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Cell
--- Structure & Function
Dr J Raju
CELL THEORY
Cell theory is based upon the work of Matthias
Schleiden, Theodore Schwann, and Rudolph
Virchow.
Cell theory states that:
All organisms are composed of cells
Cells are the basic unit of structure and
function in organisms
All cells come from preexisting cells through
cell division
Three Major Parts of Cells
1. Cell (plasma) membrane - barrier between inside and
outside (skin)
2. Cytoplasm - organelles, free proteins, ions (guts)
3. Nucleus - Control center for decision-making,
responding to environment and replicating genetic
material (nervous system)
Basic unit of structure & function in body
Highly organized molecular factory
Cell is smallest unit having properties of life
CELL STRUCTURE (ANIMAL)
Why are cells so
small?
• 10 – 100m
• Must be small to minimize
energy consumption…
• But large enough to
minimize heat loss
Size determines rate of life
• Large enough to house
organelles needed to
eat, grow, reproduce
• Small enough that little
energy & time is
needed for transport of
gases, food & waste
• Maximize volume
Observing Cells
• Light microscope
– Can observe living cells in true color
– Magnification of up to ~1000x
– Resolution ~ 0.2 µm – 0.5 µm
• Electron Microscopes
– Preparation needed kills the cells
– Images are black and white – may be colorized
– Magnification up to ~100,000
• Transmission electron microscope (TEM) 2D image
• Scanning electron microscope (SEM) 3D image
SEM
TEM
Types of Eukaryotic cells
Most only have plasma membranes
Nucleus: Control center
• Holds genetic code and
“machinery” for
replication and
transcription
• Bounded by nuclear
envelope (inner and
outer membrane)
• Nuclear pores
• Present in every cell at
• Chromatin
some life-stage
• Chromosomes
• Chromosomes?
– Hypercoiled DNA
• Chromatin?
– Histone coiled DNA
Nucleoli
• 1 - 4 per nucleus
• Produce ribosomal
RNA (rRNA)
• Site of large and
small ribosomal
subunit formation
via attachment of
rRNA and
ribosomal protein.
Where is the cytoplasm?
• Located between cell membrane & nuclear
membrane
• Consists of:
– Cytosol: intracellular fluid (mostly H20, ions &
buffering proteins)
– Organelles: structures with specific functions;
suspended in cytosol
– Inclusions: Insoluble materials (lipids)
Ribosomes
• Site of protein synthesis
• Consist of 2 subunits, each
made of rRNA + protein
• Two varieties
– Free ribosomes: produce
proteins that travel to nucleus
– Fixed ribosomes: produce
proteins for export to
Endoplasmic Reticulum (ER)
Endoplasmic Reticulum (ER)
• Network of membranes
connected to nuclear
envelope
• 4 major functions
1. Synthesis (proteins,
carbohydrates, lipids)
2. Storage
3. Transport
4. Detoxification
• Two types
- Smooth
- Rough
Smooth ER (SER)
• Why is it called smooth?
• Responsible for the synthesis and storage of:
– Phospholipids and cholesterol for maintenance
and growth of membranes of cell, ER, nucleus,
Golgi apparatus (GA)
– Steroid hormones: estrogens and androgens
– Glycerides in liver and fat cells
– Glycogen in skeletal muscle and liver cells
Rough ER (RER)
• Workshop
• Synthesizes proteins (it
has fixed ribosomes!!)
and may chemically
modify them.
– Polypeptide chains
migrate into cisternae,
assume tertiary
structure + additional
modification
• Ships proteins to GA
via transport vesicles
Golgi Apparatus
• Packing & shipping depot
• Consists of 5-6 flattened
membranous disks
(cisternae)
Functions of GA
• Produces 3 export vesicles:
1. Secretory - exocytosis
2. Membrane renewal – replacement & remodeling
3. Lysosomes - “Primary” contain inactive digestive
enzymes
GA
Lysosome Functions
Abnormal lysosomes
• Lack, or have malfunctioning enzymes
– Normal cell products accumulate & stifle
(suffocate) cells
• Tay-Sachs disease
– Lysosomes lack enzymes that break down lipids
in nerve cells
• Pompe’s disease
– Lysosomes lack hydrolytic enzyme that splits
glycogen
Peroxisomes
• Contain digestive enzymes
• Functions:
– Absorb and breakdown fatty acids and nucleic
acids - produces H2O2 (danger!)
– Convert free radicals to H2O2
– Coverts H2O2 to harmless H2O and O2, using
catalase
Mitochondria = Powerhouse
Mitochondria harness energy!
