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Cell Structure and Function Chapter 4 Biology 100. Discovery of the Cell. Robert Hooke used a simple kind of microscope to study slices of cork in 1664. He saw many cubicles fitting neatly together Hooke called these cells.

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discovery of the cell
Discovery of the Cell
  • Robert Hooke used a simple kind of microscope to study slices of cork in 1664.
  • He saw many cubicles fitting neatly together
    • Hooke called these cells.
  • van Leeuwenhoek was the first to see living cells and later was first to see bacteria

Van Leeuwenhoek’s microscope

the cell theory
The Cell Theory
  • Schleiden and Schwann came up with the theory in the 1830’s
    • All living things are made of cells
  • Virchow added in 1855 to the cell theory
    • New cells are formed only from division of pre-existing cells, not spontaneous generation
differences and similarities of cells
Differences and Similarities of Cells
  • All cells are surrounded by a plasma membrane (cell membrane) which is selectively permeable to materials.
  • Prokaryotes lack a true nucleus as well as internal membrane-bound organelles.
    • Bacteria
  • Eukaryotes have a true nucleus and have at least one membrane-bound organelle
    • Include plant, animal, fungi, protozoa and algae cells
cell size
Cell Size
  • Prokaryotic cells can be 1-10 μm, while eukaryotic cells are 10-100 μm.
  • Some eukaryotic cells are quite large, like the yolk of a chicken egg
  • Two organelles found in eukaryotes, the mitochondrion and the chloroplast, are similar in size to most bacteria.
cell size1
Cell Size
  • The ratio of surface area to cell volume limits cell size because it reflects the balance between supply rate and supply demand.
    • The surface area determines the rate at which materials diffuse into or out of the cell.
  • For a cell of a constant shape, for every time the surface area increases by L2, volume increases L3
  • Prokaryotes have no true nucleus or membrane-bound organelles, have a nucleoid region
  • Has a Cell Wall, and some may have a capsule, which encloses the cell wall
  • Forms of movement are the flagella and/or the pilli
  • Contains ribosomes where protein synthesis occurs
  • Bacteria cells have different shapes
    • Rod-shape
    • Spherical shape
    • Spiral
  • Eukaryotes, unlike prokaryotes, have a true, membrane-bound nucleus.
    • Contain a variety of organelles, specialized membrane-bound structures where cell processes occur
endoplasmic reticulum
Endoplasmic Reticulum
  • Endoplasmic Reticulum (ER) is where proteins and lipids are synthesized
    • Large surface area
    • Rough ER is embedded with ribosomes
      • Ribosomes are nonmembranous organelles that help with synthesis of proteins
    • Smooth ER is where lipids are synthesized, detox
golgi apparatus
Golgi Apparatus
  • Golgi Apparatus are smooth, flattened membranous sacs
    • Collects, packages and distributes molecules manufactured in the cell
    • Animals contain around 20 complexes, plants have hundreds.
vesicles and vacuoles
Vesicles and Vacuoles
  • Tiny, membranous sacs known as vesicles deliver molecules to and from the Golgi Complex, vacuoles are larger structures that perform the same tasks
    • Some go from ER to the Golgi Complex
    • Some go to other organelles in cell
    • Others will go to plasma membrane and combine with it
      • May contain insulin, enzymes, etc. to go outside the cell
      • In plants, some vesicles have cellulose to make new cell wall material
    • Some vesicles contain enzymes to breakdown various molecules

Fig. 4.11, pg. 77.

  • A lysosome is a tiny vesicle that buds off the Golgi Apparatus and contains enzymes that break down macromolecules, for digestion and destruction
    • These enzymes function best at a pH of 5
      • Hydrogen ions are transported into lysosome to create this acidic environment
    • Will also destroy bacteria, viruses and fungi
  • A peroxisome is an organelle that has the enzyme, catalase, that breaks down hydrogen peroxide, H2O2
    • Breaks down fatty acids into 2 carbon fragments
    • n addition it includes enzymes which synthesize cholesterol and bile acids.
  • Is not formed in the Golgi apparatus
  • Aids chemical reactions, including the breakdown of fatty acids, synthesis of cholesterol and synthesis of lipid molecules
  • Ribosomes are constructed from two subunits, which are composed of ribosomal RNA and proteins.
  • Ribosomes synthesize polypeptides from free amino acids, according to the instructions on messenger RNA.
  • Ribosomes are like CD players, producing music (proteins) according to the instructions on the CD (mRNA).
  • The nucleus is surrounded by two membranes, the nuclear envelope
  • Protein complexes at nuclear pores regulate the entry of large macromolecules into and out of the nucleus
  • The nucleus contains most of the DNA in a cell.
  • The primary function of the nucleus is to transfer the information for the synthesis of proteins from DNA to RNA.
  • The nucleolus is a dense area within the nucleus with DNA fragments, ribosomal RNA, and proteins.
  • The nucleolus organizes the RNA and proteins into the ribosomal subunits.
  • Mitochondrion (singular) has two membranes
  • Outer membrane is relatively simple, but the inner membrane is highly folded, a structure called cristae
  • Cristae are rich in enzymes for electron transfer and ATP synthesis
  • The matrix is a fluid filled space inside the inner membrane
  • It contains soluble enzymes for aerobic cellular respiration.
  • The matrix also contains DNA (mtDNA), RNA, and ribosomes.
  • Known as the powerhouse of the cell
  • It converts the energy stored in organic molecules to forms usable to the cells, especially production of ATP
  • Food + O2 → CO2 +H20 + Energy (ATP)
  • Chloroplasts are also surrounded by two membranes.
  • The outer and inner membranes, and intermembrane space are barriers, but play no specific functional role in photosynthesis.
  • Inside the inner membrane is the stroma, an aqueous space.
  • Floating in the stroma are thylakoids, flat membranous sac.
  • The thylakoid membrane contains chlorophyll and other pigments, electron transfer molecules, and enzymes that trap the energy in sunlight - photosynthesis.
  • This energy is used to generate ATP and high energy electrons in the light-dependent phase of photosynthesis.
  • These molecules pass to the stroma where CO2 and H2O are converted into sugars.
  • The stroma also contains DNA (chDNA), RNA, and ribosomes.
  • In the stroma, DNA is transcribed to RNA and RNA is translated into some chloroplast proteins.
  • Intermediate filaments are semi-permanent components of the cytoskeleton.
  • Intermediate filaments are semi-permanent components of the cytoskeleton.
  • Intermediate filaments maintain cell shape and attach to proteins in the cell membrane
  • Microfilaments are built with the beadlike protein actin
  • During cell division, motor proteins pull actin filaments together, slicing the cytoplasm in half like string around a ball of dough.
  • In muscle cells, the motor protein myosin pulls together microfilaments during contraction.
  • Microtubules are small tubes that are built with the protein tubulin.
  • During normal conditions, one of their functions is to act as roadways for motor proteins.
  • Microtubules are also the central supports for cilia and flagella.
  • Covered by just the cell membrane, cilia and flagella extend from the cell.
  • Motor proteins push/pull on the tubules within the cilium/flagellum.
  • This causes them to move back and forth.
cell membranes
Cell Membranes
  • Phospholipids are the dominant molecule in membranes.
  • Phospholipids naturally assemble into a bilayer
  • The membrane’s center is hydrophobic because of the fatty acid tails.
  • The outer edges are hydrophilic because of the phosphate groups.
cell transport
Cell Transport
  • A membrane is a selectively permeable barrier
  • Single molecules that are nonpolar or only slightly polar can pass through the hydrophobic core without problems.
  • These include O2, N2, CO2, steroids, alcohols, fatty acids, and pesticides.
  • The cell cannot regulate movement of these molecules as they follow the rules of simple diffusion
cell transport1
Cell Transport
  • Single molecules that are polar or charged require a transport protein or channel protein to pass through the core.
  • These include H20, ions, sugars, amino acids, and proteins.
  • Their movements

