CELL PHYSIOLOGY

A cell
This is the fundamental and functional unit of life.
Functions of a cell
The main functions of the cell include
1. Basic unit of life. The cell is the smallest part to which an organism can be
reduced that still retains the characteristics of life.
2. Protection and support. Cells produce and secrete various molecules that
provide protection and support of the body. For example, bone cells are
surrounded by a mineralized material , making bone a hard tissue that protects
the brain and other organs and that supports the weight of the body.
3. Movement. All the movements of the body occur because of molecules
located within specific cells such as muscle cells
4. Communication. Cells produce and receive chemical and electrical signals that
allow them to communicate with one another. For example, nerve cells
communicate with one another and with muscle cells, causing them to contract
5. Cell metabolism and energy release. The chemical reactions that occur within
cells are referred to collectively as cell metabolism. Energy released during
metabolism is used for cell activities, such as the synthesis of new
molecules,muscle contraction, and heat production, which helps maintain body
temperature.
6. Inheritance. Each cell contains a copy of the genetic information of the
individual. Specialized cells are responsible for transmitting that genetic
information to the next generation
NB: The cell of a living organism
The cell can be defined as the basic unit of structure and function in a living
organism.
This generalisation is known as the cell theory and it embraces four ideas;
A. The cell is the building block of structures in living cells
B. The cell is derived from other cells by cell division
C. The cell contains hereditary information that is passed from parent cell to
daughter cell
D. The cell is the functioning unit of life i.e. the chemical reactions of life takes
place within cells The cell theory states that “a cell is the fundamental and
functional unit of life” i.e. the cell is the basic unit of the structure and function in
living organisms.

Factors that limit cell size

1. Surface area to volume ratio Small cells have large surface area:
Volume ratio (SA: V ratio) while large cells have a small SA: V ratio. A large SA: V
ratio enables fast rate of diffusion while a small SA: V ratio slows the rate of
diffusion. Small cells have low metabolic demands and form low amount of
wastes while large cells have higher metabolic demands and form much amount
of wastes. Therefore, the large SA: V ratio in small cells enables adequate supply
of oxygen and nutrients and expulsion of wastes e.g. carbon dioxide via the
surface of the cell by simple diffusion while the small SA: V ratio in large cells
limits diffusion hence the supply of nutrients by simple diffusion is inadequate to
meet the metabolic demands of the cell.
2. Nucleocytoplasmic ratio
DNA in the nucleus provides instructions for protein synthesis hence controls
activities of the whole cell. Each nucleus can only control a certain volume of
cytoplasm. Specialization forms some long / large cells, therefore to overcome
this limitation such cells are modified to become multinucleate / coenocyte e.g.
skeletal muscle cells and fungal hyphae.
3. Fragility of cell membrane
As cell size increases, the risk of damage to the cell membrane also increases. This
limits the maximum size of cells, especially animal cells. Hence;
(i) In animals, some large sized cells take in substances in bulk by endocytosis
and expel bulk substances by exocytosis to supplement on simple diffusion.
(ii) (ii) Some animal cells increase their surface area by forming many tiny
projections called microvilli.
(iii) (iii) Some cells divide when they reach a certain size to maintain suitable
SA: V ratio. Note: SA: V ratio particularly limits the size of bacterial cells,
i.e. prokaryotic cells which are incapable of endocytosis and exocytosis.
(iv) 4. Mechanical structures that hold the cell together Cells with tough cell
walls e.g. plant cells are larger than cells with only the fragile cell
membrane e.g. animal cells because the tough walls provide support and
maintain cell shape. Cells with complex internal cytoskeleton are larger
than cells with little cytoskeleton because the cytoskeleton protects and
supports the cell structure and maintains cell shape
TYPES OF CELLS
There are two fundamentally different types of cells, the prokaryote cell and
eukaryote cell.
A. Prokaryote cell (Pro, before; karyon, nucleus) Characteristics of prokaryotic
cells
- These are cells that do not have a true nucleus.
- They have no membrane bound organelles. An organelle can be defined as a
membrane-enclosed structure with specialised functions, suspended in the
cytosol of eukaryotic cells.
- Their nuclear material lies in a free region known as a nucleoid e.g. in bacteria.
They were probably the first organisms on earth
- The cell has no distinct nucleus. The nucleoplasm appears scattered in the
cytoplasm or the nuclear materials e.g. DNA.
- The cell lacks a nuclear membrane
- Each cell has got very few cell organelles (cell parts) e.g. they do lack the
chloroplasts and mitochondria.
- The cell has a single circular chromosome in the form of a ring, of
Deoxyribonucleic Acid (DNA) in the cytoplasm, not contained in a nuclear
membrane
 - They are extremely small, ranging in size between 1-10milimetres in diameter -
Duplication of the chromosomes occurs but not on the spindle i.e. their cells are
capable of multiplication
- The cell has got a unique cell wall containing a polysaccharide Examples include
bacteria and cyanobacteria i.e. first organisms on earth.
B. Eukaryotic cell (Eu, true; Karyon Nucleus)
These are cells with a true nucleus. Their nuclear materials are found inside the
nucleus surrounded by two membranes. They probably evolved about 1000
million years ago, 2 million years after the prokaryotes. There are 2 main types of
eukaryotic cells; the plant cell and the animal cell. Cells as seen with the light
microscope A light microscope is a microscope that uses light as a source of
radiation. Under the microscope, cells are described as a small unit of living
protoplasm and always surrounded by cell surface membrane and sometimes as
in plants, surrounded by a non-living cell wall made of cellulose. The most
conspicuous structure is the nucleus which contains a deeply staining material
know as chromatin. When loose it is referred to as chromosome. Chromosomes
appear as thread like structures just before nuclear division. The living material
between the nucleus and the cell surface is known as the cytoplasm which
contains a variety of organelles.
Description of a generalised structure of a eukaryotic cell.
A cell is a small unit of living protoplasm, always surrounded by a cell surface
membrane and sometimes by a non-living cell wall (as in plants and fungi). The
most conspicuous structure of the cell is the nucleus which contains chromatin.
Chromatin is the looselycoiled form of chromosomes. Chromosomes contain
genetic material in the form of DNA. The nucleus is separated from the cytoplasm
by its nuclear membrane The cytoplasm contains organelles.
Comparison between prokaryotic and eukaryotic cells
Similarities Differences

