CELL AND CELL ORGANELLES.
A cell is the basic functional unit of a living organism. A living organism with a single cell is called a unicellular organism. In contrast, those made up of many cells are called multicellular organisms.
Examples of unicellular organisms
Examples of multicellular organisms
- human beings
Tissues, Organs and Organ Systems
The tissue is made up of many cells that are grouped to perform similar functions.
Organs are formed when various tissues which perform related functions are grouped together.
An organ system is made up of multiple organs whose functions are synchronized.
Organ systems, when brought together form the organism.
The above description shows the order of increasing the complexity of living organism from the smallest functional unit(cell) to the whole organism.
Examples of cells, tissues and organs.
- Red blood cell
- white blood cell
- neuron cell
- root hair cell
- sperm cell
- connective tissues
- epithelial tissues
- contractile tissue.
- Brain, liver
- small intestine.
- digestive system
- the nervous system
- circulatory system
- skeletal system
- breathing system
- excretory system
- reproductive system.
1. A book is made up of chapters or topics, and each topic is made up of paragraphs, each paragraph is made up of sentences, each sentence is made up of words. I make a stop at the word because it is the word that you will give you the smallest meaning or understand what is being said. In this illustration,
Book represents the organism
Chapters represent the organ systems of the organism
Paragraph represents tissues of the organism
Sentence represents tissues
Word represents the cell.
The same way you can’t get meaning if you split a word into syllables, the same way a cell becomes the smallest unit of life.
All functions of the body are performed in the cell. Within the cell, are structures which enable it to perform its functions. These structures within a cell are called organelles.
It is the cell that performs the functions that give living things their characteristics. Examples of functions performed by the cells include;
- gaseous exchange
Both plants and animals have cells. However, the structure of a plant cell differs from that of an animal cell. The differences are summarised below.
Basis Plant cell Animal cell
Size Mostly larger in size Smaller in size
Shape Regular in shape Irregular shape
Structure Made up of cellulose cell wall outside Has only the cell membrane
the cell membrane without the cellulose cell wall
Organelles Have chloroplast Chloroplast absent
Position of nucleus Peripheral nucleus Central nucleus
Size of central vacuole Large central vacuole Small central vacuoles.
Centrioles Lack centrioles Have centrioles.
These are structures within a cell
They include; cell membrane, cytoplasm, nucleus, mitochondria (plural), central vacuole, endoplasmic reticulum, Golgi bodies,
N/B Chloroplasts and cellulose cell wall are cell organelles ONLY in plants
Centrioles are organelles found ONLY in animal cells.
1. Cell membrane
This is the outermost boundary (limit) of the cell.
It forms a protective barrier of the cell to the external environment.
It is semi-permeable-only allow some (small molecules to pass through)
Its main function is to selectively allow the movement of substances in and out of the cell.
Properties of the cell membrane
The cell membrane has three properties
It is semi-permeable-only
It is sensitive to change in PH and temperature because it is a protein in nature.
It Posses electric charges on its surface.
The cell wall is only found in plant cells. It encloses the plasma membrane in plant cells.
Its function is to give the plant cell its shape and provide mechanical strength to the plant cell
Unlike the cell membrane, the cell wall is permeable and hence allows movement of materials across it without any barrier.
It is a fluid field inner part of the cell. It provides the site for chemical reactions to occur.
It is this fluid that bathes other cell organelles.
It is sausage-shaped organelle.it has a double membrane where the inner membrane is folded into structures called cristae. Cristae increases surface area for attachment of respiratory enzymes (organic catalysts)
It provides the site for respiration (chemical breakdown of glucose to release energy)
Therefore it acts as the powers house of the cell.
Only found in plant cell
It provides a site for photosynthesis. (Detailed description is in photosynthesis in plants)
5. Endoplasmic reticulum
These are an interconnected system of membrane-bound channels. The channels form a network of cisternae or tubules arranged parallel to each other and vesicles. The endoplasmic reticulum is continuous with the nuclear membrane. There are two types of the endoplasmic reticulum.
Rough endoplasmic reticulum–
They have ribosomes on their surface. It is the presence of ribosomes on their surface that gives them rough appearance hence their name.
Smooth endoplasmic reticulum-
They have no ribosomes on their surface.
Functions of endoplasmic reticulum
The rough endoplasmic reticulum is responsible for the synthesis of proteins.
Smooth endoplasmic reticulum takes part in the synthesis of lipids.
The Golgi body is made up of shallow saucer-shaped cisternae which pile one upon the other to form stacks. Golgi bodies perform a secretory role by secreting vesicles which transports the sections to the desired destination.
Mostly, the secretions are synthesized in the rough endoplasmic reticulum, the products transported to Golgi apparatus where they are stored in the form of secretory vesicles.
Functions of Golgi complex
packaging and transport of glycoproteins.
Ribosomes are found either freely in the cytoplasm or attached to the rough endoplasmic reticulum. When attached to the surface of endoplasmic reticulum, it is called the rough endoplasmic reticulum.
The function of ribosomes
Ribosomes are responsible for protein synthesis.
Contain lytic enzymes that break down worn out tissues or organelles.
Only found in animal cell
Has two functions;
Formation of cilia and flagella
Takes part in cell division
The nucleus is made up of three parts; nuclear membrane, nuclear pore and nucleolus. The nucleolus is the dense part of the nucleus. Nucleolus synthesized ribosomes.
Contains genetic material which is transferred from parents to the offsprings.
The nucleus also controls all the activities of the cell.
In plants, the vacuole plays a major role in maintaining the osmotic pressure of the cell.
They serve as storage organelles – can store food, salt and sugar. There are various types of the vacuole, depending on the substance they store. These include