B. Life Systems · Cells

Unicellular and Multicellular Organisms: Meeting Basic Needs

B2.5 A2.1 A3.1 A3.2 A1.2About 60 minutes

Learning goals

I can describe several unicellular organisms and how each one meets its basic needs.
I can compare how unicellular and multicellular organisms get food, exchange gases, remove wastes, move and reproduce.
I can use surface area to volume ratios to explain why cells are small and why large organisms need systems.
I can use code to model how fast a population of bacteria can grow, and connect it to food safety.

Words to know

unicellular organism
A living thing made of only one cell, which carries out every life process by itself.

multicellular organism
A living thing made of many cells that share the work of staying alive.

binary fission
A way unicellular organisms reproduce: one cell copies its DNA and splits into two identical cells.

cilia
Many short, hair-like structures on the outside of some cells that beat back and forth to move the cell or move things past it.

flagellum
A long, whip-like tail some cells use to swim. Plural: flagella.

pseudopod
A "false foot": a part of the cell that bulges out so an amoeba can move and surround food.

surface area to volume ratio
How much outside surface something has compared with how much space is inside it. Small objects have a larger ratio.

Remember from Grade 6

In Grade 6 you sorted living things into kingdoms. Most bacteria and many protists (like amoebas and paramecia) are unicellular: their whole body is one cell. Most plants and animals are multicellular. Fungi include both: yeast has one cell, and mushrooms have many.

A unicellular organism has to do everything a living thing does with just one cell. How does it manage?

A cluster of brown mushrooms growing on a mossy fallen log among autumn leaves. — Mushrooms are multicellular fungi. Yeast, a fungus too, is a single cell that has to meet all its needs on its own. AI-generated image

An amoeba (not to scale). It moves by pushing out pseudopods, which also surround food to form a food vacuole. A contractile vacuole pumps out extra water.

Organism	What it is	How it meets its needs
Bacteria	Very small cells (about 1 to 2 µm) with no nucleus; their DNA is loose in the cytoplasm.	Many absorb food from their surroundings (some make their own food). Some move with flagella. Reproduce by binary fission, sometimes very quickly.
Amoeba	A protist with a flexible shape.	Moves by pushing out pseudopods; surrounds food to form a food vacuole; pumps out extra water with a contractile vacuole; reproduces by splitting in two.
Paramecium	A slipper-shaped protist that lives in ponds.	Moves by beating thousands of cilia; cilia also sweep food into an opening; uses contractile vacuoles to get rid of extra water.
Euglena	A protist with a flagellum, chloroplasts and a light-sensitive eyespot.	Swims toward light and makes food by photosynthesis; can also take in food from the water when there is no light.
Yeast	A unicellular fungus, with a cell wall but no chloroplasts.	Absorbs sugars from around it and releases carbon dioxide (this makes bread rise). Reproduces by budding: a small new cell grows out and breaks off.

Basic needs: one cell versus many

Every organism needs to get food (energy), take in and give off gases, keep the right amount of water, remove wastes, respond to its surroundings and reproduce.

A unicellular organism does all of this through its cell membrane and with its organelles. Oxygen and carbon dioxide simply diffuse through the membrane. Wastes diffuse out or are pumped out.
A multicellular organism has specialized cells that each do one job very well, organized into tissues, organs and organ systems. Your lungs exchange gases, your digestive system breaks down food, and your circulatory system carries everything to and from every cell. Each cell still uses its own membrane, but it relies on the systems to deliver what it needs.

Need	Unicellular (e.g., paramecium)	Multicellular (e.g., human)
Food	Takes in food through its cell surface or an opening; digests it in food vacuoles	Digestive system breaks food down; blood carries nutrients to cells
Gas exchange	Oxygen and carbon dioxide diffuse through the membrane	Respiratory system takes in oxygen; blood carries gases to and from cells
Wastes	Diffuse out; contractile vacuole pumps out water	Excretory system (kidneys) and lungs remove wastes
Movement	Cilia, flagella or pseudopods	Muscles and bones (musculoskeletal system)
Responding	Moves toward food or light, away from harm	Nervous system with specialized nerve cells
Reproduction	Binary fission or budding: fast, offspring are identical	Usually two parents; slower; offspring vary
If one cell dies	The whole organism dies	Other cells carry on and can replace it

Why are cells so small? Why do big organisms need systems?

Everything a cell needs must come in through its surface, but the cell's whole volume needs it. Look at cubes as models of cells:

Cubes with side lengths 1 cm, 2 cm and 3 cm (drawn to scale with one another). As the cube gets bigger, its surface area to volume ratio gets smaller.

Side length	Surface area (6 faces × side × side)	Volume (side × side × side)	Surface area : volume
1 cm	6 cm²	1 cm³	6 : 1
2 cm	24 cm²	8 cm³	3 : 1
3 cm	54 cm²	27 cm³	2 : 1

As a cell grows, its volume grows faster than its surface. A very large cell would not have enough membrane to let in oxygen and food, or let out wastes, fast enough for all of its inside, and diffusion to its centre would be too slow. So cells stay small. A big organism is made of many small cells, not one giant one, and it needs transport systems (like blood vessels in animals or xylem and phloem in plants) to bring materials close to every cell.

Coding a growing population

Under ideal conditions in a laboratory, some bacteria can divide about every 20 minutes. Let's write a simple program to automate the calculation of how many there would be, starting from one cell.

Binary fission in action: every 20 minutes each bacterium splits into two, so the number doubles. After 1 hour (3 doublings) there are 2 × 2 × 2 = 8 cells.

set cells to 1

set minutes to 0

repeat until minutes = 180

change minutes by 20

set cells to cells × 2

display minutes, " minutes: ", cells, " cells"

end repeat

Output ends with: 180 minutes: 512 cells. That's 9 doublings, 2⁹ = 512. Change 180 to 600 (10 hours) and the program shows 2³⁰, more than 1 billion cells. In real life, growth slows when food or space runs out (remember carrying capacity from Grade 7).

