Download Cell Structures and Functions

Cell Biology
Cellular Functions
Cells
Tissues
Body
Organs
Body systems
• The cell Theory states that:
1. All living things are composed of one or more
cells
2. The cell is the smallest unit that retains the
properties of life
3. New cells arise from existing cells
• Any cell is defined (bounded) by a cell membrane
also called plasma membrane.
• All the material inside the cell is called the
protoplasm, in other words the cytoplasm and its
content including the nucleus.
• Inside the cell usually near the center, is the
nucleus.
• The cytoplasm is the fluid that contains all the
cellular organelles enclosed inside the cell
membrane
Animal Cell
The cell Structure:
1] The Nucleus: is the control center of the cell, it is bound by a double
membrane, called the nuclear membrane, this membrane has
numerous pores, which allow the passage of certain molecules in
and out of the nucleus, and prevents others. Inside the nucleus the
hereditary (genetic) information is organized into threadlike
structures called the chromosomes. Along the length of the
chromosomes are the genes, in other words genes are certain
sections of chromosomes, the genes are the characteristicdetermining units, each gene contains the instructions (a code) to
produce a certain protein. Each chromosome is made of nucleic
acids and proteins, the nucleic acid in chromosomes is
deoxyribonucleic acid (DNA), the genetic material of life!
Nucleolus
Chromatin
Pore
Eukaryotic Cell Nucleus
The Nucleus and Rough ER
•
Cells could be classified according to the presence
or absence of a nuclear membrane (nuclear
envelope), into two types:
1. Prokaryotic cells: are those cells which have the
nuclear material distributed all over the cell, with
no nuclear envelope.
e.g.: Bacteria and Cyanobacteria
2. Eukaryotic cells: are those cells which have the
nucleus bound with a double membrane, the
nuclear membrane.
e.g.: Plant cells and Animal cells
Cell wall
Plasma membrane
Chromatin
ribosomes
flagellum
Prokaryotic Bacterial Cell
• Inside the nucleus there is a dark stained structure called
the nucleolus. The nucleolus is involved in the synthesis
of a genetic material called ribosomal RNA, which is a
second type of nucleic acid (ribonucleic acid), ribosomal
RNA (rRNA) synthesis is directed and controlled by DNA, in
turn (rRNA) directs the synthesis of ribosomes, these are
specialized structures that are important for protein
synthesis.
• Pores in the nuclear envelope retain DNA inside, however
they allow the passage of molecules like those of
messenger RNA (mRNA) which is a copy of the genetic
information contained in DNA, this (mRNA) attaches to the
ribosomes, where the information is used for the synthesis
of proteins.
2] The cell membrane: is the outermost boundary of the cell, it is
composed of protein and a bilayer of lipid (fat) molecules, the cell
membrane holds the cell contents in place and regulates the
passage of materials in and out of the cell, it also contains receptor
sites that allow the entry of materials that affect cell activity, which
means that it is a selectively permeable membrane that allows the
entry of some materials while keeps other out.
• It is important to be aware that the cell membrane is fluid in nature,
in other words it is a lipoid double layer that bounds the cell.
• Some cells are equipped with flagella or cilia, both are continuous
with the cell membrane and have contractile proteins that allow them
to move.
e.g.: sperms have flagella (singular: flagellum)
cells of the trachea have cilia (singular: cilium)
Phospholipids
• Phospholipids are composed of a glycerol
molecule to which are attached two fatty acids
and a highly polar phosphate group.
• The phosphate group may be thought of as a
polar head and the fatty acids as long non-polar
tails.
• The polar head is hydrophilic and the non-polar
tails are hydrophobic.
Phospholipids and the cell membrane
The cell membranes are mostly composed of phospholipids.
3] The cytoplasm: is the fluid that contains all the
parts of the cell, enclosed by the cell membrane.
Most of the cell’s chemical activities take place in
the cytoplasm, absorbed nutrients are transported
and processed in the cytoplasm, the cytoplasm also
stores the waste material until proper disposal
The Cell organelles (cytoplasmic organelles)
1] Mitochondria: are tiny oval shaped organelles. Mitochondria have a
smooth outer membrane and an inner folded membrane, the finger
like projections of the inner membrane are called cristae. Each
mitochondrion provides the cell with the needed energy in a series of
chemical process called cellular respiration, it is inside the
mitochondria that this process takes place, with the help of enzymes
that speed the chemical reaction, sugar molecules combine with
oxygen to form carbon dioxide and water, with the release of energy
from the break down of the chemical bonds of sugar, the energy is
stored in a compound called adenosine triphosphate (ATP).
• It should also be noted that mitochondria have their own DNA which
means that they have to replicate before cell division.
Structure of a Mitochondrion
2] The Ribozomes: Ribosomes are the sites of protein
synthesis. Cell growth and reproduction requires
the synthesis of different protein molecules. There
are 20 different amino acids from which proteins
are composed, the properties of proteins are
determined by the number and the sequence of
amino acids in the chain.
