class 8 science cell structure

Cell - Structure and Functions

Amend Education Academy 9999908238

Cell - Structure and Functions
A microscope is an instrument used to see objects too small for the naked eye. An English scientist, Robert Hooke, discovered the existence of cells in 1665.Heis known for his book Micrographia, and for first using the word “cell” to describe the basic unit of life.


Scanning electron microscopes are used to examine the external parts of various organisms; It is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample, producing signals that contain information about the sample’s surface topography, composition, and other properties such as electrical conductivity.

The transmission electron microscope (TEM) is used to view the internal structure of a cell and its organelles. TEM is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor, such as a CCD camera.

Organisms that are made up of a single cell and perform all their vital activities, like reproduction, locomotion and digestion, are called unicellular organisms. Unicellular organisms can be found everywhere. The oldest forms of life, unicellular organisms existed 3.8 billion years ago, if not longer.

Organisms that are made up of more than one cell are called multi-cellular organisms. Most life that can be seen with the naked eye are multi-cellular, as are all animals and plants.

Amoeba uses small finger-like projections, called pseudopodia, for locomotion and to capture prey. Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells with these structures are called ameboids.

Paramecium is a single cell, and is built in such a way that can perform all vital activities, like reproduction, locomotion, digestion, and so on.

The White Blood Corpuscle (WBC) is the only animal cell that changes its shape. WBC’s are responsible for protecting our bodies against invading bacteria and harmful micro-organisms. To find invading bacteria, they travel along with blood in blood vessels. Whenever they find invading bacteria, they squeeze through the blood vessels and intercellular spaces, catch the bacteria and kill it. To perform this task, WBC’s change their shape using pseudopodia similar to those in an amoeba.

The branched structure of a neuron helps it transfer messages to all parts of the body. A neuron, also known as a nerve cell, is an electrically excitable cell that processes and transmits information by electrical and chemical signalling.

Structure and Function of Cells

The black layer or scab (a hard coating on the skin formed during the wound healing reconstruction phase) that you see on a wound is a result of the aggregation of dead Red Blood Cells (RBC’s).

The human respiratory system consists of the nostrils, nasal passage, internal nares, pharynx, larynx, trachea, bronchi, bronchioles and alveoli. Alveoli are the functional units of the lungs.

All organ systems work together to form a complex organism. Organ systems include the digestive system, respiratory system, circulatory system, excretory system and nervous system.

Cytoplasm is a thick liquid in a cell that holds the organelles, except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm. Within the cells of eukaryotes organisms, the contents of the nucleus are separated from the cytoplasm.

The cell membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It consists of the phospholipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes, such as cell adhesion, ion conductivity and cell signalling, and serve as the attachment surface for extracellular material and intercellular cytoskeleton.

The nucleus controls the activities of a cell. The nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organised as multiple long linear DNA molecules to form chromosomes. The genes within these chromosomes are the cell's nuclear genome. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression. The nucleus is, therefore, the control centre of the cell.

Methylene blue is the stain placed on plant and animal cells to differentiate the nucleus under a microscope. Solutions of this substance are blue when in an oxidising environment, but will turn colourless if exposed to a reducing agent.

Chromosomes are called the vehicles of heredity because they carry genes, and help in the inheritance or transfer of characteristics from the parents to the offspring. A chromosome is an organised structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.

Bacteria and blue-green algae that contain nucleoids are called prokaryotes.

Organisms that have a well-organised nucleus are called eukaryotes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu, meaning "good", and karyon, meaning "nut" or "kernel". Most eukaryotic cells also contain other membrane-bound organelles.

The nucleoid is an irregularly-shaped region within the cell of prokaryotes, which has nuclear material without a nuclear membrane.

The nucleolus is a small, spherical body in the nucleus. It is a non-membrane bound structure composed of proteins and nucleic acids. Ribosomal RNA is transcribed and assembled within the nucleolus.

Chloroplast is a type of plastid involved in photosynthesis in plants. These are specialised organelles found in all higher plant cells. These organelles contain the plant cell's chlorophyll, providing the green colour. They have a double outer membrane. Within the stroma are other membrane structures, the thylakoids and grana where photosynthesis takes place.

A vacuole is a membrane-bound organelle present in all plant, fungal cells, and some animal and bacterial cells. Vacuoles are essentially enclosed compartments filled with water containing inorganic and organic molecules, including enzymes, in solution. In certain cases, though, vacuoles may contain solids that have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles, and are effectively just larger forms of these. The organelle has no basic shape or size, and its structure varies according to the needs of the cell.

Cells Structure QA

Amend Education Academy 9999908238

Question 1. Indicate whether the following statements are True (T) or False (F).

1. Unicellular organisms have one-celled body. (T/F)
2. Muscle cells are branched. (T/F)
3. The basic living unit of an organism is an organ. (T/F)
4. Amoeba has irregular shape. (T/F)

Answer.

1. Unicellular organisms have one-celled body. (True)
2. Muscle cells are branched. (False)
3. The basic living unit of an organism is an organ. (True)
4. Amoeba has irregular shape. (True)

Question 2. Make a sketch of the human nerve cell. What function do nerve cells perform?.
Answer: Human nerve cell, also known as neuron, are branched like structure. A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are thin structures that gives rise to a complex "dendritic tree". An axon is a special cellular extension that arises from the cell body at a site called the axon hillock. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates.

Functions : The nerve cell or neuron processes and transmits information through electrical and chemical signals, thereby helping to control and coordinate the working of different parts of the body.  Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia and play a vital role in  Sensing   touch, sound, light and other through  sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and affect glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.

