Study notes
Cell Structure & Organisation
The structures inside typical plant and animal cells, what each one does, how plant and animal cells differ, and how a cell's structure fits its job.
Learn it step by step
The cell membrane controls what enters and leaves
Every cell, plant or animal, has a cell membrane surrounding the cytoplasm. It is partially permeable, meaning it lets some substances cross freely and holds others back, so the cell can control its own internal contents.
The cell wall is a plant-only support structure
Plant cells have a rigid cell wall, made mainly of cellulose, sitting outside the cell membrane. It gives the cell a fixed shape and mechanical support; animal cells have no cell wall, only a flexible cell membrane, which is why animal cells can change shape.
The nucleus controls the cell and holds the genetic material
The nucleus contains the cell's genetic material and directs the cell's activities, including which proteins it makes. Almost every cell has one nucleus, which is why a cell that lacks a nucleus (like a mature red blood cell) is unusual and worth explaining, not assuming to be a mistake.
The cytoplasm is where most cell chemistry happens
Cytoplasm is the jelly-like substance filling the cell, holding the organelles in place and providing the site for many chemical reactions, including parts of respiration.
Vacuoles differ sharply between plant and animal cells
A plant cell typically has one large, permanent vacuole filled with cell sap, taking up much of the cell's volume and helping keep the cell firm. An animal cell, if it has a vacuole at all, has only small, temporary ones. Size and permanence are the key contrast, not just presence or absence.
Chloroplasts are found only in plant cells that photosynthesise
Chloroplasts contain chlorophyll and are the site of photosynthesis. Only plant cells (and only the ones exposed to light, such as leaf cells) have them; animal cells never do, because animal cells cannot photosynthesise.
Mitochondria, ribosomes, ER and Golgi body each have one main job
Mitochondria are the site of (aerobic) respiration, releasing usable energy. Ribosomes make proteins. The endoplasmic reticulum is a network of membranes that transports substances, including proteins, around the cell. The Golgi body processes and packages substances, especially proteins, often for export from the cell. These are membrane systems and organelles shared by plant and animal cells.
Specialised cells trade a general shape for one job done very well
A cell's structure fits the one function it is specialised for: a muscle cell packs in many mitochondria for the extra energy needed to contract; a root hair cell grows a long thin extension to increase its surface area for absorbing water and minerals; a mature red blood cell loses its nucleus so there is more internal space to carry oxygen. Each change is a trade-off that suits one specific job, not a random variation.
Worked examples
- State what a nucleus normally does: it controls the cell's activities and directs protein production, but it also takes up physical space inside the cell.
- State the red blood cell's one main job: to carry as much oxygen as possible around the body, bound to haemoglobin filling the cytoplasm.
- Removing the nucleus frees up extra internal space that would otherwise be occupied by it, leaving more room for haemoglobin and therefore for oxygen.
- Because the red blood cell has already finished developing and does not need to divide or make new proteins for the rest of its short working life, losing the nucleus costs it nothing functionally but gains it carrying capacity.
- So the missing nucleus is a specialisation that fits the cell's one job (maximising oxygen transport), not damage or a fault: calling it a 'defect' misunderstands cause and effect, since the cell is built this way on purpose, not broken this way by accident.
- Recall what mitochondria actually do: they are the site of respiration, releasing usable energy from glucose, for whichever cell contains them.
- Recall which cells need energy: every living cell, plant or animal, needs a continuous energy supply for growth, repair, movement, and everyday chemical reactions, not plants only.
- Since every living cell respires, every living cell needs mitochondria to do it, so both plant and animal cells contain mitochondria.
- The claim confuses a structure that plant cells have and animal cells do NOT (chloroplasts, for photosynthesis) with one that both plant and animal cells share (mitochondria, for respiration); those are two different organelles with two different jobs.
- Recall the general rule: a specialised cell's structure fits its one particular job, even within the same organism.
- State the root hair cell's job: absorbing water and mineral ions from the soil, which happens across its surface.
- The root hair cell grows a long, thin extension of the cell membrane into the soil, which greatly increases its surface area available for absorption.
- State the leaf palisade cell's job instead: capturing as much light as possible for photosynthesis, which needs many chloroplasts positioned near the leaf's upper surface.
- So the two cells differ in shape and organelle content because they are specialised for two different functions (absorption vs. light capture), even though both are ordinary plant cells built from the same basic parts (cell wall, membrane, cytoplasm, nucleus, vacuole).
- Check the claim against what a cell membrane is: it is the structure controlling what enters and leaves the cell, and it is present in every living cell, plant or animal.
- So the claim is false: a cell membrane is a shared feature, not a difference at all, which makes it the wrong thing to list as a distinguishing point.
- Identify features that genuinely differ instead: (1) plant cells have a cellulose cell wall outside the membrane, animal cells do not; (2) plant cells typically have one large, permanent vacuole, animal cells have only small, temporary ones if any; (3) plant cells (in photosynthetic tissue) have chloroplasts, animal cells never do.
- The general lesson is to check whether a structure is present in BOTH cell types before listing it as a 'difference': cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER, and Golgi body are all shared, so only cell wall, large permanent vacuole, and chloroplasts are true plant-vs-animal contrasts at this level.
Mind map
Mind map for Cell Structure & Organisation.
- Shared by plant and animal cells
- cell membrane: controls what enters/leaves
- cytoplasm: site of many reactions
- nucleus: controls cell, holds genetic material
- mitochondria: site of respiration
- ribosomes: make proteins
- ER: transports substances around the cell
- Golgi body: processes and packages substances
- Plant cells only
- cell wall (cellulose): shape and support
- large, permanent vacuole: cell sap, firmness
- chloroplasts: site of photosynthesis
- Structure fits function
- muscle cell: many mitochondria
- root hair cell: large surface area for absorption
- red blood cell: no nucleus, more room for oxygen