The Cell Membrane and Diffusion Imagine Imagine if

The Cell Membrane and Diffusion

Imagine… �Imagine if you had to live inside a plastic bag. �How long would you survive? �Not long without holes so oxygen could enter. Soon you would need a way to get water and food through the plastic. �You would also need a way of removing wastes, such as urine and carbon dioxide. �In some ways the cell membrane is like the plastic bag but much more complex.

The Cell Membrane � The cell membrane controls what enters or exits the cell. � The cell membrane is called a selectively permeable membrane because only certain particles can pass through it. � Permeable means “let through. ” Impermeable means “not let through. ” The cell membrane is permeable to some materials and impermeable to others.

The Cell Membrane �The cell membrane is made of two layers of fat particles. �Proteins are fixed into these layers. Some of these proteins form pathways, called channels, that open and close to let different particles pass through. �Particles that easily pass through the membrane include water, oxygen and carbon dioxide. �Particles that do not easily pass through the membrane (because they are too large) include sugars and fats.

The Cell Membrane

Fluid Movement in Cells: Diffusion � When you place a blob of ink in water the ink gradually spreads out and colours the whole beaker of water. � At first, the concentration of particles is highest where the ink blob first dropped. � Then, the particles spread out through the container. � When the ink particles are released into the water, they collide with the water particles. The ink and water particles move and bounce off each other. � The particles spread out as they move.

Diffusion �Particles move from where they are highly concentrated (many in one place) to where they are less concentrated (few in one place). �This process is called diffusion. �When two areas have different concentrations of a certain particle, we say that there is a concentration gradient. �Diffusion can happen only when there is a concentration

Diffusion � Diffusion is important to cells. � Many materials a cell needs enter the cell by diffusion. Some wastes leave the cell by diffusion. � In your body, blood carries oxygen to cells and carries carbon dioxide waste away from cells through diffusion. � Oxygen moves from red blood cells where it is highly concentrated, to other body cells where it is less concentrated. � Carbon dioxide moves out of