Ch 8 Cell: The Unit of Life

This set covers every single thing one needs to memorize from the chapter. I've also attached images, links wherever required to understand that particular term in a better manner. All the best to whoever uses this set!

What is a cell?

The basic structural and functional unit of all living organisms

Unicellular organisms are capable of:

1. Independent Existence

2. Performing the essential functions of life such as reproduction, metabolism, and response to stimuli.

Who coined the term ‘cell’?

Robert Hooke

Who first saw and described a living cell?

Antonie van Leeuwenhoek

Who discovered the nucleus?

Robert Brown, who observed it in plant cells.

What did Schwann do?

Botanist, he reported that all plants are formed as a product of a large number of different kinds of cells, which form the tissues of the plant.

What did Schwann do?

Zoologist, he reported that all plant and animal cells have a unique boundary which we call the plasma membrane and hence stated that all organisms are composed of cells and a product of cells.

Who formulated the cell theory

Schleiden and Schwann

What could the cell theory not explain?

That how new cells were formed

Who cleared the drawback of the cell theory?

Rudolph Virchow

What did Rudolph Virchow do?

He proposed that all cells arise from pre-existing cells, addressing a key limitation of the original cell theory.

What is the current cell theory understood as?

1. All living organisms are composed of cells and products of cells

2. All cells arise from pre existing cells

Omnis cellula e cellula

“All cells arise from pre existing cells”

Describe the basic structure of an onion cell

1. Distinct cell wall as it’s outer boundary

2. Underlying cell membrane

3. Inside, a dense membrane bound structure called the nucleus

   1. Contains chromosomes which in turn contain genetic material, DNA.

4. A semi-fluid matrix occupying the volume of the cell, and carrying out metabolic reactions

Describe the basic structure of a cheek cell

1. Outer cell membrane as the delimiting structure of the cell

2. Inside, a dense membrane bound structure called the nucleus

   1. Contains chromosomes which in turn contain genetic material, DNA.

3. A semi-fluid matrix occupying the volume of the cell, and carrying out metabolic reactions

What are eukaryotic cells?

1. Cells with a membrane-bound nuclei are called eukaryotic cells

2. Contain membrane-bound distinct structures called organelles

3. More developed in nature

What are prokaryotic cells?

1. Cells in which a membrane-bound nuclei is absent.

2. Does not contain membrane bound organelles

Name some non-membranous organelles

• Ribosomes (plants, animals)

• Centrosome (animals only)

Where are ribosomes found?

• Cytoplasm

• Mitochondria

• Chloroplasts (In plants)

• Rough ER (In animals)

Smallest Cells?

Mycoplasmas (length: 0.3 micrometre)

Length of bacteria?

3-5 micrometre

Largest isolated cells?

Ostrich egg

Diameter of human RBCs?

7 micrometre

Some of the longest cells?

Nerve cells

The shape of the cell varies due to ____________

The function they perform

Identify the cells, describe their shape

Red blood cells

Round and biconcave

Identify the cells, describe their shape

White blood cells

Amoeboid (irregular)

Identify the cells, describe their shape

Columnar epithelial cells

long and narrow

Identify the cells, describe their shape

Nerve cells

branched and long

Identify the cells, describe their shape, where are they present?

• Tracheids

• elongated

• xylem

Identify the cells, describe their shape, where are they present?

• Mesophyll cells

• Round and oval

• Between the upper and lower epidermis of the leaf

Give examples for prokaryotic cells

• Bacteria

• Blue-green algae

• Mycoplasma

• PPLO (Pleuro-pneumonia Like organism)

Four basic shapes of bacteria

1. Bacillus (rod like)

2. Coccus (spherical)

3. Vibrio (comma shaped)

4. Spirillum (spiral)

Features of prokaryotic cells

• generally smaller than eukaryotic

• divide more rapidly

• vary greatly in shape and size

All prokaryotes have a ________ surrounding the cell except __________

Cell wall, mycoplasma

The semi-fluid matrix filling the prokaryotic cell is _______

cytoplasm

Where is the genetic material present in a prokaryotic cell?

The genetic material is not bound by a cell membrane and hence, it is suspended naked in a cytoplasm of a prokaryotic cell.

What is the genomic DNA in a prokaryotic cell?

Single chromosome/circular DNA

How many chromosomes does a prokaryotic cell have?

