Sub-Cellular Structures

Cells contain structures with particular jobs

A cell is the basic structural unit of a living organism. It contains sub-cellular structures, each suited to a particular function. Their arrangement is not arbitrary: genetic information must be stored, proteins must be produced, energy must be released and the movement of substances must be controlled.

The cytoplasm is the mainly watery material inside a cell. It holds the sub-cellular structures and is where many chemical reactions occur. Although different types of cell are organised differently, all the cells considered here have a cell membrane and ribosomes.

Eukaryotic and prokaryotic organisation

Animal and plant cells are eukaryotic. Their DNA is enclosed inside a nucleus, and they contain membrane-bound organelles such as mitochondria. Bacterial cells are prokaryotic: they have no nucleus, so their chromosomal DNA lies free in the cytoplasm. They also lack mitochondria and are generally smaller and structurally simpler than eukaryotic cells.

This distinction is about organisation, not whether a cell is alive or capable of functioning. A bacterium still makes proteins, controls exchanges with its surroundings and carries genetic information, but it does these things without a nucleus or mitochondria.

Structures shared by animal and plant cells

The nucleus contains the cell's genetic material, DNA. Genes within this DNA provide instructions for making proteins, so the nucleus helps control the cell's activities. It is characteristic of eukaryotic organisation, although a few highly specialised eukaryotic cells are exceptions.

The cell membrane forms the boundary between the cell contents and the surroundings. It is selectively permeable, meaning that it controls which substances enter and leave. For example, cells may need to take in oxygen and glucose and remove carbon dioxide. The membrane therefore helps maintain the conditions needed for reactions inside the cell.

Mitochondria are the site of aerobic respiration. During respiration, energy stored in glucose is released and made available for cell processes. Cells with a high energy demand tend to contain more mitochondria, showing a direct relationship between the number of these organelles and the work a cell performs.

Ribosomes are the site of protein synthesis: they join amino acids to make proteins. Cells require proteins for purposes including growth, repair, enzymes and cell structures, so ribosomes are essential in animal, plant and bacterial cells.

Additional structures in plant cells

A typical plant cell has the same nucleus, cell membrane, mitochondria and ribosomes as an animal cell, but it also has structures related to support and photosynthesis.

The cell wall is a rigid layer outside the cell membrane. In plants it is made of cellulose. It supports the cell, helps it retain its shape and contributes to the strength of plant tissues. The wall does not replace the cell membrane: the membrane still controls movement into and out of the cell.

Chloroplasts contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. Photosynthesis produces glucose, storing energy in a chemical form the plant can use. Cells exposed to light, such as many leaf cells, contain numerous chloroplasts. Cells that do not photosynthesise, such as most root cells, may have none, so chloroplasts should not be treated as present in every plant cell.

The large permanent vacuole contains a solution called cell sap. Water entering the vacuole pushes the cell contents against the cell wall. Because the wall resists this pressure, the cell becomes firm or rigid, helping to support the plant.

These structures work together. Photosynthesis in chloroplasts makes glucose, while respiration in mitochondria releases energy from glucose. The wall and fluid-filled vacuole provide support, while the membrane continues to regulate exchanges with the surroundings.

Organisation of a bacterial cell

A bacterium has a cell membrane surrounding its cytoplasm but no nucleus. Its main chromosomal DNA is usually one closed circular loop lying free in the cytoplasm. It carries most of the genes needed to control the bacterium's activities and reproduction.

Some bacteria also contain plasmid DNA. Plasmids are small circular pieces of DNA separate from the main chromosomal loop. They carry additional genes and can sometimes move between bacterial cells, spreading characteristics encoded by those genes. Plasmids are useful extras rather than a replacement for the main chromosomal DNA, and not every bacterium has them.

Bacterial ribosomes make proteins from amino acids, just as ribosomes do in eukaryotic cells. Bacteria do not have mitochondria, so reactions involved in respiration occur using enzymes associated with their cell membrane instead.

Some bacteria have one or more flagella. A flagellum moves to propel the cell, allowing the bacterium to travel through a liquid environment. This can help it reach favourable conditions, but flagella are not present in all bacteria.

Connecting structure to function

A strong biological explanation goes beyond naming a structure. It follows a chain of reasoning: identify the structure, state what happens there and explain why that process matters. For example, a mitochondrion is the site of respiration; respiration releases energy; that energy enables cell processes. Likewise, a plant cell wall is rigid; it resists pressure from the cell contents; this helps support the cell and the plant.

Cell types

• Eukaryotic: DNA enclosed in a nucleus; includes animal and plant cells; has membrane-bound organelles.
• Prokaryotic: no nucleus; bacterial chromosomal DNA lies free in the cytoplasm; no mitochondria.
• All three cell types have a cell membrane and ribosomes.

Animal cells

• Nucleus: contains DNA and controls cell activities.
• Cell membrane: controls substances entering and leaving.
• Mitochondria: site of aerobic respiration and energy release.
• Ribosomes: site of protein synthesis.

Plant cells

Plant cells have the four structures above, plus:

• Cellulose cell wall: support and shape.
• Chloroplasts: chlorophyll absorbs light for photosynthesis; not present in every plant cell.
• Large permanent vacuole: contains cell sap and helps keep the cell rigid.

The cell wall and cell membrane are separate: the wall supports; the membrane controls exchange.

Bacterial cells

• Chromosomal DNA: main closed circular loop carrying most genes; not enclosed in a nucleus.
• Plasmid DNA: small additional circles carrying extra genes; present in some bacteria.
• Cell membrane: controls exchange and carries enzymes involved in respiration.
• Ribosomes: make proteins.
• Flagella: move some bacterial cells.

Structure–function recall

Use the pattern structure → process → benefit. For example: mitochondrion → respiration → energy released for cell processes.