Transcript The Discovery of the Cell

A Tour of the Cell
(Chapter 6)
The Discovery of the Cell:
Hooke
• In 1665, Robert Hooke used
an early compound
microscope to look at a thin
slice of cork, which is a plant
material.
• He noticed that cork looked
like thousands of tiny, empty
chambers which he called
“cells”
• We now know cells are the
basic units of life.
The Discovery of the Cell:
Von Leeuwenhoek
• The existence of cells was
unknown for many years, but
this changed with the
invention of the microscope.
• Anton von Leeuwenhoek used
a single-lens microscope to
observe pond water and other
things.
• The microscope revealed a
world of tiny living
organisms.
Developing the Cell Theory
• In 1838, Matthias Schleiden
concluded that all plants were
made of cells.
• In 1839, Theodor Schwann
stated that all animals were
made of cells.
• In 1855, Rudolph Virchow
concluded that new cells were
created only from division of
pre-existing cells.
• These discoveries led to the cell
theory.
Compound Light Microscope
• Combination of lenses and
light used to magnify small
objects held on a slide
•Live specimen may be
observed (ie, pond water)
•Max magnification = 1000x
Electron Microscopes
• Image produced on a computer screen using a beam of
electrons rather than light
• More powerful (300,000x or more) than light
microscopes, but specimen cannot be alive
– Transmission
• Study of inner structure of a specimen
• Samples are cut into thin slices for viewing
• Images are 2-D
– Scanning
• Allows study of specimen surface
• Images are 3-D
Images from Electron Microscopes
The Cell Theory
• All living things are
composed of cells.
• Cells are the basic units of
structure and function in
living things.
• New cells are produced
from pre-existing cells.
Self-Assessment
• What are the three key ideas of the cell
theory?
• What scientists contributed to the
development of the cell theory? How did
they contribute?
• Identify the type of microscope most
useful for viewing each of the following:
– A group of cells in a thin layer of onion skin
– The details of the surface of a human hair
– The detailed structure of a mitochondria
inside a muscle cell
Two Types of Cells
•All cells are surrounded by a barrier called a cell membrane
and contain DNA
•Eukaryotic cells contain a nucleus & membrane
organelles. (plants, animals, fungi & protists.)
•Prokaryotic cells do NOT contain a nucleus (still have DNA)
and most organelles (do have ribsomes) and are classified as
bacteria.
Cell Boundaries
• All cells are surrounded by a thin, flexible
barrier, the plasma or cell membrane, which
acts as a gate-keeper, regulating what enters
and leaves the cell, and also provides some
protection and support
• Many cells also produce a strong supporting
layer around the membrane known as a cell
wall. The main function of the cell wall is
support and protection for the cell.
Structure of the Cell Membrane
• The composition of nearly all cell
membranes is a double-layered sheet
called a lipid bilayer.
Lipid bilayer
Functions of the Plasma Membrane
• Embedded enzyme proteins help carry out chemical
reactions of the cell
• Receptor proteins allow to receive chemical messages
• Surface molecules allow recognition & communication
between cells
• Transport proteins serve as channels or pumps to move
materials in and out of cells
• Proteins from adjacent cells allow intracellular joining
Diffusion Through Cell Boundaries
• Particles in a solution tend to move from an area
where they are more concentrated to an area
where they are less concentrated. (visualize this as a
downhill movement, [high] to [low])
• This process is called diffusion
• When the concentration of the solute is the same
throughout a system, the system has reached
equilibrium.
Osmosis
• The diffusion of water through a selectively
permeable membrane.
Osmotic Pressure & Tonicity
• If you compare two solutions, the more
concentrated solution (less water) is
hypertonic.
• The more dilute (more water) solution is
hypotonic.
• When the concentration is equal in both
solutions, the solution is described as isotonic.
Effect of Tonicity on Cells
Facilitated Diffusion
• Cell membranes have protein channels that act as
channels (pores) or carrier molecules, making it easy for
certain molecules to cross.
• The movement of specific molecules across cell
membranes through protein channels is known as
facilitated diffusion.
• This process usually involves solute molecules (rather
than solvent molecules) and is a form of passive
transport because it requires no energy, ([high] to [low])
Active Transport
• Active transport involves the use of energy (ATP) to move
substances across a cell membrane
• Ex) Movement of materials in the opposite direction from
which the materials would normally move - against a
concentration difference
• This is achieved by membrane proteins that act as pumps
• Visualize this as UPHILL, [low] to [high])
• Large materials also move by active
transport regardless of concentration
gradient
Active Transport of Large Materials
(“Bulk Transport”)
• Endocytosis is the process of taking
large materials into the cell using
vesicles, or pockets, of the cell
membrane. (endo=enter)
–Phagocytosis – cell “eating”;
actively moving solids into
the cell
–Pinocytosis – cell “drinking”;
actively moving liquids into
the cell
• Exocytosis, the membrane of the
vesicle surrounding the material
fuses with the cell membrane,
forcing the contents out of the cell
(exo=exit)
Summary Assessment
• What is the difference between active and
passive transport?
• Name and describe three examples of each.
• Distinguish between hypotonic, hypertonic and
isotonic solutions.
– Explain what will happen to an animal cell in each.
– Explain what will happen to a plant cell in each.