Lesson 2

SECTION 2.1

Cell membrane, nucleus and organelles

Identify the cell membrane, nucleus and main organelles, and relate each structure to the job it performs.

Learn

A cell is the smallest unit that can carry out all the basic activities of life, and it is far more organised than a bag of fluid. It has an outer boundary, the cell membrane, that controls what enters and leaves. Inside, a watery fluid called cytosol holds specialised structures called organelles, each with its own job. Most of the cell’s instructions are kept in the nucleus, while other organelles build proteins, release energy, package and ship products, and break down waste. Because structure and function go together, learning an organelle means learning what it is made of, where it sits and what work it performs. This section gives you that map of the cell and a worked example of how the parts cooperate.

Key Concepts & Topics

The cell membrane: a selective boundary

The cell membrane, also called the plasma membrane, is a thin boundary made mainly of a phospholipid bilayer. A phospholipid is a molecule with a water-attracting head and two water-avoiding tails, so in water the molecules line up with heads outward and tails inward, forming two layers. Embedded in this bilayer are proteins that act as channels, pumps, receptors and identity markers, along with cholesterol that adjusts how fluid and stable the membrane is. Because the proteins can move sideways within the bilayer, the arrangement is often described as a fluid mosaic. The membrane is selectively permeable: some substances cross easily, some cross only through specific proteins, and others cannot cross at all. This control is what makes the cell a distinct compartment.

For example: Oxygen and carbon dioxide cross the bilayer easily, while glucose needs a transport protein because it is too large and too water-attracting to slip through the lipid core.

The nucleus: the control centre

The nucleus is the largest organelle in most cells and contains most of the cell’s genetic material. It is enclosed by the nuclear envelope, a double membrane with pores that let selected molecules pass between the nucleus and the cytoplasm. Inside, the genetic material is stored as chromatin, a complex of DNA and proteins; when the cell prepares to divide, chromatin condenses into visible chromosomes. The nucleus also contains a darker region, the nucleolus, where components of ribosomes are assembled. Because the instructions for building proteins are kept here, the nucleus directs the cell’s activities. A useful image is a library that is also the manager’s office: the information is stored in the nucleus, and copies of individual instructions are sent out to be used in the cytoplasm.

For example: A liver cell responds to a hormone by making a new enzyme. The instruction for that enzyme is copied in the nucleus and sent to the cytoplasm, where the enzyme is built.

Detailed animal-cell cutaway showing the nucleus, membrane and intracellular organelles. — **Look closely**
Find the large nucleus, then trace the folded membranes around it. Organelles share one cytoplasm: they are cooperating structures within a cell, not separate miniature cells. This is an illustration, not a microscope photograph.
Label the cell
Image source and licence
OpenStax · Source image · CC BY 4.0. Original image, reproduced without alteration.

Organelles for protein production and shipping

Proteins are built by ribosomes, small structures found free in the cytosol or attached to a folded membrane network called the endoplasmic reticulum (ER). The rough ER is studded with ribosomes and produces proteins destined for the membrane or for export; the smooth ER has no ribosomes and makes lipids, stores calcium and helps detoxify substances. Newly made proteins travel from the ER to the Golgi apparatus, a stack of flattened sacs that modifies, sorts and packages them. The packaged products leave in vesicles, small membrane-bound bubbles that carry them to the cell membrane for release or to another destination. This route — ribosome, rough ER, Golgi, vesicle, membrane — is called the secretory pathway, and it explains how a cell such as a pancreatic cell can make a hormone and deliver it outside itself.

For example: Insulin is built on ribosomes of the rough ER, trimmed and packaged in the Golgi, then released by vesicles when blood glucose rises.

Organelles for energy, recycling and support

Mitochondria are the organelles that release usable energy. They use oxygen to break down nutrients and transfer the energy into ATP, the molecule cells use to power their work. Cells with high energy needs, such as muscle and liver cells, contain many mitochondria. Lysosomes are membrane-bound sacs of digestive enzymes that break down worn-out organelles, engulfed material and other waste; peroxisomes are similar small sacs that break down fatty acids and neutralise harmful substances such as hydrogen peroxide. The cytoskeleton is an internal network of protein filaments that gives the cell shape, anchors organelles and enables movement; centrioles help organise the cytoskeleton during cell division. Together these structures maintain the cell, supply its energy and allow it to change shape or move.

For example: A heart muscle cell is packed with mitochondria because continuous contraction needs a large, constant supply of ATP.

A mitochondrial cutaway showing the outer membrane and folded inner membrane. — **Look closely**
Compare the cutaway with the electron micrograph alongside it. Follow the smooth outer boundary and the folded inner membrane. The folds, called cristae, provide surface for processes involved in ATP production. The matrix is the space enclosed by the inner membrane.
Image source and licence
OpenStax · Source image · CC BY 4.0. Original image, reproduced without alteration.

