Structure of Eukaryotic Cells
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
The structure of eukaryotic cells, restricted to the structure and function of:
- cell-surface membrane
- nucleus (containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleoli)
- mitochondria
- chloroplasts (in plants and algae)
- Golgi apparatus and Golgi vesicles
- lysosomes (a membrane-bound organelle that releases hydrolytic enzymes)
- ribosomes
- rough endoplasmic reticulum and smooth endoplasmic reticulum
- cell wall (in plants, algae and fungi)
- cell vacuole (in plants)
Eukaryotic cells are more complex than prokaryotes and have membrane-bound organelles and other specialised structures. Common examples of eukaryotic organisms include: plants, animals, fungi and algae.
What does membrane-bound mean?
What does membrane-bound mean?
Surrounded by a membrane, separating the organelle’s contents from the cytoplasm. For example, the nucleus is surrounded by a double membrane (nuclear envelope) and mitochondria have their own double membrane. Ribosomes are not membrane-bound.
Overview of a plant cell
Organelles: Structure and Function
Cell-Surface Membrane
Structure:
- Phospholipid bilayer with embedded proteins
- Contains glycoproteins, glycolipids and cholesterol
- Fluid-mosaic model: components can move within the bilayer
Function:
- Partially permeable: controls which substances enter and leave the cell
- Cell signalling via receptor proteins
- Cell recognition via antigens on the surface
Nucleus
Structure:
- Largest organelle, surrounded by a double membrane (nuclear envelope)
- Nuclear envelope has nuclear pores for transport of mRNA and ribosome subunits
- Contains chromatin (protein-bound, linear DNA) and one or more nucleoli
Function:
- Holds the genetic information (DNA) that codes for polypeptides (proteins)
- Site of DNA replication and transcription (mRNA synthesis)
- Nuclear pores allow selective transport between nucleus and cytoplasm
- The nucleolus produces ribosomal RNA (rRNA) and assembles ribosome subunits
Mitochondria
Structure:
- Double-membrane organelle
- Inner membrane folded into cristae (increases surface area)
- Matrix contains enzymes and its own circular DNA and 70S ribosomes
Function:
- Site of aerobic respiration: produces ATP
- Krebs cycle occurs in the matrix
- Oxidative phosphorylation occurs on the inner membrane (cristae)
Chloroplasts
Structure:
- Double-membrane organelle (plants and algae only)
- Contains thylakoid membranes arranged into grana (stacks)
- Stroma is the fluid-filled matrix; contains own circular DNA and 70S ribosomes
Function:
- Site of photosynthesis
- Light-dependent reactions occur on the thylakoid membranes
- Light-independent reactions (Calvin cycle) occur in the stroma
Rough Endoplasmic Reticulum (RER)
Structure:
- System of flattened membrane-bound sacs (cisternae)
- Studded with 80S ribosomes on its outer surface
- Continuous with the nuclear envelope
Function:
- Ribosomes on the RER synthesise proteins
- Proteins are folded and processed inside the cisternae
- Proteins transported in vesicles to the Golgi apparatus
Smooth Endoplasmic Reticulum (SER)
Structure:
- System of membrane-bound tubules
- No ribosomes on its surface
- Connected to the rough ER
Function:
- Synthesises and processes lipids and carbohydrates
Golgi Apparatus
Structure:
- Stack of flattened, membrane-bound sacs (cisternae)
- Has a cis face (receiving side) and trans face (shipping side)
- Surrounded by Golgi vesicles
Function:
- Modifies proteins received from the RER (e.g. adds carbohydrates to form glycoproteins)
- Packages proteins into vesicles
- Sorts proteins for secretion, delivery to lysosomes, or insertion into membranes
Golgi Vesicles
Structure:
- Small, spherical, membrane-bound sacs
- Bud off from the trans face of the Golgi apparatus
Function:
- Transport modified proteins and lipids to the cell-surface membrane
- Release contents by exocytosis (secretion)
- Some vesicles become lysosomes
Lysosomes
Structure:
- Membrane-bound vesicle formed from Golgi vesicles
- Contains powerful hydrolytic (digestive) enzymes
- Membrane keeps enzymes safely separated from the cytoplasm
Function:
- Breaks down worn-out organelles (autophagy)
- Digests pathogens engulfed by phagocytosis
- Involved in apoptosis (programmed cell death)
Ribosomes
Structure:
- Tiny organelles made of ribosomal RNA (rRNA) and protein
- Not membrane-bound
- Eukaryotic = 80S; prokaryotic = 70S
Function:
- Site of translation: mRNA is translated to synthesise polypeptides
- Free ribosomes: make proteins for use inside the cell
- Bound ribosomes (on RER): make proteins for secretion
Cell Wall
Structure:
- Rigid layer surrounding the cell-surface membrane
- Made of cellulose (a polysaccharide of β-glucose) in plants and algae; chitin in fungi
- Freely permeable to water and dissolved substances
Function:
- Provides structural support and maintains cell shape
- Prevents the cell from bursting (lysis) when water enters by osmosis
- Found in plants, algae and fungi (not animal cells)
Cell Vacuole (Plants)
Structure:
- Large, permanent, membrane-bound organelle in plant cells
- Filled with cell sap (sugars, amino acids, mineral ions, pigments)
Function:
- Maintains turgor pressure by osmosis, which keeps the cell rigid
- Stores substances and waste products
- May contain hydrolytic enzymes
How a cell makes and secretes a protein
In Unit 4 (DNA and Protein Synthesis), you will learn about the processes involved in making proteins (transcription and translation). The steps below show where this happens in the cell, using an enzyme as an example.
