Bio crash course
4.1 Plasma membrane structure and
function
• Membrane structure
– Fluid-mosaic model
– Phospholipid bilayer
• Hydrophobic tails face inward
• Hydrophilic heads face surfaces
– Proteins
• Integral proteins-embedded
• Peripheral proteins-inner surface
– Glycoproteins, glycolipids, cholesterol
1-2
Fluid-mosaic model of membrane structure
• Fig. 4.1
1-3
Membrane protein diversity
• Fig 4.2
1-4
Permeability of plasma membrane, cont’d.
• Active transport mechanisms-require ATP, can transport against a
concentration gradient
– Active transport
• Requires a carrier protein
• Transports molecules from low concentration to high
– Exocytosis-vesicle mediated transport
• Transports cell products and wastes out of the cell by vesicle formation
– Endocytosis-vesicle mediated transport
• Transports substances into the cell by vesicle formation
• Pinocytosis-”cell drinking”
• Phagocytosis-”cell eating”
– Diffusion: a movement of particles from an area of high concentration
to low concentration across a semi permeable membrane
1-5
4.3 Diffusion and osmosis, cont’d.
• Osmosis-diffusion of water across a semi
permeable membrane
– Osmotic pressure-force that causes water to move
in a direction
– Osmotic pressure is due to the number of
nondiffusable particles in solution
– Hypotonic solutions-cause cells to swell and burst
– Hypertonic solutions-cause cells to shrink, or
crenate
– Isotonic solutions-no change
1-6
3.2 Eukaryotic cells
• Eukaryotic cells have a membrane-bound
nucleus
– Nucleus contains genetic material
– All plant and animal cells are eukaryotic
• Plasma membrane-outer boundary of cell
– Phospholipid bilayer-note arrangement of
hydrophilic and hydrophobic ends
– Embedded proteins
– Associated glycolipids and cholesterol
1-7
Eukaryotic cells, cont’d.
• Organelles-subcellular structures which
perform specific life functions for the cell
• Many organelles are found in both animal and
plant cells
• Some are found exclusively in plants or
animals
– Plants- chloroplasts, central vacuole
– Animals-centrioles
1-8
Eukaryotic cells, cont’d.
• Nucleus
– Contains the genetic material DNA
– Nucleoplasm-semifluid within nucleus
– Chromatin-threadlike DNA which has a grainy
appearance
– Nucleolus-dark regions of chromatin which
produce rRNA which composes ribosomes
– Nuclear membrane- double layered, surrounds
nucleus and has large pores
1-9
Eukaryotic cells, cont’d.
• Ribosomes– site of protein assembly
– Composed of RNA subunits
– Exist either as free ribosomes or bound to
endoplasmic reticulum
• Endomembrane system– Includes nuclear membrane, endoplasmic
reticulum (er), Golgi apparatus, and vesicles
1-10
Eukaryotic cells, cont’d.
• Endomembrane system cont’d.
– Rough endoplasmic reticulum (rer)
•
•
•
•
Complex system of sacs and channels
Has attached ribosomes
Serves as site of assembly of proteins for export
Assembled proteins enter channels for processing
– Addition of sugar chains to form glycoproteins
• Released in vesicles
– Smooth endoplasmic reticulum (ser)
• Synthesizes lipid products such as phospholipids and steroids
• Released in vesicles
1-11
Eukaryotic cells, cont’d.
• Golgi apparatus
–
–
–
–
–
packaging and processing center for cell products
Receives the vesicles from er
Vesicles fuse with Golgi and products are released inside
Further modification of glyoproteins occurs
Products are packaged into secretory vesicles and released
to the cell membrane
– Golgi also produces lysosomes-protein containing vesicles
within cells
1-12
Eukaryotic cells, cont’d.
• Lysosomes
–
–
–
–
Contain hydrolytic enzymes
Fuse with vesicles from cell membrane containing macromolecules
Digestion occurs and nutrients released to cell
Also may be involved in programmed cell death-”suicide sacs”
• Lysosomal membranes in old or damaged cells rupture and enzymes
digest the cell
1-13
Eukaryotic cells, cont’d.
• Mitochondria-another energy related
organelle
– Site of aerobic cell respiration-production of ATP
– Outer double membrane surrounds fluid-filled
matrix
– Inner folded membrane-folds are called cristae
– Cristae provide increased surface area for the
production of ATP
1-14
Metabolic reactions and energy
transformations cont’d.
• ATP- energy for cells
– Adenosine triphosphate
– Generated from ADP + P
• Avantages of ATP as energy carrier:
– Provides a common energy currency for many reactions
– Breakdown of ATP to ADP+P releases sufficient energy for
biological processes- little wasted
– ATP breakdown is coupled to endergonic reactions in such
a way that it minimizes energy loss
1-15
The ATP cycle
• Fig 6.3
1-16
Overview of cellular respiration cont’d.
