Organelles in Plant Cells

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Cell Wall and Cell Vacuole

The cell wall is a structure external to the plasma membrane. Cell vacuoles are found in plant cell cytoplasm.

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.
The middle lamella joins adjacent cell walls together and is made of pectin.

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.
Pits are dips in the cell wall that allow the passage of water and ions between neighboring cells.
- Unlike plasmodesmata, pits do not extend into the cytoplasm.

Plant cells each have a large central vacuole that occupies most of the area of the cell.
These vacuoles are surrounded by a membrane called the tonoplast and contain a weak solution of salts and sugars called cell sap.
Cell vacuoles are found in the cytoplasm of plant cells but are NOT present in animal cells.
The tonoplast is the membrane which surrounds the vacuole.

The central vacuole allows the cell to remain rigid.
When the central vacuole holds more water, the vacuole pushes against the cell wall and pressure is maintained. This stops the plant from wilting.
The vacuole also isolates certain chemicals that are unwanted by the rest of the cell.

Chloroplasts, amyloplasts, and plasmodesmata are all organelles which are found in plants. They all have different roles in plant function.

Chloroplasts have their own DNA, ribosomes and have inner and outer membranes.
The space enclosed by the inner membrane contains a set of interconnected and stacked fluid-filled membrane sacs called thylakoids.
- Each stack of thylakoids is called a granum (plural = grana).
- Grana are linked by lamellae.
- Lamellae are flat, thin parts of thylakoid membrane.
The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.

Photosynthesis is the series of reactions that use carbon dioxide, water and light energy to make glucose and oxygen.
This process happens in chloroplasts and allows plants to make their own food (e.g. sugars).
Chloroplasts are found in plant and algal cells but not in animal cells.

Plasmodesmata are small channels which allow the exchange of substances between plant cells.
The channel forms in the cell walls of plants.
Pits are small cavities in the cell walls of xylem vessels, which allow water to be exchanged between them. This is essential for transporting water up the plant.

Centrioles play an important role in cellular division and organisation.

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.

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.

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.

Plasma membranes protect cells from their surrounding environment.
The selectively-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).