1, hence single chromosome

What is a single chromosome?

A single chromosome refers to a single, continuous molecule of DNA found in both prokaryotic and eukaryotic cells, but with different structures and arrangements. In prokaryotes, it's typically a single, circular DNA molecule. In eukaryotes, it's a linear DNA molecule packaged with proteins, forming a more complex structure. 

What is circular DNA

Circular DNA refers to DNA that forms a continuous loop, with no free ends. It's a closed structure, unlike linear DNA, which has distinct ends. This type of DNA is commonly found in prokaryotic cells, as well as in the organelles of eukaryotic cells like mitochondria and chloroplasts. 

How do prokaryotic cells have single chromosome/circular DNA

A single chromosome refers to a single, continuous molecule of DNA found in both prokaryotic cells, hence called circular DNA. In prokaryotes, it's typically a single, circular DNA molecule.

Small circular DNA outside the genomic DNA is called _________

Plasmids

What do plasmids confer?

Plasmids confer (carry) phenotypic characters of the bacteria

What are phenotypic characters?

Phenotypic characters in prokaryotes are observable traits that can be used to distinguish and classify them. These traits include morphology (shape and size), growth characteristics, metabolic capabilities, and response to environmental factors. 

Give example of a phenotypic character in prokaryotic cells

Resistance to antibiotics

What is resistance to antibiotics?

It refers to the observable trait of a microorganism that allows it to survive exposure to an antibiotic, which would normally kill or inhibit its growth. This resistance can arise from genetic changes (genotypic) or through non-heritable, metabolic or physiological adaptations (phenotypic). 

https://my.clevelandclinic.org/health/articles/21655-antibiotic-resistance

Uses of plasmids

• Laboratory manipulation of genes

• Plasmids are essential tools for monitoring bacterial transformation, which is the process of introducing foreign DNA into bacteria. They serve as vectors, carrying the desired DNA into the bacterial cell. By incorporating specific marker genes, like antibiotic resistance genes, into the plasmid, scientists can easily select and identify bacteria that have successfully taken up the foreign DNA. 

Which organelles are found in prokaryotic cells?

Only ribosomes

Prokaryotes have something unique in the form of ________

Inclusion bodies

What are inclusion bodies?

Inclusion bodies are nuclear or cytoplasmic aggregates of stable substances like proteins/organic/inorganic compounds. They are used as storage units

A specialised differentiated form of cell membrane present in prokaryotes are called ________

Mesosomes

What are mesosomes?

Mesosomes are inner foldings of the cell membrane found in prokaryotic cells. These foldings:

• increase surface area,

• are sites of secretion,

• respiration,

• DNA replication and

• assist in cell wall formation.

What is cell envelope in prokaryotic cells?

The outer protective covering of the cell is called cell envelope.

https://www.aakash.ac.in/important-concepts/biology/cell-envelopes-and-cell-membrane-invagination

Components of the cell envelope of a prokaryotic cell

Glycocalyx —> Cell Wall —> Cell Membrane

Bacteria can be classified into two groups on the basis of:

• Differences in the cell envelope

• The manner in which they respond to the gram staining procedure

What is gram staining?

Gram staining is a procedure by which bacteria are classified into Gram-positive and gram-negative bacteria

Gram positive: take up the gram stain
Gram negative: do not take up the gram stain

What is the difference between gram positive and gram negative bacteria?

Essentially, gram +ve bacteria take up the gram stain and -ve do not.

https://www.technologynetworks.com/immunology/articles/gram-positive-vs-gram-negative-323007

How does glycocalyx differ in different bacteria?

On the basis of composition and thickness

Glycocalyx in the form of loose sheath is called ________

slime layer

It is a form of a thin glycocalyx layer

When the glycocalyx in some bacteria is thick and tough, it is called _________

Capsule

It is a form of a thick glycocalyx layer

Function of the cell wall

1. Determines the shape of the cell

2. Prevents the bacterium from bursting/collapsing

Explain the structure of mesosomes

• Extensions of the plasma membrane into the cell

• Vesicles, Tubules, Lamellae

Functions of mesosomes (as in NCERT)

1. Cell wall formation

2. DNA replication

3. Distribution of daughter cells

4. Respiration

5. Secretion

6. Increase surface area of the plasma membrane and enzymatic contents

Other membranous extensions in bacteria such as cyanobacteria are called _________

chromatophores

What do chromatophores contain?

