Difficulty: Beginner | Prerequisites: Introduction to Biology notes
Cells are the basic unit of life and the physical place where everything you learned in the intro (metabolism, homeostasis, genetic inheritance) happens. This unit covers the two major cell types, the structures all cells share, and the specialised organelles found in eukaryotic cells. You need to know this material before moving on to any unit on cell division, genetics, or energy pathways.
All living things are made of cells. Cells come in two main types: prokaryotic (simple, no nucleus, found in bacteria and archaea) and eukaryotic (complex, with a nucleus and membrane-bound organelles, found in plants, animals, fungi, and protists). Every cell shares a few universal features: a cell membrane, cytoplasm, and ribosomes. The organelles inside eukaryotic cells each handle a specific job, from energy production to protein transport.
Cell
The fundamental structural and functional unit of all living organisms. In simple terms, cells are the smallest thing that can be called "alive." Every organism, from a bacterium to a blue whale, is built from one or more of them.
Prokaryotic cell
A relatively simple cell that lacks a membrane-bound nucleus and membrane-bound organelles. Found in bacteria and archaea. Think of it as the stripped-down model: DNA floats freely in the cytoplasm, and the cell handles all its functions without internal compartments.
Eukaryotic cell
A more complex cell containing a true nucleus (which houses the DNA) and various membrane-bound organelles. Found in plants, animals, fungi, and protists. Think of it as the compartmentalised model: different tasks happen in different rooms.
Cell membrane (aka plasma membrane)
A thin, flexible barrier made of lipids and proteins that surrounds every cell, controlling what enters and exits. In simple terms, it is the cell's gatekeeper.
Cytoplasm
The gel-like substance filling the interior of the cell, surrounding the organelles. Composed of water, salts, and organic molecules. Think of it as the cell's internal environment, where many chemical reactions take place.
Organelle
A specialised structure within a cell that performs a specific function. In simple terms, organelles are to a cell what organs are to a body: each one has a particular job.
Mitochondria (singular: mitochondrion)
Organelles that produce energy (ATP) for the cell through cellular respiration. Often called the "powerhouse of the cell" because they convert nutrients into usable energy.
Ribosome
A small structure (not membrane-bound) responsible for protein synthesis. Think of it as the cell's protein factory, reading genetic instructions and assembling amino acids into proteins.
Endoplasmic reticulum (ER)
A network of membranes involved in protein and lipid synthesis. Rough ER has ribosomes on its surface; smooth ER does not. In simple terms, the ER is the cell's manufacturing and transport network.
Golgi apparatus (aka Golgi body)
An organelle that modifies, sorts, and packages proteins and lipids for transport to their final destinations. Think of it as the cell's post office: it receives products, labels them, and ships them where they need to go.
Nucleus
The membrane-bound organelle in eukaryotic cells that contains the cell's DNA and controls gene expression. In simple terms, it is the cell's control centre.
All cells fall into one of two categories.
Prokaryotic cells:
No membrane-bound nucleus; DNA sits in an open region called the nucleoid.
No membrane-bound organelles.
Typically very small (1 to 10 micrometres).
Found in bacteria and archaea.
Eukaryotic cells:
Have a true, membrane-bound nucleus containing the cell's DNA.
Contain membrane-bound organelles (mitochondria, ER, Golgi, etc.).
Generally larger (10 to 100 micrometres).
Found in animals, plants, fungi, and protists.
Despite their differences, all cells share three things:
Cell membrane: a lipid-and-protein bilayer that separates the cell's interior from the outside environment and regulates what moves in and out.
Cytoplasm: the gel-like interior where chemical reactions occur, filling the space between the membrane and the nucleus (or nucleoid).
Ribosomes: the sites of protein synthesis, found in both prokaryotic and eukaryotic cells (though eukaryotic ribosomes are slightly larger).
Nucleus: stores DNA and coordinates cell activities such as growth, metabolism, and reproduction.
Mitochondria: carry out cellular respiration to produce ATP, the cell's main energy currency.
Endoplasmic reticulum (ER):
Rough ER (studded with ribosomes): synthesises proteins destined for secretion or for membranes.
Smooth ER (no ribosomes): synthesises lipids and detoxifies certain substances.
