The Cell in Its Environment

Key Concepts

• How do most small molecules cross the cell membrane?

• Why is osmosis important to cells?

• What is the difference between passive transport and active transport?

Key Terms• Selectively

permeable

• Diffusion

• Osmosis

• Passive transport

• Active transport

• Some substances can pass through the membrane while others cannot.

• Molecules move from an area of higher concentration to an area of lower concentration.

• The diffusion of water molecules through a selectively permeable membrane.

• The movement of dissolved materials through a cell membrane without using cellular energy.

• The movement of materials through a cell membrane using cellular energy.

Introduction

• Cells have structures that protect their contents from the world outside.

• All cells are surrounded by a cell membrane that separates the cell from the outside environment.

• The cell membrane is selectively permeable, which lets some things enter and leave the cell.– Oxygen– Carbon Dioxide– Water– Food molecules– Waste products

Diffusion

What is Diffusion?

• The process by which molecules move from an area of higher concentration to an area of lower concentration.

• The main method by which small molecules move across the cell membrane.

When Does Diffusion Stop?

When equilibrium is reached!

Animation of Diffusion• http://highered.mcgraw-hill.com/sites/0072495855/student_

view0/chapter2/animation__how_diffusion_works.html

• How does diffusion play a role in the following situations?– smelling cookies baking in your oven– making a pitcher of lemonade– adding chemicals to a pool

• Can you think of any other examples of diffusion?

Osmosis

What is Osmosis?

• The diffusion of water molecules through a selectively permeable membrane.

• Many cellular processes depend on osmosis because cells cannot function properly without adequate water.

How is Osmosis Related to Diffusion?

• Molecules tend to move from an area of higher concentration to an area of lower concentration.

• Water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.

Effects of Osmosis on Cells

Animation of Osmosis• http://highered.mcgraw-hill.com/sites/0072495855/student_

view0/chapter2/animation__how_osmosis_works.html

• Osmosis has a number of life-preserving functions.– assists plants in receiving water– helps in the preservation of fruit and meat– used in kidney dialysis

• Osmosis can be reversed to remove salt and other impurities from water.

Passive & Active Transport

What is Passive Transport?

• The movement of materials through a cell membrane without using cellular energy.

• Diffusion and osmosis are examples of passive transport.

Animation of Facilitated Diffusion

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

What is Active Transport?

• The movement of materials through a cell membrane using the cell’s energy.– The movement of a

substance in the opposite direction than they naturally move by diffusion.

• Minerals• Some sugars• Most amino acids

Methods of Active Transport

• Transport Proteins

• “pick up” molecules

• Examples of substances that are carried . . . – Calcium– Potassium– Sodium

• Engulfing

• Cell membrane surrounds and engulfs, or encloses, a particle.

• A vacuole is formed around the engulfed particle.

• Energy is required by the cell to perform this function.

Animation of Active Transport

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

“Transport” is like Riding a Bike?

• Riding a bike down a hill . . .

• Pedaling a bike up a hill . . .

• Passive Transport • Active Transport

Why Are Cells Small?

• Cells become less efficient as they grow.

• The smaller they are, the easier it is for them to do their jobs.

• The smaller they are, the easier it is for substances to be moved in and out.

Cells . . .

. . . More than meets the eye!!