Tour of the Cell

Microscopy

Microscopes used to visualize cells

Light Microscope (LM)

Visible light is passed through a specimen, then through glass lenses which refract (bend) the light so the image is magnified.

Can magnify effectively 1000x the size of the specimen.

Cell components can be stained/labeled to enhance contrast.

Resolution is to low to study organelles, the membrane bound structures within eukaryotic cells.

Electron Microscopes (EM)

Parameters of microscopy

Def. Magnification: The ratio of an object's image size and it's actual size.

Def. Resolution: The measure of clarity of the image or minimum distance of two distinguishable points.

Def. Contrast: The visible differences in brightness between parts of the sample.

Scanning Electron Microscope (SEMs)

Focused beam of electrons onto the surface of a specimen, providing a 3-D like image.

Transmission Electron Microscope (TEMs)

Focused beam of electrons through a specimen, used to study the internal structure of cells.

Cell Fractionation

Takes cells apart and separates the major organelles enabling scientists to determine their function.

Figure 6.4

Internal Structures of Cells

Prokaryotes vs Eukaryotes

Prokaryotic Cells

Eukaryotic Cells

No Nucleus

Features of all Cells

Plasma mebrane

Semifluid substance called Cytosol (cytoplasm)

Chromosomes (Genetic Material)

Ribosomes (make Proteins)

DNA in an unbound region called the nucleoid

No membrane-bound organelles

Cytoplasm bound by the plasma membrane

DNA in Nucleus, bound by a double membrane

Membrane-bound organelles

Cytoplasm in region between the Nucleus and Plasma Membrane

Generally larger in size

Genetic Instructions and Protein Synthesis

Nucleus

Ribosomes

Contains most of the genetic material in a eukaryotic cell.

Enclosed by Nuclear Envelope, separating it from the cytoplasm.

Double membrane, each consisting of a lipid bilayer.

Pores, lined with a pore complex, regulate the entering and exit of molecules from the nucleus.

Nuclear Envelope lined by the nuclear lamina, composed of proteins to maintain the nucleus shape.

DNA is organized into units called Chromosomes.

One chromosome contains one DNA molecule associated with proteins called chromatin.

Nucleolus located within the nucleus and is the site of Ribosomal RNA (rRNA) synthesis.

Use the information from the DNA to make Proteins.

Carry out protein synthesis in two locations

In the Cytosol (Free Ribosomes)

On the outside of the Endoplasmic Reticulum or Nuclear Envelope (Bound Ribosomes)

Endomembrane System

Some components connected via transfer by vesicles.

Endoplasmic Reticulum (ER)

Connected to the Nuclear Envelope

Two regions of the ER

Smooth Endoplasmic Reticulum: Lacks bound Ribosomes

Functions

Synthesizes lipids

Metabolizes Carbohydrates

Detoxifies drugs and poisons

Stores calcium ions

Rough Endoplasmic Reticulum: Contains surface bound Ribosomes

Functions

Bound Ribosomes produce Glycoproteins

Distributes Transport Vesicles, secretory proteins surrounded by membranes

Is the membrane factory of the cell

Golgi Apparatus

Consists of flattened membranous sacs called Cisternae.

Functions

Modifies products of ER

Manufactures certain macromolucles

Sorts and packages materials into Transport Vesicles

Lysosomes

Membranous sacs of hydrolytic enzymes

Vacuoles

Hydrolytic enzymes and lysosomal membranes are produced by the rough ER and then transferred to the Golgi apparatus for processing.

Lysosomal enzymes work best within the acidic environment of the lysosome.

Some types of cells can engulf another cell by phagocytosis; forming food vacuoles.

Lysosomes recycle the cell's own organelles and macromolecules; via autophagy

Lysosomes fuse with the food vacuole to digest the molecules.

Large vesicles derived from the ER and Golgi apparatus.

Functions vary in different types of cells

Food Vacuoles: Formed by Phagocytosis

Contractile Vacuoles: Pump excess water out of the cells; mostly found in freshwater protists.

Central Vacuoles: Holds organic compounds and water; found in mature plant cells.

