Chapter 4 Cells and Organelles. Properties and Strategies of Cells The Eukaryotic Cell in Overview: Pictures at an Exhibition Virus, viroids and Prions:

Chapter 4Chapter 4

Cells and Organelles


• Properties and Strategies of Cells

• The Eukaryotic Cell in Overview: Pictures at an Exhibition

• Virus, viroids and Prions: Agents That Invade Cells

Properties and Strategies of Cells• To consider what cells

are?• How they function?• The classification of

cells based on– Their organizational

complexity

– The sizes and shapes

– The specialization that cells undergo

• Liver cells, kidney cells

• All cells are bacteria, archaea or eukarya

• Cells comes in many sizes and shapes

• Eukaryotic cells use organelles to compartmentalize cellular function

• Prokaryotes and eukaryotes differ from each other in many way

• Cell specialization demonstrates the unity and diversity of biology

Limitations on Cell Size

• Several factors limit cell size– Surface area/ Volume ratio

• The exchanges between cell and its environment

– Diffusion rates of molecules• In general, molecules move through the cytoplasm by

diffusion

– The need for adequate concentrations of reactants and catalysts

• The size increase obviously taxes the synthetic capabilities of the cell

All Organism Are Bacteria, Archaea or Eukaryotes

Ancestral cell

Bacteria

Archaea

Eukarya

Prokaryote vs Eukaryote

• Bacteria and archaebacteria

• Appeared shortly after earth cooled, 4 billion years ago.

• No membrane bound organelles

• Simple, looped DNA in nucleoid region

• Protists, plants, animals, fungi

• First appeared ~ 1.8 billion years ago

• Cell functions compartmentalized in organelles

• DNA in chrosomes in nucleus

Bacteria, Archaea and Eukaryotes Differ from Each Other in Many Ways

• Presence or absence of a membrane-enveloped nucleus

• Use of internal membrane to segregate function

• Cytoskeleton

• Exocytosis and endocytosis

• Organization of DNA

• Segregation of genetic information

• Expression of DNA

Bacteria, archaea, and eukaryotes differ from each other in many ways



• Tubules and filaments





Eukaryotic Cells Use Organelles to Compartmentalize Cellular Function

• Chloroplast– enzymes, compounds, pigments needed for

photosynthesis




• Cytoskeleton








• Cytoskeleton





Exocytosis （胞吐作用） and Endocytosis （胞吞作用）

• Eukaryotic cells- to change materials between the membrane-bounded compartment within the cell and the exterior of the cell




• Cytoskeleton








• Cytoskeleton








• Cytoskeleton





Cell Specialization Demonstrates the Unity and Diversity of Biology

• Plant cells and animal cells

• Different tissue, different cell type, different function


• Properties and Strategies of Cells

• The Eukaryotic Cell in Overview: Pictures at an Exhibition

• Virus, viroids and Prions: Agents That Invade Cells

• 細胞核 (nucleus)• 細胞質 (cytoplasm)• 細胞表面 (surface)

Four Major Structural Features(at least)

• Plasma membrane• Nucleus• Membrane-bounded organelles• Cytosol

– cytoskeleton

• Cell wall• Extracellular matrix

The Eukaryotic Cell in Overview

• The plasma membrane defines cell boundaries and retains contents

• The nucleus is the cell’s information center

• Intracellular membranes and organelles define compartment

• The cytoplasm of eukaryotic cells contains the cytosol and cytoskeleton

• The extracellular matrix and the cell wall are outside of the cell

The Plasma Membrane Defines Cell Boundaries and Retains Contents

• Lipid bilayer– Phospholipids ： amphipathic- two

hydrophobic “tail” and a hydrophilic “head”

• Proteins ： amphipathic (amphiphilic)– Enzymes– Transport proteins

• Carbohydrates– Glycoproteins

• These membranes also participate in metabolism as many enzymes are built into membranes.

• The barriers created by membranes provide different local environments that facilitate specific metabolic functions.

• The general structure of a biological membrane is a double layer of phospholipids with other lipids and diverse proteins.

• Each type of membrane has a unique combination of lipids and proteins for its specific functions.– For example, those in the membranes of

mitochondria function in cellular respiration.







• The nucleus contains most of the genes in a eukaryotic cell.– Some genes are located in mitochondria and chloroplasts.

