Cells as the basic units of living organisms

Eukaryotic cells

Eukaryotic cells contain a true nucleus and a range of membrane-bound organelles, each with a structure suited to its function.

• The nucleus stores the genetic information and controls the cell's activities.
• The rough endoplasmic reticulum and Golgi body cooperate to make and modify proteins.
• The mitochondria produce ATP through aerobic respiration.

Plant cells

Plant cells differ from animal cells by having:

• A cellulose cell wall
• Chloroplasts
• Plasmodesmata
• A large permanent vacuole bounded by the tonoplast

Both cell types use ATP from respiration to drive energy-requiring processes such as active transport and protein synthesis.

Mitochondria and chloroplasts are notable because they contain 70S ribosomes and small circular DNA, supporting their endosymbiotic origin.

Prokaryotic cells

Prokaryotic cells, such as bacteria, are much smaller and simpler than eukaryotic cells:

• They lack membrane-bound organelles.
• They contain free circular DNA that is not enclosed in a nucleus.
• They have 70S ribosomes, which are smaller than the 80S ribosomes of eukaryotes.
• They possess a peptidoglycan cell wall.

Comparing prokaryotes with eukaryotes reveals shared features (cell surface membrane, ribosomes, DNA) but striking differences in size, complexity and DNA organisation.

Viruses sit outside the cell theory entirely, as they are non-cellular particles with a nucleic acid (DNA or RNA) core and a protein capsid, sometimes surrounded by a phospholipid envelope.

Mastering organelle identification on micrographs and being able to compare cell types underpins almost every exam question on this topic.