Paper 6 - General Science and Technology

Life Science

Structure and Function of animal cell and cell organelles 

The Characteristics of Life:
Courses in Human Anatomy and Physiology concentrate on the human species so the table below includes comments on each of the characteristics as applied to humans:

1. Eating: 
   
2. Movement: Examples include movement of limbs, facial tissues, materials through the digestive system, and so on.
3. Reproduction: This is the means by which the species as a whole continues beyond the life-span of any single individual. The human reproduction process involves the exchange of genetic information and material.
4. Breathing: Breathing involves the intake of air from outside the body, absorption of oxygen gas (O2) for use within the body, and the release of the waste product carbon dioxide gas (CO2) from the body.
5. Growing: The term "growing" refers to the increase in size of a person during his/her childhood, typically until the age of about 18 years. It can be affected by a lifestyle factors - such as a balanced diet, by genetic factors - e.g. coming from a relatively "short" or "tall" family, and also by certain hormonal conditions that affect growth/development.
6. Waste (excretion): This term refers to removal from the body of waste products that may have been formed by metabolic processes. Examples include:
   
   
   Breathing out (of the body) carbon dioxide (C2O) and water (H2O), 
   the waste products from aerobic respiration.
   Sweating, removal of waste products from the body via the skin.
   Urinating. (Urea is nitrogenous waste, i.e. it is from proteins.)
   Defecating. Strictly, materials that leave the body by the process of defecation are undigestable food materials - rather than waste products formed by the metabolic processes that occur in the body. Their colour is due to the undigestable food itself, and to the effects of any excess bile.
7. Secretion: Examples of substances secreted from the human body include:
   
   
   Hormones (somtimes described simply as "chemical messengers")
   Sebum (from sebaceous glands), e.g. ear wax.
8. Circulation: In human there are two circulation systems:
   
   
   Primary Circulation = Blood Circulation,
   and
   Secondary Circulation = The Lymphatic System.

What is a Eukaryotic cell?

Eukaryotic cells are the type of living cells that form the organisms of all of the life kingdoms except monera. Protista, fungi, plants and animals are all composed of eukaryotic cells.

What about living cells that are not eukaryotic ? 

Not all cells are eukaryotic. There are also prokaryotic cells 

(see prokaryotic cell structure), such as the cells of blue-green algae and many different types of bacteria.

Examples of Eukaryotic Cells

• Examples of eukaryotic cells include plant cells and animal cells, but not bacteria (which are prokaryotic cells).

Examples of specific types of plant, animal and other eukaryotic cells could also be given as examples of eukaryotic cells, e.g.

• muscle fibres, leucocytes and neuroglia which are types of animal cells, and
• parenchyma cells, collenchyma cells and sclerenchyma cells, which are types of plant cells.
• There are numerous specific examples of eukaryotic cells including over 200 different types of cells are identifiable in human tissue alone.

Features of a general Eukaryotic Cell

Eukaryotic cells are often described by comparision with prokaryotic cells. There are many differences between these types of cells, the simplest distinction being that as prokaryotic cells are more primative than eukaryotic cells, eukaryotic cells are generally larger and much more sophisticated than prokaryotic cells due to the presence of a complex series of membranes that divide a typical eukaryotic cell into compartments and also due to the many different types of specialized organelles present in most eukaryotic cells. These features result in most eukaryotic cells being complex structures that have many self-controlled systems, e.g. for generating energy, moving materials around, and even self-destruction in appropriate circumstances. 

The following short list of key features of eukaryotic cells can be used to describe eukaryotic cells without much reference to prokaryotic cells.

