Cell Structure and Function

A TOUR OF THE CELL

Cell Theory
•

All organisms are composed of one or more cells

•

Cells are the smallest living units of
all living organisms

•

Cells arise only by division of a previously
existing cell

Cell Size
•

relatively small because as size increases,
volume increases much more rapidly.
– longer diffusion time

How are cells studied…
•

Microscopy
 Magnification
 Resolution/resolving power

•

Cell fractionation
 Technique that involves separation of
cellular organelles

Visualizing Cells
•

•

Resolution - minimum distance between two
points can be apart and be
distinguished as two separate points
Magnification – how much larger an object is
made to appear compared to its
real size

General types of Microscopes
•

•

Compound light microscope
Electron microscope
 Transmission EM
 Scanning EM

Visualizing Cells

Cell Characteristics
•

•



Genetic material (DNA)
 Single, circular in prokaryotes
 double helix in eukaryotes
Cytoplasm fills cell interior –
 sugars, amino acids,
 proteins – organelles
Plasma membrane encloses the cell
phospholipid bilayer

Living organisms as made up of either
Prokaryotic or Eukaryotic cells
Prokaryotic
(pro=before ; karyon=kernel)

Eukaryotic
(Eu=true; karyon=kernel)

Found only in bacteria and cyanobacteria

Found in Protista, Fungi, Plantae and
Animalia

No true nucleus;lack nuclear membrane

True nucleus; bounded by nuclear membrane

Genetic material in nucleoid region

Genetic material within nucleus

No membrane-bound organelles

Contains cytoplasm with cytosol and
membrane-bound organelles

Generalized Prokaryotic and Eukaryotic Cells

Generalized Animal and Plant Cells

Nucleus

•

•

•

•

Bound by nuclear membrane (two phospholipid bilayers)
– Nuclear pores – protein gatekeepers
 Usually proteins going in and RNA going out
Repository for genetic material organized
w/ proteins = chromosomes
Directs activities of the cell
Usually single, some cells several, RBC none
– Nucleolus - region of intensive ribosomal RNA synthesis

Chromosomes
•

DNA of eukaryotes is divided into linear chromosomes.
– exist as strands of chromatin, except during cell
division
– associated with packaging histones, packaging
proteins
 nucleosomes

Endomembrane System
•

Includes:
 Nuclear envelope
 Endoplasmic reticulum
 Golgi apparatus
 Lysosomes
 Vacuoles
 Plasma membrane
(not actually an endomem. but related to the system)

Importance:
Compartmentalizes cell, channeling passage of molecules through cell’s interior.

Endoplasmic reticulum
Extensive membranous network of tubules and sacs (cisternae)



Rough ER - studded with ribosomes
Smooth ER - few ribosomes

Synthesizes lipids,
phospholipids and
steroids

Smooth ER

Detoxifies drugs

Manufactures
secretory proteins

Participates in
carbohydrate
metabolism

Stores calcium ions for muscle contraction

Rough ER

Manufactures
membrane

Golgi apparatus

 Stacked, flattened membranous sacs (cisternae)\
 Has distinct polarity, cis face and trans face

 Modifies, stores, routes, collects, products
of ER

 Distribute molecules synthesized at one
location in the cell and utilized at
another location

 Modifies, stores, routes, collects, products of ER
 Distribute molecules synthesized at one location in the cell and
utilized at another location

SYNTHESIS OF MEMBRANE COMPONENTS AND THEIR
ORIENTATION ON THE RESULTING MEMBRANE

Vesicles
–

Lysosomes - membrane-bound vesicles
containing digestive enzymes
(lipase, protease, carbohydrase,nuclease)

–

Microbodies - enzyme-bearing, membraneenclosed vesicles.


Peroxisomes - contain enzymes that catalyze
the removal of electrons and
associated hydrogen atoms
- Peroxide-producing oxidases
- catalase

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Cytoplasm

Endoplasmic
reticulum

Phagocytosis
Food
vesicle

Golgi
apparatus

Lysosomes

Plasma
membrane

Extracellular
fluid

Digestion of
phagocytized
food particles
or cells

Transport
vesicle
Old or damaged
organelle
Breakdown
of old
organelle

Relationship among Endomembranes: A Summary
Is an extension of

Nuclear
envelope

Is confluent with

Rough ER

Smooth ER

Membrane and secretory proteins produced
in ER are transported in

Vesicles
Fuse with the forming
face of

Golgi apparatus
Pinch off maturing face

Vesicles
Give rise to

Lysosomes

Fuse with and add to plasma membrane
and may release cellular products to outside

Plasma membrane

Ribosomes
•

Ribosomes are RNA-protein complexes composed of
two subunits that join and attach to messenger RNA.
– site of protein synthesis
– assembled in nucleoli

Organelles With DNA
•

Mitochondria
–
–

•

bounded by exterior and interior membranes
interior partitioned by cristae

Chloroplasts
–

–

have enclosed internal compartments of stacked
grana, containing thylakoids
found in photosynthetic organisms

Mitochondria





"Powerhouse of the cell" - cellular metabolism
With outer and inner membranes, cristae
Have their own DNA

Chloroplasts
•Chloroplasts are larger and more complex than mitochondria
•Grana – closed compartments of stacked membranes
•Thylakoids – disc shaped structure – light capturing pigment
•Stroma – fluid matrix

Endosymbiosis
•

Endosymbiotic theory suggests engulfed prokaryotes
provided hosts with advantages associated with
specialized metabolic activities.

Theory of Endosymbiosis

Evidence for the endosymbiont theory is that
mitochondria and chloroplasts:
- Are appropriate size to be descendants of
eubacteria.
- Have inner membranes similar to those on
prokaryotic plasma membranes.
- Replicate by splitting, as in prokaryotes.
- DNA is circular and different from the DNA of
the cell's nucleus.
- Contain their own components for DNA
transcription and translation into proteins .
- Have ribosomes similar to prokaryotic ribosomes.
- Molecular systematics lend evidence to support
this theory.
- Many extant organisms are involved in
endosymbiotic relationships.

Cytoskeleton





Network of protein fibers throughout the cytoplasm
supporting cell shape, anchoring organelles, motility
Constructed from 3 types of fibers:
 Microtubules
 Microfilaments
 Intermediate filaments

1. Cell motility (cilia and flagella)

Microtubule
3. Make up centrioles
in animal cells
4. Separation of
chromosomes during
cell division

2. Cellular support

Structure of cilia and flagella

Microfilaments
(globular actin and long chain F actin)

Participates in
muscle contraction
Provides support

localized
contraction of cells

Intermediate filaments

Framework
of cytoskeleton

Reinforce
cell shape

Fix organelle position

Plant Cells
•

•

Central vacuole
– often found in the center of a plant cell, and
serves as a storage facility for water and other
materials
Cell wall
– primary walls – laid down while cell is
growing
– middle lamella – glues cells together
– secondary walls – inside the primary cell walls
after growth

Plant Cell

Animal Cells
•

Animal cells lack cell walls.
– form extracellular matrix
 provides support, strength, and resilience