Cell Organelles and Their Jobs

Cells are tiny, but they are well organized. Many of their jobs are done by structures called organelles (the word means ‘little organs’). This lesson tours the main organelles and what each one does, compares cells with and without a nucleus, compares plant and animal cells, and follows a protein as it travels through a cell. Along the way you will clear up five common mix-ups.

Two kinds of cells: prokaryotes and eukaryotes

Every living thing is made of cells, and there are two basic cell types.

• Prokaryotic cells (bacteria and archaea) have no nucleus and no membrane-bound organelles. Their DNA sits in the cytoplasm, gathered in a region called the nucleoid. The name means ‘before a nucleus’.
• Eukaryotic cells (animals, plants, fungi and protists) have a nucleus that holds the DNA, plus other membrane-bound organelles. The name means ‘true nucleus’.

Both kinds of cell have a cell membrane, cytoplasm (the fluid inside the membrane), DNA and ribosomes. Most bacteria also have a cell wall.

Ribosomes are not wrapped in a membrane. Some books still count them as organelles, and this lesson lists them with the organelles.

The nucleus: the cell’s DNA library

The nucleus holds most of a eukaryotic cell’s DNA, the instructions for building proteins and for running the cell. It is surrounded by two membranes, called the nuclear envelope, which have pores that let molecules such as messenger RNA (mRNA) pass through. mRNA is a working copy of one set of instructions.

Inside the nucleus is the nucleolus, a dense region where parts of ribosomes are made.

You may have heard that the nucleus is the ‘brain’ of the cell. That is a common line, but it can mislead. The nucleus does not think or decide. It stores the DNA instructions and copies the ones the cell needs, and the rest of the cell carries out the work. Some cells even work without one: mature red blood cells in mammals have no nucleus, yet they still carry oxygen around the body.

Making proteins: ribosomes, rough ER and smooth ER

Ribosomes build proteins. A ribosome reads a copy of the DNA instructions (mRNA) and joins amino acids together in the right order. The order of the instructions in the DNA sets the order of the amino acids, and that order gives each protein its shape and its job. Ribosomes are found in every cell, including prokaryotic cells. Some float free in the cytoplasm, and some are attached to the rough ER.

The endoplasmic reticulum (ER) is a network of membrane-enclosed tubes and flat sacs that is connected to the nuclear envelope. It comes in two kinds:

• Rough ER is studded with ribosomes, which gives it a rough look under a microscope. Proteins made on these ribosomes enter the ER, where they are folded and prepared. They are then sent on to be released from the cell, placed in membranes, or delivered to lysosomes.
• Smooth ER has no ribosomes. It makes lipids (including oils and steroids), and in liver cells it helps break down some harmful substances.

Free ribosomes and ER-attached ribosomes are the same kind of structure. What differs is where the protein is going. Proteins that will work in the cytoplasm are made on free ribosomes. Proteins that will be exported, built into membranes or sent to lysosomes are made on ribosomes attached to the rough ER.

Sorting and shipping: the Golgi apparatus and vesicles

The Golgi apparatus is a stack of flattened membrane sacs. It receives proteins and lipids from the ER, modifies them, sorts them and packs them into vesicles, small membrane-bound sacs that carry cargo to its destination. Some Golgi vesicles become lysosomes.

A vesicle delivers its cargo by fusing with another membrane. When a vesicle fuses with the cell membrane and releases its contents outside the cell, the process is called exocytosis.

Vesicles do not just drift. Most are carried along tracks made by the cytoskeleton, which you will meet shortly.

The protein pathway

Put the last three sections together and you get a story. Take a cell in the pancreas that makes insulin, a protein hormone that is released into the blood.

1. Nucleus: the DNA instructions for insulin are copied into mRNA, which leaves through a nuclear pore.
2. Ribosome on the rough ER: a ribosome reads the mRNA and builds the protein. Because insulin is made to be exported, the protein enters the rough ER.
3. Vesicle: a vesicle buds off the ER and carries the protein to the Golgi apparatus.
4. Golgi apparatus: the protein is finished, sorted and packed into a new vesicle.
5. Cell membrane: the vesicle fuses with the cell membrane and releases the protein outside the cell.