• Contain membranous and non-membranous organelles that have specific functions within the cell. For example, all eukaryotic cells have a nucleus(Mature red blood cells (erythrocytes) in mammals are anucleate, i.e. they have no nuclei, when they are mature. This, together with the lack of many other organelles, results in increased accomodation within red blood cells for the haemoglobin that carry oxygen to cells around the body via red blood cells inside blood vessels. However, unlike prokaryotic cells, those cells did have nuclei at earlier stages in their development.). Different eukaryotic cells contain different types and quantities of other organelles depending on the type and function of the particular cell e.g. many plant cells contain chloroplasts which are the sites of photosynthesis within those cells.
• Contain linear DNA molecules (As opposed to the circle of double-stranded DNA and sometimes short pieces of circular DNA called plasmids that are present in prokaryotic cells. (The DNA molecules in prokaryotic cells are not associated with proteins , hence prokaryotic cells don't have chromosomes.)that are larger than the (circular) DNA molecules in prokaryotic cells and, in association with proteins, form structures called chromosomes. 
• Cell division is usually by mitosis, although meiosis applies to some specific types of cells concerned with sexual reproduction.
• Endocytosis is possible for eukaryotic cells. This is a process by which small regions (sometimes called 'patches') of cell membrane leave the membrane that encloses the cell, forming instead much smaller membrane-bound vesicles that are able to deliver substances such as nutrients from the external environment to locations deep within the cell. This mechanism enables eukaryotic cells to have larger sizes than would otherwise be possible - i.e. if transport of nutrients into the cell were only possible via simple diffusion. 

Structure of Animal Cell :

What is cell?

The cell is the basic structural, functional, and biological unit of all known living organisms. Cells 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 cell biology.

Cell Structure

All animal cells are multicellular. They are eukaryotic cells.The structure of cells varies according to the type and purpose of the cell (for example, which functions it is performing and in which part of the body). All plant and animal cells contain organelles which are structures within the cell that are specialised for particular functions.

The following very simple diagram illustrates a single animal cell with simple representations of key organelles:

It is also important to know something about each of these organelles:

Organelle	Structure/Function
Cell Membrane	The cell membrane keeps the cell together by containing the organelles within it. Cell membranes are selectively-permeable, allowing materials to move both into and outside of the cell.
Centrosomes	The centrosomes contain the centrioles, which are responsible for cell-division.
Cytoplasm	Cytoplasm is a jelly-like substance that is sometimes described as "the cell-matrix". It holds the organelles in place within the cell.
Golgi Apparatus	The golgi apparatus of a cell is usually connected to an endoplasmic reticulum (ER) because it stores and then transports the proteins produced in the ER.
Lysosomes	Lysosomes are tiny sacs filled with enzymes that enable the cell to utilize its nutrients. Lysosomes also destroy the cell after it has died, though there are some circumstances (diseases/conditions) in which lysosomes begin to 'break-down' living cells.
Microvilli	"Microvilli" is the pural form; "Microvillus" is the singular form. Microvilli are finger-like projections on the outer-surface of the cell. Not all cells have microvilli. Their function is to increase the surface area of the cell, which is the area through which diffusion of materials both into, and out of, the cell is possible.
Mitochondria	"Mitochondria" is a plural term; which is appropriate as these are not found alone. The quantity of mitochondria within cells varies with the type of cell. These are the energy producers within the cell. They generate energy in the form of Adenosine Tri-Phosphate (ATP). Generally, the more energy a cell needs, the more mitochondria it contains.
Nuclear Membrane	The nuclear membrane separates the nucleus and the nucleolus from the rest of the contents of the cell.
Nuclear Pore	Nuclear pores permit substances (such as nutrients, waste, and cellular information) to pass both into, and out of, the nucleus.
Nucleolus	The nucleolus is responsible for the cell organelles (e.g. lysosomes, ribosomes, etc.).
Nucleus	The nucleus is the "Control Center" of the cell, which contains DNA (genetic information) in the form of genes, and also information for the formation of proteins. Information is carried on chromosomes, which are a form of DNA.
Ribosomes	Ribosomes interpret cellular information from the nucleus and so synthesize appropriate proteins, as required.
Rough Endoplasmic Reticulum (RER)	"Rough" indicates that there are ribosomes attached to the surfaces of the endoplasmic reticulum. The endoplasmic reticulum is where proteins and lipids are produced within the cell, and is also concerned with the transport of these materials within the cell.
Smooth Endoplasmic Reticulum (SER)	"Smooth" indicates that there are no ribosomes attached to the surfaces of the endoplasmic reticulum.The endoplasmic reticulum is where proteins and lipids are produced within the cell, and is also concerned with the transport of these materials within the cell.

Cell Membrane:
This is illustrates a cross-section of the phospholipid bilayer that forms the membrane around the outside of all animal (including human) cells.

