The Cellular level of Organization Chapter 3 The Cellular level of Organization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Cell Day!

Study of Cell Functions Cell biologists can isolate organelles to study their functions The goal of cell fractionation is to separate the major organelles of the cells so that their individual functions can be studied.

Fractionation begins with homogenization, gently disrupting the cell. This process is driven by a ultracentrifuge, a machine that can spin at up to 130,000 revolutions per minute and apply forces more than 1 million times gravity (1,000,000 g). Fractionation begins with homogenization, gently disrupting the cell. Then, the homogenate is spun in a centrifuge to separate heavier pieces into the pellet while lighter particles remain in the supernatant. As the process is repeated at higher speeds and longer durations, smaller and smaller organelles can be collected in subsequent pellets.

Cell Structure and Function

Cell Characteristics Plasma Membrane Cytoplasm Organelles Outer cell boundary Cytoplasm Cytosol Cytoskeleton Cytoplasmic inclusions Organelles Specialized structures that perform specific functions

Functions of the Cell Basic unit of life Protection and support Movement Communication Cell metabolism and energy release Inheritance

Plasma Membrane Intracellular versus extracellular Membrane potential Glycolipids and glycoproteins Fluid-mosaic model

Membrane Lipids Phospholipids form a lipid bilayer Hydrophilic (water-loving) polar heads Hydrophobic (water-fearing) nonpolar heads Cholesterol: Determines fluid nature of membrane

Membrane Proteins Integral or intrinsic Peripheral or extrinsic Extend from one surface to the other Peripheral or extrinsic Attached to either the inner or outer surfaces of the lipid bilayer

Marker Molecules Allow cells to identify one another or other molecules Glycoproteins Glycolipids Examples: Immune system Recognition of oocyte by sperm cell

Channel Proteins Nongated ion channels Ligand gated ion channel Always open Ligand gated ion channel Open in response to small molecules that bind to proteins or glycoproteins Voltage-gated ion channel Open when there is a change in charge across the plasma membrane

Receptors Receptor molecules Linked to channel proteins Exposed receptor site Linked to channel proteins Acetylcholine Linked to G proteins Alter activity on inner surface of plasma membrane

Enzymes and Carrier Proteins

Movement through the Plasma Membrane Diffusion Osmosis Filtration Mediated transport mechanisms Facilitated diffusion Active transport Secondary active transport

Diffusion Movement of solutes from an area of higher concentration to lower concentration in solution Concentration or density gradient Difference between two points Viscosity How easily a liquid flows

Diffusion

Osmosis Diffusion of water (solvent) across a selectively permeable membrane Important because large volume changes caused by water movement disrupt normal cell function Cell shrinkage or swelling Isotonic: cell neither shrinks nor swells Hypertonic: cell shrinks (crenation) Hypotonic: cell swells (lysis)

Osmosis

Osmosis

Filtration Works like a sieve Depends on pressure difference on either side of partition Moves from side of greater pressure to lower Example: In kidneys in urine formation

Mediated Transport Mechanisms Involve carrier proteins Characteristics Specificity To a single type of molecule Competition Saturation Rate of transport limited to number of available carrier proteins

Saturation of a Carrier Protein

Mediated Transport Mechanisms Facilitated diffusion Higher to lower concentration without metabolic energy Active transport Requires ATP Secondary active transport Ions or molecules move in same (symport) or different direction (antiport)

Secondary Active Transport

Endocytosis Internalization of substances by formation of a vesicle Types Phagocytosis Pinocytosis Receptor-mediated endocytosis

Pinocytosis and Receptor-Mediated Endocytosis

Exocytosis Accumulated vesicle secretions expelled from cell Examples Secretion of digestive enzymes by pancreas Secretion of mucus by salivary glands Secretion of milk by mammary glands

Cytoplasm Cellular material outside nucleus but inside plasma membrane Cytosol: Fluid portion Cytoskeleton: Supports the cell Microtubules Microfilaments Intermediate filaments Cytoplasmic inclusions

Organelles Small specialized structures for particular functions Most have membranes that separates interior of organelles from cytoplasm Related to specific structure and function of the cell

Centrioles In specialized zone near nucleus: Centrosome Each unit consists of microtubules Before cell division, centrioles divide, move to ends of cell and become spindle fibers

Cilia Appendages projecting from cell surfaces Capable of movement Moves materials over the cell surface

Flagella Similar to cilia but longer Usually only one exists per cell Move the cell itself in wavelike fashion Example: Sperm cell

Microvilli Extension of plasma membrane Increase the cell surface Normally many on each cell One tenth to one twentieth size of cilia Do not move

Ribosomes Sites of protein synthesis Composed of a large and small subunit Types Free Attached to endoplasmic reticulum

Endoplasmic Reticulum Types Rough Attached ribosomes Proteins produced and modified Smooth Not attached ribosomes Manufacture lipids Cisternae: Interior spaces isolated from rest of cytoplasm

