Structure of eukaryotic cellsAQA A-Level Biology: Revision notes

The cell theory is a unifying concept in biology: all living organisms are made of cells, the cell is the basic unit of structure and function, and new cells arise only from existing cells.

Eukaryotic cells (animals, plants, algae, fungi) have a nucleus and other membrane-bound organelles, each with a specific function. Separating reactions into organelles allows different conditions in each, for example hydrolytic enzymes kept apart from the cytoplasm.

The nucleus is surrounded by a nuclear envelope and contains chromosomes, each made of linear DNA bound to protein, and one or more nucleoli. The nucleolus makes ribosomal RNA and assembles ribosomes. The nucleus stores the genes that code for proteins.

The cell-surface membrane forms the boundary of the cell and controls the entry and exit of substances.

Mitochondria are the site of aerobic respiration, releasing energy to make ATP. They have a double membrane, with a folded inner membrane, so cells with a high energy demand (e.g. liver cells, muscle cells) have many.

Chloroplasts (in plants and algae) are the site of photosynthesis. They have a double membrane and contain chlorophyll to absorb light. They are found in the green parts of plants, such as palisade mesophyll cells.

Ribosomes are the site of protein synthesis. They are free in the cytoplasm or attached to rough endoplasmic reticulum (RER).

• RER: a system of membranes with ribosomes; it folds and transports proteins made on its ribosomes.
• Smooth ER (SER): has no ribosomes; it synthesises and processes lipids.
• Golgi apparatus: stacks of flattened membrane that modify proteins and package them into Golgi vesicles; the vesicles carry them to the cell-surface membrane for secretion.
• Lysosomes: membrane-bound organelles, formed from Golgi vesicles, that contain and release hydrolytic enzymes, for example to digest a bacterium engulfed by a phagocyte.

The route for a secreted enzyme is: ribosome on RER → RER → vesicle → Golgi → secretory vesicle → cell-surface membrane.

A cell wall is found in plants, algae and fungi. In plants and algae it is made of cellulose. It is a strong layer outside the cell-surface membrane that supports the cell and prevents it bursting when water enters.

The cell vacuole (in plant cells) is a large, permanent sac bounded by a membrane and containing cell sap. It helps to keep the cell firm (turgid) by pushing on the cell wall, and in palisade cells it pushes chloroplasts to the edge of the cell.

In complex multicellular organisms, cells become specialised for specific functions. Specialised cells of one type form a tissue, tissues combine to form an organ, and organs work together in an organ system. Example: hepatocytes → liver tissue → liver → digestive system.

You must be able to apply organelle knowledge: a pancreatic exocrine cell has lots of RER, Golgi apparatus and mitochondria for making and secreting enzymes; a palisade cell has many chloroplasts for photosynthesis.