Every tissue and organ in a living organism is built from cells, but cells are not all identical. A leaf cell absorbing light, a root hair cell taking up water and a red blood cell carrying oxygen face very different demands. Their structures therefore differ in ways that are directly related to their functions.
This topic is easiest when you keep asking one question: what job does this structure help the cell perform? Once the function is clear, the names of the organelles stop feeling like an isolated vocabulary list.
Diagnostic Quiz
What Do You Already Know About Plant and Animal Cells?
Try these five questions before reading. They check whether you can connect a cell structure to its function and recognise the first signs of cell specialisation.
5 diagnostic questions
Core cell structures and functions
Instant explanation for every answer option
Question 1 of 5
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Which cell structure controls the movement of substances into and out of a cell?
Cell wall
The cell wall supports a plant cell, but it is not the structure that selectively controls entry and exit.
Cell membrane
Correct. The cell membrane is partially permeable and regulates what passes into and out of the cell.
Cytoplasm
The cytoplasm is the site of many metabolic reactions; it does not form the cell's selective boundary.
Nucleus
The nucleus contains genetic material and controls cell activities, but it does not control movement across the cell boundary.
The key distinction is boundary versus control. A plant cell has both a cell wall and a cell membrane, but the membrane is the selectively controlling boundary.
Which structure contains chlorophyll and absorbs light energy for photosynthesis?
Chloroplast
Correct. Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis.
Mitochondrion
Mitochondria are sites of aerobic respiration, not photosynthesis.
Ribosome
Ribosomes are sites of protein synthesis.
Golgi body
The Golgi body modifies and packages substances such as proteins.
Chloroplasts are associated with photosynthesis. They are present in photosynthetic plant cells, but not in every plant cell.
What is the main function of mitochondria?
To make proteins
Proteins are assembled at ribosomes.
To store cell sap
Cell sap is stored in the large vacuole of a typical plant cell.
To carry out aerobic respiration
Correct. Mitochondria are the sites of aerobic respiration, where energy is released for cell activities.
To control cell division
The nucleus contains genetic information and is involved in controlling cell activities.
Cells that need a large supply of energy, such as muscle cells, usually contain many mitochondria because aerobic respiration occurs there.
Which statement correctly compares vacuoles in typical plant and animal cells?
Only plant cells have vacuoles.
Animal cells can have vacuoles, although they are usually smaller and temporary.
Plant cells usually have one large sap-filled vacuole, while animal cells may have small temporary vacuoles.
Correct. This is the standard O-Level comparison between typical plant and animal cells.
Animal cells have one large permanent vacuole, while plant cells have many small vacuoles.
This reverses the usual pattern.
Vacuoles are present only in cells with chloroplasts.
Vacuoles and chloroplasts are different structures; a plant cell can have a vacuole even if it has no chloroplasts.
Do not use 'vacuole present or absent' as the plant-versus-animal rule. The useful comparison is large and sap-filled in typical plant cells versus small and temporary in animal cells.
A root hair cell has a long, narrow extension. What is the main advantage of this shape?
It gives a larger surface area for absorption.
Correct. The extension greatly increases the surface area of cell membrane available for absorbing water and mineral ions from the soil.
It provides more space for haemoglobin.
Haemoglobin is found in red blood cells, not root hair cells.
It prevents water from entering the cell.
Root hair cells are specialised for uptake from the soil, so preventing water entry would oppose their function.
It allows the cell to carry out photosynthesis faster.
Root hair cells are usually underground and are specialised for absorption rather than photosynthesis.
Specialised cells illustrate the principle that structure is related to function: the root hair cell's long extension increases the membrane area available for absorption.
The Cell as the Basic Unit of Life
A cell is the smallest unit of an organism that can carry out the processes needed for life. Inside a cell, different structures carry out different roles. Some of these structures are called organelles: specialised parts of the cell that perform particular functions.
At O-Level, it is useful to separate the structures you may recognise using a light microscope from the smaller organelles that are usually identified from electron micrographs. This is because electron microscopes have much greater resolving power and can reveal much finer internal detail.
