Cell Structure and Function - · PDF fileCell Structure and Function 3-2 ... • Fluid nature provides/allows ... reassembled if membrane is damaged – Membranes can fuse with each

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Chapter 3Cell Structure and Function

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• Basic Structure of the Cell– Plasma membrane – Cytoplasm containing organelles – Nucleus

• Functions of the Cell– Basic unit of life– Protection and support through production and

secretion of various kinds of molecules– Movement. Various kinds occur because of specialized

proteins produced in the cell– Communication. Cells produce and receive electrical

and chemical signals– Cell metabolism and energy release– Inheritance. Each cell contains DNA. Some cells are

specialized to gametes for exchange during sexual intercourse

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Plasma Membrane • Separation of intracellularvs. extracellular materials

• Production of charge difference (membrane potential) across the membrane by regulation of intracellular and extracellular ion concentrations– Outside of membrane positively

charged compared to inside because of gathering ions along outside and inside

• Glycocalyx: combinations of carbohydrates and lipids (glycolipids) and proteins (glycoproteins) on outer surface.

• Fluid-mosaic model

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Membrane Lipids• Phospholipids and cholesterol

predominate– Phospholipids: bilayer. Polar

heads facing water in the interior and exterior of the cell (hydrophilic); nonpolar tails facing each other on the interior of the membrane (hydrophobic)

– Cholesterol: interspersed among phospholipids. Amount determines fluid nature of the membrane

• Fluid nature provides/allows – Distribution of molecules

within the membrane– Phospholipids automatically

reassembled if membrane is damaged

– Membranes can fuse with each other

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Marker Molecules: Glycoproteins and Glycolipids

• Allow cells to identifyone another or other molecules – Immunity– Recognition of oocyte

by sperm cell– Intercellular

communication

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Attachment Sites

• Integrins, membrane-bound proteins

• Attachment sites to other cells or to extra/intracellular molecules.

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Channel Proteins (Integral): hydrophilic region faces inward; charge determines molecules that can pass through

• Nongated ion channels: always open– Responsible for the

permeability of the plasma membrane to ions when the plasma membrane is at rest

• Gated ion channels can be open or closed– 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

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Receptor Molecules

• Proteins in membranes with an exposed receptor site

• Can attach to specific ligand molecules and act as an intercellular communication system

• Ligand can attach only to cells with that specific receptor

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Enzymes and Carrier Protein

• Enzymes: some act to catalyze reactions at outer/inner surface of plasma membrane. Surface cells of small intestine produce enzymes that digest dipeptides

• Carrier proteins: integral proteins move ions from one side of membrane to the other

– Have specific binding sites– Protein change shape to transport ions or molecules

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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

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Osmosis

• Diffusion of water (solvent) across a selectively permeable membrane. Water moves from an area of low concentration of solute to an area of high concentration of solute

• Osmotic pressure: force required to prevent water from moving across a membrane by osmosis

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Osmosis and Cells• 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)

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Filtration

• Works like a sieve• Depends on pressure difference on either side

of a partition• Moves from side of greater pressure to lower• Example: urine formation in the kidneys.

Water and small molecules move through the membrane while large molecules remain in the blood

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Mediated Transport Mechanisms• Involve carrier proteins or

channels in the cell membrane

• Characteristics– Specificity for a single type

of molecule– Competition among

molecules of similar shape– Saturation: rate of

transport limited to number of available carrier proteins

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Active Transport

• Requires ATP. The use of energy allows the cell to accumulate substances

• Rate of transport depends on concentration of substrate and on concentration of ATP

• Example: Na/K exchange pump that creates electrical potentials across membranes

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Secondary Active Transport

• Ions or molecules move in same (symport) or different (antiport) direction.

• Is the movement of glucose a symporter example or an antiporter example?

• This example shows cotransport of Na+ and glucose. 1. A sodium-potassium

exchange pump maintains a concentration of Na that is higher outside the cell than inside. Active transport.

2. Na moves back into the cell by a carrier protein that also moves glucose. The concentration gradient for Na provides the energy required to move glucose against its concentration gradient.

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Endocytosis

• Internalization of substances by formation of a vesicle

• Types– Phagocytosis– Pinocytosis– Receptor-mediated

endocytosis

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Pinocytosis and Receptor-Mediated Endocytosis

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Exocytosis

• Accumulated vesicle secretions expelled from cell• Examples

– Secretion of digestive enzymes by pancreas– Secretion of mucous by salivary glands– Secretion of milk by mammary glands

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Cytoplasm• Cellular material outside

nucleus but inside plasma membrane

• Composed of Cytosol, Cytoskeleton, Cytoplasmic Inclusions, Organelles

• Cytosol: fluid portion. Dissolved molecules (ions in water) and colloid (proteins in water)

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Cytoskeleton• Supports the cell but has to allow for

movements like changes in cell shape and movements of cilia– Microtubules: hollow, made of

tubulin.• Internal scaffold, transport, cell

division– Microfilaments: actin.

