Eukaryotic cells and their organelles
Introduced| English | Meaning |
|---|---|
| organelle/ˌɔːɡəˈnel/ | A specialised structure within a cell that carries out a particular function. |
| cytoplasm/ˈsaɪtəplæzəm/ | The contents of a cell inside its membrane and outside the nucleus, where many chemical reactions occur. |
| eukaryotic/ˌjuːkərɪˈɒtɪk/ | Describing cells with a membrane-bound nucleus and other membrane-bound organelles. |
| nucleolus/ˌnjuːklɪˈəʊləs/ | The dense region inside the nucleus where ribosomal RNA is made and ribosome subunits are assembled. |
| ribosome/ˈriːbəʊsəʊm/ | A cellular structure that joins amino acids into a polypeptide using information from messenger RNA. |
| Golgi body/ˈɡɒlɡɪ ˈbɒdi/ | A stack of flattened membranes that modifies, packages and dispatches proteins in vesicles. |
| vesicle/ˈvesɪkl/ | A small membrane-bound compartment used to store or transport substances within or between cells. |
| mitochondria/ˌmaɪtəˈkɒndrɪə/ | Organelles in eukaryotic cells where most aerobic respiration and ATP production occur. |
| lysosome/ˈlaɪsəʊsəʊm/ | A vesicle of digestive enzymes that breaks down worn-out organelles and engulfed material. |
| chloroplast/ˈklɔːrəʊplæst/ | An organelle in plants and algae that contains photosynthetic pigments and carries out photosynthesis. |
| vacuole/ˈvækjuːəʊl/ | A membrane-bound compartment containing fluid and dissolved substances, often large and permanent in plant cells. |
| tonoplast/ˈtɒnəplæst/ | The membrane surrounding the large central vacuole of a plant cell. |
| plasmodesmata/ˌplæzməʊdˈsmɑːtə/ | Channels through plant cell walls that connect the cytoplasm of neighbouring cells. |
| centrioles/ˈsentrɪəʊlz/ | Cylinders of nine microtubule triplets, usually occurring as a pair in animal cells and helping organise microtubules. |
| nuclear envelope/ˈnjuːklɪə ˈenvələʊp/ | The double membrane surrounding the nucleus of a eukaryotic cell. |
| cristae/ˈkrɪstiː/ | Folds of a mitochondrion's inner membrane that increase the surface available for respiratory electron transport and ATP production. |
| matrix/ˈmeɪtrɪks/ | The fluid-filled space inside a mitochondrion where the link reaction and Krebs cycle occur. |
| grana/ˈɡrɑːnə/ | Stacks of thylakoid membranes in a chloroplast where the light-dependent reactions occur. |
| stroma/ˈstrəʊmə/ | The fluid-filled region inside a chloroplast, surrounding the thylakoid membranes. |
| cilia/ˈsɪlɪə/ | Small hair-like structures on cells that can beat to move the cell or nearby material. |
| microvilli/ˈmaɪkrəʊvɪlaɪ/ | Tiny projections of a cell's surface that increase its area for absorption or other exchange. |
| microtubules/ˌmaɪkrəʊˈtjuːbjuːlz/ | Hollow protein tubes that support cell structure and help move chromosomes and organelles. |
A cell is a tiny factory
- Zoom into a typical plant or animal cell and you find a busy factory — a control room, power stations and packaging lines.
- These working parts are organelles 细胞器, each with a job, floating in the jelly-like cytoplasm 细胞质.
- The cell is eukaryotic 真核: most of its DNA is kept inside a nucleus. Learn each organelle's structure and its function.
The main organelles
| Organelle | Function |
|---|---|
| nucleus | holds the DNA; controls the cell (nucleolus 核仁 makes ribosomes 核糖体) |
| rough ER | makes and transports proteins (ribosomes on its surface) |
| smooth ER | makes lipids |
| Golgi body 高尔基体 | modifies and packs proteins/lipids into vesicles 囊泡 for secretion |
| mitochondria 线粒体 | site of respiration — releases energy as ATP |
| ribosomes | join amino acids to make proteins |
| lysosomes 溶酶体 | sacs of enzymes that break down waste |
A generalised animal cell and its organelles
- Explore below: tap each organelle on the diagram to see what it does.