• “powerhouse” of the
cell…makes ATP
• Double membrane
• Number per cell
varies with metabolic
activity (0% volume of RBC,
20% volume of liver cell)
Chloroplasts make food!
• Photosynthetic
eukaryotes
• Converts light energy &
CO2 to sugars
• Stroma: tubules &
membranous disks
• Grana: stacks of disks;
membranes chock full
of chlorophyll, which
traps solar energy
Microfilaments
• Thinnest cytoskeletal elements (rodlike)
• Composed of the globular protein actin
• Enable cells to change shape and move
Cytoskeleton
• Intermediate filaments
– Present only in animal cells of
certain tissues
– Fibrous proteins join to form a
rope-like structure
• Provide internal structure
• Anchor organelles in place.
Cytoskeleton
• Microtubules – long hollow
tubes made of tubulin proteins
(globular)
– Anchor organelles and act as
tracks for organelle movement
– Move chromosomes around
during cell division
• Used to make cilia and flagella
Plant cells:
Cell wall: maintains structure;
protection from environment;
limits water absorption
Central vacuole: storage of
nutrients; gets rid of waste
products; maintains pH;
enzymes for digestion;
contains pigments
Plastids: storages nutrients;
contains pigments; needed for
photosynthesis (chloroplast)
Amyloplast: makes starch
Do not contain lysosomes…in
animals only!!!
Occurring in sheets of tightly packed cells, epithelial
tissue covers the outside of the body and lines
organs and cavities within the body.
– The cells of a epithelium are closely joined and in
many epithelia, the cells are riveted together by tight
junctions.
– The epithelium functions as a barrier protecting against
mechanical injury, invasive microorganisms, and fluid
loss.
– The free surface of the epithelium is exposed to air or
fluid, and the cells at the base of the barrier are
attached to a basement membrane, a dense mat of
extracellular matrix.
Epithelia are
classified by the
number of cell
layers and the
shape of the cells
on the free
surface.
• Some epithelia, called glandular epithelia, absorb or
secrete chemical solutions.
– For example, glandular epithelia lining tubules in the
thyroid gland secrete a hormone that regulates fuel
consumption.
– The glandular epithelia that line the lumen of the
digestive and respiratory tracts form a mucous
membrane that secretes a slimy solution called mucus
that lubricates the surface and keeps it moist.
• The free epithelial surfaces of some mucous
membranes have beating cilia that move the film of
mucus along the surface.
• In the respiratory tubes, this traps dust and particles.
Why cells must control materials?
The plasma
membrane is the
boundary between
the cell and its
environment.
It is the plasma membrane’s job to:
• allow a steady supply of glucose, amino acids,
and lipids to come into the cell no matter what
the external conditions are.
• remove excess amounts of these nutrients when
levels get so high that they are harmful.
• allow waste and other products to leave the
cell.
This process of maintaining the cell’s
environment is called homeostasis.
Selective permeability is a process used to
maintain homeostasis in which the plasma
membrane allows some molecules into the
cell while keeping others out.
Plasma
Membrane
Water
Cell Membrane Functions
• Physical isolation - separates inner and outer
environments
• Sensory receptor - membrane receptor
proteins sense changes in external environment
(encrusted with peripheral nerves)
• Regulates exchange with the environment
- membrane channel proteins + carrier proteins
• Structural support - intercellular protein
attachment
Structure of cell membrane
• 6-10 nm thick
• Contains lipids, proteins and carbohydrates
– Lipids
• Phospholipids; Cholesterol; Glycolipids
– Proteins
• Integral; Peripheral
– Carbohydrates
• Form glycocalyx (identity)
oligosaccharide cholesterol
groups
phospholipid
EXTRACELLULAR ENVIRONMENT
(cytoskeletal pro-
teins beneatch open gated gated active RECEPTOR LIPID BILAYER
the plasma ADHESION
channel channel channel transpor PROTEIN
membrane) PROTEIN
protein proten proten t protein
(open) (closed) RECOGNITION
(area of PROTEIN
enlargment)
TRANSPORT PROTEINS CYTOPLASM
PLASMA MEMBRANE
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Transmembrane
proteins
Peripheral
protein
Filaments of
Cytoplasm cytoskeleton
Plasma Membrane
• Surrounds & gives cell form; selectively
permeable
• Formed by a double layer of phospholipids
– restricts passage of polar compounds
Plasma Membrane continued
• Proteins customize membranes
– Provide structural support
– Serve as transporters, enzymes, receptors &
identity markers
Plasma Membrane continued
• Carbohydrates in form of glycoproteins &
glycolipids are part of outer surface
– Impart negative charge to surface
Structure of the Plasma Membrane
Asymmetrical; the two halves are not identical
The plasma
membrane is
composed of two
layers of
phospholipids
back-to-back.
