Science Chapter 2 The Functioning Cell

Chapter 2.1 The Cell Membrane: Gatekeeper of the Cell

The Functioning Cell  In order for our cells to function, they need air, food, and water.  If someone asked you why you need a drink, you might say you are hot or thirsty, but you could also say that your cells need the water.

Cell Membrane  Think of a border separating two nations. Customs officers check the items that travelers are carrying. Some items, such as firearms and plants are not allowed to cross the border.  A cell membrane works similarly in that it allows some substances to enter or leave a cell, but not others.

Cell Membranes  When a cell selects what items get to come in and out it is called selectively permeable.  When a cell membrane allows anything to cross it, it is called permeable.  A membrane that allows nothing to cross is call impermeable.

Example  Think of what happens when you pour water into a plastic bag. The water is not going to go through the plastic. The plastic is impermeable to water.  What if you pour water into a bag made of cheesecloth? The water is going to go through the cloth; therefore, it is permeable.

Example  Imagine if you pour a mixture of water and sand into a cheesecloth bag.  Will the bag be permeable, impermeable or selectively permeable?

Answer  The cheesecloth bag will be selectively permeable to the mixture of water an sand. Water will go through but most of the sand will not.

Diffusion  What causes substances to move in the first place?  The particles in all liquids and gases are constantly moving in every direction and bumping into each other.  Thus, in a concentrated blob, particles will spread out to avoid more collisions.  This spreading-out process is diffusion.

Diffusion  Diffusion also plays a part in moving substances into and out of cells.  Imagine an amoeba living in water and the concentration of the carbon dioxide in the water is the same as inside of the cell. The carbon dioxide will move in and out of the cell at the same rate.

Diffusion  If the amoeba is producing carbon dioxide and there is more inside the cell than outside the cell; then more carbon dioxide cells will leave the cell and less will come in.  This process continues until the concentration of carbon dioxide inside and outside the cell are equal.

Osmosis  The most common substance found inside and around the cell is water.  About 70% of a cell’s content is water.  The diffusion of water though a selectively permeable membrane is called osmosis.

Example  If you have ever left carrot sticks in the fridge for awhile, you probably noticed that they became limp.  This is because they have lost their moisture through osmosis.  If you put the carrot sticks into water, then 24 hrs later, they are crisp again.

Osmosis  Remember when I said that when we are hot we get thirsty?  When we are active, we lose moisture from our body in our breath and our sweat. Water is drawn out by osmosis. We need water to restore cell water content.

Osmosis  Glucose (which cells use for energy) dissolves in water to form a glucose solution.  When water moves out of a cell, the dissolved substances inside the cell become more concentrated.  When water moves into a cell, the dissolved substance becomes more diluted.

Osmosis  Water tends to move from a dilute solution to a more concentrated solution.  Water moves from an area of high concentration to low concentration.

Transport  Small particles---such as water, carbon dioxide, and oxygen---diffuse freely into and out of cells through small openings in the cell membrane.  This only depends only on the concentrations of the particles.  It occurs without any use of energy by the cell.  Natural for these particles to equalize.

Active Transport  When certain substances need to have a greater concentration inside the cell such as glucose for energy. This is called active transport and it needs the cell’s energy to perform this task.

Active Transport  Like gates in a wall, large particles called carrier proteins that are embedded in the membrane controls the substances moving through the wall.  Each carrier protein attracts particles of a certain substance.  The protein attaches to the substance and moves in through the membrane inside the cell.

Questions  Complete questions 1-6 on page 47.