Golgi apparatus: receives products from the ER, modifies and packages them, then ships them to their final destination (inside or outside the cell).
Ribosomes: translate mRNA into proteins. Found free in the cytoplasm or attached to the rough ER.
Cells carry out metabolism (all the chemical reactions that sustain life).
Cells grow by synthesising new molecules and organelles.
Cells reproduce by dividing (mitosis in somatic cells, meiosis in sex cells).
Cells maintain homeostasis by regulating their internal environment through the selective permeability of the cell membrane.
Understanding cell structure is the basis of modern medicine. Antibiotics, for example, target features unique to prokaryotic cells (such as their ribosomes or cell walls), which is why they can kill bacteria without harming human cells.
Biotechnology relies on knowledge of organelles and cell processes. Genetic engineering inserts new DNA into a cell's nucleus; drug delivery systems are designed to cross cell membranes and reach specific organelles.
Students often think prokaryotic cells have no organelles at all. They do have ribosomes (which are not membrane-bound). What they lack is membrane-bound organelles such as a nucleus, mitochondria, or ER.
"Cell membrane" and "cell wall" are not the same thing. All cells have a membrane; only some (plants, fungi, bacteria) also have a rigid cell wall outside the membrane.
Students sometimes assume mitochondria are found only in animal cells. Both plant and animal cells have mitochondria. Plants additionally have chloroplasts for photosynthesis.
The nucleus is not "the brain" of the cell in the way a brain processes information in real time. The nucleus stores genetic instructions; the cell's behaviour depends on which genes are being expressed and how the rest of the cell responds.
⚠️ Be able to compare and contrast prokaryotic and eukaryotic cells. This is one of the most commonly tested topics in introductory biology.
⚠️ Know the function of each major organelle. Expect matching or short-answer questions that give you a function and ask you to name the organelle (or vice versa).
⚠️ Understand the structure and role of the cell membrane. Questions about selective permeability, the lipid bilayer, and transport across the membrane come up frequently.
⚠️ Be ready to explain why the cell is considered the basic unit of life. This connects back to cell theory, which may appear as a standalone question.
True or False: Prokaryotic cells have a membrane-bound nucleus.
Fill in the blank: The ______ is the organelle responsible for producing ATP in eukaryotic cells.
True or False: All cells, whether prokaryotic or eukaryotic, have a cell membrane.
Fill in the blank: The gel-like substance inside the cell that surrounds the organelles is called the ______.
True or False: Ribosomes are membrane-bound organelles.
Answers: 1. False (prokaryotes lack a membrane-bound nucleus). 2. Mitochondrion (plural: mitochondria). 3. True. 4. Cytoplasm. 5. False (ribosomes are not membrane-bound).
Q: What are the two main types of cells, and how do they differ?
A: Prokaryotic cells (bacteria, archaea) lack a membrane-bound nucleus and membrane-bound organelles. Eukaryotic cells (plants, animals, fungi, protists) have a true nucleus and membrane-bound organelles such as mitochondria and the ER.
Q: Name three features that all cells share.
A: A cell membrane, cytoplasm, and ribosomes.
Q: What is the function of the Golgi apparatus?
A: It receives proteins and lipids from the endoplasmic reticulum, modifies and sorts them, then packages them into vesicles for transport to their final destination inside or outside the cell.
Q: Explain the difference between rough ER and smooth ER.
A: Rough ER has ribosomes on its surface and is involved in protein synthesis (especially proteins destined for secretion or for membranes). Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification.
Q: Why are cells considered the basic unit of life?
A: Cells are the smallest structures capable of carrying out all the functions of life, including metabolism, growth, reproduction, and homeostasis. Every living organism is composed of one or more cells.
This material connects to the Introduction to Biology unit, where metabolism and homeostasis were introduced as concepts. Cells are where those processes physically take place.
Understanding organelle function prepares you for upcoming units on cellular respiration (mitochondria) and photosynthesis (chloroplasts), where you will trace the specific energy pathways step by step.
Cell division (mitosis and meiosis) builds directly on the nucleus and DNA storage covered here. You will need to know what the nucleus contains and how chromosomes are organised before studying how cells replicate.
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