Cell Structure

& organelles
Function
 Cell Theory
• All living things are made up of cells.
• Cells are the smallest working units of all
living things.
• All cells come from preexisting cells
through cell division.
 Definition of Cell

A cell is the smallest unit that is

capable of performing life
functions.
• Cells are the smallest units of life.
• They are a closed system, can self-
replicate, and are the building blocks of
our bodies.
• In order to understand how these tiny
organisms work, we will look at a cell’s
internal structures.
• We will focus on eukaryotic cells, cells that
contain a nucleus.
• Prokaryotic cells, cells that lack a nucleus,
are structured differently.
 Examples of Cells
Amoeba Proteus

Plant Stem

Bacteria

Red Blood Cell

Nerve Cell
 Two Types of Cells

•Prokaryotic
•Eukaryotic
 Prokaryotic
• Do not have
structures
surrounded by
membranes
• Few internal
structures
• One-celled
organisms,
Bacteria

http://library.thinkquest.org/C004535/prokaryotic_cells.html
 Eukaryotic
• Contain organelles surrounded by membranes
• Most living organisms
Plant Animal

http://library.thinkquest.org/C004535/eukaryotic_cells.html
 “Typical” Animal Cell

http://web.jjay.cuny.edu/~acarpi/NSC/images/cell.gif
 “Typical” Plant Cell

http://waynesword.palomar.edu/images/plant3.gif
• A cell consists of two major regions, the
cytoplasm and the nucleus.

• The nucleus is surrounded by a nuclear

envelope and contains DNA in the form of
chromosomes.

• The cytoplasm is a fluid matrix that usually

surrounds the nucleus and is bound by the
outer membrane of the cell.
• Organelles are small structures within the
cytoplasm that carry out functions
necessary to maintain homeostasis in the
cell.

• They are involved in many processes, for

example energy production, building
proteins and secretions, destroying toxins,
and responding to external signals.
• Organelles are considered either
membranous or non-membranous.

• Membranous organelles possess their

own plasma membrane to create
a lumen separate from the cytoplasm.

• This may be the location of hormone

synthesis or degradation of
macromolecules.
• Non-membranous organelles are not
surrounded by a plasma membrane. Most
non-membranous organelles are part of
the cytoskeleton, the major support
structure of the cell. These include:
filaments, microtubules, and centrioles.
Definition of a cell Smallest functional unit within a living organism that can
function independently

Components Plasma membrane, cytoplasm, nucleus, membranous

organelles, non-membranous organelles

Membranous Endoplasmic reticulum, golgi apparatus, mitochondria,

organelles peroxisomes, lysosomes, transport vesicles

Non-membranous Ribosomes, microtubules, cytoskeleton (actin filaments,

organelles intermediate filaments, centrioles)
Surrounding the Cell
 Cell Membrane
• Outer membrane of cell
that controls movement
in and out of the cell
• Double layer

http://library.thinkquest.org/12413/structures.html
 Cell Wall
• Most commonly found
in plant cells &
bacteria
• Supports & protects
cells

http://library.thinkquest.org/12413/structures.html
Cell Parts
Organelles
 Plasma membrane

• The plasma membrane surrounds the cell

to create a barrier between the cytosol and
the extracellular matrix.

• Plasma membranes also enclose lumens

of some cellular organelles.
• The structure of the membrane resembles
a fluid mosaic made up of phospholipids,
cholesterol, and membrane proteins.

• Phospholipid molecules, the main

structural components of the membrane,
form an amphipathic bilayer.

• An amphipathic structure is both

hydrophilic and hydrophobic; part of the
structure has a high affinity for water and
the other part is repelled by water.
• The inner surface of each layer is made up
of lipid chains and thus is hydrophobic.
The outer surface of each layer is made
up of the polar heads of the phospholipids
and is hydrophilic.
• Proteins associated with the plasma
membrane are either peripheral
membrane proteins or integral membrane
proteins.

• Peripheral membrane proteins interact

closely with the membrane through ionic
interactions.

