Cell Structure and Function

Impacts, Issues:Food For Thought

A strain of E. coli bacteria that causes severe illness or death occasionally contaminates foods such as ground beef and fresh vegetables

4.1 The Cell Theory

The cell theory, a foundation of modern biology, states that cells are the fundamental units of life

Measuring Cells

One micrometer (μm) is one-thousandth of a millimeter

Animalcules and Beasties

Van Leeuwenhoek was the first to describe small organisms seen through a microscope, which he called animalcules and beasties

Hooke was the first to sketch and name cells

Development of the Microscope

The Cell Theory Emerges

In 1839, Schleiden and Schwann proposed the basic concepts of the modern cell theory• All organisms consists of one or more cells• A cell is the smallest unit with the properties of life• Each new cell arises from division of another,

preexisting cell• Each cell passes its hereditary material to its

offspring

4.2 What Is a Cell?

Cell• The smallest unit that shows the properties of life

All cells have a plasma membrane and cytoplasm, and all start out life with DNA

The Basics of Cell Structure

Eukaryotic cell• Cell interior is divided into functional

compartments, including a nucleus

Prokaryotic cell• Small, simple cells without a nucleus

All Cells Have Three Things In Common

Plasma membrane• Controls substances passing in and out of the cell

DNA containing region• Nucleus in eukaryotic cells• Nucleoid region in prokaryotic cells

Cytoplasm• A semifluid mixture containing cell components

Prokaryotic and Eukaryotic Cells

Fig. 4-4a, p. 56

Fig. 4-4b, p. 56

Cells Have Large Surface Area-to-Volume Ratio

Cell Size

Surface-to-volume ratio restricts cell size by limiting transport of nutrients and wastes

Preview of Cell Membranes

Lipid bilayer• A double layer of phospholipids organized with

their hydrophilic heads outwards and their hydrophobic tails inwards

• Many types of proteins embedded or attached to the bilayer carry out membrane functions

Basic Structure of Cell Membranes

Fig. 4-6a, p. 57

hydrophilic head

two hydrophobic tails

A A phospholipid, the main type of lipid in cell membranes.

Fig. 4-6b, p. 57

Fig. 4-6c, p. 57

4.1-4.2 Key Concepts:What All Cells Have In Common

Each cell has a plasma membrane, a boundary between its interior and the outside environment

The interior consist of cytoplasm and an innermost region of DNA

4.3 How Do We See Cells?

We use different types of microscopes to study different aspects of organisms, from the smallest to the largest

Modern Microscopes

Light microscopes• Phase-contrast microscopes • Reflected light microscopes• Fluorescence microscopes

Electron microscopes• Transmission electron microscopes• Scanning electron microscopes

Light and Electron Microscopes

Fig. 4-7a, p. 58

Fig. 4-7b, p. 58

Fig. 4-8, p. 59

a) Light micrograph. A phase-contrast micro-scope yields high-contrast images of transparent specimens, such as cells.

b) Light micrograph. A reflected light micro-scope captures light reflected from opaque specimens.

c) Fluorescence micro-graph. The chlorophyll molecules in these cells emitted red light (they fluoresced) naturally.

d) A transmission electron micrograph reveals fantastically detailed images of internal structures.

e) A scanning electron micro-graph shows surface details of cells and structures. Often, SEMs are artificially colored to highlight certain details.

Stepped Art

Different Microscopes, Different Characteristics

4.3 Key Concepts:Microscopes

Microscopic analysis supports three generalizations of the cell theory: • Each organism consists of one or more cells and

their products • A cell has a capacity for independent life • Each new cell is descended from a living cell

4.4 Introducing Prokaryotic Cells

Bacteria and archaea are the prokaryotes (“before the nucleus”), the smallest and most metabolically diverse forms of life

Bacteria and archaea are similar in appearance and size, but differ in structure and metabolism

General Prokaryote Body Plan

Cell wall surrounds the plasma membrane• Made of peptidoglycan (in bacteria) or proteins (in

archaea) and coated with a sticky capsule

Flagellum for motion

Pili help cells move across surfaces• Sex pilus aids in sexual reproduction

Fig. 4-10, p. 60

flagellum

capsulecell wallplasma membrane

cytoplasm, with ribosomesDNA in nucleoid

pilus

Archaeans

Bacteria

4.5 Microbial Mobs

Although prokaryotes are all single-celled, few live alone

Biofilm• Single-celled organisms sharing a secreted layer

of polysaccharides and glycoproteins• May include bacteria, algae, fungi, protists, and

archaeans

A Biofilm

4.4-4.5 Key Concepts:Prokaryotic Cells

Archaeans and bacteria are prokaryotic cells, which have few, if any, internal membrane-enclosed compartments

In general, they are the smallest and structurally the simplest cells

4.6 Introducing Eukaryotic Cells

Eukaryotic (“true nucleus”) cells carry out much of their metabolism inside membrane-enclosed organelles

Organelle• A structure that carries out a specialized function

within a cell

Organelles of Eukaryotic Cells

Eukaryotes: Animal and Plant Cells

Fig. 4-14a, p. 62

vacuole

plasma membrane

mitochondrion

nucleus

1 µm(a) Human white blood cell.

Fig. 4-14b, p. 62

cell wall

central vacuole

plasma membrane

chloroplast

mitochondrion

nucleus

1 µm

(b) Photosynthetic cell from a blade of timothy grass.

