Health Professionals Foundation Programme

Texila American University

College of Medicine

Biology-I FB1100

Module 2 of Bio. I

CELL AND ITS STRUCTURE

A: Cell and its Structure

B: The Nucleus and Ribosomes
C: Membranous Organelles
D: The cytoskeleton and Others
E: Cell Membrane and Traffic across membranes
LEARNING OBJECTIVES
At the end of this Module students will be able to;

• understand the structures and purposes of basic components of

prokaryotic and eukaryotic cells (plant and animal cells), especially
macromolecules, membranes, and organelles
• understand how cellular components are used to generate and utilize
energy in cells
• Compare and contrast prokaryotic and eukaryotic cells. List their
similarities and differences
• Compare and contrast plant and animal cells. List their similarities and
differences
• Review the structure of membranes and explain the importance of
different proteins for creating intracellular membranous compartments
where different cellular functions can take place
LEARNING OBJECTIVES (Cont.)
• Learn the structure and functions of each of the following, and where
applicable relate to specific known genetic diseases;
a. Plasma membrane
b. Cytoskeleton
c. Nucleus
d. Endoplasmic reticulum (smooth and rough)
e. Golgi apparatus & vescilces
f. Lysosomes
g. Peroxisomes
h. Mitochondria

• Appreciate the structure and functions of the biological membranes

including but not limited to transport, attachment, recognition,
enzyme activities and relate those functions to specific diseases
Life’s Hierarchical Order
A: Cell and its Structure
• A cell is a unit of life and organisms are either unicellular or
multicellular. All life is composed of cells and all cells come from other
cell – cell theory.
• Cell sizes and shapes vary with functions i.e. bird egg, long muscle
cells and lengthy nerve cells, transmitting impulses rapidly between
distant parts of body.
Cells are divided into two groups;
• – prokaryotic and
• - eukaryotic cells
• Prokaryotes: (Greek-pro-before, and karyon-kernel) lacks nucleus, its
DNA is coiled into nucleoid coz no membrane surrounds the nucleoid
region, DNA is in direct contact with the rest of the cell contents, e.g.
bacteria; the cytoplasm
• The plasma membrane encloses the cytoplasm, surrounding the
plasmamembrane is the bacteria cell wall, may have capsule, pili and
flagella
 Domains of Life

Domains of Life

Three broad categories of organisms

• Archaea –Unicellular prokaryotes

• Bacteria –Unicellular prokaryotes

• Eukarya –Includes both unicellular and multicellular eukaryotes

A. Cell and its Structure Con..
A. Cell and its Structure Con..
A. Cell and its Structure Con..
A. Cell and its Structure Con..

Eukaryotes:
(Greek eu, true and karyon kernel) referring to the nucleus, all the plants,
animals, protists or fungi are fundamentally similar but profoundly
different from prokaryotes – Monera - bacteria
Taking a look at plant and animal cells as representatives of the eukaryotes.
In eukaryotic cells, membranes partition the cytoplasm into a maze of
compartments called membranous organelles and these are;
• Nucleus
• Endoplasmic reticulum
• Golgi apparatus
• Mitochondria
• Lysosomes and microbodies
• Many chemical activities, collectively called cellular metabolism occur
in the fluid filled spaces within membranous organelles, where specific
reactions occur.
A. Cell and its Structure Con..

The conditions are maintained and are at variance with others i.e.
While Endoplasmic Reticulum (ER) is making a hormone a microbody may
be making H2O2.

Compartmentalized functions of animal cells

1. A lysosome is a membranous sac of hydrolytic enzymes so the
membrane keeps its content safe from hydrolyzing the cell itself.
2. Another benefit of the internal membranes is that they greatly increase
surface area.
3. Manufacturing, processing, and shipping of molecules to appropriate
destination: DNA, proteins, carbohydrates, lipids
4. Breakdown of molecules: large structures, lipids, detoxification of
certain substances
5. Energy processing: forming cellular energy form
6. Structural support, movement, and communication
A. Cell and its Structure Con..

All the organelles above are present in both plant and animal cells. However
there are differences, like centrioles are absent in plants and animal cells
have one or more flagella, only gametes in few plant species.

Plant cell has a rigid cell wall, which is absent in an animal cell
• Plant cell walls contain the polysaccharide called cellulose
• Plant cells have a polygonal shape as opposed to the oval in animals
• Chloroplast and large central vacuole are absent in animal cell
• Eukaryotic cells also contain nonmembranous organelles these are;

1. Centrioles
2. Flagella }All contain protein tubes called microtubules
3. Cytoskeleton
4. Ribosomes
A. Cell and its Structure Con..
A. Cell and its Structure Con..
B: The Nucleus and Ribosome

• The nucleus is the cell’s genetic control centre of all eukaryotic cells.
• Its DNA is the hereditary blueprint and it directs the cell activities as does
DNA of prokaryotes nucleoid region.
• Most DNA is attached to proteins forming very long fibres called
chromatin.
• During cell’s reproduction, chromatin coils into chromosomes, thick
enough to be seen by light microscope enclosing the nucleus is a nuclear
envelope double membrane perforated with pores.
• Adjoining the chromatin is a mass of fibres and granules called nucleolus,
a combination of DNA, RNA and proteins where ribosomes are made and
assembly of ribosomal subunits for the synthesis of polypeptide (proteins)
• Transported through the pores to many areas where protein synthesis is
required.
B: The Nucleus and Ribosome (Cont.)
B: The Nucleus and Ribosome (Cont.)
C: Membranous Organelles (Cont.)

