Transcript Cell Structure and Function

Cells: The Living Unit
Biology 2121
Chapter 3
Introduction
• (1). C ells are the basic units of all living things.
• (2). The basic parts of a cell
– Plasma membrane
– C ytoplasm
– Nucleus
• (3). Organelles
– Specialized cellular components
– “little organs”
Generalized Human Cell
Nucleus
• Structure
– Uninucleated; multinucleated
(some liver cells; anucleated
(RB C )
– Envelope (nuclear lamina)
and encloses DNA
– Pores
• rRNA
– Nucleoli
• Ribosome assembly (rRNA)
• Two subunits
C hromatin
(1). DNA (30%); Histone Proteins (60%) and RNA (10%)
(2). Nucleosome
– 8 histone proteins and DNA wrapped 2x
– Linker DNA
(3). During prophase of mitosis
– C ondenses into chromosomes
(4). 46 chromosomes in each cell
– C ontains genetic material
C hromatin
Endoplasmic Reticulum
• Rough ER
– C ontinuous with nucleus
– contains ribosomes
• Smooth ER
– No ribosomes
– Lipid metabolism (fatty acids),
cholesterol synthesis
– Steroid-based hormones
– Liver
• Enzymes produced that release glucose from
glycogen
• Detox drugs, carcinogens, alcohol
– Muscle cells
• C alcium ions trigger contractions; released from
Sarcoplasmic reticulum (type of smooth ER)
Ribosomes
• (1). Site of ‘Protein Synthesis’
• (2). Free
– Floating in cytoplasm
– Produce proteins for cytosol in cytoplasm
• (3). Attached
– Produce proteins for the cell membrane or
‘export’
• Hormones, integral proteins, enzymes
• (4). Structure
– rRNA and proteins (2 subunits)
Golgi Apparatus
• Function
– Modify, concentrate and package
proteins and lipids made in the rough
ER
• Movement
– Proteins move from the ER to the Golgi
and to parts of the golgi via transport
vesicles
• Faces
– Enter on the ‘cis’ face and leave via the
‘transface’
• Vesicles
Products
1. Proteins and lipids for export
2. Proteins inserted and
function in the plasma
membrane
3. Enzymes for lysosomes
Secretory Vesicles
• Export Proteins
– Leave the Golgi and move
towards the plasma membrane
for secretion as ‘secretory
vesicles’ (granules)
• Parts of the Cell
Membrane
– Lipids, transmembrane proteins
are taken to the membrane in
this way
Lysosomes
• Function
– Dispose of bacteria, old parts of cells, waste
Animations- Lysosome
• C ontents
– Digestive enzymes (hydrolytic)
– ‘acidic hydrolases’ function in low pH environments best
• Other Functions
– Digest particles that enter the cell via endocytosis
– Degrades old organelles, tissues
– Breaks down nonuseful tissues (between fingers and toes during
development)
Endomembrane System
• System of organelles that function together
– Produce, store, export molecules
– Degradation of substances
– Vesicular Transport (animation)
• Organelles in the System
–
–
–
–
–
1. Endoplasmic Reticulum (ER)
2. Golgi Apparatus
3. Secretory vesicles
4. Lysosomes
5. Nuclear membrane
Mitochondria
• Function
– Production of ATP (Adenosine
Triphosphate)
• Structure
– Double membrane encloses inner
membrane (C ristae)
– Own DNA and RNA – self
reproduction
• Location
– All cells; more in kidney, liver cells
Peroxisomes
C ontains two important enzymes
• 1. Oxidase
– Detoxes alcohol and formaldehyde.
Formaldehyde converted into hydrogen peroxide
– Destroys free radicals; converts them into
hydrogen peroxide
• 2. C atalase
– Breaks hydrogen peroxide into water and oxygen
• Large amounts in kidney and liver
Other Important Parts
1. C ytoskeleton
– Microtubules
• Larger; attaches to organelles and pulls them to different
locations (motor proteins-ATP)
– Microfilaments
• Thin; cause cell to change shape due to actin and myosin
• C leavage furrow; endo- and exocytosis
– Intermediate fibers
• Insoluble fibers that attach to desmosomes
2. C entrioles
