CBSE Class 9 Science The Fundamental Unit of Life Notes

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THE FUNDAMENTAL UNIT OF LIFE

THE FUNDAMENTAL UNIT OF LIFE

CELL

The cell is called the fundamental unit of life. A cell is capable of independent existence and can carry out all the functions necessary for a living being. A cell carries out nutrition, respiration, excretion, transportation and reproduction, the way an individual organism does. Unicellular organisms are capable of independent existence, which shows a cell’s capability to exist independently. Because of this, the cell is called the fundamental and structural unit of life. All living beings are composed of the basic unit of life, the cell.

CELL THEORY (SCHLEIDEN, SCHWANN AND VIRCHOW)

• All living organisms are composed of one or more cells.
• The cell is the basic unit of structure, function and organisation in all organisms.
• All cells come from pre-existing, living cells.

CELL SIZE, SHAPE AND NUMBER

There is much variation in the size, shape and number of cells in different organisms, and also in various parts of the body. Most cells are only a few micrometres in diameter and are visible only with the help of a microscope.

Cells may be spherical, spindle-shaped, elongated, polyhedral or irregular. The shape of a cell is determined by the specific function it performs.

The number of cells is related to the size of the organ or body. Small organisms have a limited number of cells, while larger ones such as an elephant, a whale or a banyan tree have a countless number of cells. Some organisms also have cells of different kinds.

Figure: Various cells from the human body – smooth muscle cell, blood cells, nerve cell, fat cell, bone cell, ovum and sperm.

INTEXT QUESTIONS (PAGE 59)

Who discovered cells, and how?

Answer: Cells were discovered in 1665 by the English botanist Robert Hooke. He used a primitive microscope to observe cells in a cork slice.

Why is the cell called the structural and functional unit of life?

Answer: Cells constitute the various components of plants and animals. A cell is the smallest unit of life and is capable of all living functions. Cells are the building blocks of life, which is why they are called the basic structural and functional units of life. Cells vary in shape, size and the activity they perform; the shape and size of a cell are related to the specific function it performs.

STRUCTURE OF THE CELL

PROTOPLASM

A cell is made of a life-giving substance called protoplasm. Protoplasm is a highly organised, jelly-like, viscous, semi-fluid material composed of molecules of various chemicals, most of which are organic molecules such as proteins, carbohydrates, fats and nucleic acids. Protoplasm is commonly called the “physical basis of life”.

A plant cell consists of a cell wall and a protoplast. The cell wall is absent in animal cells. The protoplast denotes the whole of the protoplasm present in a cell. It is differentiated into the plasma membrane, the nucleus and the cytoplasm.

PLASMA MEMBRANE

The plasma membrane is a semi-permeable membrane composed of a bilayer of lipid and protein. It is the outermost covering of the cell and separates the contents of the cell from its external environment. It allows the entry and exit of some materials into and out of the cell, and prevents the movement of some other materials. Therefore the plasma membrane is called a selectively permeable membrane.

 

Figure: Structure of the plasma membrane. Labels: extracellular space (top); phospholipid; glycolipid; transmembrane protein (or integral protein); glycoprotein; carbohydrate; cholesterol; cytoskeleton and cytoplasm (bottom).

Functions of the plasma membrane

• It regulates the entry and exit of substances into and out of the cell; therefore it is called a selectively permeable (or semi-permeable) membrane.
• It provides an outer boundary to the cell and protects the cell from injury.
• It allows the flow of materials and information between different organelles of the same cell, as well as between adjacent cells.
• It provides some organic connections between adjacent cells.

INTEXT QUESTIONS (PAGE 61)

How do substances like CO₂ and water move in and out of the cell? Discuss.

Answer: The cell membrane is selectively permeable and regulates the movement of substances in and out of the cell.

Movement of CO₂: CO₂ is produced during cellular respiration, so it is present in a high concentration inside the cell and must be removed. Outside the cell its concentration is lower. Therefore, by diffusion, CO₂ moves from the region of higher concentration (inside the cell) to the region of lower concentration (outside the cell). Similarly, O₂ enters the cell by diffusion when its concentration inside the cell is lower than in the surroundings.

Movement of water: water moves from a region of higher water concentration to a region of lower water concentration through the plasma membrane. The plasma membrane acts as a semi-permeable membrane, and this movement of water is called osmosis. The movement of water across the plasma membrane is affected by the amount of substances dissolved in the water.

Why is the plasma membrane called a selectively permeable membrane?

Answer: The plasma membrane regulates the movement of substances in and out of the cell. It allows the entry of only some substances and prevents the movement of some other materials, so it is called a selectively permeable membrane.

