Chapter 4 A Tour of the Cell

SC 101General BiologyEssential Biology

CM Lamberty

The Microscopic World of Cells

• Cells are marvels of complexity• Trillions of cell in human body• Many specialized types• Main tool for exploration--Microscope

Microscopes as Windows on the World of Cells

• Light Microscope (LM)– Visible light projected through specimen– Lenses enlarge the image and project to eye– View living cells

• Magnification – Increase in size, depends on lens

• Resolving Power– Clarity of magnified image

• Electron Microscope (EM) beam of electrons– Scanning Electron Microscope (SEM)

• View cell surfaces

– Transmission Electron Microscope (TEM)• View internal structures

Microscopes as Windows on the World of Cells

Microscopes as Windows on the World of Cells

The 2 Major Categories of Cells

The countless cells on earth fall into two categories:• Prokaryotic cells — Bacteria and Archaea• Eukaryotic cells — plants, fungi, and animals

All cells have several basic features.• They are all bound by a thin plasma membrane.• All cells have DNA and ribosomes, tiny structures that

build proteins• All cells have cytosol (aka cytoplasm)

The 2 Major Categories of Cells

Prokaryotic and eukaryotic cells have important differences. Prokaryotic cells are older than eukaryotic cells.• Prokaryotes appeared about 3.5 billion years ago.• Eukaryotes appeared about 2.1 billion years ago.

Prokaryotes• Are smaller than eukaryotic cells• Lack internal structures surrounded by membranes• Lack a nucleus, DNA is contained in a nucleoid• Have a rigid cell wall

Idealized Prokaryotic Cell

The 2 Major Categories of Cells

Eukaryotes• Only eukaryotic cells have organelles, membrane-

bound structures that perform specific functions.• The most important organelle is the nucleus, which

houses most of a eukaryotic cell’s DNA.

Idealized Eukaryotic Cell

Overview of Eukaryotic Cells

• Eukaryotic cells are fundamentally similar.• The region between the nucleus and plasma

membrane is the cytoplasm.• The cytoplasm consists of various organelles

suspended in fluid.• Unlike animal cells, plant cells have• Protective cell walls• Chloroplasts, which convert light energy to the

chemical energy of food

Membrane Structure

• Separates living cell from nonliving surroundings

• Regulates traffic of chemicals in and out of cell• Key to how it works is the structure

Plasma Membrane: Lipids & Proteins

• Phospholipids– Related to dietary fats– Only 2 fatty acid tails not

3• hydrophobic

– Phosphate group in 3rd position • Charged, hydrophilic

• Phopholipid bilayer– 2-layered membrane– Proteins embedded in

bilayer• Regulate traffic

Plasma Membrane: Lipids & Proteins

• Fluid Mosaic– Not static (fluid)– Diverse proteins (mosaic)– Phospholipids and proteins free to drift about in the

plane of the membrane

• Illness can result if membrane is compromised– Superbugs: staphylococcus aureus– MRSA– Flesh eating disease!!

Cell Surfaces

• Plant cells have rigid cell wall surrounding plasma membrane– Made of cellulose– Protect the cells– Maintain cell shapes– Keep cells from absorbing too much water

• Cells connected via channels through cell walls– Join cytoplasm of each cell to neighbor– Allow water and small molecules to move between

cells

Cell Surfaces

• Animal cells lack cell wall – Extracellular matrix• Sticky coating to hold cells together• Protects and supports cells

• Cells junctions– Connect cells together– Allow cells in tissue to function in coordinated way

Genetic Control of Cell

• Nucleus chief of the cell– Genes store information necessary to produce

proteins– Proteins do most of the work of the cell– Contains “MOST” of the cells DNA

Structure and Function of Nucleus

• Nuclear Envelope– Double membrane that surrounds nucleus– Similar in structure to plasma membrane– Pores allow transfer of materials- pore complex passes

through the double membrane– Inner portion is covered in a nuclear lamina

• Nucleolus– Prominent structure– Here a type of RNA called rRNA is made by DNA

instructions

• Chromatin– Fibers formed from long DNA and associated

proteins• Chromosome– One chromatin fiber

The nucleus

DNA, chromatin and chromosomes

Ribosomes

• Responsible for protein synthesis• In eukaryotic cells, ribosomes are made in nucleus and by

RNA called ribosomal RNA (rRNA) and then transported into cytoplasm where small and large subunits join– Suspended in fluid making proteins that remain in fluid– Attached to outside of endoplasmic reticulum, making

proteins incorporated into membranes or secreted by cell

– Free ribosomes function in cytosol, ie for glysolysis

How DNA Directs Protein Production

• DNA programs protein production in cytoplasm via mRNA

• mRNA exits through pores in nuclear envelope, travels to cytoplasm, and binds to ribosomes

• As ribosomes move along mRNA, genetic message translated into protein with specific amino acid sequence.

