Cells, tissues and organelles — Edexcel International AS Level Biology
Common features of cells, tissues to organ systems, eukaryotic and prokaryotic ultrastructure, protein transport and recognising organelles in micrographs.
Revision notes
6 short notes, in the order of the specification. Each one in short:
All living organisms are made of cells. Every cell has a cell surface membrane, cytoplasm, genetic material in the form of DNA, ribosomes for protein synthesis, and metabolic reactions catalysed by enzymes. Eukaryotic cells keep their DNA in a nucleus and have membrane-bound organelles, whereas prokaryotic cells have neither.
In a multicellular organism, similar specialised cells working together form a tissue, different tissues working together form an organ, and several organs working together form an organ system. This division of labour lets each cell type do one job efficiently, so the whole organism functions in a coordinated way.
Eukaryotic cells contain organelles, each with a structure suited to its function. The nucleus holds DNA, the nucleolus makes ribosomal RNA, ribosomes make proteins, rough ER transports proteins, smooth ER makes lipids, mitochondria release ATP, centrioles form the spindle, lysosomes digest, and the Golgi apparatus modifies and packages proteins.
Proteins made on ribosomes of the rough endoplasmic reticulum enter its lumen and are packaged into transport vesicles. These fuse with the Golgi apparatus, where the proteins are modified and packaged into secretory vesicles. The vesicles move to the cell surface membrane and fuse with it, releasing the protein, for example an extracellular enzyme, by exocytosis.
A prokaryotic cell such as a bacterium has no nucleus or membrane-bound organelles. It has a cell wall, sometimes a protective capsule, circular DNA, small plasmids, smaller 70S ribosomes, and may have a flagellum for movement and pili for attachment. Each structure has a specific function.
In electron micrographs each organelle has a recognisable appearance. Mitochondria are oval with a folded inner membrane, rough ER is a stack of flattened sacs studded with dots, the Golgi apparatus is a stack of curved sacs with vesicles, and the nucleus has a double envelope with pores and a dark nucleolus.
Exam questions
8 exam-style questions (24 marks), each with its mark scheme.
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17 cards: flip them, mark what you knew, and practise the rest.
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The whole of cell structure, reproduction and development on one page, so you can see where this subtopic fits.
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Quick check questions
Name three features found in all living cells.
Show answer
Any three of: cell surface membrane, cytoplasm, DNA (genetic material), ribosomes, enzyme-controlled metabolic reactions.
What is the function of the nucleolus?
Show answer
It makes ribosomal RNA and assembles ribosome subunits.
Name the organelle that makes lysosomes.
Show answer
The Golgi apparatus.
How does prokaryotic DNA differ from eukaryotic DNA?
Show answer
It is circular, not associated with histones and not enclosed in a nucleus.
What are the features of a mitochondrion in an electron micrograph?
Show answer
Oval shape, a double membrane and an inner membrane folded into cristae.
Frequently asked questions
What is the difference between a tissue, an organ and an organ system?
A tissue is a group of similar cells doing the same job. An organ is a group of different tissues working together for one function, such as the stomach. An organ system is several organs working together for a major body function, such as the digestive system.
What does the Golgi apparatus do?
The Golgi apparatus receives proteins from the rough ER in transport vesicles, modifies them (for example by adding carbohydrate to make glycoproteins), then packages them into secretory vesicles. It also makes lysosomes, which contain hydrolytic enzymes, so it acts as the cell's processing and packaging centre.
How do prokaryotic cells differ from eukaryotic cells?
Prokaryotic cells have no nucleus or membrane-bound organelles. Their DNA is a circular loop lying free in the cytoplasm, they have smaller 70S ribosomes, and they have a peptidoglycan cell wall. Eukaryotic cells have a nucleus with linear DNA, larger 80S ribosomes and membrane-bound organelles such as mitochondria.
How are proteins secreted from a cell?
Proteins made on the rough ER enter its lumen and are carried in transport vesicles to the Golgi apparatus, where they are modified and packaged into secretory vesicles. These move to the cell surface membrane and fuse with it, releasing the protein outside the cell by exocytosis.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026