Chapter 4 A Tour of the Cell

The Art of Looking at Cells Artists have long found inspiration in the visual richness of the living world Conversely, scientists use art to illuminate their findings Micrographs show structures as scientists see them Drawings can emphasize details

INTRODUCTION TO THE CELL 4.1 Microscopes provide windows to the world of the cell A light microscope (LM) enables us to see the overall shape and structure of a cell Passes visible light through a specimen Can study living cells and cells and tissues that have been stained Can magnify only about 1,000 times Video: Euglena

Eyepiece Ocular lens Objective lens Specimen Condenser lens Light LE 4-1a Eyepiece Ocular lens Objective lens Specimen Condenser lens Light source

Magnification is the increase in the apparent size of an object; for example, 1,000X

Resolution is a measure of the clarity of an image A light microscope can resolve objects as small as 2 m

The electron microscope (EM) allows greater magnification than LM and reveals cellular details Uses a beam of electrons rather than light Has much greater resolution than LM (2 nm) Can magnify up to 100,000 times Cannot be used with living specimens

Scanning electron microscope (SEM) studies detailed architecture of cell surfaces

Transmission electron microscope (TEM) studies the details of internal cell structure

Modifications to LM use different techniques to enhance contrast and selectively highlight cellular components

4.2 Most cells are microscopic Cells vary in size and shape Minimum is determined by the total size of all the molecules required for cellular activity Maximum is limited by the need for sufficient surface area to carry out functions

LE 4-2a Human height Length of some nerve and muscle cells Unaided eye Chicken egg Frog egg Most plant and animal cells Light microscope Nucleus Most bacteria Mitochondrion Mycoplasmas (smallest bacteria) Electron microscope Viruses Ribosome Proteins Lipids Small molecules Atoms

A small cell has a greater ratio of surface area to volume than a large cell of the same shape The microscopic size of most cells ensures a sufficient surface area across which nutrients and wastes can move to service the cell

10 mm 30 mm 30 mm 10 mm Surface area of one large cube = 5,400 mm2 LE 4-2b 10 mm 30 mm 30 mm 10 mm Surface area of one large cube = 5,400 mm2 Total surface area of 27 small cubes = 16,200 mm2

4.3 Prokaryotic cells are structurally simpler than eukaryotic cells There are two kinds of cells Prokaryotic (bacteria, archaea) Eukaryotic (protists, plants, fungi, animals) All cells share some common features Plasma membrane DNA ribosomes

Nucleoid region Nucleus Organelles Eukaryotic cell Prokaryotic cell LE 4-3a Prokaryotic cell Nucleoid region Colorized TEM 15,000 Nucleus Organelles Eukaryotic cell

Usually relatively small, relatively simple cells Prokaryotic cells Usually relatively small, relatively simple cells Do not have a membrane-bound nucleus DNA is coiled into a nucleoid region in the cytoplasm Cytoplasm includes ribosomes

Other prokaryotic structures Plasma membrane Complex cell wall Capsule, pili, prokaryotic flagella in some forms

LE 4-3b Prokaryotic flagella Ribosomes Capsule Cell wall Plasma membrane Nucleoid region (DNA) Pili

4.4 Eukaryotic cells are partitioned into functional compartments Eukaryotic cells are usually larger than prokaryotic cells (10-100 (m diameter) Distinguished by a true nucleus Contain both membranous and nonmembranous organelles Compartmentalize metabolism Increase membrane surface area for reactions

Video: Cytoplasmic Streaming Animal cells Are bounded by the plasma membrane alone Lack a cell wall Contain centrioles and lysosomes Often have flagella Video: Cytoplasmic Streaming

LE 4-4a Smooth endoplasmic reticulum Rough endoplasmic reticulum Nucleus Flagellum Not in most plant cells Lycosome Centriole Ribosomes Peroxisome Golgi apparatus Microtubule Intermediate filament Cytoskeleton Plasma membrane Microfilament Mitochondrion

