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Advanced Biology
Organization of the Cell
• Cells are dramatic examples of the underlying unity of all
living things.
• Idea first expressed by Matthias Schleiden and Theodor
Schwann in 1839. They concluded that plants & animals
are made of cells.
• Rudolf Virchow saw cells dividing and making daughter
cells in 1855. Proposed cells only come from other cells.
• August Weismann added to Virchows theory that the cell
can be traced from ancient cells.
• CELL THEORY:
• 1. Cells are the basic living units of organization &
function in all organisms.
• 2. Cells come from other cells.
Cell Organization
• A cell is the smallest unit that can carry out
all activities associated with life.
• But no part of an isolated cell can survive.
• A cell hardly gets NRG in the form it needs
it to be in. It must be converted.
Advances in technology helps us to better
understand cells, their function & structure.
Cell Organization and Size
• Cell organization & size helps them maintain
HOMEOSTASIS, stable internal environment.
• A.) The organization of all cells is basically similar.
• The plasma membrane, which is a membrane that surrounds
all cells, helps keep cells separate from their external
environment.
• Plasma menbrane also serves as a SELECTIVE barrier of
what enters & exist the cell.
• Cells have ORGANELLS which carry out specialized
functions.
Cell Size
• B.) Cell size is limited.
• Most cells are too microscopic to see with the naked eye.
• The small micrometer is too small to see & count the
organelles of the cell, a nanometer is used instead.
• A human egg can be seen with the naked eye, as big as a
period in a sentence.
• A cell is small because it is much easier to maintain
homeostasis and do daily functions.
• When a cell gets bigger, the volume increases faster than
the surface area also putting a restriction on size.
Cell Size
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Cell Shape & Function
• Not all cells are spherical or cuboid.
• Fingerlike projections from the plasma membrane
are called Microvilli, increase the surface area for
absorption.
• Some cells can change shape to accommodate a
function.
• Also small because molecules inside must travel
distances.
• This distance is smaller when the cell is small.
• C.) Cell size & shape are related to function
• Size & shape are related to the function of a cell.
• Examples: sperm, epithelial cells, nerve cells.
Methods for Studying Cells
• One of the most important tools is the
microscope.
• In 1665, Robert Hook discovered cells when he
observed dead cork and said they reminded him
of the rooms monks lived in.
• Anton von Leeuwenhoek designed a good lens
for microscopes a few years later and improved
microscopy.
Cork cells
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A.) Light microscopes are used to
study stained cells
• Light microscope has a tube with glass lenses at each end,
also called a compound microscope.
• Magnification is the ratio of size of image to actual size,
usually no more than 1000X.
• Resolution is the capacity to distinguish detail.
• Minimum space where two individual points can be seen,
not blurry.
• Bright-field uses light.
• Dark-field has scattered light.
• Phase contrast & differential interference contrast uses
density.
• Fluorescence looks at molecular structure of cells.
B.) Electron Microscope
• Ultra-structure is fine detail.
• Electron microscopes magnify up to 1/4 million times.
• Transmission Electron Microscope (TEM) requires
that the specimen is enbeded in plastic & cut into very
thin strips to produce layered images.
• Scanning Electron Microscope (SEM) requires that the
specimen is coated in metal and an indirect beam of
electrons creates a 3-D image of the surface.
Microscopes
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C.) Cell Fractionation enables
study of cell organelles
• Cell fractionation is purifying organelles.
• Centrifuge spins broken cells to form a pellet.
• Pellet forms at bottom and supernatant is at the top
of solution.
• The supernatant is spun again in differential
centrifugation, at higher speeds.
• Pellets are further purified in density gradient
centrifugation.
Cell Fractionation
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Prokaryotic & Eukaryotic cells
• Prokaryotes do not have DNA in a nucleus but in a
nuclear area or Nucleoid, no membrane.
• Prokaryote means before the nucleus.
• Many prokaryotes have cell walls and flagella & have
ribosomes.
• Eukaryotes are highly organized, advanced cells.
• Eukaryote means true nucleus.
• Cells are filled with a jelly-like substance called
protoplasm.
• Outside nucleus is called cytosol.
• Inside nucleus is called nucleoplasm.
• Organelles in cytosol = cytoplasm.
Cell Membrane
• Lets things in & out of cell, keeps certain things
away from other parts of the cell.
• Membranes are work surfaces that store energy.
• Endomembrane systems are all internal
membranes.
• Some materials travel in vesicles.
Anatomy of a Cell Membrane
• A bi-phospholipid layer with integral proteins
throughout.
• Is Amphipathic due to hydrophobic & hydrophilic
regions.
• 2-D fluid, cannot move without channel.
• Lots of saturated fatty tails, become more solid.
• Unsaturated, bends where double bonds exist.
• Cholesterol acts as a buffer, either keeps tails apart or
brings them closer together.
