Cellular Biology

OUTLINE
1. Introduction
2. Celluar Anatomy
- Various Cellular Organelles
3. Cellular Functions
- Cellular transport
- Cellular Growth and Metabolism
- Protein Synthesis
- Mitochondrial Functions
4. Summary
5. Conclusion

The cell (from Latin cella, meaning "small room") (Greek kytos, "contain") is the basic
structural, functional, and biological unit of all known living organisms. They are the
smallest unit of life that can replicate independently, and are often called the "building
blocks of life".

The study of cells is called Cellular Biology. The cell was discovered by Robert Hooke in
1665. He named it for its resemblance to cells (small rooms) inhabited by Christian
monks in a monastery. The number of cells in plants and animals varies from species to
species. Humans contain about 100 trillion (1014) cells. Most plant and animal cells are
visible only under the microscope, with dimensions between 1 and 100 micrometres.

The cell theory was first developed in 1839 by Matthias Jakob Schleiden and
Theodor Schwann. It states:
- That all organisms are composed of one or more cells.
- That all cells come from pre-existing cells,
- That vital functions of an organism occur within cell
- That all cells contain the hereditary information necessary for regulating their
functions and for transmitting information to the next generation of cells.
Cells emerged on the earth at least about 3.5 billion years ago.

CELLULAR BIOLOGY
This is the scientific study of cells–including their physiological properties, their
structure, the organelles they contain and their interactions with their environment.
Knowing the components of cells and how cells work is fundamental to all biological
sciences. Appreciating the similarities and differences between cell types is particularly
important to the fields of cell and molecular biology. This has wide applications in
cancer research and developmental biology. As a result of these fundamental similarities
and differences, the principles learned from the study of one cell type can be
extrapolated and generalized to other cell types. Hence, research in cell biology is closely
related to genetics, biochemistry, molecular biology, immunology, and developmental
biology.
CELLULAR VIABILITY
The viability and normal activity of cells depends on a variety of fundamental
housekeeping functions that all differentiated cells must perform. These functions
include protection from the environment, nutrient acquisition, communication,
movement, renewal of senescent molecules, (senescent – old, growing old, ageing)
molecular catabolism, and energy generation. These specialised functions are
compartmentalized within membrane-bound intracellular organelles.

This compartmentalization allows potentially injurious degradative enzymes or reactive

metabolites to be concentrated or stored at high concentrations in specific organelles
without risking damage to other cellular constituents. Compartmentalization also allows
the creation of unique intracellular environments (e.g., low pH or high Ca2+) that may
selectively regulate the functions of enzymes or metabolic pathways.

CELLULAR ANATOMY
There are two types of cells:

Eukaryotes, which contain a nucleus, and

Prokaryotes, which do not.

Prokaryotic cells are usually unicellular organisms, while eukaryotic cells can be either
unicellular or part of multicellular organism.
CELL ORGANELLES

Cell Membrane
The cell membrane, or plasma membrane, surrounds the cytoplasm of a cell. In animals,
the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes
it is usually covered by a cell wall. The cell membrane serves to separate and protect a
cell from its surrounding environment and it is made mostly from a double layer of
phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic).
Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic
membrane.
Embedded within this membrane is a variety of protein molecules that act as channels
and pumps that move different molecules into and out of the cell.
The cell membrane is 'semi-permeable', in that it can either let a substance (molecule or
ion) pass through freely, pass through to a limited extent or not pass through at all. Cell
surface membranes also contain receptor proteins that allow cells to detect external
signaling molecules such as hormones.

 Variation
 The cell membrane has different lipid and protein compositions in distinct types
of cells and may have therefore specific names for certain cell types:
 Sarcolemma in myocytes
 Oolemma in oocytes
 Axolemma in neuronal processes - axons
Historically, the plasma membrane was also referred to as the plasmalemma
The Phospholipid bilayers are usually composed of phospholipids, which have a
hydrophilic head and two hydrophobic tails each. When phospholipids are exposed to
water, they arrange themselves into a two-layered sheet (a bilayer) with all of their tails
pointing toward the center of the sheet.

