Cell Structure and Function Flashcards

Types of Cells & Cell Size

• Characteristics of ALL Cells:

  • Bounded by a plasma membrane.
  • Contain cytosol (semifluid substance).
  • Contain chromosomes:
    • Prokaryotes: circular chromosome.
    • Eukaryotes: linear chromosomes.
  • Contain ribosomes (site of protein synthesis).
  • Cytoskeleton.

• Eukaryotes vs. Prokaryotes:

  Feature	Eukaryotes	Prokaryotes
  DNA	Linear chromosomes within a membrane-bound nucleus	Circular chromosome in the cytosol
  Size	5-100 µm	0.2-10 µm
  Organization	Often multicellular (some single-celled), some have cell walls	Single-celled, have cell walls
  Organelles	Membrane-bound organelles (mitochondria, ER, Golgi, etc.)	No membrane-bound organelles, different size ribosomes than eukaryotes
  Examples	Plants, animals, protists, fungi	Bacteria, archaea

• Structural Evidence for Eukaryotic Relatedness:

  • All eukaryotes possess:
    1. Nucleus
    2. Membrane-bound organelles (mitochondria, ER, Golgi, etc.)
    3. Endomembrane systems
    4. Linear chromosomes

• Benefits of Membrane-Bound Organelles (Eukaryotes):

  • Partitioning of functions to specialized organelles (e.g., rough ER, smooth ER, Golgi, mitochondria, chloroplast).
  • Prokaryotes lack membrane organelles but perform similar tasks along the cell membrane or within the cytosol.

• Evolution of Eukaryotes:

  • Internal membranes developed, creating internal micro-environments.
  • Advantage: specialization = increased efficiency (~2 billion years ago).
  • Infolding of the plasma membrane led to the formation of ER and the nuclear envelope.

• Endosymbiosis:

  • Mitochondria Origin:
    • Early eukaryotic heterotroph engulfed an aerobic bacterium but did not digest it.
    • Mutually beneficial relationship: Resources for mitochondria, extra ATP for host cell.
  • Chloroplasts Origin:
    • Came after mitochondria.
    • Eukaryote engulfed a photosynthetic bacterium (cyanobacteria) but did not digest it.
    • Mutually beneficial relationship: Resources for chloroplast, sugars for host cells.

• Endosymbiotic Theory Evidence:

  • Mitochondria and chloroplasts originated as separate organisms (bacteria) engulfed but not digested by an ancestral eukaryote, leading to a symbiotic relationship.
  • Evidence:
    • Mitochondria & chloroplasts divide and replicate independently of host cell division, similar to binary fission in bacteria.
    • Contain a single circular chromosome (DNA) like prokaryotes.
    • Contain protein-synthesizing ribosomes resembling prokaryotic ribosomes.
    • Double membranes; the inner one is similar in molecular structure to modern prokaryotic membranes.
    • Similar size and shape as prokaryotes.

• Animal vs. Plant Cells:

  • Unique to Plant Cells: Chloroplasts, cell wall, central vacuole, plasmodesmata.
  • Unique to Animal Cells: Gap junctions.

• Cell Size:

  • Experiment: Diffusion in Agar Cubes
    • Agar cubes of varying sizes (3x3x3 cm3, 2x2x2 cm3, 1x1x1 cm3) placed in a basic solution turn pink as the solution diffuses in.
    • Demonstrates the rate of diffusion relative to cell size.

• Why Cells are Small:

  • Surface area to volume ratio is critical.
  • As a cell increases in size, volume grows faster than surface area.
  • High surface area to volume ratio is required for efficient exchange of materials.
    1. Larger cells have longer transport distances.
    2. Smaller surface area relative to volume restricts material exchange.
  • While the rate of diffusion remains constant, cells with a higher surface area to volume ratio more efficiently exchange nutrients and waste.

• Surface Area Increase Mechanisms:

  • Small finger-like projections (root hairs, villi & microvilli) increase the surface area to volume ratio for material and energy exchange.

