Source: Cell Organelle Chart Key (Michigan State University)
Tags: cytoplasm, cytoskeleton, microtubule, microfilament, cilia, flagella, ribosome, endoplasmic reticulum, rough ER, smooth ER, Golgi apparatus, lysosome, vacuole, mitochondria, centriole, chloroplast, organelle
Difficulty: Introductory | Prerequisites: Read the companion notes on Cell Boundaries and the Nucleus first.
Once you understand how a cell is bounded and where its DNA lives, the next question is: what does all the machinery inside the cell actually do? This set of notes covers the organelles that sit in the cytoplasm and carry out the cell's day-to-day work: building proteins (ribosomes, ER, Golgi), recycling waste (lysosomes), generating energy (mitochondria, chloroplasts), storing materials (vacuoles), and maintaining structural integrity (cytoskeleton). These are the components that turn genetic instructions into a functioning, moving, dividing cell. Most introductory biology exams test these organelles heavily, so knowing what each one looks like, where it is found, and what it does is essential.
The cytoplasm is where the action happens. Ribosomes build proteins, the ER and Golgi process and ship them, lysosomes break down waste, mitochondria produce energy (ATP), and chloroplasts (plants only) capture light for photosynthesis. The cytoskeleton holds the whole thing together and enables movement.
Cytoplasm
The clear, gel-like fluid (cytosol plus organelles) that fills the interior of the cell between the cell membrane and the nucleus. Found in all cell types.
Think of it as the factory floor where all the machinery sits and chemical reactions take place.
Cytoskeleton
A network of protein filaments (microtubules, microfilaments, and intermediate filaments) that provides structural support, maintains cell shape, and enables movement. Found in animal and plant cells, with simpler versions in some prokaryotes.
In simple terms, it is the cell's internal scaffolding and transport rail system.
Cilia / Flagella
Short (cilia) or long (flagella) hair-like projections made of microtubules that extend from the cell surface. Found primarily in animal cells.
Think of cilia as tiny oars rowing in unison, and a flagellum as a single whip-like tail.
Microtubule
Hollow tubes made of the protein tubulin. Part of the cytoskeleton. Found in animal and plant cells.
In simple terms, they are the thick structural beams of the cytoskeleton, also used as tracks for moving cargo within the cell.
Microfilament (actin filament)
Long, thin fibres made of the protein actin. Part of the cytoskeleton. Found in animal and plant cells.
Think of them as the thin cables that help the cell move, contract, and hold its shape.
Ribosome
Tiny, abundant structures made of ribosomal RNA and protein. The site of protein synthesis. Found in all cell types (prokaryotic, animal, plant).
In simple terms, ribosomes are the protein-building machines. They read mRNA instructions and assemble amino acids into proteins.
Endoplasmic reticulum (ER)
A highly folded membrane system in the cytoplasm. Comes in two forms: rough ER (studded with ribosomes, processes proteins) and smooth ER (no ribosomes, produces lipids and carbohydrates, detoxification). Found in animal and plant cells.
Think of the rough ER as the protein finishing line and the smooth ER as the lipid and detox workshop.
Golgi apparatus (Golgi body)
A flattened stack of membrane-bound sacs (cisternae) located near the cell membrane. Sorts, packages, and ships proteins and lipids to their final destinations. Found in animal and plant cells.
In simple terms, the Golgi is the cell's post office: it labels, wraps, and dispatches molecular parcels.
Lysosome
A membrane-bound sac containing digestive enzymes. Breaks down food particles, bacteria, and worn-out organelles. Found in animal cells only.
Think of it as the cell's recycling centre and waste disposal unit.
Vacuole
A membrane-bound sac used for storage. In plant cells, a large central vacuole stores water, nutrients, and waste, and helps maintain turgor pressure. Animal cells may have small vacuoles or none.
In simple terms, the vacuole is the cell's storage tank.
Mitochondrion (plural: mitochondria)
A double-membrane-bound organelle that produces ATP through cellular respiration. Found in animal and plant cells. Often called the "powerhouse of the cell."
Think of mitochondria as the generators that convert food molecules into usable energy.
Centrioles / Basal bodies
Small cylindrical structures made of microtubules, found outside the nucleus in animal cells. Involved in organising the mitotic spindle during cell division and anchoring cilia and flagella.
In simple terms, centrioles help pull chromosomes apart when a cell divides.
Chloroplast
A double-membrane-bound organelle found only in plant cells. Contains the pigment chlorophyll in its inner membranes (thylakoids). Captures light energy and converts it into chemical energy (glucose) through photosynthesis.
Think of chloroplasts as solar panels that power the plant cell by harvesting sunlight.
Cytoplasm fills the cell interior. All organelles are suspended in it, and most chemical reactions occur here.
Present in all cell types: prokaryotic, animal, plant.
The cytoskeleton is the internal framework that gives the cell its 3D shape.
Microtubules: hollow protein tubes. Maintain cell shape, form the mitotic spindle during division, and build cilia/flagella.
Microfilaments: thin actin fibres. Support cell shape and enable movement (e.g. muscle contraction, amoeboid movement).
Cilia: short, numerous projections. Beat in coordinated waves to move fluid across the cell surface (e.g. lining of the trachea).
Flagella: long, whip-like projections. Propel entire cells (e.g. sperm cells).
Centrioles: found only in animal cells. Organise microtubules during cell division (mitosis) and anchor cilia/flagella at their base (basal bodies).
Ribosomes are the starting point: they translate mRNA into protein.
