Cell Structure and Function

Cell Structure & Function

• Cells are the structural and functional units of life, with over 200 types in the human body.
• Cell types vary in size, shape, components, and function.

Cell Theory

• The cell is the fundamental structural and functional unit of life
• The organism's overall function depends on the combined activities of its cells
• A cell's biochemical functions are dictated by its specific shape and subcellular structures.
• Cells come from other cells.

Cytology

• Cytology studies cells.
• Sex cells (germ or reproductive cells) include male sperm and female oocytes (which develop into ova).
• Somatic cells include all body cells except sex cells.

Generalized Cell

• All cells share basic structures and functions.
• Plasma membrane: A flexible outer boundary.
• Nucleus: The control center containing DNA.
• Cytoplasm: Intracellular fluid with organelles.

Extracellular Materials

• These are substances found outside cells.
• Extracellular fluids (body fluids)
  • Interstitial fluid: fluid in which cells are submerged.
  • Blood plasma: fluid of the blood.
  • Cerebrospinal fluid: fluid around the nervous system organs.
• Cellular secretions (e.g., saliva, mucus).
• Extracellular matrix: substance that holds for gluing cells together.

Fluid Compartments and Body Water

• Total body water is approximately 40 liters
• The intracellular fluid (ICF) compartment holds about 2/3 or 25L of total water.
• The extracellular fluid (ECF) compartment holds about 1/3 or 15L of total water.
• Plasma is 3L.
• Interstitial fluid (IF) is in spaces between cells.
• "Other" fluids include lymph, cerebrospinal fluid (CSF), humors (eye), synovial fluid, serous fluid, and gastrointestinal secretions.

Plasma Membrane

• Acts as an active barrier that separates intracellular fluid (ICF) from extracellular fluid (ECF)
• Regulates entry and exit of substances, vital to cellular activity.
• Also called the "cell membrane”.
• Consists of membrane lipids making up flexible lipid bilayer.
• Specialized membrane proteins float in the fluid membrane, creating changing patterns.
  • Pattern is referred to as the fluid mosaic model.
• Surface sugars create the glycocalyx.
• Membrane structures create cell junctions to hold cells together.

Plasma Membrane Composition

• Membrane lipids form a flexible lipid bilayer.
• Specialized membrane proteins suspended in the lipids.
• Hydrophobic tails prevent water-soluble substances from crossing, creating a boundary.
• Polar heads are hydrophilic (water-loving).
• Nonpolar tails are hydrophobic (water-fearing).
• Functions include acting as a mechanical barrier, allowing selective permeability, electrochemical gradients, communication, and cell signaling.

Membrane Lipids

• 75% phospholipids make up the bilayer:
  • Polar (charged) phosphate heads face outwards and are hydrophilic.
  • Nonpolar (no charge) fatty acid tails are inside and are hydrophobic.
• 5% are glycolipids with sugar groups on the outer surface.
• 20% is cholesterol which increases membrane stability.

Membrane Proteins

• Allow cell communication with the environment.
• Half the mass of plasma membrane.
• Function is specialized; can float freely or be anchored inside the cell.
• Integral proteins firmly insert into membrane, most spanning it.
  • These have hydrophobic regions interacting with lipid tails and hydrophilic areas interacting with water.
  • They act as transport channels/carriers, enzymes, or receptors.
• Peripheral proteins are loosely attached to integral proteins.
  • Include filaments for membrane support and can function as enzymes, facilitate cell division/muscle contraction, or connect cells.

Protein Functions

• Receptor proteins bind and respond to ligands (ions, hormones).
• Carrier proteins transport specific solutes
• Channels regulate water and solute flow.
  • Gated channels open or close for substance passage.

Membrane Proteins Tasks

• Transport of hydrophilic molecules.
• Signal transduction via receptors.
• Attachment to cytoskeleton and extracellular matrix.
• Enzymatic activity.
• Intercellular joining of adjacent cells.
• Cell-cell recognition due to glycoproteins.

How Substances Move Across Membranes

• The plasma membrane acts as a barrier, but nutrients must enter and wastes must exit.
• Permeability determines which substances can pass through.
• Impermeable membranes do not allow any substances through.
• Freely permeable membranes allow any substance to pass.
• Selectively permeable membranes allow some substances through.
  • Based on factors like size, electrical charge, molecular shape, and lipid solubility.

