THE CONCEPT OF CELL

The short history about discovery of a cell

– The cell was first named and discovered by an English biologist called Robert Hooke in 1665 when he was observing a piece of cork under the microscope he had made.

– Robert Hooke noticed that the cork was made up by many box- like units which were packed together to form a structure like a honeycomb. He called these units cells.

– Hooke observed many other living materials and noted that they were also made up of cells.

MEANING OF CELL

Cell: Is the basic unit of life.

OR

Cell: is a structural and functional unit of life in living organisms.

All living things are made up of one or more cells.

Unicellular organisms

Are organisms which are made up of one cell.

– Example: bacteria and protozoans

Multicellular organisms

Are organisms which are made up of many cells.

– Example: animals, plants and fungi

The characteristics of the cell

Characteristics of the cell include the following:

(i)    Cells are microscopic.

(ii)    Cells are capable of growing and developing

(iii) Cells can reproduce and increase in number

(iv)    Cells can move e.g. white blood cell and unicellular organisms such as bacteria

(v)    Cells are capable of detecting changes in the environment and respond to the changes.

The cell theory

The cell theories were proposed by two scientists namely Schleiden, a Belgian botanist, in 1838 and Schwann, German Zoologist, in 1839.

The following are observations noted down by the two biologists after careful study of cells

(i) New cells only come from pre-existing cells.

(ii) Cells contain structures called organelles.

(iii) All living things are composed of one or more cells.

(iv)    Cells carry hereditary information which is passed from one generation to another.

(v)    There is a relationship between structure and function of a cell.

(vi)    Cells are the building blocks of living things and life exists only in cells.

TYPES OF CELLS

There are two main types of cells, namely:

(i)    Prokaryotic cells

(ii)    Eukaryotic cells

PROKARYOTIC CELLS

Are cells with no membrane-bound nucleus.

Example: Bacteria cell

Prokaryotic organisms: are single-celled organisms

– Prokaryotic organisms are also called prokaryotes Example: bacteria and blue-green bacteria Characteristics of prokaryotic cells.

(i)    They have no true nuclei.

(ii)    They lack membrane- bound organelles such as mitochondria

(iii) They are smaller in size than eukaryotic cells.

(iv) They have few cell organelles

THE DIAGRAM OF PROKARYOTIC CELL

EUKARYOTIC CELLS

Are cells whose nuclei are bounded by nuclear membrane.

– Example: plant cells, animal cells, fungi cells, protoctists cells Eukaryotic organisms: are organisms which contain eukaryotic cells

– Eukaryotic organisms are also called eukaryotes

– Example: protoctists, fungi, plants and animal cells

Characteristics of eukaryotic cells

(i)    They have true nuclei

(ii)    They have a membrane-bound nucleus called nuclear envelope.

(iii)    They are bigger and more complex than prokaryotic cells

(iv)    They contain specialized membrane-bound organelles such as mitochondria

(v) They have many organelles

THE DIAGRAM OF EUKARYOTIC CELL

Types of eukaryotic cells

There are two main types of eukaryotic cells, namely:

(i)    Animal cells

(ii)    Plant cells

Other types of eukaryotic cells include:

-Fungi cells

– Protoctists cells such as amoeba cells, euglena cells

DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS

ANIMAL CELLS

Are cells which found in animals

Characteristics of animal cell

(i)    It has irregular shape

(ii)    It lacks cell wall

(iii) It is nucleus located at the centre

(iv)    It stores food in form glycogen

(v)    It lacks chloroplasts

ORGANELLE

Is a specialized cellular structure or part that perform a specific function.

Example of organelles

– Nucleus, mitochondria, ribosomes, chloroplasts Parts/organelles of animal cell

Animal cell has the following organelles:

(i)    Nucleus

(ii)    Cytoplasm

(iii)    Mitochondria

(iv)    Cell membrane

(v)    Ribosome

(vi)    Golgi bodies

A diagram of animal cell as seen under a light microscope

PLANT CELLS

Are cells which found in plants

Characteristics of plant cell

(i)    It has cell wall

(ii)    It has chloroplast

(iii)    It has a large permanent vacuole

(iv)    It stores food in form of starch

(v)    It is large in size Parts/organelles of a plant cell Plant cell has the following organelles:

