Cell Biology: Structure and Function

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Objectives

• Identify and describe the 3 main parts of a cell
• Describe the basic structure and function of the cytoplasm.
  • Describe the structure and function of the cytosol, including the cytoskeleton
  • Describe the structure and function of ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, proteasomes, and mitochondria
  • Describe the structure and function of the nucleus, including the nuclear envelope, pores, nucleoli, genes, and chromosomes
• Describe the composition and function of the plasma cell membrane.
  • List the different components of the plasma membrane
  • List the types of membrane proteins and describe their function
  • Explain the concepts of membrane fluidity and permeability and how these affect transport across the cell
• Explain the concept of concentration and electrical gradients and their contribution to electrochemical gradients
• Describe the structure and function of cilia, flagella, and microvilli. Include function and typical locations
• Describe the various types of transport across the cell membrane.
  • Describe passive processes of simple and facilitated diffusion
  • Define and describe osmosis and osmotic pressure
  • Describe active transport processes, comparing and contrasting them with passive processes
  • Describe the 3 types of vesicular transport: endocytosis (3 types), exocytosis and transcytosis

Case Study

• A 65-year-old female with type 2 diabetes is admitted to the ED due to altered mental status (confused, disoriented), drowsiness and dry oral mucosa.
• Blood glucose levels are dangerously high (>600 mg/dL - normal is 70-100 mg/dL)
• Treatment involves IV fluids, IV insulin, and electrolyte replacement.
• Questions:
  • Which symptom indicated the patient's cellular dysfunction?
  • What is the major cause of the patient's condition?
  • What cell part acts as a barrier between the cell's contents and surroundings?
  • Explain the role of membrane permeability response to IV fluids
  • Identify the structure responsible for insulin production
  • List cell structures vital for insulin production and secretion (synthesis to secretion)

Cell Parts

• Plasma Membrane: Outer boundary of the cell, acts as a selective barrier for substances entering/leaving the cell.
• Cytoplasm: Includes cytosol (fluid portion containing dissolved solutes, particles, and water), and organelles (specialized structures with unique functions)
• Nucleus: Contains DNA, controls cell structure and function, directing activities and producing ribosome subunits.

Cytoplasm Details

• Cytosol (Intracellular fluid) - site of chemical reactions (e.g., glycolysis, protein synthesis, translation, meiosis, mitosis), comprising 55% total cell volume and 75-90% water. Contains ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, waste products, lipid droplets, and glycogen granules.
• Organelles - specialized components within the cytoplasm, each with diverse structures and functions.

Organelles (detailed)

• Centrosome: Paired centrioles and pericentriolar material, crucial for cell division
• Cilia and Flagella: Motile projections for moving fluids (cilia) or the whole cell (flagella)
• Ribosomes: Composed of rRNA and protein; synthesize proteins
• Endoplasmic Reticulum (RER): Synthesizes proteins destined for secretion or insertion into cell membranes (in the RER region)
• Endoplasmic Reticulum (SER): Synthesizes fatty acids, steroids, and other components
• Golgi Complex: Modifies, packages, and transports proteins, forming secretory vesicles
• Lysosomes: Contain powerful digestive enzymes to break down substances and recycling worn-out cell structures
• Peroxisomes: Smaller than lysosomes, contain oxidases to perform functions like degrading H₂O₂ and protecting cells
• Proteasomes: Barrel-shaped structures used for continuous destruction of unneeded or damaged proteins.
• Mitochondria: Powerhouses of the cell, generate ATP through aerobic respiration

Cytoskeleton

• Network of protein filaments (microtubules, intermediate filaments, microfilaments) throughout the cytoplasm.
• Provides cell shape, organizes cellular contents, aids organelle movement, chromosome movement during cell division, and movement of whole cells (like phagocytes).

Nucleus: Detailed

• Nucleolus: Spherical structures within the nucleus responsible for producing ribosomal subunits.
• Nuclear Envelope: Double membrane surrounding the nucleus with nuclear pores to regulate movement of substances in and out of the nucleus.
• DNA: Hereditary units (genes) located in the nucleus which controls cell structure and directs cellular activities. DNA is packaged into nucleosomes. DNA coils into chromatin and further coils to form chromosomes

Plasma Membrane

• The cell membrane is a flexible barrier that surrounds a cell.
• The fluid mosaic model describes the plasma membrane. Its structure includes a lipid bilayer composed of phospholipids.
• The lipid bilayer is made of components like phospholipids (polar head, hydrophobic tails), cholesterol, and glycolipids.
• The membrane has proteins (integral, peripheral) embedded within the membrane and extending to or through the lipid bilayer.
• Membrane proteins perform many functions like acting as channels, carriers, receptors, enzymes, and linkers that transport ions or molecules and aid in signaling, adhering to other cells or substances, forming connections between cells.
• Glycocalyx: External carbohydrate coatings of proteins and lipids, important for cell recognition & adhesion
• Membrane permeability: Cell membranes are selectively permeable and facilitate the passing of small, uncharged substances like oxygen, carbon dioxide, and steroids across the lipid bilayer. Water and urea also pass relatively easily. Ions and large molecules like glucose do not pass easily without transport proteins.

Membrane Transport

• Passive transport (kinetic energy): Molecules move down their concentration gradient
  • Simple diffusion
  • Facilitated diffusion: Channel-mediated, carrier-mediated
  • Osmosis
• Active transport (energy required): Molecules move against their concentration gradient
  • Primary active transport- uses ATP directly (e.g., sodium-potassium pump)
  • Secondary active transport - uses an electrochemical gradient established by a primary active transport pump

Membrane Transport - Vesicles

• Types of endocytosis:
  • Receptor-mediated endocytosis
  • Bulk-phase endocytosis
  • Phagocytosis
• Exocytosis: Material released from the cell
• Transcytosis: Substances moved inside, across, and outside of a cell.

Membrane Specializations

• Cilia and Flagella: Motile projections from the cell surface
• Microvilli: Microscopic projections that increase surface area for absorption

Cell Membrane Gradients

• Concentration gradient: Difference in the concentration of a chemical between two areas
• Electrical gradient: Difference in electrical charge between two areas (e.g., membrane potential).
• Electrochemical gradient: Combined effect of concentration and electrical gradients on ion movement.

Test your knowledge of cell structures, protein components, and transport mechanisms. The quiz covers topics like microfilaments, microtubules, nuclear pores, and cellular transport.