The center of this bilayer contains almost no water and excludes molecules like sugars
or salts that dissolve in water but not in oil. This is similar to the coalescing of oil
droplets in water and is driven by the same force, called the hydrophobic effect. The cell
membrane is selectively permeable and it is able to regulate what enters and exits the
cell, thus facilitating the transport of materials needed for survival.

The movement of substances across the membrane can be either “passive", occurring
without the input of cellular energy, or “active", requiring the cell to expend energy in
transporting it. The cell membrane thus works as a selective filter that allows only
certain things to come inside or go outside the cell. The cell employs a number of
transport mechanisms that involve biological membranes:

Transport of materials across the Cell Membrane

1. Passive Osmosis and Diffusion: Some substances (small molecules, ions) such as
Carbon dioxide (CO2) and Oxygen (O2), can move across the plasma membrane by
diffusion, which is a passive transport process. Because the membrane acts as a
barrier for certain molecules and ions, they can occur in different concentrations on
the two sides of the membrane. Such a concentration gradient across a
semi-permeable membrane sets up an osmotic flow for water and ions.
2. Transmembrane protein channels and transporters: Nutrients, such as
sugars or amino acids, must enter the cell, and certain products of metabolism must
leave the cell. Such molecules diffuse passively through protein channels such as
aquaporins (in the case of water (H2O)), in facilitated diffusion. Molecules can also be
pumped across the membrane by transmembrane protein transporters. Protein
channel, also called permeases, are usually quite specific, recognizing and
transporting only a limited food group of chemical substances, often even only a
single substance.

3. Endocytosis: Endocytosis is the process in which cells absorb molecules by

engulfing them. The plasma membrane creates a small invagination, in which the
substance to be transported is captured. The invagination then creates a vesicle
containing the captured substance. Endocytosis is a pathway for internalizing solid
particles "cell eating" (phagocytosis), small molecules and ions "cell drinking"
(pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of
active transport.

4. Exocytosis: Just as material can be brought into the cell by invagination and
formation of a vesicle, the membrane of a vesicle can be fused with the plasma
membrane, extruding its contents to the surrounding medium. This is the process of
exocytosis. Exocytosis occurs in various cells to remove undigested residues of
substances brought in by endocytosis, to secrete substances such as hormones and
enzymes, and to transport a substance completely across a cellular barrier. In the
process of exocytosis, the undigested waste-containing food vacuole or the secretory
vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior
of the cell to the surface. The vesicle membrane comes in contact with the plasma
membrane. The lipid molecules of the two bilayers rearrange themselves and the two
membranes are, thus, fused. A passage is formed in the fused membrane and
the vesicles discharges its contents outside the cell.

Growth and metabolism

Between successive cell divisions, cells grow through 2 main cellular metabolic
processes by which individual cells process nutrient molecules.

Metabolism consists two (2) main distinct processes:

i) Catabolism, in which complex molecules are broken down within the cell to produce
energy.

ii) Anabolism in which the cell uses energy and reducing power to synthesize complex
molecules. Complex sugars consumed by the organism can be broken down into a
less chemically complex sugar molecule called glucose. Once inside the cell, glucose is
broken down to make adenosine triphosphate (ATP), a form of energy. Cells are
capable of synthesizing new proteins, which are essential for the modulation and
maintenance of cellular activities. This process involves the formation of new protein
molecules from amino acid building blocks based on information encoded in
DNA/RNA.
Protein synthesis generally consists of two major steps:
Transcription and
Translation.
Within the nucleus (light blue), genetic information (DNA, dark blue) are transcribed
into RNA. This RNA is then subject to post-transcriptional modification and
control, resulting in a mature (nessenger) mRNA (red) that is then transported out of
the nucleus and into the cytoplasm (peach), where it undergoes translation into a
protein. mRNA is translated by ribosomes (purple) that match the three-base codons of
the mRNA to the three-base anti-codons of the appropriate tRNA.

