Identify the main organelles of eukaryotic cells and relate their structure to function.

What this dot point is asking

Why organelles matter

Organelles are specialised structures inside the cell, most enclosed by their own membrane. By separating the cell into compartments, each with its own conditions and enzymes, the cell can run many different chemical processes at the same time without them interfering. This compartmentalisation is the great advantage of eukaryotic cells over prokaryotic ones.

The nucleus

The nucleus is the control centre. It is surrounded by a double membrane (the nuclear envelope) pierced by pores that allow molecules such as mRNA to pass out. Inside, the DNA is stored as chromatin. The nucleus controls the cell by determining which proteins are made, and it contains the nucleolus, where ribosomes are assembled.

Mitochondria

Mitochondria are the site of aerobic cellular respiration, releasing energy from glucose and storing it as ATP. Each has a double membrane, with the inner membrane folded into cristae to give a large surface area for the reactions that produce ATP. Cells with high energy demands, such as muscle and kidney cells, contain many mitochondria.

Chloroplasts

Chloroplasts are found in plant cells and some protists and are the site of photosynthesis. They contain the green pigment chlorophyll, which absorbs light. Internal membranes are arranged in stacks (grana) within a fluid (the stroma), giving a large surface for capturing light. Chloroplasts, like mitochondria, have their own small loop of DNA, evidence that both evolved from once-free-living bacteria.

Ribosomes

Ribosomes are the site of protein synthesis, reading mRNA and joining amino acids into chains. They are not membrane-bound. Some float free in the cytoplasm, making proteins for use inside the cell, and others are attached to the endoplasmic reticulum, making proteins for export.

Endoplasmic reticulum and Golgi apparatus

These two organelles form a production and delivery line for proteins and lipids:

• Rough endoplasmic reticulum is studded with ribosomes and processes the proteins they make, folding them and moving them in vesicles.
• Smooth endoplasmic reticulum has no ribosomes and makes lipids and helps detoxify substances.
• The Golgi apparatus receives proteins and lipids, modifies and packages them, and ships them out in vesicles, either to other parts of the cell or out of it by exocytosis.

Lysosomes and vacuoles

Lysosomes contain digestive enzymes that break down worn-out organelles, waste, and material brought in by endocytosis. Vacuoles store substances; in plant cells a large central vacuole stores water and dissolved substances and helps keep the cell firm by pressing the contents against the cell wall.

Plant cell extras

Plant cells have three features animal cells lack: a rigid cellulose cell wall outside the membrane for support and protection, chloroplasts for photosynthesis, and a large permanent central vacuole. Animal cells instead often contain centrioles, which help organise cell division.

Structure suits function

Across all organelles, structure matches function. Folded membranes maximise reaction surfaces, the nuclear envelope's pores control what leaves the nucleus, and the membranes of the endoplasmic reticulum and Golgi create channels for moving products. Recognising these links is the key skill: given an organelle's structure, you should be able to predict its role, and the reverse.