Cellular Organization Notes

The Cellular Level of Organization

  • Cells are the building blocks of the human body.
  • Cell Theory (developed from Robert Hooke’s research):
    • Cells are the building blocks of all plants and animals.
    • All cells come from the division of pre-existing cells.
    • Cells are the smallest units that perform all vital physiological functions.
    • Each cell maintains homeostasis at the cellular level.

Two Main Classes of Cells

  • Sex Cells (Germ Cells):
    • Contain 23 chromosomes (haploid).
    • Examples: Sperm and Oocytes.
  • Somatic Cells:
    • All other cells in the human body.
    • Contain 46 chromosomes (diploid).

Parts of a Cell

  • Plasma (Cell) Membrane
  • Cytoplasm
    • Cytosol
    • Organelles
  • Nucleus

Functions of the Plasma Membrane

  • Physical Isolation: Barrier.
  • Regulates Exchange with the Environment:
    • Ions and nutrients enter.
    • Wastes eliminated and cellular products released.
  • Monitors the Environment:
    • Extracellular fluid composition.
    • Chemical signals.
  • Structural Support: Anchors cells and tissues.

Membrane Structure

  • Lipids
  • Proteins
  • Carbohydrates

Membrane Lipids

  • Double layer of phospholipid molecules:
    • Hydrophilic Heads: Toward watery environment on both sides.
    • Hydrophobic Fatty-Acid Tails: Inside membrane.
    • Barrier to ions and water-soluble compounds.
  • Cholesterol:
    • Comprises 20% of cell membrane lipids.
    • Increases the strength of the plasma membrane.

Membrane Proteins

  • Integral Proteins: Within the membrane.
  • Peripheral Proteins: Bound to the inner or outer surface of the membrane.

Functions of Membrane Proteins

  • Anchoring Proteins: Attach to other structures, stabilize position, allow cell movement, and maintain cell shape & structure.
  • Recognition Proteins: Allow cells to recognize other similar cells.
  • Enzymes: Catalyze and speed up chemical reactions in the ECF (Extracellular Fluid) or ICF (Intracellular Fluid).
  • Receptor Proteins: Recognize specific molecules (ligands) and bind to them, triggering changes in cell activity.
  • Carrier Proteins: Bind solutes and transport them across the cell membrane, either passively or using ATP.
  • Channels: Integral proteins with pores that allow either continuous movement of small molecules (leak channels) or those that open and close in response to a certain stimulus (gated channels).

Membrane Carbohydrates

  • Proteoglycans, glycoproteins, and glycolipids:
    • Extend outside cell membrane.
    • Form sticky “sugar coat” (glycocalyx).
  • Functions of the Glycocalyx:
    • Lubrication and protection.
    • Anchoring and locomotion.
    • Specificity in binding (receptors).
    • Recognition (immune response).

Membrane Permeability

  • Is selectively permeable.
  • What passes is based on:
    • Size
    • Electrical charge
    • Molecular shape
    • Lipid solubility

Gradients Across Membrane

  • Concentration Gradient:
    • High concentration of sodium (Na+) outside.
    • High concentration of potassium (K+) inside.
  • Electrical Gradient:
    • Inside is negative relative to outside.
    • Resting potential = −70mV-70mV

Movement Across the Plasma Membrane

  • Passive Processes:
    • No energy expenditure.
    • Moves down a concentration gradient (High → Low concentration).
    • Diffusion.
  • Active Processes:
    • Requires energy expenditure.
    • Moves against a concentration gradient (Low → High concentration).
    • May utilize 40% of cells ATP supply.
    • Carrier mediated & vesicular transport.

Diffusion

  • Passive, random, molecular motion causes the even distribution of molecules in a solution.
  • The net movement of a substance from an area of higher concentration to an area of lower concentration.

Factors Influencing Rate of Diffusion

  • Concentration Gradient: The steeper the gradient, the faster diffusion proceeds.
  • Temperature: The higher the temperature, the faster the diffusion rate.
  • Ion and Molecule Size: Smaller size, faster rate.
  • Electrical Forces: Opposite charges attract & like charges repel (Inside of cell is negative relative to outside).
  • Distance: Shorter the difference, faster the rate.

Types of Diffusion

  • Simple Diffusion:
    • Lipid soluble.
    • Small uncharged.
  • Osmosis: Diffusion of water.
  • Channel-Mediated:
    • Large.
    • Charged (ions).
    • Water soluble molecules.
  • Facilitated Diffusion

Osmosis

  • Diffusion of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration.

Tonicity

  • Based upon the concentration of solute in an aqueous solution. Water will flow into or out of a cell.
  • Tonicity: Is the effect of osmotic solutions on the shape of a cell.
    • Isotonic: The solution the cell is in does not cause a net flow of water in or out of the cell.
    • Hypotonic: The solution is less concentrated than the cell and water will move into the cell.
    • Hypertonic: The solution is more concentrated than the cell and water will flow out of the cell.

