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Unit 1: The Cell

Learning Outcomes

• Compare the structure of typical animal and

plant cell

• Compare and contrast the structure of

prokaryotic cells with eukaryotic cells

• Outline the structure and function of the

following organelles:

o Nucleus, Endoplasmic Reticulum, Golgi

Apparatus and Mitochondria

• Discuss fluid mosaic model of cell membrane

single complex molecule called

sacculus.

Cell division

In prokaryotes mitosis is missing and

the cell divided by binary fission.

Cell division by mitosis.

Organelles

Organisms possessing prokaryotic

cells are called prokaryotes.

Organism possessing eukaryotic

cells is called eukaryotes.

Origin/

Evaluation

Prokaryotes present primitive stage

of evaluation.

Eukaryotes probably evolved from

prokaryotes.

Flagella

Simple, lacking microtubules

extracellular (not enclosed by cell

surface membrane) 20 nm diameter.

Complex, with 9 + 2 arrangement

of microtubules intracellular

(surrounded by cell surface

membrane) 200 nm diameter

Respiration

Mesosomes in bacteria except

cytoplasmic membrane un blue

green algae.

Mitochondria for aerobic

respiration.

Photosynthesis

No chloroplasts no membrane

stacking

Chloroplasts containing

membranes which are usually

stacked into lamellae or grana.

Nitrogen

fixation

Mainly unicellular Mainly have the ability

Form Mainly unicellular

Mainly multi-cellular (except

Protoctista, many of which are

unicellular)

Cell size Average diameter 0.5 – 20 μm

10 - 100 μm diameter common

commonly 1000-10000 times

volume of prokaryotic cells.

Examples

Prokaryotes include bacteria and

blue green algae (cyanobacteria)

Eukaryotes include all other

unicellular or multi-cellular

organisms such as animals, plants

fungi and Protista.

Comparison of Animal and Plant Cell

Figure 1.2 Differences between plant and animal cell

The figure above shows the features and differences between plant and animal cells

Animal Cell

1. Usually smaller in size.
2. Enclosed by plasma membrane

only.

3. Plastids absent.
4. Cytoplasm consists largely of

smaller vacuole.

5. Prominent and highly complex

Golgi bodies.

6. Possess centrosome with one

of two centrioles.

Plant Cell

1. Comparatively larger in size.
2. In addition to plasma membrane

mostly surrounded by a thick cell

wall.

3. Plastids (chloroplast and

chromoplast) are very common.

4. Cytoplasm peripheral, central

space occupied by a large vacuole.

5. Contain several subunits of Golgi

bodies called dictyosomes.

6. No centrioles present, instead two

small clear area called polar caps

are present.

Chemical Composition

Cell membrane contains phospholipids 20 - 40% proteins, 60 - 80% cholesterol and

polysaccharides.

• It is about 7 nm thick
• The basic structure is a phospholipid bilayer.
• The hydrophilic phosphate heads of the phospholipids face outwards. The result is the

aqueous environment inside and outside the cell.

• The hydrocarbon tails face inwards and create a hydrophobic interior.
• Most protein molecules float about in the phospholipid bilayer forming a fluid mosaic pattern.
• The proteins stay in the membrane because they have regions of hydrophobic amino acids

which interact with the fatty acid tails to exclude water.

• Some proteins and lipids have short branching carbohydrate chains like antennae forming

glycoproteins and glycolipids respectively.

• Membrane also contains cholesterol. Like unsaturated fatty acids cholesterol disturbs the

close packing of phospholipids and keeps them more fluid. This can be important for

organisms living at low temperatures when membranes can solidify. Cholesterol also

increases flexibility and stability of membranes, without it membrane break up.

• The two sides of a membrane differ in composition and function.

Functions of Membrane

• The phospholipid bilayer provides the basic structure of membrane. It also restricts entry and

exit of polar molecules and ions.

• Channel protein and carrier proteins are involved in the selective transport of polar

molecules and ions across the membrane.

Fluid Mosaic Model According to the fluid mosaic model the cell membrane consists of a double

layer of phospholipid molecules, known as a lipid bi-layers. It has proteins and other molecules.

The name fluid mosaic is used because the bi-layer is a very fluid structure and it contains a mosaic

of protein molecules.

Figure 1. 5 Fluid Mosaic Model

Transport Across The Cell Membrane

Diffusion

Diffusion is the movement of molecules

or ions from a region of their high

concentration to a region of their low

concentration. The process is passive

(does not require energy and happens

spontaneously).

Two factors affect the rate of diffusion

a) Difference in concentration between

point A and point B. The steeper the

gradient, the faster the rate of diffusion.

b) The greater the surface area of a

membrane through which diffusion is

taking place the greater the rate of

diffusion.

Facilitated Diffusion

Some substances enter and leave cells

much faster than you would expect it

only diffusion occurred. We now know

that some membrane proteins facilitate

the diffusion of some substances across

the cell membrane.

Two types of proteins are responsible

for facilitated diffusion.

• Specific carrier protein takes

particular substance from the

membrane to the other.

• Ion channels are proteins that

open and close to control the

passage of selected charged

articles.

• Channel proteins have a fixed

shape

Point to Ponder

Remember, diffusion

and passive

transport

are the

two names of the

same (^) process

• Some proteins act as receptor molecules for chemical signaling between

cells.

• Some proteins act as an antigen these act as cell identity markers.
• Glycolipids and glycoproteins help cells to recognize each other – allowing

the immune system to tell the difference between body cells and invading

bacteria.

• Energy transfer ion photosynthesis and respiration proteins take part in the

energy transfer system.

The Nucleus

§ The nucleus is the largest and most prominent organelles in the animal cell.

§ Almost all eukaryote cells have a nucleus – red blood cells in mammals and phloem cells

in plants are exception.

§ Every nucleus is surrounded by a nuclear envelope. This consists of two membranes that

are separated by gap of 20 to 40 nm.

§ Nucleus may be irregular in shape and about 10 μm in diameter. It contains the cell’s DNA,

which carries information that allows the cell to divide and carry out all its cellular

processes.

§ Nucleus controls the life and activities of the cell.

§ In animal cells nucleus is generally present in the central part. In plant cells nucleus is

pushed towards periphery due to a large central vacuole.

§ DNA/RNA and proteins (including

enzymes) forms the chemical

composition of coleus.

§ Nucleus consists of nuclear

membrane, nucleoli, nucleoplasm

and chromosomes.

§ The number of nuclear pores is

highly variable. For example, the

undifferentiated cells (e.g. eggs)

have many pores (about 30000 per

nucleus) while the differentiated

cells (e.g. erythrocytes) have only 3

or 4 pores/nucleus. Each pore has a

definite structure.

Nucleolus

Nucleolus is a darkly stained structure within the nucleus. It has no membrane.

There are two regions of a nucleolus.

The Peripheral Granular Area

It contains the materials from which ribosomal subunits are formed.

The Central Fibrillar Area

It has large molecular weight RNA and rDNA.

• The heredity material is in the form of chromosomes, which controls all the activities of

the cell. Chromosome is formed of DNA and proteins.

• Nucleus is visible when the cell is in non-dividing stage. It contains chromatin network

and soluble sap called nucleoplasm. Dark staining chromatin, consisting of tightly packed

DNA, is known as Heterochromatin. The lighter, more loosely packed material is called

Euchromatin contains the DNA that is being actively read to produce proteins. In

heterochromatin, the DNA is packed together and is not being read. Nucleus is stained

with the basic dyes because of the chromatin material. During cell division chromatin

material is converted into darkly stained thread like structures called chromosome.

Chromosome is made of arms and centromeres.

• Centromere is the place on the chromosome where spindle fibers are attached during cell

division.

o Each chromosome consists of two identical chromatids at the beginning of cell

division which are held together at centromere.

• A chromatid is exact replica of the chromosome.
• The information to control cell activities is present on the chromosomes in the form of

genes.

• The chromosome number varies which may be 2n = 2 to 2n = 1200 (Pteridophytes).
• The number of chromosomes in all individuals of the same species remains constant

generation after generation.

• Some examples of chromosome No. are: Frog = 26, Chimpanzee = 48. Fruit fly (Drosophila

melanogaster) = 8, Wheat = 42, Onion = 16, Potato. = 48. Garden pea = 14. Penicillium (a

fungus) has two chromosomes (one pair), corn 20, wheat 42, sugarcane 80, some ferns

have more than 500 pairs, mosquito 6, fruit fly 8, frog 26, honey bee 32, mouse 40 and

human cells have 46 chromosomes (23 pairs).

• The number of chromosomes in normal body cells is diploid (2n) while the gem cells

(sperms and eggs) have haploid chromosome number (11).

Endoplasmic reticulum (ER)

The nuclear envelope joins with the membrane of the endoplasmic reticulum (ER). It is a system

of complex network spread throughout the cell.

These are present in Eukaryotic cells & are of two types:

1) RER

2) SER

• On the outside surface of the RER are ribosomes.
• The main function of RER is to keep together and transport the proteins made on the

ribosomes.

• Instead of simply diffusing away into the cytoplasm, newly made proteins enter into RER.

Here they fold into three-dimensional shape.

• A mature cell that makes and secretes large amounts of protein – such as one that makes

digestive enzymes – has RER that occupies as much as 90 per cent of the total volume of

the cytoplasm.

• RER is also a storage unit for enzymes and other proteins.

Do you know?

Germ cells have n

number of chromosomes

while somatic cells have

2n number of

chromosomes

Examples

Human germ cells (eggs and sperms) = 23

chromosomes.

Drosophila germ cells = 4 chromosomes.

§ Generally, proteins that are to be used inside the cell are made on free ribosome while those

that are to be secreted out of the cell are made on ribosomes that are bound to ER

membranes.

§ New ribosomes are formed in the nucleolus.

§ Eukaryotic ribosome is 80 S (60 S + 40 S). Mg

++ controls this

attachment.

§ A group of ribosomes attached to the same mRNA are called

polysomes.

The Golgi Complex

The Golgi apparatus/Golgi complex was discovered by Camillo Golgi

in 1898. It is found in eukaryotic cells.

• Golgi apparatus consists of stacks of flattened membrane
• bound sacs or flattened cavities or vesicles called Cisternae (5 – 8).
• The whole organelle is a shifting, flexible structure; vesicles are constantly being added at one

side and lost from the other. Generally, vesicles fuse with the forming face (the one nearest

to the nucleus) and leave from the maturing face (the one nearest to the cell surface

membrane).

• Golgi complex has proteins, carbohydrates, glycoproteins and some enzymes. The Golgi

complex appears to be involved with the synthesis and modification of proteins. lipids and

carbohydrates.

• Major functions of Golgi complex are formation of conjugated molecules and Secretions.
• Proteins made on the ribosomes attached to ER are packaged into the vesicles by the ER.

Some of the vesicles join with the Golgi complex and the proteins they contain are modified

before they are secreted out of the cell.

• Golgi apparatus + Golgi vesicles = Golgi complex
• Secretions are products formed in the cells on ribosomes and then pass to the outside

through endoplasmic reticulum and Golgi apparatus.

• In plants Golgi apparatus is involved in the synthesis of cell wall.

Figure 1. Golgi Apparatus Figure 1. Ribosomes (Large and small subunits)

Mitochondria

• Mitochondria are important organelles of eukaryotic cells. They manufacture and supply

energy to the cell. Therefore, they are also called powerhouse of the cell.

• Mitochondria are particularly abundant in metabolically active cells, tissues such (is muscle

and tissues involved in active transport.

• The size (0.5 – 1.5 μn wide) and number of mitochondria varies and depend upon the

physiological activity of the cell.

• Under compound microscope mitochondria may be rod shaped, vesicles or filaments. In

Electron Microscope, Mitochondria have two membranes, outer smooth and inner with

aristae.

• The inner surface of aristae in the mitochondrial matrix has small knob like structures called

F 1 particles. These are involved in ATP formation.

• Mitochondrial matrix contains enzymes, coenzymes and organic and inorganic salts.

Mitochondria also contain DNA and ribosomes.

• Mitochondrial matrix helps in metabolic processes like Kreb's cycle, aerobic respiration and

fatty acid metabolism etc. Their main function is to make ATP via the process of aerobic

respiration. ATP diffuses into the cell and provides instant chemical energy.

• Mitochondria have a double membrane; the outer membrane is smooth while the inner one

is folded. This arrangement gives as large internal surface area on which the complex

reactions of aerobic respiration can take place.

Mitochondrion is a self-replicating organelle.

Centrioles

Centrioles are present in animal cells, some microorganisms and lower plants. They are absent

in higher plants.

