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Cell Structure and Organelles Explained

Every living thing is made of cells. Whether you are looking at a single-celled bacterium or one of the 37 trillion cells in a human body, the cell is the basic unit of life. Understanding what goes on inside a cell — the organelles, their jobs, and how they cooperate — is the foundation of all biology.

Prokaryotes and Eukaryotes

All cells fall into one of two broad categories. Prokaryotic cells (bacteria and archaea) have no membrane-bound nucleus; their DNA sits free in the cytoplasm in a region called the nucleoid. They are generally smaller (1–10 µm) and structurally simpler. Eukaryotic cells (animals, plants, fungi, and protists) have a true nucleus enclosed in a double membrane, and a system of internal membranes that create specialised compartments called organelles. All GCSE and A-level biology focuses on eukaryotic cells, so that is what we cover here.

The Cell Membrane

Every cell is surrounded by a cell membrane (plasma membrane) — a phospholipid bilayer roughly 7–10 nm thick. Phospholipid molecules have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) fatty acid tails. In water they spontaneously arrange into a bilayer: heads face the watery environments outside and inside the cell, tails hide together in the middle. Embedded in this bilayer are protein channels, carriers, receptors, and enzymes. The membrane is described as fluid (components can move laterally) and mosaic (proteins are scattered throughout) — the fluid mosaic model. Its primary role is to control what enters and leaves the cell: it is selectively permeable.

The Nucleus

The nucleus is the control centre of the cell. It is enclosed by the nuclear envelope, a double membrane pierced by nuclear pores that allow molecules such as mRNA and ribosomes to pass through. Inside, DNA is wound around histone proteins to form chromatin; during cell division this condenses into visible chromosomes. Within the nucleus, a dense region called the nucleolus assembles ribosomal RNA and packages it into ribosome subunits, which are then exported to the cytoplasm. The nucleus is the largest organelle in most animal cells and is usually visible under a light microscope.

Mitochondria

Mitochondria (singular: mitochondrion) are the site of aerobic cellular respiration — the process that converts glucose and oxygen into ATP, carbon dioxide, and water. Each mitochondrion has two membranes: a smooth outer membrane and a highly folded inner membrane. The folds, called cristae, greatly increase the surface area available for the electron transport chain and ATP synthase enzymes. The space enclosed by the inner membrane is the matrix, where the Krebs cycle reactions occur. Cells with high energy demands — muscle cells, liver cells, sperm cells — have especially large numbers of mitochondria.

Ribosomes and the Endoplasmic Reticulum

Ribosomes are tiny granular structures (20–25 nm) made of ribosomal RNA and protein. They are the site of protein synthesis: they read mRNA and link amino acids into polypeptide chains. Ribosomes are found either floating free in the cytoplasm (making proteins for use inside the cell) or attached to the rough endoplasmic reticulum (making proteins for secretion or for the cell membrane).

The endoplasmic reticulum (ER) is an extensive network of membranes continuous with the nuclear envelope. The rough ER is studded with ribosomes and folds newly made proteins into their correct three-dimensional shapes. The smooth ER lacks ribosomes and is involved in lipid and steroid synthesis, and in detoxification in liver cells.

The Golgi Apparatus

The Golgi apparatus is a stack of flattened membrane sacs called cisternae. It receives vesicles budded off from the rough ER, modifies and sorts their protein cargo (adding sugar chains, for example), and packages them into secretory vesicles for export. Think of the Golgi as the cell's post office: it receives, addresses, and dispatches molecular packages. Cells that secrete large amounts of protein — pancreatic cells making digestive enzymes, goblet cells making mucus — have a well-developed Golgi.

Key Differences: Animal and Plant Cells

Chloroplasts are the site of photosynthesis. Like mitochondria, they have two outer membranes plus an inner system of membrane sacs called thylakoids stacked into grana, where the light-dependent reactions occur. The fluid-filled space around the grana is the stroma, where the Calvin cycle (light-independent reactions) takes place. Chloroplasts contain their own circular DNA and ribosomes, evidence for their evolutionary origin as once-independent cyanobacteria.

The large central vacuole of plant cells is filled with cell sap and maintained under pressure (turgor pressure), giving the cell rigidity. When a plant wilts, it is because this turgor pressure is lost. The cellulose cell wall provides further structural support and prevents the cell from bursting when it absorbs water by osmosis.

Summary

Eukaryotic cells are compartmentalised by membrane-bound organelles, each with a specialised function: the nucleus stores and expresses genetic information; mitochondria produce ATP by aerobic respiration; ribosomes and the rough ER make and fold proteins; the Golgi apparatus modifies and dispatches them. Plant cells add chloroplasts for photosynthesis, a rigid cell wall, and a large central vacuole. Knowing each organelle's structure and function is the key to understanding every process in biology, from cell division to immunity to inheritance.