Newly synthesized proteins (black) are often further modified, such as by binding to an
effector molecule (orange), to become fully active.

New proteins destined for the plasma membrane or points beyond are synthesized in
the rough endoplasmic reticulum (RER) and physically assembled in the Golgi
apparatus. Proteins intended for the cytosol are synthesized on free ribosomes.
Smooth endoplasmic reticulum (SER) may be abundant in certain cell types such as
gonads and liver where it is used for steroid hormone and lipoprotein synthesis, as well
as for the modification of hydrophobic compounds (for example, drugs) into
water-soluble molecules.

Types of Proteins

Antibodies - are specialized proteins involved in defending the body from antigens
(foreign invaders). They are utilized by the immune system to identify and defend
against offending (injurious) agents such as bacteria, viruses and other foreign
intruders.

Enzymes - are proteins that facilitate biochemical reactions. They are often referred to
as catalysts because they speed up chemical reactions. Examples include the enzymes
lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a
digestive enzyme that works in the stomach to break down proteins in food.

Hormonal Proteins - are messenger proteins which help to coordinate certain bodily
activities. Examples include insulin, oxytocin, and somatotropin.
Insulin regulates glucose metabolism by controlling the blood-sugar concentration.
Oxytocin stimulates contractions in females during childbirth.
Somatotropin is a growth hormone that stimulates protein production in muscle cells.

Structural Proteins - are fibrous and provide support. Examples include keratin,
collagen, and elastin. Keratins strengthen protective coverings such as hair, quills,
feathers, horns, and beaks. Collagens and elastin provide support for connective tissues
such as tendons and ligaments.
Storage Proteins - store amino acids. Examples include ovalbumin and casein.
Ovalbumin is found in egg whites and casein is a milk-based protein.

Transport Proteins - are carrier proteins which move molecules from one place to
another around the body. Examples include haemoglobin, cytochromes and transferrin.
Haemoglobin transports oxygen through the blood.
Cytochromes operate in the electron transport chain as electron carrier proteins.
Transferrin is an iron transporting protein

Cellular Metabolism and Mitochondrial Function

Mitochondrion (pl mitochondria) Gr mitos – thread chondrion – granules/grains

The origin of the mitochondrion (pl mitochondria) traced to ancestral prokaryotes that
were engulfed by primitive eukaryotes about 1.5 billion years ago. Their origin explains
why mitochondria contain their own DNA (circularized, about 1% of the total cellular
DNA), encoding roughly 1% of the total cellular proteins and approximately 20% of the
proteins involved in oxidative phosphorylation.

Mitochondria carry out all the steps of DNA replication, transcription, and translation.
The ovum contributes the vast majority of cytoplasmic organelles to the fertilized zygote,
hence mitochondrial DNA is virtually entirely maternally inherited.
Mitochondrial Anatomy
1. Outer mitochondrial membrane,
2. Intermembrane space (the space between the outer and inner membranes),
3. Inner mitochondrial membrane,
4. Cristae space (formed by infoldings of the inner membrane), and
5. Matrix (space within the inner membrane).

Mitochondrial Functions
1. Energy Generation
2. Intermediate metabolism – High celluar and DNA turnout in mitotically active cells
3. Mediates Cell Death via necrosis and apoptosis
4. Protein synthesis – Most especially those involved in oxydative phosphorylation
 Antonie van Leeuwenhoek (October 24, 1632 – August 26, 1723) (aged 90))
Father of Microbiology

Robert Hooke’s Microscope

Some of Leeuwenhoek's main discoveries are:

Bacteria, (e.g., large Selenomonads from the human mouth), in 1676

The vacuole of the cell.
The spermatozoa in 1677.
The banded pattern of muscular fibers, in 1682.