- Nucleus: the gene (DNA) for the enzyme is transcribed into mRNA, which leaves through a nuclear pore.
- Ribosomes on the rough ER: the mRNA is translated and the polypeptide is made.
- Rough ER: the polypeptide is folded and processed, then carried to the Golgi apparatus in a vesicle.
- Golgi apparatus: the protein is modified (e.g. carbohydrate added to form a glycoprotein) and packaged into vesicles.
- Golgi vesicles: they carry the protein to the cell-surface membrane, fuse with it and release the protein by exocytosis.
Throughout this process, the mitochondria provide the ATP needed, e.g. for vesicle transport and exocytosis.
Cell Specialisation Note
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into tissues, tissues into organs and organs into systems.
Students should be able to apply their knowledge of these features in explaining adaptations of eukaryotic cells.
All cells in an organism contain the same genes. This means your muscle cell contains the same genes (DNA) as your skin cell. But why do they look and function so differently?
The answer is cell specialisation, whereby different genes are expressed (or not expressed) in different cell types. This is covered in Stem Cells and Cell Specialisation.
So in multicellular organisms (organisms made of many cells, like us), different cells have specific structural features that make them better adapted for their function.
Why would a muscle cell have lots of mitochondria?
Why would a muscle cell have lots of mitochondria?
Muscle cells need to contract, which requires a lot of ATP. Mitochondria are the site of aerobic respiration, so having many mitochondria means the cell can produce ATP at a high rate to meet its energy demands.
Levels of Organisation
To work efficiently, specialised cells are often organised into a hierarchy:
- Cells: the basic unit of life
- Tissues: a group of similar cells working together to carry out a particular function (e.g. epithelial tissue)
- Organs: a group of different tissues working together (e.g. the stomach contains muscle tissue, epithelial tissue, glandular tissue)
- Organ systems: a group of organs working together (e.g. the digestive system)
How this topic is tested
This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.
- Tested in 6 of 9 years (2017–2025): 14 question parts worth 31 marks.
- 32nd most-examined topic overall by marks, 5th in Unit 2.
Marks by year
Most-tested spec points
- Organelle Structure & Function: tested in 11 parts (28 marks)
- Cell Specialisation & Organisation: tested in 1 part (1 marks)
Also links to: Prokaryotic Cell Structure.
Maths and practical skills
- Percentages, ratios and fractions (MS 0.3): e.g. 2025 P1 Q9.4
Tips from examiner reports
What students commonly get wrong
- The nucleolusWatch out: the nucleolus does not contain all the cell's DNA. Its job is to make rRNA and assemble ribosomes. 2022 P1 Q1.1
- Lysosome vs lysozymeWatch out: a lysosome is an organelle, and lysozymes are the enzymes inside it, so don't mix up the two words. A lysosome is also not a cell, so it can't engulf anything: instead, a vesicle fuses with the lysosome and its hydrolytic enzymes digest the contents. 2023 P3 Q4.1
- How vesicles release a proteinWhen describing how a cell secretes an enzyme, say the vesicles fuse with the cell-surface membrane and release it by exocytosis. "Takes it out of the cell" or "to where it's needed" isn't enough. 2024 P1 Q10.2
Exam Question Practice
A few exam-style questions on this topic to try. Have a go at each one before opening the answer.
Describe the structure of the nucleus, and the functions it carries out in a eukaryotic cell.
(4 marks)
Hint
Think about what you’d see inside the nucleus, then what happens there: what is DNA used to make, and what processes copy or read it?
