cell biology

Cell biology

Introduction
Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen. The organization of these molecules into cells is one feature that distinguishes living things from all other matter. The cell is the basic unit of life.

Every living thing - from the tiniest bacterium to the largest whale - is made of one or more cells.
Before the C17th, no one knew that cells existed, since they are too small to be seen with the naked eye. The invention of the microscope enabled Robert Hooke, (1665) and Anton van Leuwenhoek (1675) to see and draw the first ‘cells’, a word coined by Hooke to describe the cells in a thin slice of cork.
The idea that all living things are made of cells was put forward in about 1840 and in 1855 came ‘Cell Theory’ – i.e. ‘cells only come from other cells’ – contradicting the earlier theory of ‘Spontaneous Generation’

Scientific theories

Cells : all organisms are composed of cells, and new cells only come from pre-existing cells.
Homeostasis: the internal environment of an organism stays relatively constant.
Genes: organisms contain coded information that dictates their form , function and behavior.
Ecosystem: populations of organisms interact with each other and the physical environment.
Evolution: all organisms have a common ancestor, but each is adapted to a particular way of life.

Cell Theory consists of three principles:

There is no spontaneous generation, but cells up rising from preexisting cells.
All living things are composed of one or more cells.
Cells are the basic units of structure and function in an organism.


Matter
Refers to anything that takes up space and has mass. It is presence as solid , liquid or gas.
Elements
An element is one of the basic building blocks of matter among about 92 naturally occurring elements, the human body compose of 4 elements only ( oxygen, carbon, hydrogen, nitrogen).
Atom
Is the smallest unit of an element that still retains the chemical and physical properties of the element. The atom composed of nucleus ( protons and neutrons)and outer shell ( electrons).

Molecules

Combination among the atoms to form a molecule, the union between atoms can occur as ionic bond ( sodium chloride) or covalent bond ( calcium chloride).

Carbohydrate (CHO)

Are characterized by the presence of ( H-C-OH), its function for quick and short-term energy storage in all organisms. It is presence as:
Simple CHO
Monosaccharide : composed of 5-7 carbon atoms such as glucose (our body), fructose (fruit) , galactose (milk).
Disaccharide : composed of 2 monosaccharides such as maltose, sucrose.
Compound CHO which are a macromolecules (polysaccharides) such as starch, glycogen.

Lipid

The most familiar lipids are fats and oil, and others:
fats usually of animal origin (butter) which are solid at room temperature, it is characterized by saturated fatty acids.
Oil usually of plant origin (corn oil, soybean oil) which are liquid at room temperature, it is characterized by unsaturated fatty acids.
Trans fat: which is creative artificially from vegetable oils by hydrogenation to make them semisolid.
Phospholipid : it seem like fat except in the 3rd fatty acid there is a phosphate group, it is the primary components of cellular membranes.
Steroids: which have a backbone of four fused carbon rings attach with a functional group. Cholesterol is the precursor of the steroids like sex hormones like testosterone and estrogen.


Proteins
Are the primary importance in the structure and function of cells, such as:
Support (keratin).
Enzymes.
Transport (plasma membrane channels, hemoglobin).
Defense (Abs).
Hormones (insulin).
Motion (actin and myosin).

Cell size

1. A few types of cells are large enough to be seen by the unaided eye. The human egg (ovum) is the largest cell in the body, and can (just) be seen without the aid of a microscope.
2. Most cells are small for two main reasons:
The cell’s nucleus can only control a certain volume of active cytoplasm.
Cells are limited in size by their surface area to volume ratio. A group of small cells has a relatively larger surface area than a single large cell of the same volume. This is important because the nutrients, oxygen, and other materials a cell requires must enter through it surface. As a cell grows larger at some point its surface area becomes too small to allow these materials to enter the cell quickly enough to meet the cell's need.

Cell shape

Cells come in a variety of shapes – depending on their function:-
The neurons from your toes to your head are long and thin; Blood cells are rounded disks, so that they can flow smoothly.

Types of cells

Prokaryotes and Eukaryotes
Organisms whose cells normally contain a nucleus and membrane –bounded organelles are called Eukaryotes.
Organisms whose cells lack a nucleus and have no membrane-bound organelles are known as Prokaryotes.


