NCERT Class 7 Science Chapter 8: Cell-Structure And Functions

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Brain Teasers

• Difference between plant cell and animal cell?

• What is cell wall and cell membrane

• Prokaryotes and eukaryotes

• Structural unit of organ – cell

• Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism.

• Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Cork is a part of the bark of a tree. As honey comb structure. These were actually dead cells.

• Improved microscope with higher magnification

Multicellular Unicellular

• Cell – basic structural units, different arrangements in design, shape and size.

• The egg of a hen represents a single cell and is big enough to be seen by the unaided eye.

• Stains (dyes) are used to colour parts of the cell to study the detailed structure.

• Human body has trillions of cells which vary in shapes and sizes

• A billion is a thousand million. A trillion is a thousand billion.

• Billion cell organism begin life as single cell. The fertilised egg cell multiplies and the number of cells increase as development proceeds.

• A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces

• Shape – amoeba -These are called pseudopodia – appear and disappear as amoeba moves and feeds

• WBC in humans can change shape similar to amoeba (it is full fledged organism)

• RBC – spherical

• Muscle cells – spindle in shape

• Nerve cell – long and branched (control and coordinate)

• Components of the cell are enclosed in a membrane. Membrane provides shape.

• Cell wall is additional covering giving shape and rigidity

• Size of Cell: The smallest cell is 0.1 to 0.5 micrometer in bacteria. The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich. A white material surrounds the yellow part. White material is albumin which solidifies on boiling. The yellow part is yolk. It is part of the single cell.

• Size of cell has no relation to size of body. Nerve cells, both in the elephant and rat, are long and branched. They perform the same function, that of transferring messages.

Nucleus, Cytoplasm Human cheek cells

roots help in the absorption of water and minerals.

Leaves, are responsible for synthesis of food

Each organ is further made up of smaller parts called tissues. A tissue is a group of similar cells performing a specific function.

The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. Between membrane there is medium. Membrane is porous and allows exchange.

Nucleus – dense round body inside, spherical and lies in centre. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane

Cytoplasm – jelly like substance between nucleus and cell – organelles are present as mitochondria, golgi bodies, ribosomes, etc.

Cell wall – for protection in plants. protection against variations in temperature, high wind speed, atmospheric moisture etc.

Cheek cell – with toothpick, add iodine and methylene blue solution – cell wall is absent in animal cell.

Plant cell Animal cell

Nucleus has nucleolus.

Nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring

Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic

Nucleus – inheritance and control center of activities.

The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.

The cells having nuclear material without nuclear membrane are termed prokaryotic cells - bacteria and blue green algae

onion cells and cheek cells having well-organised nucleus with a nuclear membrane are designated as eukaryotic cells.

Vacuole – blank looking structure. Cheek cells have smaller vacuoles. Large vacuoles are common in plant cells. Vacuoles in animal cells are much smaller.

Plastids in plants have chlorophyll

Green colored plastids are called chloroplasts. They provide green colour to the leaves.

Fact to Know: The cells in the outermost layer of our skin are dead (no nerve cells). An average adult carries around about 2 kg of dead skin. Billions of tiny fragments of the skin are lost every day. Every time you run your finger on a dusty table, you shed a lot of old skin.

-Mayank