Diversity of Specialised Tissues

Lessons

Senior 4 · Biology

Diversity of Specialised Tissues

4 lessons

Lesson 4 of 4

Levels of organisation and the advantages of being multicellular

25 min

A ball of cells that shares the work

In ponds and slow streams lives Volvox, a green alga that forms a hollow ball of hundreds to tens of thousands of cells, each with two flagella. Most of the cells only swim and photosynthesise; a few, inside the ball, specialise in reproduction and grow into the daughter colonies you can see within the parent. Its close relative Chlamydomonas is a single cell that must do everything itself. Between them, the two show the first step of a path that leads to plants and animals with billions of specialised cells.

This lesson puts the cells and tissues of this unit into order, from cell to organism, and asks why multicellular life has been so successful, and what it costs.

Definition

Organ: a structure made of several different tissues that work together to carry out a particular function. Examples: a leaf, the heart.

Organ system: a group of organs that work together to carry out a major function. Examples: the shoot system of a plant; the circulatory system of a mammal.

Organism: a complete living thing, which may be unicellular or multicellular.

Division of labour: the sharing of the body's tasks among different specialised cells, tissues and organs.

Colony: a group of similar cells or organisms living attached together, in which each cell can usually survive on its own.

From cell to organism

Each level is built from the one before it and can do things that its parts cannot do alone. A guard cell can open and close a stoma, but only a leaf, with epidermis, mesophyll and veins working together, can photosynthesise efficiently; only the whole maize plant, with roots, stem and leaves, can grow, flower and set seed. Below the cell are organelles and molecules, and above the organism are populations, communities and ecosystems (Unit 1).

Organ	Tissues it contains	What each contributes
Leaf	Epidermis, palisade and spongy mesophyll (parenchyma), xylem, phloem, collenchyma or sclerenchyma	Protection; photosynthesis and gas exchange; supply of water; export of sugar; support
Heart	Cardiac muscle, connective tissue (valves, fibrous rings), epithelium (lining), nervous tissue, blood vessels	Contraction; strength and one-way flow; smooth lining; control of rate; its own blood supply
Stomach	Columnar epithelium with gland cells, smooth muscle, connective tissue, nerves	Secretion of acid and enzymes; churning; support; coordination

The problem of size

Every cell exchanges oxygen, carbon dioxide, nutrients and wastes across its surface, but uses them throughout its volume. As an object grows, its surface area increases with the square of its length, but its volume with the cube, so its surface area to volume ratio falls (Figure 2). Diffusion is also fast only over short distances: the time taken grows with the square of the distance.

Worked example 1: surface area to volume ratio

Calculate the surface area to volume ratio of cubes of side 1 cm, 2 cm and 4 cm, and state the pattern.

Side 1 cm: $\text{SA} = 6 \times 1^2 = 6\,\mathrm{cm^2}$ ; $V = 1^3 = 1\,\mathrm{cm^3}$ ; ratio $= \frac{6}{1} = 6$ .
Side 2 cm: $\text{SA} = 6 \times 2^2 = 24\,\mathrm{cm^2}$ ; $V = 2^3 = 8\,\mathrm{cm^3}$ ; ratio $= \frac{24}{8} = 3$ .
Side 4 cm: $\text{SA} = 6 \times 4^2 = 96\,\mathrm{cm^2}$ ; $V = 4^3 = 64\,\mathrm{cm^3}$ ; ratio $= \frac{96}{64} = 1.5$ .
Pattern: for a cube, ratio $= \frac{6L^2}{L^3} = \frac{6}{L}$ . Each time the side doubles, the ratio halves. A large organism has far less surface for each unit of volume that needs supplying.

Worked example 2: how far can oxygen diffuse?

The time for a molecule to diffuse a distance $x$ is roughly $t \approx \frac{x^2}{2D}$ . For oxygen in water, $D \approx 2 \times 10^{-9}\,\mathrm{m^2\,s^{-1}}$ . Estimate $t$ for (a) 10 µm, the width of a cell; (b) 1 mm; (c) 1 cm.

(a) $x = 10\,\mu\mathrm{m} = 1 \times 10^{-5}\,\mathrm{m}$ : $t = \frac{(1 \times 10^{-5})^2}{2 \times 2 \times 10^{-9}} = \frac{1 \times 10^{-10}}{4 \times 10^{-9}} = 0.025\,\mathrm{s}$ .
(b) $x = 1 \times 10^{-3}\,\mathrm{m}$ : $t = \frac{1 \times 10^{-6}}{4 \times 10^{-9}} = 250\,\mathrm{s}$ , about 4 minutes.
(c) $x = 1 \times 10^{-2}\,\mathrm{m}$ : $t = \frac{1 \times 10^{-4}}{4 \times 10^{-9}} = 25\,000\,\mathrm{s}$ , about 7 hours.
Diffusion is ample inside a single cell but hopeless over centimetres. Large multicellular organisms therefore need flattened shapes (leaves), gas-exchange surfaces (lungs, gills) and mass transport systems (blood, xylem and phloem) to bring every cell within a short distance of supplies.