Structure and function are linked. If a cell needs to carry out a particular process rapidly or efficiently, you should expect its structure to be adapted to that demand.
Structures of Typical Plant and Animal Cells
The six structures below form the foundation of cell comparison: cell wall, cell membrane, cytoplasm, nucleus, vacuoles and chloroplasts. You should be able to identify them from diagrams and suitable light-microscope images, and state what each does.
Cell membrane
The cell membrane forms the living boundary of the cell. It is partially permeable, meaning that some substances can cross it more readily than others. Its key role is to control the movement of substances into and out of the cell.
A plant cell has a cell membrane too. The membrane lies just inside the cell wall, so the presence of a wall does not remove the need for a selectively controlling membrane.
Cytoplasm
The cytoplasm is the cell material in which many metabolic reactions take place. It contains the enzymes and dissolved substances needed for many of the chemical reactions that keep the cell functioning.
Nucleus
The nucleus contains the cell's genetic material in chromosomes and controls the activities of the cell. It is therefore closely linked to processes such as growth, protein production and cell division.
Do not turn this into an absolute rule that every animal cell must have a nucleus. A mature red blood cell is a notable exception because it loses its nucleus as part of its specialisation.
Cell wall
The cell wall of a plant cell is made mainly of cellulose. It lies outside the cell membrane and provides mechanical support, helps the cell maintain its shape and helps prevent excessive expansion when water enters the cell.
Cell wall: provides support and shape; it is outside the membrane and is not the main selective barrier.
Cell membrane: controls movement of substances into and out of the living cell.
Vacuoles
A typical plant cell usually contains one large, sap-filled vacuole. The cell sap is a watery solution containing dissolved substances. When the vacuole is filled with water, it helps press the cell contents against the cell wall and contributes to the firmness of plant tissues.
Animal cells may also contain vacuoles, but these are generally small and temporary. Therefore, saying that “animal cells have no vacuoles” is too absolute.
Chloroplasts
Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis. They are found in photosynthetic plant cells, such as many cells in leaves.
However, not every plant cell contains chloroplasts. Root hair cells, for example, are normally underground and are specialised for absorption rather than for receiving light.
Typical plant cell — structural overview
Use the numbered markers to identify the O-Level structures:
Marker
Structure
1
Cell wall
2
Cell membrane
3
Nucleus
4
Large sap-filled vacuole
5
Chloroplast
6
Mitochondrion
7
Ribosomes
8
Endoplasmic reticulum
9
Golgi body
Cytoplasm: this is the material inside the cell membrane that surrounds the organelles. Because cytoplasm fills the spaces around many structures, it is explained here rather than marked by one numbered point.
O-Level focus: learn the cell wall, cell membrane, cytoplasm, nucleus, large sap-filled vacuole, chloroplasts, mitochondria, ribosomes, endoplasmic reticulum and Golgi body. The illustration contains additional biological detail, but unnumbered structures are not the focus of this topic.
Simplified labelled teaching view; structures are not drawn to exact scale.
Plant_cells icon by SwissBioPics, adapted for Reviewston and licensed under CC BY 4.0.
Typical animal cell — verified structural overview (schematic, not to scale)
How to read this overview: the large organelle near the centre is the nucleus; mitochondria are the oval or bean-shaped organelles with folded inner membranes; the endoplasmic reticulum forms an extensive membrane network around the nucleus; and the Golgi body appears as a stack of curved, flattened membrane sacs. The cell membrane forms the cell boundary, while the cytoplasm fills the cell interior.
Microscope note: These diagrams are structural overviews, not literal light-microscope views. Larger features such as the cell wall, nucleus, vacuole and chloroplasts may be identified in suitable light-microscope preparations, but smaller organelles such as mitochondria, ribosomes, endoplasmic reticulum and Golgi bodies are studied much more clearly using electron micrographs.
Organelles Seen More Clearly with an Electron Microscope
Some cell structures are too small to be resolved clearly with the school light microscope. Electron micrographs reveal much finer detail, so you should also be able to recognise and state the functions of mitochondria, ribosomes, endoplasmic reticulum and the Golgi body.
