Eukaryotic cells

• 1. Lysosome
  2. Cytoplasm
  3. Centriole
  4. Ribosome
  5. Rough endoplasmic reticulum
  6. Plasma membrane
  7. Nucleolus
  8. Smooth endoplasmic reticulum
  9. Nuclear membrane pore
  10. Nuclear membrane
  11. Vacuole (small, not permanent)
  12. Golgi apparatus
  13. Mitochondrion

  
• The ultrastructure of the cell are the structures that can only be observed in detail using the electron microscope
• Membranes are the outer boundary of the cell and also to many intracellular organelles. Cell membranes control what enters and exits the cell. They also localise enzymes in reaction pathways (e.g respiration in mitochondria). They also keep biological molecules separate.
• The protoplasm is the cytoplasm and nucleus combined.
• The nucleus is the largest organelle in the cell. The nucleus is surrounded by a double nuclear membrane with pores. Chemical substances can pass in and out of the nucleus through these pores so that the nucleus can control events in the cytoplasm. Inside the nuclear envelope are DNA and RNA.
• When the cell is not actively dividing, the DNA is bonded to the protein to form chromatin. In the nucleus, there is at least one nucleolus - an extra dense area of almost pure DNA and protein. The nucleolus is involved in the production of ribosomes, and recent research also suggests that the nucleolus plays a part in the control of cell growth and division.
• Mitochondria are found in the cytoplasm. They have inner and outer membranes and are the site of aerobic respiration. Mitochondria contain their own genetic material so that when the cell divides, the mitochondria replicate themselves easily. Mitochondrial DNA is part of the whole genome of the organism.
• In each cell, there is usually a pair of centrioles near the nucleus. The centrioles are involved in cell division. When the cell divides, the centrioles pull apart to produce a spindle of microtubules that are involved in the movement of chromosomes.
• Protein synthesis occurs on these organelles in the cytoplasm of the cell. Ribosomes are made from ribosomal DNA and protein and consist of a large subunit and a small subunit
• The main type of ribosomes in eukaryotic cells are 80S ribosomes. 80S ribosomes are made up of a 40S small subunit and a 60S large subunit. The ratio of RNA:protein in 80S ribosomes is 1:1
• 70S ribosomes are located in the mitochondria and in the chloroplasts of plant cells. These ribosomes are usually found in prokaryotic cells. They consist of a small 30S subunit and a large 50S subunit. The ratio of RNA:protein in 70A ribosomes is 2:1
• A lysosome may fuse with the outer cell membrane to release its enzymes outside the cell as extra cellular enzymes (exocytosis)
• Lysosomes contain a mix of digestive enzymes. They frequently fuse with each other and with a membrane-bound vacuole which contains either food or worn out organelle. Their enzymes then break down the contents into molecules that can be reused. . If an entire cell is too old, needs to be removed during development, has a mutation or is under stress, then the lysosome may rupture. They release their enzymes which then digest the entire contents of the cell (programmed cell death/apoptosis)
• The cytoplasm of the cell contain the endoplasmes reticulum, a 3D network of cavities surrounded by membranes. The ER network links with the membrane around the nucleus. It is covered in 80S ribosomes, the RER has a large surface area for synthesis of proteins. The RER stores and transports the proteins after they have been made.
• The smooth endoplasmic reticulum is not covered in ribosomes and is involved in the synthesis and transport of steroids and lipids.
• The Golgi apparatus looks like a rather dense area of cytoplasm. It is made up of stacks of parallel, flattened membrane pockets formed by vesicles from the endoplasmic reticulum fusing together. Proteins are brought to the Golgi apparatus in vesicles that are separate from the RER where they were made. The vesicles fuse with the membrane sacs of the Golgi apparatus and the protein enters the Golgi stacks. The proteins are modified as they go through the Golgi apparatus
• The inner areas of the Golgi apparatus, near the RER are very rich in enzymes that modify proteins in a different way. This is where most proteins and enzymes are diverted into their final product. The Golgi apparatus also aligns the areas of the protein that need be on the outside of the cell membrane, such as receptor binding sites, so when they arrive at the membrane they are inserted facing the correct direction.