STEM Unit 2

- The single circular chromosome is found in the nucleoid region, but there is no nuclear membrane and no true nucleus.

- No membrane-bounded organelles are found in the cytosol. (Ribosomes are found, but they are not membrane-bound.)

- From the preceding table, notice how much smaller prokaryotes are than eukaryotes.

- a membrane enclosed nucleus contains the cell's linear chromosomes.

- many membrane-bounded organelles are found in the cytoplasm

- on average, eukaryotes are much larger than prokaryotes.

The surface area-to-volume ratio becomes less favorable as a cell increases in siz.e The total volume grows proportionately more than the surface area. Because a cell acquires resources through the plasma membrna, cell size is limited.

1) lysosomes

2) Centrioles

3) Flagella

- The nucleus contains most of the cell's DNA. It is in the nucleus where DNA is used as the template to make mRNA, which contains the code to produce a protein. Because the nucleus contains the genetic information, it is referred to as the control center of the cell.

- The nucleus is the most noticeable organelle in the cell because of its large relative size. The nucleus is surrounded by a double membrane, the nuclear envelope. Note that the nuclear envelope is continuous with the rough endoplasmic reticulum. The nuclear envelope contains nuclear pores that control what may enter or leave the nucleus.

- Chromatin is the complex of DNA and protein housed in the nucleus that is formed from the chromosomes. As a cell gets ready for cell division, the diffuse threads of chromatin condense back into visible chromosomes.

- The nucleolus is a region of the nucleus where rRNA complexes with proteins to form ribosomal subunits.

Nucleolus is a region of the nucleus where rRNA complexes with proteins to form ribosomal subunits.

Chromatin is the complex of DNA and protein housed in the nucleus that is formed from the chromosomes. As a cell gets ready for cell division, the diffuse threads of chromatin condense back into visible chromosomes.

The nuclear envelope is continuous with the rough endoplasmic reticulum. The nuclear envelope contains nuclear pores that control what may enter or leave the nucleus.

Ribosomes are protein factories. They are composed of rRNA and protein and sites of protein synthesis in the cell. Each ribosome consists of a large and a small subunit.

Free Ribosomes: found floating in the cytosol and generally produce proteins that are used within the cell.

Bound Ribosomes: attached to the endoplasmic reticulum and make proteins destined for export from the cell.

ER makes up more than half of the total membrane structure in many cells. The ER is a network of membranes and sacs whose internal area is called the cisternal space.

- Smooth ER has three primary functions: synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons.

- Rough ER is called because its associated ribosomes make the structure appear rough under the microscope. Ribosomes associated with ER synthesize proteins that are generally secreted by the cell. As the proteins are produced by the ER-bound ribosomes, the polypeptide chains travel across the ER membrane and into the cisternal space. Within the cisternal space the proteins are packaged into transport vesicles, which bud off the ER and move toward the Golgi Apparatus.

The golgi apparatus operates something like the postal system - proteins from the transport vesicles are modified, stored, and shipped. The golgi apparatus consists of flattened sacs of membranes called cisternae, arranged in stacks. Golgi stacks have polarity--the cis face recieves vesicles, whereas the trans face ships vesicles. Products of the ER are modified here, and the Golgi apparatus is extensive in cells specialized for secretion.

Lysosomes are membrane-bound sacs of hydrolytic enzymes that can digest large molecules, including proteins, polysaccarides, fats, and nucelic acids.

They have digestive enzymes that break down macromolecules to organic monomers that are released into the cytosol and thus recycled by the cell. The digestive or hydrolytic enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or leaks, the enzymes are not very active in the neutral pH of the cell. This is a good example of the importance of cell compartmentalization.

Vacuoles are membrane-bound vesicles. Food vacuoles such as those formed by phagocytosis or protists are one example, as are the contractile vacuoles that maintain water balance in paramecia and other protists.

central vacuoles in plant cells may concentrate and contain compounds not found in the cytosol. A large central vacuole is one of the striking differences between plant and animal cells. In plants, a vacuole can make up as much as 80% of the cell.

Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to generate ATP by extracting energy from sugars, fats, and other fuels. Mitochondria are found in both plant and animal cells.

- Mitochondria consist of an outer and inner membrane. The inner membrane is highly folded. These cristae (folds) increase the surface area, enhancing the productivity of cellular respiration.

- The inner compartment, the mitochondrial matrix, is fluid-filled and many of the reactions of cellular respiration occur here

- The mitochondrial matrix contains mitochondrial DNA seperate from nuclear DNA as well as ribosomes.

