Why Do Cells Divide Instead of Just Growing Bigger?
- The short answer is there are many fundamental problems that arise when cells get too large.
- Many of which, get solved with cell division.
Definition
Cell division
The process by which one cell splits into two daughter cells. This is essential for growth, repair, and reproduction.
Exam technique
- In the MYP eAssessment of N21, Question 2a, the question checks understanding of why cells divide in organisms.
- Credit is awarded for stating valid biological reasons such as growth, repair of damaged tissue, or replacement of worn-out cells.
Larger cells have diffusion problems
- Nutrients and oxygen must diffuse from the surface to the center, while waste products must diffuse from the center to the surface.
- As cells get bigger, this distance increases.
- The center of a large cell would be too far from the surface, making diffusion too slow.
Surface area doesn't keep up with volume
- When a cell doubles in width, its surface area increases by 4 times but its volume increases by 8 times.
- The cell needs more resources (volume) but has proportionally less membrane (surface area) to get them through.
- This is known as the surface area to volume ratio (SA:V)
Definition
Surface area to volume ratio
The relationship between a cell's surface area (outer membrane) and its volume (internal space). As cells get larger, this ratio decreases, creating problems for the cell.
The nucleus can't control a huge cell
- The nucleus produces instructions (mRNA) for the whole cell.
- In a very large cell, these instructions take too long to reach distant parts.
- The nucleus becomes overwhelmed trying to manage too much cytoplasm.
Dividing solves all these problems
- Two small cells have more total surface area than one large cell of the same volume.
- Each small cell has a short diffusion distance from surface to center.
- Each nucleus only has to control a manageable amount of cytoplasm.
Note
Organisms grow by making more cells through cell division, not by making existing cells larger.
Analogy
- Imagine a restaurant kitchen feeding customers.
- A small kitchen (small cell) with one door (membrane) can serve 20 people efficiently.
- But if you tried to serve 200 people through the same door, service would be terrible.
- Better to have 10 small kitchens, each with its own door, each serving 20 people.
Example
- Elephants are thousands of times larger than mice, but their cells are roughly the same size.
- Elephants just have many more cells.
- Both species keep cells small to maintain efficient diffusion and control.
How Do Cells Know When It's Time to Divide?
- Cells follow a controlled cycle known as the cell cycle, alternating between two phases: growth and division.
- During interphase (growth phase), the cell gets larger and copies its DNA.
- During division, the cell splits into two.
- As a cell divides, it produces two small daughter cells.
- Most of a cell's life is spent in interphase, doubling in size.
- Once large enough, they divide again, producing small cells.
- The cycle repeats: divide → grow → divide → grow.
Definition
Cell cycle
The sequence of events that a cell goes through from one division to the next. Cells alternate between growing (interphase) and dividing.
Why can't cells divide and grow at the same time?
- During growth (interphase), DNA must be unpacked and spread out so it can be copied.
- During division on the other hand, DNA must be tightly packaged into chromosomes so it can be separated.
- These two states are incompatible, DNA can't be both packed and unpacked simultaneously.
Cells divide when they receive the right signals
- Growth factors - chemical signals from other cells.
- Nutrient availability - cells need sufficient nutrients to support division.
- Cell damage - damaged tissues release signals triggering nearby cells to divide.
Different cells divide at different rates
- Frequently dividing cells: skin cells (every 2-3 weeks), intestinal lining cells (every 3-5 days), blood cells (continuously).
- Rarely dividing cells: liver cells (only when needed for repair), muscle cells (rarely after childhood).
- Never dividing cells: nerve cells and heart muscle cells (lost forever if damaged).
Example
- When you cut your finger, damaged cells release growth factors.
- Nearby skin cells receive these signals and begin dividing rapidly.
- Each division produces two small cells, which then grow during interphase before dividing again.
- New cells fill in the wound and once the wound is closed, the signals stop, and division slows back to normal.
Tip
- The complete cell cycle can take 24 hours in rapidly dividing cells, but interphase typically takes 20-23 hours while division itself only takes 1-2 hours.
- Most of the cell's time is spent growing, not dividing.
Self review
- Name two types of cells that divide frequently and two that rarely or never divide.
- What must happen to DNA before a cell can divide?
- What are checkpoints in the cell cycle and why are they important?