CELL WALL

The cell wall is present only in plant cells. It is a rigid protective covering outside the plasma membrane. The presence of the cell wall distinguishes plant cells from animal cells. Most plant cell walls are made of cellulose.

The cell wall consists of three layers: the middle lamella, the primary wall and the secondary wall. The middle lamella is a thin, amorphous, cement-like layer between two adjacent cells. The primary wall is the first-formed wall of the cell, produced inside the middle lamella. The secondary wall is a thick layer found inside the primary wall.

Functions of the cell wall: it gives a definite shape to plant cells; it provides mechanical strength; it protects the protoplasm against injury; it gives rigidity to the cell.

CYTOPLASM

A cell is enclosed in a membranous covering and is filled with a liquid substance called the cytoplasm. The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles, each of which performs a specific function for the cell.

Functions of the cytoplasm: it helps in the distribution of enzymes, nutrients and other biomolecules within the cell; biomolecules such as proteins, nucleotides and fatty acids are synthesised in the cytoplasm.

NUCLEUS

The nucleus is the major central structure in the cell. It is a dense, spherical structure embedded in the cytoplasm. The nucleus has a double-membraned envelope called the nuclear envelope, which encloses a ground substance called the nucleoplasm (karyolymph). The nuclear envelope has many pores called nuclear pores.

 

The nucleoplasm has two types of nuclear structures: (i) the nucleolus and (ii) chromatin. The nucleolus is a spherical body rich in protein and RNA; it is the site of ribosome formation, and there may be one or more nucleoli in the nucleoplasm. The chromatin is a network of fine threads composed of the genetic material DNA (deoxyribonucleic acid) and proteins. During cell division the chromatin condenses into thick, cord-like structures called chromosomes. Chromosomes contain genes, and each gene is responsible for one hereditary character of the organism. Genes contain the information for inheritance of features from parents to the next generation in the form of DNA molecules.

Functions of the nucleus: (i) it controls all the metabolic activities of the cell; (ii) it controls the inheritance of characters from parents to offspring; (iii) it controls cell division.

PROKARYOTES AND EUKARYOTES

Based on the complexity of organisation, especially of the nucleus, cells are classified into two types: prokaryotic cells and eukaryotic cells.

Prokaryotic cells. The cells of bacteria and cyanobacteria (blue-green algae) lack a well-organised nucleus and are called prokaryotic cells. Their DNA is not enclosed by a nuclear membrane. They also lack membrane-bound organelles. Organisms that possess prokaryotic cells are called prokaryotes, and are considered to be primitive organisms.

Eukaryotic cells. The cells of all plants (except bacteria and cyanobacteria) and animals possess a well-organised nucleus and are called eukaryotic cells. Their genetic material is enclosed by a nuclear membrane. They possess membrane-bound organelles such as the endoplasmic reticulum, Golgi body, mitochondria, plastids and vacuoles. Organisms that possess eukaryotic cells are called eukaryotes.

INTEXT QUESTIONS (PAGE 63)

Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.

Answer: The completed table:

CELL ORGANELLES

A cell performs a variety of functions, such as (i) synthesis of complex molecules and their breakdown, (ii) production of energy, and (iii) secretion of certain substances. These activities are performed by different cell organelles, which are enclosed by membranes.

ENDOPLASMIC RETICULUM

The endoplasmic reticulum (ER) is a complicated, interconnected system of membrane-bound channels and tubules. It is spread throughout the cytoplasm and is continuous with the plasma membrane and the nuclear membrane. There are two types of ER: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough (granular) ER is found in cells that synthesise proteins. It looks rough because ribosomes remain attached to the membrane of the ER. Smooth (agranular) ER is found in cells that synthesise lipids. Its walls are smooth because ribosomes are not attached to its membrane.

Functions of the ER: it provides a large surface area for the metabolic activities of the cell; the rough ER plays an important role in protein synthesis; the smooth ER is involved in the synthesis of steroids, hormones and lipids.

GOLGI COMPLEX (GOLGI APPARATUS)

The Golgi apparatus was first described by Camillo Golgi. The Golgi complex consists of saucer-like compartments called cisternae, a network of interconnecting tubules, and vesicles and vacuoles at the peripheral regions. In plant cells the Golgi apparatus is referred to as dictyosomes.

Functions of the Golgi complex: it is involved in the formation of lysosomes; it is also responsible for the synthesis of the cell wall and cell membrane.

LYSOSOMES

Lysosomes are small membrane-bound vesicles which contain various types of digestive enzymes. They serve as the intracellular digestive system and are therefore called “digestive bags”. They are produced by the joint activity of the endoplasmic reticulum and the Golgi apparatus. If the membrane of a lysosome ruptures, its enzymes would digest the entire cell and cause the death of the cell, so lysosomes are called “suicide bags”.