How DNA Directs Protein Production

DNA RNA Protein

The Endomembrane System

• Cytoplasm of eukaryotic cells partitioned by organelle membranes

• Some are connected – Directly by membranes (which are not always

identical)– Indirectly by transfer of membrane segments or tiny

vesicles• Together form endomembrane system– Includes nuclear envelope, endoplasmic reticulum,

Golgi apparatus, lysosomes and vacuoles

Endoplasmic Reticulum (ER)

• Main functioning facility in cell• Continuous with Nuclear Envelope• Rough ER– Ribosomes stud the surface– Produce membrane and secretory proteins (i.e. salivary glands)

– Products transferred via transport vesicles• Smooth ER– Lacks ribosomes on surface– Synthesis of lipids (steroids)– Helps liver detoxify drugs

Endoplasmic Reticulum (ER)

The Golgi Apparatus

• Refinery, warehouse and shipping center• Products made in ER reach Golgi in transport

vehicles• Receiving dock and shipping dock• Modifications by enzymes as products move from

receiving to shipping – Phosphate groups added as tags for different

destinations– CIS face is the receiving side– TRANS face is the transporting and delivery side

The Golgi Apparatus

Lysosomes

• Sac of digestive enzymes (animal cells)• Proteins• Polysaccharides• Fats• Nucleic acids

• Develop from vesicles budding from Golgi• Food vacuoles

– fuse with lysosomes, exposing food to enzymes for digestion– Small molecules from digestion leave the lysosome and nourish the

cell.• Breakdown damaged organelles• Sculpturing feature

– Digest webbing between fingers and toes

Lysosomes

Vacuoles

• Sacs that bud from ER, Golgi or plasma membranes

• Variety of size and function– Contractile vacuoles of protists pump out excess

water in the cell.– Central vacuoles of plants• Store nutrients• Absorb water• May contain pigments or poisons

Vacuoles

Review of Endomembrane System

Chloroplasts and Mitochondria

• Energy Conversion• Cellular power stations

Chloroplasts

• Photosynthetic cells of plants and algae• 3 compartments– Space between membranes that surround

chloroplast– Stroma: thick fluid – Network of disks and tubes• Grana: interconnected stacks of disks• Solar power pack

Chloroplasts

Mitochondria

• Site of cellular respiration– Harvest Energy from food and converts to ATP

• Found in all eukaryotic cells• Structure– Enveloped by 2 membranes filled with matrix– Inner membrane has several infoldings (cristae)

• Contain DNA that encodes their own protein

Mitochondria

Endosymbiont Theory

• Theory states that an early ancestor of eukaryotic cells engulfed an oxygen using non photosynthetic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host it was enclosed, becoming an endosymbiont. Eventually these 2 merged into a single cell which was a eukaryote with mitochondria.

• At least one of theses mitochondrion endosymbiont cells also took in a prokaryotic cell containing chloroplasts leading to plant life

Endosymbiont Theory Supports

• Unlike endomembranous organelles with one membrane, chloroplasts and mitochondria have double membranes surrounding them

• Both contain ribosomes and their own DNA attached to inner membranes

• Bothe are somewhat independent and grow and reproduce within the cell.

• Detoxify harmful substances that enter the cell.

• Seen especially in kidney and liver• Contain peroxidase and catalase (some

chemical reactions in the body create hydrogen peroxide which is toxic to cells. Peroxidase breaks this down.

Peroxisomes

The Cytoskeleton

• Network of fibers extending throughout cytoplasm

• Skeleton and muscles– Support and movement

Maintaining Cell Shape• Series of fibers– Microtubules• Straight hollow tubes composed of proteins• Guide movement of chromosomes when cells divide

– Intermediate filaments and Microfilaments• Both thinner and solid• Give shape much like keratin in dead skin

– ACTIN filaments- smallest fibers• Anchorage and reinforcement for organelles• Dynamic cytoskeleton (amoeboid movements)

Cytoskeleton

• Microtubules– Made of tubulin- globular protein• DIMER- molecule with 2 subunits alpha and beta

– Supports cell shape and tracks for organelles.– Animal cells have a CENTROSOME- near nucleus

that acts as an organizing center for microtubules.• CENTRIOLES are located within the centrosome and

have 9 sets of triplet microtubules arranged in a ring

ACTIN

• Actin filaments are smaller than intermediate fibers and work with other proteins to contract muscle tissue.

• In plants the interaction of actin and myosin cause a cytoplasmic streaming- circular flow of cytosol that helps speed distribution of materials in the cell

Maintaining Cell Shape

Cilia and Flagella

• Mobile appendages• Aid in movement• Flagella – Generally occur singly– Propel cell– Undulating whiplike motion

• Cilia– Shorter and more numerous than flagella– Promote movement by back and forth motion– Some function to move fluid over tissue surfaces

Cilia and Flagella

Extracellular Matrix (ECM)

• Animals have an elaborate ECM made of glycoproteins (aka proteoglycans). Most abundant proteoglycan is COLLAGEN.

• Collagen makes up 40% of protein found in humans. Collagen is secreted from the matrix

• Some cells attach to ECM by proteins called INTEGRINS. ECM integrins transmit signals to the ECM and cytoskeleton and integrate changes occurring outside and inside the cell.– Current research suggests that these ECM integrins can regulate

cell behavior and may even influence the nucleus of the cell.