CELL TRANSPORT and THE PLASMA MEMBRANE

SB1d. Explain the impact of water on life processes

(i.e., osmosis, diffusion).

What if…

What would happen if an organism could not get energy or get rid of wastes?

An organism’s cells perform all of these functions. Cells must exchange materials with their environments. The cell membrane (also called the plasma membrane) allows this exchange to take place.

FUNCTIONS OF THE PLASMA (CELL) MEMBRANE• Surrounds and protects the cell

• Controls flow in and out of the cell

• SELECTIVELY PERMEABLE: allows some molecules in, others are kept out

• Maintains homeostasis (balance within cells)

Plasma Membrane Structure

Fluid-Mosaic Model: molecules are free to move

Outsideof cell

Cellmembrane

Proteins

Proteinchannel

Lipid bilayer

Carbohydratechains

Insideof cell

(cytoplasm)

• Phospholipid bilayer:• Hydrophilic heads

• Hydrophobic tails

• Proteins embedded in the membrane help transport substances across the membrane and determine which particles can pass through.

Plasma Membrane Structure

Tonicity of SolutionsTonicity refers to the force exerted across the membrane as differing concentrations of solutes travel in and out of the cell.

• Isotonic solutions

• Hypotonic solutions

• Hypertonic solutions

Isotonic solutions

• This is a solution where the concentration of the dissolved substances outside of the cell is the same as the inside of the cell.• Movement of molecules into the cell=movement of molecule

outside of the cell

• The concentration of water is the same inside and outside of the cell.

Hypotonic solutions• The concentration of dissolved substances is

lower outside of the cell than inside of the cell

• Water will move in to equalize

• Osmotic pressure increases

• Cell expands; animal cells may burst (cytolysis)

• Plant cells don’t burst due to cell wall

Hypertonic solutions• The concentration of dissolved substances is higher outside

the cell than inside of the cell

• Water rushes out of the cell and causes it to shrivel

• This can result in plasmolysis: water goes out of plant cells causing the plant to wilt

Cytolysis & Plasmolysis

Cytolysis Plasmolysis

View Tonicity Animations

Moving Across Membranes

There are two types of transport across the cell membrane

Active Transportrequires the cell to use energy

Passive Transportno energy is used

Types of Passive Transport

DIFFUSION

molecules move from high concentration to low concentration

OSMOSIS

diffusion of water

FACILITATED

DIFFUSION

uses carrier proteins to move large molecules

The difference in the concentrations is called a gradient

Cellmembrane

Higher Concentrationof Water

Lower Concentrationof Water

Water molecules

Sugar molecules

Section 7-3

Figure 7-17 Osmosis

Go to

Section:

Osmosis

Active Transport

• Uses energy

• Moves molecules from low concentration to high concentration against the concentration gradient

• Imagine pushing clothes into a hamper that is already full

ATPenergy

Example of Active Transport

• Sodium-Potassium Pump moves 3 potassium (K+) ions into and 2 sodium (NA+) ions out of a cell

• requires energy and a transport protein

Active vs Passive Transport

Moving Very Large Particles

• Some molecules can’t use the carrier proteins

• Carbohydrates, Starches, etc.

• They must enter and leave the cell using endocytosis and exocytosis.

• This requires energy.

Endocytosis-entering the cell1. Molecule touches cell.

2. Cell membrane surrounds molecule.

3. Vesicle forms and enters cell.

Exocytosis-leaving the cell1. Molecule packaged in vesicle.

2. Vesicle fuses with cell membrane.

3. Molecule is released to outside of cell.

Cell Transport Review

1. What term refers to the diffusion of water?

2. What type of transport is used when moving from low to high concentration?

3. What structures allow molecules to cross the cell membrane in facilitated diffusion?

4. Give three examples of passive transport.

5. What structure is formed when the cell membrane surrounds a large molecule?