Download CHAPTER 4 A TOUR OF THE CELL

CHAPTER 4 A TOUR OF THE CELL
Drugs that target cells: Antibiotics are drugs that disable or kill
infectious bacteria. In 1928 Alexander Flemming discovered the
first antibiotic, penicillin. Deaths as a result of bacterial infections
after surgery were drastically reduced. The goal of treatment is to
kill invading bacteria while causing minimal harm to the host.
Most antibiotics bind to structures that are found only in bacterial
cells.pg. 53
All organisms are made up of cells, they are the building blocks of
all life. The cell is the smallest entity that exhibits all the
characteristics of life. Cells are very small (microscopic) but very
complex. They can be single celled like bacteria or some protists
or multicellular like plants and animals. To study cells we use the
microscope. Magnification increases the objects size. Clarity
depends on the resolving power, the ability to show two objects as
separate. With the naked eye, we can resolve images as close as
mm(10-3m). Light microscopes are limited to 0.2µm. Robert
Hooke looked at this slices of dead cork cells in 1665, in 1683
Leeuwenhoek looked at the first living cells in pond water. In 1838
Matthais Schleiden looked at many different plants and concluded
that all plants are made up of cells. In 1838 Theodore Schwann
concluded all animals are made up of cell. He said all living things
are made up of cells. In 1855 Rudolf Virchow proposed that new
cells come from preexisting cells. All this led to the cell theory,
which states: (1) all living things are composed of cells (2) cells
are the basic unit of structure and function (3) all cells are
produced from other cells.
In the 1950’s the electron microscope was invented. It uses a beam
of electrons which allows us to see much more detail in cells. This
was a thousand fold improvement over the light microscope. There
are two kinds. The scanning SEM which looks at the surface of
cells or molecules and the transmission TEM which is able to look
at internal organelles. See page 55 for cell sizes.
The two major categories of cells are the prokaryotic (bacteria &
Archaea) and the eukaryotic cells (protists, fungi, plants &
animals). Prokaryotic cells are about 1/10 the size of eukaryotic
cells, they appeared in the fossil record ~ 3.5 billion years ago and
they are much simpler. Prokaryotic cells lack membrane bound
organelles, and they lack a nucleus. The DNA of prokaryotic cells
is coiled in a nucleoid region not partitioned from other cell
structures. Surrounding the plasma membrane is a cell wall.
Cell membrane and transport: The key to how the plasma
membrane works is in its structure. It is able to regulate the flow of
chemicals into and out of the cell. It is described as a
phospholipids bilayer. Phospholipids are related to dietary fats but
have only 2 fatty acids instead of three. The phosphate group is
electrically charged making it hydrophilic (water-loving). The two
fatty acids (tail) are hydrophobic (water-fearing). The head mixes
with water and tails avoid it. The membrane has proteins
embedded in the bilayer. The phospholipids and proteins are free
to drift in the membrane so it is called a fluid mosaic model. The
plasma membrane is selectively permeable, allowing materials like
oxygen and nutrients to enter and allows wastes like carbon
dioxide to leave. Some substances can enter or leave by way of
transport proteins. See page 59.
Passive transport requires no energy from the cell. In diffusion
molecules (gases, solids or liquids) move randomly in all
directions until equilibrium is reached. Living cells are in a fluid
environment and molecules move from an area of greater
concentration to an area of lower concentration until evenly
distributed, called dynamic equilibrium.
Facilitated diffusion occurs when substances such as H+ and other
organic ions are transported by way of specific transport proteins
in the plasma membrane. These transport proteins act as selective
corridors.
Osmosis is the diffusion of water across a selectively permeable
membrane. Water will move from an area of greater concentration
to one of a lower concentration. If the membrane separates two
solutions with different concentrations of solutes, the solution with
a higher concentration of solutes is said to be hypertonic. The
solution with a lower concentration is hypotonic. See page 81.
Hypotonic solution (lower solute) has a higher concentration
of water
Hypertonic solution (higher solute) has a lower concentration
of water
Water diffuses from hi →lo concentration. Water molecules move
in both directions, but when they reach equal solute concentration
they are said to be isotonic.
Healthy cells are isotonic to their fluid environment. If a red blood
cell is placed an isotonic solution, the cell’s volume remains
constant. The number of water molecules entering and leaving the
cell is equal. Marine organisms are isotonic to sea water.
Osmoregulation is used to control water balance. As an example, a
fresh water fish in a hypotonic environment has kidneys and gills
that work to prevent a build up of water in the body. Water
balance in plants is different, they have rigid cell walls. A plant in
an isotonic solution will wilt. In a hypotonic solution (best) it will
take on water and become turgid. In a hypertonic solution the cell
membrane will pull away from the cell wall and it will shrivel,
called plasmolysis.
Active transport requires cell energy to move molecules across the
cell membrane. Transport proteins actively pump a solute across a
membrane against the solutes concentration gradient. Membrane
proteins use ATP as their energy source for active transport. See
page 82
Exocytosis and endocytosis use two different types of active
transport. Exocytosis occurs when large secretory proteins exit the
cell in transport vesicles that fuse with the plasma membrane and
carry the contents outside the cell. Endocytosis occurs when
molecules enter the cell. There are three types. Phagocytosis
(cellular eating), pinocytosis(cellular drinking) and receptormediated endocytosis. The cell membrane surrounds and forms a
vesicle to bring the substance into the cell.