Organelles & Structures

• Plasma Membrane:

  • Semi-permeable lipid bilayer; phospholipids are the major constituent.
  • Separates internal from external environment.
  • Involved in cell adhesion, cell signaling, and transport of materials.

• Cytoplasm:

  • The entire contents of the cell, excluding the nucleus, bounded by the plasma membrane.
  • Includes the cytosol and organelles within the cytosol.
  • In prokaryotes, it includes the entire contents of the cell.
  • Many cell reactions take place here.
  • Cytosol: semifluid substance in the cytoplasm.

• Organelles:

  • Specialized subunit within a cell with a specific function.
  • Membrane-bound (e.g., nucleus, ER, Golgi, chloroplast, mitochondria, lysosomes).
  • Non-membrane bound: ribosomes.

• Nucleus:

  • Contains the genetic material (DNA) of a eukaryotic cell.
  • Nuclear Envelope: Encloses the nucleus, separating its contents from the cytoplasm.
  • Nucleolus: Found inside the nucleus, where ribosomes are assembled from rRNA and ribosomal proteins.

• Ribosome:

  • Site of protein synthesis; translation of genetic instructions yields specific polypeptides.
  • Composed of ribosomal RNA (rRNA) and protein.

• Ribosome Protein Product Destinations:

  • Free-Ribosomes: Most proteins function within the cytosol.
  • Rough ER Membrane Bound Ribosomes:
    1. Proteins destined for secretion
    2. Proteins destined for insertion into the cell membrane
    3. Proteins for organelles (lysosomes, Golgi, Endoplasmic Reticula)

• Endoplasmic Reticulum:

  • Interconnected system of tubules; continuous with the nuclear envelope.
  • Two Types:
    • Smooth ER: Lacks ribosomes, site of lipid synthesis (fatty acids, phospholipids, steroids), drug & poison detoxification (liver cells contain lots of smooth ER).
    • Rough ER: Compartmentalizes the cell, contains ribosomes that produce proteins for secretion, insertion into cell membrane, or for organelles.

• Golgi Apparatus & Vesicles:

  • Golgi Apparatus: Consists of flat membranous sacs, receives proteins from rough ER via vesicles, modifies proteins, places them within another vesicle, and routes them to their final destination (secretion, insertion into membrane, or for organelles).
  • Vesicles: Small membrane-bound sacs used to move chemicals to other locations in or outside the cell.

• Lysosome:

  • Contains hydrolytic enzymes.
  • Functions:
    1. Breaks down bacteria.
    2. Intracellular digestion.
    3. Autophagy: breaks down unnecessary or dysfunctional cell parts and organelles.
    4. Apoptosis: programmed cell death; used in programmed development and control of growth; destroys cells if DNA is damaged and cannot be fixed (mutations in certain genes prevent apoptosis in cancer cells).

• Vacuoles:

  • A membrane-bound sac that plays roles in intracellular digestion and the release of cellular waste products.
    • Food Vacuole: Stores food taken into the cell.
    • Central Vacuole: Stores water to maintain turgor pressure in plant cells; can also store nutrients.
    • Contractile Vacuole: Found in freshwater protists; maintains water balance by pumping out excess water.

• Endomembrane System:

  • Components: Nuclear Envelope, Rough ER, Smooth ER, Golgi Apparatus, Lysosomes, Vacuoles, Vesicles, Cell Membrane.
  • Functions:
    1. Synthesis of proteins and their transport into membranes and organelles or out of the cell.
    2. Metabolism and movement of lipids.
    3. Detoxification of poisons.

• Chloroplast & Mitochondria:

  • Chloroplast: Performs photosynthesis; converts light energy to chemical energy in the form of carbohydrates; found within algae and plants.
  • Mitochondria: Site of cell respiration; uses oxygen to break down organic compounds and produce ATP; found in eukaryotic cells.

• Cytoskeleton:

  • A network of fibers that organizes structures and activities in the cell
  • Functions:
    1. Maintains cell shape (morphological integrity).
    2. Anchorage for organelles.
    3. Allows for the movement of organelles and vesicles within the cell.
    4. Helps separate the chromosome copies in dividing cells.
  • Types of fibers: microtubules, microfilaments (actin), intermediate filaments.

