The cell structure

Cell
 Cell is the basic living, structural, and functional unit of the body.
 Cells are grouped together to form tissues, each of which has a specialized
function, e.g.- Bone and blood tissue.
 Different tissues are grouped together to form a organs, e.g. liver, stomach,
and kidney etc.
 Organs are grouped together to form a system, each of which performs a
particular function responsible for maintaining homeostasis .
e.g. Urinary system, Respiratory system etc.

Cellular Level of organization

Cytology
 Cytology is the branch of science that deals with the microscopic study of
cells, their original structure and function .
 The cell is divided into two major parts :
Plasma membrane
Sub-cellular organelles :

Plasma membrane
 The thin barrier that separates the internal components of the cell from the
extracellular materials and external environment is the plasma membrane

Composition of Membrane:
 The cell membrane is principally made-up of lipids, proteins and
carbohydrates.
 The lipids are mainly phospholipids, cholesterol and glycolipids.
 The membrane linked to either proteins or lipids.

Sub –cellular organelles of cell
 Cytoplasm
 Nucleus
 Nucleolus
 Ribosomes
 Endoplasmic Reticulum(ER)
 Mitochondria
 Golgi complex
 Lysosome

Cytoplasm
 It is a gel like substance enclosed within the plasma membrane .
 Cytoplasm is transparent, viscous gel like fluid.
 It containing 75 to 90% of water, suspended and dissolved components such as
proteins, lipids, and carbohydrate, different inorganic substances and salts.
 Function of cytoplasm
• The cytoplasm functions to support and suspend organelles and cellular
molecules.
• Many cellular processes also occur in the cytoplasm, such as protein
synthesis, the first stage of cellular respiration (known as glycolysis), mitosis,
and meiosis.

Nucleus
 The nucleus is usually a spherical or oval in shape.
 It is the largest structure of the cell.
 The nuclear membrane is double membrane which separates the nucleus
from the cytoplasm.
 The nuclear membrane contains pores in membrane where inner and outer
parts of membrane fuses.
 It contains a spherical structure called as nucleolus.

 Functions of Nucleus
• It controls the heredity characteristics of an organism.
• It main cellular metabolism through controlling synthesis of particular enzymes.
• It is responsible for protein synthesis, cell division, growth and
differentiation.
• Stores heredity material in the form of deoxy-ribonucleic acid (DNA) strands.

Ribosomes:
 These are tiny spheres that contain ribosomal RNA and several ribosomal
proteins
 These are the site for proteins synthesis.
 These are made up of two sub-unit;
 Smaller sub-unit (40s)
 Larger sub-unit(50s)
 Ribosomes are two types :
 Membrane bounded ribosomes
 Free ribosomes

Endoplasmic Reticulum(ER)
 The E.R is a pattern of membrane enclosed channels called as cisterns of
varying shapes.
 It is an interconnected network of internal membrane .
 Based on it association with ribosomes the ER is divided into two types:
Rough ER
Smooth ER

Mitochondria
 The Mitochondria is called the power house of the cell.
 They are organelles that act like a digestive system which takes in nutrients,
breaks them down, and creates energy rich molecules for the cell.
 The biochemical processes of the cell are known as cellular respiration.
 The mitochondria consist of two lipoprotein Membranes :
Outer mitochondrial membrane :
Inner mitochondrial membrane :
 The region between the two membrane is called as the intermembrane space

Golgi complex
 Golgi Apparatus or complex is Present near the nucleus.
 It consists of four to six flattened sacs called as cisterns placed upon each
other like a pile of plates with expanded bulges at their ends.
 The stack of Golgi sacs has two define regions cis and trans.
 The Golgi apparatus gathers simple molecules and combines them to make
molecules that are more complex.
 It then takes those big molecules, packages them in vesicles, and either stores
them for later use or sends them out of the cell.
 It is also the organelle that builds lysosomes (cell digestion machines).

Lysosome
 Lysosomes are specialized vesicles within cells that digest large molecules
through the use of hydrolytic enzymes.
 It contains 60 kinds of powerful digestive and Hydrolytic enzymes that can
Hydrolyse large molecules such as RNA, DNA, proteins and lipids .
 Vesicles are small spheres of fluid surrounded by a lipid bilayer membrane.
 They have roles in transporting molecules within the cell.
 Lysosomes are only found in animal cells; a human cell contains around 300 of
them
 Lysosome enzymes work best at acidic PH= 5 and inactivated at neutral PH
value.

Transfer of material across the Plasma
membrane
 Movement of small molecules across the membrane:
(A)Diffusion
Simple diffusion
Facilitated diffusion
Osmosis
(B)Active transport
 Movement of large molecules across the membrane:
Endocytosis
Exocytosis

Simple Diffusion or Passive transport
 Diffusion is the movement of a substance from an area of high concentration
to an area of low concentration without utilization of energy.
 Diffusion happens in liquids and gases because their particles move randomly
from place to place.
 Diffusion is an important process for living things; it is how substances move
in and out of cells.
 Substance moves across the cell membrane by 3 basic mechanism:
The molecules remains in the aqueous phase and diffusion through aqueous
channels pores in the membrane.
The molecules leaves the aqueous phase on one side of the membrane , dissolved
in the lipid bilayer and cross it and again enters the aqueous phase on opposite
side of membrane .
The molecules combine with carrier molecules and help them across the cell
membrane .

Facilitated diffusion :
 It is called as carrier as carrier –mediated diffusion .
 The carrier protein facilitated the diffusion of the substances to the other
side membrane .
 Energy is not required for the such transfer.
 Many lipids insoluble substances like certain vitamins, glucose cross the
membrane by this process.
 The transfer is in the direction of the contraction gradient , from higher
concentration to lower concentration .

Osmosis
 Osmosis. The movement of water molecules through a selectively permeable
membrane from a region of high water concentration to low water concentration
is called osmosis.
 A solution where there is higher water concentration outside the cell is called a
hypotonic solution

Active transport :
 When the material is transport out against the concentration gradient i.e
from lower concentration to higher concentration with utilization of energy
then the process called as active transport .
 Energy is obtained from the hydrolysis of ATP .
 Active transport is of two types :
(a) Primary Active Transport :
(b) Secondary Active Transport :

Primary Active Transport :
 Energy is derived from hydrolysis of ATP which changes the shape of carrier
protein .
 The carrier protein pumps a substance across a plasma membrane against its
concentration gradient .

Secondary Active Transport
 Secondary active transport, is transport of molecules across the cell
membrane utilizing energy in other forms than ATP.
 This energy comes from the electrochemical gradient created by pumping ions
out of the cell. This Co-Transport can be either via antiport or symport.

Endocytosis :
 It is a transport mechanism that involves engulfing extracellular materials
within a segment of the cell membrane to form a vesicle called as vesicular
transport.
 For example:
Macromolecular nutrients like facts and starches , oil soluble vitamin A , D , E,
K and drug such as insulin.
 Endocytosis Includes:
 Phagocytosis:
 Pinocytosis:

Exocytosis
 Undigested substance called as residual body excreted through the cell
membrane by a process as exocytosis,
 Exocytosis' main purpose is to expel material from the cell into the
extracellular fluid.
 This is the opposite of what occurs in endocytosis.
 In exocytosis, waste material is enveloped in a membrane and fuses with the
interior of the plasma membrane.

Cell division
 Cell division is the process by which a parent cell division into two or more
daughter cells.
 These cells divide once in approximately every 24 hours.
 The duration of the cell cycle is can vary with organism and the cell type.
There are two types of the cell division:
 Somatic cell division
 Reproductive cell division

Somatic Cell division
 The cell cycle is an orderly sequence of events by which a somatic cell
duplicates its contents and division into .
 Human cell contains 23 pairs of chromosomes .
 One member of each pair is inherited from each parent.
 Somatic cells contain two sets of chromosomes ; they are called as diploid
cells, denoted as 2n.
 The cell cycle is divided into two basic phases :
Interphase
Mitotic phase