This plasma membrane consists mainly of phospholipids and proteins, most of the membrane proteins being glycoproteins.
The two components of the phospholipids are the heads (represented by black circles), and the fatty acid tails (that extend into the phospholipid bilayer).

Golgi Apparatus:

The Golgi Apparatus is also known as the Golgi complex, the Golgi body, or simply the Golgi.

Structure: 

It consists of a collection of vesicles and folded membranes. These are usually connected to an endoplasmic reticulum (ER) because the golgi apparatus stores and then transports the proteins produced in the ER.

Functions:

The Golgi apparatus modifies, sorts and packages macromolecules for delivery to other organelles or secretion from the cell via exocytosis.

Due to its functions of storing and transporting proteins, the Golgi apparatus may be creatively thought of as the "Post Office" of the cell. It is particularly well developed in cells that produce secretions.

Ribosomes:

Ribosomes are particles consisting of ribonucleic acid (RNA) and protein that interpret cellular information from the nucleus of the cell and synthesize appropriate proteins as required by the cell.

Ribosomes are either attached to the endoplasmic reticulum (specifically to the rough endoplasmic reticulum, RER) or free in the cytoplasm as polysomes.

Rough Endoplasmic Reticulum(RER):
An endoplasmic reticulum (ER) in a cell is a system of membranes which is the site of manufacture of proteins and lipids.


The "rough" component of the term "Rough Endoplasmic Reticulum" or "RER" indicates that there are ribosomes attached to the surfaces of the endoplasmic reticulum.

Smooth Endoplasmic Reticulum(SER):
An endoplasmic reticulum (ER) in a cell is a system of membranes which is the site of manufacture of proteins and lipids.


The "smooth" component of the term "Smooth Endoplasmic Reticulum" or "SER" indicates that there are no ribosomes attached to the surfaces of the endoplasmic reticulum - this is as opposed to "Rough Endoplasmic Reticulum (RER)" which does have ribosomes attached to it.

Lysosome:
Lysosome are membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers—proteins, nucleic acids, carbohydrates, and lipids. Lysosomes function as the digestive system of the cell, serving both to degrade material taken up from outside the cell and to digest obsolete components of the cell itself. For this function they are popularly referred to as "suicide bags" or "suicide sacs" of the cell. In their simplest form, lysosomes are visualized as dense spherical vacuoles, but they can display considerable variation in size and shape as a result of differences in the materials that have been taken up for digestion. Lysosomes thus represent morphologically diverse organelles defined by the common function of degrading intracellular material. Furthermore, lysosomes are responsible for cellular homeostasis for their involvements in secretion, plasma membrane repair, cell signalling and energy metabolism, which are related to health and diseases.

Mitochondria:
Mitochondria is the main energy source of the cell.
They are called the power house of the cell because energy(ATP) is created here.
Mitochondria consists of inner and outer membrane.
It is spherical or rod shaped organelle.

It is an organelle which is independent as it has its own hereditary material.

Cilia and Flagella:
Cilia and flagella are structurally identical structures.
They are different based on the function they perform and their length.
Cilia are short and are in large number per cell while flagella are longer and are fewer in number.
They are organelles of movement.

The flagellar motion is undulating and wave-like whereas the ciliary movement is power stroke and recovery stroke.

Vacuoles:
They are bound by single membrane and small organelles.
In many organisms vacuoles are storage organelles.

Vesicles are smaller vacuoles which function for transport in/out of the cell.

Nucleolus:
The nucleolus is a dense spherical structure within the nucleus of a cell. It contains ribonucleic acid (RNA) for the synthesis of ribosomes and also has an important role in the production of proteins and RNA.


The nucleolus is a part of the nucleus of the cell that disappears during cell division.

Nucleus:
The nucleus is the "Control Center" of each cell, which contains DNA (deoxyribonucleic acid, which is genetic information) in the form of genes, and also information for the formation of proteins.


DNA, which is combined with protein, is normally dispersed throughout the nucleus as chromatin. During cell division the chromatin becomes visible as chromosomes. The nucleus also contains ribonucleic acid (RNA), most of which is located in the nucleolus.