Prokaryotes
Eukaryotes
Typical organisms
Bacteria
Protista, fungi, plants, animals
Typical size
~ 1-10 μm
~ 10-100 μm (sperm cells) apart from the tail, are smaller)
Type of nucleus
Nuclear body
No nucleus
real nucleus with nuclear envelope
DNA
circular
linear molecules (chromosomes) with histone proteins
Ribosomes
70S
80S
Cytoplasmic structure
very few structures
highly structured by membranes and a cytoskeleton
Cell movement
Flagella/cilia made of flagellin
Flagella and cilia made of tubulin
Mitochondria
None
1 - 100 (though RBC’s have none)
Chloroplasts
None
in algae and plants
Organization
usually single cells
single cells, colonies, higher multicellular organisms with specialized cells
Cell division
Binary fission
(simple division)
Mitosis (normal cell replication) Meiosis (gamete production)


Parts of eukaryotic cells
The structures that make up a Eukaryotic cell are determined by the specific functions carried out by the cell. Thus, there is no typical Eukaryotic cell. Nevertheless, Eukaryotic cells generally have three main components: A cell membrane, a nucleus, and a variety of other organelles.

Cell wall

It found only in plant cell , but it absent in human or animal cells. It helps in protecting the plasma membrane and plays a vital role in supporting and protecting the cells. It is a thick outer layer made of cellulose.

Cell membrane

The cell membrane is a complex barrier separating every cell from its external environment. the term fluid mosaic model used for description of membrane structure.
The cellular membrane is a phospholipid bilayer with attached or embedded proteins. Cellular membrane have a polar hydrophilic head and non-polar hydrophobic tail.
Cellular membrane have "Selectively Permeable" membrane for regulates proteins crossing and allows certain molecules and ions to enter and exit the cytoplasm freely.
Cellular membrane functions:
Diffusion: is the random movement of certain molecules from an area of higher concentration to an area of lower concentration until they are equally distributed.
Osmosis: in the net movement of water across a semi-permeable membrane from an area of higher concentration to an area of lower concentration. Tonicity osmosis is the effect of solution on a cells (crenation), it divided into:
Isotonic solution (the cells remain unchanged).
Hypotonic solution ( the cells gain water and burst).
Hypertonic solution ( the cells loss of water and shrink).
These condition are useful in our body to water absorption control.

Passive transport (Facilitated transport) : in which certain substance transport into the cell occurs by protein carriers WITHOUT exhaustion of cell energy. So as to equal distribution of such molecule between inside and outside the cell , e.g. Glucose molecules.
Active transport: is the transport of a certain molecules from lower to high concentration by protein carriers WITH exhaustion of cell energy (breakdown of ATP), e.g. Iodine ions in the thyroid gland ,sugar absorption from digestive tract and Na+ - K pump in nerve and muscle cells.
Endocytosis: for large substances moves into the inside the cell through plasma membrane invagination to envelope a substance and fluid then forming of endocytic vesicle inside the cell. Such condition can occur during phagocytrosis.
Exocytosis: a vesicle fuse with plasma membrane as secretion occurs. Such condition can occur with signaling molecules eg. Neurotransmitters.
Cytoplasm (cytosol)


Is the jelly-like mixture in which the other organelles are suspended, so cytosol + organelles = cytoplasm. Organelles carry out specific functions within the cell. In Eukaryotic cells, most organelles are surrounded by a membrane, but in Prokaryotic cells there are no membrane-bound organelles.

Nucleus

The nucleus is normally the largest organelle within a Eukaryotic cell.
The nucleus contains chromatin which is a combination of DNA molecules and proteins (histones) that make up the chromosomes (human, 46, fruit fly 6, fern 1260) . These proteins coil up (dehydrate) at the start of nuclear division, when the chromosomes first become visible.
Whilst most cells have a single nucleus some cells (macrophages) have more than one and fungi have many nuclei in their cytoplasm.
The nucleus is surrounded by a double membrane called the nuclear envelope, which has many nuclear pores through which mRNA, and proteins can pass.
Most nuclei contain at least one nucleolus. The nucleoli are where ribosome are synthesized. When a nucleus prepares to divide, the nucleolus disappears.

