IN THE NAME OF ALLAH MOST BENEFICIAL, MOST

MERCIFUL

The Cell and its functions

Dr Sadia Amer

Learning Objectives• Organization of cell.• Physical structure of cell.

Organization of cell• A typical cell consists of: 1-Nucleus 2-Cytoplasm The nucleus is separated from

cytoplasm by nuclear membrane. The cytoplasm is separated from

surrounding fluid by cell membrane or plasma membrane.

• The different substances that make up the cell are collectively called Protoplasm.

• Protoplasm has five basic substances:-WaterElectrolytesProteins Lipids Carbohydrates

Water• Principal fluid medium.• Present in all cells except fat cells.• About 75-80 % is water.• Many chemicals are dissolved in water.• Many are suspended as solid

particulate.

Electrolytes• Important ions are Na,K, Mg, PO4 ,

So4,HCO3, Cl, Ca.• Provide inorganic chemicals for

cellular reactions• Necessary for cellular control

mechanisms.

Proteins• Most abundant after water.• Constitute 10-20 % of cell mass.• Has two types: 1-Structural proteins 2-Functional proteins

Structural Proteins• Mainly in the form of long filaments

that are polymers of many individual protein molecules.• They form microtubules which

provide the “cytoskeletons” of organelles as cilia, nerve axons, the mitotic spindles etc

• Extracellularly, fibrillar proteins are found in collagen and elastic fibers of CT and in blood vessel walls, tendons, ligaments etc

Functional Proteins• Usually composed of combination of

few molecules in tubular-globular form.• These are mainly enzymes of cell.• Mobile in the cell fluid.• Some are adherent to membranous

structures inside the cell.

Lipids• Several Types of substances that are

grouped together because of their common property of being soluble in fat solvents are called lipids.• Constitute about 2% of total cell mass.• Examples:-phospholipids ,cholestrol

and triglycerides(neutral fat)

• Lipids are insoluble in water.• So used to form cell membrane and

intracellular membranes.• In fat cells , triglycerides constitute

about 95% of cell mass.• These are the body’s main store

house of energy giving nutrients.

Carbohydrates• Constitute about 1% of total cell mass.• 3% in muscle cell.• 6% in liver cells.• Present in cell as part of glycoprotein

molecule.• Play major role in nutrition of cell.

• Glucose is always present in ECF so that it is readily available when cell is in need.• Small amount of carbohydrate is

stored in cell in the form of glycogen.• Glycogen is an insoluble polymer of

glucose.

Physical structure of cell• The cell contains highly organized physical

structures called Intracellular organelles.• Most organelles are covered by

membranes called membranous organelles.

• Includes ER, Golgi apparatus, mitochondria, lysosomes, Peroxisomes

• These membranes are composed of lipids and proteins.• It provides barrier to movement of

water and water soluble substances.• Proteins provide channels as well as

act as enzymes.

Cell Membrane• Is thin, pliable, elastic structure .• About 7.5 to 10 nanometers thick.• Composed of lipids ,proteins. And carbohydrates.• Proteins=55%• Phospholipids=25%• Cholestrol=13%• Other lipids=4%• Carbohydrates=3%

Structure (LIPID)• It is a lipid bilayer, thin double

layered, each layer only one molecule thick.• Interspersed in this are large

globular protein molecules.

• Lipid bilayer is composed of phospholipids molecules.• Its one end is soluble in water i.e

hydrophilic.• Other end soluble in fats i.e

hydrophobic

• The phosphate end is hydrophilic and fatty acid portion is hydrophobic.• The lipid layer in the middle is

impermeable to water soluble substances e.g ions, glucose, urea etc

• Fat soluble substances like oxygen, carbon dioxide, alcohol etc can penetrates this portion.

• Cholestrol molecules in membrane are lipid in nature b/c their steroid nucleus is highly fat soluble.• They determine degree of

permeability.• It also control fluidity of membrane.

Cell Membrane Proteins

Cell membrane protein

• Two types of proteins are present:-Integral Protein

Peripheral Protein Integral proteins protrude all the

way through the membrane. Peripheral proteins are present to

only one surface of the membrane.

• Integral proteins provide structural channels through which water molecules and water soluble substances esp ions can diffuse.• These channels have selective

properties.• Some integral proteins act as carrier

proteins for transport of substances.

• Sometimes these even transport substances in direction opposite to their natural direction of diffusion.This is active transport.• Some act as enzymes.• Some serve as receptor for water

soluble chemicals e.g peptide hormone

• Peripheral proteins are often attached to integral proteins.• These act as enzymes or controllers

of transport of substances.

Membrane Carbohydrate• Occur in combination with proteins or lipids

in the form of glycoproteins or glycolipids.• The glyco portion protrude to outside of cell.• Proteoglycans are bound to small porotein

cores.• Entire outer surface is coated with Glycocalyx

Functions of carbohydrates• Give cell a negative surface.• Attaches cells to one another.• Some act as receptor substances for

binding hormones.• Some have role in immune

reactions .

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