Three limits to remember. Not every protein takes this route: proteins that work inside the cytoplasm are made on free ribosomes and never enter the ER. The ER and Golgi also handle lipids. And the ‘pathway’ is a standard model: real cells have many deliveries going on at once.

Energy converters: mitochondria and chloroplasts

Mitochondria (one is a mitochondrion) have a double membrane, with the inner one folded. They are where most of a eukaryotic cell’s ATP is made. ATP is the small molecule that carries energy the cell can use right away. The first steps of breaking down glucose happen in the cytoplasm, and the later steps, which make most of the ATP, happen in the mitochondria. Cells that use a lot of energy, such as muscle cells, tend to have many mitochondria. Mitochondria are found in the cells of animals, plants, fungi and most other eukaryotes.

Mitochondria are often called the ‘powerhouse’ of the cell. That nickname needs a limit. Mitochondria do not create energy, because energy is never created. They convert the chemical energy stored in food molecules into ATP, and some of the energy is released as heat.

Chloroplasts are found in plant cells and algae, and they are where photosynthesis happens (see the Photosynthesis lesson). Chloroplasts are not found in animal cells or fungal cells. A plant chloroplast also has a double membrane. A plant cell that has chloroplasts has mitochondria too: the chloroplast uses light to make sugar, and the mitochondria release the energy in sugar as ATP.

Mitochondria and chloroplasts have one more thing in common. Each has its own small DNA and its own ribosomes. The leading scientific explanation is the endosymbiotic theory: they descended from free-living prokaryotes that were taken inside ancestor cells long ago.

Storage, cleanup and support: vacuoles, lysosomes and the cytoskeleton

A central vacuole is a large membrane-bound sac in a mature plant cell. It often takes up much of the cell. It stores water, nutrients and wastes. The water inside pushes against the cell wall, and that pressure keeps plant cells firm. When a plant loses water, its vacuoles shrink and the plant wilts. Animal cells may have small vacuoles, but not a large central one.

Lysosomes are small membrane-bound sacs full of digestive enzymes. They break down worn-out organelles, food particles and invaders such as bacteria that the cell has taken in. Lysosomes are standard in animal cells. Plant cells are usually drawn without them, and the large central vacuole can take over some of the same breakdown work.

The cytoskeleton is a network of protein fibers (such as microtubules and actin filaments) running through the cytoplasm. It gives the cell its shape, holds organelles in place, and works as tracks for moving vesicles and organelles. In some cells it also helps the whole cell move.

Boundaries: the cell membrane and the cell wall

The cell membrane is a thin, flexible boundary around every cell, prokaryotic or eukaryotic. It is made of a double layer of lipids with proteins, and it controls what enters and leaves the cell. The next lesson covers how.

A cell wall is a rigid layer outside the cell membrane. It supports and protects the cell, and in a plant cell it resists the pressure of the water inside. A cell wall does not replace the membrane: the membrane is still inside the wall and still controls what goes in and out.

Plants have cell walls (made mostly of cellulose). So do fungi (their walls contain chitin), most bacteria (their walls contain peptidoglycan), most archaea (their walls are made of other materials) and some protists, such as many algae. Animal cells have no cell walls. So the cell wall is not a plant-only feature.

Plant cell or animal cell?

Plant and animal cells are both eukaryotic, so they share most organelles.

• In both: nucleus, ribosomes, rough and smooth ER, Golgi apparatus, mitochondria, cell membrane and cytoskeleton.
• In plant cells but not animal cells: a cell wall, chloroplasts and a large central vacuole.
• In animal cells (as drawn in textbooks): lysosomes. Animal cells have no cell wall and no chloroplasts.

Two cautions. First, ‘plant only’ means ‘plant, not animal’: fungi have cell walls, and algae have chloroplasts. Second, not every plant cell has chloroplasts. A cell in a root, for example, usually does not.

Five common mix-ups

• ‘The nucleus is the brain.’ It stores the DNA instructions. It does not think.
• ‘Mitochondria are only in animals.’ Plants, fungi and most other eukaryotes have them too.
• ‘Mitochondria create energy.’ They convert the energy in food molecules into ATP.
• ‘Plant cells have chloroplasts, so they do not need mitochondria.’ They need both: chloroplasts make sugar, and mitochondria release the energy in it as ATP.
• ‘Only plants have cell walls.’ Fungi, most bacteria, most archaea and some protists have them too.