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Cells functions and structures (Tools to Study Cells: Microscopes…
Cells functions and structures
Tools to Study Cells:
Microscopes
Microscopes development helped the discovery and early study of cells
Light Microscopes (LM)
In a
light microscope (LM)
, visible light passes through a specimen and then through glass lenses
Lenses refract(bend) the light so that the image is magnified
2D: Compound Light
3D: Dissecting microscope/ stereomicroscope
Electron Microscopes (EM)
The
electron microscope (EM)
focuses a beam of electrons through the specimen or onto its surface.
3D:
Scanning electron microscopes (SEMs)
focus a beam of electrons onto the surface of a specimen, providing images that look 3-D
2D:
Transmission electron microscopes (TEMs)
focus a beam of electrons through a specimen. Is used to study the internal structure of cells.
3 Important parameters in microscopy
Magnification, the ratio of an object’s image
Resolution, the measure of the clarity of the image, or the minimum distance of two distinguishable points
Contrast, visible differences in brightness between parts of the sample
Techniques to Study Cells:
Cell Fractionation
Cell fractionation
takes cells apart and separates the major organelles and other subcellular structures from one another
An equipment called a
centrifuge
that is used for this task involves it to spin test tubes holding mixtures of disrupted cells, at a series of increasing speeds
Cell fractionation
enables scientists to determine the functions of organelles
Surface area vs. Volume area
Metabolic requirements set upper limits on the size of cells
The surface area to volume ratio of a cell is critical
Large volume = Large surface area
Small volume = small surface area
As a cell increases in size, its volume grows proportionately less than its volume (Area is proportional to a linear dimension squared, whereas volume is proportional to a linear dimension cubed.)
Thus a smaller object has a greater ratio of of surface area to volume
The
plasma membrane
is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell
Cytoskeleton
Improvements in both light microscopy and electron microscopy have revealed the
cytoskeleton,
a network of fibers extending throughout the cytoplasm
The eukaryotic cytoskeleton is composed of 3 types of molecular structures:
Microtubules
are the thickest of the three components of the cytoskeleton
Are constructed of dimers of tubulin
Functions of microtubules:
Shaping the cell
Guiding movement of organelles
Separating chromosomes during cell division
In animal cells, microtubules grow out from a
centrosome
near the nucleus
In animal cells, the centrosome has a pair of
centrioles
, each with nine triplets of microtubules arranged in a ring
Microfilaments
also called actin filaments, are the thinnest components
built as a twisted double chain of
actin
subunits
A network of microfilaments helps support the cell’s shape
They form a
cortex
just inside the plasma membrane to help support the cell’s shape
Bundles of microfilaments make up the core of microvilli of intestinal cells
Microfilaments that function in cellular motility contain the protein
myosin
in addition to actin
Cells crawl along a surface by extending
pseudopodia
(cellular extensions) and moving toward them
Cytoplasmic streaming
is a circular flow of cytoplasm within cells, driven by actin-myosin interactions
Intermediate filaments
are fibers with diameters in a middle range
larger than microfilaments but smaller than microtubules
Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes
They support cell shape and fix organelles in place
It organizes the cell’s structures and activities, anchoring many organelles
Support and motility
The cytoskeleton helps to support the cell and maintain its shape
It interacts with
motor proteins
to produce cell motility
Inside the cell, vesicles can travel along tracks provided by the cytoskeleton
Extracellular components and connections
Plant
cell walls
are made of cellulose fibers embedded in other polysaccharides and proteins
Animal cells secret glycoproteins and proteoglycans that form the
extracellular matrix (ECM)
, which functions in support, adhesion, movement, and regulation
Cell junctions connect neighboring cells . Plants have
plasmodesmata
that pass through adjoining cell walls. Animal cells have
tight junctions
,
desmosomes
, and
gap junction
.
