Structure of Cell

Love Group

Masters of Sciences in Physical Therapy

​Master of Science in Public Health

Diploma in Pharmacy

Structure of Cell

• Definition:

“Cell is defined as the structural and functional unit of the living body”

Characteristics of cell

• Needs nutrition and oxygen
• Produces its own energy necessary for its growth, repair and other activities
• Eliminates carbon dioxide and other metabolic wastes
• Maintains the medium, i.e. the environment for its survival

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Structure of Cell

• Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane.
• Cell body has two parts, namely nucleus and cytoplasm surrounding the nucleus
• structure of the cell is studied under three headings:
• 1. Cell membrane
• 2. Cytoplasm
• 3. Nucleus.

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Cell Membrane

• Cell membrane is a protective sheath, enveloping the cell body.
• It is also known as plasma membrane or plasmalemma.
• This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF).
• The cell membrane is a semipermeable membrane. So, there is free exchange of certain substances between ECF and ICF.

Composition of Cell Membrane

• 1. Proteins (55%)
• 2. Lipids (40%)
• 3. Carbohydrates (5%).

Structure of Cell Membrane

• It is three layered membrane
• one electron-lucent (central) and two electron dense layer (either side).
• Central layer formed by lipid substances
• Other two layers are protein substances
• Contains some carbohydrate molecule also

Structural model of cell membrane

Danielli-Davson model (James F Danielli and Hugh Davson in 1935)

• This model was basically a ‘sandwich of lipids’ covered by proteins on both sides.

Unit membrane model (1957, JD Robertson)

• on the basis of electron microscope studies
• Lipid bilayer sandwitched between inner and outer layer of protein.

Fluid Mosaic Model (1972, SJ Singer and GL Nicholson)

• Protein molecules embeded (floats) in a lipid bilayer in a mosaic manner.

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Lipid layer of cell membrane

It has two parts

• Phospholipid
• are the lipid substances containing phosphorus and fatty acids
• Phospholipids are soft and oily structures and cholesterol helps to ‘pack’ the phospholipids in the membrane
• Cholestrol
• Cholesterol molecules are arranged in between the phospholipid molecules
• cholesterol is responsible for the structural integrity of lipid layer of the cell membrane

Phospholipids

Head portion

• The outer part of the phospholipid molecule
• Head portion is the polar end and it is soluble in water and has strong affinity for water (hydrophilic)

Tail Portion

• the inner portion of phospholipid molecule
• It is insoluble in water and repelled by water (hydrophobic)

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Lipid Layer

• semipermeable membrane and allows only the fat-soluble substances to pass through it
• oxygen, carbon dioxide and alcohol pass through it

Protein layer

• give protection to the central lipid layer
• mostly glycoproteins
• Proteins are divided into two proteins
• Integral Proteins
• Peripheral proteins

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Integral protein

• proteins that pass through entire thickness of cell membrane from one side to the other side

Peripheral proteins

• proteins which are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane.

Functions of Proteins

• provides structural integrity and transport of substances

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Cytoplasm

• Cytoplasm of the cell is the jelly­like material formed by 80% of water.
• It contains a clear liquid portion called cytosol and various particles of different shape and size.
• 1.Ectoplasm:
• Peripheral part of cytoplasm, situated just beneath the cell membrane
• 2. Endoplasm:
• Inner part of cytoplasm, interposed between the ectoplasm and the nucleus.

Organelles in Cytoplasm

• Cellular structure present/embeded in cytoplasm
• Considered as small organs of the cell
• Some are membrane bounded organelles and some are without membrane

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Endoplasmic Reticulum ER

• network of tubular and microsomal vesicular structures which are interconnected with one another
• Membrane bounded organelle

Two types

1. Rough ER
1. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic
2. Synthesis of proteins
2. Smooth ER
1. It is the endoplasmic reticulum with smooth appearance. It is also called agranular reticulum.
2. Synthesis of non-protein substances i.e cholestrol and steroids.
3. Role in cellular metabolism

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Golgi Apparatus

• Golgi apparatus is a membrane-bound organelle, involved in the processing of proteins.
• Present in all cells except red blood cells
• Major functions of Golgi apparatus are processing, packing, labeling and delivery of proteins and other molecules like lipids to different parts of the cell.

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Lysosomes

• The lysosomes are formed by Golgi apparatus.
• The enzymes synthesized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi apparatus.
• Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes
• 1.Primary lysosome, which is pinched off from Golgi apparatus.
• 2. Secondary lysosome, which is the active lyso some. It is formed by the fusion of a primary lysosome with phagosome (macromlolecule engulf by cell membrane).

• Lysosomes are often called ‘garbage system’ of the cell because of their degradation activity.
• E.g
• Proteases hydrolyzes proteins into amino acids.
• Lipases hydrolyzes lipids into fatty acids

Peroxisomes

• Organelles like lysosomes peroxisomes are pinched off from endoplasmic reticulum and not from the Golgi apparatus
• Breakdown the fatty acids by means of a process called beta­-oxidation

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Centrosome

• Situated most in center of cell close to nucleus
• Centrioles are responsible for the movement of chromosomes during cell division

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Mitochondria

• It is known as the power house of cell
• Concerned with the energy production

Nucleus

• Nucleus is present in all the cells in the body except the red blood cells.
• The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes.
• Uninucleus
• Multinucleus

Nuclear Membrane

• Communicates with cytoplasm
• It is the outer layer of nucleus

Nucleoplasm

• is a highly viscous fluid that forms the ground substance of the nucleus. It is similar to cytoplasm present outside the nucleus.

Chromatin

• thread-like material made up of large molecules of DNA

Function of Nucleus

• Control cell activities, includes metabolism
• Protein synthesis
• Growth and reproduction

DNA

• Deoxyribonucleic acid
• Double Helical Structure
• It is formed by two chains of polynucleotide

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• Deoxyribose – sugar
• 2. Phosphate
• 3. One of the following organic (nitrogenous) bases:
• Purines – Adenine (A) – Guanine (G)
• Pyrimidines – Thymine (T) – Cytosine (C)

Cell Cycle

• Damaged and dead and worn out cells are often replaced by cell division
• Maintaining Tissue Integrity and Function
• Cell with nuclei have 46 chromosomes divide by mitosis that means into two genetically identical daughter cells.
• Self destruct (apoptosis)

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Cell division

• The period between two cell division is known as cell cycle
• It has two phases mitosis and interphase

Interphase

1^st Gap Phase (G1)

• Cell grow in size and volume
• Longest phase

• Sometimes cell do not continue the G1 phase and enters G0 phase
• Cells may stay in G0 phase for rest of life or re-enter the cell cycle

S phase synthesis phase

• Chromosome replicate and forming two copies of DNA.

Second Gap Phase (G2)

Further growth and preparation of Cell Division

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Mitosis

• a process that results in two
• Cell with nuclei have 46 chromosome

Prophase

• Replicated chromosome become tightly coiled
• Each of original chromosome is paired with its copy and double chromosome

Metaphase

• Chromatin align on the center of spindle attach by their centrosome

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Anaphase

• Centrosome separate
• One of each pair of sister chromatid

Telophase

• Mitotic spindle disappears
• Chromosome uncoil
• Nuclear envelope reforms

Cytokines

• Two identical daughter cells

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