Chapter 3A

Cells: The Living Units

3.1 Cells: The Smallest Living Units

• Cell theory:
  • A cell is the structural and functional unit of life.
  • The function of an entire organism depends on the individual and combined activities of all its cells.
  • Structure and function are complementary; biochemical functions are dictated by cell shape and specific subcellular structures.
  • Continuity of life has a cellular basis; cells arise only from preexisting cells.

Cell Diversity

• Over 250 different types of human cells exist, differing in size, shape, and subcellular components, leading to functional diversity.
• Examples:
  • Cells that connect body parts/form linings/transport gases: Fibroblasts, Erythrocytes, Epithelial cells.
  • Cells that move organs and body parts: Skeletal and Smooth muscle cells.
  • Cell that stores nutrients: Fat cell.
  • Cell that fights disease: Macrophage.
  • Cell that gathers information and controls body functions: Nerve cell.
  • Cell of reproduction: Sperm.

Generalized Cell

• All cells share common structures and functions.
• Human cells have three basic parts:
  • Plasma membrane: Flexible outer boundary.
  • Cytoplasm: Intracellular fluid containing organelles.
  • Nucleus: DNA-containing control center.

Extracellular Materials

• Substances found outside cells.
• Classes:
  • Extracellular fluids (body fluids):
    • Interstitial fluid: Cells are submersed (bathed) in this fluid.
    • Blood plasma: Fluid of the blood.
    • Cerebrospinal fluid: Fluid surrounding nervous system organs.
  • Cellular secretions (e.g., saliva, mucus).
  • Extracellular matrix: Substance that acts as glue to hold cells together.

Plasma Membrane

• Acts as an active barrier separating intracellular fluid (ICF) from extracellular fluid (ECF).
• Plays a dynamic role in cellular activity by controlling what enters and leaves the cell.
• Also known as the “cell membrane”.

3.2 Structure of Plasma Membrane

• Consists of membrane lipids that form a flexible lipid bilayer.
• Specialized membrane proteins float through this fluid membrane, resulting in constantly changing patterns (fluid mosaic model).
• Surface sugars form glycocalyx.
• Membrane structures help hold cells together through cell junctions.

Membrane Lipids

• Lipid bilayer composition:
  • 75% Phospholipids:
    • Phosphate heads: Polar (charged), hydrophilic (water-loving).
    • Fatty acid tails: Nonpolar (no charge), hydrophobic (water-hating).
  • 5% Glycolipids: Lipids with sugar groups on the outer membrane surface.
  • 20% Cholesterol: Increases membrane stability.

Membrane Proteins

• Allow cell communication with the environment.
• Make up about half the mass of the plasma membrane.
• Most have specialized membrane functions.
• Some float freely, and some are tethered to intracellular structures.
• Two types:
  • Integral proteins: Firmly inserted into the membrane, most are transmembrane (span the membrane).
    • Have hydrophobic areas (interact with lipid tails) and hydrophilic areas (interact with water).
    • Function as transport proteins (channels and carriers), enzymes, or receptors.
  • Peripheral proteins: Loosely attached to integral proteins.
    • Include filaments on the intracellular surface for plasma membrane support.
    • Function as enzymes, motor proteins (for shape change during cell division and muscle contraction), or in cell-to-cell connections.

Glycocalyx

• Consists of sugars (carbohydrates) sticking out of the cell surface, attached to lipids (glycolipids) or proteins (glycoproteins).
• Every cell type has different patterns of this “sugar coating”.
• Functions:
  • Specific biological markers for cell-to-cell recognition.
  • Allows the immune system to recognize “self” vs. “nonself”.

Clinical – Homeostatic Imbalance 3.1

• Glycocalyx of some cancer cells can change so rapidly that the immune system cannot recognize the cell as being damaged.
• The mutated cell is not destroyed by the immune system, so it can replicate.

Cell Junctions

• Some cells are “free” (not bound to other cells), e.g., blood cells, sperm cells.
• Most cells are bound together to form tissues and organs. Three ways cells can be bound together:
  • Tight junctions:
    • Integral proteins on adjacent cells fuse to form an impermeable junction that encircles the whole cell.
    • Prevent fluids and most molecules from moving between cells.
  • Desmosomes:
    • Rivet-like cell junction formed when linker proteins (cadherins) of neighboring cells interlock like the teeth of a zipper.
    • Linker protein is anchored to its cell through thickened “button-like” areas on the inside of the plasma membrane called plaques.
    • Keratin filaments connect plaques intercellularly for added anchoring strength.
    • Allow “give” between cells, reducing the possibility of tearing under tension.
  • Gap junctions:
    • Transmembrane proteins (connexons) form tunnels that allow small molecules to pass from cell to cell.
    • Used to spread ions, simple sugars, or other small molecules between cells.
    • Allow electrical signals to be passed quickly from one cell to the next (used in cardiac and smooth muscle cells).

3.2 Structure of Plasma Membrane

• Many substances must constantly move across the plasma membrane; some pass through easily, some do not.
• The plasma membrane is selectively permeable, allowing only certain molecules to cross.
• Two essential ways substances cross the plasma membrane:
  • Passive transport: No energy is required.
  • Active transport: Energy (ATP) is required.

3.3 Passive Membrane Transport

• Passive transport requires no energy input.

• Three types of passive transport:

  • Simple Diffusion
  • Facilitated diffusion
  • Osmosis

• All types involve diffusion: Natural movement of molecules from areas of high concentration to areas of low concentration (moving down a concentration gradient).

• All molecules have random, high-speed movement due to their intrinsic kinetic energy.

• Movement results in collisions between molecules.