Mitochondria
Mitochondria are the sites of aerobic respiration. During aerobic respiration, energy is released from food molecules for use in cell activities.
Cells with high energy demands often contain many mitochondria. Muscle cells are a classic example because repeated contraction requires a substantial supply of energy.
Exam wording: it is better to say that mitochondria are the sites of aerobic respiration, where energy is released, than to write simply that mitochondria “make energy”.
Ribosomes
Ribosomes are the sites of protein synthesis. They assemble amino acids into proteins according to genetic instructions.
Ribosomes may be found free in the cytoplasm or attached to the endoplasmic reticulum. When ribosomes are attached, the membrane system is commonly described as rough endoplasmic reticulum.
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a network of membranes within the cell. It helps to transport substances through the cell. Rough ER has ribosomes attached and is especially associated with proteins that will be processed and transported.
Golgi body
The Golgi body consists of stacks of flattened membrane sacs. It receives substances such as proteins from the ER, modifies and packages them, and forms vesicles that can carry the packaged materials to other parts of the cell or towards the cell surface for secretion.
Ribosome: makes the protein.
Rough ER: helps process and transport newly made proteins.
Golgi body: modifies, sorts and packages proteins into vesicles.
Comparing Typical Plant and Animal Cells
Plant and animal cells have more in common than they have differences. Both are eukaryotic cells, so both contain a nucleus and membrane-bound organelles such as mitochondria, ER and Golgi bodies. The exam skill is to recognise the distinguishing structures without turning a typical diagram into an absolute rule for every real cell.
Structure
Typical plant cell
Typical animal cell
Cell membrane
Present, just inside the cell wall
Present; forms the outer living boundary
Cytoplasm
Present
Present
Nucleus
Present in a typical living plant cell
Present in a typical animal cell, with specialised exceptions such as mature red blood cells
Cellulose cell wall
Present
Absent
Chloroplasts
Present in photosynthetic plant cells; absent from many non-photosynthetic plant cells
Absent
Vacuoles
Usually one large sap-filled vacuole
May have small temporary vacuoles
Mitochondria
Present
Present
Ribosomes
Present
Present
Endoplasmic reticulum
Present
Present
Golgi body
Present
Present
How to Read Cell Diagrams and Micrographs
Real cells rarely look as tidy as textbook diagrams. Organelles may be cut at different angles, boundaries may be faint, and staining may make some structures much more obvious than others. Instead of relying only on overall shape, look for diagnostic features.
Clue
What it suggests
Why
Thick outer boundary with a thinner membrane just inside
Plant cell
The thick boundary is the cellulose cell wall.
Large clear central region occupying much of the cell
Large plant-cell vacuole
The vacuole can push the cytoplasm and nucleus towards the edge.
Green oval structures in a suitable plant specimen
Chloroplasts
Chlorophyll gives chloroplasts their green appearance.
Small oval organelle with folded inner membranes in an electron micrograph
Mitochondrion
The folds are cristae of the inner mitochondrial membrane.
Stack of flattened membrane sacs with small vesicles nearby
Golgi body
This is the characteristic stacked appearance of the Golgi apparatus.
Membrane network, often with small dots attached
Rough ER
The dots are ribosomes attached to the ER membrane.
Specialised Cells: Structure Fits Function
In a multicellular organism, different cells become specialised for different jobs. Specialisation means that a cell develops features that make it especially effective at one function. The important examination skill is to connect each structural feature to a clear advantage.
Muscle cells
Muscle cells contract to produce movement. Contraction requires a substantial and repeated supply of energy, so muscle cells contain many mitochondria. More mitochondria provide more sites for aerobic respiration.
Good explanation: “A muscle cell contains many mitochondria, so it can carry out aerobic respiration at a high rate and release the energy needed for contraction.”
Root hair cells
Root hair cells absorb water and mineral ions from the soil. Each cell has a long, narrow extension that greatly increases its surface area of cell membrane. This gives a larger area across which absorption can occur.
Root hair cells also commonly have many mitochondria because mineral-ion uptake may require active transport. The transport mechanism itself is developed in the next topic, but the structural idea is already useful here: cells that carry out energy-demanding transport benefit from having many mitochondria.