Plant cells Are bounded by both a plasma membrane and a rigid cellulose cell wall Have a central vacuole and chloroplasts Usually lack centrioles, lysosomes, and flagella

LE 4-4b Rough endoplasmic reticulum Nucleus Ribosomes Smooth endoplasmic reticulum Golgi apparatus Microtubule Central vacuole Intermediate filament Cytoskeleton Not in animal cells Chloroplast Microfilament Cell wall Mitochondrion Peroxisome Plasma membrane

ORGANELLES OF THE ENDOMEMBRANE SYSTEM 4.5 The nucleus is the cell's genetic control center The nucleus contains the cell's DNA Controls cellular activities by directing protein synthesis Forms long fibers of chromatin that make up chromosomes

The nucleus is separated from the cytoplasm by the nuclear envelope Pores in the envelope control flow of materials in and out Ribosomes are synthesized in the nucleolus

LE 4-5 Nucleus Chromatin Two membranes of nuclear envelope Nucleolus Pore Rough endoplasmic reticulum Ribosomes

4.6 Overview: Many cell organelles are connected through the endomembrane system The endomembrane system is a collection of membranous organelles Divide the cell into compartments Work together in the synthesis, storage, and export of molecules Prime example: Endoplasmic reticulum (ER) A continuous network of flattened sacs and tubes

4.7 Smooth endoplasmic reticulum has a variety of functions Smooth endoplasmic reticulum (smooth ER) lacks attached ribosomes Synthesizes lipids Processes materials such as toxins and drugs in liver cells Stores and releases calcium ions in muscle cells

LE 4-7 Smooth ER Rough ER Nuclear envelope Ribosomes Smooth ER TEM 45,000

4.8 Rough endoplasmic reticulum makes membrane and proteins Rough endoplasmic reticulum (rough ER) is studded with ribosomes Manufactures membranes Modifies and packages proteins that will be Transported to other organelles Secreted by the cell

Transport vesicle buds off Secretary Ribosome (glyco-) protein inside trans- port vesicle Ribosome Sugar chain Glycoprotein Polypeptide Rough ER

4.9 The Golgi apparatus finishes, sorts, and ships cell products The Golgi apparatus consists of stacks of flattened membranous sacs Receives and modifies substances manufactured by ER Ships modified products to other organelles or the cell surface

LE 4-9 Golgi apparatus Golgi apparatus “Receiving” side of Transport vesicle from ER New vesicle forming “Shipping” side of Golgi apparatus TEM 130,000  Transport vesicle from the Golgi

Animation: Lysosome Formation 4.10 Lysosomes are digestive compartments within a cell Lysosomes are sacs of enzymes that form from the Golgi apparatus Function in digestion within a cell Destroy bacteria that have been ingested into white blood cells Recycle damaged organelles Animation: Lysosome Formation

LE 4-10a Rough ER Transport vesicle (containing inactive hydrolytic enzymes) Plasma membrane Golgi apparatus Engulfment of particle Lysosome engulfing damaged organelle “Food” Lysosomes Food vacuole Digestion

LE 4-10b Lysosome Nucleus TEM 8,500 

two damaged organelles LE 4-10c Lysosome containing two damaged organelles Mitochondrion fragment TEM 42,500  Peroxisome fragment

4.11 Abnormal lysosomes can cause fatal diseases CONNECTION 4.11 Abnormal lysosomes can cause fatal diseases Lysosomal storage diseases Result from an inherited lack of one or more lysosomal enzymes Seriously interfere with various cellular functions

Video: Paramecium Vacuole 4.12 Vacuoles function in the general maintenance of the cell Plant cells contain a large central vacuole Has lysosomal and storage functions Some protists have contractile vacuoles Pump excess water out of cell Video: Paramecium Vacuole Video: Chlamydomonas

