Inside a Cell

At rest, your heart beats about 60 to 100 times a minute, roughly once or a little more each second. Its muscle cells are crowded with tiny parts called mitochondria. A white fat cell under your skin is different. It is mostly one big drop of stored fat, with a thin rim of everything else. These two cells share many kinds of parts, but they use their space very differently. Why?

Explore: switch off one part of a cell

The cell-as-a-team model shows a cell as labelled blocks, one for each part. You pick a heart muscle cell, a fat storage cell or a leaf cell. Then you switch off one part at a time and watch six bars, one for each life process: growth, repair, reproduction, gas exchange, metabolism and response.

1. Pick a cell

2. Switch off one part

Reset

In this sketch the bars are relative lengths, not measurements. The more a process uses a part and the bigger that part’s share, the more its bar drops. Reproduction also loses part of growth’s drop, because a cell grows before it divides. Block widths show small, ordinary or large only, not to scale. Only the mitochondria shares and the leaf cell’s large vacuole come from the lesson. A leaf cell’s ordinary mitochondria share is a guess here. Every other part is ordinary size. Heart and fat cells are drawn with a vacuole only to keep the three cells alike. A leaf cell also has a cell wall and chloroplasts, which this sketch leaves out. Nothing is chosen by chance.

Rule to keep: the parts work as one system, so trouble in most parts slows several life processes.

Try this.

1. Pick the heart muscle cell. Predict how many bars will drop if you switch off the mitochondria. Then test it.
2. Turn the mitochondria back on. Switch off the cell membrane instead. Predict which bars drop, then check.
3. Pick the fat storage cell and switch off the mitochondria again. Predict whether its bars drop as much as the heart cell’s did. Then compare.

Explain: the parts and their jobs

Many cells contain organelles. An organelle is a small part inside a cell that does a special job. Here are the main parts most plant and animal cells share.

• The nucleus holds the cell’s instructions. These instructions guide what the cell makes and does.
• The cell membrane is the thin boundary around the cell. It controls what enters and leaves, and it lets the cell sense its surroundings.
• The cytoplasm is the jelly-like fluid that fills the cell and holds the organelles. Many reactions happen in it.
• The endoplasmic reticulum (ER) is a network of folded membranes. It helps make and move many proteins and fats.
• Mitochondria release much of the energy stored in food such as sugar (glucose), in a form the cell can use.
• A vacuole is a sac that stores water, food and wastes.

Each job uses more than one part.

Job	Parts that help
Using energy	Mitochondria, cytoplasm
Making molecules	Nucleus (instructions), ER, cytoplasm
Removing wastes	Cell membrane, vacuoles
Responding to the environment	Cell membrane, nucleus

Not every cell has every part. A mature human red blood cell has no nucleus and no mitochondria. Bacteria have no nucleus, as you will see in that lesson. Plant cells add a cell wall, chloroplasts and a large central vacuole, shown in that lesson.

Explain: the parts work as a team

A cell must grow, repair itself and reproduce. It also exchanges gases, carries out metabolism and responds. The organelles do all of this together.

• Growth and repair. Instructions from the nucleus guide which proteins and fats are made. The ER makes and moves many of them. Mitochondria release the energy for this building.
• Reproduction. Before a cell divides, it grows and copies its instructions. You will see how in that lesson.
• Gas exchange. Oxygen moves in across the cell membrane, and carbon dioxide moves out. Mitochondria use the oxygen. That lesson shows how gases cross.
• Response. A signal touches the cell membrane, and the cell changes what it does.

Metabolism means all the interactions among molecules inside the cell, such as building molecules up and breaking them down. Two examples matter most in this course. In photosynthesis, plant cells use radiant energy (light) to make sugar (glucose), as in that lesson. In cellular respiration, cells break down sugar and release its stored energy, as in that lesson.

A cell is a bit like a sports team, where each player has a role. The comparison breaks in one way: no part decides or plans anything. Each part simply follows chemistry.

Explain: same parts, different amounts

Look again at the two cells from the start. Both have a nucleus, a membrane, cytoplasm, ER and mitochondria. But the amounts are very different.

A heart muscle cell works without a break, so it releases a lot of energy. Mitochondria fill a large share of it. A typical white fat cell stores fat. One fat drop fills most of the cell, and the other parts sit in a thin rim.

Elaborate: how much space do mitochondria fill?

Scientists took very thin slices of cells and photographed them with an electron microscope. Then they laid a grid of points over each picture and counted the points on mitochondria.

Cells measured	Mitochondria (% of cell volume)
Hummingbird flight muscle (rufous hummingbird)	34.5
Heart muscle, 10 kinds of mammals	22 to 37 (range)
Human arm muscle, elite skiers	9.0
Human leg muscle, elite skiers	8.6
Guinea pig pancreas cell	8.1
Chicken brown-like fat cell, grown in a dish for 22 days	4.2
Chicken white fat cell, embryo	0.7
Human red blood cell, mature and healthy	0 (none)

Sources: Mathieu-Costello and colleagues, 1992 (hummingbird); Barth and colleagues, 1992 (heart, ten species including humans); Ørtenblad and colleagues, 2018 (skiers, Norway); Bolender, 1974 (pancreas, five guinea pigs); Mezentseva and colleagues, 2008 (chicken cells). The red blood cell row is a textbook fact, not a measurement. Most rows come from different studies; the two skier rows share one study, and so do the two chicken rows.

1. Looking only at rows with a single value, which measured cell had the largest share of mitochondria? Which had the smallest share above zero?
2. Find the mean of the two skier values.
3. How many percentage points apart are the hummingbird muscle and the pancreas cell?

Design problem

Scientists count points on a grid to measure space inside a cell. Does the spacing of the grid change your answer? Plan a test using a printed electron microscope picture of a cell and clear grids you draw yourself.

One way to do it

• Question: Does grid spacing change the measured share of a cell filled by mitochondria?
• Independent variable: spacing between grid points, such as 10 mm, 5 mm and 2 mm.
• Dependent variable: percent of points that land on mitochondria.
• Constants: the same printed picture, the same rule for a point on an edge, and the same lighting.
• Control: the finest grid (2 mm) is the comparison for the others.
• Repeated trials: three classmates count each grid separately. Find the mean for each spacing.
• Safety: use a ruler and pencil only; no sharp tools needed.
• What it means: if the coarse grid gives values far from the fine grid, it needs more points to be trusted.

Evaluate: check your thinking

A heart muscle cell and a fat cell share many kinds of organelles. The heart cell works without a break, and it gives a large share of its space to mitochondria, which release energy from food. The fat cell stores fat, so one fat drop fills most of it. In both cells, the parts work together as one team.

Science practices you used: using a model and naming its limits, reading a data table, finding a mean and a difference, and planning a fair test with repeated trials.