Energy Production

Evolutionary Similarities between Mitochondria and Chloroplasts

Mitochondria

Chloroplasts

click to edit

Similarities led to the Endosymbiont Theory

Enveloped by a double membrane

Contain free ribosomes and circular DNA molecules

Suggests that an early ancestor of eukaryotic cells engulfed an oxygen using non-photosynthetic prokaryotic cell; which formed a symbiotic relationship with the host cell, becoming an endosymbiont.

Figure 6.16

Endosymbiont later evolved into mitochondria.

At least one cell may have taken up a photosynthetic prokaryote; which later evolved into a chloroplast.

Grow and reproduce somewhat independently in cells

Site of Cellular Respiration, metabolic process that uses oxygen to generate ATP

Made up of a smooth outer membrane and an inner folded membrane called Cristae

Inner membrane crates two compartments: Intermembrane space and Mitochondrial matirx

Some steps of cellular respiration are catalyzed in the mitochondrial matrix.

Folded Cristae allows for more surface area for enzymes to synthesize ATP.

Found in plants and algae and are the sites of photosynthesis.

Contain green pigment, Chlorophyll, as well as enzymes and other molecules that function in photosynthesis

Structures

Thylakoids: Membranous sacs stacked to form Granum.

Stroma: Internal fluid

Chloroplast is one of a group of plant organelles called plastids.

Peroxisomes

Specialized metabolic compartments bound by a single membrane

Produce Hydrogen Peroxide and converts it into water.

Peroxisomes perform reactions with different functions; it is unknown how peroxisomes are related to other organelles.

Organization of Structures and Activities

Cytoskeleton

Network of fibers extending throughout the cytoplasm. That organizes cell's structures and activities, anchoring many organelles.

Composed of

Microtubules

Hollow rods about 25 nm in diameter, and 200 nm to 25 microns long. constructed from dimers of tubulin.

Functions

Shaping the cell

Guiding movement of organelles

Separating Chromosomes during cell division

Grow out from Centrosomes near the Nucleus

Each Centrosome has a pair of Centrioles; each with nine triplets of microtubules.

Cilia and Flagella

Control the beating of Flagella and Cilia, Microtubule-containing extensions that project from some cells.

Microfilaments (Actin filaments)

Many unicellular Eukaryotes are propelled through water by Cilia or Flagella; they differ by their beating pattern

Common structures between Cilia and Flagella

Group of microtubules sheathed by an extension of the plasma membrane

Basal body anchors the Cilium or Flagellum

Dynein, a motor protein, which drive the bending movement of a Cilium or Flagellum.

Has two "feet" that "walk" along microtubules. One foot maintains contact, while the other releases and reattaches on step farther along; causing the microtubules to bend, rather than slide, because the microtubules are held in place.

Solid rod about 7 nm in diameter, build as a twisted double chain of actin subunits.

Cortex, a network of microfilaments just inside the plasma membrane

Extra Cellular Components

Bundles of microfilaments make up the core of microvilli in intestinal cells

Microfilaments that function in cellular motility contain Myosin in addition to Actin.

Cells crawl along a surface by extending pseudopodia (cellular extensions) and moving towards them.

Cytoplasm Streaming: Circular flow of cytoplasm within cells, driven by actin-myosin interactions.

Intermediate Filaments

Figure 6.T01d

Figure 6.T01b

Figure 6.T01c

Range in diameter from 8 to 12 nm; larger than microfilaments bus smaller than microtubules.

More permanent cytoskelton fixtures that microfilaments and microtubules.

Supports cell shape and fixes organelles in place

Plant Cell Wall

Protects the plant cell, maintains it's shape and prevents excessive intake of water

Made up of cellulose fibers embedded in other polysaccharides and protein.

Extracellular structure that distinguishes plant cells from animal cells

Layers

Prokaryotes, Fungi, and some unicellular Eukaryotes have cell walls.

Primary Cell Wall: Relatively thin and flexible

Middle Lamella: Thin layer between primary cell walls of adjacent cells.

Secondary Cell Wall: Added between the plasma membrane and primary cell wall in some cells.

