Download Cell and Organelles SG - Bishop Seabury Academy

Biology
Cell Membranes & Organelles
Name________________________
Cell Fractionation:
This process of taking a cell apart, separating the major organelles so that their functions can be studied. Cells
are broken up to help release their contents. The contents are spun in a machine called a centrifuge separate the
parts. The heavier parts sink to the bottom and form a pellet. The liquid left over is called the supernatant. This
is then transferred to another test tube and re-spun at a higher rate. This will continue until several pellets have
been formed of decreasing size.
Prokaryotic and Eukaryotic Cells:
Prokaryotic
1. No nuclear membrane ( nucleoid region)
2. No membrane bound organelles
3. Found only in The kingdoms Eubacteria and
Archaebacteria
4. Size 0.1 um - 10 um.
Eukaryotic
1. Definite membrane bound nucleus
2. Contains membrane bound organelles
3. Found in Kingdoms: Protista, Animal,
Fungi, and Plantae
4. 10 um-100 um.
Cell Size:
...is governed by several factors: The size of the cell is controlled by metabolic requirements. DNA must be
available to produce the enzymes and proteins needed for proper functioning. A too-small cell will not have
enough DNA to support life and a cell too large will need an enormous amount of DNA to carry on its
functions.
A second restriction involves surface area to volume ratio. As the cell increases in size, its volume
increases geometrically while its surface area increases arithmetically. Eukaryotic cells cope with these
problems in that they contain membrane bound organelles. These organelles break up the volume of the cell
performing distinct functions which cut down on the raw materials needed. Each part of the cell does not need
the same material to function.
Nucleus, Nucleolus, & Ribosomes (protein synthesis):
The Nucleus: -contains most of the genes of the cell.
- size is about 5um. in diameter
- surrounded by a lipid bilayer perforated with pores (nuclear envelope)
- contains chromatin (“chromosomes” when condensed) made of DNA and protein.
The Nucleolus: - produces ribosomes at a rate of 10,000 / min.
- ribosomes undergo final assembly in the cytoplasm
- mRNA is sent out from the nucleus to control protein synthesis
(we’ll discuss this again later)
Ribosomes:
- function in protein synthesis
- not considered a “membrane-bound” organelle.
- built from 2 subunits (only functional when these subunits are joined together)
- Large subunit is composed of 45 proteins and 3 molecules of RNA
1
- small subunit consists of 33 proteins molecules and 1 RNA.
There are 2 types of ribosomes:
- bound are found attached to the ER (proteins made here become part of the
membrane system or exported from the cell).
- free ones float around in the cytosol (proteins made here usually stay in the cell)
* How does a ribosome know if it will be free or bound? Signal sequence (20 amino acids found at the
start of a protein being coded by the ribosome alerts the ribosome to attach itself to the ER. If the
sequence is missing it will remain free.
Endomembrane system = ER + Golgi + Vesicles:
Endoplasmic Reticulum:
- network of membranous tubules and sacs
- continuous with the nuclear membrane
There are 2 types of ER
- Rough called so because of the presence of ribosomes
Functions: Antibody production occurs in the rough ER. Many secretory
proteins are also produced here. These proteins are glycoproteins, proteins
and carbohydrates combined. The carbohydrate is called an
oligosaccharide. These secretory proteins are bound in special structures
called transport vesicles. The Rough ER is involved with membrane
production.
- Smooth does not contain ribosomes
Functions: synthesis of lipids (including hormones), carbohydrate metabolism, and
detoxification of drugs and poisons. It also helps in muscle contraction by regulating the
calcium ions in the muscle.
Golgi Apparatus: - a series of flattened membranous sacs (cisternae)
- synthesis, storing, sorting and shipping of materials.
- cisternae have 2 distinct faces (differing thickness and molecular composition)
- cis face receives the material (close to the ER)
- trans face ships materials and gives rise to vesicles (once full of material they pinch
off).
- product modified via enzymes: modification puts “address labels” on product
Lysosomes:
- a bag of hydrolytic enzymes (proteins, polysaccharides, fats, nucleic acids)
- optimum pH of 5 is maintained by continuously pumping of H ions into the
vesicle.
- enzymes more or less inactive in the cytosol (pH 7) unless in high concentration
- autodigestion
- some are produced by the trans face of the Golgi Apparatus
Functions: intracellular digestion (food), autophagy (recycling cellular materials), and
programmed destruction (anuran metamorphosis).
* In humans Tay-Sachs disease is caused by a lack of lipid digesting enzymes and an accumulation of
lipids in the brain. See also Pompe's disease with carbohydrate enzymes and ultimately, liver damage.
2
Non-Endomembrane System Organelles:
Microbodies: membrane bound enzyme filled structures able to function in a particular metabolic pathway.
- Peroxisomes detoxify alcohol and other poisons by producing H2O2 transferring a H from the
poison to water.
- Glyoxysomes (specialized peroxisomes) found in fat storing tissue of seeds and plants.
