The Cell in its Environment The Cell Membrane

What do you think?  Look at the pictures of the various types of cells. Do you think all cells have the same function or same task based on the pictures why or why not?

Types of Cells

What is the one thing that all cells have in common?  It’s DNA, the instructions for the functions that the cell must carry out, but each type of cell has a different function to carry out so the instructions are different

 Cells always have many activities going on at one time because there are so many functions of a cell. Some basic functions are needed for the cell to stay alive so they go on constantly other functions are only needed at specific times so they only go on when needed. Many of these activities maintain a biological balance with the cell’s surrounding environment. This balance is called homeostasis.

Homeostasis  Homeostasis is the ability of organisms to maintain their internal conditions. The amounts of molecules inside and outside the cell are kept at certain levels to maintain balance.  How do you keep your body temperature at a homeostatic level?

How Do They Do That!  Sometimes to maintain balance a cell moves molecules in or out of itself.  This is called transport- where molecules are moved from one side of a membrane to another.  Molecules naturally move from an area of high concentration to an area of low concentration

 What is meant by concentration?

Diffusion  The main method by which small molecules move into and out of cells is diffusion  Diffusion is the process by which molecules tend to move from an area of high concentration to an area of lower concentration

Why the flow?  The cell membrane is selectively permeable, which means that some substances can pass through while others can not  Permeable comes from a Latin word that means “to pass through”  Cell membrane is usually permeable to substances such as oxygen, water, and carbon dioxide but not permeable to large molecules and salts

What causes Diffusion?  Molecules are always moving. As they move they bump into one another causing collisions.  The more collisions the more the molecules move away from each other and spread out until they spread evenly throughout the area.

Diffusion in cells (an example)  Single-celled organisms in pond water get the O 2 they need to survive from the H 2 O around them.  There are more molecules of O 2 in the H 2 O outside the cell (higher concentration) than there are inside the cell (lower concentration)  The O 2 molecules diffuse from the area of high concentration to the area of low concentration

Osmosis-Diffusion of H 2 O Molecules  The diffusion of H 2 O molecules through a selectively permeable membrane is called osmosis.  H 2 O passes easily into and out of cells through the cell membrane.  By moving the water molecules around, cells can raise or lower the concentrations of other molecules

Hypotonic hypo-lower & tonic refers to the solute molecules-not the water  A solution where the solute concentration that is lower than the solute concentration of another solution.  Water molecules are more concentrated on the outside of the cell than the inside of the cell.  In this environment, water molecules move into the cell, if too much water enters the cell could burst

Hypertonic hyper-higher & tonic refers to the solute molecules-not the water  A solution where the solute concentration is higher than the solute concentration elsewhere  Water molecules are more concentrated on the inside of the cell than the outside of the cell.  In this environment, water molecules move out of the cell, if too much water leaves the cell could shrink

Isotonic  A solution where the solute concentration is equal to the inside solute concentration of another solution  Water molecules are equally concentrated on the inside of the cell as the outside of the cell.  The whole environment is in equilibrium- a state of equal concentrations in all parts of an area.

Passive Transport  Moving materials though a cell membrane through osmosis and diffusion is like cycling downhill-it requires no energy  The movement of materials through a cell membrane without using energy is called passive transport  Normally goes from an area of high concentration to an area of low concentration

Active Transport  Moving materials though a cell membrane opposite the natural direction is like cycling uphill-it requires energy  The movement of materials through a cell membrane using energy is called active transport  Normally goes from an area of low concentration to an area of high concentration

Active Transport-Transport Proteins  One method of active transport is transport proteins  Transport Proteins “pick up” molecules outside the cell and carry them in using energy in the process

Active Transport-By Engulfing  Happens when the cell membrane surrounds, or engulfs, a particle  Once the particle is engulfed, the membrane pinches off and forms a vacuole within the cell