• Amino acids are chemically bonded together by
enzymes at the ribosomes, the specific sequence of
amino acids is determined by instructions encoded
in the DNA, a change in the position of a single
amino acid can result in a different protein.
• Ribosomes exist floating freely in the cytoplasm, and also
bound to the endoplasmic reticulum (ER). ER bound to
ribosomes is called rough ER because the ribosomes
appear as black dots on the ER in electron microscope
photos, giving the ER a rough texture.
• Ribosomes are made of rRNA and proteins, measuring just
20 nm (nanometers) ribosomes are among the smallest
organelles in the cytoplasm, yet they make up a great
portion of the cytoplasm.
• Ribosomes disassemble into two subunits when not
actively synthesizing protein.
3] The Endoplasmic Reticulum (ER): is a network of
interconnecting channels that transport materials
throughout the cytoplasm, the (ER) reaches from the cell
membrane to the nuclear membrane.
• There are two types of the endoplasmic reticulum, the
rough endoplasmic reticulum (RER) that has many
ribosomes attached to it and the smooth endoplasmic
reticulum (SER) that has no ribosomes.
• The (RER) is associated with cells involved in enzyme
secretion, such as those of the liver and the pancreas.
• The (SER) is found in the cells of developing seeds and in
animal cells that secret steroid hormones.
The Rough Endoplasmic Reticulum (RER)
4] The Golgi Apparatus (Golgi Complex) (Golgi Body):
• The Golgi apparatus stores, modifies, and packages
proteins from the rough endoplasmic reticulum.
• The Golgi apparatus is a group of membranous sacs piled
on top of each other.
• These membrane-bound structures pinch off at the end to
produce smaller protein sacs called vesicles. When these
vesicles reach the cell membrane, they fuse with it and
release their content outside the cell, this process is called
exocytosis, when other vesicles coming from outside the
cell fuse with the cell membrane and release their contents
into the cell, this process is called endocytosis.
Golgi Body
5] The Lysosomes: are formed by the Golgi apparatus
and are bound by a single membrane.
• Lysosomes contain enzymes that break large
molecules into smaller ones within the cytoplasm.
• Lysosomes play an important role in the body’s self
defence mechanism, as their digestive enzymes
destroy the invading bacteria after being engulfed
by the white blood cells, this also results in the
death of the white blood cells, the result is the
formation of pus which consists of dead cells, fluid
and protein fragments.
The Cytoskeleton
• Is NOT an organelle
• Is transparent in standard light and electron microscope
preparations, and is therefore "invisible. It is usually left
out of drawings of the cell, but it is an important, complex,
and dynamic cell component. The cytoskeleton maintains
the cell's shape, anchors organelles in place, and moves
parts of the cell in the processes of growth and motility.
• The cytoskeleton includes 3 types of filaments, the
microtubules, the microfilaments (Actin) and the
intermediate filaments.
Intermediate Filaments (Model)
Cytoskeleton Functions:
Establishing cell shape
Providing mechanical strength
Locomotion
Chromosome separation in mitosis and
meiosis
• Intracellular transport of organelles
•
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Special Structures of Plant Cells
1] Plastids: are chemical factories and storehouses for food and colour
pigments.
•
There are many types of plastids, namely chloroplasts,
chromoplasts, and amyloplasts.
a.
•
Chloroplasts: are prevalent in all green plants, they contain the
green pigment chlorophyll, chloroplasts specialize in
photosynthesis, a process by which plants combine carbon
dioxide from the air, with water from the roots, in the presence of
light to produce sugar and oxygen.
Like mitochondria, chloroplasts contain their own DNA and
ribosomes, which means that during cell division they replicate.
b.
Chromoplasts: store orange and yellow pigments, found in many
fruits and flowers.
c.
Amyloplasts: are colourless plastids, and they are the storehouses
of starch, the are found in many seeds and roots, such as potato
tubers.
Chloroplast Structure
2] Vacuoles: are fluid filled spaces (usually one big
vacuole) that store sugar, minerals and proteins,
they increase the surface area of the cell allowing
more absorption, they also provide the cell with
rigidity and support, through turgor pressure.
3] Cell Wall: most of the plant cells are surrounded by
a cell wall composed of cellulose, its main function
is to protect and support plant cells.
Comparison between Animal and Plant Cells
Cell
Animal Cell
Plant Cell
Yes
No
Centrosome
Yes
No (most plant
cells)
Vacuole
Small (if present)
Yes
Plastids
No
Yes
Cell Wall
No
Yes
Structure
Lysosomes
Application: Cell Research in Medicine
• Hybridomas are cells made from white blood cells and cancer cells,
they produce large amounts of antibodies for defence against
disease
• The secretion of digestive enzymes by lysosomes (as the membrane
vesicles burst) may explain arthritis and its pain, when cortisone (a
drug) is given the swelling and pain decreases as it strengthens the
lysosome membrane
• Cells grown in tissue culture, can be used for skin, liver and other
cell transplants.
• Foetal brain tissue transplants into patients with Parkinson’s
disease, are helped greatly through this technique, however ethical
considerations remain controversial