Vacuoles:
- Sacs that are larger than vesicles
- function in digestion, pumping excess material, food & waste storage, cell stabilizer
- found in many protists and plants
Energy-Related Organelles:
Mitochondria: - converts energy to forms the cell can use (cellular respiration)
- contains own DNA
- not truly part of the endomembrane system
- membrane is self manufactured (own ribosomes) or free ribosomes
- double membrane structure
- outer membrane is smooth
- inner is folded (called cristae) to increase surface area
- The area between the 2 membranes is called the inner membrane space.
- inner most area of the mitochondrion is called the mitochondrial matrix (where
most of the rxns for cellular respiration occur)
- number of mitochondria is correlated with cell function
Chloroplasts: - specialized member of a family of closely related plant organelles called plastids. Similar in
hypothesized history to mitochondria.
- Amyloplasts store starch
- Chromoplasts contain orange and yellow pigments
- Chloroplasts contain chlorophyll and site of photosynthesis
- measure 2um- 5um in diameter
- double membrane structure
- outer membrane is smooth
- inner membrane formed of stacks of “pancakes” (thylakoid membranes). - the
individual “pancakes” are called grana.
- area between the thylakoid and outer membrane is called the stroma.
Organelles for Structure and Movement:
Cytoskeleton: - important structural (and possibly communication) component of cell
- anchoring site for organelles
- easily broken down and reassembled, allowing the cell to move
- involved in motor movement (cilia and flagella)
- vesicles may move along cytoskeletal rail systems on way to membrane
- contains microtubules and microfilaments, intermediate filaments
Actin filaments (microfilaments): 2 interwoven pieces of actin 7nm. in diameter often associated with
Myosin, another protein.
- They function in muscle contraction (actin and myosin sliding over each other)
3
- cytoplasmic streaming (plants)
- ameboid movement (pseudopodia)
- changes in cell shape (white blood cells)
Intermediate filaments seem to be most important in cell structure.
- Structure and function not well understood
Microtubules : straight hollow rods measuring about 25 nm. thick
- constructed from protein tubulin
- They shape, support and help move organelles around the cell
- Organized by the MTOC (microtubule organizing center) in the centrosome
- dynein & kinesin (protein) “walking”
Centrioles:
- not found in plants or fungi.
- 9 + 0 triplets, arrangement of microtubules
- pairs of centrioles found in animal centrosomes at at 90o angles to each other
- involved in forming the mitotic spindle
Cilia and Flagella – membrane-bound cylinders filled with matrix material
- 9 + 2 pairs, arrangement of microtubules
- centriole-like basal body
Outside the Cell:
The Plasma Membrane (Cell Membrane): A fluid mosaic of lipids, proteins, and carbohydrates
A. Selective Permeability
1. Nonpolar molecules (e.g.: hydrocarbons, O2, CO2)
2. Polar molecules (compared in a synthetic membrane)
a. Small uncharged polar molecules (e.g.: H2O, EtOH) pass easily
b. Large uncharged polar molecules (e.g.: glucose) do not pass easily
c. All ions (charged atoms or groups of atoms) have difficulty passing hydrophobic layer
B. Passive Transport: diffusion across a biological membrane
1. Spontaneous
2. Passive
3. Rate regulated by permeability of membrane
4. Water diffuses freely across most cell membranes (osmosis)
C.
Osmosis: passive transport of water
1. Relative concentration of a solution outside with respect to the inside of a cell
a. hypertonic = cells crenate (shrivel)
b. isotonic = cell volume stable
c. hypotonic = cell gains water and may lyse (burst)
D.
Facilitated Diffusion: specific proteins facilitate the passive diffusion of selected solutes
E.
Active Transport: pumping solutes against their gradients
1. Cell must expend energy
2. Helps maintain steep ionic gradients across the cell membrane (Na+, K+, Mg2+, Ca2+, Cl-)
3. Transport proteins use ATP to accomplish this
F.
Exocytosis and Endocytosis (large molecules: proteins and polysaccharides)
1. Exocytosis
a. Fusion of vesicles with cell membrane
b. Vesicle usually originates in ER or Golgi and migrates to plasma membrane
4
c. Used by secretory cells to export products
2. Endocytosis
a. Phagocytosis (cell eating)
i. Cell engulfs particle with pseudopodia
ii. Vacuole fuses with a lysosome
b. Pinocytosis (cell drinking)
i. Droplets of liquid taken into small vesicles
ii. Non-discriminating
Cell Wall: Found in plants and fungi, and some protists.
- Protection, maintains shape, prevents lysis because of water uptake
- Structure: Primary wall (plants): thin and flexible (first wall laid down)
secondary wall (plants): strong and durable, multi-layered (wood)
sticky middle lamella between adjacent cells (middle lamella rich
in pectins (polysaccharides))
Extracellular Matrix: In animal cells the glycocalyx is found between cells
- made of sticky oligosaccharides that act as glue to keep the cells together.
- collagen is the most abundant glycoprotein (about ½ the total protein in the
human body!)
- collagen fiber is used for strength and support
Intercellular Junctions:
1. Tight junctions. bind cells together in such a way that no material can pass through the intercellular spaces.
Epithelial cells are held together by tight junctions.
2. Desmosomes: These bind the cells together like rivets. They let material pass through the intracellular
spaces.
3. Gap Junctions: They connect cells but allow material to pass from one cell to another through the opening in
the center of the joint.
5