Pigments

Structure present on the walls of prokaryotic cells

• Flagella

• Pili

• Fimbriae

Motile bacteria have _______ for mobility

Flagella - thin filamentous extensions from the cell wall

Parts of bacterial flagellum

• Filament (longest portion)

• Hook

• Basal Body

Other surface structures present in bacteria

• Pili

• Fimbriae

Pili

Elongated tubular structures made up of a special protein

Fimbriae

Bristle like fibres sprouting out of the cell

Uses:

• Helping the bacteria attach to rocks in streams

• To host tissues

Ribosomes in prokaryotes

A ribosome is a complex molecular machine composed of two subunits: a large subunit and a small subunit. The complete ribosome is formed when these two subunits come together, and it is this complete structure that is capable of carrying out protein synthesis. The subunits themselves are not functional for protein synthesis until they combine. 

Size of ribosomes in prokaryotes

15-20 nm

Subunits of prokaryotic ribosomes

50S and 30S = 70S complete

Why 50S+30S=70S in prokaryotic ribosomes?

The sedimentation rate of a complete ribosome is always less than the sum of its subunits.

What is S in 50S and 30S?

S is the sedimentation rate, also called the Svedberg unit

Function of ribosomes

Protein synthesis

what is mRNA

Messenger RNA or mRNA is involved in protein synthesis

Several __________ may attach to a single strand of mRNA and form a chain called __________ or ______________

ribosomes, polyribosomes, ribosomes

Function of a polysome

the ribosome of a polysome translate the mRNA into proteins, increase protein production efficiency.

Examples of inclusion bodies

1. Phosphate granules

2. Cyanophycean granules

3. Glycogen granules

4. Gas Vacuoles

Where are gas vacuoles found (prokaryotes)

Blue green and purple and green photosynthetic bacteria

Examples of Eukaryotes

• protists

• plants

• animals

• fungi

Features of eukaryotes (other than those mentioned earlier)

• possess a variety of locomotory and cytoskeletal structures

• their genetic material is organized into chromosomes

________, ________ and ________ are structures/ organelles present in plant cells but not in animal cells

Cell wall

Plastids

Vacuoles

_________ are present in animal cells but not in plant cells

Centrosomes

Composition of the cell membrane

Proteins and lipids

Features of lipids in the cell membrane

• mainly phospholipids

• arranged in a bilayer

• polar head, hydrophobic tail

Why do the polar heads of the lipid bilayer face towards the outside?

• In a phospholipid bilayer, the polar (hydrophilic) heads of the phospholipids face the outer sides (towards the aqueous environment, both inside and outside the cell) because they are attracted to water. Conversely, the nonpolar (hydrophobic) tails are directed inwards, away from the water, forming a barrier. This arrangement is energetically favorable and is the basis for the structure of cell membranes. 

• The polar heads ensure that the nonpolar tail of the saturated hydrocarbons is protected from the aqueous environment

What is the nonpolar tail/ hydrophobic tail made up of?

saturated hydrocarbons

Apart from phospholipids, the cell membrane also contains _______

Cholesterol

Apart from the lipid bilayer, the cell structure is also composed of _________ and ________

proteins and carbohydrates

What are erythrocytes?

Oxygen-carrying RBCs

Ratio of proteins and lipids in erythrocytes

52% protein

40% lipids

Types of membranous proteins (cell membrane)

• Integral Proteins: partially/totally buried b/w the lipid layer

• Peripheral Proteins: located on the surface

The fluid mosaic model

The fluid mosaic model describes the structure of cell membranes as a dynamic combination of lipids, proteins, and carbohydrates that are constantly moving and interacting. It emphasizes the membrane's fluidity and the mosaic-like arrangement of its components. 

What is the nature of lipids in the cell membrane?

Quasi-Fluid

(A quasi-fluid is a material that exhibits characteristics of both a fluid and a solid, meaning it can flow like a fluid under certain conditions while also maintaining some structural rigidity. This "partial" fluidity is due to the material's ability to deform and move while still possessing a degree of structural integrity.)

What does this quasi-fluid nature of lipids enable?

Lateral movement of proteins within the overall lipid bilayer - fluidity

Why is the fluid nature of the cell membrane important

• cell growth

• formation of intercellular junctions

• endocytosis

• cell division

• secretion