Biologia

The cell: structure and functions explained simply

What a cell is, how it is built and how it works: membrane, cytoplasm, nucleus and organelles, prokaryotes and eukaryotes, animal and plant cells, with 5 solved exercises and 3 FAQs.

Recommended for: Grade 8 · Grade 9

The cell is the smallest part of an organism able to live on its own: it is the structural and functional unit of every living thing. In short, every cell is bounded by a plasma membrane, contains a cytoplasm where chemical reactions take place, and stores its own genetic material (DNA), which directs all its activities.

The three parts every cell has

  • Plasma membrane: the selective boundary that lets useful substances in and keeps or pushes the others out.
  • Cytoplasm: the internal fluid (cytosol) with all the structures suspended in it; the “workshop” where reactions happen.
  • Genetic material: DNA, holding the instructions for building proteins and for making the cell reproduce.

Prokaryotes and eukaryotes

In prokaryotic cells, typical of bacteria and archaea, the DNA lies free in the cytoplasm and there are no membrane-bound organelles: they are small (1-10 micrometres) and simple. In eukaryotic cells the DNA is enclosed in the nucleus and the cytoplasm is divided into specialised compartments, the organelles. This division of labour lets eukaryotic cells be much larger and more complex.

The organelles and what they do

  • Nucleus: stores the DNA and coordinates the cell’s activity; inside it the nucleolus builds ribosomes.
  • Ribosomes: assemble proteins following the instructions carried by messenger RNA.
  • Endoplasmic reticulum: rough (with ribosomes) for proteins to be exported, smooth for lipids.
  • Golgi apparatus: finishes, sorts and packages molecules into vesicles.
  • Mitochondria: the site of cellular respiration, producing energy as ATP.
  • Lysosomes: hold digestive enzymes that break down substances and worn-out structures.
  • Chloroplasts (plants only): contain chlorophyll and carry out photosynthesis.
  • Vacuoles: store water and substances; in plants the central vacuole provides rigidity.

Animal cell and plant cell compared

Both are eukaryotic and share nucleus, mitochondria, ribosomes, reticulum and Golgi. Only the plant cell, however, has a cellulose cell wall, chloroplasts and a large central vacuole; only the animal cell has centrioles and, as a rule, plenty of lysosomes. The shape gives it away at once: a plant cell is boxy and regular, an animal cell is rounded and more irregular.

In the exercises below you put this framework to work: first recognising the type of cell from its visible structures, then linking each organelle to the job it does, and finally predicting how a cell behaves in surroundings of different concentration. Every solution is explained step by step and checked.

Solved exercises

1. Under the microscope you observe a cell with a rigid outer wall, several green bodies and one large vacuole taking up most of the centre. Is it an animal or a plant cell? Explain why. base

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  1. I list the structures observed: rigid wall, green bodies, large central vacuole.
  2. The green bodies contain chlorophyll: they are chloroplasts, found only in cells that carry out photosynthesis.
  3. The rigid cellulose wall and the large central vacuole are also typical of plant cells.
  4. None of these three structures exists in an animal cell, which has only the plasma membrane and at most small vacuoles.

Answer: It is a plant cell: cell wall, chloroplasts and a large central vacuole are three features exclusive to plant cells.

2. Which of these structures are found in BOTH prokaryotes and eukaryotes? Plasma membrane, nucleus with nuclear envelope, ribosomes, mitochondria, DNA. base

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  1. I recall the key difference: prokaryotes (bacteria, for example) have no membrane-bound organelles.
  2. A nucleus with a nuclear envelope is therefore eukaryotic only: in prokaryotes the DNA lies free in the cytoplasm, in a region called the nucleoid.
  3. Mitochondria are also double-membrane organelles: eukaryotes only.
  4. What remains is the plasma membrane, ribosomes and DNA, which every cell has because it must define its boundary, build proteins and store information.

Answer: Plasma membrane, ribosomes and DNA. Nucleus with nuclear envelope and mitochondria occur only in eukaryotes.