• The nucleus averages about 5 m in diameter.• The nucleus is separated from the cytoplasm by a double

membrane- called nuclear envelope.– Inner and outer nuclear membrnes

• Where the double membranes are fused, a pore allows large macromolecules and particles to pass through– Such as ribosomal subunit, mRNA, chromosomal protein and

enzymes

The nucleus contains a eukaryotic cell’s genetic library

• Within the nucleus, the DNA and associated proteins are organized into fibrous material, chromatin（核染質） .

• In a normal cell they appear as diffuse mass.

• However when the cell prepares to divide, the chromatin fibers coil up to be seen as separate structures, chromosomes.

• Each eukaryotic species has a characteristic number of chromosomes.

– A typical human cell has 46 chromosomes, but sex cells (eggs and sperm) have only 23 chromosomes.







Intracellular Membranes and Organelles Define Compartment

• Mitochondrion

• Chloroplast

• The endoplasmic reticulum

• The Golgi complex– Secretory vesicles

• The Lysosome

• Vacuoles

• Ribosomes

Mitochondrion （粒線體）• Surrounded by two membrane- inner and outer

mitochondrial membrane

• Circular DNA (RNA and protein)

• Most of the chemical reactions involved in the oxidation of sugars and other cellular ‘fuel’ molecules occur within the mitochondria

– Transporting electrons located in or on cristae ( 皺摺 )

– TCA cycle and fat oxidation occur in matrix

Mitochondrial Disorders

• Muscle or nerve tissue

• Myopahties ： disease or disorders of muscle cells

• Leigh syndrome( 利氏病 ) ： a devastating neurodegenerative ( 神經退化 ) disorder

• Fetal infantile respiration defect

The Endosymbiont Theory(內共生學說 )

Did mitochondria and chloroplasts evolve from ancient bacteria?

• Having their own DNA and ribosome• Enable to carry out the synthesis of both RNA and

proteins– rRNA sequences, ribosome size– Sensitivity to inhibitor of RNA and protein synthesis

• DNA organization- circular form without associated with histone

• Resemble bacterial cells in size and shape• Protoeukaryotes- ancestor of eukaryotic cells

– Mitochondria ： Ancient purple bacteria– Chloroplast ： Ancient Cyanobacteria ( 藍綠菌 )

The Endoplasmic Reticulum（內質網）

• Extending throughout the cytoplasm

• A network of membrane

• Cisternae( 扁囊 )- consists of tubular membrane and flatted sacs that are interconnected

• 參與細胞內各種膜的包裝工作（佔細胞質總數的一半）

• The endoplasmic reticulum (ER) accounts for half the membranes in a eukaryotic cell.

• The ER includes membranous tubules and internal, fluid-filled spaces, the cisternae ( 扁囊 ).

• The ER membrane is continuous with the nuclear envelope and the cisternal space of the ER is continuous with the space between the two membranes of the nuclear envelope.

The endoplasmic reticulum manufacturers membranes and performs many other

biosynthetic functions

Smooth ER• The smooth ER is rich in enzymes and plays a role

in a variety of metabolic processes.• Enzymes of smooth ER synthesize lipids and

steroids– oils, phospholipids, cholesterol and steroid hormones

• The smooth ER also catalyzes a key step in the mobilization of glucose from stored glycogen in the liver.

• Other enzymes in the smooth ER of the liver help detoxify drugs and poisons.

• Muscle cells are rich in enzymes that pump calcium ions from the cytosol to the cisternae.

• Rough ER is especially abundant in those cells that secrete proteins.– As a polypeptide is synthesized by the ribosome, it

is threaded into the cisternal space through a pore formed by a protein in the ER membrane.

– Many of these polypeptides are glycoproteins, a polypeptide to which an oligosaccharide is attached.

• These secretory proteins are packaged in transport vesicles that carry them to their next stage.

Rough ER

• Rough ER is also a membrane factory.– Membrane bound proteins are synthesized

directly into the membrane.– Enzymes in the rough ER also synthesize

phospholipids from precursors in the cytosol.– As the ER membrane expands, parts can be

transferred as transport vesicles to other components of the endomembrane system.

Rough ER

The Golgi Complex（高基氏體）• Closely related to the smooth ER

– Vesicle （囊泡） arise by budding off the ER and accepted by Golgi complex

• Play an important role in the processing and packaging of secretory proteins and in synthesizing complex polysaccharides.

• Most membrane proteins and secretory proteins are glycoprotein.

– Initial steps in glycosylation take place with the lumen of the rough ER, but completed with the Golgi.

• 細胞內消化的場所 - 含許多種水解（ pH 約在 5 ）能消許多的化大分子

（如蛋白質、多糖類、脂肪以及核酸）– 吞噬作用（ phagocytosis ）– 自食作用（ autophagy ）：水解細胞本身的物質（胞器或其他大分子） - 更新作用• 生物發育• 細胞凋亡（ apoptosis ）

Autophagy

• Macrophagy– Autophagic vacuole (autophagosome)– Derived from ER

• Microphagy– Small bits of cytoplasm rather than whole

organelles

• chaperone-mediated autophagy

– p354

• The lysosomes play a critical role in the programmed destruction of cells in multicellular organisms.– This process allows reconstruction during the

developmental process.

• Several inherited diseases affect lysosomal metabolism. (more than 40 heritable lysosomal storage disease -harmful accumulation of a specific substance ：polysaccharides or lipids)– These individuals lack a functioning version of a

normal hydrolytic enzyme.– Lysosomes are engorged with indigestable

substrates.– These diseases include Pompe’s disease （龐培氏症） in the liver and Tay-Sachs disease in the brain.

龐培氏症•龐培氏症是一種溶小體（ lysosome ）肝醣（ Glycogen ）儲積症，又常被稱為肝醣儲積症第二型。其發生的原因是溶小體一種酸性麥芽糖酵素的缺乏，使得進入溶小體的肝醣無法被分解而持續堆積，進而影響到細胞的功能。

•龐培氏症影響的範圍是全身性的，但最為嚴重的是在肌肉組織。肝醣的堆積造成肌肉肥大（如舌頭及心臟），並影響其功能，造成肌肉無力。病情持續發展，肌肉則逐漸退化，而以纖維組織取代。

Tay-Sachs disease

• 病因：– 罹患此症的嬰兒因缺乏一種己醣胺酵素 A

(Hexosaminidase A) ，而此酵素的重要功能在於與神經節甘脂 (Gangliosides ：一種glycolipid) 代謝有關，特別是 GM2, ；這些GM2 物質會聚集並逐漸毀壞腦部與神經細胞，直到中樞神經系統功能完全停止。

• 臨床特徵： – 嬰兒通常症狀表現於 3~6 個月大時；罹患

此症的患孩可能會有聽力與視力方面障礙，合併四肢麻痺，大約死於 5 歲。

The Peroxisome ( 過氧化小體 )

• Surrounded by a single membrane• Found in plant and animal cells, fungi, protozoa and

algae – In animal, most found in liver and kidney

• Generating and degrading H2O2

• 將脂肪酸氧化分解成較小的分子運送至粒線體，當作細胞呼吸的燃料。– Peroxisomal enzymes: defective or absent- cause serious

disease (ALD: 腎上腺腦白質營養不良 neurological debilitation)

• 肝細胞的過氧化小體有解毒的作用（如解酒精）• May play a role in aging.

Vacuoles ( 液泡 )

• In animals- temporary storage or transport

• Some protozoa- – take up food particles-phagocytosis– 伸縮泡（ contractile vacuole ）

• In plants- central vacuoles 中央液泡– 一般植物細胞：無機離子如鉀、氯離子– 種子：堆積有機物（蛋白質）– 花瓣：色素– 其他：對動物的毒物或是味道極差之物質

• Vesicles and vacuoles (larger versions) are membrane-bound sacs with varied functions.– Food vacuoles, from phagocytosis, fuse with

lysosomes.– Contractile vacuoles, found in freshwater protists,

pump excess water out of the cell.– Central vacuoles are found in many mature plant

cells.

Vacuoles have diverse functions in cell maintenance

• The membrane surrounding the central vacuole, the tonoplast, is selective in its transport of solutes into the central vacuole.

• The functions of the central vacuole include stockpiling （儲存） proteins or inorganic ions, depositing metabolic byproducts, storing pigments, and storing defensive compounds against herbivores.

• It also increases surface to volume ratio for the whole cell.

• Cell types that synthesize large quantities of proteins (e.g., pancreas, 胰腺 ) have large numbers of ribosomes.

• Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.

• Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum.– These synthesize proteins that are either included

into membranes or for export from the cell.







• The cytoskeleton provides mechanical support and maintains shape of the cell.

• The fibers act like a geodesic dome to stabilize a balance between opposing forces.

• The cytoskeleton provides anchorage for many organelles and cytosolic enzymes.

