Cell and Transport Processes Across Cell Membrane

UNIT-2
Biochemical organization of the
cell and transport processes
across cell membrane
- P. S. Salve
P. S. Salve, Asst. Professor, KLE College of Pharmacy, KAHER, Belagavi Page 1

CELL
• Structural and functional unit of life
• Basic unit of biological activity
• The concept of cell originated from the contributions
of Schleiden and Schwann (1838)
• After 1940, the complex structure of cell was studied

Types of cell
1. Prokaryotes
(pro: before; karyon:
nucleus)
 Do not possess a well defined
nucleus
 Have a relatively simple
structure
 They lack cell organelles like
nucleus, mitochondria, ER, and
have only one chromosome
with a single molecule of DNA
 E.g. Various bacteria
2. Eukaryotes
(eu: true; karyon: cell)
 Possess a well defined
nucleus
 Have more complex
structure and functions
 Animals and plants are
composed of eukaryotic cells

Animal Cells Plant Cells
Centrioles
Cell membrane
Ribosomes
Nucleus
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Mitochondria
Cytoskeleton
Cell Wall
Chloroplasts

The Cell

Subcellular organelles
1. Nucleus
2. Mitochondria
3. Endoplasmic reticulum
4. Golgi apparatus
5. Lysosomes
6. Peroxisomes
7. Cytosol and cytoskeleton

1. Nucleus
 Largest cellular organelle
 Bordered by a porous double membrane – nuclear envelope
 Contains thin fibers of DNA – repository of genetic information –
Governs functions of cell
 Eukaryotic DNA + histones (basic protein) --- Nucleosomes

 Nucleosomes – Constitute chromatin fibres of chromosomes
 Rod shaped chromosomes
 Contains a small round nucleolus – Produces ribosomal RNA
which makes ribosomes
 Nucleoplasm – Ground material of nucleus – Contains DNA
polymerases and RNA polymerases

2. Mitochondria
 Greek - mitos: thread;
chondros: granule
Centres for –
 Cellular respiration
 Energy metabolism
 Regarded as power house of
the cell

• Composed of double membrane system
• Outer membrane – smooth, completely envelops the organelle
• Inner membrane – folded to form cristae (Latin: crests) – Contains
components of electron transport chain and oxidative
phosphorylation
• Internal chamber – matrix or mitosol – Contains enzymes
required in metabolism of carbohydrates, lipids and amino acids
• Mitochondria are principal producers of ATP in the aerobic cells

 Mitochondrial matrix contains
 A circular double stranded DNA (mtDNA)
 RNA and
 Ribosomes
 Thus, mitochondria are equipped with an independent
protein synthesizing machinery

3. Endoplasmic reticulum
• Network of membrane enclosed spaces that extend throughout
the cytoplasm
• Two types:
1. Smooth ER
2. Rough ER

1. Smooth ER:
 Does not contain ribosomes
 Involved in the synthesis of lipids (triacylglycerols,
phospholipids, sterols)
 Helps in metabolism of drugs
 Supplies Ca2+ for cellular functions
2. Rough ER:
 Contains ribosomes
 Ribosomes are involved in protein biosynthesis
 During cell fractionation – Rough ER disrupts to form small
vesicles - Microsomes

4. Golgi apparatus
• Clusters of membrane vesicles – dictyosomes – constitute the
golgi apparatus
• Newly synthesized proteins – transferred to golgi apparatus –
catalyses the addition of carbohydrates, lipids or sulfate
moieties to proteins
• These proteins are then transported across the plasma
membrane

5. Lysosomes
• Spherical vesicles enveloped
by a single membrane
• Digestive tract of the cell
• Digest proteins, lipids,
carbohydrates & nucleic acids
• Recycling Center
• Recycles cellular debris