Carrier-Mediated Transport

  • May be passive or active process.
  • Integral proteins carry a substance across the cell membrane.
  • Substrate specific.
  • Types:
    • Facilitated diffusion
    • Active transport

Facilitated Diffusion

  • Passive
  • Carrier proteins transport molecules too large to fit through channel proteins (glucose, amino acids).
    • Molecule binds to receptor site on carrier protein.
    • Protein changes shape, molecules pass through.
    • Receptor site is specific to certain molecules.

Active Transport

  • Move substrates against concentration gradient.
  • Require energy, such as ATP.
  • Ion pumps move ions (Na+, K+, Ca2+, Mg2+).
  • Exchange pump countertransports two ions at the same time.

Sodium-Potassium Exchange Pump

  • The most prevalent active transport mechanism.
  • Requires 40% of cellular ATP.
  • All cells have 1000s of them.
  • Maintains low concentration of Na+ and a high concentration of K+ in the cytosol.
  • Operates continually.

Vesicular Transport

  • Bulk transport.
  • Materials move into or out of cell in vesicles.
    • Endocytosis (endo = inside) is active transport using ATP.
      • Pinocytosis: cell drinking
      • Phagocytosis: cell eating
    • Exocytosis (exo = outside): Granules or droplets are released from the cell.

Pinocytosis and Phagocytosis

  • Pinocytosis: Nonselective drinking of extracellular fluid.
  • Phagocytosis: Pseudopods extend to form phagosome; ingestion of solid material.

Exocytosis

  • A vesicle created inside the cell fuses with the cell membrane and discharges its contents into the extracellular environment.

The Cytoplasm

  • The “stuff” between the cell membrane and the nuclear membrane.
  • Two major subdivisions:
    • Cytosol (intracellular fluid)
    • Organelles: Structures within the cell that perform a specific function/s.

Organelles

  • Nonmembranous organelles:
    • No membrane, direct contact with cytosol.
    • Includes the cytoskeleton, microvilli, cilia, ribosomes.
  • Membranous organelles:
    • Covered with plasma membrane, isolated from cytosol.
    • Includes the endoplasmic reticulum (ER), the Golgi apparatus, lysosomes, and mitochondria.

Cytoskeleton

  • Network of structural proteins.
  • Functions:
    • Cell support and shape
    • Cell & organelle movement
  • Continually reorganize.
  • Major components:
    • Microfilaments
    • Intermediate filaments
    • Microtubules

Microfilaments

  • Thin filaments composed of the protein actin.
    • Provide additional mechanical strength.
    • Pair with thick filaments of myosin for muscle movement.

Intermediate Filaments

  • Mid-sized filaments.
    • Durable (collagen).
    • Strengthen cell and maintain shape.
    • Stabilize organelles.

Microtubules

  • Large, hollow tubes of tubulin protein.
    • Strengthen cell and anchor organelles.
    • Change cell shape.
    • Move vesicles within cell.
    • Form spindle apparatus.

Microvilli

  • Finger-like projections of the cell membrane.
  • Increase surface area of the membrane.
  • Seen in areas where cells are actively absorbing materials from ECF (e.g., digestive tract).

Centrioles

  • Are paired cylinders arranged at right angles to one another.
  • Made of short microtubules.
  • During cell division, form the spindle fibers associated with the movement of DNA strands.

Cilia and Flagella

  • Cilia: Numerous, short, hair-like projections extending from the surface of a cell; move materials across the surface of the cell.
  • Flagella: Long extension of membrane; move an entire cell. In the human body, the sperm cell is the only flagellated cell.

Ribosomes

  • Perform protein synthesis.
  • Two types:
    • Free ribosomes in cytoplasm: Manufacture proteins for cell.
    • Fixed ribosomes attached to ER: Manufacture proteins for secretion.

Proteasomes

  • Organelles that contain protein-digesting enzymes (proteases).
  • Remove and recycle damaged or denatured proteins.
  • Removes abnormal “foreign” proteins thus are involved in the immune response

Endoplasmic Reticulum

  • Endo- = within, plasm = cytoplasm, reticulum = network.
  • Cisternae are storage chambers within membranes.
  • Functions:
    • Synthesis of proteins, carbohydrates, and lipids.
    • Storage of synthesized molecules and materials.
    • Transport of materials within the ER.
    • Detoxification of drugs or toxins.

Smooth Endoplasmic Reticulum (SER)

  • No ribosomes attached.
  • Synthesizes lipids and carbohydrates:
    • phospholipids and cholesterol (membranes)
    • steroid hormones (reproductive system)
    • glycerides (storage in liver and fat cells)
    • glycogen (storage in muscles)

Rough Endoplasmic Reticulum (RER)

  • Surface is covered with ribosomes.
  • Synthesizes protein and glycoprotein.
  • Encloses products in transport vesicles.

