¨       The most important parts of a cell are its borders, which separate the cell from its surroundings.

¨       The cell membrane is a thin, flexible barrier that surrounds all cells.

¨       The cell wall is a strong supporting layer around the cell membrane found only in plant cells.

Cell Membrane

¨       The cell membrane regulates what enters and leaves the cell and also provides protection and support.

¨       A lipid bilayer is a double-layered sheet, which composes nearly all cell membranes.

¨       Protein molecules are also embedded in the lipid bilayer.

¨       Carbohydrate molecules are attached to many of these proteins, resembling tails.

¨       The outer part of the cell membrane is hydrophilic (water loving).

¨       The inner part of the cell membrane is hydrophobic (water fearing).

Cell Wall

¨       Cell walls are present in many organisms, including plants, algae, fungi, and many prokaryotes.

¨       Cell walls lie outside the cell membrane and are porous to allow water, oxygen, carbon dioxide and other substances to pass through.

¨       The main function of the cell wall is to provide support and protection for the cell.

¨       Plant cells are mostly cellulose, a tough carbohydrate fiber.

¨       Cellulose is the principle component for both wood and paper.

Diffusion Through Cell Boundaries

¨       Every living cell exists in a liquid environment that it needs to survive.

¨       One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.

Measuring Concentration

¨       The cytoplasm of a cell contains a solution of many different substances in water.

¨       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.

Diffusion

¨       In a solution, particles move constantly.

¨       Diffusion is when particles move from an area of higher concentration to an area of lower concentration.

¨       Equilibrium is when the concentration of the solute is the same throughout a system.

¨       Diffusion depends upon random particle movement.

¨       Substances diffuse across membranes without requiring the cell to use energy.

Process of Diffusion

a.       There is a higher concentration of solute on one side of the membrane as compared to the other side of the membrane.

b.       Solute particles move from the side of the membrane with a higher concentration of solute to the side of the membrane with a lower concentration of solute.  The solute particles will continue to diffuse across the membrane until equilibrium is reached.

c.       When equilibrium is reached, solute particles continue to diffuse across the membrane in both directions.

Osmosis

¨       If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it.

¨       A membrane is impermeable to substances that cannot pass across it.

¨       Most membranes are selectively permeable, meaning that some substances can pass across them and others cannot.

¨       Water passes quite easily across most membranes, even though many solute molecules cannot.

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

How Osmosis Works

¨       In osmosis, the only movement is by the water molecules. 

¨       Water can cross the membrane in both directions but the solute cannot!

¨       So, there is a net movement of water from an area of high concentration to an area of low concentration.

¨       A hypotonic solution means a lesser concentration of solutes and a higher concentration of water outside the cell. (“below strength”)

¨       A hypertonic solution means a higher concentration of solutes and a lower concentration of water outside the cell. (“above strength”)

¨       An isotonic solution means the same concentration of both solute and water inside and outside the cell. (“same strength”)

Osmotic Pressure

¨       Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane.

¨       Osmotic pressure can cause serious problems for a cell.

¨       Because the cell is filled with salts, sugars, proteins, and other molecules, it will almost always be hypertonic to fresh water.

¨       Osmotic pressure should produce a net movement of water into a typical cell that is surrounded by fresh water.

¨       This will cause the volume of the cell to increase until the cell becomes swollen and may eventually burst.

¨       Most cells is large organisms do not come in contact with fresh water because they are bathed in fluids such as blood, that are isotonic.

¨       Plant cells and bacteria that do come in contact with freshwater, are surrounded by tough cell walls.

¨       The cell walls prevent the cells from expanding but increased pressure may make the cell walls vulnerable to injury.

Facilitated Diffusion

¨       Cell membranes have protein channels that make it easy for certain molecules to cross the membrane.

Ø       Red blood cells have a cell membrane protein with an internal channel that allows glucose to pass through it.  Only glucose can pass through this channel and it can move in either direction.

¨       The process of facilitated diffusion is said to help substances across the cell membrane.

¨       Hundreds of different protein channels have been found that allows particular substances to cross-different membranes.

¨       Despite facilitated diffusion being fast and specific, it is still diffusion!

¨       It will only occur if there is a higher concentration of the particular molecule on one side than on the other side.

¨       This movement does not require the use of the cell’s energy.

Active Transport

¨       Active transport requires energy and is the movement of materials in the opposite direction.

¨       Active transport of small molecules is carried out by transport proteins or “pumps” that are found in the membrane itself.

¨       Larger molecules and clumps of material are actively transported by processes known as endocytosis and exocytosis.

Molecular Transport

¨       Energy requiring pumps move calcium, potassium, and sodium ions across cell membranes.

¨       Changes in protein shape play a role in the pumping process.

¨       A large portion of the energy used by cells daily is devoted to this process.

Endocytosis and Exocytosis

¨       Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane.

¨       The pocket breaks loose from the outer portion of the cell membrane and forms a vacuole within the cytoplasm.

¨       Two types of endocytosis are phagocytosis and pinocytosis.

¨       Phagocytosis means “cell eating”.

¨       In phagocytosis, extensions of cytoplasm surround a particle and package it within a food vacuole.  The cell engulfs it.

¨       Pinocytosis means “cell drinking”.

¨       In pinocytosis, tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell.

¨       Exocytosis is the process of cells releasing large amounts of material from the cell.

¨       The membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell.