Friday, 2 November 2012

Biology Chapter 2 Form 4

2.1 Cell Structure And Function

Cellular Components of Animal and Plant Cells
Generally, a cell is made up of a plasma membrane
which contains protoplasm
. The protoplasm is made up of cytoplasm and a nucleus.
A cheek cell is an example of an animal cell. Animal cells do not have specific shapes. The cell is enclosed by an outer plasma membrane. The cell wall is absent in the animal cell. See figure human cheek cell below.
human cheek
Human cheek cell
An epidermal onion cell is an example of a plant cell. Plant cells have a specific shape as the cell wall is present. Apart from giving a specific shape, the cell walls also support and protect the plant cells. It is a rigid layer consisting of cellulose which is permeable to fluids. It acts as a pathway for the transport of nutrients across the plasma membrane. See figures onion epidermal cell below.

onion cell 1
Onion cell
In terms of the structure, all cells can be divided into three main parts or components: the plasma membrane, nucleus
and cytoplasm
. Figures below show the structures of the animal cell and plant cell as seen under a light microscope.
Under the light microscope, the plasma membrane, cytoplasm and nucleus of an animal cell can be seen. In the plant cell, however, the plasma membrane, cell wall
, cytoplasm, nucleus, vacuoles and chloroplast can be seen.
The cytoplasm in both animal and plant cells contains many organelles. Organelles are specialised structures each of which is surrounded by each own membrane and performs its own functions.
Organelles in the animal cells are the nucleus, rough and smooth endoplasmic reticulum, mitochondria, golgi apparatus, lysosomes, ribosomes, centrioles and vacuoles whereas organelles in plant cells are the nucleus, rough and smooth endoplasmic reticulum, mitochondria, golgi apparatus, lysosomes, robosomes, vacuoles and chloroplast. For more information about the organelles in animal cells and plant cells, click here.
The organelles are very tiny and can only been seen under the electron microscope.
The structure of the animal cell
The structure of the animal cell
The structure of the plant cell
The structure of the plant cell

Comparison between Animals and Plant Cell Structures
The comparisons between the structure of animal and plant cells are shown in table below. For extra information about the differences between animal and plant cell structure, click here.
Comparison between animals and plant cells

Density of Organelles in Specific Cells
The organelles are small organs of the cells that carry out specific functions. The density of the organelles refers to the total number of the organelles in a cell. Some cells have a larger number of certain organelles; thus they are said to have a high density of organelles. A high density of organelles is required to coordinate and carry out specialised functions.
For example, the cells in the meristems
of plants have a high density of organelle chloroplast so that these cells can absorb sunlight for photosynthesis to take place. Similarly, the cells of the brain, liver and kidney contain thousands of mitochondria. The function of the mitochondria is to supply large amounts of chemical energy to the cells so that the cells can perform the contraction and generation of heat. Conversely, the skin does not require much energy; thus it contains only a few hundred mitochondria.

2.2 Cell Organisation
ganisms can be classified into unicellular organisms and multicellular organisms. Unicellular organisms are made up of only a single cell each whereas multicellular organisms are larger organisms that are made up of more than one cell. Examples of the unicellular organism are the amoeba
and the paramecium
. The amoeba and paramecium are called the protozoans. Even though these unicellular organisms are simple in structure, they can perform the basic life processes like any other multicellular organisms. Examples of basic life processes are feeding
, reproduction
, respiration, excretion, locomotion, sensitivity, growth and osmoregulation

Unicellular Organisms
The following are examples of the living processes of the amoeba and paramecium.
Unicellular Organisms

