Chapter 3Cells & Their Functions

The Role of Cells• Basic unit of all life• Cytology – the study of cells• Simplest structure that shows all the

characteristics of life such as–Organization–Metabolism–Responsiveness–Homeostasis–Growth –Reproduction

The Role of Cells• It is possible for a single cell to live

independently of other cells–Protozoa–bacteria

• Cells make up all tissues• All activities of the human body result from

the activities of individual cells

Microscopes• Stains – dyes that color cells and

tissues to aid viewing• Compound light microscope

– Magnifies 1000 times– 2 lenses– Uses visible light for illumination

• Transmission electron microscope– 1,000,000 times magnification– Uses electron beam instead of

light• Scanning electron microscope

– 100,000 times magnification– Shows surface features, but in 3D

view

Cell Structure

• All cells share certain characteristics.

• Main parts:– Plasma membrane– Nucleus– Cytoplasm– Organelles

Plasma Membrane

• Formerly called the cell membrane

• Function– Encloses the cell– Participates in growth

& reproduction– Involved in cellular

interactions– Regulates what

enters and leaves the cell

Plasma Membrane

• Bilayer structure – Phospholipids – Cholesterol– Carbohydrates– Proteins

• Channels • Transporters• Receptors• Enzymes • Linkers• Cell identity markers

Plasma Membrane Proteins• Channels pores that allow specific ions to enter or leave• Transporters shuttle substances across the membrane• Receptors act as points of attachment for materials that act

on the cell• Enzymes participate in reactions occurring at the plasma

membrane• Linkers give structure to the membrane and help cells attach

to each other• Cell identity markers are proteins that are unique to an

individual’s cells. Important in immunity & tissue transplants

Cell Nucleus

• Nucleus is the biggest organelle in the cell• Organelles – specialized structures that

perform different tasks for the cells• Control center of the cell• Contains chromosomes which govern all

cellular activities• Nucleolus contains RNA, DNA, and proteins

and manufactures ribosomes in the cytoplasm• Almost all DNA is found in the cell nucleus

The Cytoplasm

Cytosol – liquid part of cytoplasm containing nutrients and organelles such as:

• Endoplasmic reticulum• Ribosomes• Mitochondria• Golgi apparatus• Lysosomes• Peroxisomes• Vesicles• Centrioles

• Endoplasmic reticulum Network of membranes that is either smooth or

rough• Smooth ER – involved in lipid synthesis• Rough ER – comes from ribosomes attached

to the ER • Ribosomes – necessary for protein synthesis and

are either attached to ER or floating free

Cell Organelles Cont.• Mitochondria – converts nutrients into ATP,

the power plant of the cell. Active cells have more mitochondria than other cells

• Golgi Apparatus – involved in sorting and modifying proteins and then packaging them for export from the cell

• Centrioles - near the nucleus; help to organize the cell and divide the cell contents during the process of cell division

Cell Organelles Cont.• Lysosomes – remove waste and

foreign materials from the cells• Peroxisomes – destroy byproducts of

metabolism • Vesicles – storage units of the cell,

can also move materials into and out of cells

Surface Organelles• Cilia – small hair-like projections that

wave, creating movement of fluids around the cell. Respiratory passage cells have cilia and cells in the female reproductive tract.• Flagellum – long whip-like extension

for movement. The only cells in humans with flagella are sperm cells.

Cellular Diversity• Size –Average size is 10-15 micrometers–RBCs are small – 7 micrometers–Muscle cells are big – 200 micrometers

• Composition – most human cells have all the organelles. Some cells have more than one type of organelles than another due to their function

Protein Synthesis: Chromosomes & Genes

• Chromosomes are the hereditary units that govern cells– 23 pair, 46 total

• Divided into genes• Genes carry the

messages for the development of particular inherited characteristics and do so by directing the manufacture of proteins in the cell.

DNA & RNA

• Genes are made of DNA – deoxyribonucleic acid –DNA is composed of nucleotides

• RNA – Ribonucleic acid – a related compound which participates in protein synthesis but is not part of chromosomes–RNA is composed of nucleotides

DNA & RNA Composition

• DNA is composed of – 4 nucleotides

• Adenine (A)• Guanine (G)• Cytosine (C)• Thymine (T)

• RNA is composed of – 4 nucleotides

• Adenine (A)• Guanine (G)• Cytosine (C)• Uracil (U)

Nucleotide Structure

• Composed of three units

• Sugar – – RNA – ribose– DNA - deoxyribose

• Phosphate - an area containing phosphorus

• Nitrogen base – an area containing nitrogen

Deoxyribose & Ribose

DNA Structure• DNA is built like a ladder

– Sugar-phosphate backbone– Nitrogen bases are the steps

• 2 DNA strands make a pair – Nucleotide adenine always

pairs with thymine– Guanine always pairs with

cytosine

• 2 DNA strands are bonded together with hydrogen bonds – form double helix

RNA

• RNA interprets the information from the DNA blueprint to synthesize proteins

• RNA is found almost entirely in the cytoplasm• RNA is a single strand of nucleotides• There are 3 types of RNA– mRNA – messenger RNA– rRNA – ribosomal RNA– tRNA – transfer RNA

The Role of RNA

Protein synthesis• Transcription – Messenger RNA (mRNA)

• Translation– Transfer RNA (tRNA)– Ribosomal RNA (rRNA)

Protein Synthesis and RNA• DNA breaks its bonds and

uncoil into single strand• Transcription – transfer of

information from DNA to mRNA in the nucleus >> matching RNA strand forms based on pairing

• mRNA leaves the nucleus and travels to a ribosome in the cytoplasm

Transcription.