– C ontained within the centrosome
– Involved in cell replication or division
The Plasma Membrane
1. Forms a ‘boundry’
– Separates the intracellular and extracellular
environments
2. Function
– Selectively permeable
3. Structure
– Phospholipid bilayer with inserted proteins,
cholesterol, glycolipids
Phospholipid Bilayer
• Fluid Mosaic model
• Heads are hydrophilic
– W ater-loving (polar)
• Tails are hydrophobic
– W ater-hating (nonpolar)
• What is allowed to pass freely through the
membrane?
– Allow for the passage of small polar molecules such as water, gases, small proteins
(amino acids) via facilitate diffusion, lipid soluble
– Impermeable to large polar molecules, large proteins, carbohydrates and ions
Plasma Membrane Permeability
May pass through the phospholipid
bilayer
May not pass through (will need
assistance)
1. Small polar molecules
1. Ions
(water, urea)
2. Small – lipid soluble
(steroids)
3. Small nonpolar
(oxygen, gases)
2. Large uncharged polar molecules
(glucose)
3. Large proteins
Role of the Membrane
Proteins
• Integral (transmembrane)
– Inserted completely through the membrane
– Function as channel or carrier proteins
• Transport substances through the membrane that cannot
normally pass through freely (large substances)
• Peripheral
– Attached to the integral proteins
– Attachment sites for the cytoskeleton filaments,
surface for enzymatic reactions to take place
Membrane proteins(Integral)
Functions
Example
1. Ion channels
Moves ions inside and
outside the membrane (ions
are charged and repelled)
Na-K pump;
Na-K movement during
depolarization of neuron cell
membranes
2. Transport
Movement of glucose
Facilitated diffusion of
glucose
3. Receptor
Binds to molecule, cells,
hormones
Insulin receptors in cells
“Ligands”
4. Enzyme Site
Surface for enzyme reaction
5. Linkers
Anchor proteins in opposite
cells – holds cell to cell
structure
* Also includes peripheral
proteins
6. Identification
Enables cells to recognize
other cells
ABO antigens
**Peripheral proteins
Support plasma membrane and
integral proteins; moves things
inside the cell; attachment
Other parts of the cell membrane
1. Cholesterol
– function to keep the ‘fluidity’ of the membrane stable
2. Glycoproteins
– Oliogosaccharides (3-10; 60 monosaccharides)
– Glycoproteins + glycolipids = glycocalyx
3. Glycocalyx
– Attachment and recognition, protection (bacteria)
• WB C s can detect a ‘foreign’ glycocalyx!
• C ell to cell attachment (protects cells from enzyme damage)
4. Microvilli
– Small extensions on the surface that increases surface area
– Absorptive cells (small intestine)
Specializations
• Tight Junctions
– Integral proteins that fuse cells together
– Particles are not able to pass through spaces
between cells
• Desmosomes
– Filaments attached to plaque, which are attached
to linker proteins that bind cells together
– Bind cells, keep cell upright and structurally intact
• Gap Junctions (nexus)
– Passage of ions, simple sugars, small molecules
from cell to cell
– C hannels are called ‘connexons’
Extensions of the Cell
• C ilia
– Small hair-like projections used for
moving substances across a membrane
(trachea)
• Flagella
– Longer whip-like projection
– Locomotion
– Sperm
Trachea - cilia
Membrane Transport
• Passive processes
– No energy required, substances move ‘down’ their
concentration gradients
• Examples are osmosis and diffusion
• Active Processes
– Require energy in the form of ATP
– Substances move ‘against’ their concentration gradients
• Examples are the sodium-potassium pump and vesicular
transport
Passive Processes
• 1. Simple Diffusion
– Nonpolar and lipid-soluble substances pass through
– Gases, fat-soluble vitamins
