A TOUR OF THE CELL

the fundamental unit of life

CELLS WERE FIRST DISCOVERED BY HIM IN 1665

ROBERT HOOKE

ROBERT HOOKE’S MICROSCOPE

ROBERT OBSERVED :

CORK COMES FROM THE BARK OF THE TREE

HONEY COMB

ROBERT CALLED

THESE LITTLE COMPARTMENTS AS CELLS.

CELL IS A LATIN WORD FOR ‘ A LITTLE ROOM’.

COMPOUND MICROSCOPE

ONION PEEL

1)THESE STRUCTURES LOOK SIMILAR TO EACH OTHER.

2) TOGETHER THEY FORM A BIG STRUCTURE LIKE AN ONION BULB.

3) THE CELLS OF AN ONION PEEL WILL ALL LOOK SAME,REGARDLESS OF THE SIZE OF THE ONION.

4) THESE SMALL STRUCTURES ARE THE BASIC BUILDING UNITS OF THE ONION BULB AND ARE CALLED CELLS.

UNICELLULAR ORGANISMS

ORGANISMS THAT HAVE A SINGLE CELL.

AMOEBA

PARAMOECIUM

CHLAMYDOMONAS

MULTICELLULAR ORGANISMS

ORGANISMS HAVING MORE THAN ONE CELL TO PERFORM VARIOUS FUNCTIONS.

ANIMALS

PLANTS

FUNGI

DIFFERENT CELLS

BONE CELL

OVUM

SPERM BLOOD CELL

NERVE CELL FAT CELL

CELL

A cell is the smallest unit that is capable of performing life functions.

CELL

THE SHAPE AND SIZE OF CELLS ARE RELATED TO THE SPECIFIC FUNCTION THEY PERFORM.

EACH LIVING CEL LHAS THE CAPACITY TO PERFORM CERTAIN BASIC FUNCTIONS.

CELL CONTAINS A SPECIFIC COMPONENT CALLED ORGANELLES.

EACH KIND OF CELL ORGANELLE PERFORMS A SPECIAL FUNCTION.

ALL CELLS ARE FOUND TO HAVE THE SAME ORGANELLES,IRRESPECTIVE OF THEIR FUNCTION AND WHICH ORGANISM THEY ARE FOUND IN.

Why are cells of different shapes and sizes found in the human body?

> The shape and size of the cells depend upon the function they perform. In a human body, each organ performs a specific function, hence the cells of different organs have different shapes and sizes.

Who discovered cells, and how?

> Cells were first discovered by Robert Hooke, in1665. He observed a thin slice of cork under his self designed microscope.This slice of cork resembled the structure of the honeycomb consisting of small compartments. He named these small compartments as cells.

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

> Cell is called the structural and functional unit of life because

a) It gives structure to the living organism.

b) It helps the living organism to perform various life functions.

Cell membrane or Plasma membrane

It is the outermost covering of the cell that separates the contents of the cell from the external environment.

Allows the entry and exit of materials in and out of the cell.It also prevents the movement of some other material and is therefore called a selectively permeable membrane.

The plasma membrane is flexible and is made up of organic molecules like proteins and lipids.

DIFFUSION

GASEOUS EXCHANGE TAKES PLACE FROM A REGION OF HIGHER CONCENTRATION TO A REGION OF LOW CONCENTRATION.

OSMOSIS

The movement of water from a region of high water concentration through a semi- permeable membrane to a region of low water concentration.

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

As the cell functions the concentration of CO2 in the cell increases whereas the concentration of CO2 in the external environment is low. CO2 moves out of the cell from a region of higher concentration to the external environment which is the region of lower concentration.

Water moves in and out the cell from a region of higher concentration through a semi- permeable membrane to a region of lower concentration by the process of osmosis.

Difference between

Diffusion Osmosis

1)It can occur in any medium.

2)It does not require a semi-permeable membrane.

1)It occurs only in a liquid medium.

2)It requires a semi-permeable membrane.

What is a hypotonic solution?

Hypotonic solution is the solution surrounding the cell which has higher water concentration than that in the cell.

Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net result is that the water enters the cell and the cell is likely to swell up.

HYPOTONIC SOLUTION

When is a solution said to be isotonic?

When the medium surrounding a cell has the same concentration of water as that in the cell it is said to be isotonic.

Water crosses the cell membrane in both the directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

ISOTONIC SOLUTION

What is a hypertonic solution?

When the medium surrounding a cell has lower concentration of water than that in the cell it is said to be hypertonic.

Water crosses the membrane in both the directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.

HYPERTONIC SOLUTION

What is Plasmolysis?

Plasmolysis is the phenomenon by which a plant cell shrinks away from the cell wall when it loses water by osmosis.

PLASMOLYSIS

What is endocytosis?

Endocytosis is the process by which a cell is able to engulf food and other material from its external environment due to the flexibility of the plasma membrane.

Example : Amoeba acquires its food through such processes.

ENDOCTYOSIS

Plasma membrane is called a selectively permeable membrane. Why?

> Plasma membrane is called a selectively permeable membrane because it allows some materials to enter and leave the cell.

Cell wall

>Hard outer covering of the cell.

>Present outside the cell membrane.

>Cell wall is composed of cellulose.

>Cellulose is a complex substance and provides structural strength to plants,mechanical strength,protection against pathogens

>Cell wall is present in the plant cell and helps the plant to withstand the environmental changes.

Nucleus

>Has a double layered covering called nuclear membrane.

Nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside (cytoplasm).

Contains chromosomes which are visible as rod-shaped structures only when the cell is about to divide.