• Phases of cellular respiration
– Glycolysis
• Breakdown of glucose to 2 molecules of pyruvate
• Oxidation by removal of hydrogens releases enough energy to
make 2 ATP
– Preparatory reaction
• Pyruvate oxidized to acetyl CoA and carbon dioxide is removed
• Prep reaction occurs twice because glycolysis produces 2
pyruvates
– Citric acid cycle
• Acetyl CoA is converted to citric acid and enters the cycle
• Cyclical series of oxidation reactions that produces 1 ATP and
carbon dioxide
• Citric acid cycle turns twice because 2 acetyl CoA’s are produced
per glucose
1-17
Overview of cellular respiration cont’d.
• Phases of cellular respiration, cont’d.
– Electron transport chain
• Series of electron carrier molecules
• Electrons passed from one carrier to another
• As the electrons move from a higher energy state to a
lower one, energy is released to make ATP
• Under aerobic conditions 32-34 ATP per glucose
molecule can be produced
– Pyruvate
• Pivotal metabolite in cellular respiration
• If no oxygen is available, pyruvate is reduced to lactate
(in animals) or ethanol and carbon dioxide (in plants) in
a process called fermentation
1-18
Cellular respiration
• Fig 7.3
1-19
Eukaryotic cells, cont’d.
• The cytoskeleton
– Maintains cell shape
– Allows cells and organelles to move
– Components include actin filaments, intermediate filaments, and
microtubules
– Actin filaments interact with motor molecule myosin in muscles
– Intermediate filaments support the nuclear membrane
– Microtubules protrude from the centrosome and form centrioles, cilia,
and flagella
1-20
Eukaryotic cells, cont’d.
• Centrioles
– Short tubules with 9+0 pattern of microtubule triplets
– In animal cells, centrosome is composed of 2 centrioles
– Believed to be involved in microtubule formation including
mitotic spindle
• Cilia and flagella
– Hair-like projections, cilia generally multiple and flagella
single or double
– 9+2 pattern of microtubules
– Whip-like action
1-21
Actin and intermediate filaments
• Fig 3.12
• Fig 3.13
1-22
Cell increase and decrease, cont’d.
• The cell cycle
– Set of events that occur between the time a cell divides
and the time the resulting daughter cells divide
• Stages of interphase –longest phase of the cycle
– Normal cell functions occur as well as preparation for
division
– G1 stage-organelles double in number, accumulates
materials needed for division
– S stage-DNA replication
– G2 stage-synthesis of proteins needed for division
1-23
The cell cycle
• Fig 5.1 a
1-24
Cell increase and decrease, cont’d.
• Mitotic stage
– Follows interphase
– Includes mitosis and cytokinesis
• Control of cell cycle
– The protein cyclin must be present for stages to progress
– G2 checkpoint-stops cycle if DNA is not done replicating or
is damaged
– M checkpoint-stops if chromosomes not aligned
– G1 checkpoint-protein p53 stops cycle if DNA damaged
1-25
Mitosis overview
• Fig 5.3
1-26
Maintaining the chromosome number
• Mitosis in detail-animal cells
– Prophase-nuclear membrane disappears, centrosomes
migrate, spindle fibers appear
– Metaphase-chromosomes line up at equator, associated
with spindle fibers
– Anaphase-centromeres divide, sister chromatids migrate
to opposite poles, cytokinesis begins
– Telophase-nuclear membranes form, spindle disappears,
cytokinesis occurs
1-27
Maintaining the chromosome number
• Mitosis in plant cells
– Occurs in meristematic tissues
– Same phases as animal cells
– Plant cells do not have centrioles or asters
• Cytokinesis in plant cells
–
–
–
–
Flattened, small disk appears between daughter cells
Golgi apparatus produces vesicles which move to disk
Release molecules which build new cell walls
Vesicle membranes complete plasma membranes
• Cytokinesis in animal cells
• Cleavage furrow forms between daughter nuclei
• Contractile ring contracts deepening the furrow
• Continues until separation is complete
1-28
Overview of meiosis
• Fig 5.9
1-29
Reducing the chromosome number cont’d.
• Phases of meiosis I
– Prophase I
• Synapsis occurs, nuclear membrane breaks down
• Homologues line up side by side and crossing over occurs
– Metaphase I
• Homologous pairs line up at equator such that maternal or
paternal member may be oriented toward either pole
– Anaphase I
• Homologous chromosomes (each still consisting of 2 chromatids)
undergo independent assortment into daughter cells
– Telophase I
• Cytokinesis produces 2 daughter cells which are haploid
1-30
Reducing the chromosome number cont’d.
• Interkinesis-period between meiosis I and meiosis II
• Phases of meiosis II
– Prophase II• Cells have 1 member of each homologous pair
– Metaphase II
• Chromosomes line up at the equator
– Anaphase II
• Centromeres divide and daughter chromosomes
migrate
– Telophase II
• Nuclei form, cytokinesis
1-31
Comparison of mitosis and meiosis con’td.
• Table 5.1
• Table 5.2
1-32
3.3 Prokaryotic cells
• Archae and bacteria are prokaryotes
– Lack a nucleus
– Smaller than eukaryotes
• Prokaryotic cell structures
–
–
–
–
–
–
Cell wall-contains peptidoglycans
Capsule present in some
Plasma membrane
Nucleoid-region which contains a single chromosome
Ribosomes
Thylakoids-in photosynthetic cyanobacteria
1-33