Revision Notes on Cell – Structure and Functions

A Cell (from Latin cella, meaning "Small Room") is the basic Structural, Functional, and Biological unit of all known Living Organisms. A cell is the Smallest Unit of life that can replicate independently, and cells are often called the "Building Blocks of Life".

Discovery of the Cell

Figure1: Robert Hooke discovered cells in 1665.

Hooke saw honeycomb-like structure when he observed thin slices of cork under a microscope. He noticed that each box was separated from the other by a partition or a wall. He was the one to name each of these boxes as cells.

*Cork is a part of the bark of a tree.

Scientists could study cells of living organisms only when improved microscopes with high magnification were invented – 150 years after the Hooke first observed the cells. In 1830s, two German scientists called Matthias Schleiden and Theodor Schwann proposed the Cell Theory which stated:

• All living organisms are made of cells.

• Cells are basic structural or functional units of living organisms.

• All cells are born out of pre-existing cells through cell division.

The Cell

While cells are often referred to as 'bricks' making up a building, cells are complex living structures that can have a variety of shapes, sizes and functions.

Did you know?

Figure 2: An egg of a hen is a single cell which can be seen by the naked eye.

Organisms Show Variety in Cell Number, Shape and Size

To study cells, scientists use microscopes to magnify them. They often use stains (dyes) to colour parts of the cell and study them in detail.

Number of Cells

There are billions and trillions of cells in a tall tree or a large animal. A human body also has trillions of cells which have different shapes, sizes and functions.

Organisms can be of two types:

• Multicellular Organisms are made up of more than one cell. For Example, Mango Tree and Frog.

• Unicellular Organisms are made up of a single cell. For Example, Amoeba and Paramecium.

Did you know?

Multicellular organisms with billions of cells also start their life as a single cell. The fertilised egg (which is a single cell) multiplies to form more cells as the organism develops.

Single-celled organisms also perform all the necessary life functions like multicellular organisms, including:

• ingestion and digestion of food,

• respiration,

• excretion,

• reproduction, and

• growth.

The only difference is that while a single cell performs all the functions in the unicellular organisms, multicellular organisms have a specialised group of cells to perform different functions. The specialised cells form tissues, which in turn form organs.

Shape of Cells

Amoeba, which is a single-celled organism, does not have a definite shape. Its shape keeps changing as it moves or feeds, protruding parts of its body to form 'pseudo' (meaning 'false') 'podia' (meaning 'feet'). These projects are known as pseudopodia.

Humans have millions of cells, such as white blood cells (found in blood), cells that make up muscles, and cells that form nerves. Their shapes vary according to the functions they perform.

Most cells are round, spherical or elongated. Some are spindle-shaped which are long and pointed at both ends. Nerve cells or neurons are quite long and are branched out to receive and transfer messages.

Figure 3: Different shapes of human cells

Shapes of the cells are maintained by a covering called Cell Membrane or Plasma Membrane.

Bacterial cells and plant cells have an additional rigid covering called a Cell Wall.

Size of Cells

The size of the cells may vary from a millionth of a metre (known as a micron) to a few centimetres but most of them cannot be seen with the naked eye. We need microscopes to see these microscopic cells.

It is not necessary that the size of cells of a large animal (such as an elephant) will be larger than cells of a small animal (such as a rat). The size of the cell depends on the function it performs. For example, nerve cells of a rat and nerve cells of an elephant perform the same functions and hence, are of the same size.

Cell Structure and Function

In a unicellular organism, a single cell performs all the basic functions of life but in multicellular organisms there is division of labour.

Figure 4: Parts of cells

To observe the basic components of a plant cell:

• Peel an onion.

• Place a small piece of the dry and thin onion peel in a drop of water on a glass slide.

• Put a drop of methylene blue solution on this thin layer and place a coverslip on it (while ensuring that no air bubbles get trapped in the coverslip).

• Place the slide under the microscope.

Figure 5: Onion peel cells under microscope

You will see:

• The boundary of the onion cell is a cell membrane, which is covered by another thick covering called the cell wall.

• The dense round body in the centre of the cell is called the nucleus.

• The jelly-like substance between the cell membrane and the nucleus is called the cytoplasm.

To observe the basic components of an animal cell:

• Scrape the inside of your cheek lightly with a clean toothpick.

• Place it in a drop of water on a glass slide.

• Add a drop of iodine solution or methylene blue solution and place a coverslip on it.

• Place the slide under the microscope.

Figure 6: Cheek cells

You will see:

• Cell membrane (Cell wall is absent in animal cells),

• Nucleus, and

• Cytoplasm.

There are some other organelles (or components of cells) too. These include mitochondria, ribosomes, and golgi bodies.

Organelles found in both Plant and Animal Cells are:

1. Endoplasmic Reticulum

Figure 7:  Endoplasmic reticulum- SER and RER

Endoplasmic Reticulum (ER) is a network of canals made up of membranes that encloses a fluid-filled lumen. It is of two types:

• Rough Endoplasmic Reticulum (RER): It is lined with ribosomes and hence, look rough.

  • Function of RER: It plays a key role in synthesis of protein as ribosome are attached to it.

• Smooth Endoplasmic Reticulum (SER): It does not have any ribosomes and hence, look smooth.

  • Function of SER: It plays a key role in synthesis of lipids.

