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HBIO1 > Cell Function
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Fluid-Mosaic Model

  • Membranes consist of a phospholipid bilayer studded with proteins, polysaccharides, lipids
  • The lipid bilayer is semipermeable - H2O and some small, uncharged, molecules (O2, CO2) can pass through
  • Phospholipids have two parts
    • "Head": hydrophilic → attracts and mixes with H2O
    • Two "fatty acid tails": hydrophobic

Passive Transport


  • Uses energy from moving particles (kinetic energy)
  • Substances move down their conc. gradient until the conc. are in equilibrium
  • Fick's lawrate of diffusion across an exchange surfaces (e.g. membrane, epithelium) depends on
    • Surface area across within diffusion occurs (larger)
    • Thickness of surface (thinner)
    • Difference in conc. gradient (larger)
    • (surface area * difference in conc.) / thickness of surface
  • Microvilli
    • Extensions of the plasma membrane
    • They increase the surface area of the membrane
    • Accelerate the rate of diffusion
  • Temperature increases rate of diffusion due to increasing K.E. (kinetic energy)

Facilitated Diffusion

  • Transmembrane proteins form a water-filled ion channel
    • Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. Gradient
    • NB: this is a passive process → no ATP required
    • Some channels use a gate to regulate the flow of ions
    • Selective permeability → not all molecules can pass through selective channels
  • Transport mechanism
    • Carrier protein binds to substrate (specific molecule)
    • Molecule changes shape
    • Release of the diffusing molecule (product) at the other side of the membrane
  • Example
    • If you want to move a muscle, a nerve impulse is sent to this muscle
    • The nerve impulse triggers the release of a neurotransmitter
    • Neurotransmitter binds to a specific transmembrane protein
    • The protein opens channels that allow the passage of Na+ across the membrane
    • In this specific case, this causes muscle contraction
    • These Na+ channels can also be opened by a change in voltage


  • Special term used for the diffusion of water through a differentially permeable cell membrane
  • Water is polar and able to pass through the lipid bilayer
  • Transmembrane proteins that form hydrophilic channels accelerate osmosis, but water is still able to get through membrane without them
  • Osmosis generates pressure called osmotic pressure
    • Water moves down its conc. gradient
    • When pressure is equal on both sites net flow ceases (equilibrium)
    • The pressure is said to be hydrostatic (water-stopping)

Water Potential

  • Measurement of ability or tendency of water molecules to move
  • Water potential of distilled water is 0, other solutions have a negative water potential
  • Hypotonic
    • Solution is more dilute / has a lower conc. of solute / gains water by osmosis
    • Cells placed in a hypotonic solution will increase in size as water moves in
    • For example, red blood cells would swell and burst
    • Plant cells are unable to burst as they have a strong cellulose cell wall
  • Hypertonic
    • Solution with a higher conc. of solutes / loses water by osmosis
    • Cells will shrink in hypertonic solutions
  • Isotonic
    • Solutions being compared have equal conc. of solutes
    • Cells which are in an isotonic solution will not change their shape
    • The extracellular fluid of the body is isotonic
  • Molecules collide with membrane / creates pressure, water potential
  • More free water molecules, greater water potential, less negative
  • Solute molecules attract water molecules which form a "shell" around them
    • water molecules can no longer move freely
    • less "free water" which lowers water potential, more negative

Active Transport

  • Movement of solute against the conc. gradient, from low to high conc.
  • Involves materials which will not move directly through the bilayer
  • Molecules bind to specific carrier proteins / intrinsic proteins
  • Involves ATP by cells (mitochondria) / respiration
    • Direct active transport - transporters use hydrolysis to drive active transport
    • Indirect active transport - transporters use energy already stored in gradient of a directly-pumped ion
  • Bilayer protein transports a solute molecule by undergoing a change in shape (induced fit)
  • Occurs in ion uptake by a plant root; glucose uptake by gut cells


  • 20-30nm in size
  • Small organelles often attached to the ER but also found in the cytoplasm
  • Large (protein) and small (rRNA) subunits form the functional ribosome
    • Subunits bind with mRNA in the cytoplasm
    • This starts translation of mRNA for protein synthesis (assembly of amino acids into proteins)
  • Free ribosomes make proteins used in the cytoplasm. Responsible for proteins that
    • go into solution in cytoplasm or
    • form important cytoplasmic, structural elements
  • Ribosomal ribonucleic acid (rRNA) are made in nucleus of cell

Endoplasmic Reticulum (ER)

  • Rough ER
    • Have ribosomes attached to the cytosolic side of their membrane
    • Found in cells that are making proteins for export (enzymes, hormones, structural proteins, antibodies)
    • Thus, involved in protein synthesis
    • Modifies proteins by the addition of carbohydrates, removal of signal sequences
    • Phospholipid synthesis and assembly of polypeptides
  • Smooth ER
    • Have no ribosomes attached and often appear more tubular than the rough ER
    • Necessary for steroid synthesis, metabolism and detoxification, lipid synthesis
    • Numerous in the liver

Golgi Apparatus

  • Stack of flattened sacs surrounded by membrane
  • Receives protein-filled vesicles from the rough ER (fuse with Golgi membrane)
  • Uses enzymes to modify these proteins (e.g. add a sugar chain, making glycoprotein)
  • Adds directions for destination of protein package - vesicles that leave Golgi apparatus move to different locations in cell or proceed to plasma membrane for secretion
  • Involved in processing, packaging, and secretion
  • Other vesicles that leave Golgi apparatus are lysosomes

Endocytosis and Exocytosis

  • Substances are transported across plasma membrane in bulk via small vesicles
  • Endocytosis
    • Part of the plasma membrane sinks into the cell
    • Forms a vesicle with substances from outside
    • Seals back onto the plasma membrane again
    • Phagocytosis: endocytosis brings solid material into the cell
    • Pinocytosis: endocytosis brings fluid materials into the cell
  • Exocytosis
    • Vesicle is formed in the cytoplasm // may form from Golgi apparatus
    • Moves towards plasma membrane and fuses with plasma membrane
    • Contents are pushed outside cell
    • Insulin is secreted from cells in this way


  • 1µm in diameter and 7µm in length
  • Mostly protein, but also contains some lipid, DNA and RNA
  • Power house of the cell
  • Energy is stored in high energy phosphate bonds of ATP
  • Mitochondria convert energy from the breakdown of glucose into adenosine triphosphate (ATP)
  • Responsible for aerobic respiration
  • Metabolic activity of a cell is related to the number of cristae (larger surface area) and mitochondria
  • Cells with a high metabolic activity (e.g. heart muscle) have many well developed mitochondria