Extracellular Matrix (ECM)

Made up of glycoproteins, such as collegen, proteoglycans, and fibronectin

ECM can regulate cells behaviors by communicating with a cell through integrins

Animal cells lack cell walls but are covered by an elaborate extracellular matrix

ECM proteins bind to receptor proteins in the plasma membrane called integrins

Can influence the activity of genes in the nucleus

Cell Junctions

Plasmodesmata

Types of Cell junctions in Epithelial Tissue

Neighboring cells often adhere, interact, and communicate through physical contact

Channels that perforate plant cell walls; through which water, small solutes, and sometimes proteins or RNA can pass from cell to cell.

Tight Junctions: Membranes of neighboring cells are pressed together preventing leakage of extracellular fluids

Desmosomes (anchoring junctions): fastens cells together into strong sheets

Gap Junctions (Communicating junctions): Provide cytoplasmic channels between adjacent cells

Plasma Membrane

Surface area to volume ratio of a cell is critical; as a cell increases it's volume increases proportionally more than it's surface area

Figure 6.7

Differences in Membrane Composition

Fluid Mosaic of Lipids and Proteins

Phospholipids, the most abundant lipid in the plasma membrane, are amphipathic molecules. Containing hydrophobic and hydrophilic regions.

The hydrophobic tails of the phospholipids are sheltered inside the membrane, while the hydrophilic heads are exposed to water on either side.

Fluid Mosaic Model: Membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Proteins are not randomly distributed in the membrane.

Fluidity of Membranes

Held together mainly by weak hydrophobic interactions.

Most of the lipids, and some proteins, can move sideways within the membrane. Rarely a lipid may flip-flop across the membrane from one phospholipid layer to the other.

As temperatures cool, membranes switch from a fluid state to a solid state.

The temperature at which a membrane solidifies depends on the types of lipids

Variations of lipid composition of cell membranes appear to be adaptations to specific environmental conditions.

Ability to change lipid composition have evolved in organisms that live where temperatures vary.

Membranes rich in unsaturated fatty acids are more fluid than those rice in saturated fatty acids.

Membranes must be fluid to work properly

Steroid cholesterol effects membrane fluidity at different temperatures

At warm temperatures, cholesterol restrains movement of phospholipids.

At cool temperatures, cholesterol maintains fluidity by preventing tight packing.

Plants use a related steroid lipid to buffer membrane fluidity.

Role of Membrane Carbohydrates

Membrane Proteins and their Functions

Membrane is a collage of different proteins, clustered in groups, embedded in the fluid matrix of the lipid bilayer.

Proteins determine most of the membrane's functions

Phospholipids form the main fabric of the membrane.

Def. Peripheral Proteins: Proteins bound to the surface of the membrane.

Def. Integral Proteins: Proteins that penetrate the hydrophobic core.

Proteins that span the membrane are called Transmembrane Proteins.

Cell-surface membranes carry out several functions

Signal Transduction

Cell-Cell recognition

Enzymatic activity

Intercellular joining

Transport

Attachment to the cytoskeleton and extracellular matrix

Cells recognize each other by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane

Carbohydrates maybe covalently bonded to lipids, or to proteins.

Carbohydrates vary among species, individuals, and even cell types in an individual.

Structure results in Selective Permeability

Cells must exchange materials with it's surrounding, the plasma membrane is selectively permeable to regulate the cell's molecular traffic.

Permeability of the Lipid Bilayer

Hydrophobic (Nonpolar) molecules can dissolve into the lipid bilayer and pass through the membrane rapidly.

Hydrophilic molecules, such as ions and polar molecules, cannot cross the membrane easily.

Proteins built into the membrane regulate transport.

Types of Molecule Transport

Passive

Synthesis and Sidedness of Membranes

Have distinct inside and outside faces.

asymmetrical distribution of proteins, lipids, and carbohydrates in the membrane are determined when the membrane is built by the ER and Golgi apparatus.

Diffusion

Def. Diffusion: The tendency for molecules to spread evenly into the available space.

Each molecule moves randomly, diffusion of a population of molecules may be directional.

Def. Dynamic Equilibrium: As many molecules cross the membrane in one direction as the other.

Substances diffuse down their own concentration gradient, the region along which the density of a chemical substance increases or decreases.

No work is required

Diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen.

Water Balance

Osmosis is the diffusion of water across a selectively permeable membrane.

Water diffuses from a region of lower solute concentration to a region of higher solute concentration until the solute concentration is equal on both sides.

Tonicity

Def. Tonicity: The ability of a surrounding solution to cause a cell to gain or lose water.

Types of Solutions

Depends on it's concentration of solutes that cannot cross the membrane relative to that inside the cell.

Def. Hypertonic Solution: Solute concentration is greater than that of inside the cell; Cell loses water.

Def. Hypotonic Solution: Solute concentration is less than that of inside the cell; Cell gains water.

Def. Isotonic Solution: Solute concentration is the same as inside the cell; no net water movement.

Effects of Tonicity on Cells With and Without Cell Walls

Hyper/Hypotonic environments create osmotic problems for organisms that lack rigid cell walls.

Cells without cell walls with shrivel in hypotonic environments and lyse (burst) in hypertonic environments.

Cells with cell walls can better maintain water balance.

Cells in hypotonic solutions swell entill the cell wall opposes intake; cell becomes turgid (firm)

If the cells surroundings are isotonic the cell becomes flaccid (limp)

In a hypertonic solution the cell loses water and the membrane pulls wasy from the cell wall.

In plant cells this causes the plant to wilt with a potentially lethal effect, Plasmolysis.

Osmoregulation

Def. Osmoregulation: The control of solute concentrations and water balance.

Bacteria and Archaea that live in hypersaline environments have cellular mechanisms to balance internal and external solute concentrations.

Transport Proteins

Bulk Transport

Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins, Large molecules cross the membrane in bulk via vesicles.

Def. Exocytosis: When transport vesicles migrate to the membrane, fuse, and release their contents outside the cell.

Secretory cells use endocytosis to export products.

Def. Endocytosis: When cells take in macromolecules forming vesicles from the plasma membrane.

Types

Def. Pinocytosis ("Cellular Drinking"): Molecules dissolved in droplets are taken up when extracellular fluids are "gulped" into tiny vesicles.

Def. Receptor-Mediated Endocytosis: When specific solutes bind to receptors on the cell's surface which triggers the formation of a vesicle.

Def. Phagocytosis ("Cellular Eating"): Cell engulfs a particle in a vacuole which then fuses with a lysosome to digest the particle.

Emptied receptors are recycled into the plasma membrane.

Human cells use receptor-mediated endocytosis to take in cholesterol, which is carried in particles called low-density lipoproteins (LDLs).

Receptor Proteins, Receptors and other molecules from extracellular fluid are transported in vesicles.

Allow the passage of hydrophilic substances across the membrane.

Some, called Channel Proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel.

Channel Proteins, called aquaporins, facilitate passage of water molecules.

Others, called Carrier Proteins, bind to molecules and change shape to shuttle them across the membrane.

Some ion channels, called Gated Channels, open or close in response to stimulus.

Transport Proteins are specific for the substance it moves.

Facilitated Diffusion

Transport proteins speed up passive movement of molecules across the plasma membrane, includes channel proteins and carrier proteins.

Facilitated Diffusion is still passive, because solutes move down it's concentration gradient, without requiring energy.

Active

Some Transport Proteins can move solutes against their concentration gradient. Allowing cells to maintain concentration gradients that differ from their surroundings.

Requires energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradient.

All proteins involved are Carrier Proteins.

Ion Pumps/Membrane Potential

Membrane potential is the voltage across the membrane.

Voltage is created by differences in a distribution of positive and negative ions across the membrane.

The Cytoplasmic side is negatively charged relative to the Extracellular side.

Def. Electrochemical Gradient: Drives the diffusion of ions across the membrane.

Def. chemical Force: Ions concentration gradient.

Def. Electrical Force: The effect of the membrane potential on the ion's movement.

Def. Electrogenic Pump: Transport proteins that generates voltage across the membrane.

The main pump of plants, Fungi and Bacteria is a Proton Pump; which actively transports Hydrogen Ions (H+) out of the cell.

Helps store energy that can be used for cellular work.

Cotransport

Def. Cotransport: Occurs when active transport of a solute indirectly drives transport of other substances.

Diffusion of an actively transported solute down it's concentration gradient is coupled with he transport of a second substance against it's own concentration gradient.