§ Centrioles are short bundles of filaments, set at right angles to each other. They are found in

a clear area of cytoplasm known as the centrosome.

§ In a cross section, each centriole consists of a cylindrical array of 9 microtubules, each

microtubule has 3 tubules. The tubules are composed of special protein caned tubulin. The

chemical composition and structure is similar to that of cilia and flagella.

Centrioles Mitochondria

A compound microscope has

different magnification powers. The

ocular lenses may he 5 X and 10 X

while objective lenses may be 20 X,

40 X, 100 X etc.

Additional Notes

The magnification power of microscope is determined by

multiplying X values of ocular lens and X value of objective lens.

For example: A microscope with 10 X ocular lens and 40 X

objective lens will have 10 X 40 = 400 X magnifying power.

The source of illumination in compound

microscopes is visible light. In electron

microscope the source of illumination is

a beam of electrons.

In multi-cellular organisms there is a

division of labour. Examples from animals

are:

Muscle cells contract and relax

Nerve cells transmit impulses

Gland cells secrete

Red blood cells carry oxygen

Some stomach cells secrete gastric juice

White blood cells (WBC) produce

antibodies.

Eye cells detect and respond to light.

The resolution of electron microscope is 2

• 4 Angs trom. It is 500 X greater than that

of the compound microscope and 250,

X greater than that of the naked eye.

Examples from animals are:

Xylem cells conduct water and mineral salts from

soil to the aerial parts of the plant

Phloem cells translocate

(^) food Sclerenchymatous cells give support to the plants

Chlorenchymatous cells carry out photosynthesis

Parenchymatous. cells store surplus food and

Meristernatie cells produce new cells for growth

and development of the plant

Due to different functions the cells

have different shapes and sizes.

The function of an organism is due to

activities and interactions of different

cells and cell components.

To study cell parts, modern techniques are used.

Most modern technique is

cell fractionation.

The process of grinding to get a uniform

composition/structure is called

homogenization.

The tissues are taken and are

homogenized by special instruments

(like homogenizers).

Cell size is measured in

micrometer (μm). One μm is

0.000,001 meter or 1 x 10 –

6 of a metre.

The process of separation of different parts of the cell into different

layers (on the basis of their size and weight and density of the

medium) in the centrifuge tube using a centrifuge machine at medium

speed is called density gradient centrifugation.

In most plant cells, cell

membrane is surrounded by

cell wall.

Ostrich's egg is the biggest egg

The electron

microscope has shown

that the cell wall is

formed by three main

layers:

Middle lamella

Primary cell wall

Secondary cell wall

Additional Notes

Secondary cell wall is formed on Inner

surface of primary wall. It is thick and rigid

than primary wall. Chemically it is

composed of inorganic salts, silica, wax,

cutin and lignin etc.

Primary wall is a true wall and develops in

newly growing cells. It is mainly made up

of cellulose with some deposition of pectin

and hemicellulose.

First of all Middle Lamella is formed between

the primary wall s of the neighboring cells. It is

not the true wall and is composed of pectin or

calcium pectate.

The polysaccharides in bacterial cell wall and

cellulose in plant cell wall are carbohydrates.

Cell wall protects the cell from osmotiolysis.

The protoplasm of a eukaryotic cell is

divided into nucleus and cytoplasm.

Cytoplasm is formed by an aqueous ground substance

which contains:

Many cell organelles

Insoluble wastes and storage products (called inclusions)

Cytosol (the soluble part of cytoplasm is called cytosol)

Cytosol is 90 % water and 10

inorganic and organic molecules.

Cytosol has true solutions and colloidal

solutions. The colloidal solution may be sol or

gel. Sol is non-viscous and gel is viscous.

The material present in between

the plasma membrane and the

nuclear membrane is called

cytoplasm.

Cytosol is 90 % water and 10 inorganic and organic molecules

Lysosomes are involved in:

Autophagy/self-eating (Autophagosomes)

Phagocytosis

Extra-cellular digestion and^ autolysis

The peripheral part of the cell is

like a gel.

The process of separation of different parts of the cell into

different layers (on the basis of their size and weight and

density of the medium) in the centrifuge tube using a

centrifuge machine at high speed is called

density gradient

ultracentrifugation.

The free floating cell organelles (e.g.

