Chapter 4: Cells and Their Environment

The Fluid Mosaic Model of the Plasma Membrane

Parts of the Plasma Membrane

Phospholipid – has a polar phosphate “head” which is hydrophilic. It has two nonpolar lipid “tails” which are hydrophobic.

Parts of the Plasma Membrane

Cholesterol – helps to stabilize the membrane.

The only thing that keeps the plasma membrane together is the orientation of the lipid tails (they stay away from water)

Parts of the Plasma Membrane

Functions of Proteins:

1.Transport. They allow larger molecules and charged molecules/ions to go through the membrane.

2.They can act as markers that other cells can recognize.

3.They can act as enzymes.

Selective Permeability

The plasma membrane is selectively permeable – it allows some substances to pass through, but not all. (It is selective).

Substances that can easily pass through are:

O2

CO2

H2O

Aquaporin – water into the cell

Brownian Motion

Molecules are in constant, random motion, called Brownian motion.

Animation

Membrane (Cell) Transport

Transport of materials in and out of the cell

Passive Transport – requires no energy expenditure from the cell

Active Transport – requires energy from the cell

Passive Transport

Three types:

1. Simple diffusion

2. Facilitated diffusion

3. Osmosis

What is Diffusion?

Diffusion – the net movement of particles from an area of high concentration to an area of low concentration

Concentration

• High concentration: a lot of molecules in a given space.

• Low concentration: a few molecules in a given space.

• Concentration gradient: a difference in the concentration of a substance.

Diffusion & Concentration GradientIf molecules are going from an area of HIGH concentration to an area of LOW concentration, they are going down the concentration gradient.

Dynamic Equilibrium

A condition in which the concentration of a substance is equal throughout a space. The molecules are still moving, but there is no change in concentration.

What is Facilitated Diffusion?

Diffusion of substances across a membrane with the help of transport proteins

Facilitated diffusion

What is Osmosis?

Osmosis is the diffusion of water through a selectively permeable membrane.

NOT!

Osmosis

Note that osmosis ONLY refers to the diffusion of WATER!

Osmosis: Isotonic SolutionIsotonic solution has the same solute/ water concentrations in and out of the cell. Water is in a dynamic equilibrium.

Osmosis: Hypertonic Solution

Hypertonic solution: the water concentration is higher inside the cell than outside. Water will diffuse out of the cell. The cell will shrink (crenate)

“Hyper” means “more”: there is more solute outside of the cell (therefore, less water)

Osmosis: Hypotonic Solution

Hypotonic solution – the concentration of water is greater outside of the cell as compared to the inside. Water will enter the cell. The cell will expand and may burst.“Hypo” means “less” or “under”. There is less solute outside of the cell.

Osmosis

Osmosis in Plants• Turgor pressure –

pressure of water inside of a cell.

• The large central vacuole in plants regulates the amount of water in the cell.

Is turgor pressure higher when a cell is in a hypertonic, hypotonic or isotonic solution?

Plasmolysis

A loss of turgor pressure is called plasmolysis

Plasmolysis in Elodea

Normal Plasmolyzed

Plasmolysis in Red Onion

Red Onion Cells - normal Red Onion Cells - plasmolyzed

Which solution is isotonic, hypertonic and hypotonic to the cell?

A B C

Active TransportExamples:

1. Solute pumping – molecules go from an area of low concentration to an area of high concentration.

2. Endocytosis• Phagocytosis• Pinocytosis• Receptor-Mediated Endocytosis

3. Exocytosis

Contractile Vacuoles

• Some species of protists live in a hypotonic environment (what will happen to them??)

• Contractile vacuole – a structure in some protists that expels water.

Contractile vacuole

Endocytosis and Exocytosis• Endocytosis – the

cell takes in particles

• Exocytosis – the cell releases particles into its environment

Phagocytosis – How your immune cells get rid of some pathogens• Phagocytosis is a type

of endocytosis in which a cell engulfs a large particle.

• Your white blood cells engulf bacteria and then merge them with a lysosome to digest them.