Cell signaling: Communication begins with the reception of an
extra cellular signal such as a hormone. This signal triggers a
transduction pathway (relay) and leads to chemical responses. See
fig. 5.19.
ATP adenosine triphosphate. A-P-P~P It is the triphosphate tail
that is the part that provides energy for cellular work. Each
phosphate is negatively charged. To release (power) energy the last
phosphate is broken off. Leaving ADP. STP energizes other
molecules in cells by transferring phosphate groups to those
molecules. This helps cells perform three kinds of work,
(1)mechanical such as muscle contraction (2) transport such as
active transport using membrane proteins (3) chemical such as
dehydration synthesis in cellular respiration.
Special enzymes catalyze the phosphate transfers that energize the
working parts of cells.
ATP cycle: Your cells spend (currency) ATP continuously. It can
be restored by adding a phosphate group back to ADP that takes
energy. Cellular respiration regenerated the cells supply of ATP.
Cellular Work spends ATP recycled from ADP and a phosphate
through cellular respiration. Energy coupling- the transfer of
energy from processes that yield energy to processes that consume
energy (such as muscle contraction).
Energy is defined as the capacity to do work. It is work that moves
things.
Kinetic energy is the energy of motion- wheels turning
Potential energy is the energy of position-water behind the dam,
the ball at rest
Conservation of matter states that matter cannot be created or
destroyed, it can only be converted from one form to another.
Conservation of energy states you cannot destroy or create energy
only convert it from one form to another.
Entropy is a measure of disorder or randomness- everything that
happens in nature increases entropy(the snow boarders energy was
lost to the atmosphere as heat).
Cell organelles are ‘little” organs that divide up the labor within
the cell. These are found in eukaryotic cell.
The nucleus has a double membrane called the nuclear envelope, it
is filled with tiny pores that allow for the passage of material in
and out of the nucleus. Within the nucleus are dark, loosely packed
fibers of DNA. Each long fiber is a chromosome made up of a
combination of DNA and proteins. Also in the nucleus is the
nucleolus which produces RNA.
The ribosomes in the cytoplasm functions to build and assemble
proteins. Some are free floating and some are attached to the
endoplasmic reticulum.
The endoplasmic reticulum(ER) is a system of tubules. Some are
called rough since they have bumps of ribosomes on them and
some ER is called smooth since it has no bumps on it. In rough ER
the ribosomes produce membrane proteins and secretory proteins.
Smooth ER has many enzymes built into the membrane, these
enzymes enable the ER to perform many function- one is the
synthesis of lipids, including steroids.
The golgi apparatus is a refinery, warehouse and shipping center.
Products made in the ER reach the golgi in transport vesicles. The
golgi enzymes modify and distribute to other organelles or send
product to the plasma membrane in vesicles to be secreted outside
the cell.
Lysosomes are a membrane enclosed sac of digestive enzymes.
The lysosome provides a compartment where the cell can digest
macromolecules of proteins, polysaccharide, fats and nucleic acids
safely without committing suicide. Some lysosomes help destroy
harmful bacteria. White blood cells ingest bacteria into vacuoles
and enzymes rupture the bacteria cell walls. Lysosomes recycle
damaged organelles. (Tay-Sachs disease)
Vacuoles are membrane sacs that bud from ER, golgi, or the
plasma membrane. There are many different sizes and functions
such as food and contractile vacuoles. Plants have a large central
vacuole.
Chloroplasts are the organelles of plants and protists that are
involved in photosynthesis. There are three major compartments:
(1) the space between the 2 membranes (2) the stroma, a thick fluid
within the chloroplast (3) a network of membrane enclosed tubes
and disks called the grana (traps light energy).
Mitochondria are the site of cellular respiration, the process that
harvest energy from sugars and converts it into ATP (direct
energy). Mitochondria have a double membrane which contains a
thick fluid. The inner membrane folds back on itself many times
this is called the cristae. The cristae on the extensive membrane
maximize the ATP output.
Cytoskeleton is a network of fibers extending throughout the cell
that function both in support and movement. The cytoskeleton
contain several types of fibers, one of the most important is
microtubules. Microtubules are hollow, they provide anchorage
and reinforcement for many organelles. Some organelles move
along tracks made of microtubules, they also guide the movement
of chromosomes during cell division.
Cilia and Flagella re made up of microtubules and are used as
appendages providing whip-like movement. They extend from the
plasma membrane.
Plant cell walls and cell junctions: Plasmodesmata cell junction,
channels that connect one cell to another and allow water to pass
from one cell to another as well as other materials.
Animal cell surfaces and cell junctions: (1) tight junctions bind
cells together-leak proof (2) anchoring junctions attach adjacent
cells to each other but allow materials to pass between cells (3)
communicating junction- channels that allow water and small
molecules to flow