Chapter 3

Cells

Foundation for bodily functions

• foundation for the study of A&P

Most human cells range from 10-15 micrometers in diameter. What limits how large a cell can be?

Relationship b/w its volume & surface area

Cytology

Study of cellular structure & function

• 200 kinds of cells in body

What are the major components of a cell?

1. Plasma membrane (cell membrane)

• forms a cell’s surface boundary

2. Cytoplasm

• material b/w plasma membrane & nucleus (largest organelle)

Cytoplasm

Parts:

1. Cytoskeleton: framework of filaments & tubules

• structure, shape, transport & organize

1. Organelles: diverse structures performing tasks for the cell

2. Inclusions: accumulated cell products; lipids, pigments, bacteria

3. Cytosol: clear gel embedding other components

• aka intracellular fluid (ICF) & organelles

  ECF(extracellular fluid) is all other body fluids

• B/w cells is called tissue fluid

What is the most useful measurement for cells?

Micrometer (10^-6 meter)

What activities happen at the cell surface?

• binding of signal molecules

• Stimulation of cellular activity

• Attachment of cells

• Transport or materials in & out

Plasma membrane

Defines the boundary of the cell & governs interactions

• maintains chem composition differences b/w ECF & ICF

• two-layered lipid film with embedded proteins

Lipids in the plasma membrane?

1. Phospholipids

• 75%

• Phospholipids bilateral

2. Cholesterol

• 20%

• Affects membrane fluidity; too much inhibits proteins & enzymes; too little becomes fragile

3. Glycolipids

• 5%

Membrane proteins

1. Integral proteins penetrate cell membrane

• glycoproteins

2. Peripheral proteins do not protrude into phospholipid bilayer

Functions

1. Receptors: receive & bind chemical signals from other cells

2. Enzymes: carry out chemical reactions at membrane surface

EX: degrading signal molecules, breaking down dietary nutrients

3. Channel proteins: allow water & hydrophilic solutes to enter or leave

• gated channels- open & close at diff times (pacemaker)

4. Carriers: bind substances on one side of membrane & release it on the other

• transport glucose, amino acids, ions

5. Cell identity markers

6. Cell adhesion molecules (CAMs)

Glycocalyx

Fuzzy carb coat covering cells; made of glycolipids & glycoproteins

Functions

• cell-adhesion

• Helps distinguish healthy from diseased cells

Cell surface extensions

• Microvilli (finger like) brush border

• Cilia (hair like) primary cilium- solitary, nonmotile

• Flagella tail of sperm

• Pseudopods vary in shape; macrophages use these

Aid in absorption, movement, sensory processes, phagocytosis

Cell junctions

Formed by proteins at surface & link cells together & attach them to material

Enable:

• grow & divide

• Resist stress

• Communicate with each other

1. Tight

2. Desmosomes

3. Gap

Selective permeability

Allows some substances to pass, & holds back others

Filtration

Pressure forces fluid through selective permeable barrier

• holds large particles back & water/ small particles to pass

EX: kidneys cleanse the blood

Simple diffusion

Net movement of particles from high to low concentration

• no energy needed

• Nonpolar & hydrophobic substances through LIPID regions

• Hydrophilic diffuse through protein channels

Osmosis

Net movement of water from lower to higher solute concentration

• water moves up a solute concentration gradient

Plays role in homeostasis

EX: cell volume

osmotic balance is crucial

Tonicity

Ability of a solution to change intracellular pressure & volume

1. Isotonic: fluid has = ECF concentration of solutes as the ICF

   ex: saline fluid therapy

2. Hypertonic: ECF has a higher concentration of solutes than ICF

Shrinks

3. Hypotonic: ECF has a lower concentration of solutes

Swells

Facilitated diffusion

Carrier mediated transport: employs carrier proteins in plasma membrane to move solutes that could not get through membrane on their own

• movement of solute down concentration gradient

• No energy required

EX: absorption of sugars from digested food

Active transport

Carrier-mediated transport

• requires energy input

• Moves solutes up concentration gradient

• Ceases immediately if cell stops producing ATP

• Sodium potassium pump

Vesicular transport

Substances move through membrane in vesicles

• requires ATP

Endocytosis: brings matter into cell

Exocytosis exports material from cell

3 forms of Endocytosis

1. Phagocytosis: cell eating (white blood cells)

2. Pinocytosis: cell drinking

3. Receptor-mediated: more selective

Cytoskeleton

Network of proteins filaments & tubules in cytoplasm that structurally supports cell

• determines shape & organize content

• Transports substances within cell

• Contributes to cell movements

• Connected to proteins in plasma membrane

Components

microfilaments, intermediate filaments, microtubules

Microfilaments

Thinnest

• made of actin, in the cores of micovilli

• form dense fibrous mesh on the internal plasma membrane terminal web

Intermediate Filaments

Stiff

Contribute to the strength of desmosomes

Include the protein keratin

• also fills cells of epidermis; gives strength to the skin

Micotubules

Thickest

Holds organelles in place

Form bundles that maintain cell shape

Inclusions

Two kinds

1. Stored cellular products ( pigments, fat globules, glycogen granules)

2. Foreign bodies (viruses, bacteria, dust particles)

Never enclosed in a membrane

Not essential to cell survival

Organelles

Play individual roles in survival of cell

Compartmentalized components of the cell

Nucleus

Largest organelle

Most cells only have 1

• mature red blood cells have none

• Skeletal muscle cells have many

Nuclear envelope: two parallel membranes surrounding nucleus

• nuclear pores: perforated

Regulate traffic in & out nucleus

Bind the two membranes together

Nucleoli: one or more dense masses in nucleus; subunits of ribosomes are made

Chromosomes: threadlike bodies of DNA & protein found in dividing cells

• human cells have 46

• Chromatin: form that DNA takes in nondividing cells

  fine filaments dispersed throughout

Endoplasmic reticulum (ER)