Ribosome

Unlike most other organelles, ribosome are not surrounded by a membrane.
Ribosome are the site of protein synthesis in a cell.
Some are free in the cytoplasm (Prokaryotes); others attach to the membranes of rough endoplasmic reticulum (rough ER) so it hence its name.
They exist in two sizes:
70s are found in all Prokaryotes, chloroplasts and mitochondria, suggesting that they have evolved from ancestral Prokaryotic organisms.
80s found in all eukaryotic cells – attached to the rough ER (they are rather larger).

Endoplasmic reticulum

The ER is a system of membranous tubules and sacs.
The primary function of the ER is to act as an internal transport system, allowing molecules to move from one part of the cell to another.
The quantity of ER inside a cell fluctuates, depending on the cell's activity. Cells with a lot include secretary cells and liver cells.
The rough ER is studded with ribosome and is the site of protein synthesis. It is an extension of the outer membrane of the nuclear envelope, so allowing mRNA to be transported swiftly to the ribosome, where they are translated in protein synthesis.
The smooth ER is where polypeptides are converted into functional proteins and where proteins are prepared for secretion. It is also the site of phospholipids of cell membrane and steroid synthesis, and is associated with the Golgi apparatus. Smooth ER has no ribosome and is also involved in the regulation of calcium levels in muscle cells, and the breakdown of toxins by liver cells.

Golgi apparatus

1. The Golgi apparatus is the site of processing, packaging and secreting of the cell, so it is much more common in glandular cells.
2. The Golgi apparatus is a system of membranes, made of flattened sac-like structures called cisternae.
3. It works closely with the smooth ER, to modify proteins for export by the cell.

Mitochondria

Mitochondria are found scattered throughout the cytosol, and are relatively large organelles.
Mitochondria are the sites of aerobic respiration, in which energy from organic compounds is transferred to ATP. For this reason they are sometimes referred to as the ‘powerhouse’ of the cell.
ATP is the molecule that most cells use as their main energy.
Mitochondria are more numerous in cells that have a high energy requirement - our muscle cells contain a large number of mitochondria, as do liver, heart and sperm cells.
Mitochondria are surrounded by two membranes:
The smooth outer membrane serves as a boundary between the mitochondria and the cytosol.
The inner membrane has many long folds, known as cristae, which greatly increase the surface area of the inner membrane, providing more space for ATP synthesis to occur.
Mitochondria have their own DNA, and new mitochondria arise only when existing ones grow and divide. They are thus semi-autonomous organelles.


Lysosomes (garbage collectors)
Lysosomes are small spherical sac, contain hydrolytic enzymes within a single membrane. WBCs is the most common site of Lysosomes.
Lysosomes are the site of protein digestion – thus allowing enzymes to be re-cycled when they are no longer required. They are also the site of food digestion in the cell, and of bacterial digestion in phagocytes.
Lysosomes are formed from pieces of the Golgi apparatus that break off.

Cytoskeleton

Just as your body depends on your skeleton to maintain its shape and size, so a cell needs structures to maintain its shape and size.
In animal cells, which have no cell wall, an internal framework called the cytoskeleton maintains the shape of the cell, and helps the cell to move.
They seen by electron microscope.
The cytoskeleton consists of two structures:
microfilaments (contractile). They are made of actin, and are common in motile cells.
microtubules (rigid, hollow tubes – made of tubulin) which have the following functions:
To maintain the shape of the cell.
To serve as tracks for organelles to move along within the cell.
They form the spindle fiber to assist of chromosomes movement .
Intermediate filaments: it is intermediate in size between microfilaments and microtubules.

Cilia and flagella

Cilia and Flagella are structures that project from the cell, where they assist in movement.
Cilia (sing. cilium) are hair-like, short structure.
Flagella (sing. flagellum) are whip-like, longer structure.
Cilia found in our body , such as respiratory tract for lung cleaning from debris, the loss of theses cilia will lead to severe respiratory infection., cilia move the egg along the oviduct where it will be fertilized by flagellated sperms.
Flagella found in sperms, and certain bacteria cell.