Cells and Cellular Processes

Cell Composition and Structure

• All human cells vary considerably in size, although all are microscopic.

• Cells differ notably in shape depending on their function.

• Cells contain cytoplasm, a substance found only in cells.

• Organelles are specialized structures within the cytoplasm.

• The cell interior is surrounded by a plasma membrane.

• Internal living material of cells is called cytoplasm, which fills the space between the plasma membrane and the nucleus.

The Plasma Membrane

• The plasma membrane forms the outer boundary of the cell.

• It is composed of a thin, two-layered membrane of phospholipids containing proteins.

• Other structural components include cholesterol and carbohydrate chains.

• The plasma membrane is selectively permeable, controlling what enters and exits the cell.

Cellular Organelles: Structure and Function

• Ribosomes:
    - Known as "protein factories."
    - May be attached to the rough endoplasmic reticulum (ER) or lie free in the cytoplasm.
    - Made of two tiny subunits consisting mostly of ribosomal RNA.
    - Function to manufacture enzymes and other protein compounds.

• Endoplasmic Reticulum (ER):
    - A network of connecting sacs and canals that carry substances through the fluid cytoplasm.
    - Rough ER: Has ribosomes attached; collects, folds, and transports proteins.
    - Smooth ER: Involved in chemical processing.

• Golgi Apparatus:
    - Described as the "chemical processing and packaging center."
    - Consists of a group of flattened sacs near the nucleus.
    - Collects chemicals into vesicles that move from the smooth ER outward to the plasma membrane.
    - The Cell’s Protein Export System (Process):
      1. Proteins assembled by ribosomes are folded in the ER and pinch off in membrane vesicles.
      2. ER vesicles move to the Golgi apparatus for processing and packaging.
      3. Inside Golgi chambers, proteins undergo chemical modifications and move via vesicles from chamber to chamber for further processing.
      4. Processed molecules are packaged in a membranous vesicle that pinches off and is pulled to the cell surface.
      5. The secretory vesicle "pops open" at the cell surface to release contents into extracellular space.

• Mitochondria:
    - Composed of inner and outer membranous sacks.
    - Involved with energy-releasing chemical reactions.
    - Contains one DNA molecule.

• Lysosomes:
    - Membranous-walled organelles containing digestive enzymes.
    - Formerly thought responsible for apoptosis (programmed cell death).
    - Provide a protective function by "eating" microbes.

• Centrosome:
    - The region of cytoplasm near the nucleus that serves as the microtubule-organizing center.
    - Contains centrioles: paired organelles lying at right angles to each other that function in cell reproduction.

Cell Extensions

• Microvilli: Small, fingerlike extensions of the plasma membrane that increase the absorptive surface area of the cell.

• Cilia: Fine, hairlike extensions found on free or exposed surfaces of some cells; capable of moving in unison in a wavelike fashion.

• Flagella: Single projections extending from cell surfaces that are much larger than cilia. In humans, the "tails" of sperm cells are the only example.

The Nucleus

• The nucleus controls the cell because it contains DNA, the genetic code—instructions for making proteins, which determine cell structure and function.

• Nuclear Envelope: Made up of two separate membranes containing nuclear pores.

• Nucleolus: A dense region of nuclear material.

• Nucleoplasm: The fluid inside the nucleus.

• Chromatin and Chromosomes:
    - Chromatin granules are composed of DNA.
    - During cell division, DNA molecules become tightly coiled into chromosomes.
    - Each human cell has $46$ chromosomes in the nucleus.

Movement of Substances Through Cell Membranes

• Passive Transport Processes:
    - Do not require added energy.
    - Movement occurs "down a concentration gradient."
    - Diffusion: Substances scatter themselves evenly through an available space from high to low concentration.
    - Osmosis: Specialized diffusion of water across a selectively permeable membrane when some solutes cannot cross.
    - Dialysis: Specialized diffusion of solutes across a selectively permeable membrane.
    - Filtration: Movement of water and solutes caused by hydrostatic pressure on one side of the membrane; responsible for urine formation.

• Active Transport Processes:
    - Movement is "up the concentration gradient."
    - Requires energy from adenosine triphosphate ($ATP$).
    - Occurs only in living cells.
    - Ion Pumps: A protein complex in the cell membrane (e.g., sodium-potassium pump or $Na^+ - K^+$ ATPase; calcium pump) that uses $ATP$ to move substances against gradients.
    - Phagocytosis: A protective mechanism used to destroy bacteria (cell eating).
    - Pinocytosis: Used to incorporate fluids or dissolved substances into cells (cell drinking).

Relationship of Cell Transport to Disease

• Cystic Fibrosis: Characterized by abnormally thick secretions in airways and digestive ducts, resulting from failed $Cl^-$ (chloride ion) transport.

• Cholera: A bacterial infection causing $Cl^-$ and water to leak from cells lining the intestines, resulting in severe diarrhea and water loss.

Cell Growth and Reproduction

• DNA Structure:
    - Resembles a long, narrow ladder twisted into a double helix.
    - Made of a sugar (deoxyribose), phosphate units, and nitrogen-containing bases.
    - Complementary Base Pairing: Adenine pairs with Thymine ($A-T$) and Cytosine pairs with Guanine ($C-G$).

• Genetic Code:
    - A gene is a specific segment of base pairs in a chromosome.
    - The sequence of base pairs determines heredity and controls protein/enzyme production.
    - Enzymes facilitate chemical reactions that determine cell structure and function.

• Protein Synthesis:
    - Transcription: Occurs in the nucleus. Double-stranded DNA separates to form messenger RNA ($mRNA$). $mRNA$ leaves the nucleus through nuclear pores.
    - Translation: Occurs in the cytoplasm at the ribosome. Ribbon-like $mRNA$ is read in series of three nucleotide bases called codons. Transfer RNA ($tRNA$) molecules bring specific amino acids into place. Amino acids are joined by peptide bonds to form polypeptides.

The Cell Life Cycle and Mitosis

• Interphase: The period when the cell is not actively dividing. Includes cell growth and DNA replication (each half of a DNA molecule becomes a whole identical molecule).

• Mitosis: Division of the nucleus and cytoplasm into two daughter cells. Heredity is made possible by distributing identical chromosomes.

• Stages of Mitosis:
    1. Prophase: Chromatin granules organize into visible chromosomes (pairs of linked chromatids attached at a centromere). Centrioles move away from the nucleus, spindle fibers appear, and the nuclear envelope/nucleolus disappear.
    2. Metaphase: Spindle fibers attach to each chromatid. Chromosomes align across the center of the cell.
    3. Anaphase: Centromeres break apart. Separated chromatids (now chromosomes) move away from the center toward opposite ends of the cell. A cleavage furrow begins to develop.
    4. Telophase: Cell division is completed. Nuclear envelopes and nucleoli appear in two new daughter cells. Cytoplasm is divided via cytokinesis.

Changes in Cell Growth and Reproduction

• Hypertrophy: Increase in the size of individual cells, increasing the overall size of tissue.

• Atrophy: Decrease in the size of individual cells, decreasing the overall size of tissue.

• Hyperplasia: Increase in cell reproduction, increasing the size of tissue.

• Anaplasia: Production of abnormal, undifferentiated cells; often associated with cancer.

• Cancer: Specifically mentioned in relation to lung and respiratory epithelium (pseudostratified ciliated) where cancer cells can invade smooth muscle, connective tissue, blood vessels, and lymphatic vessels.