Movement of Material Across the Cell Membrane

Movement of Material Across the Cell Membrane

Diffusion – the random movement ofmolecules from a place of high to a place of low concentration. At some point you will not notice the movement of particles, this is called Dynamic Equilibrium.

Osmosis – the diffusion of water through a semi-permeable membrane. Water will move along a concentration gradient from high to low.

There are three situations with respect to how water moves:

Hypertonic

Hypotonic

Isotonic
Passive Transport:

When no energy is required to move material across the cell membrane. Molecules will follow the concentration gradient.

Simple Diffusion:

If particles are small and have a neutral charge they will move from a place of high concentration to a place of low concentration.

Facilitated Diffusion

There are three protein mechanisms involved in facilitated diffusion.

Channel proteins

Carrier Proteins

Carrier Gate Proteins

Particles that have the correct shape fir into a receptor which cause the protein to change shape allowing material access to the cell. Protein will reset shape once particle enters the cell.

One molecule fits a receptor (insulin) opening a channel for a second molecule (glucose) to gain access to the cell.

Active Transport:

Molecules require energy to move from a (low) to a (high). Energy is supplied in the form of a chemical called ATP.

Protein Pumps:

The movement of sodium across a membrane is a good example of active transport. Marine fish are able to control the flow of sodium allowing them to maintain a life in salty water.

Endocytosis:

The cell membrane can also engulf structures that are much too large to fit through the pores in the membrane protein. In this process the membrane itself wraps around the particle and pinches off a vesicle inside the cell. Amoeba engulf food particles and feed in this manner.

There are three types of endocytosis:

Pinocyctosis: engulfing a large liquid particle

Phagocytosis: engulfing of large solid food particles

Receptor mediated – receptors on the cell membrane fit a shape of a molecule. The membrane folds in, forming a vesicle with the molecules inside.

Exocytosis:

The reverse process where a cell produces large molecules packaged inside a vesicle. The vesicle fuses with the cell membrane becoming a part of the membrane, releasing the material from the cell. For example, the release of insulin from pancreatic cells.