Organelles (Anatomy)

Question 1

Describe the basic ultra structure of the nucleus

Answer 1

• Usually spheroid all to elliptical in shape.
• Enclosed in a double membrane called the nuclear envelope.
• The nuclear envelope is perforated with nuclear pores, formed by a ring of proteins called the nuclear pore complex.
• Immediately inside the nuclear envelope is a narrow, densely fibrous zone called the nuclear lamina, composed of a web of intermediate filaments.
• The nucleoplasm is inside the nucleus and contains chromatin (fine thread-like matter composed of DNA and protein), and a nucleolus (site of ribosome production.

Question 2

Describe the structural components of the cell membrane

Answer 2

• Phospholipid bilayer composed of amphipathic molecules.
• Integral proteins penetrate into the phospholipid bilayer or all the way through it (transmembrane proteins).
• Peripheral proteins do not penetrate into the phospholipid bilayer but adhere to one face of the membrane.

Question 3

Describe the process of endocytosis

Answer 3

Phagocytosis

• A phagocytic cell encounters a particle of foreign matter.
• The cell surrounds it with its pseudopods.
• The particle is phagocytised and contained in a phagocyte.
• The phagocyte fuses with a lysosome and becomes a phagolysosome.
• The lysosome releases hydrolytic enzymes which digest the foreign matter.

Pinocytosis

• The plasma membrane becomes dimpled at points.
• These pits separate from the surface membrane and form membrane-bounded pinocytotic vesicles in the cytoplasm.
• The vesicles contain droplets of ECF with whatever molecules happen to be there.

Receptor-mediated endocytosis

• Extracellular molecules bind to receptors on the plasma membrane.
• The receptors cluster together.
• The plasma membrane sinks inwards, forming a clathrin-coated pit.
• The pit separates from the plasma membrane to form a clathrin-coated vesicle containing concentrated molecules from ECF.

Question 4

Describe the process of exocytosis

Answer 4

• A secretory vesicle approaches the plasma membrane and docks on it by means of linking proteins.
• The plasma membrane caves in at this point to meet the vesicle.
• The plasma membrane and vesicle unite to form a fusion pore through which the vesicle contents are released.

Question 5

Describe the ultrastructure of rough and smooth endoplasmic reticulum

Answer 5

Rough endoplasmic reticulum

• A system of parallel, flattened sacs called cisternae.
• The cisternae are covered with ribosomes.
• Adjacent cisternae are connected to each other by bridges.
• The RER is continuous with the outer membrane of the nuclear envelope.

Smooth Endoplasmic Reticulum

• Cisternae are more tubular.
• The cisternae are branched more extensively.
• No ribosomes.
• The cisternae of the SER are continuous with those of the RER, so the two are different parts of the same network.

Question 6

Describe the structure of a typical mitochondrion

Answer 6

• Can be spheroidal, rod-shaped, kidney-shaped, or threadlike.
• Surrounded by a double membrane.
• The inner membrane has folds called cristae (increase SA) which project across the matrix (space between the cristae).
• The matrix contains the enzyme ATP synthase (generates ATP), ribosomes, and mitochondrial DNA.

Question 7

Describe the ultrastructure of the Golgi apparatus

Answer 7

• A small system of cisternae.
• Each cisterna is a flattened, curved sac with swollen edges.
• Golgi vesicles containing newly synthesised proteins bud off the swollen rim of a cisterna.

Question 8

Describe the ultrastructure of the lysosome and state where they are produced

Answer 8

• Spherical-shaped membrane-bound organelles containing hydrolytic enzymes.
• Produced from the Golgi apparatus.

Question 9

Describe the ultrastructure of peroxisomes

Answer 9

• Small, spherical-shaped membrane bound organelles containing oxidative catalase enzymes.

Question 10

Describe the ultrastructure of proteasomes.

Answer 10

• Large, barrel-shaped protein complexes located in both the cytoplasm and nucleus.

Question 11

Describe the ultrastructure of the cytoskeleton

Answer 11

• Network of protein filaments and cylinders.
• Microfilaments are made of the protein actin and are especially concentrated in a fibrous mat called the terminal web on the cytoplasmic side of the plasma membrane.
• Intermediate filaments are thicker and stiffer than microfilaments.
• Microtubules are cylinders made of 13 parallel strands called protofilaments and radiate from an area of the cell called the centrosome. Each protofilament is a long chain of globular proteins called tubulin.

Question 12

Distinguish between microvilli, cilia, flagellae,and centrioles

Answer 12

• Microvilli are thin membrane folds projecting from the apical surface and supported by actin microfilaments. They serve primarily to increase a cell’s surface area. They are only 1-2 micrometer long. Abundant in epithelial cells of the intestines and kidneys.
• Cilia are hair-like appendages 10-15 micrometers in length with microtubules at the centre, which occur on exposed membrane surfaces of some cells. Arranged as a ring of 9 special double microtubules with a pair of sing microtubules at the centre. They move substances e.g. mucus, over the cell surface. Found in the respiratory tract and female reproductive tract.
• Flagella are long singular membrane extensions supported by microtubules. Same arrangement of microtubules as in cilia. Much longer than cilia. Complex sheath of coarse fibres between the atonement and plasma membrane which stiffens the tail and gives it more propulsive power. Contains the motor protein dynein.
• A centriole is a short cylindrical assembly of microtubules, arranged in nine groups of three microtubules each. Centrioles are involved in spindle assembly during mitosis and are also found at the basal end of cilia and flagella.