Mitochondria) move in the cytoplasm

due to cytoplasmic streaming

movements and is called active mass

movement of cytoplasm.

De Duve in 1949 isolated lysosomes. They

are found in most eukaryotic cells

Lysosomes are rich in acid phosphatases and

several other hydrolytic enzymes.

In the primary cell wall the cellulose fibers

are arranged in a criss-cross^ manner which

give strength to the cell wall.

The autophagosomes working on digestive

vacuoles are also known as secondary Lysosomes.

De-Duve and coworkers isolated peroxisomes in

1965 from liver cells and other tissues. These are

single membrane or

ganelles (0.5 μm in diameter).

TAY-SACH'S disease is due to the absence of an

enzyme that is involved in the catabolism of

lipids. Accumulation of lipids in brain cells

leads to mental retardation and even death.

If an enzyme that breaks glycogen

to glucose is absent from

Lysosomes, the result is a disease

Glycogenosis type II.

The living content of the cell is called

protoplasm.

Practice Questions

1. Which of the following are colorless?

a) Chloroplasts c) Leucoplasts

b) Chromoplasts d) None of these

2. Which of the following is most slender in structure?

a) Microtubules c) Intermediate filaments

b) Microfilaments d) Both A and B

3. Soluble part of the cytoplasm is termed as:

a) Cisterna c) Endocytosis

b) Cytosol d) Both A and B

4. Which of the following modifies proteins and lipids by adding carbohydrates?

a) Golgi Apparatus c) Plasma membrane

b) Polysome d) None of these

5. The nucleus contains:

a) Mitochondria b) Cytosol

c) Enzyme d) DNA

6. Another name for Golgi complex is:

a) Dictyosome b) Endoplasmic Reticulum

c) Cyto-membrane system d) None of the above

7. Leucoplasts are a kind of:

a) Lysosomes b) Chloroplasts

c) Plastids d) Grannum

8. Vacuole in plants is responsible for:

a) Photosynthesis b) Cellular excretion

c) Turgor pressure d) Starch storage

9. By which of the following can movements of materials across animal cell

membranes be accomplished?

a) Active transport b) Diffusion

c) Pinocytosis only d) All of the above

10. Cell membranes are composed of:

a) Lipids and proteins b) Phospholipids

c) Proteins and carbohydrates d) Lipids and terpenoids

11. Hydrogen peroxide degradation in a cell is a function of:

a) Peroxisomes b) Ribosomes

c) Mitochondria d) Microbodies

12. Movement of substances from lower concentration to higher concentration

across semi-permeable membrane is termed as:

a) Facilitated diffusion b) Diffusion

c) Passive transport d) Active transport

13. Robert Hook was the first person to see cells in:

a) Various plants b) Fungi

c) Animals d) Cork

14. The concept “OMNIS cellula-e-cellula” means that, new cells are formed from:

a) Non-living material b) Pre-existing living cells

c) Dead organic matter d) As a result of chemical reactions

15. In generalized plant cell the nucleus is:

a) Present in the middle of the cell

b) Displaced to the peripheral site of the cell

c) Absent

d) Modified into endoplasmic reticulum

16. Plasma membrane is found in the cells of:

a) Animals only b) Plants only

c) Both in plants and animals d) Bacteria only

17. The semi-circular channels and system of tubes found in the cytoplasm are

known as:

a) Ribosomes b) Endoplasmic reticulum

c) Glyoxysomes d) Vacuoles

18 ) The structures that are involved in the manufacture and supply of energy to the

cell are:

a) Centrioles b) Nucleolus

c) Plastids d) Mitochondria

19. In a plant cell chlorophyll is present in:

a) Chromoplasts b) Stroma

c) Leucoplasts d) Chloroplasts

20. Ribosomes have equal amount of:

a) DNA and Protein b) mRNA and Protein

c) RNA and Protein d) None

21. Plastids are only found in the:

a) Animals and plants b) Plants

c) Animals d) Viruses

22. The command center of an eukaryotic cell is:

a) Nucleoid b) Small vacuoles

c) Endoplasmic reticulum d) Nucleus

23 ) Lipids synthesis / metabolism takes place in which of the following organelle?

a) Mitochondria

b) Vacuoles

c) Rough endoplasmic reticulum

d) Smooth endoplasmic reticulum