chapter 3

Cells: Basic Units of the Body

• The cell is the fundamental unit of structure and function in the body.

• Cells are measured in micrometers (μm).

• Differentiation refers to the process when cells develop specialized characteristics.

• Cells exhibit a variety of sizes and shapes, and their structure is closely linked to their function.

Composite Cell

• No single typical cell exists, but a composite cell illustrates the components commonly found in most cells.

Major Parts of a Cell:

• Nucleus:

  • Contains genetic material.

  • Directs the cell's activities.

• Cytoplasm:

  • Comprises organelles with specialized functions suspended in a fluid called cytosol.

• Cell (plasma) membrane:

  • Acts as the outer boundary of a cell.

  • Maintains cell integrity and controls entry and exit of substances.

Cell Membrane

• The Cell (Plasma) Membrane:

  • Serves as the outer boundary of the cell.

  • Maintains the cell's integrity.

  • Flexible, thin, and elastic.

  • Separates intracellular fluid (cytosol) from extracellular fluid.

  • Selectively permeable, allowing regulation of substance entry and exit.

  • Involved in signal transduction, enabling the cell to receive and respond to messages.

  • Mainly composed of lipids and proteins, with some carbohydrates.

Structure of the Cell Membrane:

• Phospholipid bilayer:

  • Framework of the cell membrane.

  • Hydrophilic heads (water-soluble) face outside and inside the cell.

  • Hydrophobic tails (water-insoluble) form the interior.

  • Permits lipid-soluble substances to pass but not water-soluble ones.

• Cholesterol:

  • Stabilizes the membrane and helps keep it impermeable to water-soluble substances.

• Fluid Mosaic Model:

  • Some lipids and proteins are movable within the membrane.

  • Membrane proteins perform various functions: pores, channels, receptors, enzymes, and adhesion molecules (CAMs).

• Carbohydrates are involved in cell recognition and interaction.

Types of Membrane Proteins (Table 3.1):

Organelles Occupy the Cytoplasm

• Cytoplasm:

  • Consists of networks of membranes and organelles suspended in cytosol.

  • Cytoplasm = cytosol + organelles.

  • Cytosol: The fluid part of cytoplasm.

  • Organelles: Tiny solid structures with specific functions in the cell.

  • Contains the cytoskeleton, the supportive framework of protein rods and tubules.

Organelles: Ribosomes and Endoplasmic Reticulum

• Ribosomes:

  • Composed of protein and RNA, located free in cytoplasm or on Rough Endoplasmic Reticulum (RER).

  • Provide structural support and enzyme activity to link amino acids in protein synthesis.

• Endoplasmic Reticulum (ER):

  • Membrane-bound sacs, canals, and vesicles that serve as a tubular transport system.

  • Rough ER: Contains ribosomes, involved in protein synthesis.

  • Smooth ER: Lacks ribosomes and conducts lipid synthesis.

Organelles: Vesicles and Golgi Apparatus

• Vesicles:

  • Membranous sacs that store or transport substances.

• Golgi Apparatus:

  • Composed of flattened, membranous sacs.

  • Refines, packages, and delivers proteins made on the RER.

Organelles: Mitochondria, Lysosomes, and Peroxisomes

• Mitochondria:

  • Membrane-bound, fluid-filled sacs.

  • House chemical reactions that extract energy from nutrients (cellular respiration producing ATP).

  • Often referred to as the powerhouse of the cell.

• Lysosomes:

  • Small membranous sacs containing enzymes that digest proteins, carbohydrates, nucleic acids, bacteria, debris, and worn-out cell parts.

  • Known as the garbage disposals of the cell.

• Peroxisomes:

  • Membranous sacs similar to lysosomes.

  • Contain enzymes that digest lipids, alcohol, and hydrogen peroxide.

Cytoskeleton

• Composed of three types of thread-like structures in cytoplasm:

  • Microfilaments:

  • Tiny rods of actin; smallest diameter of cytoskeletal filaments.

  • Involved in cellular movement, such as muscle contraction.

  • Microtubules:

  • Larger tubes of tubulin; largest diameter of cytoskeletal filaments.

  • Maintain cell shape and make up structure of cilia, flagella, and centrioles.

  • Assist in moving organelles.

  • Intermediate filaments:

  • Comprised of various proteins with a diameter between those of microfilaments and microtubules.

  • Support the nuclear envelope.

Cell Nucleus

• Contains genetic material and controls cell activities.

• Nuclear Envelope:

  • Double-layered membrane surrounding the nucleus.

  • Separates nucleoplasm from cytoplasm.

  • Nuclear pores allow passage of certain substances.

• Nucleolus:

  • Dense body of RNA and protein; site of ribosome production.

• Chromatin:

  • Composed of the cell’s chromosomes containing DNA wound around proteins.