Red blood cells
Mature red blood cells transport oxygen. They contain the protein haemoglobin, which binds oxygen. Their biconcave shape gives a large surface-area-to-volume ratio and a short diffusion distance, helping oxygen move in and out efficiently.
They also lack a nucleus, leaving more internal space for haemoglobin. Their flexible shape helps them pass through narrow capillaries.
Specialised cell
Structural adaptation
How the adaptation helps
Muscle cell
Many mitochondria
Provides many sites for aerobic respiration to release energy for contraction.
Root hair cell
Long extension
Increases surface area of cell membrane for absorption from the soil.
Red blood cell
No nucleus when mature
Leaves more space for haemoglobin and therefore oxygen transport.
Red blood cell
Biconcave and flexible
Provides a large surface-area-to-volume ratio, short diffusion distance and allows passage through narrow capillaries.
Common Mistakes
Common mistake
Why it is wrong
Better approach
“Animal cells do not have vacuoles.”
Animal cells may have small temporary vacuoles.
Compare the usual size and permanence: large sap-filled vacuole in a typical plant cell; small temporary vacuoles in animal cells.
“The cell wall controls what enters and leaves a plant cell.”
The wall mainly provides support and is porous. Selective movement is controlled by the cell membrane.
Keep the functions separate: wall for support; membrane for selective control.
“All plant cells contain chloroplasts.”
Only photosynthetic plant cells need chloroplasts. Root hair cells usually do not have them.
Write “photosynthetic plant cells contain chloroplasts” unless the question clearly shows a typical leaf cell.
“Mitochondria make energy.”
Energy is not created by the organelle.
State that mitochondria are the sites of aerobic respiration, where energy is released for cell activities.
“The Golgi body makes proteins.”
Proteins are assembled at ribosomes.
Use the sequence: ribosome makes → ER processes/transports → Golgi modifies/packages.
“Plant cells are rectangular and animal cells are round.”
Real cells have many shapes depending on their function and position.
Identify cells from structures such as the cell wall, chloroplasts and vacuole, not from outline alone.
What you should be able to do
Identify core cell structures
Recognise the cell wall, cell membrane, cytoplasm, nucleus, vacuoles and chloroplasts in suitable diagrams and light-microscope images.
State functions accurately
Give the function of each cell structure using precise biological language rather than vague descriptions.
Recognise organelles in electron micrographs
Identify mitochondria, ribosomes, endoplasmic reticulum and Golgi bodies from their characteristic appearance.
Compare plant and animal cells
State both similarities and differences without relying on oversimplified rules such as “animal cells have no vacuoles”.
Link structure to function
Explain how muscle cells, root hair cells and red blood cells are adapted to their particular jobs.
Interpret unfamiliar cells
Use a set of observed features to infer the likely cell type and explain what those features reveal about function.
Mastery Quiz
Can You Apply Cell Structure and Specialisation?
These ten questions test whether you can compare plant and animal cells, interpret organelle clues and explain how specialised cells are adapted to their functions.
10 mastery questions
Plant, animal and specialised-cell reasoning
Instant explanation for every answer option
Question 1 of 10
Quiz complete
How did you do?
A cell has a cellulose cell wall, chloroplasts and a large sap-filled vacuole. What is the best conclusion?
It is a typical plant cell.
Correct. The combination of a cellulose cell wall, chloroplasts and a large sap-filled vacuole identifies a typical photosynthetic plant cell.
It is a typical animal cell.
Animal cells do not have cellulose cell walls or chloroplasts.
It must be a red blood cell.
A mature red blood cell lacks a nucleus and does not have these plant-cell structures.
It must be a muscle cell.
Muscle cells are animal cells and do not have cell walls or chloroplasts.
One feature alone can sometimes mislead, but a set of features is powerful. Cell wall + chloroplasts + large sap-filled vacuole is the typical plant-cell combination.
An electron micrograph shows an organelle with a double membrane and many folds of the inner membrane. Which organelle is it, and what happens there?