(i)    Nucleus

(ii)    Cellulose cell wall

(iii)    Cell membrane

(iv)    Mitochondria

(v)    Cytoplasm (vi) Ribosome

(vii)    Golgi bodies

(viii)    Cell sap vacuole

Parts/organelles of a plant cell

Plant cell has the following organelles:

(i) Nucleus

(ii) Cellulose cell wall

(iii) Cell membrane

(iv) Mitochondria

(v) Cytoplasm

(vi) Ribosome

(vii) Golgi bodies

(viii) Cell sap vacuole

A diagram of plant cell as seen under a light microscope

SIMILARITIES AND DIFFERENCES OF PLANT AND ANIMAL CELLS

Similarities between animal cell and plant cell

(i)    Both have cell membranes

(ii)    Both contain cytoplasm

(iii) Both have nucleus

(iv)    Both have mitochondria

(v)    Both have Golgi bodies

(vi)    Both have ribosomes

Differences between plant cell and animal cells

Note: Animal cell and plant cell have common three main structures or organelles, namely:

(i) Nucleus

(ii)    Cell membrane

(iii)    cytoplasm

FUNCTIONS OF DIFFERENT PARTS OF PLANT AND ANIMAL CELLS

1. NUCLEUS

Is a dark spherical body which is surrounded by double membrane called a nuclear membrane

– Nucleus is the largest organelle of the cell

– The nucleus and cytoplasm form the protoplasm.

– The nucleus is made of a viscous fluid called nucleoplasm.

Functions of nucleus

The following are the functions of the nucleus:

(i)    Nucleus controls all activities of the cell.

(ii)    Determines the cell’s size, shape and function. (iii)Determines hereditary characteristics of a cell.

(iv)    The nucleus contains chromosomes which carry genetic material.

(v)    It contains nucleolus which manufactures ribosomes.

(vi)    Determines the chemical processes that take place in cell.

2. CYTOPLASM

Is a transparent jelly-like fluid in which chemical reactions take place.

– It contains up to 90% water and dissolved substances such as carbondioxide, oxygen, proteins, sugars and minerals.

– The movement of cytoplasm is known as cytoplasmic stream

Functions of cytoplasm

(i)    It is a place where chemical reactions take place

(ii)    It is a site where cell organelles such as nucleus, vacuoles are suspended. (It is a medium in which cell organelles are found)

(iii)It store useful substance such as starch, glycogen, fat droplets, proteins, sugars and minerals.

3. CELL MEMBRANE

Is a semi-permeable membrane made of protein and oil.

– Cell membrane is also known as plasma membrane or plasmalemma.

– It has pore or microscopic holes to allow in and out movement of materials of the cell.

– The cell membrane is the living part of the cell.

Functions of cell membrane

(i)    Cell membrane controls the movement of materials in and out of the cell.

(ii)    Cell membrane encloses the contents of the cell.

(iii) It allows cell to communicate with another cells.

Question: Why Cell membrane is said to be selective permeable membrane?

Answer: Cell membrane is said to be selective permeable membrane because it allows selective movement of substances into and out of the cell.

4. CELL WALL

Is a rigid outer layer that surrounds the cell membrane in plants, fungal, algae and bacterial cells.

– Animal cell has no cell wall

– The cell wall in plant cell is made up of cellulose.

– The cell wall is non-living part of the cell.

Functions of the cell wall

(i)    It gives the cell its shape.

(ii)    It is freely permeable to all kinds of molecules.

(iii) It supports and protects the cell.

(iv)    It provides mechanical support for plant body in herbaceous plants.

(v)    It prevents the cell from bursting.

(vi)    It allows gases, water and substances to go through.

Differences between cell wall and cell membrane

5. VACUOLE

Is a fluid-filled space surrounded by a membrane called tonoplast.

– Animal cells have small and temporary vacuoles. In animal cells, vacuoles are small droplets of fluid in the cytoplasm variable in size and position

– Plants cells have a large and permanent vacuole that usually occupies the central part of the cell. Vacuole in plant cell is made up of the fluid called sap hence the name cell sap vacuole

– The cell sap may contain salts, sugar and pigments dissolved in water.

– The outward pressure of the vacuole on the cell wall makes the plant cells firm, giving strength and resilience to the tissues.

Functions of vacuole

(i) It is responsible for food storage and osmoregulation.

(ii) It excretes and secretes waste from the cell.

(iii)It stores substances such as sugar, minerals and wastes

(iv) It maintains the shape of a plant cell

6.  MITOCHONDRION (plural: Mitochondria)

Is a sausage –shaped (oval- shaped) organelle bounded by double membrane.