  • Stores information for protein synthesis.

Movements Into and Out of the Cell

• Movements across cell membranes involve passive and active mechanisms.

Passive Mechanisms:

• Do not require cellular energy (ATP).

• Examples include:

  • Diffusion: Movement of atoms, molecules, or ions from regions of higher concentration to regions of lower concentration.

  • Facilitated Diffusion: Diffusion across the cell membrane through membrane proteins (e.g., ion channels, transporters).

  • Osmosis: Movement of water across a selectively permeable membrane.

  • Filtration is also included as a passive mechanism.

Osmosis:

• Defined as the movement of water from a region of higher water concentration to a region of lower water concentration, sometimes referred to as "diffusion of water."

• Water moves through aquaporins, which are channels between phospholipids in the cell membrane.

  • This process is passive and requires no ATP.

Tonicity and Osmotic Pressure:

• Osmotic Pressure: Ability to generate pressure needed to lift a volume of water; increases with the concentration of impermeant solutes.

• Tonicity: Refers to the ability of a solution outside the cell to affect the cell's internal water volume.

  • Isotonic Solution: Equal osmotic pressure; resulting in no net water gain or loss in cells.

  • Hypertonic Solution: Higher osmotic pressure; cells lose water.

  • Hypotonic Solution: Lower osmotic pressure; cells gain water.

Active Mechanisms

• Require cellular energy (ATP) to move substances across the cell membrane.

• Include:

  • Active Transport: Movement from lower to higher concentration (against the concentration gradient).

  • Endocytosis: Movement of substances into the cell within a vesicle.

  • Exocytosis: Release of substances from the cell.

  • Transcytosis: Involves receptor-mediated endocytosis followed by exocytosis and quickly transports substances.

Active Transport:

• Movement from lower concentration to higher concentration using carrier molecules - commonly termed pumps.

• Examples include sugars and amino acids.

• Secondary Active Transport: A process using a gradient established by another substance; does not use ATP directly.

Endocytosis:

• Movement of substances into cells via vesicles.

• Pinocytosis: Engulfs droplets of liquid.

• Phagocytosis: Engulfs solid particles.

• Receptor-mediated Endocytosis: Engulfs specific substances after binding to receptor proteins on the cell membrane.

Exocytosis and Transcytosis:

• Exocytosis: Release of substances or particles from the cell as vesicles fuse with the cell membrane.

• Transcytosis: Involves endocytosis followed by exocytosis, used to transport substances across tightly connected cells (example: transport of HIV across the lining of mucosal barriers).

The Cell Cycle

• Defines the changes a cell undergoes from formation to division.

Stages of the Cell Cycle:

• Interphase: Cell growth and maintenance of normal functions.

• Mitosis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

Interphase Phases:

• S Phase: DNA replication occurs here.

• G1 and G2 Phases: Structures and other molecules duplicate.

Mitosis Overview:

• Involves karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis Phases:

• Prophase: Chromatin condenses into chromosomes; centrioles move apart; nuclear envelope disperses; microtubules form.

• Metaphase: Chromosomes align midway between centrioles.

• Anaphase: Chromosomes separate and move towards opposite poles.

• Telophase: Chromosomes return to chromatin state; nuclear envelopes form around each chromatin mass.

Control of Cell Division

• Control over frequency of cell division is essential and varies by cell type:

  • Skin, intestinal, and blood cells divide frequently.

  • Neurons divide a limited number of times.

• Tumors may arise if the control of mitosis fails due to mutations in oncogenes or tumor suppressor genes.

Tumors Overview:

• Types include benign tumors (localized) and malignant tumors (invasive, capable of metastasizing).

• Abnormal mutations in oncogenes (which control the cell cycle) and tumor suppressor genes can lead to cancer.

• Cancer cells exhibit characteristics like loss of cycle control, dedifferentiation, invasiveness, and the ability to metastasize.

Stem and Progenitor Cells

• Differentiation: The process through which cells specialize.

• Stem Cell: Can divide to form two new stem cells (self-renewal) or differentiate into various cell types.

• Progenitor Cell: A partially specialized daughter of a stem cell; can differentiate into a limited number of cells.

• Totipotent Cells: Can become any cell type (e.g., a fertilized egg).

• Pluripotent Cells: Can become a limited number of cell types.

Stem Cells in Medicine:

• Utilized in regenerative medicine, using the body’s ability to generate new cells to treat diseases.

• Sources include donor stem cells, umbilical cord cells, or reprogrammed differentiated cells.

Cell Death

• Apoptosis: Programmed cell death, a normal part of cellular development and maintenance due to tissue remodeling (e.g., eliminating webbing between fetal fingers).

• Necrosis: Unplanned cell death resulting from damage, representing a pathological process.