Chloroplast; protein synthesis
Chloroplasts carry out photosynthesis and contain internal membrane systems, but they are not the organelle described here for aerobic respiration.
Mitochondrion; aerobic respiration
Correct. Mitochondria have an inner membrane folded into cristae and are sites of aerobic respiration.
Golgi body; photosynthesis
The Golgi body is a stack of flattened membrane sacs and does not carry out photosynthesis.
Ribosome; aerobic respiration
Ribosomes are much smaller particles and are sites of protein synthesis.
Electron micrographs can reveal ultrastructure that cannot be resolved clearly with a school light microscope. Folded inner membranes are a strong clue for mitochondria.
A gland cell secretes large amounts of a protein enzyme. Which pair of structures would you expect to be especially well developed?
Rough endoplasmic reticulum and Golgi body
Correct. Rough endoplasmic reticulum is closely associated with the production and transport of proteins, while the Golgi body modifies and packages them for secretion.
Cell wall and chloroplasts
These are plant-cell structures and are not the main organelles involved in protein secretion.
Large central vacuole and chloroplasts
These structures are associated with typical plant cells and do not explain high protein secretion.
Mitochondria and cell wall
Mitochondria release energy by aerobic respiration, but a cell wall is not part of the protein-processing and secretion pathway and is absent from animal gland cells.
Protein-secreting cells need a well-developed protein-handling pathway. Ribosomes on rough ER make the protein, the rough ER helps transport it, and the Golgi body modifies and packages it into vesicles for secretion.
Why does a plant cell need a cell membrane even though it already has a cell wall?
The cell wall is completely impermeable.
The cell wall is porous; it is not an impermeable barrier.
The membrane controls movement of substances, whereas the wall mainly provides support and protection.
Correct. The membrane is partially permeable and controls entry and exit, while the wall provides mechanical support and helps maintain shape.
The membrane makes cellulose for the wall.
Cellulose is part of the cell wall, but this is not the reason the membrane is required as a selective boundary.
The membrane contains chlorophyll for photosynthesis.
Chlorophyll is found in chloroplasts, not in the cell membrane.
The cell wall and cell membrane have different jobs. Support does not replace selective control of what crosses into and out of the living cell.
Which adaptation of a mature red blood cell allows it to carry more oxygen?
It has many chloroplasts.
Red blood cells are animal cells and do not have chloroplasts.
It has a large central vacuole.
Red blood cells do not have the large central vacuole typical of plant cells.
It lacks a nucleus, leaving more space for haemoglobin.
Correct. Loss of the nucleus leaves more internal space for haemoglobin, which binds oxygen.
It has a cellulose cell wall.
Animal cells do not have cellulose cell walls.
A mature red blood cell is a useful exception to the simplified idea that 'animal cells have a nucleus'. Its lack of a nucleus is an adaptation for oxygen transport.
Why do muscle cells usually contain many mitochondria?
They need a high rate of aerobic respiration to release energy for contraction.
Correct. Contraction requires a large supply of usable energy, so muscle cells contain many mitochondria for aerobic respiration.
They need to photosynthesise during exercise.
Muscle cells are animal cells and do not photosynthesise.
They need to store large amounts of cell sap.
Large sap-filled vacuoles are characteristic of typical plant cells, not muscle cells.
They need to make a cellulose cell wall.
Animal cells do not make cellulose cell walls.
Whenever a specialised cell has a high energy demand, many mitochondria are a logical adaptation because mitochondria are the sites of aerobic respiration.
Which feature of a root hair cell most directly increases the rate at which it can absorb water and mineral ions from the soil?
A long extension that increases surface area
Correct. The root hair extension greatly increases the surface area of cell membrane in contact with soil water.
A biconcave shape
A biconcave shape is an adaptation of red blood cells.
A lack of a nucleus
Root hair cells normally have a nucleus; lacking one is not their key adaptation.
A thick cellulose wall around the entire root
A thick wall around the whole root would not explain the specialised absorbing surface of the root hair cell.
The important phrase is surface area of cell membrane. More membrane in contact with soil solution provides more area across which absorption can occur.
Which statement about typical plant cells is correct?
Every plant cell contains chloroplasts.