Explore the organelles
Tap each organelle on the diagram — explore all eight to see what each one does.
What is the main function of mitochondria?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Mitochondria are the site of aerobic respiration, producing ATP.
The tiny organelle that joins amino acids to build proteins is the ______.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Ribosomes carry out protein synthesis; the nucleolus makes them.
Smooth ER is studded with ribosomes and makes proteins, while rough ER makes lipids.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
False. It is the other way round: rough ER has ribosomes on its surface and makes proteins; smooth ER has none and makes lipids.
The Golgi body:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The Golgi processes and packages molecules into vesicles to be secreted.
Plant-cell extras
- Chloroplasts 叶绿体 — site of photosynthesis (green, stacked membranes).
- Cell wall — strong cellulose layer; supports the cell and stops it bursting.
- A large permanent vacuole 液泡 (with a tonoplast 液泡膜 membrane) — stores water and keeps the cell firm.
- Plasmodesmata 胞间连丝 — channels linking neighbouring cells' cytoplasm.
A plant cell adds a cellulose cell wall, chloroplasts and a large central vacuole
Which organelle carries out photosynthesis in a photosynthetic plant cell?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Chloroplasts trap light energy for photosynthesis; they are found in plant cells, not animal cells.
Plant vs animal cells
- Compared with animal cells: a cellulose cell wall and large permanent vacuole; chloroplasts in photosynthetic plant cells.
- Animal usually has: centrioles 中心粒 (absent in most plant cells).
- Typical cells of both groups have: cell surface membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER, Golgi.
A root hair cell is a plant cell without chloroplasts. Identify a cell using several features, not one missing organelle.
Match each organelle to its function.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Mitochondrion = respiration; chloroplast = photosynthesis; lysosome = digestion of waste; Golgi = packaging.
Select all the parts found in BOTH a typical plant cell and a typical animal cell.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Nucleus, mitochondria, ribosomes and Golgi are in both. Chloroplasts and a cellulose cell wall are plant-only.
Which is found in plant cells but NOT animal cells?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Cell wall, chloroplasts and a large permanent vacuole are plant-only; the others are in both.
The nucleus and cell boundaries
- The nuclear envelope 核膜 has two membranes and pores. It separates the main DNA store from the cytoplasm while allowing controlled exchange; the nucleolus makes ribosomal RNA and assembles ribosomal subunits.
- The cell surface membrane is a thin, partially permeable boundary controlling movement into and out of the cell. A plant cell wall lies outside this membrane; its cellulose fibres give support, but it is not the selective boundary.
A cell makes many ribosomes before increasing its protein production. Distinguish the roles of its nucleus, nuclear envelope and nucleolus. Explain why the envelope needs pores.
The nucleus contains most of the cell’s DNA, including genes used in protein production. The nuclear envelope is a double membrane separating nuclear contents from the cytoplasm. Pores permit controlled exchange, including RNA and ribosomal subunits. The nucleolus makes ribosomal RNA and assembles ribosomal subunits; it does not translate proteins for secretion.
Follow a protein out of a cell
- Rough ER has flattened membrane sacs with 80S ribosomes on the outside; free cytoplasmic ribosomes are also 80S. Ribosomes make proteins; proteins for secretion enter rough ER and travel in transport vesicles to the Golgi body.
- Smooth ER is a network of membrane tubes without attached ribosomes; it makes lipids.
- The Golgi body is a stack of flattened sacs with nearby vesicles. It modifies and packages the protein; a secretory vesicle moves to the cell surface membrane and fuses with it to release the contents.
- A gland cell secretes a protein enzyme. Many ribosomes on rough ER support protein synthesis; Golgi bodies and vesicles process and export the enzyme.
- A cell producing a lipid secretion instead needs more smooth ER, whose membranes lack ribosomes. Explain the organelle difference using the product, not just the word “secretion”.