 The protoplasm Prokaryotes Eukaryotes

is surrounded
by a membrane The nuclear material is not enclosed The nuclear material is enclosed by a
that is by a nuclear membrane nuclear membrane
selectively
Genetic material is circular double Most DNA is linear and associated with
permeable
strand of DNA histone proteins to form chromosomes
 The binding
protein is made No membrane bound organelles Has membrane bound organelles
up of lipid –
protein No mitosis or meiosis Mitosis , meiosis or both occur
complex
No spindle formation There’s spindle formation
 The cells have
got ectoplasm Ribosomes are smaller They are bigger
and nuclear
regions Rigid cell wall containing murein Cell walls of plants and algae contain
(peptoglycan) cellulose, fungi contain chitin and
animal cells have no cell walls

No mitochondria (mesosomes in Mitochondria present and refered to

bacteria, and plasma membrane of as power house of the cell
cyano bacteria contain respiratory
enzymes)

Advantages of having membrane bound organelles

1. Potentially harmful reactions (enzymes) can be isolated inside an organelle so
that they do not harm the rest of the cell
2. The rate of any metabolic reactions inside an organelle can be controlled by
regulating the rate at which the membrane allows the first reaction to occur or to
enter
3. The containment of enzymes for a particular metabolic pathway within the
organelle means that the products of the reaction will always be in close
proximity to the next enzymes within the sequence. This increases the rate of
metabolic reactions
4. Many metabolic processes which involve enzymes occur in the membrane.
Structure of the cell

Functions of cellular organelles of the eukaryotic cell

Cellular organelle Function

Plasma Membrane Transport of molecules in and out of cell, receptors for hormones and
neurotransmitters

Lysosome Intracellular digestion of macromolecules and and hydrolysis of nucleic acid ,

protein , glycosaminoglycans , glycolipids, sphingolipids

Golgi apparatus Post transcriptional modification and sorting of proteins and export of proteins

RER Biosynthesis of protein and secretion

SER Biosynthesis of steroid hormones and phospholipidids , metabolism of foreign

compounds

Nucleus Storage of DNA, replication and repair of DNA , transcription and post
transcriptional processing

Peroxisomes Metabolism of hydrogen peroxide and oxidation of long chain fatty acids

Nucleolus Synthesis of rRNA and formation of ribosomes

Mitochondrion ATP synthesis , site for tricarboxylic acid cycle, fatty acid oxidation ,
oxidative phosphorylation, part of the urea cycle and part of heme
synthesis

Cytosol Site for glycolysis , pentose phosphate pathway, part of gluconeogenesis, urea
cycle , and heme synthesis , purine and pyrimidine nucleotide synthesis