CHAPTER 8CELLS & THEIR ENVIRONMENT

• Review: Cell (or plasma) membrane – is composed of a lipid bilayer with embedded proteins that float. The cell membrane is a selectively permeable membrane, which means it controls what enters and what leaves the cell. Remember that the cell membrane allows to cell to remain separate from the environment.

• The currently accepted model is the

Fluid Mosaic Model.• According to this model, the cell

membrane consists of a double layer of phospholipids in which large protein molecules float.

Cellular Transport• The cell membrane helps organisms maintain

homeostasis.• Homeostasis is the maintenance of the internal

environment despite changes in the external environment.

• Smaller molecules like H20, CO2, and 02 pass easily through the cell membrane.

• Larger molecules like glucose, amino acids, ions, and most polar molecules cannot pass through. Ions (which have a charge—Na+, Cl-) and most polar molecules cannot diffuse across the cell membrane because they cannot pass through the nonpolar interior of the lipid bilayer.

Cellular Two Types of Cellular Transport:1. Passive Transport 2. Active

Transport• 1. Passive Transport (going down the

concentration gradient)• The passage of substances through the

membrane from regions of higher concentration into regions of lower concentrations.

• Movement across the cell membrane that does NOT require energy from the cell.

Three Types of Passive Transport:1. Diffusion 2. Osmosis 3. Facilitated Diffusion

In all 3, the movement of a substance is from a region of higher concentration to regions of lower concentration

High

Low

Three Types of Passive Transport:1. Diffusion 2. Osmosis 3. Facilitated Diffusion

• 1. Diffusion• Simplest type of passive transport.• If there is a concentration gradient in the

solution, the substance will move from an area of higher concentration to an

area of lower con- centration.• This movement results because of the random move- ment of particles.https://www.youtube.com/watch?

v=VY0mZUDvbH4

Three Types of Passive Transport:1. Diffusion 2. Osmosis 3. Facilitated Diffusion

• Diffusion depends on a concentration gradient, which is the difference in concentration of a substance across a space.

• Equilibrium is a state that exists when the concentration of a substance is the same throughout a space.

• In order for diffusion to occur, there must be a concentration gradient. Why? If there is no concentration gradient, then the substance would be in equilibrium.

• Ion channel is a doughnut-shaped transport protein with a polar pore through which ions can pass. An ion that enters the pore can cross the cell membrane without contacting the nonpolar interior of the lipid bilayer.

Three Types of Passive Transport:1. Diffusion 2. Osmosis 3. Facilitated Diffusion

• 2. Osmosis• The diffusion of water molecules through a

selectively permeable membrane from an area of higher concentration to an area of lower water concentration.

• In living organisms, water enters and leaves cells by osmosis.

Osmosis Cont.Three Types of Solutions Describing Water

Balance in Regions Surrounding a Cell:1) Isotonic solution• No net water movement.• The concentration of dissolved substance or particles

inside the cell is equal to the concentration outside the cell.

• Example: if a cell is placed in an isotonic solution, water still moves into and out of a cell at random but there is not new movement of water.

• **The cell stays the same size.**

OsmosisThree Types of Solutions Describing Water Balance in

Regions Surrounding a Cell:2) Hypotonic solution• Water moves into the cell.• The concentration of dissolved substance or particles

in water outside the cell is lower than the concentration inside the cell; therefore, the concentration of water is higher outside the cell than inside the cell.

• **The cell swells.**• Turgor pressure is the pressure that exists in a cell. As

water diffuses into the cell, the cell swells and its internal pressure increases.

Osmosis Cont.Three Types of Solutions Describing Water

Balance in Regions Surrounding a Cell:3) Hypertonic solution• Water moves out of the cell.• The concentration of the dissolved substance

or particles is greater in the water outside the cell than the water inside the cell.

• **The cells shrinks.** Plasmolysis is the loss of water from a cell resulting in a drop in turgor pressure. This causes a plant to wilt.

Three Types of Passive Transport:1. Diffusion 2. Osmosis 3. Facilitated Diffusion

• 3. Facilitated Diffusion • The passage of substances through the

cell membrane by means of transport proteins, called a carrier protein, from regions of higher concentration into regions of lower concentration.

• A carrier protein is a type of transport protein, which binds to a specific substance on one side of the cell membrane, carries the substance across the cell membrane, and releases it on the other side. Substances such as amino acids and sugars (glucose) cross the cell membrane by facilitated diffusion.

• This process is used for molecules that cannot diffuse through the cell membrane.

Cellular Two Types of Cellular Transport:1. Passive Transport and 2.Active Transport

1. Active Transport **Requires energy from the cell**• Some substances move in and out of a cell

against a concentration gradient.-The passage of substances across a membrane

using energy—ATP. Substances are moved from regions of lower concentration into regions of higher concentrations.

-Some active-transport processes involve carrier proteins. Like the carrier proteins used in facilitated diffusion, the carrier protein used in active transport bind the specific substances on one side of the cell membrane. But in active transport, the substances bind to carrier (channel) proteins where they are in low concentration and are released where they are higher in concentration. Thus, carrier proteins in active transport function as “pumps” the move substances against their concentration gradient.

Two Types of Active Transport:1)Endocytosis and 2)Exocytosis

1) Endocytosis • The process in which cells TAKE IN large

materials that cannot pass through the cell membrane, like proteins and polysaccharides.

• The movement of a substance INTO a cell by a vesicle in called endocytosis.

• Large molecules, groups of molecules, or even whole cells are engulfed and enclosed by a portion of the cell’s membrane. That portion of the mem-

brane then breaks away, and the result- ing vesicle with its contents moves into the inside of the cell.

Endocytosis Cont.1) Endocytosis • Two Types of Endocytosis:

1) Pinocytosiso Involves the transport of solutes or fluids.2) Phagocytosiso Involves the transport of large particles or

whole cells.Example: An amoeba engulfs food particles by phagocytosis.

Pinocytosis

Phagocytosis

Two Types of Active Transport:1)Endocytosis and 2)Exocytosis

1) Exocytosis-The process where certain substances are

moved OUT of the cell.-Vesicles travel to the cell membrane and substances in- side vesicles are ejected from the cell. An example would be waste products.