Cell Division and Growth
3. Growth and Repair
Learning outcomes
- I can explain how organisms grow through cell division.
- I can describe the role of mitosis in tissue repair.
- I can explain why cell division is necessary throughout life.
- I can identify examples of growth and repair in living organisms.
- I can relate growth and repair to the cell cycle.
Growth and Repair
Living organisms grow and maintain their bodies by producing new cells.
In multicellular organisms, growth does not happen simply because existing cells become larger and larger. Instead, organisms mainly grow by increasing the number of cells.
This happens through the cell cycle, which includes growth, DNA replication, mitosis, and cytokinesis.
Growth Through Cell Division
A multicellular organism usually begins as a single fertilized cell.
That cell divides.
Then the daughter cells divide again.
For example:
1 cell → 2 cells → 4 cells → 8 cells → 16 cells
Repeated cycles of cell division eventually produce the enormous number of cells that make up tissues, organs, and organ systems.
Therefore, one of the main ways organisms grow is by:
increasing cell number through mitosis
Why Cells Do Not Just Keep Getting Bigger
Cells can grow, but they cannot continue becoming larger indefinitely.
As a cell becomes larger:
- its volume increases
- its surface area also increases
- volume increases faster than surface area
- surface area-to-volume ratio decreases
This makes exchange of substances less efficient.
Smaller cells also have shorter diffusion distances.
Therefore, organisms usually grow by producing more cells, rather than by allowing individual cells to become extremely large.
Growth and the Cell Cycle
Growth depends on the cell cycle.
The major sequence is:
Interphase → Mitosis → Cytokinesis
During interphase:
- the cell grows
- DNA is replicated
- organelles are produced
- the cell prepares for division
During mitosis:
- chromosomes are separated
- two genetically identical nuclei form
During cytokinesis:
- the cytoplasm divides
- two daughter cells are produced
These daughter cells can then grow and later enter another cell cycle.
Growth in an Embryo
One of the clearest examples of growth through cell division occurs during embryonic development.
A fertilized egg, called a zygote, starts as one cell.
It divides repeatedly:
1 → 2 → 4 → 8 → 16 → many more
As the number of cells increases:
- tissues begin to form
- organs begin to develop
- cells become specialized
- the organism becomes larger and more complex
Cell division is therefore essential for development from a single cell into a multicellular organism.
Growth in Children and Adolescents
Growth continues after birth.
During childhood and adolescence, mitosis increases the number of cells in many tissues.
Examples include:
- bones lengthening
- muscles increasing in size
- skin expanding
- organs growing
- blood-cell production increasing
Growth involves both:
- cell division
- changes in cell size
However, increasing cell number is a major part of normal growth.
Growth in Plants
Plants also grow through cell division.
Rapid cell division occurs in regions called meristems.
Meristems are found in places such as:
- root tips
- shoot tips
- buds
Cells in meristems divide actively by mitosis.
The new cells then:
- grow
- elongate
- differentiate into specialized cell types
This allows roots and shoots to become longer.
Mitosis and Tissue Repair
Growth is not the only reason organisms need cell division.
Cells are also damaged or lost during everyday life.
Mitosis produces replacement cells.
This allows tissues to repair themselves.
Examples include:
- healing cuts in the skin
- replacing damaged skin cells
- repairing some damaged plant tissues
- replacing cells lining the digestive system
- producing new blood cells
What Happens When Skin Is Cut?
Imagine a person gets a small cut.
Cells in the damaged area may be destroyed.
The body responds by beginning a repair process.
Nearby cells can enter the cell cycle.
They:
- grow
- replicate their DNA
- undergo mitosis
- complete cytokinesis
The daughter cells replace cells that were damaged or lost.
Over time, the tissue closes and heals.
Repair Requires Genetically Similar Cells
Repair works because mitosis produces cells that are genetically very similar to the original cells.
For example, when skin tissue is repaired, the body needs to produce more skin cells.
It would not be useful if the new cells had completely different genetic information.
Mitosis maintains the genetic instructions needed for the tissue to continue functioning properly.
Therefore:
mitosis → genetically similar cells → effective tissue repair
Why Cell Division Is Necessary Throughout Life
Cell division is not only important while an organism is growing.
Even adults need constant cell replacement.
Many cells have limited lifespans or are regularly damaged.