• Molecules in higher concentration areas collide more, resulting in molecules being scattered to lower concentration areas.

• Speed of diffusion is influenced by:

  • Concentration: The greater the difference in concentration between two areas, the faster diffusion occurs.
  • Molecular Size: Smaller molecules diffuse faster.
  • Temperature: Higher temperatures increase kinetic energy, resulting in faster diffusion.

• Equilibrium is reached when there is no net movement of molecules in one direction only.

• Molecules have a natural drive to diffuse down concentration gradients that exist between extracellular and intracellular areas.

• Nonpolar, hydrophobic lipid core of plasma membranes stops diffusion and creates concentration gradients by acting as selectively permeable barriers (also called “differentially permeable” barrier).

• Molecules that can passively diffuse through the membrane include:

  • Lipid-soluble and nonpolar substances.
  • Very small molecules that can pass through the membrane or membrane channels.
  • This is referred to as simple diffusion.

• Larger or non-lipid soluble or polar molecules can cross the membrane but only with the assistance of carrier molecules; this is referred to as facilitated diffusion.

• Osmosis is a special name for the movement of solvent (usually water), not molecules.

Clinical – Homeostatic Imbalance 3.2

• If the plasma membrane is severely damaged, substances diffuse freely into and out of the cell, compromising concentration gradients.
• Example: Burn patients lose precious fluids, proteins, and ions that weep from damaged cells.

Simple Diffusion

• Nonpolar, lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer.
• Examples: Oxygen, carbon dioxide, steroid hormones, fatty acids.
• Small amounts of very small polar substances, such as water, can pass.

Facilitated Diffusion

• Certain hydrophobic molecules (e.g., glucose, amino acids, and ions) are transported passively down their concentration gradient by:

  • Carrier-mediated facilitated diffusion: Substances bind to protein carriers.
  • Channel-mediated facilitated diffusion: Substances move through water-filled channels.

• Carrier-mediated facilitated diffusion:

  • Carriers are transmembrane integral proteins.
  • Each carrier transports specific polar molecules, such as sugars and amino acids, that are too large for membrane channels.
  • Binding of a molecule causes the carrier to envelope it and change shape, resulting in the molecule being moved across the membrane.
  • Binding is limited by the number of carriers present; carriers are saturated when all are bound to molecules and are busy transporting.

• Channel-mediated facilitated diffusion:

  • Channels with aqueous-filled cores are formed by transmembrane proteins.
  • Channels transport molecules such as ions or water (osmosis) down their concentration gradient.
    • Specificity is based on pore size and/or charge.
    • Water channels are called aquaporins.
  • Two types:
    • Leakage channels: Always open.
    • Gated channels: Controlled by chemical or electrical signals.

Osmosis

• Movement of solvent (not molecules), such as water, across a selectively permeable membrane.

• Water diffuses across plasma membranes:

  • Through the lipid bilayer (even though water is polar, it is so small that some molecules can sneak past nonpolar phospholipid tails).
  • Through specific water channels called aquaporins (AQPs).

• Flow occurs when water (or other solvent) concentration is different on the two sides of a membrane.

• Osmolarity: Measure of the concentration of the total number of solute particles in a solvent.

• Water concentration varies with the number of solute particles because solute particles displace water molecules.

  • When solute concentration goes up, water concentration goes down, and vice versa.

• Water moves by osmosis from areas of low solute (high water) concentration to areas of high solute (low water) concentration.

• When solutions of different osmolarities are separated by a membrane permeable to all molecules, diffusion of solutes and osmosis of water occur across the membrane until equilibrium of solutes and water is reached.

  • Equilibrium: The same concentration of solutes and water molecules on both sides, with equal volume on both sides.

• When solutions of different osmolarities are separated by a membrane that is permeable only to water, only osmosis (not diffusion) will occur until equilibrium is reached.

  • Water will have net movement across the membrane until osmolarity is the same on both sides.
  • This results in volume changes on both sides:
    • Low solute side volume decreases.
    • High solute side volume increases.

• Movement of water involves pressures:

  • Hydrostatic pressure: Outward pressure exerted on the cell side of the membrane caused by increases in the volume of the cell due to osmosis (also referred to as “back pressure”).
  • Osmotic pressure: Inward pressure due to the tendency of water to be “pulled” into a cell with higher osmolarities. The more solutes inside a cell, the bigger the pull on water to enter, resulting in higher osmotic pressures inside the cell.

• When hydrostatic pressure equals osmotic pressure, no further net movement of water occurs.

  • Water trying to get out equals water trying to get in.

• Plant cells are surrounded by strong cell walls that act to limit hydrostatic pressure levels, which, in turn, limit osmotic pressure (plant cells will fill with only so much water then stop).

• Animal cells do not have cells walls, therefore they cannot limit hydrostatic and osmotic pressures (animal cells will burst if too much water is taken in).

• Water can also leave a cell, causing it to shrink.

• Tonicity:

  • Ability of a solution to change the shape or tone of cells by altering the cells’ internal water volume.
    • An isotonic solution has the same osmolarity as inside the cell, so volume remains unchanged.
    • A hypertonic solution has a higher osmolarity than inside the cell, so water flows out of the cell, resulting in cell shrinking (crenation).
    • A hypotonic solution has a lower osmolarity than inside the cell, so water flows into the cell, resulting in cell swelling (can lead to cell bursting, referred to as lysing).

• Osmolarity is equal to molarity times the number of ions (particles).

  • Example: NaCl (1 particle) ionizes to Na^+ and Cl^− (2 particles).
  • Therefore, a 1 M solution of NaCl equals a 2 Osm solution.

• Osmolarity is expressed in osmoles/liter (osmol/L).