• Centrosomes:

  • Centrosomes are the microtubule organizing centers that forms the mitotic spindle in dividing cells.

• Cell Wall:

  • Maintenance of cell shape and skeletal support.
  • Provides protection for plant cells.
  • Found in plant cells, fungi, & bacteria.
    • Plant cell wall: made of cellulose.
    • Fungal cell wall: made of chitin.
    • Bacteria cell wall: made of peptidoglycan.

• Cell Movement:

  • Flagella: Tail-like projection powered by motor proteins; involved in cell movement.
  • Cilia: Short finger-like projections powered by motor proteins; used for cell movement.
  • Pseudopod: Temporary arm-like cellular extension; used for cell movement and ingestion.

• Prokaryotic Cell Function:

  • Prokaryotic cells lack membrane-bound organelles found in eukaryotes but perform the same functions.

  Eukaryotic Organelle	How Prokaryotes Carry Out the Function
  Nucleus	Hereditary information/DNA/chromosomes or RNA synthesis in cytosol.
  Ribosomes	Prokaryotes have ribosomes too for protein synthesis
  Rough ER	Protein synthesis/transport in cytosol; transcription & translation may be linked
  Smooth ER	Lipid synthesis or detoxification occurs in cytosol.
  Mitochondria	Other membranes or cytosolic molecules function in ATP synthesis.
  Chloroplasts	In Phototrophic Bacteria: Other membranes or cytosolic molecules function in light absorption/photosynthesis
  Cilia or flagella	Motility via bacterial flagella.
  Vacuole, vesicles	Inclusion bodies/granules/large molecules in cytosol.

• Intercellular Junctions:

  • Plasmodesmata: Channel in the cell wall connecting two plant cells, allowing water and solutes to pass between cells.
  • Gap Junction: Intercellular connection between animal cells that allows water and solutes to pass between cells.

Plasma Membrane

• Phospholipids:
  • Phosphate head (hydrophilic).
  • Fatty acid tails (hydrophobic).
  • Arranged as a bilayer.
• Cell Membrane:
  • Serves as a cellular barrier/border.
  • Controls traffic in & out of the cell.
  • Semipermeable: allows some substances to cross more easily than others.
    • Permeable: small nonpolar molecules (lipids, CO2, O2).
    • Impermeable: polar and charged molecules (sugar, water, ions).
• Membrane Fluidity & Phospholipid Saturation
  • Fluid: Unsaturated hydrocarbon tails with kinks.
  • Viscous: Saturated hydrocarbon tails.
• Cholesterol:
  • Stabilizes the fluidity of the cellular membrane.
• Polar and Charged Molecules Transport:
  • Specific protein channels and pumps enable polar and charged molecules to cross the membrane.
  • Protein channels and pumps are specific (e.g., Na^+ channels only allow sodium across).
• Protein Domains Anchor the Protein Channel:
• Within membrane Nonpolar Amino Acids anchors protein into the membrane
• Outer surfaces of membrane (polar heads) and aqueous environment Polar amino acids extend into extracellular fluid & into cystosol
• Functions of Membrane Proteins:
  • Transport, cell surface receptors, enzyme activity, cell surface identity markers, attachment to the cytoskeleton, cell adhesion.
• Membrane Carbohydrates:
  • Glycoproteins & Glycolipids: Play a key role in cell-cell recognition; important in tissue & organ development; basis for rejection of foreign cells by the immune system.
• Fluid Mosaic Model:
  • Mosaic: The cell membrane is made of many components (phospholipids, cholesterol, proteins, glycoproteins, glycolipids).
  • Fluid: All the components flow past each other.

Transport

• Phospholipid Bilayer Permeability:

  • What molecules can get through directly? Small & Nonpolar (lipids, O2, CO2).
  • What molecules cannot get through directly? Polar molecules (H2O, NH3), ions (Na^+, K^+, Cl^-, Ca^{2+}), large molecules (polymers: starch, proteins).