Careers and food safety

Cooks, chefs, bakers and butchers use this science every day. Bacteria grow fastest at warm temperatures, so Health Canada advises keeping cold food at 4 °C or colder and hot food at 60 °C or hotter. The range in between is called the danger zone. Leaving food out for a few hours can let a few bacteria multiply into millions. Bakers, on the other hand, want yeast cells to grow: they keep dough warm so the yeast releases carbon dioxide and the bread rises.

Worked example

Let's compare how two organisms get oxygen

Explain why a paramecium has no lungs or blood, but a frog needs both.

A paramecium is one tiny cell. Its large surface area to volume ratio means oxygen can diffuse through its membrane quickly enough to reach all of it.
A frog is made of millions of cells, many of them deep inside the body and far from the air.
Diffusion alone is far too slow over such distances, so the frog needs lungs (and its moist skin) to take in oxygen, and blood pumped by a heart to carry the oxygen to every cell.

One small cell can rely on diffusion through its membrane; a large multicellular organism needs organ systems to exchange and transport gases.

Worked example

Let's use the program's pattern

Using the program above, how many cells would there be after 2 hours? After how many minutes would there first be more than 1000 cells?

2 hours = 120 minutes = 120 ÷ 20 = 6 doublings. 2⁶ = 64 cells.
Keep doubling: 7 doublings = 128, 8 = 256, 9 = 512, 10 = 1024.
10 doublings × 20 minutes = 200 minutes.

64 cells after 2 hours; more than 1000 cells after 200 minutes (3 h 20 min).

Just for fun

Funny but true: a slime mould called Physarum is one giant cell with many nuclei, and no brain at all. In experiments it has found the shortest path through a maze to reach food. One cell, zero brains, still better at mazes than most of us.

Hands-on activity

Fair test: what do yeast cells need to grow?

You need: 2 identical clear jars or glasses, warm water (about the temperature of a comfortable bath), 2 packets of quick-rise or traditional baker's yeast (about 8 g each), sugar, a measuring cup and spoons, masking tape, a marker, a ruler, a timer.

Question: do yeast cells need sugar (food) to be active? Write a hypothesis.
Label the jars A: sugar and B: no sugar. In each, pour 125 mL of warm water.
Add 15 mL (1 tbsp) sugar to jar A only and stir.
Add one packet of yeast to each jar and stir each the same number of times.
Put tape up the side of each jar and mark the top of the liquid. Every 5 minutes for 20 minutes, mark and measure the height of the foam (in mm).
Record your data in a table and make a line graph with time on the x-axis.
Write a conclusion. The foam bubbles are carbon dioxide released by the yeast cells as they break down sugar. What does that tell you about how unicellular yeast meets its need for food and energy?

Safety: use warm tap water, not boiling (very hot water also kills yeast). An adult runs the hot tap. Don't drink the mixtures.

Extension: test temperature instead (cold vs warm water, both with sugar). Only change one variable.

What to look for: jar A foams much more. A good conclusion says yeast cells absorb sugar as food and release carbon dioxide as a waste, and names the controlled variables (water amount, temperature, yeast amount, stirring).

Your turn

Name three unicellular organisms.

How does a paramecium move? How does an amoeba move?

How does a unicellular organism get oxygen?

What is binary fission?

Euglena can make its own food. Which organelle lets it do this?

Give one advantage of being multicellular.

Give one advantage of being unicellular.

More practice: warm-up

Is a mushroom unicellular or multicellular?

Is yeast unicellular or multicellular?

What do bacteria lack that plant and animal cells have?

What gas makes bread dough rise?

What is the surface area to volume ratio of a 1 cm cube?

More practice: on your own

Find the surface area to volume ratio of a cube with sides of 4 cm.

Use your answer above to explain why cells don't grow to be as big as a 4 cm cube.

Compare how a paramecium and a human get rid of wastes.

In the bacteria program, what would you change to model bacteria that divide every 30 minutes for 3 hours? How many cells would there be?

Why is the "danger zone" for food between 4 °C and 60 °C?

Challenge

A single bacterium lands on potato salad left out at a summer picnic. If it divides every 20 minutes, how many bacteria could there be after 4 hours? Why might the real number be lower?

Euglena has chloroplasts like a plant but swims and can take in food like an animal. Explain why this makes it hard to classify.

Explain, using surface area to volume ratio, why an elephant needs a heart and blood vessels but a paramecium doesn't.

Check your understanding

Quiz

Which is a unicellular organism?

How does a paramecium move?

A unicellular organism gets oxygen mainly by…

A cube with 2 cm sides has a surface area to volume ratio of…

Why do large organisms need transport systems?

Starting with 1 bacterium that doubles every 20 minutes, how many are there after 1 hour?

Yeast foam in a sugar-water jar is made of…

For parents and teachers

Big idea: a unicellular organism meets all its needs with one cell, using its membrane, organelles and structures like cilia or pseudopods. Multicellular organisms divide the work among specialized cells and organ systems, which they need because diffusion only works over short distances.

How to help at home:

Do the yeast test together, and talk about bread baking.
Practise the cube calculations with sugar cubes or dice if you have them.
Connect the bacteria program to real food safety: put leftovers in the fridge promptly.

Safety: warm, not boiling, water; adults handle the hot tap.

Common difficulties:

Thinking unicellular organisms are "simple" or can't do much. They carry out every life process.
Surface area to volume: students compute correctly but can't explain. Ask: Where does food enter the cell? Where is it needed?
Assuming real populations double forever. Remind them of limiting factors from Grade 7.