Cytoplasm, cytosol and how the parts cooperate

Two words are easy to confuse. Cytosol is the watery fluid inside the cell, containing dissolved ions, nutrients and enzymes. Cytoplasm is everything inside the cell membrane except the nucleus: the cytosol plus all the organelles. Keeping the distinction matters because many reactions happen in the cytosol, while others happen inside membrane-bound organelles. The organelles are not independent: the nucleus supplies instructions, ribosomes and the ER build proteins, the Golgi packages them, mitochondria supply the ATP that powers every step, and lysosomes recycle worn parts. When one part fails, the effect is felt by the others. This interdependence is why a cell is described as a system rather than a collection of separate structures, and it is the reason organelle disorders produce effects far beyond the organelle itself.

For example: If mitochondria produce less ATP, protein building slows, the membrane’s pumps work less effectively and the cell may swell — one failure spreading through the system.

Key facts to remember

The cell membrane is a selectively permeable phospholipid bilayer with embedded proteins.
The nucleus contains most of the genetic material, enclosed by a double nuclear envelope with pores; the nucleolus assembles ribosome components.
Ribosomes build proteins; the rough ER and Golgi process and package them; vesicles carry them out of the cell.
Mitochondria transfer energy into ATP; lysosomes and peroxisomes break down waste and harmful substances.
Cytosol is the fluid; cytoplasm is the cytosol plus all organelles except the nucleus.

Example · Consider a pancreatic cell that releases insulin. The instruction is copied in the nucleus and leaves through a nuclear pore. Ribosomes on the rough ER build the protein chain. The rough ER folds it and sends it to the Golgi, which trims and packages it into vesicles. Mitochondria supply the ATP for every step. When blood glucose rises, a vesicle fuses with the cell membrane and releases insulin outside the cell. Each organelle performs one part of a single, coordinated job.

Read full notes

Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.

The membrane as a controlled gate

If the membrane were a solid wall, the cell could not take in nutrients or release waste; if it were completely open, the cell would lose its identity. The phospholipid bilayer solves this by being fluid enough to allow movement but stable enough to keep the cell together. Small, uncharged molecules such as oxygen and carbon dioxide cross the lipid core directly. Water crosses through both the lipid and dedicated channel proteins. Ions and larger molecules such as glucose need specific transport proteins, and the cell can open, close or make more of these proteins as its needs change. Embedded proteins also act as receptors that detect hormones and as markers that identify the cell to the immune system. The membrane is therefore not just a barrier but an active, information-processing surface.

The nucleus and gene expression

Most cells in the body contain the same genes, yet a nerve cell and a muscle cell behave very differently. The difference lies in which genes are active, and the nucleus is where that control begins. When a gene is switched on, its instruction is copied into a message that leaves the nucleus and is used to build a protein. Switching genes on or off is called gene expression, and it is how differentiation produces specialised cells. The nuclear pores are selective gates in this process, allowing messages and certain proteins to pass in both directions while keeping the genetic material itself inside. Because almost every cell activity depends on which proteins are present, the nucleus indirectly influences transport, metabolism, structure and division.

Producing and shipping proteins

The secretory pathway is one of the clearest examples of organelles working in sequence. A ribosome reads a message and joins amino acids into a chain. If the protein is destined for export, the ribosome is attached to the rough ER, and the growing chain enters the ER interior, where it folds and may be modified. A vesicle then carries it to the Golgi, which acts like a sorting and finishing department: it trims, adds chemical groups and labels the product for its destination. From the Golgi, vesicles deliver proteins to the cell membrane, to lysosomes or to other targets. Each step adds something the previous step could not do, and the route explains why a fault at any one stage can block the whole output, even when the protein itself is made correctly.

Energy, recycling and structural support

Mitochondria are unusual among organelles because they have their own DNA and their own membranes, and they can divide independently within the cell. Their main job is to transfer energy from nutrients into ATP through a process that requires oxygen. Cells that use a lot of energy contain many mitochondria, and the number changes with training and with disease. Lysosomes handle the cell’s recycling: they digest material brought in from outside and break down worn organelles, returning the components for reuse. Peroxisomes neutralise reactive substances and process fatty acids. The cytoskeleton provides the internal scaffolding that keeps the cell’s shape, positions organelles and allows the whole cell to move or change shape. Together, these systems keep the cell supplied, clean and structurally sound.

The cell as a coordinated system

It is easy to memorise organelles as a list, but they only make sense as a system. The nucleus issues instructions, ribosomes and the ER build proteins, the Golgi finishes and sorts them, mitochondria supply energy, lysosomes recycle, and the membrane controls the boundary and receives signals. A change in any one affects the others, which is why organelle diseases rarely produce a single isolated symptom. It is also why cells with different jobs have different proportions of organelles: a muscle cell is rich in mitochondria, a cell that exports enzymes is rich in rough ER and Golgi, and a phagocytic white blood cell is rich in lysosomes. Reading those proportions is a quick way to infer what a cell does, and it is a skill you will reuse when you study tissues.

