We have come to the end of our cell organelle series! Today we are going to be looking at the remaining organelles in the A-Level specification which don’t need covering in as much depth. Some exam boards also need a little information about cilia, eukaryotic flagella and amyloplasts, so make sure to revise those too if you need to – I will put brief notes at the bottom of the article.
The cell wall and cell membrane also need a bit of attention, but these are not technically organelles so we will cover those in a different series. For now, let’s wrap up cell organelles with the smooth endoplasmic reticulum, vacuole and centriole.
The smooth endoplasmic reticulum (SER)
We already know about the rough endoplasmic reticulum (RER), but what about the SER? It is similar in many ways, for example it consists of interconnected membrane-bound flattened sacs, but there are no ribosomes attached to the membranes which makes it appear smooth. The SER is involved in synthesising and processing lipids rather than proteins. For example, the SER synthesises phospholipids which are needed for cell membranes. The SER is particularly important in the liver where it helps to break down toxic substances such as alcohol.
Vacuoles
Although vacuoles are rarely found in animal cells, they are very often found in plant cells. The membrane of a vacuole is called the tonoplast, and it encloses a space filled with cell sap. Sap contains substances such as sugars, salts, water, enzymes and minerals. The tonoplast controls what goes in and out.
The vacuoles help to keep a plant from wilting (becoming droopy). Water is drawn into the vacuole by osmosis which maintains a high pressure inside. This keeps the plant cells turgid (rigid). Vacuoles can also perform some of the same functions as lysosomes (plant cells don’t normally have lysosomes). They contain enzymes which can break down waste and unwanted cell components.
Centrioles
You will have come across the centrioles if you have studied mitosis. During mitosis (and meiosis), the centrioles form the spindle fibres that help the sister chromatids to separate. They are found in animal cells but only some plant cells.
From one end, the centriole looks a bit like a star shape. It is made up of very small hollow protein tubes (microtubules).
Summary
- The smooth endoplasmic reticulum is a collection of interconnected membrane-bound flattened sacs which synthesise and process lipids.
- In plants, the vacuole contains cell sap and is surrounded by the tonoplast. It helps to keep the cell rigid and breaks down cell waste.
- The centriole is a collection of protein microtubules which form the spindle fibres during mitosis and meiosis.
Extra organelles
- Amyloplasts – found in plant cells. Surrounded by a membrane and contain starch granules which can be hydrolysed to release glucose when needed.
- Cilia – found on the surface of some animal cells. Hair-like projections which have an outer membrane and ten pairs of protein microtubules (nine pairs around the edge, one pair through the middle). They can move, which helps movement of substances across the cell surface – sort of like a brush.
- Eukaryotic flagella (singular – flagellum) – similar in structure to cilia but longer. The protein microtubules can contract to create movement. An example is the tails of sperm cells which drive the sperm forward.
Download my revision notes here for more information about all the organelles relevant to your exam board.