LE 4-12a Nucleus Chloroplast Central vacuole Colorized TEM 8,700 

LE 4-12b Nucleus Contractile vacuoles LM 650

Animation: Endomembrane System 4.13 A review of the endomembrane system The various organelles of the endomembrane system are interconnected structurally and functionally Animation: Endomembrane System

Transport vesicle from Golgi to plasma membrane Rough ER from ER to Golgi Transport vesicle from Golgi to plasma membrane Rough ER Plasma membrane Nucleus Vacuole Lysosome Smooth ER Nuclear envelope Golgi apparatus

ENERGY-CONVERTING ORGANELLES 4.14 Chloroplasts convert solar energy to chemical energy Chloroplasts are found in plants and some protists Are the site of photosynthesis Have a complex membranous structure for capturing and converting light energy

LE 4-14 Stroma Chloroplast Inner and outer membranes Granum TEM 9,750 Intermembrane space

4.15 Mitochondria harvest chemical energy from food Mitochondria are found in nearly all eukaryotic cells Divided into two membranous compartments Intermembrane space Second compartment enclosed by inner membrane Contains fluid mitochondrial matrix Membrane folded into cristae

Carry out cellular respiration Convert the chemical energy in food to ATP for cellular work

LE 4-15 Mitochondrion Outer membrane Intermembrane space Inner Cristae TEM 44,880 Matrix

THE CYTOSKELETON AND RELATED STRUCTURES 4.16 The cell's internal skeleton helps organize its structure and activities The cytoskeleton is network of three types of protein fibers Microfilaments Rods of globular proteins Enable cells to change shape and move

Intermediate filaments Ropes of fibrous proteins Reinforce the cell and anchor certain organelles Microtubules Hollow tubes of globular proteins Give the cell rigidity Anchor organelles and act as tracks for organelle movement

Intermediate filament LE 4-16 Tubulin subunit Actin subunit Fibrous subunit 25 nm 7 nm 10 nm Microfilament Intermediate filament Microtubule

Video: Paramecium Cilia Animation: Cilia and Flagella 4.17 Cilia and flagella move when microtubules bend Eukaryotic cilia and flagella are locomotor appendages that protrude from certain cells Move whole cells or materials across the cell surface Video: Paramecium Cilia Animation: Cilia and Flagella

The structure and mechanism of cilia and flagella are similar Microtubules wrapped in an extension of the plasma membrane 9 + 2 arrangement Extend into basal bodies Movement of dynein arms produces microtubule bending

LE 4-17c Flagellum Electron micrographs of cross sections: Outer microtubule doublet Central microtubules TEM 206,500 Radial spoke Dynein arms Flagellum Plasma membrane TEM 206,500 Basal body (structurally identical to centriole) Basal body

CELL SURFACES AND JUNCTIONS 4.18 Cell surfaces protect, support, and join cells Cells interact with their environments and each other via their surfaces Many cells are protected by more than the plasma membrane

Plant cell walls Made largely of cellulose Provide protection and support Connect by plasmodesmata, channels through the wall

Walls of two adjacent plant cells Vacuole Plasmodesmata Layers LE 4-18a Walls of two adjacent plant cells Vacuole Plasmodesmata Layers of one plant cell wall Cytoplasm Plasma membrane

Connect by cell junctions Animal cells Embedded in an extracellular matrix that binds cells together in tissues Connect by cell junctions Tight junctions bind cells into leakproof sheets Anchoring junctions link cells into strong tissues Gap junctions allow substances to flow from cell to cell

LE 4-18b Tight junctions Anchoring junction Gap junctions Extracellular matrix Space between cells Plasma membranes of adjacent cells

Animation: Tight Junctions Animation: Desmosomes Animation: Gap Junctions

FUNCTIONAL CATEGORIES OF ORGANELLES 4.19 Eukaryotic organelles comprise four functional categories Eukaryotic organelles fall into four functional categories that work together to produce the cell's emergent properties Manufacturing Breakdown Energy processing Support, movement, and communication between cells