Chloroplasts found in plants and algae, are the sites of photosynthesis.

Inside the chloroplast is another membranous system in the form of clattened, interconnected sacs called thylakoids. In some regions, thylakoids are stakced like poker chips; each stack is called a granum. The fluid outisde the thylakoids is the stroma, which contains the chloroplasts DNA and ribosomes as well as many enzymes.

- both organelles have a double membrane structure
- both organelles have their own ribosomes and circular DNA molecules
- both reproduce independently within the cell.

Peroxisomes are single-membrane-bound compartments in the cell responsible for various metabolic functions that involove the transfer in hydrogen from compounds to oxygen, producing hydrogen peroxide (H2O2).

Peroxisomes break down fatty acids to be sent to the mitochondria for fuel and detoxify alcohol by transferring hydrogen from the poison to oxygen.

The cytoskeleton is a network of protein fibers that runs throughout the cytoplasm, where it is responsible for support, motility, and regulating some biochemical activities. Three types of fibers make up the cytoskelton: Microtubules, microfilaments, and intermiediate filaments.

made of protein tubulin, are the largest of the cytoskeleton fibers.

Microtubules shape and support the cell and also serve as tracks along which organelles equipped with motor molecules can move.

They also separate chromosomes during mitosis and meiosis (forming the spindle) and are the structural components of cilia and flagella.

are composed of the protein actin. Much smaller than microtubules, microfilaments funciton in smaller-scale support.

When coupled with the motor molecule myosin, microfilaments can be involved with the movement. Examples: ameoboid movement, cytoplasmic streaming, and contraction of muscles.

are slightly larger than microfilaments and smaller than microtubules. Intermediate fibers are more permanent fixtures in the cell, where they are important in maintaining the shape of the cell and fixin the position of certain organelles.

Centrosomes are a region located near the nucleus from which microtubules grow (the area is also called the microtubule-organizing center). Centrosomes contain centrioles in animal cells.

Centrioles are located within the centrosomes of animal cells, where they replicate before cell division.

Flagella are usually long and few in number. Many unicellular eukaryotic organisms are propelled through the water by flagella, as are the sperm of animlas, algae, and some plants.

Cilia are usually much shorter and more numerous than flagella. Cilia can also be used in locomotion or, when held in place as part of a tissue layer, they can move fluid over the surface of the tissue. For example, the lining of the trachea moves mucus-trapped debris out of the lungs in this manner.

The cell wall of a plant protects the plant and helps maintain its shape. It is outside the plasma membrane. The primary component of cell walls is the carbohydrate cellulose.

Plasmodesmata are channels that perforate adjacent plant cell walls and allow the passage of some molecules from cell to cell.

ECM of animal cells is situated just external to the plasma membrane; it is composed of glycoproteins secreted by the cell (the most prominent of which is collagen).

The ECM greatly strengthens tissues and serves as a conduit for transmitting external stimuli into the cell, which can turn geners on and modify biochemical activity.

- Tight junctions: these are sections of the animal cell membranes where two neighboring cells are fused, making the membranes watertight.

- Desmosomes: fasten adjacent animal cells together, functioning like rivets to fasten cells into strong sheets.

- Gap Junctions: provide channels between adjacent animal cells through which ions, sugars, communication molecules, and other small molecules can pass.

Integral membrane proteins reside within the bilayer membranes that surround cells and organelles, playing critical roles in movement of molecules across them and the transduction of energy and signals.

Peripheral proteins form temporary bonds with the cell membrane, allowing them to detach and reattach at specific times, with specific signals. This allows cells to coordinate and communicate using networks of proteins and reactions.

The key difference between integral peripheral and surface proteins is that integral and surface proteins are permanently embedded within the plasma membrane while peripheral protein is temporally associated with the plasma membrane.

Carbohydrates on the membrane are crucial in cell-cell recognition (which is necessary for proper immune function) and in developing organisms (for tissue differentiation). Cell-surface carbohydrates vary from tissue to tissue and are the reason that blood transfusions must be type-specific.

The hydrophobic core of the membrane impedes the passage of ions and polar molecules, which are hydrophillic. However, hydrophillic substances can avoid the lipid bilayer by passing through transport proteins that span the membrane.

Perhaps the most important molecule to move across the membrane is water. Water moves through special transport proteins termed aquaporins.

Aquaporins greatly accelerate the speed at which water can cross membranes.