Explore the diagram →

Explore the diagram

Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.

Inside an animal cell

Enlarged cutaway of a generalised cell. Follow a protein from the ribosomes on rough ER to the Golgi and a vesicle. Organelle sizes and numbers vary between cells.

Cell membrane

The thin outer boundary separates cytoplasm from extracellular fluid. Its proteins control exchange and receive signals; it is not a rigid cell wall.

Teaching SVG · schematic, not to scale. Select a numbered structure to read its explanation.

Three routes across a membrane

Compare the route, direction and energy source. Arrows show net movement; the membrane is greatly enlarged. Water can cross through aquaporin channels.

Through the lipid bilayer

Small nonpolar molecules such as oxygen diffuse through the lipids, from higher to lower concentration. The polar heads face water; the tails face inward.

Teaching SVG · schematic, not to scale. Select a numbered structure to read its explanation.

Connect the illustrations to the key ideas

MEMBRANE · selective phospholipid bilayer
The membrane controls what enters and leaves and carries receptors and identity markers.

NUCLEUS AND CONTROL · genetic material and instructions
The nucleus stores the genes, and copies of instructions leave through nuclear pores.

ORGANELLES AND OUTPUT · ribosome → ER → Golgi → vesicle; mitochondria power it
Organelles build, finish and ship products while mitochondria, lysosomes and the cytoskeleton support the work.

Core Concepts Defined

Cell (plasma) membrane: The selectively permeable boundary of the cell, made of a phospholipid bilayer with proteins.
Phospholipid bilayer: Two layers of phospholipids with water-attracting heads outward and water-avoiding tails inward.
Selectively permeable: Allowing some substances to cross while restricting others.
Cytoplasm: Everything inside the cell membrane except the nucleus: cytosol plus organelles.
Cytosol: The watery fluid inside the cell, containing dissolved substances and enzymes.
Organelle: A specialised structure inside a cell that performs a particular job.
Nucleus: The organelle that contains most of the cell’s genetic material and directs its activities.
Nuclear envelope: The double membrane around the nucleus, perforated by pores.
Chromatin: The complex of DNA and proteins that stores genetic material in the nucleus.
Chromosome: A condensed, visible form of chromatin during cell division.
Nucleolus: A region of the nucleus where ribosome components are assembled.
Ribosome: The structure that builds proteins by joining amino acids.
Endoplasmic reticulum (ER): A folded membrane network; rough ER has ribosomes and makes proteins, smooth ER makes lipids and stores calcium.
Golgi apparatus: A stack of flattened sacs that modifies, sorts and packages proteins and lipids.
Vesicle: A small membrane-bound bubble that carries substances within or out of the cell.
Mitochondrion: The organelle that uses oxygen to transfer energy from nutrients into ATP.
ATP: Adenosine triphosphate, the molecule cells use to power their work.
Lysosome: A membrane-bound sac of digestive enzymes that breaks down waste and worn-out structures.
Peroxisome: A small sac that breaks down fatty acids and neutralises harmful substances.
Cytoskeleton: The internal network of protein filaments that supports the cell and enables movement.
Centriole: A structure that helps organise the cytoskeleton during cell division.

Go deeper · worked example

From instruction to exported insulin

Start in the nucleus. The gene for insulin is copied into a portable message, which leaves the nucleus through a pore in the nuclear envelope.
Build the protein. The message reaches a ribosome on the rough endoplasmic reticulum, where amino acids are joined into the insulin chain.
Fold and process. Inside the rough ER the chain folds, and it is then carried to the Golgi apparatus, which trims it and packages the mature hormone.
Package and position. The Golgi releases the hormone inside a vesicle that moves to the cell membrane and waits for the right signal.
Release and power. When blood glucose rises, the vesicle fuses with the membrane and releases insulin outside the cell. Mitochondria supplied the ATP used at every earlier step.

Explain it yourself · Explain why a fault in the Golgi apparatus would affect a cell that exports a hormone, even though the Golgi does not make the hormone.

Reveal a model explanation

The Golgi modifies, sorts and packages proteins made in the rough ER. Without it, the newly built hormone would not be trimmed or packed into vesicles, so it could not be stored or released at the right time. The protein would still be made, but the export route would fail. This shows that a job shared across organelles cannot be completed by one organelle alone.

References and review status

OpenStax, Anatomy and Physiology 2e, 3.1 The Cell Membrane and 3.2 The Cytoplasm and Cellular Organelles; Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 3. Author review only; medical and French-language expert review pending.

Original teaching schematics. English and French wording is author-reviewed for study; anatomy and French-language expert review is pending. Checks are practice only, not a graded examination.

Practice progress is saved on this device for the current learner.