Golgi Apparatus

  • The Golgi complex consists of three to twenty stacked, flattened membranous sacs (cisterns).
  • Communicates with ER.

Functions of Golgi Apparatus

  • Processes, sorts, and delivers proteins, carbs, and lipids to the plasma membrane, lysosomes, and secretory vesicles.
  • Secretory vesicles: Contain secretions that will be discharged from the cell via exocytosis.
  • Membrane renewal vesicles: These vesicles are composed of new lipids and proteins that will replace “old” lipids and proteins of the cell membrane.
  • Lysosomes: Vesicles that contain digestive enzymes and remain in the cytosol.

Lysosomes

  • Membrane-enclosed vesicles that contain powerful digestive enzymes; internal pH reaches 5.0.
  • Functions:
    • Clean up inside of cell: Break down large molecules, attack bacteria, recycle damaged organelles.
    • Autolysis: Self-destruction of damaged cells.

Peroxisomes

  • Are enzyme-containing vesicles.
  • Break down fatty acids, organic compounds, and toxic substances (e.g., alcohol).
  • Produce hydrogen peroxide.

Mitochondria

  • Powerhouses of the cell.
  • Have smooth outer membrane and inner membrane with numerous folds (cristae).
  • Take chemical energy from food (glucose).
  • Produces energy molecule ATP.
  • Mitochondria are the site of ATP production in the cell by the catabolism of nutrient molecules.
  • Mitochondria self-replicate using their own DNA; increases with need for ATP.
  • Mitochondrial DNA (genes) are usually inherited only from the mother.

The Nucleus

  • The “control center” of the cell.
  • Usually the largest structure in the cell.
  • Contains genetic material.
  • Controls cellular function and structure.

Contents of the Nucleus

  • Nuclear envelope
  • Nucleoplasm: Fluid containing ions, enzymes, nucleotides, and some RNA.
  • Nucleolus: Site of ribosome synthesis; related to protein production.
  • Genetic material DNA:
    • Chromatin contains DNA in thread-like form (cell is not dividing).
    • Chromosomes contain DNA in short, rod-like structures (cell is reproducing).

Contents of Nucleus (DNA Packing)

  • DNA has various degrees of packing represented by:
    • Nucleosomes: DNA coiled around histones.
    • Chromatin: loosely coiled DNA (cell is not dividing).
    • Chromosomes: tightly coiled DNA (cell is dividing).
  • Human somatic cells have 46 chromosomes arranged in 23 pairs.

Information in the Nucleus

  • DNA molecule: a pair of DNA strands held together.
  • Information is stored in the sequence of nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G).
  • Three bases specify a specific amino acid.
  • A gene contains all the DNA triplets needed to produce one specific protein.

Relaying the Information

  • The “message” encoded in the DNA is copied or “transcribed” into messenger RNA (mRNA).
  • The mRNA leaves the nucleus once it has copied the DNA information into RNA language. (Process called transcription).

The Making of a Protein

  • mRNA arrives at a ribosome with the “recipe” for a protein.
  • tRNA transports the appropriate amino acid to the ribosome where it will be bonded (dipeptide bond) to other amino acids.
  • The finished chain of amino acids is now a protein. (Process is called translation).

The Cell Life Cycle

  • Cell division is necessary for growth, replacement, and repair of cells.
  • Nuclear Division:
    • Somatic cell nuclear division (MITOSIS)
    • Sex cell nuclear division (MEIOSIS)
  • Cytoplasmic Division - Cytokinesis

Interphase

  • Metabolic phase - between cell division.
  • Replication of chromosomes.
  • Replication of centrioles.
  • Time - highly variable; normally about 18 to 24 hours.

Mitosis and Cytokinesis

  • Mitosis: This is the division of the nucleus and the partitioning of the DNA within it into two identical daughter cells. Involves four major stages: Prophase, Metaphase, Anaphase, Telophase.
  • Cytokinesis: Division of cytoplasm.

Prophase

  • Chromosomes become visible as nuclear chromatin packages for the division, and the nuclear membrane breaks down.
  • Centrioles move to poles, spindle fibers appear.

Metaphase

  • All 92 chromosomes line up on a narrow central zone called the metaphase plate.

Anaphase

  • Each chromosome pulls away from its replicated partner and heads to an opposite side of the cell.
  • Leaving 46 at one end and an identical set of 46 at the other end.

Telophase

  • Two new nuclei begin reforming, the nuclear membranes form.
  • The chromosomes uncoil and relax.
  • This stage marks the end of mitosis.

Cytokinesis

  • Is the cytoplasmic division of the daughter cells.
  • Organelles, the cytoplasm, and the membranes are partitioned into two cells.
  • The completion of cytokinesis marks the end of cell division.

Development of Cancer

  • Normal cell control is disrupted (mutation).
  • Cell growth is uncontrollable and takes on abnormal shape.
  • Malignant cells can no longer perform normal cell functions.
  • Organ function deteriorates.