Mark Scheme
Max 4 marks: max 2 for structure, max 2 for function
Structure
- A nuclear envelope (double membrane) with pores (1 mark)
- Chromosomes (chromatin): DNA associated with histones (1 mark)
- A nucleolus (1 mark)
Function
- Holds the genetic information that codes for polypeptides (1 mark)
- DNA replication takes place here (1 mark)
- Transcription (making mRNA) takes place here (1 mark)
- The nucleolus makes rRNA (ribosomes) (1 mark)
Also accepted / rejected
Tips
- Give at least two functions as well as structures: you can only score 2 marks from structure alone
- For function, say the DNA codes for polypeptides, and name processes such as DNA replication, transcription or making ribosomes; “controls cell activities” is GCSE level
- The nucleolus makes ribosomes (rRNA); it doesn’t hold all the cell’s DNA
The Golgi apparatus is an organelle found in eukaryotic cells.
Describe two functions of the Golgi apparatus.
(2 marks)
Hint
Give two DIFFERENT functions. Processing and packaging are related: consider what else the Golgi does.
Mark Scheme
Max 2 marks
- Modifies and packages proteins, e.g. adds carbohydrate to make glycoproteins (1 mark)
- Modifies and packages lipids, e.g. makes glycolipids (1 mark)
- Forms vesicles or lysosomes (1 mark)
Also accepted / rejected
Also accepted
- “Processes” for modifies, or “transports” (points 1 and 2)
- Makes or packages polysaccharides (not cellulose), as an extra point
Not accepted
- Two answers that describe the same function, e.g. “processes lipids” and “makes glycolipids”
Tips
- Give two different functions: e.g. modifying/packaging proteins, and forming vesicles or lysosomes
- “Processing lipids” and “producing glycolipids” are the same function, so they only score once
- The Golgi apparatus doesn’t make ribosomes
When a cell no longer needs one of the receptor proteins in its cell-surface membrane, the receptor is taken into the cell inside a vesicle. The receptor is then destroyed by a lysosome.
Suggest how the receptor protein is destroyed by a lysosome.
(2 marks)
Hint
The receptor ends up inside a vesicle. What does a lysosome contain, and how does it meet the vesicle?
Mark Scheme
- (The vesicle) fuses/binds with a lysosome (1 mark)
- Lysozyme(s) hydrolyse/digest (the receptor)
OR Hydrolytic enzyme(s) break down/hydrolyse/digest (the receptor) (1 mark)
Also accepted / rejected
2. Accept protease hydrolyses/digests (the receptor).
Tips
- The receptor is enclosed in a vesicle, so the vesicle fuses with a lysosome and its enzymes hydrolyse the receptor
- Don’t describe phagocytosis, antigens or osmotic lysis: a lysosome is an organelle, not a macrophage
- Don’t mix up lysosome (the organelle) and lysozyme (an enzyme)
Cells in the pancreas make digestive enzymes and release them into the small intestine.
Describe the role of organelles in the production and release of these enzymes.
Do not include details of transcription in your answer.
(5 marks)
Hint
Follow the pathway from gene to secreted enzyme. Which organelle is involved at each step? Don’t forget energy requirements.
Mark Scheme
Max 5 marks
- DNA in the nucleus codes for the enzyme (1 mark)
- Ribosomes on the rough endoplasmic reticulum make the enzyme (translation) (1 mark)
- The rough endoplasmic reticulum transports and modifies the enzyme (1 mark)
- Mitochondria make the ATP needed (1 mark)
- The Golgi apparatus modifies and packages the enzyme into vesicles (1 mark)
- Vesicles move to and fuse with the cell-surface membrane, releasing the enzyme (exocytosis) (1 mark)
Also accepted / rejected
Also accepted
- “Protein” or “polypeptide” instead of “enzyme” (all points)
- “Gene” or “genetic code” for DNA (point 1)
- “Golgi body” (point 5)
Not accepted
- Mitochondria “producing energy” (point 4)
Tips
- Focus on organelles: nucleus (DNA codes for the enzyme), ribosomes/rough ER (translation), rough ER (transport), Golgi apparatus (modifies and packages), vesicles fuse with the cell-surface membrane
- Include mitochondria producing ATP (not “producing energy”)
- Say vesicles fuse with the cell-surface membrane, not just take the enzyme “where it is needed”
- Don’t describe translation or protein structure in detail, or phagocytosis and DNA replication
What earns marks
- Ribosomes, packaging in the Golgi apparatus and exocytosis are the most common marks
Scientists investigated cells taken from the stomach wall of mice.
The stomach wall contains several different types of specialised cells.
Name the level of organisation made of specialised cells that are:
- all of one type
- of more than one type.
(1 marks)
Hint
Think about the levels of organisation in a multicellular organism. What is a group of one type of specialised cell called, and what is a structure made of several types called?
Mark Scheme
A tissue (all one type) and an organ (more than one type) (1 mark, both needed, in this order)
Tips
- Cells of one type form a tissue; cells of more than one type form an organ. Give them in this order
- Don’t use terms such as unicellular, multicellular, organ system, genus or phylum
Also accepted