Ultrastructure of Eukaryotic Cells 2

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Ribosomes and the Endoplasmic Reticulum

Proteins are made in ribosomes. Proteins may then be folded and processed by the rough endoplasmic reticulum (RER). The smooth endoplasmic reticulum makes and processes lipids.

Structure of ribosomes

Ribosomes can be free floating in the cytoplasm or attached to the cytoplasmic side of the endoplasmic reticulum (ER).
Ribosomes are very small organelles made of protein subunits. This means that ribosomes are not covered by a membrane.
Eukaryotic ribosomes are found in the cytoplasm, attached to the RER, and in mitochondria and chloroplasts.
- Ribosomes in the cytoplasm and RER are 80S in size.
- Ribosomes in mitchondira and chloroplasts are 70S in size.

Function of ribosomes

Ribosomes are in charge of protein synthesis.
Protein synthesis is an essential function of all cells. This is why ribosomes are found in practically every cell.

Structure of the ER

The ER is a series of interconnected membranous sacs and tubules.
The membrane of the ER is a phospholipid bilayer embedded with proteins.
The smooth ER (SER) membrane has no ribosomes but the rough ER (RER) membrane has many ribosomes on its surface.

Function of the RER and SER

The RER is responsible for processing and folding proteins.
The SER is responsible for making and processing lipids.

The Plasma Membrane and Cell Wall

The plasma membrane is a partially-permeable barrier between the cell's interior and its surroundings. The cell wall is a structure external to the plasma membrane found in plant, algal and fungal cells.

The plasma membrane

Structure of plasma membrane

The fluid mosaic model describes the structure of plasma membranes.
This model says that plasma membranes are made up of a variety of components (e.g. phospholipids, proteins and cholesterol) that are continuously moving around.
Phospholipids are the primary component of the membrane. They are well-suited to this role because they are amphipathic (contain hydrophobic and hydrophilic regions).
- The hydrophilic region faces outwards and the hydrophobic region inwards. This means that polar substances can't easily pass through the membrane.

Function of plasma membrane

Plasma membranes protect cells from their surrounding environment.
The selectively-permeable (partially-permeable) nature of plasma membranes means that they can selectively allow certain molecules to pass through and stop others.
- In this way, these membranes mediate the interactions between a cell's interior and its surroundings.
Plasma membranes play a role in several cellular processes (e.g. cell signalling).
Plasma membranes are also the attachment point for some extracellular (e.g. cell wall) and intracellular structures (e.g. cytoskeleton).

Microvilli

Sometimes the plasma membrane forms multiple small projections from its surface.
These projections are called microvilli. They increase the surface area of the cell.
- These are typical in cells lining the digestive tract, as the increases surface area speeds up absorption.

Structure of cell walls

The cell wall is a rigid covering that protects the cell.
Plant, fungal and algal cells all have cell walls.
- The major organic molecule in fungal cell walls is chitin.
- Plant and algal cell walls are made of cellulose.
  - Cellulose is a polysaccharide made up of glucose units.

Function of cell walls

The cell wall provides structural support and gives shape to the cell.
Plasmodesmata are thin cytoplasmic connections that run through the cell wall of plant cells.
- Plasmodesmata link neighboring plant cells through the cell wall to allow communication.

Centrioles, Flagella and Cilia

Centrioles play an important role in cellular division and organisation. They are also very important in the synthesis of cilia and flagella.

Structure of centrioles

Centrioles have a cylindrical shape. They are made up of parallel microtubules that surround a central cavity.
In most eukaryotic cells, centrioles are found as pairs that are arranged at right angles to one another.
Centrioles are only found in isolation when in the basal regions of flagella and cilia.

Function of centrioles

Centrioles are a key component of centrosomes, which are crucial for organising microtubules in the cell.
- Centrioles help to organise the mitotic spindle during cell division.

Structure of flagella

Flagella are hair-like structures.
- Eukaryotic flagella are described as having a nine-plus-two arrangement (nine pairs of microtubules surrounding a central pair of microtubules). All of the microtubules are made from the protein tubulin.
- Bacterial flagella are helical and are mainly made up of the protein flagellin.

Function of flagella

Flagella are used by some organisms for locomotion.
- Eukaryotic flagella generate propulsion using a whipping motion.
- Bacterial flagella generate propulsion by moving in a propeller-like way.

Structure of cilia

Like flagella, cilia have a central core called the axoneme.
- Primary cilia have a nine-plus-zero arrangement.
- Motile cilia have a nine-plus-two arrangement.

Function of cilia

Cilia generate the locomotion of the phylum Ciliophora.
Cilia can also help to move substances internally in organisms by synchronising their beating.