3.1.2Topic 3 · Voice of the Genome
Eukaryotic Cells — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 9 parts Lesson 5 of 26 1 quiz
Lesson summary
3.1.2 Eukaryotic Cells — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the structure and function of the organelles of animal and plant cells and explain how they work together.Eukaryotic cells have their DNA enclosed in a nucleus and contain membrane-bound organelles.
- Golgi apparatus
- Mitochondria
- Centrioles
- Cytoskeleton
- Cell surface membrane
- Cellulose cell wall
Section 1 of 9Eukaryotic organisation
Eukaryotic cells have their DNA enclosed in a nucleus and contain membrane-bound organelles. Compartmentalisation allows different reactions to take place under different conditions, for example the acid enzymes of the lysosome are kept apart from the rest of the cytoplasm. Animals, plants, fungi and protoctists are all eukaryotes.
Section 2 of 9Nucleus and protein-making organelles
Nucleus
Nucleus surrounded by a double membrane (the nuclear envelope) containing nuclear pores.
- It contains chromatin and one or more nucleoli, which make ribosomal RNA and assemble ribosomes.
- (DNA wound around histone proteins)
- It controls the cell by determining which proteins are made.
Ribosomes
80S in the cytoplasm and on rough endoplasmic reticulum (RER).
- Made of rRNA and protein, with no membrane.
- They are the site of translation.
Rough ER
Rough ER flattened sacs (cisternae) studded with ribosomes. Proteins made here enter the lumen and are folded and transported.
Smooth ER
Smooth ER no ribosomes; makes and processes lipids, including steroids, and in some cells stores calcium ions.
Golgi apparatus
Golgi apparatus a stack of flattened membrane sacs.
- It modifies proteins, packages them into vesicles and makes lysosomes.
- (for example adding carbohydrate to make glycoproteins)
Section 3 of 9Energy, digestion and support
Mitochondria
Mitochondria double membrane, the inner one folded into cristae to give a large surface area for the electron transport chain and ATP synthase.
- The matrix contains enzymes of the Krebs cycle, mitochondrial DNA and 70S ribosomes.
- Site of aerobic respiration.
Lysosomes
Lysosomes single-membrane sacs containing hydrolytic enzymes that digest worn-out organelles and engulfed material.
Centrioles
Centrioles pairs of short microtubule cylinders in animal cells that organise the spindle in cell division.
Cytoskeleton
Cytoskeleton protein filaments and microtubules that give shape, move organelles and vesicles and allow some cells to move.
Cell surface membrane
Cell surface membrane phospholipid bilayer controlling what enters and leaves.
Section 4 of 9Features found only in plant cells
Features found only in plant cells
Cellulose cell wall
Cellulose cell wall a rigid layer of cellulose microfibrils outside the membrane that resists bursting when water enters by osmosis.
Chloroplasts
Chloroplasts double membrane, internal membrane stacks (grana made of thylakoids) holding chlorophyll,
- and a fluid stroma where the Calvin cycle occurs.
- They also contain circular DNA and 70S ribosomes.
Large permanent vacuole
Large permanent vacuole surrounded by a membrane called the tonoplast; stores cell sap and keeps the cell turgid.
Plasmodesmata
Plasmodesmata channels through the walls linking neighbouring cells.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell wall | Absent | Cellulose |
| Chloroplasts | Absent | Present in photosynthetic cells |
| Vacuole | Small, temporary | Large, permanent |
| Centrioles | Present | Absent in higher plants |
| Carbohydrate store | Glycogen | Starch |
Section 5 of 9How organelles cooperate: making a secreted protein
A digestive enzyme such as amylase shows the system working together.
1
The gene in the nucleus is transcribed and the mRNA leaves through a nuclear pore.
2
Translation takes place on ribosomes on the RER, and the polypeptide passes into the cisternae.
3
Transport vesicles bud off and carry it to the Golgi apparatus, where it is modified and packaged into secretory vesicles.
4
These move along the cytoskeleton, fuse with the cell surface membrane and release the enzyme by exocytosis.
