Class 8 Cell Structure and Functions
Robert Hooke coined the term “Cell” in Latin cell means “Little room”
He observed The Cork (the bark of the tree) under the microscope
Cell Theory was proposed by
German Biologist
- Mattias Jakob Scheilden
- Theodar Schwann
- Robert Virchow
Cell Theory
- All living organisms are made up of cells
- Cells are the basic structural and the functional unit of living organisms.
- New cells arise from pre existing cells by Cell Division.
- No cell can originate on its own.
- Every organism starts its life as a single cell.
Two instruments are used to study the cells, they are Compound Microscope and Electron Microscope.
Difference between Compound Microscope and Electron Microscope??
Compound Microscope | Electron Microscope |
It is used to observe the tiny objects. | It is used to observe the internal organelles of the cells. |
It uses glass lenses and visible light for observation | It uses electromagnets instead of glass lenses and beam of electrons instead of light |
Magnification Power ranges from 100 to 1500 Times | Magnification power of electron microscope ranges from 1,00,000 to 5,00,000 times. |
We can only visualise through compound microscope. | The highly magnified images obtained on a photographic film or screen |
Define Staining and Why it is used??
- Cells are transparent and colourless.
- To observe and study the internal structure of the cells stains are used.
- Some common stains are Safranin, Eosin, Methylene blue, Crystal violet.
Difference between Unicellular and Multicellular organism??
Unicellular organism | Multicellular organism |
Organism made up of single cells are called unicellular organism. | Organism made up of many cells are called as Multicellular Organism. |
Ex: Amoeba, Paramecium, Euglena, Bacteria Yeast etc | Ex: Trees, insects, cow, birds, humans etc. |
All the basic functions of the cells are performed by single cell | Different parts of multicellular organism perform different function. They show division of labour. |
Ingestion, digestion respiration movement growth and reproduction all the functions are performed by single cell. | In multicellular organism each part of the body perform different functions. Ex: In Humans Heart pumps blood, Stomach digest food. |
What are the various levels of Cell organisation??
- Cells
- Tissues
- Organs
- Organ
- System
- Organism
In multicellular organism
- The Cells are the lowest level of organisations
- Group of similar cells performing specific function join together to form the Tissue
- Different tissues join together to form the Organs
- Every Organ performs specific function.
- All the Organs that work together to perform specific life function form an organ system.
- The various Organ system makes up and Organism
Ex:
- All the skin linings are formed of epithelial cells.
- The Cells Unite to from the Epithelial tissues. These are found in Stomach, Liver, Pancreas etc.
- In stomach these tissues help in the digestion of the food.
- Not only in stomach but also in oesophagus, Stomach, Small intestine, Large intestine, Liver, Pancreas they work together for the function of the food. This forms the Digestive System
- Similarly, there are Respiratory system, Excretory system, Reproductive system that work together to carry out the life process.
Shell shape and Size:
The shape and the size of the cells are related to the functions perform and not on the size of the organism.
Cells are of different types:
- Spherical – Egg of many animals.
- Spindle shape – Smooth muscle fibres
- Elongated – Nerve cells
- Oval shape – Red blood cell
- Branched – Osteocyte.
- No definite shape – WBC, Amoeba
- Nerve cells are long and have wire like projections.
- These help to carry the messages and send to all parts of the body
- Muscle cell expand and contract this is because the muscle cells are long and thin shape
Cell size:
Cells and their organelles are measured by fractions or millimetre (mm)
- 1 micron = 10-3 mm
- Size of the cell varies from 0.1 – 0.5 micrometre to very large eggs of Ostrich (170mm × 130 mm)
- Red blood cells are 7 micrometer smallest, Nerve cells 100 cm.
- The cell size are not related to the size of the organism
- Nerve cells of the rat and elephant both are long and branched and they perform similar function.
Parts of the cell
The cell consists of Cell membrane, Nucleus and Cytoplasm.
Cell membrane
- Every cell is surrounded by a thin delicate membrane called cell membrane or plasma membrane. It is Porus
- It is called as selectively permeable membrane because it allows the movement of only certain substances.
- It acts as a boundary to the cell and separate the cells from one another.
- It protect the cell from injury.
- In plants and bacteria they have additional outer wall called Cell Wall.