System of interconnected channels called cisternae enclosed by a membrane

Little network within cystoplasm

• synthesizes sterioid & other lipids

• Detoxifies drugs

• Manufactures cell’s membranes

• Produces phosolipids & plasma membrane proteins

• Produces proteins

Rough (RER): cisternae are covered with ribosomes

Proteins

Smooth (SER): cisternae lack ribosomes

Hormones

• continuous with RER

Ribosomes

Small granules of protein & ribonucleic acid (RNA)

• produced in nucleus, but then moved to cytoplasm

Read genetic messages from nucleus

Golgi Complex

Small cluster of cisternae

• each flattened, curved sac

Synthesize carbohydrates

Complete protein & glycoprotein synthesis

Golgi vesicles: membranous sacs pinch off of the golgimcomplex; are filled with complex’s secretory product

Lysosomes

Packages of enzymes enclosed in a membrane

Breakdown molecules, microbes, worn out organelles

Help carry out apoptosis, programmed cell death

Mitochondria

Synthesize ATP

Surrounded by a double membrane

• inner membranes have folds cristae bear enzymes that produce most of the ATP

• Mitochondrial matrix space b/w the cristae; contains mitochondrial DNA (mtDNA)

Centrioles

Short cylindrical assembly of microtubules

Centrosome

• area of cytoplasm near nucleus that contains. Pair of centrioles

• Plays role in cell division

Protein synthesis

Steps

1. Transcription: DNA is used to make mRNA inside nucleus

• messenger RNA (mRNA) is a copy of a gene

• Migrates into cytoplasm

• Begins at the site of a gene on DNA

• Enzymes unzips the DNA double helix

• Thymine → adenine

• Guanine → cytosine

• Adenine → uracil

• mRNA “edited” in nucleus;noncoding segments are removed

• Exported through nuclear pore into cytoplasm

2. Translation: mRNA is used to make protein

• reading of mRNA by ribosomes

• Ribosomal subunits, ribosomal RNA (rRNA)

• Genetic code on mRNA in the form of codons

• binds a transfer RNA (tRNA) contains, anticodon

• Amino acids are assembled in correct order

Cell Cycle

All cells arise from existing cells

• must accurately copy DNA & separate copies into two daughter cells

Divided into 4 phases

1. G1

2. S

3. G2

4. M

First Gap Phase (G1)

Interval b/w cell division & DNA replication

• time of protein synthesis & growth

• Completion of normal cell tasks

• Begin to replicate centrioles

• Accumulation of materials needed for DNA replication

Synthesis (S)

DNA replication phase

DNA molecules “unzip” & separate into 2 strands

DNA polymerase

• reads base sequence on 1 strand & assembles nucleotides to make complementary strand

• 2 enzymes work in opposite directions

Each new DNA molecules is composed of 1 strand of old DNA & a newly made one

Second gap (G2) phase

Brief

Interval b/w DNA replication & cell division

• centrioles replication finishes

• Synthesis of enzymes that control cell division

• Along with G1, and S, termed interphase - the time b/w cell divisions

Mitotic (M) phase

Period of mitosis

Nucleus is replicated, the DNA is divided into 2 identical sets

At the end of the M phase, cytokinesis

• cell pinches in half & forms 2 genetically identical daughter cells

Length of cell cycle varies considerably; some divide continuously, some not

Mitosis

2 mechanisms of cell division: mitosis & meiosis (produce eggs & sperm)

Used for growth: fertilized egg into individual

Continues growth of organs after birth

Repairs damaged tissues

Responsible for embryonic development & tissue growth

4 phases

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

Chromatin condenses into sister chromatids

Chromosomes coil into short dense rods

• easier to distribute than chromatin of interphase

Chromosomes have 2 genetically identical bodies sister chromatids joined at centromere (pinched spot)

• 46 chromosomes in each cell; 2 chromatids per chromosome (92 DNA molecules total)

Nuclear envelope disintegrates, chromosomes are released into cytosol

• spindle fibers form

Elongated microtubules sprout from centrioles

Push centrioles apart; eventually 1 pair at each pole of cell

Attach to centromeres of chromosomes & tug them until they line up along midline

Metaphase

Sister chromatids line up along the center of the cell

Chromosomes are aligned on cell equator

Spindle fibers now form a mitotic spindle

• some reach centrioles → chromosomes

• Some anchor assembly to inside of plasma membrane

Anaphase

Sister chromatids are pulled apart at the centromere

Centromere is split in 2 by an enzyme

• separated sister chromatids daughter chromosomes are genetically identical

One daughter is moved to each pole of cell

• centromere leads the ay, arms trail behind

Almost all body cells are genetically identical

• exceptions: egg & sperm cells

Telophase

The cleavage furrow divides the cell into 2 new daughter cells

Clustering of chromatids on each side of cell

• new nuclear envelop forms around each cluster

• Chromatids uncoil, return to chromatin

• Mitotic spindle breaks up

• End of nuclear division

Cytokinesis

Division of the nucleus

• overlaps with telophase, which is division of the cytoplasm

• Starts with a crease called cleavage furrow around the equator of cell

• Cell pinches in 2

• New cells transition into interphase; a new cell cycle begins

Where are the cytoskeleton, inclusions, & organelles located within the cell?

Cytosol

Mitochondrion

Involved in ATP synthesis

• evolved from bacteria

• muscle cells contain numerous to serve their high demand for ATP

What term refers to a segment of DNA that codes for the production of a protein?

Gene