B5.1 How do organisms develop

B5.1 How do organisms develop

Cells are the building blocks of all living things – multicellular organisms are made up of collections of cells. The cells can become specialised to do a particular job.

Groups of specialised cells working together are called tissues and a group of tissues working together are called organs.

When an egg is fertilised by a sperm it becomes a ZYGOTE.

The zygote then divides many times by mitosis to form an embryo – up to (and including) the eight-cell stage all the cells are identical and can produce any sort of cell required – embryonic stem cells

After the eight-cell stage, most of the embryo cells become specialised and from different types of tissue

Some cells remain unspecialised – these are adult stem cells. At a later stage they can become specialised however, unlike embryonic stem cells, adult cells cannot become any type of cell.

Plants have cells that are like stem cells in animals – the cells are in areas called meristems. Only cells within meristems can divide repeatedly (mitotically active)

Cells in the meristem are unspecialised but they can develop into any type of plant cell. Under normal hormonal conditions, unspecialised plant cells can become specialised to form different types of tissues (including xylem and phloem) within organs (including flowers, leaves, stems and roots)

There are two types of meristems:

  • Apical meristems – those that result in increased height and longer roots
  • Lateral meristems – those that result in increased girth

Xylem is made from specialised cells to transport water and soluble mineral salts from the plant roots to the stem and leaves, and to replace water lost during transpiration and photosynthesis

Phloem is made from specialised cells to transport dissolved foods made by photosynthesis throughout the plant for respiration or storage

When a stem is deliberately cut, special plant hormones can be added – these can send messages to the meristems to start to produce roots. As the cutting already has a stem and leaves, it will then grow into a clone of the parent plant. This may be done to reproduce a plant with desirable features.

A cut stem from a plant can develop roots and then grow into a complete plant which is clone of the parent, and that the rooting can be promoted by the presence of plant hormones (AUXINS). Auxins mainly affect cell division at the tip of a shoot, because that is where the meristems are.

The growth and development of plants are affected by the environment e.g. phototropism and geotropism (where roots and shoots grow towards and away from the source of gravity)

PHOTOTROPISM is a response by the plant to light – a plant’s survival depends on its ability to photosynthesis therefore plants need strategies to detect light and to respond to changes in intensity. This is demonstrated by the way in which plants will grow towards a light source.

The cells furthest away from a light source grow more, due to the presence of auxin which is sensitive to light. Auxin is produced at the shoot tip and migrates down the shoot

A – When the tips are removed, no auxin is made so the stems do not grow

B – When the tips are covered, auxin moves to all parts of the stem causing all parts to grow

C – When the tips are lit from one side auxin accumulates on the shaded side causing it to grow more than the illuminated side