• 2. Osmosis
– Movement of water from areas (gradients) of higher concentration to
areas of lower concentration through a semi-permeable membrane
• Diffusion: same, but movement of particles other than water.
• 3. Facilitated Diffusion
– Membrane protein channels or carriers
– Example glucose, amino acids, ions
Animations
• Animation: How Diffusion Works
• Animation: How Facilitated Diffusion Works
• Animation: How Osmosis Works
C hannel proteins: ion
movement
Transport proteins:
change shapes allows for
movement of molecules
Osmosis and Diffusion
1. Hydrostatic Pressure: As water builds up –
pressure increases; moves water back to the other
side
2. Equilibrium: reached when as many molecules
move across due to hydrostatic pressure (equals
number of molecules moving across due to
osmosis)
Solute molecules: exert pressure ‘osmotic
pressure’ - if solutes cannot pass across the
membrane causes greater OP.
•May prevent the movement of water
molecules (not help to move them)
• Definition – effects of pressure on
animal cells causes changes in cell
shape
Tonicity
• Isotonic
– Solutions with the same concentration of
nonpenetrating solutes (0.9% saline or
5% glucose)
• Hypertonic
– Solutions with higher solute
concentration
– C renate cells
• Hypotonic
– Solutions with lower solute
concentrations and high water
concentration
– C ells will ‘lyse’
Normal- isotonic
Crenated –
hypertonic
Swollen – lysis
possible
Review
• A beaker is filled with water which is 80% Na C l . A
dialysis bag is filled with a solution of water which
contains 20% Na C l. The dialysis bag is
impermeable to Na C l but permeable to water.
• W hich direction will water move?
Active Processes
• Active Transport
– Requires ‘energy’ (ATP) and a protein carrier
– Called ‘solute pumps’
• Types of Active Transport
– 1. Primary – ‘Sodium potassium pump’
• Uses energy from the ‘hydrolysis of ATP’
• ATP causes the protein to change conformation
– 2. Secondary- Sodium and glucose (cotransport)
• Uses energy stored in in an ionic concentration gradient
• Movement of ions or particles ‘down’ a concentration gradient
• 1. As Na+ moves across the cell membrane, drives the transport of another ion.
– Same for hydrogen, calcium
• For example
– Movement of Na drives glucose; movement of Na drives amino acid transport
Symport and Antiport
Systems
• 1. Symporters
– Two substances in the same direction
• 2. Antiporters
– Substances move in opposite directions
Animations
• Animation: How the Sodium Potassium Pump Works
• Animation: Sodium-Potassium Exchange Pump (Quiz
1)
Vesicular Transport
• Vesicles
– Phospholipid membranes ‘pinch’ off and form tiny vesicles
• Exocytosis
– Movement of particles or substances out of the cell
• Endocytosis
– Movement of particles or substances into the cell
• http://www.abdn.ac.uk/~clt011/flash/samples/liposomes.swf
Vesicular Transport
Endocytosis:
(1). Receptor-mediated: ligands
attach to receptors. Receptor
sites are coated pits called
‘clathrin-coated pits’. Clathrin
protein cause membrane to
invaginate and pinch off . Clathrin
coat is lost in a process called
‘uncoating’. Now fuses with an
endosome (special vesicle),
becomes a transport vesicle
which fuses with lysosome.
Degradation occurs.
Exocytosis
Phagocytosis
Bulk-Phase Endocytosis –
pinocytosis
Cell Cycle and Mitosis
• The Cell Cycle
– Period of time between cell divisions
• Interphase
– G1 , G2 , S
– Longest cell cycle phase
• Mitosis
– Division of the cell’s nucleus
– Cytokinesis – division of the cytoplasm
of the cell
• Phases of Mitosis
– Prophase is the longest phase of
mitosis!
– Figure 3.32
– End product: duplicate daughter cells
Phases of Mitosis
 Prophase