• 2nd Law of Thermodynamics

  • governs biological systems universe tends towards disorder (entropy) High to Low concentration – molecules move down the gradient

• Passive Transport:

  • No energy is required from the cell.
  • Molecules move from areas of higher concentration to areas of lower concentration (down the concentration gradient).
    1. Simple Diffusion: Molecules that can naturally cross the cell membrane without help (small, nonpolar molecules e.g., O2, CO2, Lipids). Increased temperature speeds up the rate of diffusion.
    2. Facilitated Diffusion: Diffusion of polar and charged molecules through transport proteins (channel proteins, carrier proteins, gated channels). Most transport proteins are specific.
    3. Osmosis

• Active Transport:

  • Requires the cell to input energy into the system.
    1. Protein Pumps: Moves molecules against the concentration gradient (from Low to High). Energy from ATP causes a conformational shape change in the pump, transporting solute from one side of the membrane to the other. Generates a gradient which is a form of potential energy.

• Active Transport Example Sodium-potassium pump:

• Moves sodium and potassium against their concentration gradient (requires energy – breakdown of ATP)

• Each cycle moves 3 Na^+ out and 2 K^+ in

• Makes the interior of the cell relatively negative compared to the extracellular fluid

• The purpose of pumps is to generate an electrochemical gradient. Gradients are forms of potential energy that can drive other reactions.

• Electrochemical Gradient

  • Ions move down the concentration gradient based upon the interaction between two forces
    • 1. Chemical (diffusional) force molecules move high to low concentrations
    • 2. Electric force + ions attracted to negative membrane potential and vice versa Represent

• Membrane Potential

  • The difference in charge across the membrane (voltage)
  • The greater the difference in charge (voltage) the more potential energy the gradient contains
  • The cytoplasm of the cell is negative in charge compared to the extracellular fluid because of the unequal distribution of anions and cations

• Active Transport Cotransport

• Coupled Transport

• Coupling the movement of a substance down its gradient (releases energy) to drive a different substance against its gradient (requires energy)

• Sometimes called secondary active transport ATP is used to generate the original gradient through active transport

  • Plant Cell Example: H^+ Moving [high] to [low] drives the movement of sucrose from [low] to [high] Proton pump sets up a high concentration of H^+ outside the cell, which is a source of potential energy.

• Cotransport

  • Uniport only transports one molecule at a time
  • Symport transports two different molecules in the same direction, either into or out of the cell.
  • Antiport transports two different molecules in opposite directions, one into the cell and one out of the cell

• Active Transport Bulk Transport

• movement of large molecules or large quantities of smaller molecules across the membrane

• A form of active transport - Requires energy to move and form the vesicle

• e.g. – Exocytosis, Endocytosis

  • Exocytosis the cellular secretion of macromolecules through the fusion of vesicles with the plasma membrane Requires energy to form and move the vesicle
  • Endocytosis
    The cellular uptake of molecules by a region of the plasma membrane surrounding the substance and pinching off to form an intracellular vesicle Requires energy to form and move the vesicle

• Endocytosis Three Types
  1. Phagocytosis cellular intake of a large substance (e.g. polymer) or a small organism such as a bacterium via a vesicle
  2. Pinocytosis cellular intake of extracellular fluids and its dissolved solutes
  3. Receptor-mediated endocytosis
  The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taking in enables a cell to acquire bulk quantities of a specific substance Ligand
  A molecule that binds specifically to a receptor site of another molecule e.g. - LDL

• Osmosis

  • Diffusion (passive transport) of water across a selectively permeable membrane
  • Most water will pass through a protein channel (water is polar)

• Concentration of Water

  • Direction of osmosis is determined by comparing the total solute concentrations between the different environments
  • Only solutes that are unable to diffuse across the membrane affect osmosis
    • Hypertonic – Higher [solute], Lower [water]
    • Hypotonic – Lower [solute], Higher [water]
    • Isotonic – Equal [solute] & [water]

• Managing Water Balance

  • Cell survival depends on balancing water uptake & loss
  • In relation to Freshwater, Balanced, and Saltwater environments

• Water movement Rapidly into & out of cells

• evidence that there were water channels- aquaporins. Water moves rapidly into & out of cells, evidence that there were water channels water channels allowing flow of water across cell membrane Some water technically can leak through the membrane via simple diffusion, but only at a very slow rate

