Structure of a Generalized Cell

The Cell• The cell is the basic structural and functional unit of life.

– The cells structural anatomy will determine its overall function.• All cells contains organelles which are the equivalent to organs of

an organism. – They provide a specific function within the cell.

• produces different types of proteins (enzymes)– That carry out overall cell activity.

• There are many different types of cells that contain different proportions on organelles and enzymes. – This will dictate the cells overall specific function.

• It is necessary to understand how the individual cell functions if you want to understand:– Function of the whole organism– Disease: start at a cellular level.

• i.e. cancer

Variations in cells types

Fluid Mosaic Model of the Cell Membrane

• Phospholipids bilayer with imbedded, dispersed proteins• Cholesterol provides stability to the membrane.

– You can make these from essential fatty acids.• glycolipids: are lipids with bound carbohydrate • Glycocalyx is a glycoprotein important for cell recognition.

Plasma Membrane

• Plays a dynamic role in cellular activity by controlling the movement of chemicals into/out of the cell

• The cell membrane can control the passage of various substances including– ions– monosaccharides – amino acids– nucleotides – water

Phospholipids in a Bilayer

• polar heads

• non-polar tails

• The polar heads: of each layer face outward towards the water molecules both in and out of the cell.

• The non-polar tails face inward toward each other creating a barrier against the movement of polar substances into or out of the cell

• Many organelles of the cell have one or more phospholipid bilayers .– endoplasmic reticulum– mitochondria – nucleus– Golgi complex– lysosomes

Integral or Transmembrane Proteins• Completely pass through the bilayer

Peripheral Proteins• Do not penetrate through the membrane. • They associated with integral membrane

proteins.

• Mitochondria: Is known as the powerhouse of the cell. – In the presence of oxygen it can convert organic macromolecules

into energy for the cell. (ATP)

Endoplasmic Reticulum (ER)

Interconnected network tubes within cytoplasm.

Rough ER Smooth ER

Rough ER

• External surface is studded with ribosomes.– Roles in the synthesizing both integral proteins,

antibodies and phospholipids.– proteins synthesized in the ribosomes are taken

inside (cisterna) the ER where they are modified into functional proteins.

– ( formation of H-bonds to create it’s 3 dimensional structure)

• The proteins that are synthesized in the rough ER are shuttled to the Golgi apparatus for further modification.

Ribosomes

• Site for protein synthesis: – The assembly of amino acids to form proteins .i.e.

enzyme, membrane-bound protein and muscles.• Free ribosomes in the cytoplasm

– synthesize proteins that remain inside the cell• Membrane-bound ribosomes

– can synthesize proteins such as integral proteins or proteins that get exported out of the cell.

Ribosome RER

Chaperones Proteins

• Drags new protein from ribosome into cisterna of ER where they function to:– assist in proper folding of amino acids into

proper 2nd and tertiary molecular structure• This is a result of the formation of disulfide bridges

and H-bonds

– remove some amino acids and adding carbohydrates when necessary.

– Escort protein to Golgi Apparatus

Golgi Apparatus

Golgi Apparatus

• Stacked and flattened membranous sacs that functions in modifying, concentrating and packaging proteins.– Create secretary vesicles contain modified proteins

that leave the Golgi and export them through the cell membrane

• This functions as the UPS of the cell.• Produces glycoproteins and proteoglycans

– Lysosomes and peroxiosomes

Lysosomes

• Vesicles containing digestive enzymes which hydrolyze macromolecules (intracellular digestion.)

• Lysosomes function to:– hydrolyze ingested bacteria and viruses and toxins.– Recycle organic molecules such as amino acids,

monosaccharides, fatty acids and nucleotides to make new macromolecules

– Degrade nonfunctional organelles– Breakdown glycogen and release thyroid hormone– Breakdown bone to release Ca2+

Peroxisomes

• Detoxify harmful or toxic substances such as alcohol and other drugs.– Abundant in the liver and kidneys

• Neutralize dangerous free radicals– Free radicals – highly reactive chemicals with

unpaired electrons (i.e., O2–)

– H2O2 H2O +O2

Catalase:

converts hydrogen peroxide to water and oxygen.