5
Mitochondria supply the ATP for the whole sequence.
6
Cells that secrete a lot of protein, such as pancreatic acinar cells, therefore have extensive RER and Golgi and many mitochondria.
Section 6 of 9Related structures and exam links
1
The nuclear envelope is continuous with the endoplasmic reticulum in many cells,
- so the space between the two membranes is connected to the ER lumen.
- Mitochondria and chloroplasts are thought to have evolved from engulfed prokaryotes,
- which explains their double membranes, circular DNA, 70S ribosomes and ability to divide independently.
- (the endosymbiont theory)
2
When describing a function, aim for a chain of events rather than a label.
- For example, instead of 'the Golgi packages proteins', say that it modifies the protein, sorts it and packages it into vesicles for transport to the cell surface membrane.
- Remember that a vesicle is not an organelle with a fixed function in itself, but a transport container.
3
Link also to cell fractionation: homogenisation breaks the cells open in cold, isotonic, buffered solution,
- and differential centrifugation separates the organelles in order of density, nuclei first,
- then mitochondria and so on.
4
Summary of key comparisons: the nucleus stores and protects DNA; the nucleolus builds ribosomes; the rough ER and Golgi make and process proteins; the smooth ER handles lipids; mitochondria release energy as ATP; lysosomes digest; the cytoskeleton gives shape and movement.
Related structures and exam links
Plant cells add a wall, chloroplasts and a large vacuole.
Learn to describe each organelle with its structure, then a function, then one named cell type in which it is especially abundant.
Section 7 of 9Exam-style questions
Describe the roles of the rough endoplasmic reticulum and the Golgi apparatus in the production of a secreted enzyme. [4]4 marks
11. Ribosomes on the RER translate mRNA to make the polypeptide. [1]22. The polypeptide enters the RER cisternae, where it is folded. [1]33. It is carried in vesicles to the Golgi apparatus, which modifies it (for example adds carbohydrate) [1]44. The Golgi packages it into secretory vesicles that fuse with the cell surface membrane to release it by exocytosis. [1]Explain why a liver cell contains many mitochondria and a well-developed smooth endoplasmic reticulum. [3]3 marks
Exam-style questions1. Liver cells are metabolically very active, so need a lot of ATP from aerobic respiration, made in mitochondria. [1]2. Cristae give a large surface area for ATP synthesis. [1]3. Smooth ER synthesises lipids and is involved in detoxification and lipid and cholesterol metabolism, so a large amount is needed. [1]State two structures found in a palisade mesophyll cell but not in a cheek epithelial cell and give the function of each. [4]4 marks
Cellulose cell wall: prevents bursting and gives support. [2]
Chloroplast (or large vacuole): site of photosynthesis (or maintains turgor and stores sap). [2]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Organelle | A specialised, usually membrane-bound structure inside a cell with a particular function. |
| Compartmentalisation | Separation of cell activities into membrane-bound areas so that different conditions can be maintained. |
| Cristae | Folds of the inner mitochondrial membrane that increase surface area. |
| Cisternae | Flattened membrane sacs of the ER and Golgi apparatus. |
| Tonoplast | The membrane surrounding the plant vacuole. |
| Plasmodesmata | Narrow cytoplasmic channels through plant cell walls linking adjacent cells. |
| Chromatin | DNA associated with histone proteins in the nucleus. |
Section 9 of 9Exam technique
1
Saying ribosomes have a membrane. Ribosomes are not membrane-bound, so they are not true organelles in the membrane sense.
2
Giving 'powerhouse of the cell' without detail. Mark schemes want cristae, ATP made by aerobic respiration and the matrix enzymes.
3
Saying plant cells have no mitochondria. They have both mitochondria and chloroplasts.
4
Confusing the nucleolus with the nucleusthe nucleolus makes rRNA and ribosomes, not mRNA.
5
Stating that lysosomes are produced in the nucleus. They are formed from Golgi vesicles.
Marked practiceQuizzes for this lesson
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