Water Potential

• Water Potential
  • The physical property predicting the direction water will flow (osmosis), governed by solute concentration and applied pressure
  • Ψ{cell} = ΨP + Ψ_S
    • Ψ_P = pressure potential (called turgor pressure in plants)
      • represents the pressure in addition to atmospheric pressure
      • At atmosphere pressure, pressure potential is zero, Ψ_P = 0
    • Ψ_S = solute potential (also called osmotic potential)
      • represents the effect of dissolved solutes on water potential; addition of solutes will always lower the water potential
      • solute potential represents the ability to dissolve solute. The more solute present the more negative the solute potential.
      • Pure water has a solute potential of zero, Ψ_S = 0
  • Pure water at atmospheric pressure has a water potential of zero
  • Water will move from high water potential to Low water potential
  • The addition of solutes Reduces solute potential (more negative) Reduces water potential
• Turgor Pressure The force directed against a cell wall after the influx of water and the swelling of a walled cell due to osmosis Turgid
  A walled cell that has a greater solute concentration than its surrounding hypotonic environment, resulting in an entry of water
• If a cell is placed in a solution with a lower solute concentration (hypotonic solution)
  The cell will gain water and become turgid
• The Solute Potential of the Solution Ψ_s = – iCRT
  • i = ionization constant
    • For sucrose this is 1.0 because sucrose does not ionize in water,
    • For NaCl it would be 2.0 since NaCl ionizes into 2 ions, Na + and Cl-
  • C= molar concentration
  • R= pressure constant (R = 0.0831 liter bars/mole K)
  • T = temperature in Kelvin (273 + oC)

Osmoregulation

• Conformers vs. Regulators:
  • Two different evolutionary paths
    • Regulators
      Maintain relatively constant internal conditions no matter the outside environment (requires energy)
    • Conformers
      Allow internal conditions to match the external environment conformer thermoregulation regulator conformer osmoregulation regulator
• Homeostasis
  • “steady state,” maintain internal body conditions despite fluctuating external conditions
  • Maintained by Negative Feedback
    • e.g. Maintain pH, temperature, osmolarity, blood sugar levels, metabolic rate, blood calcium levels etc.
• Homeostatic control mechanisms include at least three interdependent components
  * Receptor monitors the environment, sends signals to the control center
  * Control Center responds to changes in the environment
  * Effector a muscle or gland receives signals from the control center and corrects the deviation
• Osmoregulation
  • Osmolarity
    A measure of the concentration of solute particles in a solution that contribute to osmotic pressure (Osm/L)
  • Osmoregulation
    The process of monitoring and maintaining a certain osmolarity in a solution such as blood through negative feedback mechanisms
• Contractile Vacuole
• found in freshwater protists
• Maintains water balance (homeostasis) by pumping excess water out of a cell
• Osmoregulation in Vertebrates
  • Kidneys are the major excretory organs of vertebrates
  • Function in both excretion and osmoregulation
  • Excretion The disposal of nitrogen-containing metabolites and other waste compounds
  • Osmoregulation Regulation of solute concentrations and water balance by a cell or organism
• Osmoregulation Maintaining Water Balance (Homeostasis)
  * Hypothalamus monitors blood osmolarity
  * When blood osmolarity level is too high
  * too many solutes in blood due to dehydration or a high salt diet
  * Hypothalamus stimulates thirst = drink more
  * Hypothalamus stimulates posterior pituitary gland release ADH
  * increases water permeability of collecting duct of the kidney increases water absorption back into blood concentrates urine (darker yellow) decrease urine volume
• Fish Osmoregulation
  • Freshwater Environment
    • Hypotonic environment compared to the fish cells
    • Fish must actively uptake salt ions from the environment via the gills
    • kidneys excrete large amounts of water in dilute urine
  • Saltwater Environment
    • Hypertonic environment compared to the fish cells
    • Fish must actively pump salt ions out of gills
    • kidneys excrete salt ions in a concentrated urine to limit water loss