• Cholesterol makes the membrane more fluid at lower
temperature.
Types of membrane Proteins
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Receptor- recieves materials
Channel-lets things travel from one protein to another.
Enzymes- breaks down unwanted materials
Carrier- allows specific materials& ions to pass in & out
Recognition- identifies
Signal- gives signal for what is needed.
Aquaporins- gated water channels
Anatomy of a Cell Membrane
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Methods of Movement across
Membranes with no NRG
• Passive Transport- no NRG needed
• Diffusion- particles randomly move from high
concentrations to low concentrations
• Osmosis- diffusion of water through a membrane
• Facilitated Diffusion- carrier protein facilitates the
movement of certain ions or other polar molecules from
high concentration to low.
• A Concentration Gradient is created when there is
different concentration of a substance on each side of a
membrane.
Diffusion & Osmosis
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Isotonic, Hypertonic, &
Hypotonic Solutions
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• Isotonic- same concentration of
solutes on both sides of membrane,
Equal movement of water.
• Hypertonic- more solute on outside
of cell membrane. Movement of
water is out of cell.
• Hypotonic- less solute on outside of
cell. Movement of water is into cell
Plasmolysis
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• Loss of turgor pressure
• Water potential is from
inside cell to outside
• Caused by putting cells in a
hypertonic solution.
• Causes plant to wilt
Facilitated Diffusion
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Methods of Movement across
Membranes with NRG required
• Active Transport- ATP energy required
• Sodium-Potassium Pump- a carrier mediated active
transport system that imports 2 potassium cations into the
cell and exports 3 sodium cations.
• This creates an unequal charge (Electrical gradient) due to
more positive charge on outside cell membrane.
• This also creates a Membrane Potential because of the
concentration difference and charge difference.
• Important in nerve impulses.
• Endocytosis- taking in large particles by fusion to the cell
memnbrane to create a vacuole. (phago & pinocytosis)
• Exocytosis- ridding large particles by fusion of a vesicle to
the cell membrane.
Sodium-Potassium Pump
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Types of Endocytosis
• Phagocytosis- cell eating (WBC & bacteria)
• Once inside, the large substance is digested by
lysosome enzymes.
• Pinocytosis- cell drinking
• Tiny droplets are taken in by folds in the
membrane that trap & pinch off inside.
• Receptor- mediated endocytosis- special
receptor molecule called ligands combind with
specific molecules on a coated pit & forms a
coating around it before it pinches off inside the
cell.
Exocytosis, Phagocytosis, &
Pinocytosis
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Receptor-Mediated Endocytosis
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Cell Signaling
• Mechanisms by which cells communicate with one
another.
• Most often use chemicals.
• Signaling molecule from one cell will combine with a
receptor on another cell.
• Example: cAMP,&neurotransmitters are signaling
molecules, GTP is a receptor molecule.
• Enzymes are also used to catalyze the production on
secondary messenger molecules. See fig. 5-21
• Signal Transduction is a process where cells convert
and amplify an extracellular signal into an intracellular
signal
Cell Junctions
• Special intercellular connections
• Allow neighboring cells to do one or more of
the following:
• form strong connections
• prevent passage of materials
• communicate with each other.
Types of Cell Junctions
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1.) Anchoring Junctions
Tightly bound to each other.
Example: epithelial cells in the outer skin.
Cadherins are transmembrane proteins that play an
important role in these junctions.
Two types of anchoring junctions: Desmosomes &
Adhering junctions.
Adhering junctions cement cells together.
Cadherins form a continuous adhesion belt around each
cell connecting the microfilaments of the cytoplasm
Create a path for signaling from outside to inside cell.
Desmosomes
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• Form points of attachment
between cells like rivets.
• Substances still pass freely
through the space between
the membranes.
• Anchor to intermediate
filaments inside the cell.
Cell Junctions cont.
• 2.) Tight Junctions
• Seal off intercellular spaces between some
animal cells.
• No space remains between cells.
• Substances cannot leak between them.
• Seal off body cavities.
• Example: lining of intestine, blood-brain
barriers
Cell Junctions cont.
• 3.) Gap Junctions
• Bridges the space between cell but leaves very narrow
spaces.
• Are communicating junctions.
• Contain channels that connect the cytoplasms of adjacent
cells.
• Composed of connexin, an integral membrane protein.
• Groups of 6 connexin molecules cluster to form a cylinder
that spans the plasma membrane.
• Small organic molecules, ions, and cAMP pass through
the channels.
• Allow for rapid chemical & electrical communication.
Tight junctions & Gap Junctions
Gap Junction
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Tight Junction
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Cell Junctions cont.
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4.) Plasmodesmata
Formed between plant cells.
Form between cell walls.
Connect the cytoplams.
Form cylindrical membranous
structures called desmotubule
that connect the ER of ajacent
cells.
• Allow molecule & ions to pass,
not organelles.
• Can dilate their diameters.