Translation.

Cell Division• Meiosis– Sex cells

• Mitosis– Somatic cells

Cell Division - Mitosis• Mitosis – the process of cell division where an

original parent cell becomes two identical daughter cells

• Before mitosis can occur, DNA must double – therefore, the 46 chromosomes in the nucleus must double

Interphase• The stage of life in a cell between one mitosis and the next• During this phase, DNA uncoils and each strand takes on a

matching strand of nucleotides to make up 2 new DNA strands, each one identical to the original and each other

• The new, uncoiled DNA strandsare held in the centromere until mitosis• A typical cell spends most of its cycle in interphase and a relatively short time in mitosis

Mitosis

• 4 stages• Prophase• Metaphase• Anaphase• Telophase

Mitosis - Prophase• The new double strands of DNA coil tightly

into a double helix • The nucleolus and the nuclear membrane

begin to disappear• All other organelles disappear except those

needed for mitosis• Two centrioles in the cytoplasm move

towards opposite ends of the cell• Thin spindle fibers begin to form between

the centrioles

Mitosis - Metaphase

Chromosomes attach to spindle fibers and line up across the center of the cell

Mitosis - Anaphase

The centromere splits and the duplicatedchromosomes separate and begin to move

towards opposite ends of the cell

Mitosis - Telophase

• A membrane appears around each group of separated chromosomes, forming 2 new nuclei

• The plasma membrane pinches off to divide the cell

Mitosis : Summary• At the end of mitosis

there 2 new cells • Each daughter cell has

exactly the same DNA as the original

• All organelles reappear in the daughter cells

Movement of Substances Across the Plasma Membrane

Travel across the membrane is based on several factors:

• Molecular size• Solubility• Electrical charge

Movement That Does Not Require Cellular Energy

Passive transport • Diffusion• Osmosis• Filtration• Facilitated diffusion

Diffusion

• Concentration gradient - constant movement of particles from a region of higher concentration to one of lower concentration

• Equilibrium - diffusing substances spread throughout available space until their concentration is equal everywhere

• Passage is limited to those particles small enough to pass through spaces between molecules of the plasma membrane

Osmosis• The diffusion of water through a semi-

permeable membrane • Water moves from an area where there is

more water molecules to an area where there is less water molecules

• The solvent water moves from an area of lower solute concentration to an area of higher solute concentration

• Water follows salt

Osmotic Pressure

• Describes the tendency of a solution to draw water into it– Directly related to concentration: the higher the

concentration of a solution, the greater the tendency to draw water in

• A measure of the force driving osmosis– Determined by applying enough pressure to the

surface of a liquid to stop the flow of water by osmosis

Filtration

• Passage of water containing dissolved materials through a membrane as a result of a “pushing” force on one side– Movement of materials out of the capillaries and

into the tissues under the force of blood pressure– Movement of materials out of the blood and into

urine in the kidneys

Facilitated Diffusion

• Movement of materials across the plasma membrane in the direction of the concentration gradient but using transporters to move the material at a faster rate– Glucose

Movement That Requires Cellular Energy

Active transport• Bulk transport or vesicular transport– Endocytosis• Phagocytosis• Pinocytosis

– Exocytosis

Active Transport

• Moves solute particles in or out of the cell opposite of the direction they would normally flow by diffusion

• Transporters – proteins in the plasma membrane that are required to transport the particles– Nerve and muscle cells depend on active transport

of Na, K and Ca for proper function

• Endocytosis – bulk transport of large quantities of materials into or out of the cells– Phagocytosis – large particles are engulfed by the

plasma membrane and moved into the cell• White blood cells rid the body of foreign materials

– Pinocytosis – cell membrane engulfs droplets of fluid

How Osmosis Affects Cells• Isotonic – solutions with concentrations equal to the

concentration of the cytoplasm– Blood plasma, interstitial fluid, normal saline (0.9% salt),

5% dextrose• Hypotonic – a solution less concentrated than intracellular

fluid– A cell in hypotonic solution will draw water in and may

burst– Hemolysis – a red blood cell drawing too much water in

and bursting• Hypertonic – a solution more concentrated than intracellular

fluid– Crenation – the process by which a cell that loses water to

surrounding fluids and shrinks

Cell Aging

As cells multiply, changes occur that may lead to their damage or death•Free radical injury•Enzyme injury•Gene alteration or mutation•Slowing cell activity•Apoptosis

Cells and Cancer

Certain mutations may cause changes in cells• Uncontrolled reproduction of cells• Cells spread (metastasize), producing cancer• Cancer cells form tumors, crowding out

normal cells