Selective Permeability

• Selective Permeability allow some materials to move freely
• Restricts other materials based on size, electrical charge, molecular shape and lipid solubility
• Transport can be passive (no energy) or active (requiring energy).

Transport

• Diffusion and Osmosis = Passive
• Carrier-mediated transport = Passive or Active
  • Primary active transport such as the sodium-potassium exchange pump
• Can Pump solutes can be done against a concentration gradient via ATP
• Vesicular transport (active).

Diffusion and Osmosis

• Diffusion is the net movement of a substance from high to low concentration areas.
• Diffusion occurs because ions and molecules are constantly in motion.
• Molecules move randomly.
• Random motion leads to mixing.
• A concentration gradient is the difference between high and low concentrations.
• Nonpolar and lipid-soluble (hydrophobic) diffuse.
• Examples: oxygen, carbon dioxide, steroid hormones, fatty acids.
• Small polar substances like water can pass as well.
• The speed depends on concentration, molecular size, and temperature.
• Equilibrium: no net movement of molecules in one direction.

Influences on Diffusion Rate

• Diffusion rates are influenced by:
  • concentration (the greater the difference, the faster the rate).
  • molecular size (smaller molecules diffuse quicker).
  • temperature (higher temperatures increase kinetic energy).
• Equilibrium is reached when there is no net movement of molecules.

Facilitated Diffusion

• Certain hydrophobic molecules (e.g., glucose, amino acids, and ions) are transported passively.
  • With carrier-mediated mechanisms that have substances binding to protein carriers
  • Channel-mediated diffusion where Substances move through filled channels

Carrier-Mediated Facilitated Diffusion

• In Carrier-mediated diffusion Carriers are transmembrane integral proteins
• Binding of the molecule prompts the carrier protein to change shape and move the molecule.
• Binding is limited by the number of carriers present; carriers become saturated when all are transporting molecules.
• Each carrier transports specific polar molecules that are too large for membrane channels

Channel-Mediated Facilitated Diffusion

• It occurs through aqueous-filled transmembrane protein cores.
• Channels transport molecules such as ions or water (osmosis)
  • This follows with molecules following down their concentration gradient
  • Has specificity- based on pore size and/or charge
• Water channels are called aquaporins.
• Two types of aquaporin channels:
  • leakage (always open)
  • gated (regulated by chemical or electrical signals).

Osmosis

• Water diffuses across a selectively permeable membrane towards a solution with more solutes.
• Movement of water involves pressures, including Hydrostatic and Osmotic
  • Hydrostatic pressure: outward pressure on the cell membrane caused by increased cell volume
  • Osmotic pressure: The pulling pressure by higher solute concentrations.

Hydrostatic and Osmotic Pressure

• Hydrostatic pressure is the outward force exerted on the cell membrane due to the increase in volume of the cell
• Osmotic pressure is the inward pressure resulting from the tendency of water being pulled into a cell with higher osmolarity
  • This causes higher osmotic pressures within the cell.

Osmolarity

• Osmolarity (osmotic concentration) refers to the total solute concentration in a solution.
• Osmosis occurs until equilibrium when solutions of different osmolarity are separated by a membrane
• Water concentration varies inversely with the number of solute particles.
• Water then moves by osmosis from areas of low concentration to areas of high solute concentration

Tonicity

• Tonicity is the ability of a solution to alter the shape/tone of cells by changing their internal water volume.
  • Isotonic solution: has the same osmolarity as inside the cell, no change.
  • Hypertonic solution: higher osmolarity than inside, causing water outflow and cell shrinkage (crenation).
  • Hypotonic solution: lower osmolarity than inside, causing water inflow and cell swelling (can lead to bursting, i.e., lysing).

Membrane Transport: Active Processes

• Use ATP to move solutes across living plasma membranes.
• Active transport is used to transports solutes against a concentration gradient and requires carrier proteins.
  • Primary active transport uses ATP hydrolysis- sodium-potassium pump.
    • Most studied pump/ Basically uses Na+-K+ATPase to pump Na+ out and K+ in
  • Secondary active transport where energy is obtained indirectly from ionic gradients

Vesicular Transport

• Used for large particles, macro-molecules
• Endocytosis transports into the cell
  • It can occur through phagocytosis, pinocytosis, or receptor-mediated.
• Exocytosis transports out
• Transcytosis involves transporting molecules into, across, and out of a cell
• Vesicular transport occurs within the cell

Nucleus

• Largest organelle that contains the genetic library for protein synthesis.
• Cells can have one (uninucleate), many (multinucleate), or none (anucleate)
• The nucleus responds by synthesizing proteins.

