Cell Structure and Function for 1st year MBBS delivered by Miss Sumaira on 25 jan 2010
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Transcript of Cell Structure and Function for 1st year MBBS delivered by Miss Sumaira on 25 jan 2010
Sumaira Sarwar Khan SENIOR LECTURER
DEPARTMENT OF BIOCHEMISTRY
IM&DC
Cell Structure & Function
cell
Definition of Cell
A cell is the smallest unit that is capable of performing life functions.
cell
Cell Theory
• All living things are made up of cells. • Cells are the smallest working units of all living
things. • All cells come from preexisting cells through
cell division.
cell
Examples of Cells
Amoeba Proteus
Plant Stem
Red Blood Cell
Nerve Cell
Bacteria
cell
Two Types of Cells
•Prokaryotic•Eukaryotic
cell
Prokaryotic
• Do not have structures surrounded by membranes
• Few internal structures
• One-celled organisms, Bacteria
cell
Eukaryotic• Contain organelles surrounded by membranes• Most living organisms
Plant Animal
cell
Difference between prokaryotic and eukaryotic cell
• Prokaryotic• Single membrane• No well defined nucleus • No membrane bounded
organelles• Lysosome absent• RNA and Protein
synthesis in same compartment
• Eukaryotic• Bilayer membrane• Nucleus containing DNA
+ histones• Membrane bounded• Lysosome present• RNA in nucleous and
proteins in cytosol
cell
Cell PartsOrganelles
cell
• In the cell the biomolecules like nucleic acids, proteins, lipids and carbohydrates are organized into complex structures called the organelles
• These organelles are the functional structures of the cell
cell
Arrangements inside a typical eukaryotic cell
• cell
• protoplasm (organic+inorganic matter)
• cell inclusions cell organelles
storage granules membranous starch and Glycogen granules nonmembranous
cell
• Membranous non membranous
• Nucleus ribosomes• Mitochondria chromosomes• ER centrioles• Golgi apparatus• Lysosome
cell
“Typical” Animal Cell
cell
http://waynesword.palomar.edu/images/plant3.gif
“Typical” Plant Cell
cell
Surrounding the Cell
cell
Cell Membrane
• Outer membrane of cell that controls movement in and out of the cell
• Double layer • The plasma membrane
regulates what enters and exits the cell.
cell
PLASMA MEMBRANE
• Maintains cellular stability and integrity• Is a protective and selective barrier• Controls and directs cellular activity• The plasma membrane borders the entire cell
separating the internal environment from the external environment
cell
2007-2008
The Cell Membrane
cell
Overview• Cell membrane separates living cell from nonliving
surroundings– thin barrier = 8nm thick
• Controls traffic in & out of the cell– selectively permeable– allows some substances to cross more easily than
others• hydrophobic vs hydrophilic
• Made of phospholipids, proteins,carbohydrate
cell
Structure – Organization of the Plasma Membrane
• Lipids • Proteins• Carbohydrates
cell
Lipids in plasma membrane
1) Phospholipids (glycerol backbone)2) Sphingolipids (sphingosine base)3) Sterols (non-polar & only slightly water soluble4) glycolipids
cell
Phospholipids
• A special type of lipid• 2 fatty acid chains
making a hydrophobic tail.
• Chains have no charge and are non-polar / insoluble.
• Phosphate group is hydrophilic = has a charge so is soluble.
cell
Phospholipids
Fatty acid
Phosphate
• Fatty acid tails– hydrophobic
• Phosphate group head – hydrophilic
• Arranged as a bilayer
Phospholipid bilayer
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
cell
Cholesterol
Cholestrol helps to keep the lipid layer free to move.
cell
Membrane fat composition varies• Fat composition affects flexibility– membrane must be fluid & flexible– % unsaturated fatty acids in phospholipids• keep membrane less viscous
– cholesterol in membrane
cell
More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed
that membrane proteins are inserted into the phospholipid bilayer
It’s like a fluid…It’s like a mosaic…It’s the Fluid Mosaic Model!