Organelles of Protein Production and Excretion

Smooth ER• Smooth ER

– Storage reserve for calcium (Ca2+) required for the functioning of the muscular contractions and transmission of nerve impulses.

– Produce enzymes which: • Synthesize lipoproteins and metabolize lipids

– Production of steroids based hormones such as testosterone.

– Detoxify drugs and toxins such as alcohol and convert them to a less toxic water soluble form which can be excreted by the kidneys.

• Long term use of drugs increase SER development resulting in more efficient drug detoxifying and greater drug tolerance

Cytoskeleton

• The “skeleton” of the cell

• Dynamic, elaborate series of rods running through the cytosol

• Consists of microtubules, microfilaments, and intermediate filaments

Nucleus

Nucleus

• Largest organelle: functions as the control center of the cell. – Contains (DNA) which provides instructions for the

synthesis of all proteins. • DNA determines what proteins are synthesized

– The proteins produced determine cellular function.• Some cells such as the RBC are anuclear

– (no nuclei)• The liver and muscle cells are multinucleate

– (More than one nucleus)

Nuclear Envelope/ Nucleoli

• Nuclear Envelope :double membrane (phospholipid bilayer) structure containing many nuclear pores (allow passage in and out of the nucleus):

• Raw material for the production of DNA and RNA• Enzymes that are made in the cytoplasm but function in

the nucleus.

• Nucleoli : Dark-staining spherical bodies within the nucleus contain the RNA that produces rRNA – code for the production of ribosome subunits.

• These subunits can leave the nucleus through nuclear pores and join to form functional ribosomes.

Membrane Transport

• Movement of molecules through a membrane. • Passive transport: No ATP required!

• Filtration• Simple diffusion• Facilitated diffusion• Osmosis

• Active transport: Requires the break down of ATP!• Primary• Secondary• Exocytosis• Endocytosis

Passive Transport

• Substances diffuses across the cell membrane from a high concentration to a low.

• Like a ball rolling down a hill.– There is no energy expenditure! (ATP)

• Various substances may accomplish this by different mechanisms:– nonpolar molecules such as lipids diffuse through the

phospholipid bilayer unimpeded by a process called simple diffusion.

– Some smaller polar molecules such as electrolytes and use integral membrane proteins. (channels)

– Larger molecules such as sugar and amino acids require help (facilitation) of to cross the bilayer in a process called facilitated diffusion.

Diffusion• Molecules that are closer to each other collide

and ricochet off each other. This results in a natural tendency to scatter.

• A substance will always move down its gradient unless energy is put into the system.

Facilitated DiffusionRequired for larger substances such as glucose, amino

acids, and some ions.

Travel down their concentration gradient binding to a

specific carrier protein. The carrier protein changes configuration and releases to substance into the cell.

Summery of Diffusion• Simple diffusion – nonpolar and lipid-soluble substances

– Non polar molecules diffuse directly through the lipid bilayer• Polar molecules diffuse through channel proteins

Factors Affecting the Rate of Diffusion

1. Concentration (gradient) • The greater the difference in the concentrations

of a substance, the greater the rate of diffusion.2. Temperature

• As temperature increases, the rate of diffusion increases.

3. Size of the substance • As the size of substances increase, the rate of

diffusion decreases.

Osmosis• The diffusion of water (solvent) across a selectively

permeable membrane through water channels (aquaporins)

• Water has a natural tendency to diffuse toward the area with the greatest amount of solutes– (dissolved substances in the water)

• Think of making orange juice from concentrate. The concentrated orange juice is are the solutes. You must add the solvent (water) to it.

• Osmotic pressure

– the greater the difference in concentration of solutes the greater the osmotic pressure.

– the more water will move toward the area of higher concentration.