• Integral membrane proteins are embedded

within or pass through the lipid bilayer.
• There are six broad categories of integral membrane proteins:
pumps, channels, receptors, linkers, enzymes, and structural
proteins.
• Pumps - transport ions, sugars, and amino acids across membranes

• Channels - allow small ions and molecules to pass freely in and out
of the cell

• Receptors - recognise and bind with ligands

• Linkers - anchor the cytoskeleton to the extracellular matrix

• Enzymes - have many roles, for example ATPases participate in ion

pumping

• Structural proteins - form junctions with neighboring cells

 Is cell membrane and plasma
membrane the same?
• No, they are not the same thing. While cell
membrane covers the entire components of a
cell, plasma membrane covers only the cell’s
organelles.
• Some main differences between the two are the
fact that the plasma membrane encloses the
organelles, whereas the cell membrane
encloses the entire cell.
• Moreover, the cell membrane can protect the
cell from outside viruses and bacteria, the
plasma membrane, however, cannot do this.
Membranous organelles
 Endoplasmic reticulum
• The endoplasmic reticulum (ER) is a large
network of membranes responsible for the
production of proteins, metabolism and
transportation of lipids, and detoxification of
poisons.
• There are two types of endoplasmic reticulum
with separate functions: smooth endoplasmic
reticulum and rough endoplasmic reticulum.
• The presence or absence of ribosomes in the
ER’s plasma membrane determines whether it is
classified as smooth or rough ER.
• The outer plasma membrane of rough endoplasmic
reticulum (rER) is carpeted with ribosomes, causing it to
appear dotted under a microscope.

• Protein production occurs in the ribosomes of rER. The

ribosomes synthesize a peptide strand which enters the
lumen of the rER and folds into its functional shape.
From there it will be transported to the Golgi apparatus in
a membrane bound vesicle formed from budding of the
rER membrane.
• Smooth endoplasmic reticulum, abbreviated sER, lacks
ribosomes and thus appears smooth under a
microscope.
• Its functions vary among cell types. For example, sER in
cells of the liver have detoxifying functions while sER in
cells of the endocrine system mainly produce steroid
hormones.

• Detoxification occurs through enzymes associated with

the sER membrane and usually involves adding hydroxyl
groups to molecules.
• The presence of hydroxyl groups makes the molecules
more water soluble and therefore able to be flushed from
the body through the urinary tract. Steroid hormone
synthesis occurs through reactions that modify the
structure of cholesterol.
 Endoplasmic Reticulum
• Moves materials around
in cell
• Smooth type: lacks
ribosomes
• Rough type (pictured):
ribosomes embedded in
surface

http://library.thinkquest.org/12413/structures.html
 Golgi apparatus
• The Golgi apparatus appears as a series of flattened, membranous sacs,
or cisternae, that resemble a stack of pancakes just off the rough
endoplasmic reticulum. It receives vesicles containing proteins recently
produced by the rER.

• The Golgi apparatus can be compared to a warehouse or post office for

newly formed proteins. Here the proteins are further modified, packaged,
and sent off to their final destinations in the cell or body.
• Because the Golgi apparatus receives and sends off vesicles from opposite
sides of its stack of cisternae, it is considered polar, meaning it has a
directional structure.
• The cis-face is located near the rER and receives vesicles. The trans-face is
on the opposite side of the organelle and releases vesicles through budding
of the plasma membrane. The quantity of stacks depends on the function of
the cell.
 Golgi Bodies
• Protein 'packaging
plant'
• Move materials within
the cell
• Move materials out of
the cell

http://library.thinkquest.org/12413/structures.html
 Mitochondria
• The mitochondrion, mitochondria denotes plural, is a double
membrane bound organelle. Its inner membrane contains many
infoldings called cristae.
• The space between the outer and inner membranes is referred to as
the intermembrane space and the matrix is the space inside the
inner membrane. Free ribosomes and mitochondrial DNA can be
found in the matrix. Mitochondrial DNA is unique in that it is entirely
maternally inherited.
• Mitochondria are the powerhouses of the cell. Cellular respiration,
the generation of energy from sugars and fats, occurs in these
organelles.
• Some of the enzymes that catalyze respiration are found within the
matrix. Other proteins involved in these reactions are built into the
wall of the inner membrane.
• The cristae of the inner membrane are highly convoluted to increase
surface area. This allows for more proteins lining the membrane and
thus greater productivity.
 Mitochondria
• Produces energy through
chemical reactions –
breaking down fats &
carbohydrates
• Controls level of water and
other materials in cell
• Recycles and decomposes
proteins, fats, and
carbohydrates

http://library.thinkquest.org/12413/structures.html
 Peroxisomes
• Peroxisomes are single membrane
compartments that contain enzymes used
to remove hydrogen atoms from
substrates. The free hydrogen atoms then
bind to oxygen and create hydrogen
peroxide.
• Peroxisomes are especially important in
the liver because transferring hydrogen
from poisons or alcohol to oxygen atoms
detoxifies harmful compounds.
 Lysosomes
• Lysosomes are membranous sacs that hydrolyze
macromolecules to carry out intracellular digestion.

