Chapter 6 A Tour of the Cell

Size of a Nanometer demo

• http://learn.genetics.utah.edu/content/cells/scale/

• Glucose = .9 nm

• Ribosome = 30 nm

• Mitochondria = 4000 nm long

• Skin Cell = 30,000 nm long = 30 um

• 1000 nm = 1 um

Light Microscope - LM

• Uses visible light to illuminate the object.

• Relatively inexpensive type of microscope.

• Can examine live or dead objects.

Resolution• Ability to detect two discrete points as

separate from each other.• As Magnification increases, Resolution

decreases. • LM working limits are100 - 1000X.

Limitations - LM

• Miss many cell structures that are beyond the magnification of the light microscope.

• Need other ways to make the observations.

Light Microscope Variations• Fluorescence: uses dyes to make parts

of cells “glow”.• Phase-contrast: enhances contrasts in

density.• Confocal: uses lasers and special

optics to focus only narrow slides of cells.

Before

After

Electron Microscopes

• Use beams of electrons instead of light.

• Invented in 1939, but not used much until after WWII.

• Electron beam scans surface of the sample, exciting electrons which have their signals detected by a device that translates the pattern of electrons into an electronic signal to a video screen.

TEM – tracheal cell, long cilia were cut longitudinally, others cut to reveal

cross section

SEM rabbit trachea

covered in cilia

Advantages

• Much higher magnifications.

• Magnifications of 50,000X or higher are possible.

• Can get down to atomic level in some cases.

Disadvantages

• Need a Vacuum.

• High cost of equipment.

• Specimen preparation.

• Specimen must be dead

Other Tools for Cytology (study of Cells)

• Cell Fractionation – break the cell apart and separate out the pieces based on their density.

Cell Fractionation

History of Cells

• Robert Hooke - Observed cells in cork.

• Coined the term "cells” in 1665.• 1833 - Robert Brown, discovered the

nucleus.• 1838 - M.J. Schleiden, all plants are

made of cells.• 1839 - T. Schwann, all animals are

made of cells.

Cell Theory

• All living matter is composed of one or more cells.

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

• All cells come from cells.

• http://www.youtube.com/watch?v=mF9U5x6Nxnw

Types of Cells

• Prokaryotic - lack a nucleus and other membrane bounded structures.

• Eukaryotic - have a nucleus and other membrane bounded structures.

Both Have:

• Membrane

• Cytoplasm

• Ribosomes (but the size is different)

Prokaryotic Eukaryotic

Nucleus

Prokaryotic

Eukaryotic

Basic Cell Organization

• Membrane

• Nucleus

• Cytoplasm

• Organelles

Animal Cell

Plant Cell

Membrane

• Separates the cell from the environment.

• Boundary layer for regulating the movement of materials in/out of a cell.

Cytoplasm or Cytosol

• Cell substance between the cell membrane and the nucleus.

• The “fluid” part of a cell.

• Composed of water, dissolved salts and organic molecules (molecules that contain Carbon, dissolved proteins, and the cytoskeleton)

Organelle

• Term means "small organ”. Formed body (or compartment) in a cell with a specialized function.

• Important in organizational structure of cells.

Organelles - function

• Way to form compartments in cells to separate chemical reactions.

• Keeps various enzymes separated in space.

You must be able to:

• Identify the major organelles

• Give their structure

• Give their function

Nucleus

• Most conspicuous organelle.

• Usually spherical, but can be lobed or irregular in shape.

Structure

• Nuclear membrane

• Nuclear pores

• Nucleolus

• Chromatin

Nuclear Membrane

• Double membrane separated by a 20-40 nm space.

• Inner membrane supported by a protein matrix which gives the shape to the nucleus.

Nuclear Pores

• Regular “holes” through both membranes.

• 100 nm in diameter.

• Protein complex gives shape.

• Allows materials in/out of nucleus.

Nucleolus

• Dark staining area in the nucleus.

• 0 - 4 per nucleus.

• Ribosomes are made here

Chromatin

• Chrom: colored

• - tin: threads

• DNA and Protein in a “loose” format. Will form the cell’s chromosomes.

Nucleus - Function

• Control center for the cell.

• Contains the genetic instructions.

Ribosomes

• Structure: 2 subunits made of protein and rRNA. No membrane.

• Function: protein synthesis.

Subunits

• Large:• 45 proteins• 3 rRNA molecules

• Small:• 23 proteins• 1 rRNA molecule

Locations

• Free in the cytoplasm - make proteins for use in cytosol.

• Membrane bound - make proteins that are exported from the cell.

Endomembrane System

• Membranes that are related through direct physical continuity or by the transfer of membrane segments called vesicles.

Endomembrane System

Endoplasmic Reticulum

• Often referred to as ER.

• Makes up to 1/2 of the total membrane in cells.

• Often continuous with the nuclear membrane.

Structure of ER

• Folded sheets or tubes of membranes.

• Very “fluid” in structure with the membranes constantly changing size and shape.

Types of ER

• Smooth ER: no ribosomes.

• Used for lipid synthesis, carbohydrate storage, detoxification of poisons/drugs. Stores Ca ions

• Rough ER: with ribosomes.

• Makes secretory proteins.

Golgi Apparatus

• Structure: parallel array of flattened cisternae. (looks like a stack of Pita bread)

• 3 to 20 per cell.

• Likely an outgrowth of the ER system.

Function of Golgi Bodies

• Processing - modification of ER products (proteins for secretion).