Fluid-mosaic Model of Plasma Membrane Structure
cell
Membrane Proteins• Proteins determine membrane’s specific functions– cell membrane & organelle membranes each have unique
collections of proteins
• Membrane proteins:– peripheral proteins
• loosely bound to surface of membrane• cell surface identity marker (antigens)
– integral proteins • penetrate lipid bilayer, usually across whole membrane • transmembrane protein• transport proteins
– channels, permeases (pumps)cell
Proteins domains anchor molecule• Within membrane– nonpolar amino acids • hydrophobic • anchors protein
into membrane
• On outer surfaces of membrane– polar amino acids • hydrophilic• extend into extracellular
fluid & into cytosol
Polar areasof protein
Nonpolar areas of protein
Mosaic• Combination of proteins
makes membrane unique• Membrane proteins may
be fluid or anchored• Proteins may penetrate the bilayer fully (integral) or
reside on the surfaces of membranes (peripheral)• Integral proteins typically have hydrophobic regions
that span the bilayer as a result of nonpolar amino acids arranged as helices
• Anchored proteins strengthen membranes
cell
Membrane is a nexus of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments ofcytoskeleton
Peripheralprotein
Glycoprotein
Phospholipids
cell
Types of Membrane Proteins
– Channel Proteins– Carrier Proteins– Cell Recognition Proteins– Receptor Proteins– Enzymatic
cell
cell
Many Functions of Membrane ProteinsOutside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
Membrane carbohydrates • Play a key role in cell-cell recognition– ability of a cell to distinguish one cell from
another• antigens
– important in organ & tissue development
– basis for rejection of foreign cells by immune system
cell
Carbohydrates diversify membranes
• Membrane carbohydrates are only found on the outside (external) face of membranes
• Attach to lipids or protein (glycolipid/ glycoprotein)
• Enable cells to distinguish/ recognize one another
cell
Carbohydrates present
• The carbohydrate structure is called the glycocalyx
• These contain• galactose • Mannose• Fucose• N-acetyl glucose amine• N-acetyl neuraminic acid
cell
Any Questions??
cell
Movement across the Cell Membrane
cell
Fluidity• Membranes are maintained by
hydrophobic interactions of the phospholipids resulting in the alignment of the polar phosphate regions toward the aqueous environment and the nonpolar regions’ hydrocarbon chains toward each other.
• Membranes are in motion with fast drifting lipids and slower drifting proteins
cell
•Membrane fluidity may be influenced by presence/absence of unsaturated FA chains and Cholesterol•Fluidity of membranes is important for proper function
Permeability of the Plasma Membrane
1. Diffusion– Diffusion is the movement of molecules from an
area of higher to lower concentration– Oxygen and Carbon Dioxide are two gases that
can diffuse through the plasma membrane
cell
Diffusion• Move from HIGH to LOW concentration– “passive transport”– no energy needed
diffusion osmosis
movement of water
cell
Diffusion across cell membrane• Cell membrane is the boundary between
inside & outside…– separates cell from its environment
INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O
OUTwasteammoniasaltsCO2
H2O products
cell needs materials in & products or waste out
IN
OUT
Can it be an impenetrable boundary? NO!
Diffusion through phospholipid bilayer• What molecules can get through directly?– fats & other lipids
inside cell
outside cell
lipid
salt
aa H2Osugar
NH3
What molecules can NOT get through directly?
polar molecules H2O
ions salts, ammonia
large molecules starches, proteins
cell
Channels through cell membrane• Membrane becomes semi-permeable with
protein channels – specific channels allow specific material across
cell membrane
inside cell
outside cell
sugaraaH2O
saltNH3
Facilitated Diffusion
• Diffusion through protein channels– channels move specific molecules across
cell membrane– no energy needed
“The Bouncer”“The Bouncer”
open channel = fast transport
facilitated = with help
high
low
Active Transport
“The Doorman”“The Doorman”
conformational change
• Cells may need to move molecules against concentration gradient– shape change transports solute from
one side of membrane to other – protein “pump”– “costs” energy = ATP
ATP
low
high
Getting through cell membrane• Passive Transport– Simple diffusion
• diffusion of nonpolar, hydrophobic molecules– lipids– high low concentration gradient
– Facilitated transport• diffusion of polar, hydrophilic molecules• through a protein channel
– high low concentration gradient
• Active transport– diffusion against concentration gradient
• low high– uses a protein pump– requires ATP
ATP
cell
Permeability of the Plasma Membrane
-Transport by Carrier Proteins – Carrier proteins combine with a molecule or ion
to be transported across the membrane.
– Carrier proteins are required for:• Facilitated Transport• Active Transport
cell
Facilitated Transport
• Small molecules that are not lipid-soluble• Molecules combine with carrier proteins• Molecules follow the concentration gradient• Energy is not required
cell
Permeability of the Plasma Membrane
-Active Transport– Small molecules– Molecules combine with carrier proteins– Molecules move against the concentration
gradient– Energy is required
cell
The Sodium-Potassium Pump
cell
Transport summarysimplediffusion
facilitateddiffusion
activetransport
ATP
Permeability of the Plasma Membrane
3-Osmosis– Osmosis is the diffusion of water across a
differentially permeable membrane.– Osmotic pressure is the pressure that develops
in a system due to osmosis.
cell
Isotonic:the solute concentration is equal inside and outside
of a cell
Hypotonic: a solution has a lower solute concentration than the inside of a cell
Hypertonic: a solution has a higher solute concentration than the inside
of a cell
cell
How about large molecules?• Moving large molecules into & out of cell– through vesicles & vacuoles– endocytosis• phagocytosis = “cellular eating”• pinocytosis = “cellular drinking”
– exocytosis
exocytosiscell
– 4-Endocytosis –
Vesicles form as a way to transport molecules into a cell
• Phagocytosis: Large,particulate matter
• Pinocytosis: Liquids and small particles dissolved in liquid
• Receptor Mediated Endocytosis: A type of pinocytosis that involves a coated pit
cell
Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered bymolecular signal
cell