Question 3. Write short notes on the following:

1. Cytoplasm
2. Nucleus of a cell

Answer.

1. Cytoplasm : Cytoplasm is a jelly-like substance between the nucleus and the cell membrane. Cytoplasm, along with membrane and nucleus, is one of the basic components of a cell. The cytoplasm has three major elements; the cytosol, organelles and inclusions. Cytosol makes up about 70% of the cell volume and is composed of water, salts and organic molecules. Organelles which are also called "little organs", are usually membrane-boundand and includes mitochondria, golgi bodies, ribosomes etc. The inclusions are small particles of insoluble substances suspended in the cytosol
2. Nucleus of a cell : It is an important component of the living cell. It is generally spherical and located in the centre of the cell. It can be stained and seen easily with the help of a microscope. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus. In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.Nucleus, in addition to its role in inheritance, acts as control centre of the activities of the cell.

Question 4. Which part of the cell contains organelles?
Answer: Cytoplasm, which is a jelly-like substance between the nucleus and the cell membrane, contains organelles.Organelles which are also called "little organs", are usually membrane-boundand and includes mitochondria, golgi bodies, ribosomes etc.
Question 5. Make sketches of animal and plant cells. State three differences between them.
Answer.

 

Features	Animal Cell	Plant Cell
Cell wall:	Absent	Present (formed of cellulose)
Shape:	Round (irregular shape)	Rectangular (fixed shape)
Vacuole:	One or more small vacuoles (much smaller than plant cells)	One, large central vacuole taking up 90% of cell volume.
Chloroplast:	Animal cells don't have chloroplasts	Plant cells have chloroplasts because they make their own food
Plastids:	Absent	Present
Plasma Membrane:	only cell membrane	cell wall and a cell membrane
Lysosomes:	Lysosomes occur in cytoplasm.	Lysosomes usually not evident
Cilia:	Present	It is very rare

Question 6. State the difference between eukaryotes and prokaryotes.
Answer. Eukaryotes ( eu + karyon meaning true nucleus ) are made up of eukaryotic cells, which,like onion cells and cheek cells have a well organised nucleus with a nuclear membrane . . All organisms other than bacteria and blue green algae are called eukaryotes. Where as Prokaryotes (pro + karyon - primitive nucleus) are made up of prokaryotic cells, in which nuclear material exists without nuclear membrane. The organisms with these kinds of cells are called prokaryotes. These are of type primitive nucleus. Examples are bacteria and blue green algae.
Question 7. Where are chromosomes found in a cell? State their function.
Answer : Chromosomes are found in nucleus.They are thread-like structures carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.
Question 8. ‘Cells are the basic structural units of living organisms’. Explain.
Answer :Every living organism has many organs, which performs different functions such as digestion, assimilation and absorption. Different organs of perform specific functions. For example, roots help in the absorption of water and minerals.Leaves, are responsible for synthesis of food. Each organ is further made up of tissues which in turn, are made up of cells. This is for the reason, why in in a living organism, cell is the basic structural unit.
Question 9. Explain why chloroplasts are found only in plant cells?
Answer : The chloroplasts are found only in plant cells, because, chloroplasts contain green pigment called chlorophy, which is essential for photosynthesis in plants leaves for making food. As we know there are several small coloured bodies in the cytoplasm of the cells of plant leaf. They are scattered in the cytoplasm of the leaf cells. These are called plastids. They are of different colours. Some of them contain green pigment called chlorophyll. Green coloured plastids are called chloroplasts.
Question 10. Complete the crossword with the help of clues given below: 

 

Across 1. This is necessary for photosynthesis. 3. Term for component present in the cytoplasm. 6. The living substance in the cell. 8. Units of inheritance present on the chromosomes. Down 1. Green plastids. 2. Formed by collection of tissues. 4. It separates the contents of the cell from the surrounding medium. 5. Empty structure in the cytoplasm. 7. A group of cells.

1.C - - 2.O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3.O - - - - - - - - - - - - - - - - - - - 4.M - - - - - - - - 5.V - - - 6.P - - 7.T - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 8.G - - - -

Answer :

Across 1. This is necessary for photosynthesis. 3. Term for component present in the cytoplasm. 6. The living substance in the cell. 8. Units of inheritance present on the chromosomes. Down 1. Green plastids. 2. Formed by collection of tissues. 4. It separates the contents of the cell from the surrounding medium. 5. Empty structure in the cytoplasm. 7. A group of cells.

C H L O R O P H Y L L H - - R - - - - - - - L - - G - - - - - - - O R G A N E L L E S - R - - N - - - - - M - O - - - - - - V - E - P R O T O P L A S M - L - - I - - - C - B - A - - S - - - U - R - S - - S - - - O - A - T - - U - - - L - N - - - - E - - G E N E S

Things to remember...

1. All organisms are made of smaller parts called organs.
2. Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
3. Cells were first observed in cork by Robert Hooke in 1665.
4. Cells exhibit variety of shapes and sizes. Number of cells also varies from organism to organism.
5. Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.
6. Some organisms are single-celled, while others contain large number of cells.
7. The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
8. The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.
9. Nucleus is separated from cytoplasm by a nuclear membrane.
10. Cells without well organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.
11. Plant cells differ from animal cells in having an additional layer around the cell membrane termed cell wall.
12. Coloured bodies called plastids are found in the plant cells only. Green plastids containing chlorophyll are called chloroplasts.
13. Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.