Many plant cells, especially cells in non-photosynthetic tissues such as roots, do not contain chloroplasts.
The cell membrane lies just inside the cell wall.
Correct. In a typical plant cell, the cellulose cell wall is outside the cell membrane.
The cell wall controls which dissolved substances enter the cell.
Selective control of entry and exit is mainly a function of the cell membrane, not the cell wall.
The nucleus occupies most of the cell because plant cells have no vacuole.
Typical plant cells often have a large central vacuole, which can push the nucleus towards the edge of the cell.
Remember the order at the boundary of a plant cell: cell wall outside, cell membrane just inside. Chloroplasts are not universal to all plant cells.
Which structure is the direct site of protein synthesis?
Ribosome
Correct. Ribosomes assemble amino acids into proteins.
Golgi body
The Golgi body modifies and packages proteins after they have been made; it is not the site where the polypeptide chain is assembled.
Mitochondrion
Mitochondria are sites of aerobic respiration.
Vacuole
Vacuoles are storage compartments; they are not sites of protein synthesis.
A common exam trap is to confuse the protein-handling organelles. Ribosomes make proteins, rough ER helps transport/process them, and Golgi bodies modify and package them.
A student says, “Animal cells do not have vacuoles.” Which correction is best?
Animal cells have one large permanent vacuole.
One large permanent sap-filled vacuole is typical of a plant cell, not an animal cell.
Animal cells may have small temporary vacuoles.
Correct. Animal cells may contain small, temporary vacuoles.
Only red blood cells have vacuoles.
Vacuoles are not exclusive to red blood cells.
Vacuoles occur only when chloroplasts are present.
Vacuoles and chloroplasts are different cell structures; one does not require the other.
For O-Level comparison, state the difference precisely: plant cells usually have a large sap-filled vacuole, while animal cells may have small temporary vacuoles.
Frequently asked questions
Plant and Animal Cells: Questions Students Often Ask
Once the basic organelles are familiar, most confusion comes from rules that were memorised too rigidly. These questions help refine the distinctions that matter in examination answers.
Do all plant cells have chloroplasts?
No. Chloroplasts are needed in photosynthetic cells, so they are common in green parts of plants such as leaves. Many cells in roots, including root hair cells, normally do not contain chloroplasts because they are not exposed to enough light for photosynthesis.
What is the difference between the cell wall and the cell membrane?
The cellulose cell wall is a supporting layer outside the membrane in plant cells. The cell membrane is the living, partially permeable boundary that controls movement of substances into and out of the cell. Animal cells have a membrane but no cellulose cell wall.
Do animal cells have vacuoles?
They can. At O-Level, the useful comparison is that a typical plant cell has a large sap-filled vacuole, whereas animal cells may contain small temporary vacuoles.
Why are mitochondria numerous in muscle cells?
Muscle contraction requires a large supply of energy. Mitochondria are the sites of aerobic respiration, so having many mitochondria provides more sites where respiration can release energy for contraction.
How can I tell rough ER from the Golgi body in an electron micrograph?
Rough ER appears as a membrane network with many tiny ribosomes attached to its surface. The Golgi body appears as a stack of flattened membrane sacs, often with small vesicles nearby. The presence of attached dots is therefore a useful clue for rough ER.
Which organelle actually makes proteins: ribosomes, ER or Golgi?
Ribosomes are the direct sites of protein synthesis. Rough ER helps process and transport newly made proteins, while the Golgi body modifies, sorts and packages them. A useful sequence is: ribosome makes → ER handles → Golgi packages.
Why does a mature red blood cell have no nucleus?
Losing the nucleus leaves more internal space for haemoglobin. Since haemoglobin binds oxygen, this increases the red blood cell's capacity to transport oxygen around the body.
Should I identify a plant cell simply because it looks rectangular?
No. Shape alone is unreliable because real cells vary greatly. Look for structural evidence such as a cellulose cell wall, a large vacuole and, where appropriate, chloroplasts.
Once you can connect each structure to its job, cell biology becomes much more logical. The next step is to use these same cell boundaries and organelles to understand how substances move into and out of cells.
