Growth

Back to: BIOLOGY SS1

 

Welcome to class! 

In today’s class, we will be talking about growth. Enjoy the class!

Growth

Growth is defined as an irreversible increase in size and mass due to formation of new protoplasm in the cell.

The three distinct processes that contribute to growth are:

1. cell division,
2. cell enlargement and
3. cell differentiation.

They are called the basis of growth.

Basis of growth

Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle.

In cell division, the nucleus and then the cytoplasm of the parent cell divides to form two daughter cells, then into four and so on. This brings about an increase in the number of cells.

There are two types of cell division:

• Mitosis: the type of cell division that occurs in somatic cells i.e body cell.
• Meiosis: the type of cell division that occurs in reproductive cells.

Mitosis cell division

Mitosis is how somatic—or non-reproductive cells—divide. Somatic cells make up most of your body’s tissues and organs, including skin, muscles, lungs, gut, and hair cells. Reproductive cells (like eggs) are not somatic cells.

Cell division by mitosis occurs in four successive phases namely prophase, metaphase, anaphase and telophase.

In mitosis, the important thing to remember is that the daughter cells each have the same chromosomes and DNA as the parent cell. The daughter cells from mitosis are called diploid cells. Diploid cells have two complete sets of chromosomes.  Since the daughter cells have exact copies of their parent cell’s DNA, no genetic diversity is created through mitosis in normal healthy cells

Meiosis Cell Division

Meiosis is the other main way cells divide. Meiosis is cell division that creates sex cells, like female egg cells or male sperm cells.  What is important to remember about meiosis? In meiosis, each new cell contains a unique set of genetic information. After meiosis, the sperm and egg cells can join to create a new organism.

Meiosis is why we have genetic diversity in all sexually reproducing organisms

Cell Enlargement and cell differentiation

The daughter cells then increase in mass and size (i.e enlargement) and eventually develop into a special type of cell (i.e. differentiation) by changing its shape and structure to carry out a particular function.

The kind of cell it becomes depends on its position in the body of the organism. It may develop into a nerve cell in the brain, a muscle cell in the heart or a ciliated lining cell in the trachea.

Factors affecting growth

These factors may be divided into external and internal factors. The external factors include the following:

1. Nutrient availability
2. Toxic by-products of metabolism
3. Temperature
4. Light
5. PH

The main internal factors controlling growth are

1. hormones.   2. enzymes.

Regulation of growth by hormones

Hormones in plants that affect growth are:

Auxins, cytokinin, gibberellin etc

Hormones that regulate growth in animals are Thyroxin and those produced by the pituitary gland and sex hormones.

Responsive growth movements: Tropisms

Responsive growth movements toward or away from an external stimulus are called tropisms. If the plant movement is toward the stimulus, it is a positive tropism; away from the stimulus, a negative tropism.

The tropic response to unidirectional light is called phototropism. Gravitropism is the plant response to gravity.

The growth response of a plant or a plant part to the touch of a solid object is called thigmotropism.

Evaluation

1. Define growth
2. List three processes that contribute to the growth
3. Define mitosis and list the four main phases of mitosis
4. Name five external factors and two internal factors that affect growth
5. Name two plant hormones and state their influence on plant growth

 

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