• Structure, support for microvilli, contractility, movement

– Intermediate filaments: mechanical strength

• Cytoplasmic inclusions: aggregates of chemicals such as lipid droplets, melanin

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Organelles

• Small specialized structures with particular functions

• Most have membranes that separate interior of organelles from cytoplasm

• Related to specific structure and function of the cell

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Nucleus

• Membrane-bound• Nucleoplasm,

nucleolus and nuclear envelope

• Much of the DNA in a cell located here

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Chromosome Structure

• Chromatin: DNA complexed with proteins (histones)

• During cell division, chromatin condenses into pairs of chromatidscalled chromosomes. Each pair of chromatids is joined by a centromere

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Centrioles and Spindle Fibers• Located in

centrosome: specialized zone near nucleus

• Center of microtubule formation

• Before cell division, centrioles divide, move to ends of cell and organize spindle fibers

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Cilia• Appendages

projecting from cell surfaces

• Capable of movement

• Moves materials over the cell surface

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Flagella

• Similar to cilia but longer• Usually only one per cell• Move the cell itself in wave-like fashion• Example: sperm cell

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Microvilli• Extension of plasma

membrane• Increase the cell

surface area• Normally many on

each cell• One tenth to one

twentieth size of cilia• Do not move

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Ribosomes

• Sites of protein synthesis• Composed of a large and

a small subunit• Types

– Free– Attached (to endoplasmic

reticulum)

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Endoplasmic Reticulum• Types

– Rough• Has attached ribosomes• Proteins produced and

modified here– Smooth

• No attached ribosomes• Manufactures lipids

• Cisternae: Interior spaces isolated from rest of cytoplasm

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Golgi Apparatus

• Modification, packaging, distribution of proteins and lipids for secretion or internal use

• Flattened membrane sacs stacked on each other

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Function of Golgi Apparatus

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Action of Lysosomes

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Peroxisomes and Proteasomes• Peroxisomes

– Smaller than lysosomes– Contain enzymes to break down fatty acids 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

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Mitochondria• Major site of ATP synthesis• Membranes

– Cristae: Infoldings of inner membrane

– Matrix: Substance located in space formed by inner membrane

• Mitochondria increase in number when cell energy requirements increase.

• Mitochondria contain DNA that codes for some of the proteins needed for mitochondria production.

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Overview of Cell Metabolism• Production of ATP

necessary for life• ATP production takes place

in the cytosol (anaerobic) and mitochondria (aerobic)– Anaerobic does not require

oxygen. Results in very little ATP production.

– Aerobic requires oxygen. Results in large amount of ATP.

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Overview of Protein Synthesis

• Transcription: DNA used to form RNA

• Translation: synthesis of a protein at the ribosomes using mRNA, tRNA and rRNA

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Transcription

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Post-transcriptional Modification of mRNA

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Translation

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Regulation of Protein Synthesis• All nucleated cells except germ cells have the full

complement of DNA.• During development, differentiation occurs and some

segments of DNA are turned off in some cells while those segments remain “on” in other cells.

• During the lifetime of a cell, the rate of protein synthesis varies depending upon chemical signals that reach the cell.– Example: thyroxine from the thyroid causes cells to

increase their metabolic rate. More thyroxine, higher metabolic rate; less thyroxine, lower metabolic rate.

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Cell Life Cycle

• Interphase: phase between cell divisions– Replication of DNA– Ongoing normal cell activities

• Mitosis: series of events that leads to the production of two cells by division of a mother cell into two daughter cells. Cells are genetically identical.– Prophase– Metaphase– Anaphase– Telophase

• Cytokinesis: division of cell cytoplasm

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Mitosis

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Cellular Aspects of Aging

• Cellular clock. After a certain amount of time or certain number of cell divisions, cells die.

• Death genes. Turn on late in life, or sometimes prematurely causing cells to deteriorate and die. Apoptosis.

• DNA damage. Telomeres at ends of chromosomes TTAGGG. During replication, nucleotides are lost. Telomerase protects telomeres, enzymes seem to be lost with aging.

• Free radicals. DNA mutation caused by free radicals (atoms or molecules with an unpaired electron.

• Mitochondrial damage. Mitochondrial DNA may be more sensitive to free radicals. Loss of energy, cell death.