• Lysosomal enzymes – hydrolases – include α-glucosidase (glycogen),
cathepsins (proteins), lipases (lipids), ribonucleases (RNA)
• These enzymes – help in maintaining the cellular compounds in a
dynamic state – by degradation and recycling
• Escape of these enzymes to the cytosol – Destroys functional
macromolecules – results in occurrence of various diseases (e.g.
arthritis, muscle diseases, allergic disorders)
• Inclusion cell (I-cell) disease – Rare condition – Due to lack of
enzymes in lysosomes – They are synthesized and seen in blood but do
not reach the correct protein siteP. S. Salve, Asst. Professor, KLE College of Pharmacy, KAHER, Belagavi Page 18

6. Peroxisomes
• Single membrane cellular organelles
• Called as microbodies
• Spherical or oval in shape
• Contains enzyme catalase, which protects the cell from toxic
effects of H2O2 by converting it to H2O and O2
• Peroxisomes are involved in oxidation of long chain fatty
acids, synthesis of plasmalogens and glycolipids

7. Cytosol and cytoskeleton
• Cellular matrix is collectively called as
cytosol
• It is a compartment containing enzymes,
metabolites and salts in an aqueous gel
like medium
• Cytoplasm contains a network of protein
filaments – cytoskeleton
• They are responsible for the structure,
shape and organization of the cell

Cytoskeleton
• Filaments & fibers
• Made of 3 fiber types
1. Microfilaments/actin
filaments
2. Microtubules
3. Intermediate filaments
• 3 functions:
1. Mechanical support
2. Anchor organelles
3. Help move substances

Sr.
No.
Subcellular
organelles
Functions
1. Nucleus DNA replication, transcription
2. Mitochondria Centres for energy metabolism, ATP generation
3.
Endoplasmic
reticulum
Biosynthesis of proteins, glycoproteins,
lipoproteins, drug metabolism
4.
Golgi
apparatus
Maturation of synthesized proteins and their
transport across cell membrane
5. Lysosomes
Digestion/degradation of proteins, carbohydrates,
lipids and nucleotides
6. Peroxisomes
Cointains catalase which protects cell from toxic
effects of H2O2
7.
Cytosol and
cytoskeleton
Responsible for structure, shape and organization
of the cellP. S. Salve, Asst. Professor, KLE College of Pharmacy, KAHER, Belagavi Page 22

TRANSPORT PROCESSES
ACROSS CELL MEMBRANE

Plasma membrane
• It is an envelope surrounding the cell
• It separates and protects the cell from the external hostile
environment
• Also provides a connecting system between the cell and its
environment
• Composed of lipids, proteins and carbohydrate

Structure of plasma membrane
• Lipid bilayer model –
originally proposed by
Davson and Danielle in
1935
• Fluid mosaic model –
Proposed by Singer and
Nicolson in 1972

Fluid mosaic model
• Large number of PROTEINS are present in the cell membrane
. They “float about “ in the phospholipid bilayer
• There are two types of proteins in the membrane
1. INTRINSIC proteins
2. EXTRINSIC proteins

• Tightly bound to lipid bilayer
• Can be separated by
detergents/organic solvents
• e.g. hormone receptor, CYP450
Intrinsic
(integral)
proteins
• Loosely held to surface of membrane
• Can be easily separated
• e.g. Cytochrome C of mitochondria
Extrinsic
(peripheral)
proteins

Phospholipids
• Most abundant lipid
• Polar/ hydrophilic head
(attracted to water)
• Pair of nonpolar/
hydrophobic tails (repelled
by water)
Hydrophilic
head
Hydrophobic
tails

Polar heads - outside & inside – Facing outward
Nonpolar tails in the interior cell membranes – facing each
other
Hydrophilic head
phospholipid
Hydrophilic head
Hydrophobic tail
outside
inside

Transport across cell membranes
1. Passive transport
2. Active transport

ATP
ATP

Passive transport
Simple
diffusion
Facilitated
diffusion
Osmosis
Active transport
Ion pumps
Endocytosis
Exocytosis

Simple diffusion
• Depends on concentration gradient of particular substance
across the cell membrane
• Passage of water and gases through cell membranes
• Rate of entry is proportional to the solubility of that solute
in the hydrophobic core of the membrane
• This process does not require energy
• It is a very slow process

No Barrier:
• Substances “spread out”
• High concentration to low concentration
e.g.: Red dye placed in glass of water