- It provides protection against the Temperature, High wind speed and Atmospheric pressure and Moisture
Cytoplasm
- Cytoplasm is a thick jelly like fluid inside the cell bound by Cell membrane.
- It is present between the Nucleus and Cell membrane.
- All the chemical reactions occur here.
- There are many small cytoplasmic bodies suspended in the cytoplasm called as cell organelles. Ex: Mitochondria, Ribosomes, Golgi bodies etc.
Nucleus
- The most important component of the cell is the Nucleus.
- It is large spherical structure in cells.
- In animals it is present at the centre of the cell, in plants they are present at the Periphery of the cell due to the presence of large vacuole.
- Nucleus has 4 parts. Nuclear membrane, Nucleoplasm, Nucleolus, Chromosomes
Nuclear Membrane
- It is separated from the cytoplasm by a membrane called a Nuclear Membrane.
- It is Porus and allows only selectively particle to pass between the Cytoplasm and the Nucleus
Nucleolus
- It is small spherical body present inside the Nucleus.
- It is rich in RNA (Ribo nucleic acid )
Nucleoplasm
- It is dense fluid present in the nucleus.
Chromosomes
- DNA Deoxy Ribonucleic acid are the fine thread like structure suspended in the nucleoplasm. These are called as Chromatin.
- In dividing cells, Chromatin are dense and form chromosomes. These carry the genes.
- Genes are responsible for passing genetic information from the parent to the off spring
- Child receives one copy from the mother and another copy from the father.
Functions of chromosomes
- Nucleus controls all the metabolic activities of the cell.
- It is responsible for passing the genetic information from the parent to off spring.
- Ex: A daughter may receive the brown colour of the eyes genes from her father.
Note
- Nucleus and cytoplasm together called protoplasm. Protoplasm living content of the cell.
Cell organelles
Mitochondria
- They are small rod like structures present in all the cells
- They provide energy to all the cells by oxidation of the food.
- They are often called as “Powerhouse of the Cell”
Vacuoles
- These are sac like structures
- Plants have large vacuoles
- They stores soluble food, waste, secretions of the cell dissolved in water as cellsap.
Endoplasmic reticulum
- It is a network of tubules and channel
- It has involved in synthesis storage and transport of the cell products
Ribosomes
- These are small granular structures
- These are sites of protein synthesis
Golgi bodies or Golgi apparatus
- These are made up of tubules and vesicles
- It is responsible for the secretion of enzymes hormones and proteins.
Plastids
- These are present only in the Plant cells
- They contain pigments that perform specific roles.
- Chloroplast are also called as Green Plastids
- They contain chlorophyll and helps in photosynthesis.
- Chromoplast are non green plastids
- They give colour to the fruits and flowers.
- Leucoplast are colourless plastids. They help in the storage of the food.
Centrosomes
- They are located near the Nucleus
- Involved in cell division
- They are found only in the animal cells
Difference between Plant cell and Animal cell
Plant cell | Animal cell |
Plant cells are larger in size | Animal cells are smaller than the plant cell |
Cell wall is present | Cell wall is absent |
Centrosome is absent | Centrosome is present |
Plastids are present | Plastids are absent |
Large vacuole is present | Vacuoles are absent or maybe small in size. |
Golgi bodies or Dictyosomes scattered in the Cytoplasm. | Well developed Golgi body is present near the Nucleus |
Difference between Prokaryotic cells and Eukaryotic cells ??
Prokaryotes | Eukaryotes |
In Greek Pro means primitive karyon means nucleus. | In Greek Eu means True and karyon means Nucleus |
The cells do not have a well organised nucleus. | The cells have well organised nucleus. |
Nuclear membrane is absent. | Nuclear membrane is present. |
Ex: Bacteria, Blue green algae etc. | Ex: Plants, Animals, Protozoa, Fungi etc. |
Cell Division
- Cells divide to form more cells.
- It is responsible for the growth of organism.
- Damaged and old cells are replaced.
- Millions of cells die and replaced every second in our body.
- Bacteria and amoeba reproduce through cell division.
- During division of the cells first the nucleus splits into two and then cytoplasm divides into two separate parts across the middle.
- Newly formed the cells are called as daughter cells.
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