Cell Nucleus Details

• DNA contains code in form of molecules
• DNA directs synthesis for proteins
• DNA is contained in a nuclear envelope for protections
• The envelope also has nuclear pores for movement
• DNA joins with histones to then form chromatin
• Chromatin coils to then make chromosomes.
• Genetic code
  • Chemical language of DNA instructions
  • The process with base sequence (A, T, C, G)
  • Triplet code where Three bases = amino acid
• Gene is used DNA instructions to make one protein with heredity.

Information Storage in the Nucleus

• Genetic code constitutes the chemical language of DNA.
  • Uses sequence of bases A, T, C, and G
  • Triplet code is a three base = an amino acid
• Genes are the DNA instructions to make one protein with heredity.

Protein Synthesis

• DNA (master blueprint) transfers coded information to mRNA through process of transcription.
• mRNA exits the nucleus through pores; travels to cytoplasm to bind to ribosomes.
• Then ribosomes move along the mRNA translating genetic message with a specific amino acid sequence- also called translation.

Cytoplasm Composition

• A gel-like solution called cytosol.
  • composed of water and soluble molecules such as proteins, salts and sugars
• Inclusions are molecules that are cell specific e.g glycogen granules, ,pigments.
• Organelles are machinery and a required structures for specialised jobs in cell;

Organelles

• Nonmembranous organelles have:
  • No membrane
  • Direct contact with cytosol
  • Include cytoskeleton, centrioles, ribosomes, proteasomes, microvilli, cilia, and flagella
• Membranous organelles have: -Isolated from cytosol by a plasma membrane
  • Include endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and mitochondria

Cytoskeleton

• Made of structural proteins that determine cell shape and strength.
• Microfilaments:
  • thin filaments composed of actin.
  • provide mechanical strength.
  • interact with the protein myosin for muscle contraction.
• Intermediate filaments:
  • strengthen cell and maintain shape.
• Microtubules:
• strengthen cell, anchor organelles.

Ribosomes

• Nonmembranous organelles and location protein synthesis
• Composed of protein and ribosomal RNA (rRNA)
• Two switchable forms:
  • Free ribosomes found in cytosol and site of synthesis
  • Membrane: bound ribosomes of endoplasmic reticulum (ER with proteins for transport)

Endoplasmic Reticulum (ER)

• Storage chambers known as cisternae.
• Functions:
  1. Synthesis of proteins, carbohydrates, and lipids.
  2. Storage of synthesized molecules.
  3. Transport of materials within the ER.
  4. Detoxification of drugs or toxins.

Rough and Smooth Endoplasmic Reticulum

• Rough ER has Studded ribosomes whereas Smooth has non
• Rough manufactures proteins.
• Rough include plasma membrane with proteins and phospholipids
• Smooth synthesizes lipids (steroid hormones), lipid and cholesterol metabolism (liver cells), and absorption, synthesis, and transport of fats (intestinal cells).
• Smooth store and release for calcium used in Sarcoplasmic reticulum and specialzied smooth ER that skeletal and cardiac muscle cells

Golgi Apparatus

• Vesicles enter a face and then exit maturing face
• Modifies and packages proteins, e.g., hormones/enzymes.
• Adds/removes carbohydrates on proteins.
• Modifies plasma membrane.
• Packages special enzymes with lysosomes for internal use.

Lysosomes

• Spherical bags of digestive enzymes (acid hydrolases)
• Digest bacteria, viruses, and toxins (intracellular).
• Degrade non-functional organelles.
• Destroy cells in injured or non-useful tissue (auto-lysis).
• Break down/release glycogen and break down bone to release calcium.

Peroxisomes

• Are membranous sacs of oxidases and catalases to detoxify.
• Contain antioxidants which neutralize harmful substances.

Mitochondria

• Produce energy.
• Use breakdown of foods and oxygen to make ATP - called aerobic metabolism (cellular respiration).
• They produces 95 percent of ATP.
• Has its own DNA, RNA, and ribosomes.

Cellular Extensions

• Cilia is projections and hair-like of cell
  • It helps move surfaces e.g. respiratory with uterine tubes
• Flagella is sperm used in tails
• Microvilli is the for the surface for fingerlike extensions.
  • used for absorption in intestine.