Cell - Structure and Function Class 8 Notes

Early Days of Cells

Cells

Cells are the fundamental structural units of living organisms and the basic units of life.

Robert Hooke discovered the cell in the year 1665 from a thin section of cork under a self-made microscope.
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World of Cells

Prokaryotic & Eukaryotic cells

Prokaryotic – Size: generally small ( 1-10 µm) Where 1 µm = 10 6m.

  • The nucleus is not well defined and known as a nucleoid.
  • Single, circular chromosome.
  • Membrane-bound organelles absent. Eg: Bacterial cell

Eukaryotic – Size: generally large ( 5-100 µm).

  • The nucleus is well defined and surrounded by a nuclear membrane.
  • Possess more than one and linear chromosome.
  • Membrane-bound cell organelles are present. Eg: Human cell.

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Cell structure in Eukaryotic cells

  • Most eukaryotes are multicellular.
  • The nucleolus is present in the nucleus.
  • It contains cell organelles such as plastids, mitochondria, Golgi bodies, etc.
  • Fungi, plant, and animal cells are eukaryotic.

Cell membrane

  • The cell membrane separates cells from one another and also the internal contents from the surrounding medium.
  • It is porous and allows the movement of substances or materials both inward and outward.

Cytoplasm

  • It is the jelly-like substance present between the cell membrane and the nucleus.
  • Various other components, or organelles, of cells are present in the cytoplasm.

Cytoplasm

Cell walls in plants

  • A cell wall is an additional covering over the cell membrane in plant cells.
  • It gives shape and rigidity to these cells.
  • It gives a rigid structure to plant cells and that is why they can withstand the stress of high-velocity wind, rain, high temperature etc.

Cell Factory

Organelles

  • The various components present within the cell are known as cell organelles.
  • All these components are special and specific to their functions.
  • For example, mitochondria, lysosomes etc.

Nucleus

  • It is generally spherical and located in the centre of the cell.
  • The nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.
  • It uses information in the chromosomes to decide what each organelle should do and the functions of the cell.
  • The nucleus contains an even smaller concentrated material called nucleolus.

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Chromosomes

  • The nucleus contains thread-like structures called chromosomes.
  • These carry genes and help in inheritance or transfer of characters from the parents to the offspring.
  • Chromosomes are composed of DNA in the form of Chromatin and protein.
  • Chromosomes contain information for the inheritance of features from parents to next generation in the form of DNA molecules.

DNA

  • DNA molecules contain the information necessary for constructing and organising cells.

Functional segments of DNA are called genes.

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Vacuoles, Endoplasmic Reticulum and Golgi Apparatus

Vacuoles

  • Vacuoles are storage bubbles of irregular shapes which are found in cells.
  • The vacuole stores the food, a variety of nutrients that a cell might need to survive or waste.
  • In plant cells, vacuoles are much larger than in animal cells and they provide rigidity to the plant cells.

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Golgi Apparatus

  • Golgi Apparatus also termed as Golgi Complex is a membrane-bound nucleus.
  • Its main function is to modify proteins.
  • These modified proteins are placed in Golgi so that it can transport elsewhere in the cell when required.

Endoplasmic Reticulum

  • It is a membranous organelle.
  • It is found in all eukaryotic cells.
  • The ER is divided into two types based on their physical and functional properties viz. Smooth and Rough ER.

Cell Structure and Function 01

Rough ER

  • It synthesizes and secretes proteins in the cells.

Smooth ER

  • The SER is involved in the synthesis of phospholipids, the main lipids in cell membranes and are very important in the process of metabolism.
  • They also transport the products of the rough endoplasmic reticulum to other cell parts like the Golgi apparatus.

Lysosomes

  • Lysosomes are small membrane-bound sac-like structures which release digestive enzymes that break down food.
  • They also remove the old and damaged or dead organelles from the cell and thus, protect the cell from further damage.

Mitochondria

  • Mitochondria is a double membrane-bound sausage shaped organelle.
  • Also known as ‘powerhouse of the cell’.
  • Mitochondria are the sites of aerobic respiration in the cell and it produces energy in the form of ATP.

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Plastids

  • Plant cells have tiny, coloured structures called plastids.
  • Plastids carry pigments of various colours like orange, red, green and even colourless.
  • The plastids, which carry green-coloured pigments i.e., chlorophyll are called chloroplasts.

Plants vs Animals

Comparison of plant and animals cells

Cell Organelle Plant Cell Animal Cell
Cell wall Yes No
Cell membrane Yes Yes
Cytoplasm Yes Yes
Nucleus Yes Yes
Chloroplast Yes No
Vacuole Yes Yes

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