7-3 Cell Boundaries & Cell Transport

Copyright Pearson Prentice Hall 7-3 Cell Boundaries All cells are surrounded by a thin, flexible barrier known as the cell membrane. Many cells also produce a strong supporting layer around the membrane known as a cell wall.

Homeostasis – Maintaining a Balance Cells must keep the proper concentration of nutrients and water and eliminate wastes. The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

Copyright Pearson Prentice Hall Cell Membrane The cell membrane regulates what enters and leaves the cell and also provides protection and support. Outside of cell Cell membrane Inside of cell (cytoplasm) Protein channel Proteins Lipid bilayer Carbohydrate chains

Cell Wall Copyright Pearson Prentice Hall Cell walls are found in plants, algae, fungi, and many prokaryotes. The cell wall is used for support and to maintain structure.

Discuss the function of the cell membrane and the cell wall Write your answer in box #1

What Does Selective Permeability mean? It is a membrane that only certain substances can cross. Ex: A window screen is selectively permeable because it lets dust and air in, but not flies A coffee filter is selectively permeable because it lets water through but not the coffee grounds

The cell membrane is said to be selectively permeable or semipermeable. This means that some substances are allowed to move freely across the cell membrane whereas other are not.

Discuss… why is the cell membrane is semipermeable? Write your answer in box #2

inside cell outside cell lipid “repelled by water” “attracted to water” phosphate Cell Membrane Structure A phospholipid molecule consists of a phosphate head with two lipid tails, which are long chains of carbon and hydrogen atoms. The phosphate head is polar, which means it has a slight charge. The lipid tails are nonpolar, which means they have no charge.

Important Point: The lipid bilayer is not solid. The individual phospholipid molecules float within the bilayer. Because of this semi-fluid state, very tiny molecules like water, carbon dioxide, and oxygen can freely pass across the lipid bilayer. Lipid Mosaic Model.

Copyright Pearson Prentice Hall Diffusion Through Cell Boundaries Measuring Concentration A solution is a mixture of two or more substances. The substances dissolved in the solution are called solutes. The concentration of a solution is the mass of solute in a given volume of solution, or mass/volume.

Two Methods of Transport Across Cell Membrane: I.Passive Transport a.Diffusion (simple) b.Osmosis c.Facilitated diffusion II.Active Transport a.Transport proteins b.Endocytosis c.Exocytosis

Discuss… Two methods of transport across a cell membrane.

Passive Transport Does not require the use of energy. The most common method of passive transport is called diffusion. Movement of that substance across the cell membrane will continue until equilibrium is established.

Copyright Pearson Prentice Hall Diffusion Through Cell Boundaries Particles in a solution tend to move from an area of high concentration to an area of low concentration This process is called diffusion. When the concentration of the solute is the same throughout a system, the system has reached equilibrium.

when a substances flows from a high concentration to a low concentration they flow with the concentration gradient Concentration Gradient

Copyright Pearson Prentice Hall Diffusion Through Cell Boundaries

Discuss… How are diffusion through the cell membrane and the concentration gradient related? Write your answer in box #3

Copyright Pearson Prentice Hall Osmosis Osmosis is the diffusion of water through a selectively permeable membrane.

Copyright Pearson Prentice Hall How Osmosis Works Movement of water Dilute sugar solution (Water more concentrated) Concentrated sugar solution (Water less concentrated) Sugar molecules Selectively permeable membrane

Osmosis – A type of simple diffusion that refers specifically to the movement of water across the cell membrane. Important Point: Water has a tendency to move from areas of lower solute concentration (more pure water) to areas of higher solute concentration (less pure water).

Discuss… What is osmosis? Is osmosis flowing with the concentration gradient or against the concentration gradient? Explain. Write your answer in box #4

Copyright Pearson Prentice Hall Water tends to diffuse from a highly concentrated region to a less concentrated region. If you compare two solutions, three terms can be used to describe the concentrations: hypertonic (“above strength”). hypotonic (“below strength”). isotonic (”same strength”)

Copyright Pearson Prentice Hall Osmotic Pressure Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane.

Three Types of solutions: - Hypertonic solutions - Hypotonic solutions - Isotonic solutions

Osmosis in Red Blood Cells IsotonicHypotonicHypertonic

Discuss… How does osmotic pressure (tonicity) affects cells?

Isotonic Solution NO NET MOVEMENT OF H 2 O (equal amounts entering & leaving) Hypotonic Solution Cell Bursts Hypertonic Solution Wilting

Facilitated Diffusion Polar molecules or molecules that may be too large to pass through the lipid bilayer may require the use of channel proteins which provide a pathway across the cell membrane.

Discuss… Compare and contrast diffusion (simple) and facilitated diffusion. Write your answer in box # 5

Active Transport - Requires the use of energy. - Active transport (energy) is required whenever a substance needs to move against the concentration gradient. The direction of movement of the substance is from areas of low concentration to areas of high concentration. - Active transport usually involves the use of specialized carrier proteins embedded in the cell membrane that function as pumps.

Discuss… The differences between passive transport and active transport. Write your answer on box #6

Channel Proteins Need to make “doors” through membrane protein channels allow substances in & out specific channels allow specific material in & out H 2 O channel, salt channel, sugar channel, etc. inside cell outside cell sugaraa H2OH2O salt waste

Copyright Pearson Prentice Hall Active Transport Endocytosis and Exocytosis Endocytosis is the process of taking material into the cell. Two examples of endocytosis are: phagocytosis pinocytosis During Exocytosis, materials are forced out of the cell.

- Two other types of active transport: Exocytosis – The exporting of large molecules or large amounts of a substance out of the cell. Endocytosis – The importing of large molecules or large amounts of a substance into the cell.

Endocytosis

Exocytosis Large molecules that are manufactured in the cell are released through the cell membrane. Inside Cell Cell environment

Discuss… The 3 types of active transport. Write your answer on box #7

40 Three Forms of Transport Across the Membrane

Copyright Pearson Prentice Hall 7-3 Unlike a cell wall, a cell membrane is composed of a lipid bilayer. provides rigid support for the surrounding cell. allows most small molecules and ions to pass through easily. is found only in plants, fungi, algae, and many prokaryotes.

Copyright Pearson Prentice Hall 7-3 The concentration of a solution is defined as the volume of solute in a given mass of solution. mass of solute in a given volume of solution. mass of solution in a given volume of solute. volume of solution in a given mass of solute.

Copyright Pearson Prentice Hall 7-3 If a substance is more highly concentrated outside the cell than inside the cell and the substance can move through the cell membrane, the substance will move by diffusion from inside the cell to outside. remain in high concentration outside the cell. move by diffusion from outside to inside the cell. cause water to enter the cell by osmosis.

Copyright Pearson Prentice Hall 7-3 The movement of materials in a cell against a concentration difference is called facilitated diffusion. active transport. osmosis. diffusion.

Copyright Pearson Prentice Hall 7-3 The process by which molecules diffuse across a membrane through protein channels is called active transport. endocytosis. facilitated diffusion. osmosis.