Chapter 6: A Tour of the Cell

Observation Is the keystone of science. Need: Techniques to observe cells.

Question ? Can cells be seen with the naked eye? Yes, a few are large enough, but most require the use of a microscope.

Microscope History Janseen Brothers invent the compound microscope Robert Hooke “discovers” cells in cork. Early 1700’s - von Leeuwenhoek makes many observations of cells including bacteria.

Light Microscope - LM Uses visible light to illuminate the object. Relatively inexpensive type of microscope. Can examine live or dead objects.

Light Microscope Occular Lens Objective Lens Stage with specimen Light Source

Magnification Increase in diameter or size.

Resolution Ability to detect two discrete points as separate from each other. As Magnification increases, resolution decreases. LM working limits are X.

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.

Electron Microscopes Use beams of electrons instead of light. Invented in 1939, but not used much until after WWII.

TEMSEM

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. Specimen must stop the electrons. High cost of equipment. Specimen preparation.

Transmission Electron Microscope - TEM Sends electrons through thinly sliced and stained specimens. Gives high magnification of interior views. Many cells structures are now visible.

TEM Limitations Specimen dead. Specimen preparation uses extreme chemicals so artifacts are always a concern.

Scanning Electron Microscope - SEM Excellent views of surfaces. Produces 3-D views. Live specimens possible.

Limitations Lower magnifications than the TEM.

EM Variations High Voltage TEM Tunnel SEM Elemental Composition SEM

TEM - interiorSEM - surface

Cell Biology or Cytology Cyto = cell - ology = study of Should use observations from several types of microscopes to make a total picture of how a cell is put together.

Other Tools for Cytology Cell Fractionation Chromatography Electrophoresis

Cell Fractionation Disrupt cells. Separate parts by centrifugation at different speeds. Result - pure samples of cell structures for study.

Cell Fractionation

Chromatography Technique for separating mixtures of chemicals. Separates chemicals by size or degree of attraction to the materials in the medium. Ex - paper, gas, column, thin-layer

Electrophoresis Separates mixtures of chemicals by their movement in an electrical field. Used for proteins and DNA.

History of Cells Robert Hooke - Observed cells in cork. Coined the term "cells” in 1665.

History of Cells Robert Brown, discovered the nucleus M.J. Schleiden, all plants are made of cells T. Schwann, all animals are made of cells J.E. Purkinje, coined the term “protoplasm”.

Cell Theory All living matter is composed of one or more cells. The cell is the structural and functional unit of life.

R. Virchow “Omnis cellula e cellula” All cells are from other cells.

Types of Cells Prokaryotic - lack a nucleus and other membrane bounded structures. Eukaryotic - have a nucleus and other membrane bounded structures.

ProkaryoticEukaryotic Nucleus

Prokaryotic Eukaryotic

How small can a cell be? Mycoplasmas - bacteria that are.1 to 1.0  m. (1/10 the size of regular bacteria).

Why Are Cells So Small? Cell volume to surface area ratios favor small size. Nucleus to cytoplasm consideration (control). Metabolic requirements.

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 Cell substance between the cell membrane and the nucleus. The “fluid” part of a cell. Exists in two forms:  gel - thick  sol - fluid

Organelle Term means "small organ” Formed body 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.

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 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 per nucleus. Storage area for ribosomes.

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. Rough ER: with ribosomes. Makes secretory proteins.

Golgi Apparatus or Dictyosomes 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.

Structure Has 2 Faces Cis face - side toward the nucleus. Receiving side. face - side away from the nucleus. Shipping side. Trans face - side away from the nucleus. Shipping side.

Function of Golgi Bodies Processing - modification of ER products. Distribution - packaging of ER products for transport.

Golgi Vesicles Small sacs of membranes that bud off the Golgi Body. Transportation vehicle for the modified ER products.

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Lysosome Single membrane. Made from the Trans face of the Golgi apparatus.

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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, Pompe’s Disease

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 Large single vacuole when mature making up to 90% of the cell's volume. Tonoplast - the name for the vacuole membrane.

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: use up hydrogen peroxide. Glyoxysomes: lipid digestion.

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.

Mitochondria Have ribosomes. Have their own DNA. Can reproduce themselves. May have been 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. Contain DNA. Can reproduce themselves. Often contain starch. May have been independent cells at one time.

Plastids Group of plant organelles. Structure - single membrane. Function - store various materials.

Examples Amyloplasts/ Leucoplasts - store starch. Chromoplasts - store hydrophobic plant pigments such as carotene.

Ergastic Materials General term for other substances produced or stored by plant cells. Examples:  Crystals  Tannins  Latex  Resins

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.

Components Microtubules Microfilaments Intermediate Filaments

Microtubules Structure - small hollow tubes made of repeating units of a protein dimer. Size - 25 nm diameter with a 15 nm lumen. Can be 200 nm to 25  m in length.

Tubulin Protein in microtubules. Dimer -  and  tubulin.

Microtubules Regulate cell shape. Coordinate direction of cellulose fibers in cell wall formation. Tracks for motor molecules.

Microtubules Form cilia and flagella. Internal cellular movement. Make up centioles, basal bodies and spindle fibers.

Cilia and Flagella Cilia - short, but numerous. Flagella - long, but few. Function - to move cells or to sweep materials past a cell.

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Cilia and Flagella Structure arrangement of microtubules, covered by the cell membrane. Dynein - motor protein that connects the tubules.

Dynein Protein A contractile protein. Uses ATP. Creates a twisting motion between the microtubules causing the structure to bend or move.

Centrioles Usually one pair per cell, located close to the nucleus. Found in animal cells. 9 sets of triplet microtubules. Help in cell division.

Basal Bodies Same structure as a centriole. Anchor cilia and flagella.

Basal Body

MTOCs Microtubule Organizing Centers - sites that microtubules grow from. Assist in cell division by anchoring spindle fibers. May be anchored by centrioles.

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.

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Intermediate Filaments Fibrous proteins that are super coiled into thicker cables and filaments 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.

Intercellular Juctions Plants-Plasmodesmata

Plasmodesmata Channels between cells through adjacent cell walls. Allows communication between cells. Also allows viruses to travel rapidly between cells.

Intercellular Juctions Animals:  Tight junctions  Desmosomes  Gap junctions

Tight Junctions Very tight fusion of the membranes of adjacent cells. Seals off areas between the cells. Prevents movement of materials around cells.

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Desmosomes Bundles of filaments which anchor junctions between cells. Does not close off the area between adjacent cells. Coordination of movement between groups of cells.

Gap Junctions Open channels between cells, similar to plasmodesmata. Allows “communication” between cells.

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Summary Answer: Why is Life cellular and what are the factors that affect cell size? Be able to identify cellular parts, their structure, and their functions.