A1262303104cjFAb - PowerPoint PPT Presentation

Title: A1262303104cjFAb


1
The Cell
2
The Cell Membrane
The cell membrane is a lipid bilayer containing
phospholipids, glycolipids, cholesterol and
proteins. The overriding role played by the cell
membrane is to regulate the passage of ions,
molecules and larger particles between the
intracellular and extracellular environments.
3
Chemical Composition of the Cell Membrane
Outside the cell
Plasma membrane
Inside the cell
4
A. Membrane Lipids 1. The phospholipids in the
membrane form a bilayer in which the hydrophilic
phosphate ends of the molecules face the aqueous
environments outside and inside the cell. The
hydrophobic fatty acid ends face each other
within the membrane. 2. Cholesterol and other
lipids are closely associated with the
hydrophobic tails of the phospholipids. 3.
Ions and other water soluble materials do not
pass across the hydrophobic barrier of the
membranes interior. This helps to isolate the
cytoplasm from the surrounding fluid environment.
5
B. Membrane Proteinsa. They account for about
55 of the weight of the cell membrane. b.
Proteins are associated with the membrane in two
basic ways 1. Integrated proteins are part of
the membranes structure. The degree of
penetration and integration of proteins depends
on the relative proportions of hydrophilic and
hydrophobic regions on the protein. 2.
Peripheral proteins are more loosely bound to the
inner and outer surfaces of the membrane. Most
membrane proteins are integral.
6
Functions of membrane proteins include1.
Anchoring membrane protein may attach the
membrane to the cytoskeleton inside the cell and
to extracellular fibers. 2. Recognition
Proteins immune cells recognize the cells of
the body by the presence of special recognition
proteins. 3. Enzymes will catalyze
reactions in the extracellular or intracellular
fluid depending on the position of the enzyme
molecule within the membrane. 4. Receptor
proteins will bind specifically to
extracellular molecules called ligands. This
binding will trigger changes in the activity of
the cell. 5. Carrier Proteins will bind
molecules and transport them across the cell
membrane. This binding involves a shape change
in the carrier protein. After the solute
molecule has made the transit across the
membrane, the carrier returns to its original
shape. 6. Channel Forming Proteins many
integral proteins contain a central pore or
channel to allow a passive movement of small
molecules into and out of the cell. Some
channels are gated, requiring a specific signal
to open.
7
C. Membrane Carbohydrates 1.
Constitute about 3 of the membranes
weight. 2. Are components of larger molecules.
For example, proteoglycans, glycoproteins and
glycolipids are large integral molecules. The
carbohydrate portions of these molecules extend
out from the cells surface forming a layer
called the glycocalyx. 3. The glycocalyx
functions to a. lubricate and protect the
cells surface b. helps to anchor the cell in
place c. serve as receptor molecules for
hormones d. be used as recognition molecules
by cells of the immune system. This property of
the molecules of the glycocalyx is determined
genetically. For example, blood type is based on
the presence or absence of specific membrane
glycolipids on the surfaces of the red blood
cells.
8

• Structural Modifications of the Cell Membrane
• Intercellular junctions
• Microvilli
• Cilia
• Flagella