Multicellular Organisms
A multicellular organism
is an organism
which has more than one cell. The larger organisms need many different types of cells which differ in terms of their structures and are much specialised in their functions to carry out specific tasks. Examples of multicellular organisms are plants, human beings and all vertebrates and invertebrates. The cells of multicellular organisms differentiate and undergo specialisation in order to perform tasks more efficiently. Each cell in an organism specialises in one particular function, each one of them contributing to the functioning of the multicellular organism as a whole. The specialisation of the functioning of each cell is known as the division of labour
. Click here for further information about multicellular organisms.
Cell Organisation in Multicellular Organisms
Cells are organised into tissues, organs and systems. A tissue is a group of similar cells. The cells in a tissue are similar in structure and perform a specific function. An example of a tissue is the muscle tissue. An organ consists of different types of tissues grouped together to perform a special function. An example of an organ is the liver. The various organs that work together are known as a system. Examples of systems are the respiratory system
and the digestive system
. All systems work together to form a multicellular organism.
Therefore, the cell organisation in multicellular organisms consists of the following five levels of organisation:
five levels of organisation
Cell Organisation in Animals
The cell is a basic unit of life in all organisms. Examples of animal cells are:
Level One : The cell
Examples of animal cells
Examples of animal cells
Level Two : The tissue
In animals, tissues
can be classified into four main types. These are the epithelial tissue, muscular tissue, connective tissue and nervous tissue.
Types of animals tissue
Types of animals tissue
Level Three : The organ
Organs are formed by several tissues that are bundled together to carry out specific functions
. Examples of organs are the ear, eye, lungs, etc. For example, the heart consists of muscle tissue, connective tissue, nerve tissue and epithelial tissue that work together so that the organ can perform a specific function as a single unit.

Level four : The system
The fourth level of the organisation of the multicellular organism is the system; we shall discuss the system in the human body. There are 11 major systems which carry out major functions in the human body.
Major systems which carry out major functions in the human body
Major systems which carry out major functions in the human body

Cell Organisation in Plants
As in animals, the cell organisation in plants also consists of five levels of organisation in the hierarchy from the lowest to the highest; the hierarchy is as follows:
five levels of organisation in the hierarchy from the lowest to the highest
We shall go through each level and provide examples.
Level One : The cell
Examples of plant cells are the epidermal, the xylem cell and the mesophyll palisade. See the figure below.
Examples of plant cells
Examples of plant cells

Level Two : The tissue
In general, plants have simpler body organisms compared to humans and animals. There are two main types of tissues in plants; these are the meristematic tissue and the permanent tissue. The permanent tissue is further divided into the epidermal tissue, the ground tissue and the vascular tissue. The function of each of this tissue is shown in table below.
Types and functions of tissues in plants
Types and functions of tissues in plants

Level Four : The system
The system in plants can be classified into two groups:
i. The root system The root system consists of all the roots of a plant. It functions to absorb the water and minerals from the soil and transport them to the entire plant. The root also acts as an anchor so that the plant is stable in its position on the ground.
ii. The shoot system The shoot system consists of stems, leaves, buds, flowers and fruits. The stems and branches support the leaves so that they are in the most suitable position to receive maximum sunlight. The stems and branches also support the flowers and position them accordingly for the pollination processes.


Uniqueness of a Cell
The cell is unique and specialised in terms of the functions it carries out. For example, the nerve cell is specialised in transmitting and receiving nerve impulses. Muscle cells
, on the other hand, specialise in causing movement by contraction and relaxation and sperm cells
specialise in fertilising eggs in the reproduction process. Red blood cells carry and transport oxygen.
What makes each of the cells unique and specialised? The uniqueness of a cell depends on its cellular components. Examples of the cellular components are the plasma membrane, cell wall, cytoplasm, nucleus, rough and smooth endoplasmic reticulum, mitochondria, golgi apparatus, lysosomes, ribosomes, chloroplast, centrioles and vacuoles. (We discussed this in Section 2.1). These cellular components work together as a single unit thus specialising in a particular function and making it unique and highly specialised.
What happens if one of the cellular components is lacking or absent in a cell? If this happens, the cell is unable to carry out its function and the cell will die. For example, if there is no chloroplast in the cell, the cell cannot carry out photosynthesis and will die. Similarly, if there is no mitochondrion in the cell, there is no respiration, and the cell lacks energy to carry out its function.
The following are the effects on the cell if the cellular components are absent.
Cellular component and the effects on the cell
Cellular component and the effects on the cell
As can be seen from the table, each cellular component is important for the cell to function optimally. The cell as a unit ceases to function properly if any of its cellular components is defective or absent.
The uniqueness of the cell lies in its coordination of all the functions of the cellular components and working as a single unit. The function of the cell is highly correlated with its shape. For example, if the cell is small, circular and disc-like in shape, its function normally relates to the transportation of oxygen within the whole cell. If it is spindle in shape, its function normally relates to contraction and relaxation for movement purposes.
The following are examples of cells with their corresponding shapes and functions.
Examples of animal cells with their corresponding shapes and functions
Examples of animal cells with their corresponding shapes and functions
Examples of plant cells with their corresponding shapes and functions
Examples of plant cells with their corresponding shapes and functions

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