أسئلة الأولى هستولوجي (قبل التعديل)

Cell Membrane

• The cell membrane is the outer limiting membrane that surrounds the cell.
• It is also known as the plasma membrane or plasmalemma.
• It is 7.5-10 nm thick and visible only by electron microscopy (E.M.).
• It has a trilaminar appearance with two electron-dense lines separated by an electron-lucent one.
• It is composed of phospholipids, cholesterol, proteins, and carbohydrates.
• Phospholipids are organized into a double layer (bilayer) with hydrophilic heads facing the water and hydrophobic tails directed toward the middle.
• Cholesterol regulates the fluidity of the phospholipid bilayer and stabilizes it.
• Integral proteins are firmly embedded in the lipid bilayer and not easily extracted.
• Peripheral proteins are loosely attached to the outer or inner membrane surfaces and easily extracted.
• Trans-membrane proteins completely span the lipid bilayer and are usually attached to microfilaments in the cytoplasmic side.
• Carbohydrates are located on the external surface of the membrane, forming the Cell Coat or Glycocalyx.

Functions of Cell Membrane

• Passive diffusion: for gases and ions, according to concentration gradient.
• Active transport: for amino acids, glucose, and fatty acids, needs enzymes and energy.
• Selective transport: for hormones, drugs, and bacteria, needs receptors and energy.
• Phagocytosis: for solid (cell eating), example: macrophages can engulf bacteria.
• Pinocytosis: for fluid (cell drinking).
• Receptor-mediated endocytosis: for large molecules, needs receptors, example: protein hormones and some drugs.
• Exocytosis: opposite to phagocytosis, the cell expels waste products.
• Adhesive function: for adjacent cells.
• Immunity: recognizes the cell of its own type and foreign cells.
• Participates in the formation of the basement membrane.

Mitochondria

• Membranous organelles containing enzymes specialized for the production of energy (ATP).
• It is the power house of the cell.
• Mitochondria vary in size and shape, being elongated, rod-shaped, or spherical.
• The number of mitochondria is variable depending on the cell's activity, with liver cells containing numerous mitochondria and lymphocytes very few.
• The number of mitochondria is modified by mitochondrial division.
• Mitochondria are mobile and localize at sites of maximum energy requirement, for example, between myofibrils in cardiac muscle cells.

Light Microscopy (L/M) of Mitochondria

• Hematoxylin (H): basic (alkaline), stains blue, reacts with acidic structures inside the cell.
• Eosin (E): acidic, stains red, reacts with basic structures inside the cell, usually cytoplasm.
• Cytoplasm rich in ribosomes and RER is basophilic, for example, plasma cells.
• Cytoplasm rich in mitochondria is acidophilic, for example, renal tubules.

Electron Microscopy (E.M.) of Mitochondria

• The mitochondria is a double-membranous organelle.
• Each mitochondrion consists of four compartments:
  • Outer membrane: smooth, no folds.
  • Inner membrane: forms complex folds called cristae, the number of cristae increases in more active cells.
  • Intermembranous space: between inner and outer membrane.
  • Mitochondrial matrix: contains mitochondrial DNA and ribosomes.

Functions of Mitochondria

• Provide the cell with ATP by aerobic respiration occurring within the matrix and on the inner membrane.
• The mitochondrial matrix contains DNA for division and ribosomes for protein synthesis.

Ribosomes

• Non-membranous organelles.
• The ribosome is the protein factory of the cell.
• They are very small (20-30 nm in diameter) and too small to be seen by L.M.
• Aggregation of ribosomes leads to basophilia of the cytoplasm due to rRNA.
• Ribosomes are small electron-dense particles composed of two subunits: small and large subunits.
• The two subunits are formed of rRNA and about 80 different protein molecules.

Types of Ribosomes

• Free ribosomes: many ribosomes bound to a single mRNA molecule, synthesize proteins for use by the cell (cytoplasmic and cytoskeletal proteins).
• Attached ribosomes (polyribosomes or polysomes): single ribosome or polysomes attached to the surface of the endoplasmic reticulum forming RER, synthesize proteins to be secreted outside the cell as secretory proteins, or to remain in the cytoplasm as primary lysosomes.

Cell Membrane

• The outer limiting membrane surrounding the cell
• 7.5-10 nm thick
• Visible only by electron microscopy
• Appears trilaminar: two electron dense lines (black) separated by an electron lucent one (white)
• Composed of phospholipids, cholesterol, proteins, and carbohydrates
• Phospholipids form a double layer (bilayer) with hydrophilic heads facing outwards and hydrophobic tails inwards
• Cholesterol stabilizes the phospholipid bilayer
• Integral proteins are firmly embedded and not easily extracted, while peripheral proteins are loosely attached
• Transmembrane proteins span the lipid bilayer and can act as channels
• Carbohydrates form a "cell coat" or "glycocalyx" on the external surface

Functions

• Passive diffusion for gases and ions based on concentration gradients
• Active transport for amino acids, glucose, and fatty acids requiring enzymes and energy
• Selective transport for hormones, drugs, and bacteria needing receptors and energy
• Phagocytosis: engulfing solid particles (e.g., macrophages engulfing bacteria)
• Pinocytosis: engulfing fluids (cell drinking)
• Receptor-mediated endocytosis: engulfing large molecules using receptors (integral proteins)
• Exocytosis: expelling waste products, opposite to phagocytosis
• Adhesive function: for adjacent cells
• Immunity: recognizing self and foreign cells
• Participates in basement membrane formation

Mitochondria

• Membranous organelles containing enzymes for energy (ATP) production
• Powerhouse of the cell
• Vary in size and shape (elongated, rod-shaped, or spherical)
• Number varies depending on cell activity (e.g., liver cells have many, lymphocytes have few)
• Mitochondria are mobile and localize at sites of high energy demand (e.g., between myofibrils in cardiac muscle cells)
• Visible with light microscopy (L/M) using H&E staining
• Basophilic (blue) due to DNA and RNA
• Acidophilic (red) due to abundance of mitochondria
• Visible with electron microscopy (E.M) and appears double membranous
• Composed of four compartments: outer membrane, inner membrane, intermembranous space, and mitochondrial matrix
• Outer membrane is smooth without folds
• Inner membrane forms folds called cristae, with more cristae in active cells
• Intermembranous space is between the inner and outer membrane
• Mitochondrial matrix contains DNA (allowing division) and ribosomes (synthesizing some of its own proteins)

Functions

• Provide ATP through aerobic respiration occurring within the matrix and on the inner membrane
• Contain DNA for division and ribosomes for protein synthesis

Ribosomes

• Non-membranous organelles
• Protein factories of the cell
• Small (20-30 nm diameter)
• Individually too small for L/M, but aggregations (polyribosomes or polysomes) are basophilic (blue)
• In E.M., they appear as small electron-dense particles
• Composed of two subunits: small and large
• Subunits are made of rRNA and proteins

Types

• Free ribosomes (polyribosomes or polysomes): multiple ribosomes bound to a single mRNA molecule, synthesize proteins for cell use (cytoplasmic and cytoskeletal)
• Attached ribosomes (to endoplasmic reticulum): single ribosomes or polysomes attached to the surface of ER, synthesize proteins for secretion outside the cell, remaining in cytoplasm as primary lysosomes