Page 22 - 02 Cells Biology Notes

chloroplasts, which are the form in which plants store the food that
they make in photosynthesis.

• A tissue is a group of similar cells which work together to carry out a
particular function. Tissues are grouped into organs, and organs are
grouped into organ systems.

Movement in and out of cells

• Particles in gases, liquids and solutions are in constant random motion.
As a result of this, there is a net movement from where they are in
a high concentration to where they are in a low concentration.
This is diffusion.

• Diffusion is important to cells. For example, oxygen enters a
respiring cell by diffusion, and carbon dioxide diffuses out of it.

• Water molecules are small and can diffuse through a partially
permeable membrane. Larger molecules dissolved in the water
cannot do this. The diffusion of water through a partially permeable
membrane is called osmosis.

• Osmosis is important to cells. In a dilute solution, water passes into
a cell through its partially permeable cell membrane. The cell gets
bigger. Animal cells may burst, but plant cells do not because of
their strong cell wall.

• In a concentrated solution, water passes out of a cell by osmosis
through its partially permeable membrane. The cell shrinks. Plant
cells may become plasmolysed – that is, the cell membrane pulls
away from the cell wall.

• A solution containing a lot of water is said to have a high water
potential. A solution containing only a little water has a low water
potential. Water moves by osmosis down a water potential
gradient, from a high water potential to a low water potential.

• Cells can use energy to move substances up their concentration
gradient, from a low concentration to a high concentration. This is
called active transport. It uses energy that the cells release
by respiration.