Chromosomes contain information for inheritance of features from parents to the next generation in the form of DNA molecules. (Deoxyribo Nucleic Acid).

Nucleus

DNA molecules contain information necessary for constructing and organising cells.

Functional segments of DNA are called genes. When the cell is not dividing, DNA is present as a part of the

chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide , the chromatin material gets organised into chromosomes.

Plays an important role in cellular reproduction. It helps in determining the way the cell will develop and what

form it will exhibit on maturity by directing the chemical activites of the cell.

Significance of membranes

Example : Viruses

Viruses lack any membranes and hence do not show any characteristics of life until they enter a living body and use its cell machinery to multiply.

Cytoplasm

• It is the fluid content inside the plasma membrane.

• Contains specialised cell organelles.

Cell organelles

1) Endoplasmic reticulum

2) Golgi apparatus

3) Lysosomes

4) Mitochondria

5) Plastids

6) Vacuoles

Endoplasmic Reticulum

ER is a large network of membrane bound tubes and

sheets.

Extends throughout cytoplasm

Two types - Rough Endoplasmic Reticulum (RER)

Smooth Endoplasmic Reticulum (SER)

Serves as channels for the transport of materials

between various regions of the cytoplasm or between

the cytoplasm and the nucleus.

Functions as a cytoplasmic framework providing a

surface for biochemical activities.

RER

Arranged into flattened sacs Ribosomes on surface give it a rough

appearance Ribosomes are the sites of protein synthesis. Cells that specialize in secreting proteins

have lots of rough ER

SER

A series of interconnected tubules .

No ribosomes on surface. Helps in the manufacture

of fat molecules or lipids. In liver cells of

vertebrates, SER detoxifies many poisons and drugs.

Membrane Biogenesis

It is the process of formation of the cell membrane from proteins and lipids.

GOLGI APPARATUS

First discovered by Camillo Golgi. It consist of a system of membrane- bound vesicles arranged parallel to each other in stacks called cisterns. The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its functions include storage , modification and packaging of products in vesicles.Involved in the formation of the lysosome.

LYSOSOMES

>Waste disposal system of the cell.Keep the cell clean by digesting any foreign material as well as worn out cell organelles. Contain powerful digestive enzymes capable of breaking down all organic material.When the cell gets damaged, lysosomes may break open and the enzymes digest their own cell. Thus known as suicide bags of a cell.

Mitochondria

Known as the powerhouse of the cell. The energy required for various chemical activities is released by the mitochondria in the form of ATP molecules.( Adenosine triphosphate).ATP is known as the energy currency of the cell. Mitochondria have two membrane coverings. Outer membrane is very porous.Inner membrane is deeply folded.Mitochondria are able to make their own proteins as they have their own DNA and ribosomes .

Plastids

Present in plant cells. Two types- 1) Chromoplasts (coloured)

2) Leucoplasts (colourless)

> Plastids containing green pigment chlorophyll are called Chloroplasts.

Leucoplasts store materials as starch, oils and protein granules.

Plastids contain membrane layers embedded in a material called stroma.

Has a similar structure like the mitochondria. Have their own DNA and ribosomes.

CHLOROPLASTS

Vacuoles

Storage sac for solid or liquid contents. Small size vacuoles are present in animals. Large in plants. In plant cells, vacuoles are full of cell sap and provide

turgidity and rigidity to the cell. Vacuoles store amino acids, sugars, various organic acids

and some proteins. In Amoeba, the food vacuole contains the food items that

the Amoeba has consumed. Vacuoles play an important role in expelling excess water

and some wastes from the cell.

Overview of a plant cell

Overview of an animal cell

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

Ans : Mitochondria and plastids are the two organelles that contain their own genetic material. Both these organelles have their own DNA and ribosomes.

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

Animal cell Plant cell

Animal cells are generally small in size.

Plants cells are usually larger than animal cells.

Cell wall is absent. Cell wall is present.

Except the protozoan Euglena, no animal cell possesses plastids.

Plastids (chromoplasts and leucoplasts) are present.

Vacuoles are smaller in size. Vacuoles are larger in size.

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

(i) Membranes of the Golgi apparatus are often connected to ER membranes. It collects simpler molecules and combines them to make more complex molecules. These are then packaged in small vesicles and are either stored in the cell or sent out as per the requirement. Thus, if the Golgi apparatus is absent in the cell, then the above process of storage, modification, and packaging of products will not be possible.

(ii) The formation of complex sugars from simple sugars will not be possible as this takes place with the help of enzymes present in Golgi bodies.

(iii) The Golgi apparatus is involved in the formation of lysosomes. Thus, if the Golgi body is absent in a cell, the synthesis of lysosomes will not be possible in the cell.

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

Mitochondria are known as the powerhouse of cells. Mitochondria create energy for the cell, and this process of creating energy for the cell is known as cellular respiration. Most chemical reactions involved in cellular respiration occur in the mitochondria. The energy required for 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 cells.

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

Lipids and proteins constituting the cell membrane are synthesized in the endoplasmic reticulum.

1)SER (Smooth endoplasmic reticulum) helps in the manufacturing of lipids.

2)RER (Rough endoplasmic reticulum) has particles attached to its surface, called ribosomes. These ribosomes are the site for protein synthesis.

How does an Amoeba obtain its food?

Amoeba obtains its food through the process of endocytosis. The flexibility of the cell membrane enables the cell to engulf the solid particles of food and other materials from its external environment.

What is osmosis?

The movement of water molecules from a region of high concentration to a region of low concentration through a selectively permeable membrane is called osmosis.