Cells may be lost because of:
- friction
- mechanical damage
- exposure to chemicals
- digestive processes
- normal aging
- programmed cell death
These lost cells must be replaced.
Therefore, mitosis continues throughout life in many tissues.
Replacing Skin Cells
The outer layers of the skin are constantly exposed to:
- friction
- sunlight
- chemicals
- drying
- physical damage
Surface cells are continually lost.
New cells are produced deeper in the skin by mitosis.
As new cells form, older cells are pushed toward the surface.
This allows the skin to maintain a protective barrier.
Replacing Cells in the Digestive System
Cells lining the digestive system are exposed to:
- digestive chemicals
- enzymes
- friction from food
- changing conditions
Because these cells experience significant wear, they need to be replaced frequently.
Cells in deeper regions divide by mitosis and produce new cells.
This helps maintain a healthy intestinal lining.
Blood Cell Production
Blood cells also need continuous replacement.
Many blood cells have limited lifespans.
New blood cells are produced from stem cells in bone marrow.
These cells divide and develop into different blood cell types.
Examples include:
- red blood cells
- some white blood cells
- platelet-producing cells
Cell division therefore helps maintain healthy blood throughout life.
Growth and Repair in Bones
Bone is living tissue.
It can grow, remodel, and repair damage.
During childhood, cell division contributes to bone growth.
If a bone breaks, specialized cells help repair the damaged area.
New tissue is produced and gradually rebuilt.
This shows that even hard tissues depend on living cells and cell division.
Growth and Repair in Plants
Plants can also repair some damaged tissues.
If part of a plant is damaged, nearby cells may divide and produce replacement cells.
Plants also continue growing throughout much of their lives because meristems remain active.
This means cell division contributes to:
- new leaves
- longer roots
- longer shoots
- new branches
- some forms of tissue repair
Cell Division and Specialized Cells
Not all cells divide at the same rate.
Some tissues require frequent cell division.
Examples include:
- skin
- intestinal lining
- bone marrow
- plant meristems
Other highly specialized cells divide rarely or may not divide under normal conditions.
This means the rate of cell division depends on the function and needs of the tissue.
Stem Cells and Repair
Stem cells are unspecialized cells that can divide and produce new cells.
Some daughter cells can remain as stem cells.
Others can become specialized.
Stem cells are important in several tissues because they provide a continuing source of replacement cells.
For example:
Bone marrow stem cells can produce new blood cells.
Stem cells in the intestinal lining help replace cells lost from the surface.
Cell Growth vs Organism Growth
It is important to distinguish between:
cell growth
and
organism growth
A cell grows during interphase.
However, an organism mainly becomes larger because many cells go through repeated cycles of:
growth → division → growth → division
This increases the total number of cells.
The Cell Cycle and Repair
Repair depends on cells moving through the cell cycle.
A simplified repair sequence is:
- Tissue is damaged.
- Some cells are lost.
- Nearby cells receive signals that division is needed.
- Cells enter the cell cycle.
- DNA is replicated.
- Mitosis occurs.
- Cytokinesis produces new cells.
- New cells help replace damaged tissue.
This connects tissue repair directly to the cell cycle.
Why DNA Replication Is Important for Repair
Before mitosis, each cell copies its DNA.
This ensures that the daughter cells receive complete genetic information.
For repair, this is essential.
For example, if a skin cell divides:
Parent skin cell → DNA replication → mitosis → two genetically similar skin cells
These new cells contain the instructions needed to function as part of skin tissue.
Growth, Repair, and Cell-Cycle Control
Growth and repair must be carefully controlled.
Cells should divide:
- when new cells are needed
- when tissue needs repair
- when normal replacement is required
Cells should stop dividing when enough new cells have been produced.
This control is important because uncontrolled cell division can cause serious problems.
What Happens When Cell Division Is Uncontrolled?
Sometimes mutations affect the genes that control the cell cycle.
Cells may begin dividing when they should not.
This can lead to the formation of a mass of abnormal cells called a tumour.
Some tumours can become cancerous.
Normal growth and repair therefore depend on a balance between:
- cell division
- cell specialization
- cell death
- cell-cycle control
Worked Example: Growth
A young plant grows from 5 cm to 25 cm tall.
How does cell division contribute?