Facilitated diffusion
• Occurs through carrier/transport protein
• Occurs from higher to lower concentration
• No energy is required but rate of transport is
more rapid than simple diffusion process

Process of Facilitated Transport
• Protein binds with molecule
• Shape of protein changes
• Molecule moves across membrane

Ping-pong model
• According to this mechanism, a transport protein exists
in two conformations – Ping and pong states

Ping-pong model
1. Pong state: Active site is exposed to the exterior – with
high solute conc. – Solutes bind to the specific sites –
Conformational change occurs
2. Ping state: Active site is facing the interior of the cell –
where concentration of the solute is minimal – solute
molecule is released
Hormones regulate facilitated diffusion
e.g. insulin increases glucose transport in muscle and adipose
tissue

Water osmosis
• Osmosis – movement of water from low osmotic pressure
(dilute solution) to high osmotic pressure (concentrated
solution) across biological membranes
• Does not require ATP
• Disturbances in osmosis may cause edema, diarrhea, cholera,
inflammation of tissues

Passive Transport and Facilitated Diffusion
• Passive transport &
facilitated diffusion do
NOT require
ATP

Active Transport
DOES require the input of
ATP
Transport proteins
AGAINST
concentration gradient
outside cell
inside cell

Active transport
• Active transport occurs against a concentration gradient
• It is dependent on the supply of metabolic energy (ATP)
• It is also a carrier mediated process like facilitated
diffusion
• It mainly occurs through –
Ion-pumps (through the involvement of pump ATPases
or ion transporting ATPases)

Sodium potassium pump (Na+-K+ pump)
• The cells have a high intracellular K+ concentration and a low
Na+ concentration
• This is needed for the survival of the cells.
• High cellular K+ – Required for the optimal glycolysis (pyruvate
kinase is dependent on K+) and for protein biosynthesis
• Na+ and K+ gradients across plasma membranes – needed for
transmission of nerve impulse

• High K+ and low Na+ concentration in the cells is maintained by an
integral plasma membrane protein, namely the enzyme Na+ – K+
ATPase
• It consists of two α and two β subunits
• Na+ – K+ ATPase pumps 3 Na+ ions from inside the cell to outside
and brings 2 K+ ions from the outside to the inside with a
concomitant hvdrolvsis of intracellular ATP

Transport systems
1. Uniport system: Movement of a single molecule through the
membrane e.g. transport of glucose to the erythrocytes.
2. Symport system: The simultaneous transport of two different
molecules in the same direction e.g. transport of Na+ and
glucose to the intestinal mucosal cells from the gut
3. Antiport system: The simultaneous transport of two different
molecules in the opposite direction e.g. exchange of Cl- and
HCO3
- in the erythrocytes
All the above three systems are considered as secondary active
transport systemsP. S. Salve, Asst. Professor, KLE College of Pharmacy, KAHER, Belagavi Page 48

Transport of macromolecules
• Macromolecules like proteins, polysaccharides and
polynucleotides are transported across the membranes through
–
1. Endocytosis: Intake of macromolecules by the cells e.g.
uptake of LDL by cells
2. Exocytosis : Release of macromolecules from the cells to the
outside e.g. secretion of hormones like insulin, PTH

Endocytosis
• Movement of large material
– Particles
– Organisms
– Large molecules
• Movement is into cells
• Types of endocytosis
– bulk-phase (nonspecific)
– receptor-mediated (specific)

Process of Endocytosis
• Plasma membrane surrounds material
• Edges of membrane meet
• Membranes fuse to form vesicle
• Approximately 2% of the exterior surface of plasma membrane
possesses characteristic coated pits
• These pits can be internalized to form coated vesicles which
contain an unusual protein called clathrin

Endocytosis
• Phagocytosis – cell eating
• Pinocytosis – cell drinking
• Vesicle moves to cell surface
• Membrane of vesicle fuses
• Materials are expelled

Cell and Transport Processes Across Cell Membrane

More Related Content

What's hot (20)

Similar to Cell and Transport Processes Across Cell Membrane (20)

Recently uploaded (20)

Cell and Transport Processes Across Cell Membrane