9
Intercellular Junctions
Intercellular Junctions are found at the places
where the cell membrane of one cell makes contact
with the membrane of another. 1. Desmosomes
hold two cells together very strongly. In the
epidermis, they contain a tough fiber called
keratin. 2. Zona occludens or tight
junctions consist of an apparent fusion of the
cell membranes of adjacent cells. Found in the
lining of the small intestine. 3. Gap Junctions
very small pores in the membranes between two
cells. They may be sites of cell to cell
communication.
10
DESMOSOMES
Desmosomes (arrows) are regions where the
membranes of two cells are held together very
strongly. In the epidermis, a tough fiber called
keratin joins the cells at desmosomes.
11
TIGHT JUNCTIONS
Zona occludens or tight junctions consist of an
apparent fusion of the cell membranes of adjacent
cells. In the lining of the small intestine
these junctions seem to prevent materials from
the intestinal lumen from leaking into the
intercellular spaces.
12
GAP JUNCTIONINTERCALATED DISK
Gap Junctions appear to be a series of very small
(20 A) pores in the membranes between two cells.
They may be sites of cell to cell communication.
Gap junctions can be found in the intercalated
disks between cardiac muscle cells (arrow).
13
MICROVILLI - DIFFERENT VIEWS
Microvilli are closely bunched, tiny projections
from the free surface of cells lining the
intestinal cavity (striate border) and kidney
tubules (brush border). In these tissues their
chief function is absorption. In the stomach and
uterine epithelium they function primarily for
secretion.
14
Microtubules -Composed of molecules of tubulin
arranged to form hollow tubes about 25 nm in
diameter and possibly many micrometers (um) long.
They produce movement of certain cellular
structures by assembly and disassembly of the
microtubules attached to those structures, e.g.,
the chromosomal movement during cell
division. Produce changes in cell shape
associated with cell motility and
phagocytosis. ..Microtubules within cilia and
flagella produce the whip-like motions of these
organelles.
MICROTUBULES
CROSS SECTION OF MICROTUBULES WITHIN CILIA
(ARROW)
15
CILIA - TWO VIEWS
Cilia are short extensions of the cytoplasm and
cell membrane at the free surface of some
body cells. As revealed by electron microscopy,
a cross section of a cilium shows a central pair
of tiny, hollow microtubules surrounded by nine
peripheral pairs of microtubules. Cilia are
anchored just below the cell membrane to a basal
body. Cilia beat rhythmically on the surface of
cells lining the respiratory tract and oviduct
tubes in mammals.
16
FLAGELLUM
Flagella (singular flagellum) closely resembles a
cilium internally in microtubule arrangement and
anchorage on a basal body. They are much longer
than cilia and usually found one to a cell.
Flagella move sperm cells from where they are
deposited in the females vagina to the oviduct
where they fuse with the egg cell.
17

• Cell Transport
• Active Transport
• Exocytosis
• Endocytosis
• 2. Passive Transport
• Diffusion
• Osmosis

18
Active Transport Exocytosis and Endocytosis
Cells can eliminate materials by a process called
exocytosis. Small vesicles move to the cell
membrane and fuse with it. The contents of the
vesicle are released outside the cell. Cell can
bring molecules or small particles across the
cell membrane into the cell by a process that
looks like exocytosis in reverse. This is called
endocytosis.
19
Passive Transport - Diffusion
Passive transport is the movement of
materials across the membrane without the cell
using its own energy. An example of passive
transport is diffusion. Diffusion is the random
movement of particles from an area of high
concentration to an area of low concentration.
This leads to an even distribution of the
particles. In the example below, a perfume
bottle is opened at one end of a room (A). After
a period of time (B), the perfume molecules (P)
are distributed throughout the room.
P
A
B
20
Passive Transport - Osmosis
Osmosis is the diffusion of water from an area
where water is in high concentration to an area
where water is in low concentration. In the
body, osmosis occurs across the plasma membranes
of cells. Normally, the concentration of
water inside the cells and outside the cells is
equal. The water inside and outside the cells
contains dissolved substances called solutes such
as sodium and chloride ions. Since the
concentration of water, inside and out is equal,
the concentration of solutes must also be equal.
Two solutions which have equal concentrations of
solutes are said to be isotonic. Under this
condition, water diffuses at the same speed into
and out of the cell.
Water
Fluid outside cell contains 99.1 water and 0.9
NaCl
Cell fluid is 99.1 water and 0.9 NaCl
Water
21
Hypertonic
When the fluid outside the cells has a greater
concentration of dissolved substance in it (1.5)
than the fluid inside the cells (0.9) it is
called hypertonic. The outside water
concentration (98.5) is lower than the
concentration of water inside the cells (99.1).
By osmosis water diffuses from high to low
concentration. This results in the water moving
out of the cells faster than the water enters.
The cells shrink. The shrinkage of cells due to
the loss of water is called crenation. When a
fluid surrounding cells causes them to shrink, it
is called hypertonic.
Fluid outside cell contains 98.5 water and 1.5
NaCl
Cell fluid is 99.1 water and 0.9 NaCl
Water
Cell
Crenated Cell
22
Hypotonic
When the fluid outside the cells has a lower
concentration of dissolved substance in it (0)
than the fluid inside the cells (0.9) it is
called hypotonic. Here the concentration of
water outside the cells (100) is higher than
inside (99.1). As a result, the water continues
to diffuse into the cells until the cells burst.
The bursting of the cells is called hemolysis. A
fluid which causes cells to burst is called
hypotonic.
Water
Fluid outside cell contains 100 water and 0 NaCl
Cell fluid is 99.1 water and 0.9 NaCl
Hemolysis
Water
Cell
23
Organelles of the Cell