into / out of the cell

can be regulated by

modifying the

proteins involved.

  • Materials can move pass membranes:
    • A) as single molecules (diffusion and active transport) or in large quantities (vesicular transport)
    • B) without input of energy (passive transport) or requiring energy (active transport)
    • C) without the help of a protein (simple diffusion) or with the help of a protein (facilitated and channel-mediated diffusion).
  • During diffusion, molecules move from areas of higher concentration to areas of lower concentration, “down” the concentration gradient.
  • At equilibrium, movements of molecules in one direction are balanced by movements in the opposite direction.
  • Diffusion rates depend on:
    • a) the distance over which molecules must move:
      • shorter (thinner membrane) = faster
    • b) the size of the molecule:
      • smaller = faster
    • c) the surface available for diffusion:
      • wider = faster
    • d) the speed that molecules are moving = temperature:
      • higher = faster
    • e) the concentration gradient between two points
      • greater concentration difference = faster.
  • Diffusion rates also depend on how permeable the membrane is to a particular kind of molecule
  • The diffusion rates of one type of molecule are independent of the concentrations of any other types of molecules
  • In simple diffusion, molecules move

past the membrane through the lipid


  • In channel mediated diffusion, molecules pass the membrane through a protein pore.
  • Osmosis is the movement of “free” water down its concentration gradient.
  • Some water molecules surround solutes as part of spheres of hydration.
  • If a membrane is not permeable to that solute, then these water molecules cannot pass either.
  • A solution with few dissolved molecules (low osmolarity) will have more free water molecules than a solution with more dissolved molecules (high osmolarity).
  • Imagine that we have a selectively permeable membrane separating a 40% sugar solution from a 10% sugar solution.
  • There are more “free” water molecules on the 10% sugar side than there are on the 40% sugar side.
  • Because there are more “free” water molecules in the 10% side, water will move by osmosis (diffusion of water) to the 40% side.
  • If the concentration of dissolved materials is equal in the surrounding solution as in the cell, then no net movement of water occurs.
  • If the concentration of dissolved materials is greater in the solution than in the cell, then water will leave the cell. The solution is hypertonic compared to the cell.
  • If the concentration of dissolved materials is lower in the solution than in the cell, then water will enter the cell. The solution is hypotonic compared to the cell.
  • Specific carrier proteins allow materials that are not hydrophobic to pass through a membrane.
  • In facilitated diffusion, the carrier protein allows molecules to move from high concentration to low concentration.
  • Osmosis, facilitated diffusion, and standard diffusion are all examples of passive transport.
    • movement of materials down a concentration gradient without expenditure of energy
active tranpsort
Active tranpsort
  • Active transport is the movement of materials across a membrane against its concentration gradient.
  • Molecules are pumped from low concentration to higher concentration.
  • Active transport requires metabolic energy to do the pumping.

In exocytosis a membrane-bound sac (a vesicle) fuses with a membrane and dumps the fluid contents outside the membrane (usually outside the cell).

  • Endocytosis is the reverse of exocytosis.
    • A region of membrane forms a pocket around the external materials, pinches off a vesicle, and transports this material inside.
  • In phagocytosis, the materials being brought insides are solid particles.
  • In pinocytosis, the materials being brought inside are fluids.
endocytosis and exocytosis
Endocytosis and exocytosis
  • White blood cells actively use endocytosis and exocytosis in their role as defenders of the body from invaders.
  • When they detect a microbe, they extend fingers of membrane and cytoplasm to surround it.
  • When the membrane fingers meet, they fuse.