CELLS: The Living Units

BIO 200 Chp 3

The Living Units

Cell Theory:• The cell is the basic structural and

functional unit of life• Organismal activity depends on individual

and collective activity of cells• Biochemical activities of cells are dictated

by subcellular structure• Continuity of life has a cellular basis

Figure 3.1

Cell Structure

Cell Structure

• Plasma Membrane• Separates intracellular fluids from

extracellular fluids• Plays a dynamic role in cellular activity• Glycocalyx (a glycoprotein) bordering the

cell that provides highly specific biological markers by which cells recognize one another

Figure 3.3

Plasma Membrane Structure The Fluid Mosaic Model

Fluid Mosaic Model

• Double bi-layer of lipids with imbedded proteins

• Forms the basic “fabric” of the cell membrane

• Bi-layer consists of phospholipids, cholesterol and glycolipids– Hydrophilic – attracts water (polar head)– Hydrophobic – repel water (nonpolar tails)

Functions of Membrane Proteins• Transport• Enzymatic activity• Receptors for signal transduction

Cell Plasma Membrane

Cell Plasma Structure• The plasma aims to maintain

homeostasis• Lipid molecules of the by-layer move

freely• Polar-nonpolarity interactions keeps

stability• Microvilla – (hairs) increase the plasma

membrane surface

Plasma MembraneMembrane Junctions – help to knit or adhere cellular tissue (enzymes)Tight junction – impermeable junction that

encircles the cell, prevents molecules from passing through

Desmosome – anchoring junction scattered along the sides of cells, aid in mechanical stress

Gap junction – a nexus that allows chemical substances (electrical activity) to pass between cells

Functions of Plasma MembraneMembrane transport

• Cells are surrounded by extacelluar or interstitial fluid

• Interstitial fluid is rich and nutritious • Derives from the blood stream • Ingredients: amino acids, sugars, fatty

acids, vitamins, hormones, salts, waste products.

Functions of Plasma MembraneMembrane transport

• Substances move continuously across the plasma membrane

• It allows some substances to pass and excludes others

• Selective barrier • Differential barrier • Permeable barrier

Characteristics ofa healthy cell

*Damage barriers will imbalance homeostasis

Passive Transport:

Diffusion1. Simple diffusion – nonpolar and lipid-

soluble substances • Diffuse directly through the lipid bilayer• Diffuse through channel proteins• Molecules disperse evenly

Figure 3.6

Passive Transport

Diffusion2. Facilitated diffusion • Allows transport of glucose, amino acids,

and ions • Transported substances bind carrier

proteins or pass through water-filled protein channels

Passive TransportFacilitated diffusion 2. Carrier Proteins• Are integral transmembrane proteins• Show specificity for certain polar

molecules like sugars and amino acids • Molecules too large to pass so they are

carried through by transport receptor carriers

Passive Transport3. Diffussion through Osmosis• Occurs when concentration of a solvent is

different on opposite sides of a membrane • Diffusion of water across a semi-permeable

membrane• Osmolarity – total concentration of solute

particles in a solution

• Tonicity – how a solution affects cell volume

Figure 3.7

Figure 3.8

Active Transport• Uses ATP to move solutes across a

membrane• Requires carrier proteins• Types of Active Transport

– Primary active transport – hydrolysis of ATP phosphorylates the transport protein causing conformational change

– Secondary active transport – use of an exchange pump (such as the Na+-K+ pump) indirectly to drive the transport of other solutes

Figure 3.10

Vesicular Transport

• Transport of large particles and macromolecules across plasma membranes

• Exocytosis – moves substance from the cell interior to the extracellular space

• Endocytosis – enables large particles and macromolecules to enter the cell

Vesicular Transport

• Transcytosis – moving substances into, across, and then out of a cell

• Vesicular trafficking – moving substances from one area in the cell to another

• Phagocytosis – pseudopods engulf solids and bring them into the cell’s interior

Figure 3.12

Figure 3.13a

Figure 3.13b

Membrane Potential• Voltage (electrical potential) across a membrane• Resting membrane potential – the point where

K+ potential is balanced by the membrane potential

• range -50 to -100 millivolts (mV)• Cells become polarized• Results from Na+ and K+ concentration

gradients across the membrane• Steady state – maintained by active transport of

ions

Cell Membrane

• Cell adhesion molecules - anchor cells to the extracellular matrix, assist in movement,

• Membrane Receptors - important in immunity, regulates voltage in nerve and muscle tissue and neurotransmitters

Cytoplasm

• Cytoplasm – material between plasma membrane and the nucleus

• Cytosol – viscous semi-fluid, largely water with dissolved protein, salts, sugars, and other solutes

• Cytoplasmic organelles – metabolic machinery of the cell

• Inclusions – chemical substances such as glycosomes, glycogen granules, and pigment

