CELL AND CELL ORGANELLES.
Cell is the basic functional unit of a living organism. Living organism that are made up of a single cell are called unicellular organisms while those made up of many cells are called multicellular organisms.
Examples of unicellular organisms-bacteria, amoeba, paramecium.
Examples of multicellular organisms-human beings, plants.
When various cells are grouped together they form a tissue.
When various tissues which perform closely related functions are grouped together, they form organs.
various organs whose functions are synchronised form organs system.
Organ systems when brought together form the organism.
The above description show the order of increasing the complexity of living organism from the smallest functional unit(cell) to the whole organism.
Examples of;
Cells-Red blood cell, white blood cell, platelets, neuron cell, root hair cell, sperm cell,
Tissues-connective tissues, epithelial tissues, blood,contractile tissue.
Organs-Brain, liver, kidney, heart, spleen, lungs, mouth, small intestine.
Organ systems-digestive system, nervous system, circulatory system, skeletal system, breathing system, excretory system, reproductive system.
Mastery tips
1.A book is made up of chapters or topics, 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 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 represents 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.
Self test
Arrange the following in order from the smallest to the largest.
Circulatory system, red blood cell, Human being, heart, Blood.
From the above list, identify what is an organism, organ system, organ, tissue, cell.
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.
From the introductory topic, characteristics of living things were discussed. It is the cell that performs the functions that give living things their characteristics. Examples of functions performed by the cells inclue; respiration, gaseous exchange, excretion, nutrition etc.
Both plants and animals have cells. However, the structure of a plant cell differs to 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 the cell membrane | Has only 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. |
Cell organelles
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 outer most 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; it has three properties
It is semi-permeable-only
It is sensitive to change in PH and temperature because it is protein in nature.
Posses electric charges on its surface.
2.Cell wall
It 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 cell membrane, cell wall is permeable and hence allows movement of materials across it without any barrier.
3. Cytoplasm
It is fluid field inner part of the cell. It provides site for chemical reactions to occur.
It is this fluid that bathes other cell organelles.
3.Mitochondria (plural)
Singular is mitochondrion.
It is sausage shaped organelle.it has 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 site for respiration (chemical breakdown of glucose to release energy)
Therefore it acts as the powers house of the cell.
4. Chloroplast
Only found in plant cell
It provides site for photosynthesis. (Detailed description is in photosynthesis in plants)
5. Endoplasmic reticulum
These are interconnected system of membrane bound channels. The channels form a network of cisternae or tubules arranged parallel to each other and vesicles. Endoplasmic reticulum are continuos with the nuclear membrane.There are two types of endoplasmic reticulum
Rough endoplasmic reticulum-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-have no ribosomes on their surface.
Functions of endoplasmic reticulum
Rough endoplasmic reticulum; synthesis of proteins.
Smooth endoplasmic reticulum; synthesis of lipids.
6.Golgi bodies
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 form of secretory vesicles.
Functions of Golgi complex; packaging and transport of glycoproteins.
7.Ribosomes
Ribosomes are found either freely in the cytoplasm or attached to the rndoply reticulum. When attached to the surface of endoplasmic reticulum, it is called rough endoplasmic reticulum.
Function of ribosomes; protein synthesis.
8.Lysosomes.
Contain lytic enzymes that break down worn out tissues or organelles.
9.Centrioles.
Only found in animal cell
Has two functions;
Formation of cilia and flagella
Takes part in cell division
10. Nucleus
Nucleus is made up of three parts; nuclear membrane, nuclear pore and nucleolus. Nucleolus is the dense part of nucleus. Nucleolus synthesised ribosomes.
Contains genetic material which is transferred from parents to the offsprings.
The nucleus also controls all the activities of the cell.
11.Vacuoles
In plants, the vacuole plays a major role in maintaining osmotic pressure of the cell
They serve as storage organelles – can store food, salt and sugar. There are various types of vacuole depending on the substance they store. These include
Food vacuole
Sap vacuole
