower concentration of dissolved particles, through the cell membrane to the interior of the egg (where there was a higher concentration of dissolved particles).

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Facilitated diffusion

• Facilitated diffusion – is the diffusion of molecules across a cell membrane through transport proteins.

Facilitated diffusion is for molecules that cannot easily diffuse across a cell membrane.

The word facilitate means to make easier – thus, the transport proteins facilitate (make easier) the passage of some molecules into our out of the cell.

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Facilitated diffusion

This is still passive transport since the cell does not need to use any of its energy.

There are many types of transport proteins – most types only allow specific molecules to pass through.

The transport proteins act as tunnels through the cell membrane.

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Question

• What will happen to a houseplant if you water it with salt water (a hypertonic solution)?

• Why are transport proteins needed in the cell membrane?

• Describe facilitated diffusion using glucose as an example.