In passive diffusion, a substance travels from where it is more concentrated to where it is less concentrated, diffusing down its concentration gradient. Hydrocarbons, carbon diozide, and oxygen (O2) are hydrophobic substances that can pass easily across the cell membrane by passive diffusion. This type of diffusion requires that no work be done, and it relies only on the thermal motion energy intrinsic to the moleucle in question. It is called "passive" becuase the cell expends mo energy in moving the substance.

The diffusion of water across a selectively permeable membrane is osmosis.

- In an isotonic solution there will be no net movement of water across the plasma membrane. Water crosses the membrane but at the same rate in both solutions

- In a hypertonic solution the the cell will lose water to its surroundings. The hyper- prefix refers to more solutes in the water around the cell; hence, the movement of water to the higher (hyper-) concentration of solutes. In this case the cell loses water to the environment, will shrivel, and may die.

- In a hypotonic solution water will enter the cell faster than it leaves. The hypo-prefix refers to fewer solutes in the water around the cell; hence, the movement of water into the cell where solutes are more heavilt concentrated. In this case the cell will swell and may burst.

Water moves from a hypotonic solution to a hypertonic solution. Hypo-->Hyper

Ions and polar molecules cannot pass easily across the membrane. The process by which ions and hydrophillic substances diffuse across the cell membrane with the help of transport proteins is called facilitated diffusion. transport proteins are specific (like enzymes) for the substance they transport. They work in one of two ways:

- They provide a hydrophillic channel through which the molecules in question can pass
- They bind loosely to the molecules in question and carry them through the membrane.

In active transport, substances are moved against their concentration gradient, from the region where they are less concentrated to the region where they are more concentrated. This type of transport requires energy, usually in the form of ATP.

It is an example of active transport. This transmembrane protein pumps sodium out of the cell and potassium into the cell. The sodium-potassium is necessary for proper nerve transmission and is a major energy consumer in your body as you read this.

The inside of the cell is negatively charged compared with outside the cell. The difference in electric charge across a membrane is expressed in voltage and termed the membrane potential.

Because the inside of the cell is negatively charged, a positively charged ion on the outside, like sodium, is attracted to the negative charges inside the cell. Thus two forces drive the diffusion of ions across a membrane.

- a chemical force, which is the ion's concentration gradient
- a voltage gradient across the membrane attracts positively charged ions and repels negatively charged ions.

This combination of forces is called an electrochemical gradient.

In exocytosis, vesicles from the cell's interior fuse with the cell membrane, expelling their contents.

In endocytosis, the cell forms new vesicles from the plasma membrane; this is basically the reverse of exocytosis, and this process allows the cell to take in macromolucules. Examples: engulfing of froeign particles by white blood cells or amoebas.

- Phagocytosis: a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes.

- Pinocytosis: an energy-consuming process where extracellular fluid and solutes are taken up into a cell via small vesicles.

- Receptor-mediated endocytosis: a means to import macromolecules from the extracellular fluid. More than 20 different receptors are internalized through this pathway. Some receptors are internalized continuously, whereas others remain on the surface until a ligand is bound.

The target cell's detection of a signal molecule coming from outside the cell.

The conversion of the signal to a form that can bring about a specific cellular response

The specific cellular response to the signal molecule.

Ligand

- Intracellular receptors are found inside the plasma membrane in the cytoplasm or nucleus. The signal molecule must cross the plasma membrane and, therefore, be hydrophobic, like the steroid hormone testosterone.

- Plasma membrane receptors bind to water-soluble ligands

- Step 1: The ligand or signaling molecule has bound to the G protein-couped receptor. This causes a conformational change in the receptor so that it may now bind to an inactive G protein, causing a GTP to displace the GDP. This activates the G-protein.

- Step 2: The G protein binds to a specific enzyme and activates it. When the enzyme is activated, it can trigger the next step in a pathway leading to a cellular response. All the molecular shape changes are temporary. To continue the cellular response, new signal molecules are required.

Step 1: shows the binding of signal molecules to the receptors and the subsequent formation of a dimer. In the dimer configuration each tyrosine kinase adds a phosphate from an ATP molecule.

Step 2: shows the fully activated receptor protein as it initiates a unique cellular response for each phosphorylated tyrosine.

The ability of a single ligand to activate multiple cellular responses is a key difference between G protein-couped receptors and receptor tyrosine kinases.

Specific signal molecules cause ligand-gated ion channels in a membrane to open or close, regulating the flow of specific ions. When the ligad opens the gate and ions flow into the cell, the change in ion concentration can result in a change in cell activity.

