Cells

What are cells?

Cells are what make up living things. They are also called building of life.

Below are the cell structures of animal and plant cells.

Movement in and out of cells

Water is the main component of all cells. We find water in the cytoplasm and in cell sap. This water contains many dissolved substances and these substances plus the water enter and leave the cells through the cell membrane.  The cell membrane allows certain particles through it but it blocks the passage of others. Because of this nature it is described as a partially (or selectively) permeable membrane. Particles enter and leave cells by three processes:

Diffusion

Osmosis

Active Transport

Diffusion

Many substances can pass through the membrane by diffusion. Diffusion happens when a substance is more concentrated in one place than another. 

Osmosis

Osmosis is a special kind of diffusion involving water molecules. It occurs when two solutions are separated by a partially permeable membrane.

 The definition of osmosis is as follows:

Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.

Active Transport

Sometimes organisms require certain substances and therefore need to move dissolved substances from a region of low concentration to a region where they are at a higher concentration. This process is of course the opposite to the direction in which particles would normally move in diffusion. This is active transport. In active transport particles move against a concentration gradient and therefore require energy which must be supplied by the cell. Carrier proteins that are found in the cell membrane of cells use energy to transport molecules or ions across the membrane, against the concentration gradient. When organisms utilise active transport the energy for the process comes from respiration. Due to this the cells capable of active transport usually have more mitochondria, in which respiration takes place than other cells.