The Cell Membrane

Homeostasis

• Survival of a cell depends on the cell’s ability to maintain proper conditions within itself. In other words, it needs to maintain homeostasis.
• Homeostasis means maintaining stable internal conditions when the outside conditions change.
• Balance
• How can the cell membrane help a cell maintain homeostasis?

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• The cell membrane (also called the plasma membrane) is a flexible boundary between the cell and its environment.
• It regulates the substances that go in and out of the cell.
• Examples: nutrients enter and wastes exit
• This membrane is selectively permeable; it is “picky” about what it will let enter/exit the cell.

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The Cell Membrane

The Plasma Membrane

The Plasma Membrane

Phospholipids have 2 parts: polar heads (love H2O) and the non-polar tails (dislike H2O).

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Transport proteins are large proteins that span the cell membrane and allow for stuff to enter or exit the cell.

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Surface proteins help cells by identifying chemical signals, preventing cells from infection, and attaching cell’s membrane to its internal structure.

Cellular Transport

• moving substances in and out of the cell
• How can something get into a cell?
• How can something get out of a cell?

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Diffusion

• The net movement of particles from an area of HIGH concentration to an area of LOW concentration.
• Diffusion goes with the concentration gradient.
• It does not require the cell to spend extra energy to move substances; it relies on the random movement of particles.
• If there are no external processes interfering, then equilibrium will be reached, which eliminates the gradient.

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High Concentration

Low

Concentration

Concentration Gradient

Movement of Particles

Particles Move!!

Example of Diffusion

• The smell of air spray on one side of the classroom can eventually be detected on the other side of the classroom. This is because the scent particles diffused throughout the air.
• Concentration, temperature, and pressure can all affect the rate of diffusion.

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Diffusion of Smell

• Raise your hand when

you smell the air spray.

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What happened to the air spray?

Use CER to write at least 5 sentences on the next blank page.

Osmosis

• This is a special form of diffusion that deals only with the movement of water.
• It regulates the water flow in and out of a cell.

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Osmosis

• There is a concentration

gradient but a semipermeable membrane is in the way so the particles can’t move.

• The water moves to

balance it out.

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High Water Concentration

Low Water

Concentration

Concentration Gradient

Movement of Water

Water Moves!!

Isotonic solution- concentration of dissolved substances is the same inside the cell as it is outside of the cell.

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- No net movement of water.

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Hypertonic solution- concentration of dissolved substances is greater outside of the cell.

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- In an effort to reach equilibrium, water will move out of the cell.

Hypotonic solution- concentration of dissolved substances is greater inside the cell.

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- In an effort to reach equilibrium, water will move into the cell.

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1. When the egg was placed in the water, which direction did the water molecules move? Explain your answer.

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2. When the egg was placed in the salt water, which direction did the water molecules move? Explain your answer.

this happens.

Osmosis Questions

3. Why do grocery stores spray their fresh produce with water?

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4. If a shipwrecked crew drank salt water, they could die. Explain why.

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5. If a bowl of fresh strawberries is sprinkled with sugar, a few minutes later they will be covered with juice. Explain why.

Cellular Transport

Passive Transport- moving molecules across the plasma membrane without the cell expending energy.

Facilitated diffusion is passive transport that requires the use of the transport proteins. This sort of diffusion always goes with the concentration gradient.

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High Concentration

Low

Concentration

Concentration Gradient

Movement of Tiny Particles

Tiny Particles Move!!

High Concentration

Low

Concentration

Concentration Gradient

Movement of Larger Particles

Larger Particles Move!!

Active Transport

Active Transport- occurs when the cell needs to move substances AGAINST the concentration gradient.

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- This requires the cell to spend energy and the use of protein.

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High Concentration

Low

Concentration

Concentration Gradient

Movement of Particles

Particles Move!!

Cellular Transport

• The transport of large molecules is done by either endocytosis or exocytosis.

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• Endocytosis is the taking in of large molecules, or even whole cells.
• Exocytosis is the expulsion of molecules from the cell.

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Recap!!

Cut and Paste These Processes

• For each process, cut and paste the two pictures in the two rectangles.
• Label on each picture the area of high particle concentration and low particle concentration.
• For passive transport, use arrows to show the direction the particles will move and then the final result. How many particles should be on each side?
• Label equilibrium on the picture for the final result.

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Cut and Paste These Processes

• For facilitated diffusion, use arrows to show the direction the particles will move and then the final result. How many particles should be on each side?
• Label equilibrium on the picture for the final result.
• For active transport, draw the particle and energy input on the beginning picture.
• For the final result, show where the particle will end up and use an arrow to show its direction.

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