2. Ribosomes

Figure 8: Ribosomes- Protein Factory of cells

Ribosomes are spherical bodies made up of RNA (ribonucleic acid) and protein enzyme. They do not have membranes and are present separately in cytoplasm.

Function of Ribosome: Ribosomes are the sites where the protein synthesis takes place.

3. Golgi Bodies

Figure 9: Golgi Apparatus

Smooth, flattened sac-like structures called Cisternae stack together in parallel rows to form Golgi bodies.  Golgi Cis face receives protein from Endoplasmic reticulum and modifies, packages and stores it.

It also dispatches proteins in vesicles to various destinations.

Function of Golgi bodies: The Golgi apparatus are responsible for the secretion of enzymes, hormones and proteins.

4. Mitochondria

Figure 10: Mitochondria

Mitochondria are rod-shaped organelles with a double membrane. The outer membrane is smooth while the inner membrane folds over many times to form cristae. Cristae increase the surface area of the inner membrane by several times. Matrix is  the fluid inside the mitochondria.

Function of Mitochondria: Mitochondria act as energy production sites and are hence, known as the Powerhouses of the Cell.

Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)

5. Lysosome

Figure 11: Lysosome - A single membrane organelle

Lysosomes are sac-like structures surrounded by single-layered membranes. They contain powerful digestive enzymes that can break down all organic material. It acts as a mini digestive system within the cell.

Function of Lysosomes: Lysosomes digest damaged cells and a variety of extra- and intra-cellular material. Since they remove cell organelles that are worn out or are not functioning properly and may even digest the entire cells (that are damaged or dead), they are also known as Suicidal Bags.

6. Vacuoles

Figure 12: Vacuoles

Vacuoles are organelles enclosed by a membrane and filled with fluid. Plant cells usually have a large vacuole filled with a liquid called ‘cell sap’. Cell Sap contains dissolved sugar and salts.

Animal cells may or may not have vacuoles. If they do have vacuoles, they are much smaller than the ones found in plant cells.

Function of Vacuoles in Plant Cells: Vacuoles in plant cells keep the cells firm or turgid. They store various substances (including waste products of the cell).

Function of Vacuoles in Animal Cells: Vacuoles in animal cells store food, water, sugar, minerals and waste products of the cell. In Amoeba, vacuoles that contain food particles are also referred to as food vacuoles.

7. Plastids

Plastids are also type of organelles that are found only in Plant Cells.

Figure 13: Types of Plastids

With double-layered membrane, these organelles are found in cytoplasm of the cells. They are of two types (depending on the colour of the pigment they contain);

• Leucoplasts: These colourless organelles store starch or other plant nutrients. For Example, Starch stores in potato cells. Leucoplast are of different types :

  • Amyloplast: stores starch

  • Elaioplast: stores fat

  • Proteinoplast/ Aleuroplast: stores protein

• Chromoplasts: These contain different-coloured pigments. Most important type of chromoplasts is chloroplast which contain green-coloured pigments called Chlorophyll.

Function of Chloroplast: Chloroplasts are the sites where photosynthesis takes place. Here, carbon dioxide and water combine in the presence of energy from the sunlight to produce food. Hence, chloroplasts help in synthesis of food by green plants.

CO₂ + H₂O → Glucose + Oxygen

(in the presence of chlorophyll + sunlight)

How do molecules move in and out of the cell?

Molecules use two main methods of passive transport to move in or out of the cell:

• Osmosis, and

• Diffusion.

Osmosis

When the solvent moves from the area of dilute solution to the area of concentrated solution through a semipermeable membrane in order to equalize the concentration level of both the solutions, the process is known as ‘Osmosis’.

For Example: Roots absorb water from the soil through osmosis.

Diffusion

When the particles of any material move from a region of higher concentration to that of lower concentration until the equilibrium is reached, the process is known as ‘Diffusion’.

For Example: When you spray perfume, its fragrance spreads in air through diffusion.

Differences between Osmosis and Diffusion

Why do plant cells have cell walls?

A cell wall is the outer thick layer in plant cells that protect the cell membrane. Since plants cannot move from their place, this cell wall serves to protect their cells from the possible damage by temperature variations, high wind speed, atmospheric moisture etc.

Prokaryotic Cells	Eukaryotic Cells
DNA is clumped in an area but there is no organised nucleus with a membrane.	True nucleus is present, which is well-organised and has a nuclear membrane.
Do not have any organelles (except ribosomes)	Usually have organelles like Golgi bodies, lysosomes, endoplasmic reticulum etc.
Smaller in size	Larger in size
Examples: Bacteria and Blue-green algae	Examples: Plant and Animal cells

Comparison of Plant and Animal Cells

Now, we know that both plant and animal cells have:

• Cell Membrane,

• Cytoplasm,

• Nucleus, and

• Nuclear Membrane.

We also know that only plant cell, Bacterial and fungal cells have Cell Walls.

Figure 14: Detailed structure of Plant cell

Figure 15: Detailed Structure of Animal Cell

There are some other differences in plant and animal cells that you need to know:

• Plants have larger vacuoles while animals have smaller vacuoles. Vacuoles store water or food for plants and even store waste products until they can be discarded. Plants need bigger vacuoles as they cannot move to satisfy their hunger or thirst.

• Only plant cells have plastids. They help in manufacturing or storing food.

• Only animal cells have centrioles. Centrioles help in the division of cells.

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