Found in all cell types. Can be free in the cytoplasm or attached to rough ER.
Made of rRNA and protein, assembled in the nucleolus.
Rough ER (endoplasmic reticulum with ribosomes): folds and processes newly made proteins. Continuous with the nuclear envelope.
Smooth ER (no ribosomes): produces lipids and carbohydrates, stores calcium, and handles detoxification.
Golgi apparatus: receives proteins and lipids from the ER, modifies them further, sorts them, and packages them into vesicles for transport to their final destination (the cell membrane, lysosomes, or secretion outside the cell).
Lysosomes (animal cells only): contain digestive enzymes that break down ingested food, bacteria, and damaged organelles. Operate at an acidic pH.
Vacuoles: membrane-bound storage sacs.
Plant cells: one large central vacuole stores water, ions, nutrients, and waste. Maintains turgor pressure (the internal pressure that keeps the plant upright).
Animal cells: small vacuoles or none.
Mitochondria (animal and plant cells): carry out cellular respiration, converting glucose and oxygen into ATP. Double-membrane structure with folded inner membranes (cristae) that increase surface area.
Chloroplasts (plant cells only): carry out photosynthesis, converting light energy, water, and CO2 into glucose. Contain chlorophyll, the green pigment, in thylakoid membranes. Also double-membrane-bound.
Mitochondrial dysfunction is linked to diseases such as Parkinson's and Alzheimer's. Understanding how mitochondria produce energy helps explain why these conditions affect high-energy-demand tissues like brain and muscle.
Lysosomal storage diseases (e.g. Tay-Sachs) occur when lysosomes lack a specific enzyme and cannot break down certain molecules. The undigested material accumulates and damages the cell. This is a direct, clinical consequence of a single organelle failing.
Students often think only plant cells have vacuoles. Animal cells can have small vacuoles too; the difference is that plant cells have one large central vacuole.
Students frequently confuse the rough ER and the Golgi apparatus. The rough ER makes and folds proteins; the Golgi modifies, sorts, and ships them. They are sequential steps, not the same thing.
Students sometimes believe mitochondria are found only in animal cells. Both animal and plant cells have mitochondria. Plant cells need them for cellular respiration just as animal cells do; chloroplasts handle photosynthesis separately.
Students often think ribosomes are only on the rough ER. Ribosomes also float freely in the cytoplasm. Free ribosomes make proteins used inside the cell; ER-bound ribosomes make proteins destined for membranes or export.
⚠️ Know which organelles are found in all three cell types (prokaryotic, animal, plant): cell membrane, cytoplasm, and ribosomes.
⚠️ Be able to trace the protein production pathway: ribosome to rough ER to Golgi to vesicle to destination.
⚠️ Know the difference between rough ER and smooth ER in both structure and function.
⚠️ Understand that both mitochondria and chloroplasts are double-membrane-bound and have their own DNA (relevant to the endosymbiotic theory, often tested).
⚠️ Be ready to compare plant vs. animal cells: chloroplasts, cell wall, and large central vacuole are plant only; lysosomes and centrioles are animal only.
True or False: Ribosomes are found only in eukaryotic cells. (False: ribosomes are found in prokaryotic, animal, and plant cells.)
Fill in the blank: The organelle that sorts, packages, and ships proteins is the ______. (Golgi apparatus)
True or False: Chloroplasts are found in both animal and plant cells. (False: chloroplasts are found only in plant cells.)
Fill in the blank: The "powerhouse of the cell" is the ______. (mitochondrion / mitochondria)
True or False: Lysosomes are found in plant cells. (False: lysosomes are found only in animal cells.)
Q: Describe the pathway a protein takes from synthesis to secretion outside the cell.
A: A protein is synthesised by a ribosome on the rough ER, folded and processed within the ER lumen, transported in a vesicle to the Golgi apparatus where it is further modified and sorted, then packaged into a secretory vesicle that fuses with the cell membrane and releases the protein outside the cell.
Q: What are the structural and functional differences between rough ER and smooth ER?
A: Rough ER has ribosomes on its surface and is involved in protein processing and folding. Smooth ER lacks ribosomes and is involved in lipid and carbohydrate production, calcium storage, and detoxification.
Q: Compare mitochondria and chloroplasts. What do they have in common and how do they differ?
A: Both are double-membrane-bound, contain their own DNA, and are involved in energy conversion. Mitochondria convert glucose into ATP (cellular respiration) and are found in both animal and plant cells. Chloroplasts convert light energy into glucose (photosynthesis), contain chlorophyll, and are found only in plant cells.
Q: Name two organelles found only in animal cells and two found only in plant cells.
A: Animal only: lysosomes and centrioles. Plant only: chloroplasts and the large central vacuole (also the cell wall, though it is not strictly an organelle).
Q: What role do microtubules play during cell division?
A: Microtubules form the mitotic spindle, which attaches to chromosomes at their centromeres and pulls them apart to opposite poles of the cell during anaphase.
The protein production pathway (ribosome to ER to Golgi) connects directly to the study of the endomembrane system, a topic most courses treat as its own unit. Understanding the sequence here saves time later.
Mitochondria and chloroplasts both connect to the endosymbiotic theory, which proposes that these organelles were once free-living prokaryotes engulfed by ancestral eukaryotic cells. Their double membranes and own DNA are key evidence.
The cytoskeleton connects to cell division (mitosis and meiosis), muscle physiology, and cell motility, all of which appear in later units.
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