The Cell
visit: http://en.wikipedia.org/wiki/Cell_(biology)#Prokaryotic_cells
The Cell is the basic unit of life.
Robert Hooke (1635 – 1723) discovered the ‘cell’ while looking at slices of cork under a
primitive microscope. The Latin word, ‘cellulae’, meaning “little rooms”.
The generally accepted parts of cell theory include:
1. The cell is the fundamental unit of structure and function in living things.
2. All cells come from pre-existing cells by division.
3. All known living things are made up of cells.
1.
2.
3.
4.
5.
More recently added were the following
Energy flow (metabolism and biochemistry) occurs within cells.
Cells contain hereditary information which is passed from cell to cell during cell division
All cells are basically the same in chemical composition.
Some organisms are unicellular, made up of only one cell.
Others are multicellular, composed of countless number of cells.
•
A cell is a compartmentalized unit of life, and can containing specialised structures,
known as ‘organelles’. Most Cells are microscopic.
Organelles are membrane bound structures that perform specialized functions.
Did You Know?
•
•
•
The largest cell in the world is the Ostrich egg.
The smallest cell is a bacteria cell.
The longest cell is a nerve cell.
The size of a cell is restricted by Surface Area.
Large cells have a smaller surface area than small cells. Size matters. Large cells are then limited
in uptake of nutrients and cell supplies.
The 1st cell formed 3.5 BYA, it was prokaryotic. One billion years later, 2.5 BYA, prokaryotic cells
began membrane infolding and endosymbiosis of two prokaryotes cells gave rise to a eukaryotic
cell .
Prokaryotic Cells and Their Characteristics
•
These are cells of the Kingdom Monera which contain all bacteria cells.
•
Size = Most bacteria are from 2 – 8 micrometers in size.
•
Pro = primitive; karyon = nucleus. These have primitive nucleus. The DNA is freefloating, with no nuclear envelope. Also known as ‘nucleoid’.
•
No organelles are present.
•
Ribosomes are present. They make proteins.
•
Plasma membrane encloses the cytoplasm.
•
•
Cell wall surrounds the plasma membrane.
Some bacteria have a protective, mucillagenous sheath around the cell-wall, known as
‘capsule’.
•
•
Some bacteria have pili (singular = pilus) to attatch to surfaces.
Some bacteria have long projections – flagella – help in locomotion.
Diagram of a typical prokaryotic cell
Differences Between Prokaryotic and Eukaryotic Cells
•
Prokayotic Cells include all bacteria (Kingdom Monera), are simple, have no nucleus (false nucleus
= nucleoid), Organelles are absent / without membranes. The Flagellar structure is different.
•
Eukaryotic Cells include all living organisms besides bacteria (Protista, Fungi, Plant andAnimal), are
complex, have a true nucleus, have organells and the Flagellar structure is different from that in
prokaryotes.
Two Types of Eukaryotic Cells: Plant and Animal
PLANT Cells are AUTOTROPHIC
•
•
•
•
•
•
•
Cell wall made of cellulose is present.
Chloroplasts containing the green pigment, chlorophyll – are present.
Autotrophs – can make their own food.
Cells are angular shaped.
Large central vacuole is present.
Centrioles absent.
Flagellae are uncommon.
Animal Cells are HETEROTROPHIC.
•
•
Cell wall is absent.
Chloroplasts are absent.
•
•
•
•
•
Heterotrophs – depend on other organisms for food.
Cells are rounded and not very regular.
Lysosomes (instead of vacuoles) are small and scattered .
Centrioles are present.
Flagellae may be present.
Diagram of a typical eukaryotic cell, showing subcellular components. Organelles: (1) nucleolus (2)
nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7)
Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13)
centrioles
Plant Cell and Animal Cell
Eukaryotic Cellular Organelles:
1. Nucleus: Houses the genetic material (DNA) of the cell.
DNA associated with proteins are present as long, thread-like fibers, known as ‘chromatin’.
(During cell-division, chromatin coils up into thick, condensed strands known as
‘chromosomes’.
The nucleus is surrounded by a double-membrane with pores, known as the ‘nuclear
envelope’. Within the nucleus is the nucleolus. This is where ribosomes are synthesized and
assemble.
Identify the Nucleus
the nuclear envelope
the pores of the nucleus
now identify the endoplasmic reticulum
Endomembrane System is the prevailing theory today: This is a system of membranes
that surround the organelles and may or may not form a continuous interconnectedness
with each other).This system would serve to further divide the cell into compartments and this
increases its surface area. Organelles included in the endomembrane system are numbered 2-5:
Label all structures inside the cell :=).
Which are considered as a part of the
endomembrane system?
2. Endoplasmic Reticulum: is a network of interconnected membranes and is of two types:
Rough E.R. and smooth E.R.
•
Rough E.R. is rough due to the presence of ribosomes on the surface. This is where
most proteins are made in cells (site of protein synthesis)
•
Smooth E.R.
Have no ribosomes on the surface.
Help to synthesize lipids.
Regulate blood-sugar released from the liver cells.
Breakdown drugs and harmful substances.
Helps in muscle contraction.
3. Golgi Apparatus
•
The Golgi apparatus is not continuous or interconnected.