Phospholipids are lipids with a phosphate
attached to them.
The lipids in a Phosphate Group
plasma membrane
have a glycerol Glycerol
backbone, two Backbone
fatty acid chains,
and a phosphate Two Fatty
group. Acid
Chains
Makeup of the phospholipid bilayer
The phosphate Phosphate
group is critical for Group
the formation and
function of the
plasma
membrane.
Membrane Models
• Robertson- Unit membrane
• Singer and Nicolson - Fluid-Mosaic Model
- Membrane structure is not rigid (fluid)
- Membrane comprised of diff. molecules (mosaic)
- Proteins float around the surface of the cells
- Proteins, Carbohydrates, Phospholipids can be
added/removed from the surfaces of cells
Membrane Models
Makeup of the phospholipid bilayer
The fluid mosaic model describes the plasma
membrane as a flexible boundary of a cell. The
phospholipids move within the membrane.
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins can
move around freely within the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the
scattered protein molecules when the membrane is
viewed from above.
Other components of the plasma membrane
Cholesterol plays the important role of
preventing the fatty acid chains of the
phospholipids from sticking together.
Cholesterol
Molecule
Membrane Movement and Cholesterol
• Most of the lipids and some
proteins can drift laterally in
the plane of the membrane,
but rarely flip-flop from one
layer to the other.
• Cholesterol is wedged
between phospholipids
molecules in the plasma
membrane of animals cells. It
restrains the movement of
the phospholipids in warm
temps. and maintains fluidity
by preventing tight packing at
cold temps.
Membranes are fluid
• The higher the concentration of unsaturated fats
the more fluid the membrane.
• Fluidity of membrane structure helps maintain a
pliable (flexible) membrane very important for
example red blood cells.
Glycolipids have a structure similar to
phospholipids except that the hydrophilic head is
a variety of sugars joined to form a straight or
branching carbohydrate chain.
Experiment to demonstrate lateral
movement of proteins
• Tagged membrane receptors
move in the membrane at
about 2m per second
• When two cells (which have
different receptor proteins)
are fused.
• The receptors move and
become evenly dispersed.
MEMBRANE PROTEINS
• Each cell: 10-50 different types of membrane proteins.
• Proteins determine membrane’s specific functions.
• Cell membrane & organelle membranes each have unique collections
of proteins
Peripheral proteins (Cell surface identity marker (antigens))
- Attached loosely to membrane spanning proteins or polar
regions of phospholipids
- Can be removed w/o destroying membrane
- Include enzymes and binding proteins
that anchor cell to membrane.
Integral proteins (Transmembrane proteins)
- Tightly bound to phospholipid bilayer
- Can’t be removed w/o destroying membrane
- Most span the entire membrane
Protein Functions
• Channel Proteins - Involved in passage of molecules
through membrane.
• Carrier Proteins - Combine with substance to aid in
passage through membrane.
• Cell Recognition Proteins - Help body recognize foreign
substances.
• Receptor Proteins - Allow molecule binding, causing
protein to change shape and bring about cellular change.
• Enzymatic Proteins - Carry out metabolic reactions
directly.
Many integral proteins are glycoproteins, which have
an attached carbohydrate chain, similar to glycolipids.
Therefore it can be said that the plasma membrane is
‘Sugar coated’
Protein Functions
Classes of Amino acids
What do these amino acids have in common?
Nonpolar & hydrophobic
Classes of amino acids
What do these amino acids have in common?
Polar & hydrophilic
Proteins domains anchor molecule
• Within membrane Polar areas
of protein
– nonpolar amino acids
• hydrophobic
• anchors protein
into membrane
• On outer surfaces of membrane
– polar amino acids
• hydrophilic
• extend into extracellular
fluid & into cytosol
Nonpolar areas of protein
Membrane carbohydrates
Play a key role in cell-cell recognition
(The carbohydrate chains of glycolipids and glycoproteins
serve as the “fingerprints” of the cell)
Glycolipids and glycoproteins vary from species to species
and even from cell to cell in the same individual.
– ability of a cell to distinguish one cell from another.
– important in organ & tissue development.
– basis for rejection of foreign cells by immune system.
– Person’ particular blood group is due to the presence
of particular glyoproteins in the membrane of RBC.
Thank U
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