B3 - Growth and development

• Bacteria cells differ from plant and animal cells in that they lack a 'true' nucleus, chloroplast and mitochondria.
• As bacterial cells do not have a nucleus, DNA is found in the cytoplasm as a single circular strand or chromosome.

• The main phases of human growth involve: rapid growth - 1 that is right after birth, the other in adolescence.
• Dry Mass is the best measure of growth.
• Measuring growth by lentgh is easy to do, but only measure in one direction.
• Measuring the wet mass is hard to do e.g. trees, but is easy for animals. However the water content or arganisms can vary with time.
• Dry mass can only be measured by killing the organism and drying off the water but it does measure the true growth of the whole organism.
• Different parts of an organism may grow at different rates compared to the whole organism. This is beacause different parts of the organism may be needed at different times in the life of the organism.

• Stem cells remain undifferentiated, they can develop into different types of cells.
• Stem cells can be obtanied from embryos and could potentially be used to treat medical conditions.
• However, there are issues arising from stem cell research in animals. Some people think that it is wrong because the embryo is being destroyed. Others think that it is acceptable because it can treat life-threatining diseases.
• Stem cells can be found in the adult aswell as in the embryo. Embryonic stem cells can form a greater range of cell types and are easier to find.

• Differences between plant and animal growth include the following:

- Animals tend to only grow to a certain size, but plants can carry on growing.

- Plant cell division only happens in areas called meristems found at the tips of roots and shoots.

- The main way that plants gain height is by cells enlarging rather than dividing.

- Many plant cells keep the ability to differentiate, but most animals lose this ability at an early stage.