• This may occur for a variety of reasons. Single-celled organisms,

such as amoebas, use lysosomes to digest food products. This
process is referred to as phagocytosis.

• Phagocytosis occurs in human cells as well, however in humans this

process is used in defense to destroy invaders and bacteria.

• Lysosomes are also used to recycle the cell’s own materials. This
processes is referred to as autophagy. Damaged organelles that are
broken down in the lysosome and its organic monomers are
returned to the cell cytosol for reuse. In this way the cell is
constantly renewing itself.
 Lysosome
• Digestive 'plant' for
proteins, fats, and
carbohydrates
• Transports undigested
material to cell
membrane for removal
• Cell breaks down if
lysosome explodes

http://library.thinkquest.org/12413/structures.html
 Transport vesicles
• Transport vesicles are membrane bound
sacs used to transport materials through
the cytoplasm.
• They are formed from budding of the
plasma membrane of other organelles and
release their contents through exocytosis.

• Transport vesicles are used to move

proteins around the cell and to
release neurotransmitters into the synaptic
space.
Non-membranous organelles
 Ribosomes
• Ribosomes, either free in the cytosol or associated with
rER, synthesize proteins as polypeptide chains. This occurs through
the translation of RNA.

• Specifically, ribosomes bind to messenger RNA, abbreviated

mRNA. The ribosome reads a series of nucleotide bases in groups
of three called codons.

• The first codon read is the start codon. Each codon following the
start codon represents a specific amino acid that is then brought to
the ribosome by transfer RNA, abbreviated tRNA. The tRNA
carrying the amino acid is bound into the A site of the ribosome.
Here the amino acid is linked to the amino acid that precedes it, in
the P site. The bond between two amino acids in a polypeptide
chain is referred to as a peptide bond. After the peptide bond is
created the ribosome translocates to the next three nucleotide bases
on the mRNA strand and repeats the process until a stop codon is
reached.
 Ribosomes
• Each cell contains
thousands
• Make proteins
• Found on ribosomes
& floating throughout
the cell

http://library.thinkquest.org/12413/structures.html
 Microtubules
• Microtubules are involved in the movement of organelles and other
structures, for example lysosomes and mitochondria. They are
elongated, nonbranching polymers made up of dimers of α-
tubulin and β-tubulin.

• Microtubules contain approximately 13 circular dimeric tubulin

molecules. Dimers can be added or removed to change the length of
the microtubule.

• This process is termed dynamic instability and

requires GTP hydrolysis. All of the tubulin dimers are arranged in a
specific pattern so that they have the same orientation.

• Because of this orientation microtubules are considered polar, with a

plus and minus end. Growth occurs at the plus end. The minus end
of the microtubule does not grow.
 Actin filaments
• Actin filaments are nearly ubiquitous among all
cell types. Their structure is similar to that of
microtubules in that they are formed by a helical
arrangement of smaller molecules.

• However, actin filaments are thinner and

more flexible than microtubules. Multiple cell
functions require participation of actin. Actin
filaments are, for instance, used as anchors in
movement of membrane proteins and they form
the core of microvilli.
 Intermediate filaments
• The size of intermediate filaments, as their name implies,
is between that of microtubules and actin filaments.
Intermediate filaments consist of a rod domain with
globular domains on either end.

• The rod domain is made up of a pair of helical

monomers twisted around each other to form coiled-coil
dimers. Although the subunits of intermediate filaments
are diverse and tissue specific, the filaments generally
perform a structural role in the cell.
• They primarily form a linked continuum of filaments in
the nucleus, cytosol, and extracellular environment. They
are especially involved in the formation of cell-to-cell and
cell-to-extracellular matrix junctions.
 Centrioles
• Centrioles are structural organelles
consisting of nine microtubule triplets
organized into cylinders. The two main
functions of centrioles are the formation of
basal bodies and mitotic spindles. Basal
bodies are used as building blocks for
flagella and cilia. Mitotic spindles are
involved in the separation of
chromosomes during cell division.
Centrioles determine the location of mitotic
spindles during anaphase.