– The outer membrane is smooth but the inner membrane is folded to form cristae.

– Mitochondria are known as “power houses of the cells” because they are used to produce energy for the cells.

Function of mitochondria

(i)    To produce energy for activities of the cell.

(ii)    It is a site where respiration process takes place

DIAGRAM OF MITOCHONDRION

7. CHLOROPLAST

Is an oval-shaped organelle that contain the green pigment known as chlorophyll − Chloroplasts are found in plant cells and algae cells.

– Each chloroplast is enclosed by a double membrane, the outer and inner membranes

– They consist of layers of membranes known as lamellae suspended in an aqueous matrix called stroma.

– Chloroplast is an organelle in which photosynthesis takes place, producing sugars from carbon dioxide and water using light energy trapped by chlorophyll.

Functions of chloroplast

– It is a site where photosynthesis process takes place.

DIAGRAM OF CHLOROPLAST

CHLOROPHYLL

Is a green pigment in chloroplasts of green plants

– It is found in the grana of the chloroplast

Function of chlorophyll

– Used to absorb sunlight needed for photosynthesis.

8.    RIBOSOMES

Are small spherical structures attached to the endoplasmic reticulum

– Ribosomes are manufactured in the nucleolus. Some ribosomes are found scattered in the cytoplasm and on the surface of the nuclear membrane.

Function of ribosome

– It is a site where protein synthesis takes place.

9.    ENDOPLASMIC RETICULUM

Is a network of interconnected channels running throughout the cytoplasm.

Function of endoplasmic reticulum

– Serves as a network of pathways through which materials are transported from one part of the cell to another

Types of endoplasmic reticulum

There are two types of endoplasmic reticulum, namely:

(a)    Rough endoplasmic reticulum

(b)    Smooth endoplasmic reticulum

(a)    Rough endoplasmic reticulum

Is the endoplasmic reticulum which is covered with ribosomes Function of rough endoplasmic reticulum

– Transports protein synthesized in the ribosomes

Diagram of rough endoplasmic reticulum

(b)    Smooth endoplasmic reticulum

Is the endoplasmic reticulum with no ribosomes Function of smooth endoplasmic reticulum

– Transports lipids within the cell

Diagram of smooth endoplasmic reticulum

10.    GOLGI BODIES

Are stacks of flattened, membrane-bound sacs.

– Golgi bodies are also called Golgi apparatus

Functions of Golgi bodies

(i)    They form lysosomes

(ii)    They produce materials for making plant cell walls.

(iii) They replenish the cell membrane

(iv)    They process many secretory substances such as mucus

(v)    They add carbohydrates component to the protein packaging of the finished products before leaving the cells.

(vi)    They produce a substance, which makes the cuticle of insects

Diagram of Golgi body

THE CELL DIFFERENTIATION

Cell differentiation

Is the process by which cells are specialized to perform a particular function OR

Is the process in which a cell changes from simple to complex form and become more specialized to perform a specific function.

– Cell differentiation is also called division of labour.

Importance of cell differentiation

(i)    It leads to division of labour

(ii)    It helps the body to carry out all life processes at the same time (iii)It makes the cell to perform a particular function.

SPECIALIZED ANIMAL CELLS

The following are common examples of specialized animal cells:-

1. WHITE BLOOD CELLS

These cells can change their shape in order to engulf and destroy harmful microorganisms. − White blood cell is a cell which moves and feeds like amoeba

– Some white blood cells contain digestive enzymes which destroy the microorganisms

Function of white blood cell

– Used in body defence against infections

Diagram of white blood cell

2. RED BLOOD CELLS

These cells lack nuclei and are biconcave in shape

– Lack of nucleus in the red blood cells provides a large surface area for transporting oxygen in the body

Function of red blood cell

– Used to transport oxygen from the lungs to various body parts

Adaptation of red blood cell to its function

(i)    It lacks nucleus at maturity to increase the surface area for transporting oxygen in the body.

(ii)    It has hemoglobin which carries oxygen to different parts of the body.

Diagram of red blood cell

4.    SPERM CELL

This cell fertilize the female egg in order for reproduction to take place

– Sperm cell is a motile cell

Function of sperm cell

– Used in sexual reproduction to fertilize the egg

Adaptation of the sperm cell to its function

(i)    It has a long tail that enables it to swim to the egg

(ii)    It has numerous mitochondria for energy supply needed for swimming

(iii) It has a pointed head with acrosome which enables it to penetrate the egg.

