Mitochondria Structure and Function

Mitochondria

• Mitochondria are thread-like organelles found in unicellular and multicellular organisms.
• They are known as the "powerhouses" of cells as oxidation of food and energy release occur within them.
• Mitochondria are membrane-bound organelles.
• They are spherical or filamentous, approximately 0.5-1 µm wide and up to 12 µm long.
• The number of mitochondria is roughly consistent per cell type.
• More specialized cells, like those in the kidney and liver have more.
• Amoeba cells can have up to 500,000.
• Liver cells have between 500-2500 per cell.
• Sperm cells have as few as 20.
• Some oocytes can have up to 300,000.

Mitochondrial Structure

• Mitochondria are sac-like structures.
• They have an outer and inner membrane.
• Each membrane is approximately 5-6 nanometers thick.
• The outer membrane is smooth and continuous.
• The inner membrane folds inward, forming cristae.
• The inner membrane is similar in appearance to the outer membrane but is folded.
• The folds are called cristae.
• The space between the outer and inner membranes is called the intermembrane space.
• The space enclosed by the inner membrane is called the matrix (intercristae space).
• Cristae shape varies depending on the cell the mitochondria are located in.
• Most mitochondria have flat shelf-like cristae, whereas steroid-secreting cell mitochondria are tubular.

Mitochondrial Functions

• Mitochondria degrade metabolites.
• They contain tricarboxylic cycle enzymes that generate nucleotides from citric acid.
• They assist in steroid and nucleic acid synthesis.
• They are greatly involved in cell metabolism, especially of fats and amino acids.
• They are responsible for forming the tail sheath of spermatozoa.

Ribosomes

• Ribosomes are complex molecular machines in living cells that produce proteins from amino acids during a process called protein synthesis or translation.
• All living cells synthesise proteins in this way.
• Ribosomes are made of RNA and protein, a ribonucleoprotein.
• Ribosomes are composed of two subunits.
• Ribosomes are non-membrane-bound organelles.
• Ribosomes are formed from nucleolus proteins and pass through the nuclear pores to the cytoplasm.

Ribosomes: Structure & Types

• Ribosomes consist of two subunits: small and large.
• Small ribosomal RNA reads RNA.
• Large ribosomal RNA links amino acids to form a polypeptide chain.
• Ribosomes can be:
  • Free in the cytoplasm (e.g., embryonic cells)
  • Attached to areas of the cell (e.g., endoplasmic reticulum).
  • Appear as minute basophilic granules (light-microscopy).
  • Appear as dense granules in electron microscopy.
• Ribosomes can be clustered (polyribosome/polysome).

Ribosomes Functions

• Free ribosomes produce proteins for cell growth and organelle regeneration.
• Attached ribosomes create proteins for secretory granules and secretions.
• Help create peptidyl transfer and peptidyl hydrolysis.
• During protein synthesis, tRNA moves through three primary binding spots on the ribosome:
  • A site (aminoacylated tRNA)
  • P site (peptidyl tRNA)
  • E site (exit site for deacylated tRNA)

Endoplasmic Reticulum (ER)

• ER is the largest organelle in the cell, and is a network of membranes.
• It's present in the cell's cytoplasm.
• Organelles have a network of tubules, vesicles, and flattened cisternae.
• There are two main types: rough and smooth ER.

Rough ER (rER) Structure and Function

• Present in protein-producing cells, such as those of the plasma cell.
• Its surface has glycoprotein receptors called ribophorins.
• These receptors attach ribosome subunits to the cell's surface.
• Light microscopy shows it as basophilic areas (darker areas)
• Electron microscopy shows flattened cisternae with ribosomes on the surface

Rough ER (rER) Functions

• Protein synthesis occurs on the surface.
• The formed protein accumulates within the cavities.
• Transfer vesicles bud off the ER move proteins to the Golgi apparatus.

Smooth ER (sER) Structure and Function

• Found in lipid and steroid-hormone producing cells.
• The structure cannot be observed through light microscopy. (Not basophilic)
• Electron microscopy shows membrane-bound tubules and vesicles, lacking ribosomes.

Smooth ER (sER) Functions

• Synthesizes and stores lipids and cholesterol.
• Creates steroid hormones (testosterone, cortisone).
• Aids muscular contraction by regulating calcium levels.
• Stores and breaks down glycogen in liver and muscle cells.
• Detoxifies some drugs and hormones.
• Participates in intracellular transport connecting to the Golgi apparatus.