Advantages and costs of being multicellular

Advantages	Costs and problems
Division of labour: specialised cells each do one job very well, so the whole organism works more efficiently	Specialised cells cannot survive on their own; damage to one organ can kill the whole organism
Larger size: harder for predators to eat, better able to compete for light (tall plants), to move far and to store food	Small surface area to volume ratio: needs exchange surfaces and transport systems
Longer life: worn-out cells are replaced while the organism lives on	Needs coordination by nervous and hormonal systems (animals) or hormones (plants)
A stable internal environment (homeostasis): cells inside the body are protected from changes outside	Takes longer to grow and reach the age of reproduction; uses more resources
Complex behaviour and structures: eyes, brains, flowers, seeds	Risk of uncontrolled cell division (cancer) and of diseases that attack one organ

Worked example 3: colony or organism?

Evaluate whether Volvox should be called a colony or a multicellular organism.

For "colony": its cells are all of a similar simple type, each with flagella and a chloroplast, much like a single Chlamydomonas cell; there are no tissues or organs.
For "multicellular organism": it has two differentiated cell types (swimming somatic cells and reproductive cells), its cells are joined by cytoplasmic strands and swim in a coordinated way with a front and a back, and the somatic cells cannot reproduce or survive alone.
Conclusion: Volvox is best seen as a simple multicellular organism at the border of colonial life, showing the beginning of division of labour, but without tissues.

MooMooMath and Science: "Why are cells so small?". Watch the cube demonstration and relate it to Worked example 1 and to the need for transport systems.

Warning

Common mistakes

Saying large organisms have a "smaller surface area". They have a larger surface area but a smaller surface area to volume ratio.
Calling the heart a tissue. It is an organ, made of several tissues.
Thinking that every multicellular organism has organ systems. Sponges have specialised cells but no true tissues or organs.
Forgetting the units when giving SA (cm²) and V (cm³); the ratio itself has units of cm⁻¹, often left out.

Tip

Memory hook. "Cells Together Organise Our Organisms": Cell, Tissue, Organ, Organ system, Organism. For a cube, SA : V $= \frac{6}{L}$ : double the size, halve the ratio.

✦

Practice: levels and multicellularity

Put in order from simplest to most complex: digestive system, smooth muscle, stomach, smooth muscle cell, goat.
Explain why a leaf is described as an organ and not a tissue.
Calculate the surface area to volume ratio of a cube of side 5 cm.
A cell is a sphere of radius 10 µm. Using $\text{SA} = 4\pi r^2$ and $V = \frac{4}{3}\pi r^3$ , calculate its SA : V ratio. How does it compare with a sphere of radius 20 µm?
Using $t \approx \frac{x^2}{2D}$ with $D = 2 \times 10^{-9}\,\mathrm{m^2\,s^{-1}}$ , estimate the time for oxygen to diffuse 0.1 mm.
Give three advantages of being multicellular.
Flatworms have no blood system, yet some are several centimetres long. Suggest how they survive.

Show answersHide answers

Smooth muscle cell → smooth muscle → stomach → digestive system → goat.
It is made of several different tissues (epidermis, mesophyll, xylem, phloem and others) working together for one function, photosynthesis; a tissue contains only one or a few similar cell types.
$\text{SA} = 6 \times 25 = 150\,\mathrm{cm^2}$ ; $V = 125\,\mathrm{cm^3}$ ; ratio $= \frac{150}{125} = 1.2$ (also $\frac{6}{L} = \frac{6}{5} = 1.2$ ).
Ratio $= \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r} = \frac{3}{10} = 0.3\,\mu\mathrm{m^{-1}}$ . For $r = 20\,\mu\mathrm{m}$ : $\frac{3}{20} = 0.15$ , half as large.
$x = 1 \times 10^{-4}\,\mathrm{m}$ : $t = \frac{1 \times 10^{-8}}{4 \times 10^{-9}} = 2.5\,\mathrm{s}$ .
Any three: division of labour and efficiency; larger size; longer life with cell replacement; stable internal environment; more complex structures and behaviour.
They are very flat and thin, so every cell is only a short distance from the surface; oxygen and carbon dioxide can diffuse in and out fast enough, and the large surface area to volume ratio of a flat shape supplies the whole body.

Quick check

Put the levels of organisation in order, from simplest to most complex.

Tap the cards in the right order.

Key points

Levels of organisation: cell → tissue → organ → organ system → organism; each level does more than its parts.
Surface area to volume ratio falls as size increases ( $\frac{6}{L}$ for a cube); diffusion time rises with the square of distance.
Multicellularity brings division of labour, larger size, longer life and homeostasis, but needs exchange surfaces, transport and coordination systems.
Volvox shows an early stage of division of labour between cells.

1A ball of cells that shares the work

2From cell to organism

3The problem of size

4Advantages and costs of being multicellular

A ball of cells that shares the work

From cell to organism

The problem of size

Advantages and costs of being multicellular