Cells as the basic units of living organisms

Eukaryotic Organelles: Structure and Function

A eukaryotic cell is a cell whose genetic material is enclosed in a membrane-bound nucleus, with cytoplasm divided into specialised compartments called organelles.

Each organelle carries out a specific role, and many work together in pathways. For example, ribosomes, rough endoplasmic reticulum and Golgi body cooperate to make and export proteins. The table below summarises the structure and function of each organelle named in the syllabus.

The presence of 70S ribosomes and circular DNA in mitochondria and chloroplasts supports the endosymbiotic origin of these organelles. This detail is frequently tested, so memorise it precisely.

MisconceptionStudents often confuse the cell surface membrane with the cell wall. The cell wall is a rigid external cellulose layer found only in plants. The cell surface membrane is a thin phospholipid bilayer present in all cells. Exam cue: never call the plant cell wall “the membrane” — they are different structures.

Interpreting Photomicrographs and Electron Micrographs

A photomicrograph is an image taken using a light microscope, while an electron micrograph is taken using an electron microscope. Each reveals different levels of detail in plant and animal cells.

Light microscopes use visible light and have a maximum useful magnification of about ×1500, with a resolution limited to around 200 nm. They show large structures such as the nucleus, vacuole, chloroplasts and cell wall in colour or stained sections.

Electron microscopes use beams of electrons, achieving magnifications above ×500 000 and resolution down to about 0.5 nm. They reveal ultrastructure: the detailed internal arrangement of organelles such as cristae in mitochondria, thylakoids in chloroplasts, and ribosomes on the RER.

When interpreting micrographs, identify shape, location and characteristic features. A double membrane indicates a nucleus, mitochondrion or chloroplast. Stacked, flattened sacs near the nucleus suggest the Golgi body or RER. Numerous small dots on a membrane identify ribosomes attached to RER.

Drawing Guidance: Plan and Cell Drawings of Plant and Animal Cells Use a sharp pencil to produce single, unbroken lines with no shading or colouring. Draw plant cell walls as two parallel lines (three lines where two cells touch). Show correct relative proportions, for example a vacuole occupying most of a plant cell. Add only structures actually visible on the micrograph. Use ruled label lines that touch each structure without crossing.

Examiner InsightMarks for “describe what is shown” require named organelles linked to visible features in the image. Vague answers like “small structures” gain nothing. Exam cue: always quote the visual evidence, e.g. “double membrane with cristae indicates a mitochondrion.”

Comparing Plant and Animal Cells

Plant and animal cells share many features because both are eukaryotic, but plant cells contain additional structures linked to photosynthesis, support and storage.

The presence of a rigid cellulose cell wall allows plant cells to develop turgor pressure when the vacuole swells with water, providing structural support without a skeleton.

Cells and ATP

All living cells require energy to drive processes such as active transport, protein synthesis, muscle contraction and cell division.

This energy is supplied by ATP (adenosine triphosphate), which is produced during respiration in mitochondria and the cytoplasm. ATP transfers energy by losing a phosphate group to become ADP, releasing energy that the cell uses immediately. Because ATP cannot be stored, cells continuously regenerate it through respiration of glucose and other respiratory substrates.

Structure of a Typical Prokaryotic Cell

A prokaryotic cell lacks a true nucleus and any membrane-bound organelles, and is structurally much simpler than a eukaryotic cell.

A typical bacterium has the following key features:

• Single-celled (unicellular) organism
• Generally 1–5 µm in diameter, much smaller than eukaryotic cells (typically 10–100 µm)
• Cell wall made of peptidoglycan [a polymer of sugars cross-linked by short peptides]
• A single loop of circular DNA lying free in the cytoplasm, not enclosed in a nucleus
• Smaller 70S ribosomes for protein synthesis
• Absence of organelles surrounded by double membranes (no nucleus, no mitochondria, no chloroplasts)

Many bacteria also carry small extra rings of DNA called plasmids, and some have a flagellum for movement or a capsule for protection. These features support survival but are not always present.

Comparing Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in size, complexity and the way genetic material is organised, but share the basic features of all cells.

Both cell types share a cell surface membrane, cytoplasm, ribosomes and DNA as the genetic material, confirming a common evolutionary origin.

Viruses as Non-Cellular Structures

A virus is a non-cellular infectious particle that can only reproduce inside a living host cell.

All viruses share two essential components. The nucleic acid core contains either DNA or RNA, never both, and carries the genes needed to produce new virus particles. Surrounding this core is a capsid, a protein coat made of repeating subunits that protects the genetic material and helps the virus attach to host cells.

Some viruses, such as HIV and influenza, additionally possess an envelope: an outer layer of phospholipids stolen from the host cell membrane during release. The envelope often carries glycoproteins that recognise and bind to specific host receptors.

Because viruses lack cytoplasm, ribosomes and a metabolism of their own, they are not classified as living cells. They sit on the boundary of life and require host cells for replication.

MisconceptionViruses are not prokaryotes and they are not “small bacteria”. They have no cell membrane, no cytoplasm and no ribosomes, so they cannot carry out metabolism. Exam cue: always describe viruses as “non-cellular” rather than “single-celled”.