Chapter 4
Chemistry of Life
Section 4
Homeostasis and transport:
Passive transport the movement of substances across a membrane without energy
Diffusion:
 The simplest form of active transport
 Movement of molecules from an area of high concentration to an area of low
concentration
 The difference of concentration is called the concentration gradient
 Example: a sugar cube is drop in water and goes to the bottom, as it dissolves it spreads
out from the bottom to the top
 Diffusion is driven by kinetic energy
 Molecules are in constant motion
 Molecules tend to move down a concentration gradient
Equilibrium concentration of molecules of a substance is the same throughout
 even at equilibrium molecules are in constant motion
Diffusion across a membrane:
 cell membrane allows certain materials to pass in and out of the cell
 ability of a molecule to move depends on size and shape
 molecules that dissolve in lipids can pass freely ( carbon dioxide and oxygen)
Osmosis:
 Solution is a solute dissolved in a solvent
 The solute is the material being dissolved, solvent is material doing the dissolving
 Example: sugar water, sugar is solute water is solvent together they are a solution
 In the case of cells solutes are organic and the solvent is water
 Water diffuses across the membrane from an area of higher concentration to an area of
lower concentration
 Because water moves down the concentration gradient a cell does not need to expend
energy
Direction of osmosisnet direction depends on the amount of solutes on the two sides of
the membrane
1. Hypotonic solution Concentration of solute in the cell is higher than
concentration outside the cell
2. Hypertonic solution concentration of solute outside the cell is higher
in than the concentration in the cell
3. isotonic concentration of solutes is the same inside and outside of
the cell
Facilitated Diffusion:


Some molecules are so large they can not pass easily through a membrane
Transport proteins are imbedded in the membrane and act like a fast food window
letting large molecules into the cell and out of the cell without using energy
Active transport The movement of molecules across a membrane with energy used
1. Endocytosis- the movement of large molecules into the cell by surrounding the material
with part of the cell membrane
2. Exocytosis- the movement of large molecules out of the cell by surrounding the material
with part of the cell membrane
Section Review:
1. Explain how cell membranes are selectively permeable?
2. Compare and contrast the processes of osmosis and diffusion.
3. Why are endocytosis and exocytosis important processes in cells?
Critical thinking:
Why are fresh fruits and vegetables sprinkled with water at produce markets?