# From Single Cells to Multicellular Bodies

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/from-single-cells-to-multicellular-bodies

## The Advantages of Staying Together

Multicellularity evolved independently many times — in animals, plants, fungi, brown algae and elsewhere — which tells you the advantages are real and reachable by several routes.

The gains are size, **specialisation** and homeostasis. A single cell must do everything itself; a multicellular body can divide the labour, with each cell type doing one job well. It also escapes the surface-area-to-volume ceiling, since a large body built from small cells keeps every cell's own ratio favourable.

The costs are equally real. Cells must **cooperate**, which means suppressing their own reproduction for the good of the whole, and a lineage that stops cooperating is a cancer. Development must be coordinated, and only the germ line passes to the next generation, so most cells in a body are evolutionary dead ends.

Development starts from one cell and proceeds by three processes acting together:

| Process | What happens |
|---|---|
| Cell division | One cell becomes many |
| Differentiation | Cells become different types |
| Morphogenesis | Cells arrange into shapes |

**Differentiation** is the striking one: nearly every cell in a body carries the same genome, and they differ because they express *different subsets* of it. A liver cell and a neuron are running different programs off identical code.

**Stem cells** retain the ability to divide and to become other types. **Totipotent** cells can form a whole organism, **pluripotent** ones any body cell, and **multipotent** ones a limited family. Adult tissues keep multipotent stem cells for renewal, which is why skin and gut lining replace themselves continuously while neurons largely do not.

Growth from one cell is exponential: after $n$ divisions there are $2^n$ cells, so about 10 divisions give a thousand and 20 give a million. Reaching the roughly $10^{13}$ cells of an adult human needs only around 43 doublings.

> **Common pitfall:** thinking differentiated cells have lost the genes they no longer use. They have not, the genome stays essentially complete, which is exactly why cloning from an adult cell works and why induced pluripotent stem cells can be made by switching expression back on.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. An adult human has roughly $10^{13}$ cells, all descended from one fertilised egg. Set the approximate number of successive doublings this represents.

**Answer:** 43 (within ±4)

**Why:** Because $2^{10} \approx 10^3$, reaching $10^{13}$ needs about $13/0.301 \approx 43$ doublings. Exponential growth covers an enormous range in remarkably few steps, which is also why uncontrolled division is so dangerous.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/an-adult-human-has-roughly-10-cells-all-descended-from-one

### 2. A differentiated cell has permanently deleted the genes it no longer needs.

**Answer:** False

**Why:** False, the genome stays essentially complete, and cells differ by which genes they *express*. Dolly the sheep and induced pluripotent stem cells are both direct demonstrations: switch the expression pattern back and the cell regains its options.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/a-differentiated-cell-has-permanently-deleted-the-genes-it-no-longer

### 3. Arrange these stages of early animal development in order.

**Answer:**

1. A fertilised egg, a single cell
2. A hollow ball of undifferentiated cells
3. Distinct germ layers form
4. Organs begin to take shape
5. A functioning multicellular organism

**Why:** One cell divides into a hollow ball, germ layers are established, organs form from those layers, and the body becomes functional. The order is highly conserved, the same broad sequence runs across almost all animals.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/arrange-these-stages-of-early-animal-development-in-order

### 4. Complete the account of stem-cell potency.

**Answer:** A **totipotent** stem cell can give rise to an entire organism, a **pluripotent** one to any cell of the body but not to the supporting tissues, and a **multipotent** one only to a limited family of related types. Adult tissues retain the last kind, which is why the **epithelial** lining of the gut renews itself continuously.

**Why:** Potency narrows as development proceeds. Adult tissues keep multipotent cells for renewal, which is why gut lining and skin replace themselves constantly while neurons, lacking such a reserve, largely do not.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/complete-the-account-of-stem-cell-potency

### 5. Which are genuine advantages of multicellularity?

A. Every cell passes its genes to the next generation
B. Better internal regulation of conditions
C. Division of labour between specialised cell types
D. Larger body size without a surface-area penalty for each cell

**Answer:** B. Better internal regulation of conditions; C. Division of labour between specialised cell types; D. Larger body size without a surface-area penalty for each cell

**Why:** Specialisation, size and homeostasis are the payoffs. The last statement is precisely a *cost*: somatic cells give up reproduction entirely, which is the sacrifice that makes a coordinated body possible.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/which-are-genuine-advantages-of-multicellularity

### 6. Sort each outcome by whether it results from cooperation between cells or from its breakdown.

**Answer:**

- Cooperation working: Cells divide only when the tissue requires it, Damaged cells undergo programmed death
- Cooperation breaking down: A cell lineage divides without restraint, Cells ignore signals telling them to stop growing, A lineage invades tissues it does not belong to

**Why:** A multicellular body is a truce: cells forgo their own reproduction for the whole. Cancer is that truce failing, a lineage reverting to reproducing for itself, which is why it is sometimes described as a return to unicellular behaviour.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/sort-each-outcome-by-whether-it-results-from-cooperation-between

### 7. Multicellularity requires somatic cells to act as evolutionary dead ends. What explains why natural selection favours cells that forfeit their own direct reproduction?

A. Their individual survival takes evolutionary precedence over reproduction
B. Their sacrifice allows somatic lineages to mutate without consequence
C. They regain reproductive capacity once the organism reaches maturity
D. They ensure the transmission of their shared genome via the germ line

**Answer:** D. They ensure the transmission of their shared genome via the germ line

**Why:** Selection acts on the shared genetic lineage; somatic cells propagate their genes indirectly through germ cells. Confusing somatic sacrifice with tissue longevity or assuming cells regain fertility overlooks how division of labour strictly separates propagation from maintenance.

Page: https://tryals.app/practice/biology-i/from-single-cells-to-multicellular-bodies/multicellularity-requires-somatic-cells-to-act-as-evolutionary-dead