• 1. The Nucleus
• The Mitochondrion
• Endoplasmic Reticulum
• The Ribosome
• The Golgi Body

24
The nucleus (N) is the largest organelle
(5um in diameter) in the cell. It is usually
centrally located. The nucleoplasm contains the
nucleic acids Ribonucleic acid and
Deoxyribonucleic acid, as well as, their building
blocks, the nucleotides. Dyes like methylene
blue stain this material deeply. For this
reason, the content of the nucleus is referred to
as chromatin. .The two principal activities of
DNA are1. Replication each DNA molecule
copies itself precisely before the cell, as a
whole, divides.2. Transcription DNA
transcribes its coded message into a
complementary molecule of RNA.
THE NUCLEUS
N
N
25
The Nucleus (continued)
The nucleus stores the vital, chemically coded
information needed by the cell to carry out the
life activities of the cell. This chemical, DNA,
has the structure of a double helix or a twisted
ladder. The sides of the ladder consist of
alternating molecules of a sugar called
deoxyribose, and phosphate. The rungs of the
ladder are formed by the nitrogen bases adenine,
thymine, guanine and cytosine.
The Nucleus
26
Transcription
In the nucleus of the cell, the DNA molecule
separates into two strands. One side of the
molecule is used to construct a molecule of
messenger RNA. The code represented by the
sequence of bases in DNA is reflected in the code
sequence in the RNA molecule. This process is
called transcription.
mRNA
27
NUCLEAR ENVELOPE
The nucleus (N) is surrounded by a series of
flattened membranous sacs making up the nuclear
envelope (ne). The envelope separates the
contents of the nucleus from the cytoplasm (cy).
cy
ne
N
28
NUCLEAR PORES
The nuclear envelope (ne) is perforated
with nuclear pores (9nm). These pores allow for
chemical communication between the nucleoplasm
(np) and cytoplasm (cy). For example, mRNA
manufactured in the nucleus leaves through these
pores to reach the ribosomes in the cytoplasm.
np
cy
cy
ne
np
29
ENDOPLASMIC RETICULUM (ER)
The ER consists of a network of intracellular
membranes forming hollow tubes, flattened sacs
and rounded chambers called cisternae. This
network is connected to or continuous with the
nuclear envelope surrounding the nucleus. Protein
synthesis is primarily carried out in that
portion of ER covered with ribosomes. called
Rough ER or RER. Lipid and Carbohydrate
synthesis is accomplished in that portion of the
ER lacking ribosomes. From its appearance it is
called Smooth ER or SER.
30
Ribosomes
Ribosomes can be seen as very small dot-like
structures on the sacs of the rough endoplasmic
reticulum. The ribosomes are the site of protein
synthesis. This is where amino acids are joined
together under the direction of mRNA to form
complex protein molecules.
31
The mRNA leaves the nucleus through the
nuclear pores and arrives at the ribosome. Each
ribosome is made up of two subunits, a large and
a small portion. The mRNA enters the
ribosome one triplet of bases at a time. A
molecule of transfer RNA with a triplet code that
corresponds to the mRNA enters the ribosome
carrying a specific amino acid. As each tRNA
molecule enters the ribosome, the amino acid it
caries is chemically bonded to the previous amino
acid. This process continues until the
entire mRNA molecule is translated into the
placement of specific amino acid molecules in a
protein chain. The protein chain enters the
cisterna of the endoplasmic reticulum.
Translation
Amino acids
Amino acids linking to form a protein
tRNA
tRNA
ribosome
mRNA
32
Golgi Body
The Golgi body consists of a stack of five or six
flattened sacs (B). A single cell may have
several Golgi bodies. Generally, the Golgi body
is located near the nucleus. The sacs of the
Golgi body receive newly synthesized protein
from the rough endoplasmic reticulum. Within the
sacs, the proteins are modified by having
phosphate, lipid or sugars attached. The
finished product leaves the Golgi in vesicles for
storage or are transported out of the cell
through the cell membrane (exocytosis).