Signal transduction pathways often involve a phosphorylation cascade. Because the pathway is usually a multistep, the possibility of greatly amplifying the signal exists. At each step enzymes are called protein kinases phosphorylate and thereby activate many proteins at the next level. This cascade of phosphorylation greatly enhances the signal, allowing for a large cellular response.

Protein phosphatases are enzymes that remove phosphate groups and inactive protein kinases. Thus, the signal can be turned on by kinases and off by phosphatases.

Not all components of signal transduction pathways are proteins. Many signaling pathways incole small, nonprotein water-soluable molecules or ions called second messengers. Calcium ions and cyclic AMP are two common second messengers. The second messenger, once activated, can initate a phosphorylation cascade resulting in a cellular response.

Cell suicide. During apoptosis, the cell is systematically dismantled and digested. This protects neighboring cells from damage if a dying cell merely leaks out its digestive and other enzymes.

- Apoptosis is triggered by signals that activate a cascade of "suicide" proteins in the cells.
- In vertebrates, apoptosis is a normal part of the development and is essential for a normal nervous system, the immune system's operation, and normal morphogenesis of hands and feet in humans.

somatic cells (all body cells except gametes) have 46 chromosomes, which is the diploid chromosome number.

Mitosis is the process by which somatic cells divide, forming daughter cells that contain the same chromosome number as the parent cell.

Gametes-sperm and egg cells-are haploid and have half the number of chromosomes as a diploid cell. Human gametes have 23 chromosomes.

A special type of cell division called meiosis results in gametes.

Mitosis is the divison of the cell's nucleus

division of the cell's cytoplasm

It is 90% of the cell cycle. During interphase, the cell grows, replicates its DNA and prepares for mitosis.

- G1 phase
- S phase
- G2 phase

First phase of interphase. In the G1 phase the cell grows while carrying out cell functions unique to its cell type.

In the S phase the cell continues to carry out its unique functions but does one other important process-it duplicates its chromosomes. This means it faithfully makes a copy of the DNA that makes up the cell's chromosomes.

The G2 phase is the period after the chromosomes have been duplicated and just before mitosis. The cell continues to grow and carry out its functions during this time.

1) prophase

2) prometaphase

3) metaphase

4) anaphase

5) telophase

1. The chromatin becomes more tightly coiled into discrete chromosomes.

2. The nucleoli disappear.

3. The mitotic spindle ( consisting of micro tubules extending from the two centrosomes) begins to form in the cytoplasm.

1. The nuclear envelope begins to fragment, allowing the microtubules to attach to the chromosomes.

2. The two chromatids of each chromosome are held together by the centromere. The centromere contains protein kinetochores on each chromatid, which is where the microtubules will attach.

1. The microtubules move the chromosomes to the metaphase plate at the equator of the cell. The microtubule complex is referred to as the spindle.

2. The centrioles have migrated to opposite poles in the cell, riding along on the developing spindle.

1. Sister chromatids begin to separate, pulled apart by motor molecules interacting with kinetochore microtubules.

2. The cell elongates, as the nonkinetochore microtubules ratchet apart, again with the help of motor molecules.

3. By the end of anaphase, the opposite ends of the cell both contain complete and equal sets of chromosomes.

1. The nuclear envelopes re-form around the sets of chromosomes located at opposite ends of the cell.

2. The chromatin fiber of the chromosomes becomes less condensed.

3. Cytokinesis begins, during which the cell's cytoplasm is divided. In animal cells, a cleavage furrow forms that eventually divides the cytoplasm; in plant cells, a cell plate forms that divide the cytoplasm.

4. Prokaryotes replicate their genome by binary fission rather than
mitosis.

G1 phase checkpoint, G2 phase checkpoint, and M phase checkpoint.

The G1 phase checkpoint seems to be most important. If the cell gets the goahead signal at this checkpoint, it usually completes the whole cell cycle and divides. If it does not receive the go-ahead signal, it enters a nondividing phase called the G0 phase.

Kinases are the protein enzymes that control the cell cycle.

They exist in the cells at all times but are active only when they are connected to cydin proteins. Thus, they are called cydin-dependent kinases (Cdks). Specific kinases give the go-ahead signals at the G1 and G2 checkpoints.

The phenomenon in which crowded cells stop dividing.

Normal cells must be attached to a substratum, like the extracellular matrix of a tissue, to divide.

Transformation is the process that converts a normal cell to a cancer cell.

A tumor is a mass of abnormal cells within otherwise normal tissue.

- The lump is a benign tumor if the abnormal cells remain at the original site.
- A malignant tumor becomes invasive enough to impair the functions of one or more organs.