Chp 3

THE CELL

Sherwood Chp 2

OUTLINE

I- CELL STRUCTURE

II- MOVEMENT ACROSS CYTOPLASMIC MEMBRANE

III- CELL CYCLE

IV- REPLICATION-TRANSCRIPTION-TRANSLATION

I- CELL STRUCTURE

Cell - is the basic, living structural, and functional unit of the body.

http://www.microscopy.fsu.edu/cells/animalcell.html

Plasma (cell) membrane or cell membrane or plasmalemma – (Sherwood chp 3) Outer, limiting membrane separating the cell's internal components from the external environment. It is the gate keeper that regulates passage of substances into and out of the cell. It is formed by a phospholipid bilayer, the fatty acid chains being sandwiched between the outer phosphoric "heads". Each molecule of phospholipids is free to move relative to each other. This is called the fluid mosaic model. Within this phospholipid bilayer are embedded three different types of proteins.

Plasma Membrane Structure

Receptor proteins span the width of the membrane, form a special shape that can recognize (or bind to) a signal, such as a hormone, a neurotransmitter circulating in the blood or present in the interstitial tissue.

Integral proteins span the entire width of the membrane. They are usually involved in molecular transport (passive and active) and form pores.

Peripheral proteins are located inside the plasma membrane. They tend to have enzymatic reactions. They react to signals received by the receptor and initiate a cell reaction.

The plasma membrane is soluble to small, neutral molecules, such as H2O, CO2, O2, urea, and other lipids such as cholesterol, steroids, fatty acids ... This membrane does not allow the passage of larger molecules such as peptide, glucose and charged molecules such as Na and Cl ...

Cytoplasm

Cytosol - Is the fluid in which the cell organelles are bathing. Composed mostly of water with salts, nutrients, gases, wastes ...

Nucleus -largest organelle, contains genetic blueprint, the genome = DNA wrapped around proteins called histones. The unraveled DNA (during interphase) is a long, thin thread not visible under a microscope. Just prior mitosis, the DNA coils around the histones (like a spool ---> nucleosomes) to form visible (under microscope) chromosomes (46 of them in human). In the nucleus is a darker stained area, the nucleolus, which corresponds to the site of ribosomal RNA (rRNA) synthesis. The nuclear membrane, a double fold bilayer membrane has pores to allow material to exit the nucleus.

The Cell Nucleus Ribosome Structure and Function

Ribosomes - Tiny spheres, formed by proteins and rRNA, sites of protein synthesis. Some are free in the cytosol (or clumped together to form polysomes) and synthesize proteins needed for the cell itself. Others are attached to the endoplasmic reticulum. There, they synthesize proteins to be exported.

Endoplasmic reticulum- network of channels formed by a membrane similar to the plasma membrane. With ribosomes attached to it, it forms the rough endoplasmic reticulum, free of ribosome, it is smooth endoplasmic reticulum. Rough ER synthesizes proteins, smooth ER lipids.

Endoplasmic Reticulum and Nuclear Envelope Golgi Apparatus

Golgi apparatus- Series of elongated channels, stacked on the top of each other. They are involved in protein maturation, packaging.

Lysosomes- Contains digestive enzymes. They associate with a vacuole containing a trapped bacteria or debris and digest it.

Lysosome Peroxisome

Mitochondria- called the power house of the cell. Formed by a double fold membrane, the outer layer being smooth, the inner one folded, forming the cristae, along which are positioned a series of enzymes (the Electron Transport Chain = ETC) involved in ATP regeneration.

Mitochondria Structural Features

Centrosome- formed by 2 centrioles. One of them will migrate to the other side of the nucleus during prophase and form, with the spindle, the mitotic apparatus.

Centriole Structure Ultrastructure of Cilia and Flagella

Flagellae and cilia- flagellae are unique "tail" wiped around to propel the cell. Found only in the sperm cell. Cilia are usually numerous in a cell, moving in the same motion, found in numerous tissues such as the digestive tube, the lungs...

Microfilaments, intermediate filaments and microtubules- involved in cells contraction (filament) and cell skeleton (tubule).

Microfilament Structural Organization

II- MOVEMENT ACROSS CYTOPLASMIC MEMBRANE Sherwood Chp 3

The phospholipid bilayer is permeable to H2O, lipids, small neutral molecules (such as urea) and gases (such as O2 and CO2). It is impermeable to large and/or charged molecules (ex: glucose, proteins, electrolytes).

- Substances permeable through the cell membrane are under the influence of the force of diffusion.

- Substances impermeable through the cell membrane must pass through some sort of channels (or pores). Under these conditions, the forces controlling the movement of the substance vary, depending the substances and the type of channel.

A- PASSIVE TRANSPORT- always occurs along a gradient of concentration = from the highest concentration to the lowest. Energy or ATP is never needed.

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html

1- Diffusion - is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration until equilibrium is reached.

2- Osmosis - occurs when a gradient of concentration exists across a semipermeable membrane (or membrane permeable to some compounds but not to others). The water will be moving along its own gradient across the semipermeable membrane.

Osmosis is very important in physiological exchanges.

Ex: Red blood cells placed in an hypotonic solution (less than isotonic) will swell and burst, because of influx of water inside the cell. If placed in an isotonic solution (same solution as the one existing within the cell), the RBC will be unchanged. If placed in a hypertonic solution, the RBC will crenate and shrink because of water moving out of the cell.