1. Longest phase of ‘mitosis’!
2. Chromatin condenses into ‘chromosomes’
3. Spindle fibers appear from the area around the ‘centrioles’.

Centrioles migrate to the opposite poles of the parent cell.

Spindle fibers come from asters (protein microtubules).
4. Nuclear membrane breaks down and the chromosomes are free to move (manipulated by the spindle
fiber proteins)
 Metaphase

1. Spindle fibers move the chromosomes to the middle or center area of the cell or ‘equator’ (metaphase
plate)
 Anaphase

1. Spindle fibers separate the two sister ‘chromatids’; each chromatid is moved to the opposite pole.
 Telophase
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


1. Begins as chromosome movement stops!
2. A ‘reversal’ of prophase
3. ‘Binucleate’ for a short time
4. Cytokinesis follows.
Animations
• 1. Cell Cycle
• 2. Mitosis and Cytokinesis
• 3. Mitosis (Alt)
How is Cell Division Controlled?
• 1. Before cell division, cell volume increases (64 fold) more than cell
surface area enlargement (16 fold).
– Daughter cells produced have appropriate volume to surface area
ratios
• 2. C hemical Signals (notes)
– Growth factors, hormones released by other cells
• 3. C ontact Inhibition
– When cells come into contact with one another or touching, growth
may stop
• Proteins Involved (notes)
– C yclins
– C yclin-dependent kinases (Cdks)
Animation
• 1. Control of the Cell Cycle
S-Phase and DNA Replication
• Steps are listed on page 100
– 1. helicase
– 2. RNA primer is laid down (primase)
– 3. Formation of ‘complementary strands’
– 4. DNA polymerase III attaches the
correct base pairs to the old strand
– 5. DNA ligase ‘glues’ together the ‘lagging
strand’
• Semi-C onservative Replication
Duplicate this strand of DNA:
AATCGGATC
Animation:
1. DNA Replication
2. DNA Replication II
Protein Synthesis
• Genes are sequences of DNA that code for
one polypeptide sequence
– A ‘triplet’ sequence of bases - one amino acid
– Example: AAA codes for the amino acid Lysine
• See the genetic code (figure 3.35)
• Genes contain exons (coding sequences)
and introns (non-coding sequences)
– Introns will be cut-out before the making of
the polypeptide
How is the information in DNA copied?
Transcription is a process by which DNA is copied
into mRNA (messenger RNA)
– Takes place inside the nucleus (DNA does not leave
the nucleus)
• After transcription is complete, the mRNA will
leave the nucleus through the nuclear pores
– Each 3-base sequence in mRNA is called a ‘codon’
which codes for one amino acid
• Editing: introns are lost and exons are left on
the final mRNA strand (leaves the nucleus)
More About Transcription
 After DNA is unzipped, the process begins at the place where a
DNA promoter binds to the start point of the gene.
 RNA synthesis
 RNA nucleotides are laid down, using DNA as the template
 RNA polymerase
 C odons
 Three nucleotides
 64 possible codons made (43); 3 stop codons and one start
 Redundancy (3.35- Genetic code)
 Animation
 1. Transcription
• Base change in mRNA: U (uracil)
replaces T (thymine)
• READ FROM THE mRNA!
• DNA: A C C
• mRNA:
AG C
How is a polypeptide chain made?
• Translation is the process of converting the language of DNA
into amino acid language.
– Occurs in the cytoplasm
• 1. mRNA reaches the ribosomes on the surface of the ER
(page 108)
– mRNA will base pair to the rRNA (ribosomes)
• 2. tRNA (transfer RNA) brings the appropriate amino acid to
the surface of the mRNA
– Anticodon on the tRNA will base pair with the mRNA
codon
– The first codon is always a ‘start’ codon (AUG)
How is a polypeptide chain made?
• 3. The first codon base pairs with the first anti-codon in
the P-site
– The second codon and anticodon base pair in the Asite
• The P and A site amino acids then form a peptide bond.
– The P-tRNA then is released to the E site and goes to
pick up another amino acid
– The A-site tRNA now holding the chain moves to the Psite
• Another tRNA carrying an amino acid then moves into the
A site
• Sequence ends with a ‘stop’ codon (UGA)
More Transcribing and Translating Practice
• DNA:
• mRNA:
• tRNA:
• AA:
C AC
TTT TAG C C C
Animations
• 1. Translation