Cytroplasmic Organelles

Cytoplasmic Organelles

• Membranous - mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus

• Nonmembranous - cytoskeleton, centrioles, and ribosomes

Figure 3.17

Mitochondrion

Mitochondria

• Double membrane structure with shelf-like cristae

• Provide most of the cell’s ATP via aerobic cellular respiration

• Contain their own DNA and RNA

Ribosomes

• Granules containing protein and rRNA• Site of protein synthesis• Free ribosomes synthesize soluble

proteins• Membrane-bound ribosomes synthesize

proteins to be incorporated into membranes

Endoplasmic Reticulum (ER)

• Interconnected tubes and parallel membranes enclosing cristernae (cristae)

• Continuous with the nuclear membrane• Two varieties – rough ER and smooth ER

Figure 3.18

Endoplasmic Reticulum (er)

Rough (ER)• External surface studded with

ribosomes• Manufactures all secreted proteins• Responsible for the synthesis of

integral membrane proteins and phospholipids for cell membranes

Smooth (ER)• Looping tubule network• Catalyzes the following reactions in various

organs of the body:• Liver – lipid & cholesterol metabolism,

breakdown of glycogen, detoxification of drugs • In the testes – synthesis steroid-based

hormones• In the intestinal cells – absorption, synthesis, and

transport of fats• In skeletal and cardiac muscle – storage and release of

calcium

Golgi Apparatus• Stacked and flattened membranous sacs• Functions in modification, concentration,

and packaging of proteins• “Traffic director” for cellular protein • Transport vesicles from the ER and are

received by Golgi apparatus

Figure 3.20

Golgi Apparatus

Lysosomes• Spherical membranous bags containing

digestive enzymes• Digest ingested bacteria, viruses, and

toxins• Degrade nonfunctional organelles• Breakdown glycogen and release thyroid

hormone• Autolysis – self-digestion of the cell

Lysosomes

• Breakdown nonuseful tissue• Breakdown bone to release Ca2+• Secretory lysosomes are found in white

blood cells, immune cells, and melanocytes

Figure 3.22

Lysosomes

Figure 3.23

The Endomembrane System

Endomembrane System• System of organelles that function to: • Produce, store, and export biological

molecules• Degrade potentially harmful substances• Contains the following system:

Nuclear envelope, smooth and rough ER, lysosomes, vacuoles, transport vesicles, Golgi apparatus, and the plasma membrane

Peroxisomes

“Peroxide bodies”• Membranous sacs containing oxidases

and catalases• Detoxify harmful or toxic substances• Neutralize dangerous free radicals• Free radicals – highly reactive chemicals

with unpaired electrons

Cytoskeleton• The “skeleton” of the cell• Dynamic, elaborate series of rods running

through the cytosol• Consists of microtubules, microfilaments,

and intermediate filaments

CytoskeletonMicrotubules• Dynamic, hollow tubes made of the

spherical protein tubulin• Determine the overall shape of the cell

and distribution of organellesMicrofilamentsDynamic strands of protein Actin• Attached to the cytoplasmic side of the

plasma membrane• Braces and strengthens the cell surface

Cytoskeleton

• Intermediate Filaments• Tough, insoluble protein fibers with high

tensile strength• Resist pulling forces on the cell and help

form desmosomes

Pg 91

Centrioles• Small barrel-shaped organelles located in

the centrosome near the nucleus• Pinwheel array of nine triplets of

microtubules• Organize mitotic spindle during mitosis• Form the bases of cilia and flagella

– Whip-like, motile cellular extensions on exposed surfaces of certain cells

– Move substances in one direction across cell surfaces

Cilia

Figure 3.26

Figure 3.27a

Cellular MotionCELIA

• Cellular extensions that provide motility in a whiplike motion.

• Typically found in large numbers

• Located in the exposed surface of the cell

• Move substances in one direction across cell surface

Figure 3.27c

Cellular MotionFlagella

• Projections are longer

• A single propulsive flagellum

• Movement is achieved by propelling itself across the surface or environment

• Basal bodies in the centrioles form the bases for ceia and flagella

Nucleus

• The control center containing genetic • Largest cytoplasmic organelle - 5µm• Nuclear envelop –dbl membrane barrier• Nucleoli – DNA & RNA for genetic

synthesis• Chromatin – threadlike coils that form

chromosomes in cell division. Genes

Figure 3.28

Figure 3.31b

DNA Replication

Cell Growth and ReproductionCell Life Cycle

• Cell division – essential for growth and tissue repair.

• Cells die and continuously reproduce• Some reproduce faster than others (skin,

intestinal vs. liver).• Some loose ability to divide @ maturation (nervous tissue, skeletal muscle, heart, RBCs)• The DNA replicates before cell division

Cell Growth and Reproduction

Cell Division - M Phase (Mitotic)• 2 phases: Mitosis & Cytokinesis• Phase 1: Mitosis – nuclear division

a) prophaseb) metaphasec) Anaphased) telophase

• Phases merge together

Cell Division - Mitosis

Phase 2 – Cytokinesis• Cytokinesis - cytoplasmic division• Cleavage furrow formed in late anaphase

by contractile ring• Cytoplasm is pinched into two parts after

mitosis ends• The forming of 2 daughter cells

Figure 3.32

Figure 3.32