A fluorescent label first appears in a secreted protein at ribosomes on rough ER, later in a stack of flattened sacs, and finally outside the cell. Identify the stack and explain how the protein travels from it to the outside.
The stack is the Golgi body. It modifies and packages the protein into secretory vesicles. Vesicles move to and fuse with the cell surface membrane, releasing their contents by exocytosis. Ribosomes make the protein; the Golgi does not join amino acids to make it.
Folded membranes and ATP
- A mitochondrion has two membranes. Its inner membrane is folded into cristae 嵴, increasing the area for reactions that make ATP during aerobic respiration; the inside is the matrix 基质.
- Mitochondria contain small circular DNA and 70S ribosomes. Chloroplasts also have two outer membranes, small circular DNA and 70S ribosomes, but have internal photosynthetic membranes stacked into grana 基粒 within the stroma 叶绿体基质.
- A cell actively transports mineral ions. It needs ATP for this energy-requiring process, so many mitochondria support its function.
- “The cell needs energy” is incomplete reasoning. Name ATP, the process that uses it and the role of respiration in supplying it.
Two muscle cells have the same volume. One contracts repeatedly for long periods; the other contracts only occasionally. Predict which cell is likely to contain more mitochondria and explain the link to its function.
The repeatedly contracting cell is likely to have more mitochondria. Contraction requires ATP; aerobic respiration in mitochondria produces ATP to support the greater demand. This is a functional prediction, not a claim that mitochondria themselves contract or that cell volume alone determines ATP demand.
A typical photosynthetic plant cell contains ribosomes free in the cytoplasm, on rough ER, inside mitochondria and inside chloroplasts. State the ribosome type at each location. What job do all of them perform?
Free cytoplasmic and rough-ER-bound ribosomes are 80S. Mitochondrial and chloroplast ribosomes are 70S. All carry out protein synthesis by joining amino acids. Finding 70S ribosomes in a sample is therefore not enough by itself to identify a whole cell as a bacterium.
Cilia, microvilli and microtubules
- Cilia 纤毛 are projections that beat to move fluid, such as mucus in an airway. A typical motile cilium contains microtubules arranged as nine outer pairs around two central microtubules; microvilli 微绒毛 are supported by actin filaments and increase surface area for absorption.
- Microtubules 微管 are hollow protein tubes that support the cell and help movement of chromosomes and organelles. Centrioles are cylinders of nine microtubule triplets, usually found as a pair in animal cells; do not confuse their structure with the 9 + 2 pattern of a motile cilium.
Both cilia and microvilli project from a surface, but they do different jobs. Do not give microvilli the microtubule pattern of cilia or claim that their function is to sweep mucus.
One epithelial surface has projections with a 9 + 2 arrangement of microtubules; another has shorter projections supported by actin filaments. Identify each and compare their functions.
The first projections are typical motile cilia, which beat to move fluid or mucus over the surface. The second are microvilli, which increase surface area for absorption. Their different internal structures and functions distinguish them even though both project from a cell surface.
A student labels a cylinder containing nine groups of three microtubules as the shaft of a typical motile cilium. Correct the label and explain the structural difference. Give one role of microtubules elsewhere in the cell.
Nine microtubule triplets form a centriole, not the usual ciliary shaft. A typical motile ciliary shaft has nine outer microtubule pairs and two central microtubules. Elsewhere microtubules form the spindle that moves chromosomes during cell division, or help support the cell and move organelles. The triplet and 9 + 2 arrangements must not be interchanged.
Membranes, digestion and plant connections
- A lysosome is bounded by a single membrane and contains digestive enzymes. It breaks down material such as worn-out organelles; its membrane separates these enzymes from the cytoplasm.
- The tonoplast surrounds the plant vacuole and controls exchange with the cytoplasm. Plasmodesmata are channels through adjacent cell walls linking neighbouring cytoplasm; they are not spaces between the cells.
A molecule moves from one plant cell into a neighbouring cell through a cytoplasmic connection. Another molecule moves from cytoplasm into the vacuole. Name the connection and the membrane crossed in the second movement.
