Plasma Membrane

The outer layer of the cell is semi-permeable. This means it allows some things to pass and blocks others.

The Plasma Membrane

The plasma membrane is flexible due to lipids.

The lipids form a barrier that separate the outside and inside of the cell.

The cell membrane is made of phospholipids.

Cell Membrane Composition

Phospholipids are molecules made of two fatty acids and a phosphate group.

The lipids have a head and a tail.

The head is hydrophilic (water loving), and the tail is hydrophobic (water fearing).

Phospholipid Bilayer

The Hydrophilic Heads stay outside near the water of the cell and the hydrophobic tails stay inside away from water.

Phospholipid Bilayer

The plasma membrane contains many proteins as well as the phospholipid bilayer.

These proteins are: Cell marker proteins identify cell type (Muscle Cell,

Brain Cell etc) Enzymes to help with chemical reactions Receptor proteins bind to specific substances (such as

signal molecules) Transport Proteins move substances in and out

Membrane Proteins

The plasma membrane is described as a fluid mosaic.

A fluid is a liquid A mosaic is a type of art made of very small

pieces that together compose a larger image. The plasma membrane is not solid. Due to the

phospholipid bilayer it is a flexible structure made of many different pieces (lipids, proteins, carbohydrates).

Fluid Mosaic Model

Movement into or out of a cell (across the plasma membrane) that does not require energy is passive transport.

There are two types of passive transport: Fascilitated Diffusion Osmosis

Passive Transport

In diffusion molecules move from an area of high concentration to an area of low concentration due to random molecular motion.

Diffusion

Diffusion continues until the molecules are evenly distributed. At that point there is no net diffusion (the molecules are moving out and in and even rates…this is equilibrium)

Diffusion

Very small molecules or nonpolar molecules can travel though the cell membrane by diffusion.

The diffusion of water through a membrane is called osmosis.

Osmosis

Inside and outside cells are solutions of water, ions, polar and nonpolar molecules.

If a solution in or out of the cell contains more dissolved molecules then water will move to the side with more dissolved molecules to try and even out the concentrations.

What causes osmosis?

Water can either move into a cell, out of a cell, or stay where it is. This movement of water can greatly effect a cell or even kill it.

Types of Water Movement

Water will move out of a cell if there are more dissolved particles OUT OF the cell.

The cell is in a hypertonic solution.

When water flows out the cell shrivels or shrinks(animal cells) or plasmolyzed (plant cells).

Hypertonic Solutions

In plasmolysis the cytoplasm of plants pulls away from the cell walls (but the cell walls keep their shape). So only the inside of plant cells shrivels.

The cell is then plasmolyzed.

Plasmolysis

Water will move INTO a cell if there are more dissolved molecules in the cell.

The cell is in a hypotonic solution. When water flows into an animal

cell the cell will pop (known an lysing). When water flows into a plant cell the cell puffs out (known as turgid).

Hypotonic Solution

In a turgid plant cell the vacuole swells and pushes on the cell wall. The cell doesn’t pop like an animal cell it puffs outward.

Turgid

When cells are in a solution with equal molecules outside and inside, and water flows in at the same rate it flows out this is an isotonic solution.

Cells in an isotonic solution are normal (no lysing and no shriveling)

Isotonic Solution