Cells in the plant's meristems divide by mitosis.
This produces additional cells.
The new cells then grow and elongate.
Together, these processes increase the size of the plant.
Worked Example: Tissue Repair
A student scratches their arm and damages some skin cells.
Why is mitosis necessary?
Mitosis produces new skin cells.
These replace cells that were damaged or lost.
Therefore:
mitosis allows the damaged tissue to repair itself
Worked Example: Why Adults Still Need Mitosis
A student says:
"Adults have stopped growing, so they do not need mitosis."
This is incorrect.
Adults still lose cells through normal wear, damage, and aging.
Mitosis is needed to replace many of these cells.
Therefore, cell division remains important throughout life.
Worked Example: Cell Cycle Connection
A cell is preparing to replace a damaged cell.
What must happen before mitosis?
The cell must pass through interphase.
During interphase:
- the cell grows
- DNA is replicated
- the cell prepares for division
Then mitosis and cytokinesis can produce new cells.
Growth and Repair Compared
| Growth | Repair |
|---|---|
| Increases cell number | Replaces lost or damaged cells |
| Important during development | Important after injury or normal wear |
| Uses mitosis | Uses mitosis |
| Produces genetically similar cells | Produces genetically similar cells |
| Depends on the cell cycle | Depends on the cell cycle |
Both growth and repair depend on controlled cell division.
Examples of Growth and Repair
Growth
- embryo developing from a fertilized egg
- child growing taller
- plant roots extending through soil
- shoots growing toward light
- organs increasing in size
Repair and Replacement
- healing a skin cut
- repairing some damaged plant tissue
- replacing intestinal cells
- producing new blood cells
- healing a broken bone
All of these involve the production of new cells.
Common Misconceptions
Organisms grow mainly because their cells become enormous.
Incorrect. Multicellular organisms mainly grow by increasing the number of cells.
Mitosis is only needed during childhood.
Incorrect. Mitosis is needed throughout life for replacement and repair.
Tissue repair means damaged cells fix themselves.
Sometimes cells can repair minor damage, but damaged or lost cells are often replaced by new cells produced through division.
All cells divide at the same rate.
Different tissues have very different rates of cell division.
Growth and repair are completely different processes.
They have different purposes, but both depend heavily on the cell cycle and mitosis.
Mitosis creates genetically different replacement cells.
Mitosis normally produces daughter cells that are genetically very similar to the parent cell.
Did You Know?
The lining of your digestive tract experiences constant wear because it is exposed to food, digestive chemicals, and enzymes.
For this reason, many cells in the lining must be replaced regularly.
Specialized regions contain dividing cells that continually produce replacements.
This is a good example of why mitosis remains important even when an organism is no longer increasing greatly in overall size.
Key Terms
Growth – An increase in the size of an organism, often involving an increase in cell number.
Repair – Replacement or restoration of damaged tissues.
Mitosis – Division of the nucleus to produce genetically similar nuclei.
Cell cycle – The sequence of stages involving cell growth, DNA replication, mitosis, and cell division.
Cytokinesis – Division of the cytoplasm to produce daughter cells.
Daughter cells – Cells produced by cell division.
Stem cell – An unspecialized cell capable of dividing and producing new cells.
Meristem – A region of actively dividing cells in a plant.
Tissue – A group of similar cells working together to perform a function.
Cell replacement – Production of new cells to replace old, damaged, or lost cells.
Key Takeaways
- Multicellular organisms grow mainly by increasing their number of cells.
- New cells are produced through the cell cycle.
- Mitosis produces genetically similar nuclei.
- Cytokinesis divides the cell into daughter cells.
- Growth begins very early, when a fertilized egg repeatedly divides.
- Plant growth occurs especially in regions called meristems.
- Mitosis is important for tissue repair.
- Damaged or lost cells can be replaced by new cells produced through division.
- Cell division remains necessary throughout life, even after overall growth slows.
- Skin, intestinal tissues, and blood-producing tissues require regular cell replacement.
- DNA must be copied before mitosis so new cells receive complete genetic information.
- Stem cells provide important sources of new cells in some tissues.
- Growth and repair both depend on the same basic sequence: cell growth, DNA replication, mitosis, and cytokinesis.
- Cell division must be carefully controlled so that cells divide when needed and stop when enough new cells have been produced.