The cytoplasmic connection is a plasmodesma (plural plasmodesmata), a channel through adjacent cell walls. The vacuole is bounded by the tonoplast. The cell wall and the cell surface membrane are not names for the tonoplast.
A white blood cell takes a bacterium into a membrane sac. A small organelle containing digestive enzymes fuses with that sac. Name the organelle, describe its boundary and explain why keeping the enzymes in a compartment is useful.
The organelle is a lysosome, surrounded by a single membrane. Its enzymes digest the bacterium after the compartments fuse. Keeping the enzymes within membranes separates them from other cell contents and allows digestion in a controlled compartment. The lysosome is not a ribosome: ribosomes make proteins rather than digesting the bacterium.
Read a micrograph using evidence
- First describe visible features: membrane number, folds or stacks, surface dots, and position. Folded inner membranes suggest a mitochondrion; parallel membrane sacs with ribosomes suggest rough ER.
- Use several observations and the scale. A round outline may be a cross-section of an elongated structure; a part outside the thin section can be absent from the image without being absent from the cell.
- An unknown organelle has two bounding membranes and a folded inner membrane, but no stacks of photosynthetic membranes. These observations support a mitochondrion rather than a chloroplast.
- A nearby stack of flattened sacs with budding vesicles and no attached ribosomes supports a Golgi body rather than rough ER. Shape alone is weaker evidence than the combination.
A thin section shows a cell wall, a large vacuole and several mitochondria, but no nucleus or chloroplast. A student concludes that this is a bacterium. Evaluate the conclusion.
The evidence does not support that conclusion. Mitochondria are membrane-bound organelles absent from typical bacteria, and the wall plus large vacuole supports a plant cell. A thin section may miss the nucleus; some plant cells lack chloroplasts. Absence from one section is not proof of absence from the whole cell.
Study this electron micrograph. Describe two visible features of the large oval structures, then identify the organelle and link one feature to its function. Can you tell from this image alone whether the structures contain circular DNA and 70S ribosomes? Explain.
Photo: Louisa Howard, Dartmouth Electron Microscope Facility, public domain; unchanged.
The oval structures have an enclosing boundary and repeated dark folded membranes inside; their outlines and internal folds differ in the two sections. They are mitochondria. The internal folds are cristae, increasing membrane area for reactions involved in ATP production during aerobic respiration. Circular DNA and 70S ribosomes are known mitochondrial features, but their identity and type cannot be established from these dark textures alone. Do not label an individual dot as DNA or a 70S ribosome without evidence. Different section shapes do not by themselves show different organelle types.
You've got it
- mitochondria = respiration (ATP); ribosomes = protein synthesis
- rough ER makes proteins, smooth ER makes lipids; Golgi packs them into vesicles
- plant/animal comparison: cellulose cell wall, large vacuole, and chloroplasts in photosynthetic plant cells
- cilia move fluid; microvilli increase surface area; explain how structure supports each job
- nuclear DNA is not the only DNA: mitochondria and chloroplasts contain small circular DNA
- typical plant and animal cells have membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER, Golgi
Cell P secretes a protein hormone. Cell Q makes a lipid hormone. Predict one difference in their endoplasmic reticulum and explain how the structure supports the product made.
P is expected to have relatively more rough ER because its attached ribosomes make proteins. Q is expected to have relatively more smooth ER, which lacks ribosomes and makes lipids. Neither prediction means that the other type of ER is completely absent.
An organelle has a double membrane, folded inner membranes and small circular DNA. Which identification is best supported?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The folded inner membrane and circular DNA support a mitochondrion. Golgi sacs and rough ER do not have this combination of features.
A root hair cell and an animal gland cell are both active living cells. Compare their structures using three shared structures and two differences. Explain why not seeing chloroplasts cannot decide which of these cells is the animal cell.
Both have a cell surface membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER and Golgi bodies (any three). The root hair cell has a cellulose cell wall and a large permanent vacuole bounded by a tonoplast; the animal gland cell lacks a cellulose wall and does not have this large permanent plant vacuole. A root hair cell is a non-photosynthetic plant cell and normally has no chloroplasts, so chloroplast absence is shared by these two examples.