The cell is the basic unit of life, and inside it organelles such as the nucleus, mitochondria, endoplasmic reticulum and Golgi apparatus divide up the work.
The nucleus stores the genetic information in DNA, ribosomes build proteins from that information, and mitochondria make ATP by cellular respiration.
Proteins meant for export are made on the rough endoplasmic reticulum and leave through the Golgi apparatus and the cell membrane, and plant cells add a cell wall, chloroplasts and a large vacuole, which animal cells lack.
In the diagram you can pick the organelles one by one to see their shape and role, and in the full view follow the secretory path from the ER through the Golgi apparatus to the membrane.
The Cell, the Basic Unit of Life
Inside a cell, managing genetic information, producing energy, building and processing proteins and breaking down worn-out material all go on at once. If they were mixed in one space, digestive enzymes could wreck newly made proteins and raw materials could drift off to the wrong place. So eukaryotic cells divide their space with membranes: the nucleus guards the DNA, mitochondria carry out respiration and lysosomes keep digestive enzymes locked away. Before memorizing organelle names, it helps to grasp what each membrane-bound zone does and where a protein goes next; then each organelle's place falls into order.
🔬 A Cell as a City
①The nucleus is city hall: it stores genetic information and directs which proteins to make
②Mitochondria are power plants: they make ATP by cellular respiration
③Ribosomes and the ER are factories and the Golgi apparatus is a distribution center: proteins are made, finished, packed and shipped where they are needed
④Lysosomes are recycling centers: they break down worn-out organelles and material taken into the cell
Exploring the Organelles
A cell with no visible nucleus is still a cell. Bacteria are prokaryotic cells with no nuclear membrane, and human red blood cells lose their nucleus as they mature, yet both are cells. It is also easy to assume every organelle is wrapped in membrane, but ribosomes are membraneless particles, the nucleus and mitochondria have a double membrane, and the ER, Golgi apparatus and lysosomes have a single one. When comparing animal and plant cells, read shared structures and one-sided structures separately: both have a nucleus, mitochondria, ribosomes, ER and Golgi apparatus, while a cell wall, chloroplasts and a large vacuole belong to plant cells, and centrosomes and lysosomes mainly to animal cells.
Nucleus
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Main organelles of an animal cell and the secretory path from the ER through the Golgi apparatus to the membrane
The large oval is the cell membrane, and six organelles sit in fixed places inside it. Picking one with the slider makes only that organelle stand out and shows its name and role on a line below. The nucleus is a double circle (the double membrane) with a nucleolus inside, the mitochondrion an oval with inner folds (cristae), and the ER a set of wavy membranes; the small dots on its top two lines are ribosomes, marking rough ER. The Golgi apparatus is four flat sacs, the six separate dots are ribosomes in the cytoplasm and the purple circle is a lysosome. In the full view at the end all six light up, and a dashed arrow from the ER through the Golgi apparatus to the membrane shows the secretory path.
💡 What to Look for in Each Organelle
①Nucleus: wrapped in a double membrane (nuclear envelope), it stores DNA, and the rRNA of ribosomes is made in the nucleolus
②Mitochondria: double membrane with the inner membrane folded into cristae; they make ATP by cellular respiration and have their own DNA
③ER: rough ER, studded with ribosomes, makes proteins for export, and smooth ER, without ribosomes, makes lipids
④Golgi apparatus: processes proteins arriving from the ER and ships them out in vesicles
⑤Ribosomes are membraneless particles that translate mRNA into protein
Organelles at a Glance
Organelles
What the Main Organelles Do
Nucleus
double membrane, DNA, nucleolus (makes rRNA)
stores genetic information, controls its expression
Mitochondria
double membrane, own DNA, cristae
makes ATP (cellular respiration)
Rough ER (rER)
ribosomes attached to its surface
makes and transports proteins for export
Smooth ER (sER)
no ribosomes; abundant in liver cells
makes lipids, breaks down toxins
Golgi apparatus
a stack of flattened sacs
processes, packs and secretes proteins
Ribosome
rRNA and protein, no membrane
protein synthesis
Lysosome
keeps hydrolytic enzymes behind a membrane
digestion inside the cell
Animal Cells and Plant Cells
Cell comparison
How Animal and Plant Cells Differ
Item
Animal
Plant
Cell wall
absent
present (cellulose)
Chloroplast
absent
present (photosynthesis)
Centrosome
present
absent (in most plants)
Vacuole
small (several, if any)
large (one, in the center)
Lysosome
present
almost none (the vacuole takes over)
Worked Examples and Exam Practice
Example 1
List in order the path by which a protein secreted from the cell, such as a pancreatic digestive enzyme, is made and leaves the cell.
1
A secreted protein is made on ribosomes attached to the rough ER and enters the ER.
2
The protein inside the ER is carried to the Golgi apparatus in transport vesicles.
3
Processed and packed in the Golgi apparatus, the protein goes into a secretory vesicle, which fuses with the cell membrane and releases it outside the cell.
The dashed arrow in the full view of the diagram is exactly this route. Membrane-bound organelles take turns at making, finishing and shipping the protein.
Example 2
A cell is seen to have a cell wall, chloroplasts and one large vacuole, but no centrosome or lysosomes. Is it an animal cell or a plant cell?
1
A cellulose cell wall and chloroplasts are found only in plant cells.
2
A single large central vacuole is also typical of plant cells, and most plant cells have no centrosome or lysosomes.
▸ a plant cell
The nucleus, mitochondria, ribosomes, ER and Golgi apparatus are found in both animal and plant cells. If even one of a cell wall, chloroplasts or a large vacuole shows up, it is a plant cell.
CSAT-style problem
Which statement about the organelles of an animal cell is correct?
①Ribosomes are wrapped in a single membrane.
②Mitochondria have no DNA of their own.
③Ribosomes have no membrane, and protein synthesis (translation) takes place on them.
④The Golgi apparatus carries out photosynthesis.
⑤Lysosomes produce ATP.
▸ ③ Ribosomes have no membrane, and protein synthesis (translation) takes place on them.
1
Ribosomes are membraneless organelles made of rRNA and protein that translate mRNA into protein, so ① is wrong and ③ is correct.
2
Mitochondria have their own DNA (②), photosynthesis is done by chloroplasts (④) and ATP is made mainly by mitochondria (⑤), so the rest are wrong.
Summary
The Way Out for Secreted Proteins
nucleus (DNA → mRNA) → ribosomes on rough ER (synthesis) → ER → Golgi apparatus (processing, packing) → secretory vesicle → cell membrane
several organelles take their turn on the way from genetic information to a protein leaving the cell