3. A pancreatic cell makes large amounts of digestive enzymes (which are proteins) and exports them outside. Which organelles do you expect to be highly developed, and in what order do they work? intermedio

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  1. Enzymes are proteins, and proteins are assembled by ribosomes.
  2. Ribosomes for proteins destined for export are attached to the rough endoplasmic reticulum, where the new protein enters and is folded.
  3. From the reticulum the protein travels in vesicles to the Golgi apparatus, which finishes, sorts and packages it.
  4. The Golgi releases secretory vesicles that reach the plasma membrane and release their contents outside (exocytosis).

Answer: Abundant ribosomes and rough endoplasmic reticulum plus a well-developed Golgi apparatus. Route: ribosomes → rough ER → Golgi → secretory vesicles → outside.

4. A heart muscle cell contracts non-stop and uses a great deal of energy. Which organelle will be present in large numbers, what process takes place there and which molecule is produced? intermedio

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  1. Muscle contraction needs energy the cell can spend immediately.
  2. The organelle that extracts energy from nutrients is the mitochondrion.
  3. Inside it cellular respiration takes place: glucose is broken down using oxygen, releasing carbon dioxide and water.
  4. The energy released is stored in ATP, the molecule the cell spends in order to contract.

Answer: Very many mitochondria, the site of cellular respiration, which produces ATP (consuming glucose and oxygen).

5. You place an animal cell and a plant cell first in distilled water (hypotonic surroundings), then in a very salty solution (hypertonic surroundings). Describe (a) what happens to the animal cell in distilled water, (b) what happens to the plant cell in distilled water, (c) what happens to both in the salty solution. avanzato

Show solution
  1. I apply the rule of osmosis: water crosses the membrane and moves towards the more concentrated solution.
  2. In distilled water the outside is less concentrated than the inside, so water enters the cell.
  3. (a) The animal cell has only the plasma membrane, elastic but fragile: it swells more and more until it bursts (lysis).
  4. (b) The plant cell swells in the same way, but the rigid cell wall resists and stops the expansion: the cell becomes turgid and does not burst.
  5. (c) In the salty solution the outside is more concentrated, so water leaves: the animal cell shrivels, while in the plant cell the cytoplasm pulls away from the wall (plasmolysis).

Answer: (a) It swells and bursts (lysis); (b) it becomes turgid but the wall protects it; (c) both lose water: the animal cell shrivels, the plant cell undergoes plasmolysis.

FAQ

What is the difference between a prokaryotic and a eukaryotic cell?

A prokaryotic cell (bacteria and archaea) has no nucleus and no membrane-bound organelles: its DNA lies free in the cytoplasm in a region called the nucleoid, and it is almost always a single circular molecule. A eukaryotic cell (protists, fungi, plants, animals) keeps its DNA inside the nucleus and has a cytoplasm divided into specialised compartments such as mitochondria, endoplasmic reticulum and Golgi apparatus. Both, however, have a plasma membrane, cytoplasm and ribosomes.

Why does a plant cell have a wall and chloroplasts while an animal cell does not?

Because they live in different ways. A plant is autotrophic and builds its own food through photosynthesis, which happens in the chloroplasts; an animal cell is heterotrophic and gets its nutrients from food. A plant also does not move and needs support and protection against taking in too much water: the cellulose wall does exactly that, whereas an animal cell, which must change shape and move, would only be hindered by a rigid wall.

What is the plasma membrane and how does it decide what enters the cell?

It is the thin covering that separates the cell from its surroundings, made of a double layer of phospholipids with many proteins embedded in it (the fluid mosaic model). It is selectively permeable: small molecules such as oxygen, carbon dioxide and water cross on their own by diffusion or osmosis, from the more concentrated to the less concentrated side, with no energy cost. Larger substances, or those that must move against the gradient, pass through transport proteins and require energy in the form of ATP (active transport).