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Cell Theory
• All organisms are composed of one or more cells
• Cells are the smallest living things
• Cells arise only by division of a previously existing cell
4.1 Cells
Fig. 4.1 The size
of cells and their
contents
20 mm
2 mm
0.2 mm
20 µm
2 µm
0.2 µm
20 nm
2 nm
0.2 nm
Natural laws limit cell size
• At minimum, a cell must be large enough to house
the parts it needs to survive and reproduce
• The maximum size of a cell is limited by the
amount of surface needed to obtain nutrients from
the environment and dispose of wastes
• A small cell has a greater ratio of surface
area to volume than a large cell of the same
shape
30 µm
Figure 4.3
Surface area
of one large cube
= 5,400 µm2
10 µm
Total surface area
of 27 small cubes
= 16,200 µm2
4.3 Prokaryotic Cells
• There are two major
kinds of cells
– Prokaryotes
– Eukaryotes
Fig. 4.9
Rod
• Prokaryotes include
bacteria and archaea
– Over 5,000 species are
recognized
• Prokaryotes come in
three main shapes
Spherical
Spiral
• Prokaryotes have a very simple architecture
– They lack a nucleus and organelles
– But they make all their own vitaminsand us complicated humans can’t!!
Fig. 4.8
Pilus
Found in all
prokaryotes
4.4 Eukaryotic Cells
• Appeared about 1.5 billion years ago
• Include all cells alive today except bacteria and
archaea
• Are larger than prokaryotic cells
• Can be single cells (yeast) or multicellular
• Have a much more complex architecture
– Possess nucleus and a variety of organelles
Fig. 4.10 Structure of
a plant cell
Fig. 4.11 Structure of
an animal cell
4.2 The Plasma Membrane
• Surrounds all living cells
• Made of phospholipids + proteins
• Membranes are selectively permeable
-They control the flow of substances into and out of a cell
• Membranes organize the chemical reactions making up metabolism
Membranes contain phospholipids
• Phospholipids each have a
hydrophilic head and two
hydrophobic tails
Head
Symbol
Tails
Fig. 4.4
Membrane phospholipids form a bilayer
• In water, phospholipids form a stable bilayer
– The heads face outward and the tails face
inward
Hydrophilic
heads
Water
Hydrophobic
tails
Fig. 4.5
Water
Fig. 4.6 Proteins are embedded within the lipid bilayer
•The fluid-mosaic model
–Phospholipid bilayer with embedded proteins
Proteins Within the Membrane
• Two main types
– Cell-surface proteins
• Project from the surface of the membrane
• Act as markers or receptors
– Transmembrane proteins
• Extend all the way across the bilayer
• Provide channels in and out of the cell
• The plasma (outside) membrane controls
the cell’s contact with the environment
• The cytoplasm contains organelles
most with membranes as boundaries
-This allows the cell to carry out a variety of
activities simultaneously
4.5 The Nucleus:
The Cell’s Control Center
• Usually the largest organelle
• The nucleus is separated from the cytoplasm by the
nuclear envelope
• The nucleus contains the DNA
–
–
–
–
–
It directs all of the cell’s activities
The DNA is associated with proteins
During cell division, it condenses into chromosomes
Messenger RNA (mRNA) is made in the nucleus
Rbosomal RNA (rRNA) is made in the nucleolus
Passage for RNA
and proteins
Site of assembly of
ribosome subunits
Fig. 4.12 The nucleus
Free ribosomes
• Ribosomes are made of ribosomal RNA
(rRNA) and protein and are part of the
machinery that make proteins
• Free ribosomes make proteins in the cytosol
– Enzymes that work in the cytosol,
– Stuctural proteins like microtubules
– Nuclear proteins that transcription factors,
histones, ribosomal proteins
– Mitochondrial proteins
4.6 The Endomembrane System
• More than half of the membranes in a eukaryote
are within the cell (endo)
• The Endomembrane System is a collection of
organelles surrounded by membranes
–
–
–
–
Endoplasmic reticulum
Golgi complex
Lysosomes
Peroxisomes
• These organelles divide the cell into
compartments, manufacture and distribute cell
products
Endoplasmic Reticulum (ER)
• Consists of two distinct regions
– Rough ER
• Studded with ribosomes
• Involved in protein synthesis
• Also makes membranes
– Smooth ER
• no ribosomes
• Embedded with enzymes
• Involved in lipid and carbohydrate synthesis
Rough ER makes proteins and membrane
Fig. 4.13 The endoplasmic reticulum
Rough ER makes proteins and membrane
• When ribosomes encounter an ER signal sequence on the
mRNA they bind the surface of the ER
• Makes and modifies proteins that are inserted in membranes
• Makes and modifies proteins that are secreted (i.e. insulin)
-most secreted proteins are glycoproteins, covalently bonded
to carbohydrates
• Transport vesicles bud off
Transport vesicle
buds off
4
Ribosome
Sugar
chain
1
Figure 4.8
Polypeptide
3
Secretory
(glyco-) protein
inside transport
vesicle
Glycoprotein
2
ROUGH ER
Smooth endoplasmic reticulum
has a variety of functions
• Smooth ER no ribosomes on surface
• Smooth ER makes lipids (fatty acids, phospholipids and
steroids)
• In liver cells, it regulates carbohydrate metabolism and
breaks down toxins and drugs
• In muscle cells it stores calcium needed for muscle
contraction
The Golgi Complex
– The Golgi receives transport vesicles from the ER
– Finishes, sorts, and then ships cell products to other
organelles OR to the plasma membrane for secretion
Fig. 4.14
• Golgi bodies are flattened stack of membranes that are scattered
throughout the cytoplasm
– These are collectively referred to as the Golgi complex
Lysosomes
• Bud off from the Golgi complex
• They contain enzymes that break down macromolecules
• Function in intracellular digestion of
– Worn-out cellular components
– Substances taken into cells
– The digested material is then recycled
Peroxisomes
•Bud off from the ER
–In animals peroxisomes have enzymes that detoxifies various harmful molecules
Vacuoles
• In plants
– Store dissolved
substances
– Can increase the
cell’s surface area
• In protists
– Contractile vacuoles
are used to pump
excess water
Fig. 4.23
Fig. 4.15 The
various organelles of the endomembrane
system are interconnected structurally and functionally
Path of Protein
Production