Functions of lysosomes: they are involved in the intracellular digestion of food particles ingested by the cell through endocytosis; the lysosomes of WBCs (white blood cells) destroy pathogens and other foreign particles and thus take part in the natural defence of the body.

RIBOSOMES

Ribosomes are small granular structures made up of ribonucleic acids (RNA) and proteins. They occur free in the cytoplasm as well as attached to the outer surface of the rough ER. Each ribosome consists of two subunits, a small subunit and a large subunit. At the time of protein synthesis many ribosomes attach to messenger RNA and form a structure called a polyribosome (polysome).

Function: ribosomes play an important role in protein synthesis, so they are called the “protein factories” of the cell.

VACUOLES

Vacuoles are fluid-filled sacs bound by a single membrane. They are present in plant cells and also in certain protozoans as food vacuoles and contractile vacuoles. In plant cells a major portion of the cell is occupied by vacuoles, which are bound by a definite membrane called the tonoplast. The vacuoles of plants are filled with cell sap containing minerals, sugars, amino acids and dissolved waste products.

Functions of vacuoles: they store and concentrate mineral salts as well as nutrients; they maintain proper osmotic pressure in the cell for its turgidity and absorption of water.

Concept map: Vacuoles

• Vacuoles store nutrients and water; contain small molecules and waste; are made of a membrane (structure similar to lysosome); can be central vacuole or food vacuole.
• Water creates pressure, called turgor pressure, which provides structural support in plant cells; a shriveled cell provides little structural support.
• Central vacuole: allows for cell growth (by absorbing water); stores cell sap; only in plant cells.
• Food vacuole: created through endocytosis.

MITOCHONDRIA

Mitochondria are globular or cylindrical organelles. Each mitochondrion is bound by two membranes: an outer continuous membrane and an inner membrane thrown into folds called cristae. The cristae divide the inner chamber incompletely. The inner chamber is filled with a homogeneous dense material called the matrix. The cristae have pin-headed bodies called F₁ particles (oxysomes), which play an important role in respiration.

The matrix of mitochondria contains the enzymes necessary for the oxidation of food during respiration and the release of energy in the form of ATP molecules. Therefore mitochondria are called the “power houses” of the cell. Mitochondria contain proteins, lipids and a small amount of DNA.

Functions of mitochondria: they synthesise energy-rich compounds such as ATP; they provide important intermediates for the synthesis of several biochemicals such as chlorophyll, cytochromes, steroids and amino acids.

PLASTIDS

Plastids are disc-shaped or oval organelles which occur in plant cells only. There are three types: leucoplasts, chromoplasts and chloroplasts.

1. Leucoplasts: colourless plastids which store food in the form of starch, lipids and proteins.
2. Chromoplasts: yellow or reddish plastids, coloured by pigments other than chlorophyll. They give colour to many flowers and fruits.
3. Chloroplasts: green plastids which possess the photosynthetic pigment chlorophyll.

Each chloroplast consists of a double-membraned envelope and a matrix. The inner membrane is arranged along the length of the plastid as lamellae. At certain regions the lamellae are thickened and look like a pile of coins; these are called grana. Each granum consists of disc-shaped membranous sacs called thylakoids, inside which the chlorophyll is located. The non-thylakoid portion of the matrix is called the stroma; it contains a number of enzymes involved in photosynthesis.

Functions of plastids: leucoplasts store food such as carbohydrates, proteins and lipids; chromoplasts impart various colours to plant parts; chloroplasts make a leaf green and are the site of photosynthesis.

CENTROSOME

The centrosome is present in animal cells and in certain lower plants. It is absent in prokaryotic cells and in higher plant cells. It is located near one pole of the nucleus and contains a pair of small, hollow granules called centrioles.

Function of centrioles: centrioles play an important role in the formation of spindle fibres during cell division.

SUMMARY: HISTORY OF DISCOVERY OF CELLS

Structure of the cell (concept map)

• Cells
  • Kinds: animal cells, plant cells
  • Parts: endoplasmic reticulum (rough ER, smooth ER); nucleus; mitochondria; cell membrane; ribosome; Golgi apparatus; cell wall (only in plant cells); chloroplast (only in plant cells)

• Robert Hooke was the first to discover cells (1665).
• Leeuwenhoek was the first to discover free-living cells in pond water (1674).
• Robert Brown discovered the nucleus (1831).
• Purkinje coined the term “protoplasm” (1839).
• Schleiden (1838) and Schwann (1839) proposed the cell theory. Virchow (1855) made a further addition to the cell theory.
• The discovery of the electron microscope (1940) made it possible to study the structures of cell organelles.

What do you know about the cells?