Osmosis

• Diffusion of water through a semi-permeable membrane – from area of more water

to area of less water– from low to high

concentration of solute.• Aquaporins = channel

proteins specialized for osmosis

Osmotic Pressure

• The water shifts to the left by osmosis.– Where there is a higher

concentration of solutes.

• Osmosis will stop due to filtration of water back across membrane due to hydrostatic pressure– The pressure of gravity

on the left side.

Tonicity• Tonicity - ability of a solution to affect fluid volume

and pressure within a cell.– depends on concentration and permeability of solute

• Hypotonic solution (dilute outside the cell) – low concentration of solutes (high water concentration) – cells absorb water, swell and may burst (lyse)

• Hypertonic solution (concentrated outside cell)– has high concentration of solutes (low water concentration)– cells lose water + shrivel (crenate)

• Isotonic solution = (Equal concentrations both in and out of the cell.)– Cell will be at an osmotic equilibrium with solution

• Water will always move to where it is more concentrated with solutes.

Tonicity of Red Blood Cells

Hypotonic Isotonic Hypertonic

(Swell) (No change) (Crenate/Shrink)

Clinical Application

You are a nurse working in the hospital and your patient’s doctor orders an IV because their blood work suggested they were dehydrated. What would you give the patient.

a. Distilled water

b. Normal saline .9%NACL

c. 2% NACL saline

d. All of the above

e. None of the above

Clinical Application

You are a cyclist finishing a 2 hour ride on a hot summer day. What would be the drink of choice to replace your fluid loss.

A. Water

B. Gatorade

C. Protein shake

D. Orange juice

E. Corona

Filtration• Uses hydrostatic pressure

(fire hose) Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area.

• The movement of water, nutrients and gases through capillary walls to supply the cells with nutrients. The pressure also allows the kidneys to filter the blood. (filtrate)

Types of Active Transport• Primary active transport – hydrolysis of ATP

provides the energy to move solutes against their concentration gradient.

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

• Exocytosis-Transport of substances out of the cell enclosed with in a membranous vesicle. The secretion of neurotransmitters, hormones, mucus and cellular waste.

• Endocytosis - Substances are moved into the cell by engulfing them in a vesicle

Primary Active Transport• Process that hydrolyzes the high energy bond in a

molecule of ATP (releasing energy) • Breaking the high energy bonds converts chemical

energy within the bond to mechanical energy. • A substance can be forced from a region of

lesser to greater concentration :– High Energy Bonds / lower energy bonds

Examples of Primary Active Transport• Na+,K+ Pump

– located in the plasma membrane– actively pumps:– 3 Na+ from the ICF →ECF – 2 K+ from the ECF →ICF – maintains a Na+,K+ gradient across the cell membrane

Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation by ATP.

1

2

3

4

Phosphorylation causes the protein to change its shape.

The shape change expels Na+ to the outside, and extracellular K+ binds.

5 Loss of phosphate restores the original conformation of the pump protein. K+ binding triggers

release of the phosphate group.

6 K+ is released and Na+ sites are ready to bind Na+ again; the cycle repeats.

Concentration gradients of K+ and Na+

Extracellular fluid

Cytoplasm

Sodium-Potassium Pump

Figure 3.10

Na+, Glucose Cotransporter

Example of Secondary Active Transport

• Na+, glucose cotransporter – cotransports: Moves in the same direction as Na+

• Na+ moves down its concentration gradient into the cell as a result of the gradient created by the Na+,K+-ATPase located at the basal (bottom) surface of cells.

• SGLT(Na+ glucose transport protein): At the apical surface glucose is taken into the cell with Na+ down their concentration gradient by facilitated diffusion.

– SGLT are important for reabsorbing glucose in kidney

Exocytosis

Types of Endocytosis

• Phagocytosis (cell eating) – endocytosis of few very large substances (bacteria,

viruses, cell fragments)– vesicles containing cells fuse with lysosomes which

digest the cells • Pinocytosis (cell drinking)

– endocytosis of extracellular fluid

Phagocytosis