Diagram of sperm cell

5.    NERVE CELL

Is a longest cell in the human body

– It is thread-like structure

Function of nerve cell

– It carries and conducts nerve impulses from one part of the body to another

Adaptation of nerve cell to its function

(i)    It is long to ensure the impulse is transmitted to a long distance of the body.

(ii)    It has numerous mitochondria for energy supply

Diagram of nerve cell

6.    Egg cell

This cell is bigger in size with a lot of cytoplasm.

– The outer layer of this cell can produce chemicals to prevent any more sperms from entering the egg after fertilization.

– It is also called ovum

Function of egg cell

(i)    It fuses with the sperm cell to form a zygote

(ii)    It develops into embryo after fertilization

(iii) It carries the maternal genetic material

Adaptations of egg cell to its function

(i)    The outer layer of this cell can produce chemicals to prevent any more sperms from entering the egg after fertilization.

(ii)    It is large in size with a lot of cytoplasm to carry genetic materials and enough food reserve for developing zygote

DIAGRAM OF EGG CELL

7.    MUSCLE CELL

Is a cell with strong contractile ability

◊ Muscle cells contract and relax to bring about movement

Function of muscle cell

– Brings about movement by contracting and relaxing

Adaptation of muscle cell to perform its function

(i)    It has numerous mitochondria for energy supply

(ii)    It is multinucleated to increase coordination

DIAGRAM OF MUSCLE CELLSPECIALIZED PLANT CELLS

The following are common example of specialized plant cells;-

(I) ROOT HAIR CELLS

These cells absorb water and mineral salts from the soil

– They are elongated and have thin membrane to increase the surface area for absorption of water and nutrients from the soil.

Function of root hair cell

– Absorbs water and mineral salts from the soil

Adaptation of root hair cell to perform its function

(i) It is elongated to increase the surface area for absorption of water and nutrients from the soil.

(ii)It has thin membrane to increase the surface area for absorption of water and nutrients from the soil

(iii) It has root hairs to increase the surface area for absorption of water and mineral salts from the soil

Diagram of root hair cell

(ii) GUARD CELL

Is a cell surrounded by stomata

– The inner walls of guard cell are thicker than the outer walls

– Stomata are tiny pores found on the leaf used for gaseous exchange and water loss

– When the guard cells expand, the stoma opens and when they contract the stoma closes

Function of guard cell

– Controls opening and closing of stomata to allow gaseous exchange

DIAGRAM OF GUARD CELL

(iii) PALISADE CELLS

Are plant cells that contain a lot of chloroplasts

– Palisade cells are found at the top of the plant leaves.

Function of palisade cells

– Used to carry out photosynthesis

Adaptation of palisade cells to perform its function

(i)    They have large number of chloroplasts for carrying out photosynthesis

(ii)    They contain chlorophyll to trap sunlight energy during photosynthesis

Diagram of palisade cell

FORMATION OF TISSUE, ORGAN AND BODY SYSTEM

– Cells in multicellular organism can be organized into tissues

– Tissues can be organized into organs

– Organs can be organized into organ system (body system)

– Organ system (body system) can be organized into organism

The following below is the organization showing the correct order from simplest to the most complex

Organelles > Cells>Tissues>Organs>Organ system>
Organism

TISSUE

Is a group of cells that perform the same function

Example of tissues

Note:

Xylem is a plant tissue used to transport water and mineral salts from the roots to the upper plant parts.

Phloem is a plant tissue used to transport manufactured food from the leaves to the other plant parts.

ORGAN

Is a group of different tissues that perform a certain function.

Example of organs

Note: The following are types of animal organs:

(i) Respiratory organs e.g. lungs, trachea

(ii) Digestive organs e.g. mouth, stomach

(iii) Reproductive organs e.g. testes, ovaries, vagina, penis

(iv)    Sense organs e.g. eyes, Nose, Tongue, ears and skin

(v)    Circulatory organs e.g. Heart

(vi)    Excretory organs e.g. Kidneys, Liver, Skin

ORGAN SYSTEM

Is a group of specialized organs that work together to perform a certain function.

Examples of systems

ORGANISM

Is a combination of several systems working together

Example of organisms

– Human being

– Cow

– Frog

– All green plants e.g. maize plant, bean plant