33
Golgi Apparatus The Golgi apparatus consists of
several flattened membranous sacs. The Golgi
receives newly made proteins and lipids from the
endoplasmic reticulum and packages them for
storage in the cell or release from the cell.
The lysosomes are formed this way.
34
Types of Secretory Vesicles 1. Secretory
products move to the cell membrane for release
within secretory vesicles. 2. Lysozymes are
powerful enzymes packaged in vesicles called
lysosomes used to degrade intracellular food
particles, released by certain white blood cells
to destroy microbes or released within the cell
to carry out autolysis following trauma (suicide
sacs). 3. Peroxisomes are vesicles generally
smaller than lysosomes. They remove and
neutralize toxins within the cell such as free
radicals. They are most abundant in liver cells
where they neutralize and degrade toxins absorbed
by the digestive tract.
35
LYSOSOMES
Lysozymes (c) are packaged in specialized
vesicles called lysosomes (A). Lysozymes are
powerful enzymes used to degrade intracellular
food particles, released by certain white blood
cells to destroy microbes or released within the
cell to carry out autolysis following trauma
(suicide sacs).
36
THE MITOCHONDRION
Mitochondrion - The powerhouse of the cell. Very
small organelles about the size of bacteria (1.5
um x 2 to 8 um), they are surrounded by an
unusual double membrane. The inner membrane
contains many folds called cristae. The surface
of the inner membrane facing the interior or
matrix of the mitochondrion is covered with
respiratory enzymes and other molecules which are
responsible for the reactions which release the
stored energy in glucose. Some of that energy
is stored in molecules of adenosine triphosphate,
a form the cell can readily use for life
activities.
37
Cytoskeleton Nonmembranous structures of the
cell
The Cytoskeleton is a micro scaffolding
within the cell produced by a number of
structural elements. Besides giving the
cell shape and structure, these filamentous
elements produce movements of structures within
the cell and movements of the cell itself.
There are three types of filaments making up the
cytoskeleton1. Microtubules2. Microfilaments
3. Intermediate filaments
The yellow fibers in these cells are
microtubules. The pale fibers are actin.
38
1. Microtubules a. Composed of molecules of
tubulin arranged to form hollow tubes about 25 nm
in diameter and possibly many micrometers (um)
long. b. Produce movement of certain cellular
structures by assembly and disassembly of the
microtubules attached to those structures, e.g.,
chromosomal movement during cell division. c.
Produce changes in cell shape associated with
cell motility and phagocytosis. d. Microtubules
can serve as an anchorage or footpath for
proteins that behave as molecular motors. These
specialized proteins, kinesin and dynein produce
the intracellular movements of small structures
like vesicles. e. Microtubules within cilia and
flagella produce the whip-like motions of these
organelles.
39
2. Microfilaments a. Very slender protein
strands less than 6nm in diameter. b. The most
abundant microfilaments are composed of actin, a
contractile protein. c. Actin filaments are
most common in the periphery of the cell and rare
in the region around the nucleus. They attach
the cell membrane and the nuclear envelope to the
cytoplasm. d. Actin interacts with the thicker
(18 nm) myosin microfilaments to produce cell
contractions. This arrangement is abundant in
skeletal muscle cells.
40
3. Intermediate filaments a. Intermediate in
thickness between the actin (thin) and myosin
(thick) microfilaments. b. They are insoluble
and the most durable of the cytoskeletal
components. c. Keratins are the tough,
waterproof proteins found in the skin, hair and
nails. d. Neurofilaments provide the structural
support for the very long cell processes called
axons in nerve cells. e. Desmin filaments are
the anchorage for the intercellular junctions
(desmosomes) which hold cells together in
epithelial tissues.