Cell Ultrastructure

State the cell theory

1. The cell is the smallest, most basic unit of life that carries out all the essential processes of life such as metabolism and inheritance.

2. All living organisms are made up of cells. They can be unicellular or multicellular.

3. All cells arise from pre-existing cells by cell division.

Describe the structure of the nucleus

1. Spherical or ovoid structure, about 10-20 micrometre in diameter.

2. Largest organelle within the eukaryotic cell and can be easily observed under the light microscope.

3. Surrounded by a nuclear envelope which is a double membrane.

4. Outer membrane is continuous with the rough and smooth ER.

5. Space between outer and inner membrane is directly connected with the lumen of the ER for transport of proteins.

6. Inner membrane is lined by the nuclear lamina, a network of protein filaments that provides support for the nucleus.

7. Together with the nuclear matrix, they help to organise DNA.

8. Nuclear envelope is perforated with nuclear pores.

9. Nuclear pore complexes surround the nuclear pores to allow polar substances and macromolecules to pass through the hydrophobic core of the phospholipid bilayer.

10. Nucleoplasm is a semi-fluid matrix that fills the nucleus and contains the nucleolus.

Describe the function of the nucleus

1. Contain and store genetic information in the form of DNA.

2. Control cellular activities by regulating the rate of protein synthesis.

Describe the structure of the nucleolus

1. Darker and denser structure within the non-dividing nucleus.

2. Not membrane-bound.

3. Contains rRNA genes which code for rRNA. rRNA combines with ribosomal proteins to form ribosomes.

Describe the function of the nucleolus

1. Site of synthesis of rRNA and partial assembly of ribosomes.

Describe the structure of the ribosome

1. 20-25 micrometre in diameter.

2. Non-membrane bound.

3. Consists of 40S small ribosomal subunit and 60S large ribosomal subunits.

4. Each subunit is composed of rRNA and ribosomal proteins.

Describe the function of the ribosome

1. Site of protein synthesis.

2. Free ribosomes in the cytosol synthesise cytosolic proteins that function within the cytosol.

3. Ribosomes attached to the rER synthesise proteins for secretion out of the cell, as enzymes in lysosome or incorporated into the cell surface membrane.

Describe the formation of the ribosome in the eukaryotic cell

1. In the nucleolus, rRNA is transcribed from rRNA genes.

2. rRNA combines with ribosomal proteins to form the immature small and large ribosomal subunit.

3. They exit the nucleus via nuclear pores into the cytoplasm where they combine with more ribosomal proteins to form the mature small and large ribosomal subunit.

4. The small and large ribosomal subunits are only assembled during translation.

Describe the formation of polyribosomes in the eukaryotic cell

1. An mRNA molecule translated simultaneously by several ribosomes in cluster is called polyribosome.

2. Each ribosome assemble at the start codon then moves along mRNA until it reaches the stop codon at the 3’ end.

3. As soon as each ribosome moves a sufficient distance from the start codon, the next ribosome attaches to the mRNA and begins its translation activity.

Describe the significance of polyribosomes

1. Many of the same polypeptide chain is formed at the same time.

2. This increases the rate of translation and hence the rate of polypeptide synthesis.

Describe the structure of the rough endoplasmic reticulum

1. Numerous single membrane flattened sacs known as cisternae, which are interconnected with each other and continuous with the outer membrane of the nuclear envelope.

2. Ribosomes are attached, making the rER appear “rough”.

Describe the function of the rough endoplasmic reticulum

1. Ribosomes attached provide the site of protein synthesis.

2. Surface of rER has receptor sites for attachment of ribosomes for translation of mRNA.

3. The polypeptide synthesised by the ribosome passes through the channel of the receptor protein into the lumen of the rER as protein synthesis proceeds.

4. Packaging of proteins into ER vesicles for transport to other parts of the cell

5. The ER vesicle containing the protein buds off, travels along the microtubules and fuses with cis-face of the GA.

Describe the synthesis of a polypeptide and packaging into an ER vesicle

1. A short polypeptide chain (ER signal peptide) is synthesised by a free ribosome.

2. ER signal peptide attaches to receptor site so that ribosome is attached to the rER.

3. The polypeptide synthesised passes through the channel of the receptor protein into the rER lumen.

4. When the protein is released from the ribosome, it folds into its tertiary structure.

5. The protein is enclosed in an ER vesicle, which buds off from the rER, travels along the microtubules and fuses with cis-face of the GA where it undergoes chemical modifications.

Describe the structure of the smooth endoplasmic reticulum

1. Appears smooth due to the absence of ribosomes on its surface.

2. More tubular than rER.

3. Numerous single membrane flattened sacs known as cisternae with numerous vesicles budding off.

Describe the function of the smooth endoplasmic reticulum

1. Site of synthesis of lipids, phospholipids, cholesterol and steroid hormones.

2. Site of detoxification of poisons and drugs in liver cells.

3. Site of synthesis of glycogen.

4. Storage of calcium ions in muscle cells.

Describe the structure of the Golgi Apparatus

1. Stacks of flattened, curved membranous sacs known as cisternae which are not interconnected.

Describe the function of the Golgi Apparatus

1. Site where substances made in the rER and sER are chemically modified, sorted and transported for secretion out of the cell or delivery to other parts of the cell/ organelles. (Refer to protein secretory pathway)

2. Site of formation of lysosomes.

3. Site of synthesis of certain polysaccharides (pectin and other non-cellulose polysaccharide) used for the synthesis of the cell wall.

Describe the process of cytokinesis in plant cells

1. In plant cells, a series of Golgi vesicles appear in the middle of the parent cell.

2. The contents of the Golgi vesicles are used to form the cell walls that separates the daughter cells.

3. The membranes of the Golgi vesicles are used to form the cell surface membrane of the daughter cells.

4. The Golgi vesicles fuses to form the cell plate, which grows outwards.

5. The cell plate eventually fuses with the parent cell wall and cell surface membrane, separating the two daughter cells.

Describe the structure of the lysosome

1. Small spherical vesicles, about 0.1-0.5 micrometre in diameter, formed from the GA.

2. Single-membrane.

3. Contain hydrolytic enzymes such as proteases, lipases, nucleuses which break down proteins, carbohydrates, lipids and nucleic acids respectively.

4. These enzymes require low pH condition to work. The low pH is maintained by a proton pump on the lysosomal membrane, which pumps protons from the cytosol into the lysosome.

Describe the function of the lysosome

1. Digest and break down substances taken in via endocytosis.

2. Autophagy: damaged or worn-out organelles are engulfed and digested within the cell.

3. Autolysis: contents of lysosome are released within the cell to break it down.

4. Release of enzymes outside the cell.