Cell Biology

Lec.6 Cell Biology

Movement across Cell Membranes

The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it.

There are two ways in which substances can enter or leave a cell:

1) Passive ways

a) Simple Diffusion
b) Facilitated Diffusion
c) Osmosis (water only)

2) Active ways

Active Transport
Vesicle Transport


1) Passive ways
Diffusion
Diffusion is the net passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration (down a concentration gradient) . It continues until the concentration of substances is uniform throughout.
An example: gas exchange for respiration — oxygen from blood to tissue cells, carbon dioxide in opposite direction.

Facilitated Diffusion

This is the movement of specific molecules down a concentration gradient, passing through the membrane via a specific carrier protein. Each carrier has its own shape and only allows one molecule (or one group of closely related molecules) to pass through. Selection is by size; shape ; and charge. Common molecules entering/leaving cells this way include glucose and amino-acids. It is passive and requires no energy from the cell.

Facilitated Diffusion

Osmosis
Osmosis is a special example of diffusion. It is the diffusion of water through a partially permeable membrane from a more dilute solution to a more concentrated solution – down the water potential gradient).
Note: diffusion and osmosis are both passive, i.e. energy from ATP is not used.

The Effects of Osmosis

When an animal cell ( for example, the red blood cell) is placed in a medium, which is a water solution , the possible consequences are listed below:
If the water concentration inside the cell is the same as that in the surrounding medium (the medium is an isotonic solution) there will exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell and so the cell will retain its original size.
If the water concentration of the cell is lower than that of the medium (the medium is a hypotonic solution) surrounding the cell then osmosis will result in the cell gaining water.
The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst.
If the water concentration inside the cell is higher than that of the medium ( the medium is a hypertonic solution) the number of water molecules diffusing out will be more than that entering and the cell will shrink due to osmosis.

2) Active ways

a. Active Transport
Active transport is the energy-demanding transfer of a substance across a cell membrane against its concentration gradient, from lower concentration to higher concentration. Special proteins within the cell membrane act as specific protein ‘carriers’. The energy for active transport comes from ATP generated by respiration (in mitochondria). An example: Sodium/potassium pump in cell membranes (especially nerve cells).

b. Vesicle Transport

Some molecules or particles are just too large to pass through the plasma membrane or to move through a transport protein. So cells use two other active transport processes to move these macromolecules (large molecules) into or out of the cell. Vesicles or other bodies in the cytoplasm move macromolecules or large particles across the plasma membrane .There are two types of transport, endocytosis and exocytosis . Both processes are active transport processes, requiring energy.
Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane. The membrane folds over the substance and it becomes completely enclosed by the membrane. At this point a membrane-bound sac, or vesicle, pinches off and moves the substance into the cytosol. There are two main kinds of endocytosis:

Pinocytosis (‘cell drinking’) This is the uptake of large molecules (DNA, protein) from solution, by a form of endocytosis – the vesicles formed are minute and short-lived.
Phagocytosis (‘cell eating’) This is the uptake of solid particles by a cell e.g., Phagocytes engulfing bacteria.

Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. Exocytosis occurs when a cell produces substances for export, such as a protein, or when the cell is getting rid of a waste product or a toxin. Newly made membrane proteins and membrane lipids are moved by exocytosis.