• Distribution - packaging of ER products (proteins for secretion) for transport.

Vesicles

• Small sacs of membranes that bud off the Golgi Body.

• Transportation vehicle for the modified ER products.

Lysosome

• Single membrane.

• Made from the Golgi apparatus.

Function

• Breakdown and degradation of cellular materials.

• Contains enzymes for fats, proteins, polysaccharides, and nucleic acids.

• Over 40 types known.

Lysosomes

• Important in cell death.

• Missing enzymes may cause various genetic enzyme diseases.

• Examples: Tay-Sachs,

Vacuoles

• Structure - single membrane, usually larger than the Golgi vesicles.

• Function - depends on the organism.

Protists

• Contractile vacuoles - pump out excess water.

• Food vacuoles - store newly ingested food until the lysosomes can digest it.

Plants

• Can have a large single vacuole when mature making up to 90% of the cell's volume.

• Function• Water regulation.

• Storage of ions.

• Storage of hydrophilic pigments. (e.g. red and blues in flower petals).

Function: Plant vacuole

• Used to enlarge cells and create turgor pressure.

• Enzymes (various types).

• Store toxins.

• Coloration.

Microbodies

• Structure: single membrane.

• Often have a granular or crystalline core of enzymes.

Function

• Specialized enzymes for specific reactions.

• Peroxisomes: break down fatty acids, detoxify, adds H2 to toxis (it has enzymes in it that convert H2O2 to H20)

• Glyoxysomes: convert lipids into sugars (useful for plant seeds)

Enzymes in a crystal

Mitochondria

• Structure: 2 membranes. The inner membrane has more surface area than the outer membrane.

• Matrix: inner space.

• Intermembrane space: area between the membranes.

Inner Membrane

• Folded into cristae.

• Amount of folding depends on the level of cell activity.

• Contains many enzymes.

• ATP generated here.

Function

• Cell Respiration - the release of energy from food.

• Major location of ATP generation.

• “Powerhouse” of the cell. Comment – be careful NOT to overuse this phrase.

Mitochondria

• Have ribosomes (small size).

• Have their own DNA.

• Can reproduce themselves.

• Likely were independent cells at one time.

Chloroplasts

• Structure - two outer membranes.

• Complex internal membrane.

• Fluid-like stroma is around the internal membranes.

Inner or Thylakoid Membranes

• Arranged into flattened sacs called thylakoids.

• Some regions stacked into layers called grana.

• Contain the green pigment chlorophyll.

Function

• Photosynthesis - the use of light energy to make food.

Chloroplasts

• Contain ribosomes (small size).

• Contain DNA.

• Can reproduce themselves.

• Often contain starch.

• Likely were independent cells at one time (cyano-bacteria).

Cytoskeleton

• Network of rods and filaments in the cytoplasm.

Functions

• Cell structure and shape.

• Cell movement.

• Cell division - helps build cell walls and move the chromosomes apart.

Cytoskeleton Components

• Microtubules

• Microfilaments

• Intermediate Filaments

Microtubules

• Structure - small hollow tubes made of repeating units of a protein dimer.

• Size - 25 nm diameter. Can be 200 nm to 25 m in length.

Microtubules

• Regulate cell shape.

• Coordinate direction of cellulose fibers in cell wall formation.

• Tracks for motor molecules.

Microtubules

• Form cilia and flagella.

• Movement of organelles and chromosomes within the cell

Dynein Protein (Type of microtubule)

• A contractile protein.

• Uses ATP.

• Creates a twisting motion between the microtubules causing the structure to bend or move.

Microfilaments

• 5 to 7 nm in diameter.

• Structure - two intertwined strands of actin protein.

Microfilaments are stained green.

Functions

• Muscle contraction.

• Cytoplasmic streaming.

• Pseudopodia.

• Cleavage furrow formation.

• Maintenance and changes in cell shape.

Intermediate Filaments

• Fibrous proteins that are super coiled into thicker cables and filaments 8 - 12 nm in diameter.

• Made from several different types of protein.

Functions

• Maintenance of cell shape.

• Hold organelles in place.

Cytoskeleton

• Very dynamic; changing in composition and shape frequently.

• Cell is not just a "bag" of cytoplasm within a cell membrane.

Cell Wall

• Nonliving jacket that surrounds some cells.

• Found in:• Plants• Prokaryotes• Fungi• Some Protists

Plant Cell Walls

• All plant cells have a Primary Cell Wall.

• Some cells will develop a Secondary Cell Wall.

Primary Wall

• Thin and flexible.

• Cellulose fibers placed at right angles to expansion.

• Placement of fibers guided by microtubules.

Secondary Wall • Thick and rigid.• Added between the cell membrane and

the primary cell wall in laminated layers.• May cover only part of the cell; giving

spirals. • Makes up "wood”.

Middle Lamella

• Thin layer rich in pectin found between adjacent plant cells.

• Glues cells together.

Cell Walls

• May be made of other types of polysaccharides and/or silica.

• Function as the cell's exoskeleton for support and protection.

Extracellular Matrix - ECM• Fuzzy coat on animal cells.• Helps glue cells together.• Made of glycoproteins and collagen.• Evidence suggests ECM is involved

with cell behavior and cell communication.

Why Are Cells So Small?

• Cell volume to surface area ratios favor small size.

• Nucleus to cytoplasm consideration (control).

• Metabolic requirements.

• Speed of diffusion.