Cells and Their Environment

Cell membranes – function to communicatebetween neighboring cells. They also serve as a selectively permeable barrier. It functions incellular metabolism.

Ways materials can pass through the cell membrane:

1. Passive mechanisms – diffusion, facilitated diffusion, osmosis, and filtration.

These mechanisms require no input of cellular energy in order to occur. They occur spontaneously.

With the concentration gradient.

Diffusion – movement of particles from an area of high concentration to an area of lower concentration.

(Concentration gradient – when a difference exists between the concentration inside and outside the cell.)

Equilibrium

Equilibrium – the condition in which the concentrationof a substance is equal throughout a space.

Electrical Charge and Ion Transport:

Cells typically have a negative charge inside. Since opposite charges are attracted to each other, the positively chargedions are more likely to diffuse across the membrane into the cell.

This also means that a negatively charged ion inside the cellwill be more likely to diffuse toward the outside of the cell.

Ion channels provide a way for ions to pass through themembrane, across the nonpolar lipid interior of the cellmembrane.

(Ex. Calcium Ca2+, Sodium Na+, Potassium K+, and Chlorine Cl- channels)

Sodium Potassium Pump – transports three sodium ions out of a cell and two potassium ions into the cell.

The concentration of sodium ions is greater outside the cell than inside, and the concentration of potassium ions is greater inside the cell than outside the cell.

Therefore the sodium Potassium Pump works against the concentration gradient.

Facilitated diffusion – permease assists a molecule permeate a membrane that it would not normally beable to penetrate. (Permease, a protein carrier molecule, is a special protein which carries other molecules across the membrane.)

Osmosis – water molecules diffuse from a region of higher concentration to a region of lower concentration acrossa selectively permeable membrane, such as a cell membrane.

Filtration – this is a process that forces particles through membranes. This process is commonly used to separate solids from liquids.

Isotonic – a solution that has the same osmotic pressure asthe body’s fluids.

In an isotonic solution, equal volumes of water enter and exit the cells, so the shape and size of the cells remains unchanged.

Hypertonic – a solution that has higher osmotic pressure than the body’s fluids.

In hypertonic solutions, the cell loses more water than it gains, so the cell shrivels.

Ex. concentrated salt solution

Hypotonic – solutions with lower osmotic pressure than body Fluid.

In hypotonic solutions, the cells take in more water than they lose, so the cell swells, and may even burst.

Ex. distilled water, which has nothing dissolved in it.

Diffusion Through Ion Channels:

Ions, such as Potassium (K+), Sodium (Na+), Calcium (Ca2+), and Chloride (Cl-), are involved in many important cell functions.

For example, ions are essential to the nerve cells’ abilityto send electrical signals throughout your body.

Muscle cells in your heart could not beat without movementof ions between the cells.

Ions cannot diffuse through the nonpolar interior of the celllipid bilayer, but they can cross the cell membrane through ion channels.

Polar “Head”

Nonpolar “Tails”(The space between the lipid bilayer is nonpolar. This means charged particles can’t pass through it.)

The inside of the cell is typically negatively charged.

Since opposite charges attract, positively charged particles outside the cell are more likely to diffuse into the cell.

Negatively charged particles inside the cell are more likely to diffuse out of the cell.

In facilitated diffusion, a carrier protein changes shape in order to guide the ion into the cell.

2. Active Mechanisms

Ex. Endocytosis, exocytosis, pinocytosis, phagocytosis, receptor mediated endocytosis, transcytosis.

Active Transport – requires cellular energy to take place. Energy is supplied to the transporter system to transport molecules in the direction opposite the concentration gradient.

Active transport is where substances move across the cell membrane against the concentration gradient.This requires input of cellular energy (ATP).

Endocytosis – molecules that are too large to enter the cells by diffusion, facilitated diffusion, or active transport are conveyed within a vesicle formed from a pinched off section of the cell membrane.

CellInside

Cell Outside

Endocytosis – the ingestion of material by cells. (Ex. 1 Phagocytosis – “cell eating.” Pinocytosis – “cell drinking.”)

Exocytosis - membranes of vesicles within the cell fuse with the plasma membrane in order to let wastes exit the cell.

Outside Cell

Cell Inside

Receptor mediated endocytosis – a specific molecule binds to a receptor site protein, forming a receptor-ligand combination. This combination stimulated the cell membrane to indent. The resulting vesicle transports the substance into the cytoplasm.

Transcytosis – combines endocytosis and exocytosis.

THE END