Cell Organelles: Nucleus, Cytoplasm, and Membrane

The cell membrane, cytoplasm, nucleus and cytoskeleton are the basic structures of a eukaryotic cell: the membrane controls what enters and leaves, the cytoplasm is where most reactions occur, the nucleus stores the DNA and the cytoskeleton gives shape and movement. In brief: think of the cell as a small factory. The membrane is the gate, the cytoplasm is the workspace, the nucleus is the control office and the cytoskeleton is the frame and the conveyor belts.

These four structures are common to nearly all eukaryotic cells, so they are the first ones students need to learn. This guide explains the structure and function of each, compares them in a table, works through an example that matches structures to functions, and covers common mistakes.

What is the cell membrane?

The cell membrane, or plasma membrane, is a thin, flexible boundary that surrounds every cell. It is only about 7 to 10 nanometers thick, so it cannot be seen with a light microscope as a distinct layer.

Its structure is described by the fluid mosaic model. The membrane is a double layer of phospholipid molecules, with embedded proteins and some cholesterol in animal cells. Each phospholipid has a water-loving head and two water-fearing tails, so the tails face inward and the heads face the watery environments on both sides.

The membrane is selectively permeable. Small nonpolar molecules such as oxygen pass through easily, while ions and large molecules need protein channels or carriers. This control keeps the inside of the cell chemically different from the outside. Proteins in the membrane also act as receptors that detect signals, such as hormones.

What is the cytoplasm?

The cytoplasm includes everything inside the cell membrane except the nucleus. It consists of the cytosol, a jelly-like fluid that is mostly water with dissolved salts, sugars and proteins, and the organelles suspended in it.

Many important reactions take place in the cytosol. Glycolysis, the first stage of respiration, occurs there, and protein synthesis begins on ribosomes that float in it. The cytoplasm also stores materials and supports the organelles. Its consistency can change from more liquid to more gel-like, and in many cells it moves in a streaming motion.

What is the nucleus?

The nucleus is usually the largest organelle in an animal cell, often about 5 to 10 micrometers in diameter. It holds most of the cell’s DNA and controls the activities of the cell by directing which proteins are made.

• Nuclear envelope: A double membrane that separates the nucleus from the cytoplasm. It has pores that allow RNA and proteins to pass.
• Chromatin: DNA wound around proteins. When a cell divides, chromatin condenses into visible chromosomes.
• Nucleolus: A dense region inside the nucleus where parts of ribosomes are made.
• Nucleoplasm: The fluid inside the nucleus.

Most cells have one nucleus, but there are exceptions. Mature red blood cells in humans have none, while skeletal muscle fibers have many. A cell without a nucleus cannot make new proteins from DNA and generally has a limited life.

What is the cytoskeleton?

The cytoskeleton is a network of protein fibers that runs through the cytoplasm. It has three main types of fibers.

• Microfilaments are thin fibers made of actin. They support the cell surface and help in cell movement and in muscle contraction.
• Intermediate filaments are rope-like fibers that give strength and anchor organelles.
• Microtubules are hollow tubes made of tubulin. They act as tracks for moving organelles, form the spindle that separates chromosomes during cell division, and make up cilia and flagella.

The cytoskeleton is dynamic. It can be built up and taken apart rapidly, so cells can change shape and move.

Basic cell structures compared

Worked example: matching structures to functions

Four statements describe events in a cell. Identify the structure responsible for each.

1. A cell takes in oxygen but keeps most large proteins inside. This selective control is the job of the cell membrane.
2. Enzymes in the fluid break glucose into smaller molecules during glycolysis. The reaction takes place in the cytosol, which is part of the cytoplasm.
3. Instructions for making a protein are copied from DNA into messenger RNA. DNA is stored in the nucleus, so the copying occurs there, and the messenger RNA then leaves through nuclear pores.
4. During cell division, chromosomes are pulled to opposite ends of the cell. The spindle fibers are microtubules, part of the cytoskeleton.

A quick way to answer such questions is to ask three things: Does the event involve a boundary? Does it involve information in DNA? Does it involve movement or shape? Then choose membrane, nucleus or cytoskeleton, and treat the cytoplasm as the setting for most other reactions.

How were these structures discovered?

Robert Brown described the nucleus in plant cells in 1831, while studying orchids. The fluid mosaic model was proposed by S. J. Singer and Garth Nicolson in 1972, building on earlier ideas that membranes are lipid bilayers. The cytoskeleton was recognized in detail only after electron microscopy and fluorescent labeling became widely available in the second half of the twentieth century.

Why does this knowledge matter?

Many medicines act on these structures. Some cancer drugs interfere with microtubules to stop dividing cells. Local anesthetics block channels in nerve cell membranes, and many diseases arise from faulty membrane proteins, such as the chloride channel that is affected in cystic fibrosis.

Research on the nucleus is important for gene therapy and cloning. The first cloned mammal, Dolly the sheep in 1996, was produced by placing a nucleus from an adult cell into an egg cell whose nucleus had been removed.

Which mistakes are common?

Mistake 1: Treating the cell membrane as a solid wall

It is a flexible, selectively permeable boundary. Plant cells have a separate cell wall outside it.

Mistake 2: Using cytoplasm and cytosol as exact synonyms

The cytosol is the fluid. The cytoplasm includes the fluid and all the organelles in it.

Mistake 3: Saying that the nucleus makes proteins

The nucleus holds DNA and makes RNA. Proteins are assembled on ribosomes in the cytoplasm.

Mistake 4: Thinking the cytoskeleton is a fixed skeleton like bones

It is a changing network that reorganizes constantly as the cell moves and divides.