INTEXT QUESTIONS (CELL ORGANELLES)

Can you name the two organelles we have studied that contain their own genetic material?

Answer: Mitochondria and plastids. Both these organelles have their own DNA and ribosomes.

If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

Answer: The cell is the smallest unit of life, capable of all living functions. If its organisation is destroyed, the ability of the cell to perform all living functions such as respiration, nutrition and excretion would be affected.

Why are lysosomes known as suicide bags?

Answer: Lysosomes are membrane-bound vesicles that contain powerful digestive enzymes, which can break down any foreign food particle or microbe entering the cell. Sometimes lysosomes can cause the self-destruction of a cell by releasing these digestive enzymes within the cell. Hence they are known as “suicide bags”.

Where are proteins synthesised inside the cell?

Answer: Ribosomes are the site of protein synthesis. They are very small structures found either free in the cytoplasm or attached to the surface of the endoplasmic reticulum (the rough ER), and are composed of ribonucleic acids and proteins.

EXERCISE QUESTIONS (PAGES 66–67)

Make a comparison and write down ways in which plant cells are different from animal cells.

Answer: Differences between a plant cell and an animal cell:

How is a prokaryotic cell different from a eukaryotic cell?

Answer: Differences between a prokaryotic cell and a eukaryotic cell:

What would happen if the plasma membrane ruptures or breaks down?

Answer: If the plasma membrane ruptures, the cell will die. The plasma membrane regulates the movement of substances in and out of the cell by diffusion or osmosis, so if it ruptures the cell will leak out its contents.

What would happen to the life of a cell if there was no Golgi apparatus?

Answer: If there was no Golgi apparatus, most of its activities would not take place:

1. The membranes of the Golgi apparatus are often connected to the ER membranes. It collects simpler molecules and combines them to make more complex molecules, which are packaged in small vesicles and either stored in the cell or sent out as required. Without the Golgi apparatus, this storage, modification and packaging of products would not be possible.
2. The formation of complex sugars from simple sugars would not be possible, because this takes place with the help of enzymes present in the Golgi bodies.
3. The Golgi apparatus is involved in the formation of lysosomes or peroxisomes. Thus, if the Golgi body is absent in a cell, the synthesis of lysosomes or peroxisomes will not be possible in the cell.

Which organelle is known as the powerhouse of the cell? Why?

Answer: Mitochondria. Mitochondria create energy for the cell through cellular respiration; most of the chemical reactions of cellular respiration occur in the mitochondria. The energy required for the various chemical activities needed for life is released by the mitochondria in the form of ATP (adenosine triphosphate) molecules. For this reason mitochondria are known as the powerhouse of the cell.

Where do the lipids and proteins constituting the cell membrane get synthesised?

Answer: Lipids and proteins of the cell membrane are synthesised in the endoplasmic reticulum. The smooth ER (SER) manufactures lipids. The rough ER (RER) has ribosomes attached to its surface, and these ribosomes are the site of protein synthesis.

How does an Amoeba obtain its food?

Answer: An Amoeba obtains its food by endocytosis. The flexibility of its cell membrane enables the cell to engulf solid food particles and other materials from the external environment, forming a food vacuole in which the food is digested.

Cell engulfing food particle

What is osmosis?

Answer: Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a selectively permeable membrane. It is a special case of diffusion in which the medium is water.

For example, if the medium surrounding a cell has a higher water concentration than the cell, i.e. it is a dilute solution, the cell will gain water by osmosis.

Carry out the following osmosis experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now, (a) keep cup A empty, (b) put one teaspoon of sugar in cup B, (c) put one teaspoon of salt in cup C, (d) put one teaspoon of sugar in the boiled potato cup D. Keep these for two hours. Then observe the four potato cups and answer: (i) Explain why water gathers in the hollowed portion of B and C. (ii) Why is potato A necessary for this experiment? (iii) Explain why water does not gather in the hollowed-out portions of A and D.

Experimental set up (cups A, B with sugar, C with salt, D boiled potato with sugar)

Answer: (i) Water gathers in the hollowed portions of B and C because water enters the potato by osmosis. The medium surrounding the potato cells has a higher water concentration than the cells (the sugar or salt in the cup makes the solution there more concentrated), so water moves in by osmosis and collects in the hollow.

(ii) Potato A acts as the control set-up in the experiment. It shows that no water gathers in the cup when there is no sugar or salt in it, so the water in B and C is due to the sugar and salt.

(iii) Water does not gather in A because the cup is empty (there is no solute to draw water in); it is the control. Water does not gather in D because the potato used is boiled. Boiling denatures the proteins in the cell membrane and disrupts it. Osmosis needs a semi-permeable membrane, which has been destroyed, so osmosis does not occur and water does not enter the boiled potato cup.

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