Types of Transport: Movement through the Cell Membrane

Transport systems within the cell are like a highway system, they provide for the constant movement of molecules, in and out of the cell.

Transport systems are needed because the cells membrane is “selectively permeable”. Some molecules can pass through (permeate) the cell membrane, while others cannot.

– Because cells naturally move and collide, passive transport requires no energy to move molecules into or out of the cell.
– They move on their own.

1. DIFFUSION (Simple)
– The movement of molecules from an area where they are highly concentrated to an area where they are less concentrated or more spread out.
– Molecules move across the cell membrane from an area of high concentration to an area of low concentration. It stops when the concentration is evenly distributed or equal.

– It is simple diffusion of water ONLY.
– -OSOMOTIC PRESSURE (the pressure exerted by dissolved particles in water) moves the water across the cell.
– Water moves from where the osmotic pressure is low to where the osmotic pressure is high.
– The osmotic pressure is always working to make the solution on both sides of the cell membrane equal.
– When two solutions of different concentrations are compared the solution with the higher concentration is called HYPERTONIC and the solution with the lower concentration, HYPOTONIC. When two solutions have the same concentration they have EQUILIBRIUM.
– Equilibrium: when the osmotic pressure is equal on both sides. When the osmotic pressure is the same on both sides of the cell membrane it is ISOTONIC or ISOSMOTIC. Both sides of the cell membrane have the same amount of particles and pressure, so there is no movement on either side.
– HYPERTONIC or HYPEROSMOTIC: when a cell is in an area of higher concentration. A higher osmotic pressure is placed on the cell, so the water will move out of the cell into the surrounding area.
– HYPOTONIC or HYPOOSMOTIC: when a cell has a higher concentration of particles inside the cell then are in the solution that the cell is in. The water will move into the cell to try to make both sides of the membrane equal.

– It is diffusion that is helped by the use of protein carrier molecules.
– It works in both directions, in and out of the cell.
– It is similar to simple diffusion, but it allows larger molecules, that need extra help to get across the cell membrane.
– Ex: Glucose molecules are too large to cross the cell membrane. So a glucose carrier protein, that is located in the cell membrane, combines with the glucose molecule and helps it cross the cell membrane.
– The molecule is picked up on the one side of the membrane and released on the other side.

– Movement of molecules, which do not normally move in this direction, from an area of low concentration to an area of high concentration.
– The cell must use energy to move these molecules.
– Like facilitated diffusion, active transport uses protein carrier molecules.
– The active transport system uses the high energy molecule (ATP) ADENOSINE TRIPHOSPHATE.
– ATP is split during active transport providing free energy to power the transportation process.

– A way for cells to move very large molecules into a cell.
– The cell membrane surrounds the molecule and forms a vacuole. The vacuole will move the molecule into the cell.

– The active transport that moves molecules out of a cell.
– The molecule is surrounded and pushed out of the cell.
– This process is the exact reverse of endocytosis.