Other applications are intramuscular injections and kidney dialysis. Intramuscular injections must always be isotonic to the cell (tissue) to avoid tissue damage. In kidney dialysis, the blood is circulated around a coil containing a cleansing fluid and separated by a semipermeable membrane. The semipermeable membrane must allow the waste products to go through but retain blood cells, nutrients, salts. Nutrients, salts are unfortunately too small and will go through (by diffusion). To avoid their loss during dialysis, they are placed in equal concentrations in the cleansing fluid. To remove excess water gained by a patient in kidney failure, large proteins will be added to the cleansing fluid to draw water from the blood through osmosis.

3- Filtration- forced passage of molecules through a sieve-like membrane due to pressure. ex: kidney filtration.

4- Facilitated diffusion- molecules are too large to go through the plasma membrane (ex: glucose) and need a carrier-protein to bring them through. The force driving the passage is the difference in concentration between the 2 sides.

B- ACTIVE TRANSPORT- Transport of molecules against the concentration gradient (or from low concentration to high concentration). Energy or ATP is needed.

1- Active transport - Ex: Na/K pump. A group of proteins with enzymatic activity carries 3 molecules of Na+ toward the outside of the cell and 2 molecules of K+ toward the inside of the cell. Present in all cell. Help maintain cell membrane potential.

2- Phagocytosis- A cell, often a macrophage engulfs a bacteria or debris.

3- Pinocytosis- A cell absorbs a drop of liquid (drink)

4- Receptor-mediated endocytosis- Similar to phagocytosis but material entrapped must have been first caught by receptors. Ex: antibodies.

III- CELL CYCLE

Cell division- process in which the cells reproduce themselves.

Somatic division- a parent cell divides to form 2 identical daughter cells. Most somatic (body) cells undergo cell division (exception: neuron, muscle cells..). Nuclear division is mitosis, followed by cytoplasmic division, cytokinesis.

Mitosis- nuclear division

more of chromosome condensation: http://www.cbs.dtu.dk/staff/dave/roanoke/genetics980218.html

Cytokinesis- cytoplasmic division

G0 phase- resting stage (occurs in some cells)

G1 phase- cell growth

S phase- DNA replication (the amount of DNA is doubled 2n --> 4n)

G2 phase- preparation to mitosis

G1 + S + G2 = interphase

MITOSIS

Prophase- nuclear membrane dissolves

-centrosome splits to form the mitotic apparatus with migration of a centriole

toward the opposite pole of the nucleus

- the chromatin condenses and chromosomes become visible

Metaphase- the chromosomes migrate to the equator and attach to the spindle --> form

the equatorial or metaphasic plate.

Anaphase- chromosomes migrate toward the poles of the cell (or the centrioles)

Telophase- the opposite of prophase. In addition, the cytoplasm starts to form a furrow (beginning of cytokinesis).

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmito.html

MEIOSIS

Meiosis is a succession of 2 cell divisions, the first one called reductional division because the cells resulting from the division have only one set from chromosomes (1N and not 2N as in diploid cell). The second division is a normal mitotic division. Meiosis occurs only in stem cells that will become the gametes. Meiosis results in the formation of 4 different gametes.

IV- REPLICATION-TRANSCRIPTION-TRANSLATION

	Replication	Transcription	Translation
Time	S phase	G1 and G2	G1 and G2
Location	Nucleus	Nucleus	Cytoplasm
Action	DNA-->DNA	DNA-->RNA	RNA-->protein

Replication

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/DNAReplication.html

For an animation of the process: http://www.johnkyrk.com/DNAreplication.html

Transcription

http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/TT/TscriptD.html

DNA to RNA

During transcription a blueprint of the gene is taken by mRNA, according the laws of complementarity (A-->U, T-->A ...) and carried into the cytoplasm.

http://kvhs.nbed.nb.ca/gallant/biology/biology.html

Translation

A series of three nucleotides in a row on the mRNA is called a codon and codes for an amino acid. The mRNA attaches on a ribosome and is translated: tRNA anti-codons matching the mRNA codons will attach to the ribosome sites. The amino acids at the other end of the tRNA form a peptide bond, and the beginning of the protein.....

Mutations: A change in the coding sequence on the DNA is a mutation. There are several types of mutations: Point mutations, where a single base was replaced by a different one; frameshift mutations where 1 or several bases are deleted or added, thus shifting the reading of the code; translocations where a piece of chromosome is broken and reattached somewhere else.

Mutations induce modification in protein sequences which often result in an inactive protein.

http://press2.nci.nih.gov/sciencebehind/cancer/cancer42.htm

http://www.stanford.edu/group/hopes/basics/dna/b5.html

http://www.ucl.ac.uk/~sjjgsca/DNAmutation.html

Internet sites of interest:

http://ampere.scale.uiuc.edu/~m-lexa/cell/cell.html

http://skeleton-key.MIT.EDU:8001/bio/ch/

http://infonet.welch.jhu.edu/~omie/LectureLinks/CellPhysiologyLinks.html

http://skeleton-key.MIT.EDU:8001/bio/ch/sq.html

http://lenti.med.umn.edu:80/~mvd/cell_www/cell.html

http://dcn.davis.ca.us/carl/cellhome.htm