Tour of the Cell
All cells have
Cell membrane/ plasma membrane
Semifluid substance (called cytosol)
Ribosomes (make proteins)
Genetic material
Prokaryotic cells
Have no nucleus
DNA is in an unbound region called the
nucleoid
No membrane-bound organelles
Cytoplasm bound by the plasma membrane
Eukaryotic cells
DNA is in a nucleus that is bounded by a double membrane
Ribosomes use the information from the DNA to make proteins
Ribosomes
are complexes made of ribosomal RNA and protein
Ribosomes carry out protein synthesis in two locations:
In the cytosol (free ribosomes)
On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)
The
nuclear envelope
encloses the nucleus, separating it from the cytoplasm
Pores, lined with a structure called a pore complex, regulate the entry and exit of molecules from the nucleus
The nuclear size of the envelope is lined by the
nuclear lamina
, which is composed of proteins and maintains the shape of the nucleus
The nuclear envelope is a double membrane; each membrane consists of a lipid bilayer
DNA is organized into discrete units called
chromosomes
Each chromosome contains one DNA molecule associated with proteins, called
chromatin
Chromatin condenses to form discrete chromosomes as a cell prepares to divide
The
nucleolus
is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis
Membrane-bound organelles
Cytoplasm is in the region between the plasma membrane and nucleus
Endomembrane system (Category consists of)
Nuclear envelope
Endoplasmic reticulum
The
endoplasmic reticulum (ER)
accounts for more than half of the total membrane in many eukaryotic cells
The ER membrane stays connected with the nuclear envelope
There are 2 regions of ER
Smooth ER
, which lacks ribosomes
Functions
Synthesizes lipids
Metabolizes carbohydrates
Detoxifies drugs and poisons
Stores calcium ions
Rough ER
, whose surface is studded with ribosomes
Functions
Has bound ribosomes, which secrete
glycoproteins
(proteins covalently bonded to carbohydrates)
Distributes
transport vesicles
, secretory proteins surrounded by membranes
Is a membrane factory for the cell
Golgi apparatus
The
Golgi apparatus
consists of flattened membranous sacs called cisternae
Functions
Modifies products of the ER
Manufactures certain macromolecules
Sorts and packages materials into transport vesicles
Lysosomes
A
lysosome
is a membranous sac of hydrolytic enzymes that can digest macromolecules
Lysosomal enzymes work best in the acidic environment inside the lysosome
Hydrolytic enzymes and lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing
Some types of cell can engulf another cell by
phagocytosis
; this forms a food vacuole
A lysosome fuses with the food vacuole and digests the molecules
Then the digestion product pass into the cytosol and become nutrients for the cell
Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy
Vacuoles
Vacuoles
are large vesicles derived from the ER and Golgi apparatus
Vacuoles perform a variety of functions in diff. cells
Animal cells
Food vacuoles
are formed by phagocytosis
Unicellular eukaryotes
Contractile vacuoles
pump excess water out of cells
Plant cells
Central vacuoles
hold organic compounds and water
Plasma membrane
These components are either continuous or connected via transfer by
vesicles
Energy organelles (Category consist of)
Mitochondria
Mitochondria
are the sites of cellular respiration, a metabolic process that uses oxygen to generate ATP
They have a smooth outer membrane and an inner membrane folded into
cristae
The inner membrane creates two compartments: intermembrane space and
mitochondrial matrix
Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix
Cristae present a large surface area for enzymes that synthesize ATP
Chloroplasts
Chloroplasts
, found in plants and algae, are the sites of photosynthesis
Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis
Chloroplasts are found in leaves and other green organs of plants and in algae
Structures
Thylakoids, membranous sacs, stacked to form a granum
Stroma, the internal fluid
The chloroplast is one of a group of plant organelles, called
plastids
Mitochondria & Chloroplasts
Similarities
Enveloped by a double membrane
Contain free ribosomes and circular DNA molecules
Grow and reproduce somewhat independently in cells
These similarities lead to
endosymbiont theory
Suggests these organelles (mitochondria and chloroplasts) were once engulfed in a eukaryotic cell
Endosymbiont
(a cell living within a cell)
Peroxisomes
Peroxisomes are oxidative organelles
Chloroplasts and mitochondria cooperate with peroxisomes in certain metabolic functions
The
peroxisome
is specialized metabolic compartment bounded by a single membrane
Peroxisomes produce hydrogen peroxide and convert it to water
Peroxisomes perform reactions with many different functions
How peroxisomes are related to other organelles is still unknown
Cellular Membrane
In the
fluid mosaic model, amphipathic
proteins are embedded in the phospholipid bilayer
Phospholipids and some proteins move sideways within the membrane. The unsaturated hydrocarbon tails of some phospholipids keep membranes fluid lower temperatures, while cholesterol helps membrane resist changes in fluidity caused by temperature changes
Membranes are held together mainly by weak hydrophobic interactions
Membrane fluidity
Most of the lipids and some proteins can move sideways within the membrane
Rarely, a lipid may flip-flop across the membrane, from one phospholipid layer to the other
As temperatures cool, membranes switch from a fluid state to a solid state
As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Membranes must be fluid to work properly; membranes are usually about as fluid as salad oil
Membrane lipid composition
Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
Membrane proteins and their functions
Somewhat like a tile mosaic, a membrane is a collage of different proteins, often clustered in groups, embedded in the fluid matrix of the lipid bilayer
Phospholipids form the main fabric of the membrane
Proteins determine most of the membrane’s functions
Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core
Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into α helices
Types of diffusion
Diffusion
is the spontaneous movement of a substance down its concentration gradient
In
facilitated diffusion
, a transport protein speeds the movement of water or a solute across a membrane down its concentration gradient
Ion channels
facilitate the diffusion of ions across a membrane
Water diffuses out through the permeable membrane of a cell (osmosis)
Isotonic solution
: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
Hypertonic solution
: Solute concentration is greater than that inside the cell; cell loses water
Hypotonic solution
: Solute concentration is less than that inside the cell; cell gains water
Cells without cell walls will shrivel in hypertonic solution and lyse (burst) in a hypotonic solution