An image shows a cell packed with mitochondria. What sort of cell is it?

A mitochondrion supplies usable energy. Many of them means a high, constant energy demand. Muscle works all the time. Most likely a muscle cell

Why does a gland cell have a large rough endoplasmic reticulum?

A gland cell secretes proteins. Proteins are built on the ribosomes of the rough reticulum. Large-scale production needs a large surface. Because it makes protein for export, in quantity

What does the mitochondrion gain from its internal folds?

Folding increases surface area without increasing volume. The energy-producing systems sit on that membrane. More membrane means more of those systems. More energy output from the same volume

Cell Structure and Function · Grade 10

What are the organelles and how do you infer function from structure?

Each organelle is a room built for one job: the nucleus keeps the instructions, the ribosome builds proteins, the mitochondrion supplies energy, the endoplasmic reticulum and Golgi process and package. The job can usually be inferred from the structure — internal folding, for instance, always means a large surface area.

Learning objectives

Name the main organelles and state the job of each one
Explain why a muscle cell holds many mitochondria and a gland cell holds a large rough endoplasmic reticulum
Explain how the inner folds of a mitochondrion serve its function
Identify an unlabelled cell as a muscle cell, a gland cell or a leaf cell from its organelles

Division of labour inside one cell

The nucleus holds the genetic material, and the instructions issue from there. Ribosomes build proteins according to them. The rough endoplasmic reticulum — rough because ribosomes sit on it — receives the new proteins and folds them.

The Golgi apparatus takes over from the reticulum, finishes the processing, sorts by destination and packages into vesicles. The mitochondrion supplies usable energy from the breakdown of sugar. The lysosome breaks down waste.

The aim is not to memorise the list but to see that it is ordered: there is a place where things are written, a place where they are built, a place where they are packed, and a place that supplies the energy paying for all of it.

Structure gives the job away

A mitochondrion has an inner membrane thrown into deep folds. Folding means a large surface area inside a small volume, and on that surface sit the systems that produce energy — so the folding translates directly into more output.

The same logic recurs everywhere in biology: a folded intestine, a branching lung, a branching root. Every time you see folding, it is worth asking what reaction happens on the surface.

Identifying a cell by what fills it

A muscle cell works constantly and needs a great deal of energy, so it is crowded with mitochondria. A gland cell makes protein for export, so it has a large rough endoplasmic reticulum and a well-developed Golgi.

A leaf cell is full of chloroplasts — the same organelles met in photosynthesis. You can be handed an unlabelled image and work out what the cell does, purely from what fills it.

Worked examples

An image shows a cell packed with mitochondria. What sort of cell is it?
1. A mitochondrion supplies usable energy
2. Many of them means a high, constant energy demand
3. Muscle works all the time
Answer: Most likely a muscle cell
Why does a gland cell have a large rough endoplasmic reticulum?
1. A gland cell secretes proteins
2. Proteins are built on the ribosomes of the rough reticulum
3. Large-scale production needs a large surface
Answer: Because it makes protein for export, in quantity
What does the mitochondrion gain from its internal folds?
1. Folding increases surface area without increasing volume
2. The energy-producing systems sit on that membrane
3. More membrane means more of those systems
Answer: More energy output from the same volume

Common mistakes

Learning the organelles as a list of names: A list does not help with a question showing an unfamiliar image. Someone who knows what each does can infer the cell type from the picture.
Confusing rough and smooth reticulum: The rough carries ribosomes and handles proteins; the smooth handles lipids and detoxification. The roughness is the ribosomes themselves.
Saying the mitochondrion produces energy: Energy is not created. It converts energy stored in sugar into a form the cell can use immediately.

What to remember

Each organelle is a room for one job.
Internal folding means large surface area.
What fills a cell gives away what it does.
Rough = ribosomes = proteins.