Providing structural support to the cell, the cytoskeleton also functions in cell motility

and regulation

Intermediate filaments

• The most stable and the least soluble constituents of the cytoskeleton.

• As a scaffold that supports the entire cytoskeleton framework.

• In contrast to microtubules and microfilaments, intermediate filaments differ in their protein composition from tissue to tissue






• Beyond the cell ： Cell Adhesions, Cell Junctions, and Extracellular Structures (Ch 17)

Beyond the Cell ： Cell Adhesion, Cell Junctions, and Extracellular Structures

• Cell-Cell Recognition and Adhesion

• Cell-Cell Junctions

• The Extracellular Matrix of Animal Cells

Cell-Cell Recognition and Adhesion

• Transmembrane proteins mediate cell-cell adhsion

• Carbohydrate Groups are Important in Cell-Cell Recognition and Adhesion– Lectins– Selectins and Leukocyte Adhesion

Transmembrane proteins mediate cell-cell adhsion

• CAMs- cell adhesion molecules (IgSF: immunoglobulin superfamily)

• Cadherins- glycoproteins

Carbohydrate Groups are Important in Cell-Cell Recognition and Adhesion

• Lectins ： carbohydrate-binding proteins– Promote cell-cell adhesion by binding to a

specific sugar.

• Selectins and Leukocyte Adhesion

Cell-Cell Junctions Animal have 3 main types of

intercellular links

• Adhesive junctions link adjoining cells to each other

• Tight junctions prevent the movement of molecules across cell layers

• Gap junctions allow direct electrical and chemical communication between cells

Adhesive Junctions Link Adjoining Cells to Each Other

• Adhesive Junctions- common in epithelia– Connect the external environment to the actin

microfilaments or inetrmediate filaments– Provide a potential pathway for signals to be transmitted

from the cell exterior to the cytoplasm.– Fasten cells together into strong sheets,

• such as desmosome- much like rivets ( 鉚釘 ).

• Adhesive Junctions– Focal adhesion– Hemidesmosome– Adherens Junctions ： cadherin-medeated adhesive junctions

(Fig 17-3a)

– Desmosome

Tight Junctions

• Prevent the movement of molecules across cell layers

• Membranes of adjacent cells are fused, forming continuous belts around cells.

– Sealing spaces between cells

• Transmembrane junctional proteins

– Prevents leakage of extracellular fluid.

Gap junctions

• allow direct electrical and chemical communication between cells

• communicating junctions provide cytoplasmic channels between adjacent cells.

– Special membrane proteins surround these pores.

– Salt ions, sugar, amino acids, and other small molecules can pass.

– In embryos, gap junctions facilitate chemical communication during development.

Extracellular Structure

• Animal cells- Extracellular matrix (ECM)– Collagen fibers and proteoglycans

• Plant cells and fungal cells- Cell wall– Cellulose microfibrils embedded in a matrix

other polysaccharides and small amounts of proteins.

• Bacteria- Cell wall– peptidoglycans

ECM

• Primary function- Support

• Other functions ：– Cell motility– Cell migration– Cell division– Cell recognition– Cell adhesion– Cell differentiation during embryonic

development

• Lacking cell walls, animals cells do have an elaborate extracellular matrix (ECM).

• The primary constituents of the extracellular matrix are glycoproteins, especially collagen fibers, embedded in a network of proteoglycans.

• In many cells, fibronectins(纖維結合素 ) in the ECM connect to integrins(整合速 ), intrinsic membrane proteins.

• The integrins connect the ECM to the cytoskeleton.

The extracellular matrix (ECM) of animal cells functions in support, adhesion,

movement, and regulation

• The ECM can regulate cell behavior.– Embryonic cells migrate along specific pathways by

matching the orientation of their microfilaments to the fibers in the extracellular matrix.

– The extracellular matrix can influence the activity of genes in the nucleus via a combination of chemical and mechanical signaling pathways.• This may coordinate all the cells within a tissue.

• The cell wall, found in prokaryotes, fungi, and some protists, has multiple functions.

• In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water.

• It also supports the plant against the force of gravity.

• The thickness and chemical composition of cell walls differs from species to species and among cell types.

The Plant cell Surface-Cell Wall

Cell Walls

• Provide a structural framework and serve as a permeability barrier

• A network of culluose microfibrils, polysaccharides, and glycoproteins

Documents

Chapter 4 Cells and Organelles. Properties and Strategies of Cells The Eukaryotic Cell in Overview: Pictures at an Exhibition Virus, viroids and Prions: