THE CELL
The cell is the smallest unit of living material capable of carrying on all activities necessary
for life. It is the smallest unit that can remain alive when removed from the living organism.
This has been demonstrated by the removal of cells from plants and animals and growing
those cells in cell and tissue culture. When placed in a favorable environment and supplied
with essential nutrients and organic molecules, cells can be kept alive in isolation in
laboratory glassware indefinitely and can be made to grow, to divide and to differentiate.
The cell is made up of molecules, atoms and ions, just as are non-living things, but the cell is
capable of carrying on metabolic processes and is capable of self-replication.
Major cell parts:
Plant, fungi, protist, and bacteria cells are composed of a cell wall and the enclosed
protoplast. Only animal cells are without a cell wall.
Protoplast is the living part of the cell inside the cell wall and includes the cell membrane and
the protoplasm.
Protoplasm is the cell contents inside the protoplast and consists of cytoplasm and the
nucleus.
Cytoplasm is the cell content inside the cell membrane but outside the nucleus. Contains the
organelles.
Nucleus is composed of nuclear membrane and nucleoplasm.
Nucleoplasm is the material inside the nucleus.
CELL WALL
Present in plants, algae, bacteria and fungi.
Located exterior to the cell membrane.
In plants, made predominantly of cellulose & hemicellulose, organized as microfibrils.
Pectin present in middle lamella. Pectin helps cement cells together.
May also have lignin.
May also have suberin.
Primary cell wall - mainly cellulose, with microfibrils arranged at random.
Secondary cell wall - much thicker than the primary and deposited in specifically-ordered
layers.
Pits - thin places in secondary cell wall - allow water and dissolved materials to pass between
cells.
Plasmodesmata - cytoplasmic connections between cells that allow continuous chemical
communication between adjacent cells.
CELL MEMBRANE (PLASMA MEMBRANE, PLASMALEMMA)
Plasmalemma is composed of a lipid bilayer with phospholipids, glycolipids and cholesterol.
Also has proteins embedded in it, which can move around in membranes.
Model of membrane structure called "Fluid Mosaic Model."
Membrane Function:
1) Regulates passage of materials into and out of protoplast.
2) Coordinates synthesis and assembly of cell wall microfibrils.
3) Translates hormonal and environmental signals involved in the control of cell growth and
differentiation.
CYTOSKELETON - Microtubules, Microfilaments, etc.
MICROTUBULES
Long, slender, hollow tubes in the cytoplasm made of tubulin (protein).
Important in maintaining cell shape and in moving cell components (e.g. chromosomes during
cell division). They are the main strucutral component of cilia and flagella.
MICROFILAMENTS
Smaller than microtubules, not hollow, but are made of protein.
Important to cell shape and in moving organelles around in the cell. The microtubules and the
microfilaments together form the cytoskeleton.
MICROTRABECULAR LATTICE
Slender protein threads that interconnect essentially everything in the cell and anchor it all to
the cell membrane. Seems to be actually anchoring the so-called "free" ribosomes.
MITOCHONDRIA
Site of cellular respiration, ATP synthesis. Each mitochondrion has 2 membranes - smooth
outer membrane and an inner membrane which is highly folded forming sheets called cristae.
The enzymes (protein molecules) involved in respiration are embedded in these membranes,
esp. the inner one. Each membrane is composed of a lipid bilayer. The fluid material inside
the inner membrane is called the matrix.
RIBOSOMES
Made of 2 subunits. Each subunit is made of protein and rRNA.
Ribosomes funtion in protein synthesis. Can bind with mRNA and tRNA.
Catalyzes formation of peptide bond between amino acids forming proteins.
ENDOPLASMIC RETICULUM
Structure: Network of fluid filled sacs and tubes, forming a largeinterconnected maze of
channels. If ribosomes are not attached, it's called smooth ER (sER), or if ribosomes are
attached, it's called rough ER (rER).
Function: (1) Site of membrane synthesis. (2) Serve to anchor enzymes. (3) Serve as storage
areas, e.g. calcium and certain proteins. (4) Serve as transport channels. Proteins can be
made on ER at ribosomes, shipped across the ER membrane and to site of eventual use or
location. (5) Serve to anchor ribosomes (not the only site of ribosome anchorage).
GOLGI APPARATUS (DICTYOSOME, GOLGI BODY, GOLGI STACK, GOLGI
COMPLEX)
Golgi body funtions as the processing and packaging plant of the cell. Molecules made in the
ER are packaged in little sacs or vesicles that move to the Golgi body, to the forming face.
The vesicles fuse with the Golgi stack, get shuttled through the stack and are released on the
maturing face.
During the trip through the Golgi body, the molecules within the vesicles get chemically
modified. E.g., protein may have a carbohydrate molecule attached to make a glycoprotein.
In secretory cells, the vesicles fuse with the cell membrane and dump their contents outside
the cell.
MICROBODIES - group of organelles surrounded by only one membrane
Peroxisomes, Glyoxisomes, Lysosomes
PEROXISOMES
Mostly occur in leaf tissue associated with chloroplasts and mitochondria. Participate
in photorespiration and help scavenge free radicals.
GLYOXISOMES
Have a role in lipid metabolism. Help convert fats to carbohydrates.
LYSOSOME
Lysosomes contain enzymes that can digest or chemically break down proteins,
polysaccharides, nucleic acids and lipids. Digest large molecules within the cells to
smaller ones that can be used to build whatever the cell needs. Lysosomes don't seem
to occur in plants.
PLASTIDS - a group of membrane bound organelles capable of producing starch
Chloroplast, Chromoplast, Leucoplast, Amyloplast
CHLOROPLAST
Has lamellae (sheets) that extend through the stroma and overlap in sections. The
areas where the lamellae overlap the most, so it has the appearance of stacks of
membranes are called the grana. The areas where the lamellae are not so tightly
stacked, but extend through the stroma between stacks, are called stromal lamellae. In
the grana, each lamella is actually shaped like a flattened disc or sac and each fluidfilled disc is called a thylakoid.
Function:
site of photosynthetic reactions.
involved in amino acid synthesis
involved in fatty acid synthesis
CHROMOPLASTS
Plastids with a disorganized internal structure and lacking chlorophyll. Usually
develop by degeneration of chloroplasts. Contain yellow, orange and sometimes red
pigments (mostly carotenoids) that give the color to carrots, tomatoes, and some
autumn leaves.
LEUCOPLASTS
Colorless plastids, having no pigments. Also have no internal membrane structure,
only the outer double membrane. They store starch, lipids, proteins and other food
reserves. Starch in potato tubers is stored in leucoplasts.
AMYLOPLASTS
Plastid in which starch is stored, occurring especially in roots of plants. They seem to
have a role in gravity perception, but the mechanism is not well understood.
VACUOLE
Vacuole is often very prominent in certain plant cells. It is bound by a single membrane,
called a tonoplast, and serves as a storage compartment and to give the cell turgidity. Fluid
inside is called the cell sap.
NUCLEUS
The nucleus contains DNA, has 2 membranes forming the nuclear membrane or the nuclear
envelope. Nuclear membranes are fused at nuclear pores. In the nucleoplasm are strands of
DNA called chromatin. This is DNA from chromosomes as they become spread out in
between periods of cell division.
Major functions of nucleus:
1) regulates metabolic activity of cell through protein synthesis
2) stores genetic information and passes it on to newly forming cells
NUCLEOLUS
Small structure within the nucleus. Place where cell makes RNA that helps make up
ribosomes.