•
They are stacks of flattened sacs – named after the Italian scientist, Camillo Golgi, who
discovered them.
•
They receive transport vesicles
with proteins from the E.R., modify them
and then transport them back out where they may be needed.
4. Lysosomes / ‘Suicide Capsules’. Typically considered animal digesting organelle,
but plants can have lysosomes.
•
Lysosome is produced by the rough E.R. and the Golgi apparatus.
•
It contains hydrolytic digestive enzymes which:
•
digest food by fusing with food vacuoles.
•
Digest harmful bacteria contained in white blood cells.
•
Digest worn out / damaged organelles.
•
Aid in embryonic development – prevent webbing of fingers of embryo by destroying
cells of the web.
Abnormalities of Lysosomes: This can lead to fatal diseases such as:
1. Lysosome storage disease – the abnormal lysosomes get full of indigestible
substances, which then interfere with cellular functions.
2. Pompe’s disease – harmful amounts of glycogen accumulate in the liver cells.
3. Tay Sachs disease – lysosomes lack a lipid digesting enzyme and nerve cells in the
brain are damaged due to excess lipid accumulation.
5. Vacuoles: Only found in Plants
•
These are membraneous sacs like lysosomes. Eg. Food vacuole; plant’s large central
vacuole, contractile vacuole in paramecium, etc..
•
May contain nutrients and water for growth of cells.
•
May contain pigments in case of flowers.
•
May contain waste products.
May
May
May
May
•
•
•
•
contain important chemicals for cellular metabolism.
contain poisons.
help to maintain internal environment of cells eg. In paramecium.
help in cell-enlargement.
Non-Endomembrane Organelles: Organelles that are membrane-bound but not a part of the
endomembrane system are organelles 6-:
6. Chloroplasts (found only in plant cells)
•
•
•
Chloroplasts convert solar energy to chemical energy in sugar molecules.
They help to carry out photosynthesis in plants.
Has three compartments: intermembrane space, stroma and grana (part of
thyllakoid membrane system).
Which structure is the
thyllakoid membrane
which is the stroma?
7. Mitochondrion = Power House of a Cell (found in plant and animal cells)
•
Converts energy from one chemical form to another.
•
Carries out cellular respiration where chemical energy of foods is converted into
chemical energy of ATP = cellular fuel molecule.
•
Has two compartments: Mitochondrial matrix and cristae.
•
Mitochondria are all maternally derived.
•
Mitochondrial diseases include various mitochondrial myopathies.
Simplified structure of a typical mitochondrion
. Identify the cristae and the
matrix.
Internal Skeleton
The cellular organelles are held in place and given structural support by a
meshwork of fine proteins which form the cytoskeleton of the cell. The
cytoskeleton includes:
Microfilament
7nm, Actin subunit
Intermediate
filament 10 nm,
Fibrous Subunits
Microtubule, 25 nm,
tubulin subuntis
Microfilaments, Intermediate filaments and Microtubules (arranged from smallest to
largest).
● Microfilaments help cells change shape. They are long filamentous proteins made
of beads of actin protein linked together. They move cells and cell content and help
change cell shape.
●
●
Intermediate Filaments help determine permanent structure. They are intermediate in
size.. They are made of a combination of proteins. In animals IF produces hair, nails,
feathers and scales.
Microtubules are the largest and play a role in cell movement. They are long hollow
tubes made of spherical proteins call alpha and beta tubulin. They are a part
of
the
propelling appendages “cilia” and “flagella”. They are less common because few plant
cells move (only sperm). They are used by plants to transport packages of substances in
a monorail-like fashion.
Cytoplasm/Cytosol: inside the membrane of the cell aqueous semi-fluid in which structures
and organelles are found. The pH of the cytoplasm kept constant with buffers.
Plasmamembrane: all cells are surrounded by a hydrophobic structure called the
plasmamembrane. The plasmamembrane is semi-permeable (selectively).
Cell Walls Protect Plant cells and define cell shape: Most water enters cells by osmosis through
the plasma membrane. The primary cell wall is outside of the cell membrane and is comprised of
cellulose. Mature plants and woody plants produce a secondary cell wall that is thicker than the
primary wall. Cell walls are made of cellulose. Cellulose microfibrils can be linked together
by pectins (protein jelly like substances) or hemicellulose (glue or gumlike carbohydrates).
When linked together they form bigger, stronger strands called macrofibrils.
Between two cells is a thin lay called a middle lamella, The middle lamella is composed
mainly of pectin. They are used for cooking. They thicken jellies.
The secondary walls of plants become embedded with lignin (tar-like). The secondary
cell wall forms in between the plasma membrane and the primary cell wall. Pits are formed in
secondary wall. The secondary cell wall becomes thinner or disappears. Pits allow for more
rapid transfer of water and minerals from cell to cell.
Plasmodesmata are channels that connect the cytoplasm of two or more cells. Strands of cell
content flow through the plasmodesmata.
The Middle Lamella: Two plant cells are joined together by the middle lamella, which is largely
pectin.
Animal cells can be connected via 3 different junctions
Tight Junctions: Form a tight seal:Urinary bladder
Connecting or gap junctions: Cell to cell communication:Cardiac cells
Anchoring Junctions(Desmosomes): Cells anchored in their own Extracellular matrix: Skin cells