Golgi Apparatus Modification, packaging, distribution of proteins and lipids for secretion or internal use Flattened membrane sacs stacked on each other

Function of Golgi Apparatus

Action of Lysosomes

Peroxisomes and Proteasomes Smaller than lysosomes Contain enzymes to break down fatty and amino acids Hydrogen peroxide is a by-product of breakdown Proteasomes Consist of large protein complexes Include several enzymes that break down and recycle proteins in cell

Mitochondria Provide energy for cell Major site of ATP synthesis Membranes Cristae: Infoldings of inner membrane Matrix: Substance located in space formed by inner membrane

Nucleus DNA dispersed throughout Consists of : Nuclear envelope: Separates nucleus from cytoplasm and regulates movement of materials in and out Chromatin: Condenses to form chromosomes during cell division Nucleolus: Assembly site of large and small ribosomal units

Chromosome Structure

Overview of Cell Metabolism

Overview of Protein Synthesis

Overview of Protein Synthesis Transcription Copies DNA to form mRNA tRNA carries amino acids to ribosome Translation Synthesis of a protein at ribosome

Translation

The cell cycle “Omnis cellular e cellular” Every cell forms from existing cell: where a cell forms there must be a preexisting cell Cell cycle is the sequence of growth and division of cells Cell divide in order to reproduce: production of offspring Cell division functions in Growth structural changes increase living material in the organism Repair and renewal : replacing cells that die from normal wear and tear or accident Cell division enables sexually reproducing organisms to develop from a single cell Fertilized egg or zygote

Human somatic (Body cells) contain 46 chromosomes Cell division involves the distribution of identical genetic information DNA to two daughter cells Cell cycle involves two general periods Interphase: At this phase, the chromosomes are duplicated in preparation for the period of cell division Mitosis: the period of division, the nucleus and it’s cytoplasm divide to form two daughter cells The process of nucleus division followed by division of the cytoplasm when chromosome are distributed equally to daughter cells is known as MITOSIS Human somatic (Body cells) contain 46 chromosomes This is the Diploid or 2N Reproductive (sex) cells or Gamete (sperm and egg cells) have half as many chromosomes as the somatic cells i.e.. 23 This is the haploid or N

Mitosis produces two daughter cells Each daughter cell has a diploid (2N number of chromosomes (in pair and look alike) Mother cell: the cell that divides Daughter cells: the cells that resulted from the division In mitosis the daughter cell contain the number of chromosome as the mother cell duplication of a mother cell chromosome results in a pair of chromatid The two chromatids are held together at the center by the contromere INTRPHASE The non dividing stage of cell The period between cell division increase in cell size duplication to produce chromatid pair

Cell Life Cycle Interphase Mitosis Cytokinesis Phase between cell divisions Mitosis Prophase Metaphase Anaphase Telophase Cytokinesis Division of cell cytoplasm

PROPHASE METAPHASE First visible phase of division The chromatin material shortens and thicken Pair of centriols begin to move towards the opposite pole Spindle fiber appear between the separating centriols Nuclear membrane and nucleus disappear METAPHASE Spindle fully formed and stretches between the centrioles at the poles Each chromatid pair is attached to the spindle fiber by the contromere Contromeres line up at the middle of the (equator) of the spindle Methphase is characterized by a fully formed spindle with the chromatid pair arranged across the center

ANAPHASE TELOPHASE BINARY FISSION The centromeres divide and the chromatids of each pair separate and migrate towards the opposite pole TELOPHASE This phase is characterized by the disappearance of the mitotic apparatus and the reappearance of formed nuclei Two nucleus instead of one nucleus appear, one for each daughter cell The cytoplasm divide by pinching in of opposite sides (in-furrowing) of the membrane Daughter cells with complete cell membrane enclosing it BINARY FISSION Asexual reproduction in which the cell divides into two equal parts This is the method of cell division used by the prokaryotes (bacteria) and some other unicellular organisms

The process of binary fission involves Copy of single chromosome copies of chromosome attached to plasma membrane Growth and separation Partition forms between the newly formed cells

Mitosis

Mitosis

Meiosis

Comparison of Mitosis and Meiosis Property Mitosis Meiosis Time of DNA replication Occurs during Interphase before mitosis begins Occurs during Interphase before meiosis begins Number of division One including PMAT Two, each including PMAT Number of cells Two, each diploid (2n) and genetically identical to the parent cell Four, gametes are each different from the parent and from each other, haploid number (n) of chromosome, in males 4 gametes (sperm cells) in females, one gamete and 3 polar bodies Synapsis Does not occur During prophase 1, tetrads and crossing over Functions New cells are formed during growth or tissue repair; new cells have identical DNA and can perform the same function as the parent cell Gametes are produced for reproduction; during fertilization the chromosome from the haploid gamete unite to restore the diploid number in somatic cells, increase genetic variability as a result of crossing over

Cellular Aspects of Aging Cellular clock Death genes DNA damage Free radicals Mitochondrial damage