• Endoplasmic Reticulum ER (Greek word; network within cell) the

organelles formed of interrelated membranes.
• Some are physically connected some are not, but collectively are called
endomembrane system.
• Many work together in the synthesis, storage and export of important
molecules.
There are two ER;
1. The rough
2. The smooth
which differ in structure & function
• Rough ER;
• This refers to its appearance with many ribosomes doted around
Rough ER makes;
• 1. Membranes Proteins
C: Membranous Organelles (Cont.)

Some of these are inserted into ER membrane, enlarging it and some of this
membrane ends up in other organelles.
The proteins made are secreted by the cell e.g. (antibody) Ig

Smooth ER;
Has many functions, it is continuous with rough ER is a network of
Interconnected tubules that lack ribosomes. Enzymes embedded in its
membrane gives it one of the most important functions of smooth ER;
Synthesis of:
1. lipids
2. fats
3. phospholipids
4. steroids
C: Membranous Organelles (Cont.)
C: Membranous Organelles (Cont.)
Golgi apparatus
Golgi apparatus (Cont.)
Lysosomes
Lysosomes
Endomembrane System
The Chloroplast
The Chloroplast (Cont.)
Mitochondria

The mitochondria harvests chemical energy from food substances.

• Much less complex than the chloroplast converting chemical energy to
another chemical energy as opposed to converting solar E to chemical E.
• Carry out the process of cellular respiration, in which chemical E of foods is
converted into the chemical E of a cellular fuel molecule ATP.
• Has 2 compartments; intermembrane fluid filled and the fluid filled
• mitochondria matrix.
• Enclosed by the inner membrane, these contain different sets of enzymes.
• Enzymes molecules that make ATP are embedded in inner membrane
Mitochondria (Cont.)
C. The Cytoskeleton and Others

• In cells it provides structural support

• It contains a supportive meshwork of fine fibres collectively called
Cytoskeleton. This extend throughout the cytoplasm.

Contains 3 main types of fibres:

1. The microfilament, thinnest, solid helical rods consists of globular actin

2. The intermediate filament fibrous rope like protein – non globular

3. The microtubule, straight hollow tubes – globular proteins - tubulins

C. The Cytoskeleton and Others (Cont.)
Cilia and Flagella

• Cilia and flagella move when microtubules bend

• These are locomotor appendages that protrude from certain cells

• Both have similar structures and mechanisms of movement

• However cilia is shorter than the flagella and more in number than

• flagellum.

• Composed of core of microtubules wrapped in extensions of plasma

• membrane

• They form a 9 + 2 pattern, but is different at the base – no wrapping

• Here the nine doublets extend into anchoring structure; –Calles a basal
body
Cilia and Flagella (Cont.)
D. Cell Membrane and Traffic Across Membranes

Cell plasma membrane

• Cell membrane is a fluid mosaic of phospholipids and proteins

• Mosaic denotes a surface made of small fragments, like pieces of coloured

tile cemented together in a mosaic floor. both in and outside the cell

• The cell plasma membrane has diverse protein molecules embedded in a

framework of phospholipids

• Its fluid in the sense that most of the individual proteins and phospholipid
molecules, can drift laterally in the membrane even though phospholipid
bilayer remains intact
D. Cell Membrane and Traffic Across Membranes (Cont.)
Membrane Proteins
Membrane Proteins (Cont.)
Cell Membrane
Traffic across membranes

There are two types of traffic across plasma membranes;

1. Passive transport:

This transport is where the cell does not do work. Meaning energy is not
expended in order to cross the membrane. E.g.;

1. diffusion; particles from high conc. To where they are less conc. through a
p/memb.

2. Osmosis; is a special diffusion of water molecules from hypotonic soln to

hypertonic soln.

3. Facilitated diffusion; this is the movement of substances down its conc.

Gradient by the aid of proteins without the use of energy.
Traffic across membranes (Cont.)
Traffic across membranes (Cont.)
Traffic across membranes (Cont.)
Active transport

• This is transport through the plasma membrane where cells expend energy.
• This is where a cell expend energy in order to move molecules across
plasma membrane.
• It begins by solute inside cell binding to transport protein and ATP
transfers phosphate to protein, protein releases the solute outside cell,
second solute binds to protein, phosphate detaches from protein, protein
releases second solute into the cell.
These are;
1. exocytosis and endocytosis of larger molecules.
2. phagocytosis (cell eating) e.g. amoeba.
3. pinocytosis (cell drinking) process of taking in droplets of fluid from the
surroundings into a vesicle.
4. receptor mediated endocytosis is a very highly specific process.
The pit on the cell is lined with receptor proteins. Which have picked up
particular molecules from the surroundings. The pit will pinch closed to form
a vesicle
Receptor mediated endocytosis
 Youtube

Biology: Cell Structure I Nucleus Medical Media

https://www.youtube.com/watch?v=URUJD5NEXC8