Plant cell structure

These are plant cells seen through a light microscope. You may have seen diagrams similar to this before, but you can't tell much about the structure.

1. Ultrastructure

With the invention of the electron microscope, we can now see the ultrastructure of a plant cell.

The lovely labelled electron micrograph above clearly shows different organelles inside the cell, and their functions are in the table below.

Cell Organelle	Function
nucleus	Contains genetic information
nucleolus	Non-membrane bound, found in the nucleus and has proteins and nucleic acids
cell wall	Helps maintain structure
vacuole	Store water, nutrients, waste products and help maintain structure
chloroplast	Granule that captures light energy for photosynthesis
tonoplast	Semi-permeable membrane surrounding vacuole
mitochondrion	Provide energy for respiration, cell division and other energy releasing reactions

2. The cell wall

The plant cell wall is made up of cellulose which maintains its structure:

• Cellulose is an insoluble polymer made of beta glucose
• Glucose units are held together by 1,4 glycosidic bonds in long straight chains
• Inverted bonding means hydroxyl groups stick out on both sides of each monomer
• This causes hydrogen bonds to form cross links
• 10-10,000 cellulose molecules form microfibrils 

Cellulose and Starch:

Similarities	Differences
Made of glucose	Cellulose is insoluble, starch is soluble
Have glycosidic bonds	Cellulose is indigestible, starch is digestible
Long chain polysachharides	Alpha glucose in starch, Beta glucose in cellulose

cross section of a plant cell wall:

• Microfibrils are held down in layers by a matrix of hemicelluloses and other short chain carbohydrates (mannose, xylose, arabinose)
• The combination of microfibrils in the flexible matrix makes a composite material
• Properties are combined, making cells turgid and firm when needed, and flaccid when water is in short supply
• The cell wall is usually permeable, but can become impermeable if impregnated with lignin (wood) or suberin (cork)

Process:

1. Middle lamella forms when cell divides (made of pectin)
2. Pectin's negative carboxyl groups combine with positive calcium ions to form calcium pectate
3. Cellulose microfibrils and matrix build up on wither side of the lamella forming primary cell walls
4. As the plant ages, secondary thickening forms secondary cell walls

3. Plasmodesmata and Pits

• Lignified plant xylems have gaps called pits
• These allow water to pass through to neighboring cells

• Holes in cell walls are called plasmodesmata
• These let nutrients in and waste out

To test yourself on the structure of plant cells and learn more, check out this cool interactive animation http://www.cellsalive.com/cells/cell_model.htm

Revision links:

BBC Bitesize: http://www.bbc.co.uk/bitesize/standard/biology/investigating_cells/cells_and_diffusion/revision/2/

s-cool:
http://www.s-cool.co.uk/a-level/biology/cells-and-organelles/revise-it/introduction-to-cells