4.7 Visual Summary of Eukaryotic Cells

4.7 Visual Summary of Eukaryotic Cells

4.8 The Nucleus

• The nucleus keeps eukaryotic DNA away from potentially damaging reactions in the cytoplasm

• The nuclear envelope controls when DNA is accessed

The Nuclear Envelope

• Nuclear envelope– Two lipid bilayers pressed together as a single

membrane surrounding the nucleus– Outer bilayer is continuous with the ER– Nuclear pores allow certain substances to

pass through the membrane

The Nucleoplasm and Nucleolus

• Nucleoplasm– Viscous fluid inside the nuclear envelope,

similar to cytoplasm

• Nucleolus– A dense region in the nucleus where subunits

of ribosomes are assembled from proteins and RNA

The Chromosomes

• Chromatin– All DNA and its associated proteins in the nucleus

• Chromosome– A single DNA molecule with its attached proteins– During cell division, chromosomes condense and

become visible in micrographs– Human body cells have 46 chromosomes

Chromosome Condensation

4.9 The Endomembrane System

• Endomembrane system– A series of interacting organelles between the

nucleus and the plasma membrane– Makes lipids, enzymes, and proteins for

secretion or insertion into cell membranes– Other specialized cell functions

The Endoplasmic Reticulum

• Endoplasmic reticulum (ER) – An extension of the nuclear envelope that forms a

continuous, folded compartment

• Two kinds of endoplasmic reticulum– Rough ER (with ribosomes) folds polypeptides into

their tertiary form– Smooth ER (no ribosomes) makes lipids, breaks

down carbohydrates and lipids, detoxifies poisons

Vesicles

• Vesicles– Small, membrane-enclosed saclike organelles

that store or transport substances

• Peroxisomes– Vesicles containing enzymes that break down

hydrogen peroxide, alcohol, and other toxins

• Vacuoles– Vesicles for waste disposal

Golgi Bodies and Lysosomes

• Golgi body– A folded membrane containing enzymes that finish

polypeptides and lipids delivered by the ER– Packages finished products in vesicles that carry

them to the plasma membrane or to lysosomes

• Lysosomes– Vesicles containing enzymes that fuse with vacuoles

and digest waste materials

The Endomembrane System

The Endomembrane System

The Endomembrane System

4.10 Lysosome Malfunction

• When lysosomes do not work properly, some cellular materials are not properly recycled, which can have devastating results

• Different kinds of molecules are broken down by different lysosomal enzymes– One lysosomal enzyme breaks down

gangliosides, a kind of lipid

Tay Sachs Disease

• In Tay Sachs disease, a genetic mutation alters the lysosomal enzyme that breaks down gangliosides, which accumulate in nerve cells– Affected children

usually die by age five

4.11 Other Organelles

• Eukaryotic cells make most of their ATP in mitochondria

• Plastids function in storage and photosynthesis in plants and some types of algae

Mitochondria

• Mitochondrion– Eukaryotic organelle that makes the energy molecule

ATP through aerobic respiration– Contains two membranes, forming inner and outer

compartments; buildup of hydrogen ions in the outer compartment drives ATP synthesis

– Has its own DNA and ribosomes– Resembles bacteria; may have evolved through

endosymbiosis

Mitochondrion

Plastids

• Plastids– Organelles that function in photosynthesis or

storage in plants and algae; includes chromoplasts, amyloplasts, and chloroplasts

• Chloroplasts– Plastids specialized for photosynthesis– Resemble photosynthetic bacteria; may have

evolved by endosymbiosis

The Chloroplast

The Central Vacuole

• Central vacuole– A plant organelle that occupies 50 to 90

percent of a cell’s interior– Stores amino acids, sugars, ions, wastes,

toxins– Fluid pressure keeps plant cells firm

4.12 Cell Surface Specializations

• A wall or other protective covering often intervenes between a cell’s plasma membrane and the surroundings

Eukaryotic Cell Walls

• Animal cells do not have walls, but plant cells and many protist and fungal cells do

• Primary cell wall– A thin, pliable wall formed by secretion of cellulose

into the coating around young plant cells

• Secondary cell wall– A strong wall composed of lignin, formed in some

plant stems and roots after maturity

Plant Cell Walls

Fig. 4-22a, p. 70

Fig. 4-22b, p. 70

Fig. 4-22c, p. 70

Plant Cuticle

• Cuticle– A waxy covering that protects exposed

surfaces and limits water loss

Matrixes Between Animal Cells

• Extracellular matrix (ECM)– A nonliving, complex mixture of fibrous

proteins and polysaccharides secreted by and surrounding cells; structure and function varies with the type of tissue

– Example: Bone is mostly ECM, composed of collagen (fibrous protein) and hardened by mineral deposits

ECM

• A bone cell surrounded by extracellular matrix

Cell Junctions

• Cell junctions allow cells to interact with each other and the environment

• In plants, plasmodesmata extend through cell walls to connect the cytoplasm of two cells

• Animals have three types of cell junctions: tight junctions, adhering junctions, gap junctions

Cell Junctions in Animal Tissues

4.6-4.12 Key Concepts:

Eukaryotic Cells

• Cells of protists, plants, fungi, and animals are eukaryotic; they have a nucleus and other membrane-enclosed compartments

• They differ in internal parts and surface specializations

4.13 The Dynamic Cytoskeleton

• Eukaryotic cells have an extensive and dynamic internal framework called a cytoskeleton

• Cytoskeleton– An interconnected system of many protein filaments –

some permanent, some temporary– Parts of the cytoskeleton reinforce, organize, and

move cell structures, or even a whole cell

Components of the Cytoskeleton

• Microtubules– Long, hollow cylinders made of tubulin– Form dynamic scaffolding for cell processes

• Microfilaments– Consist mainly of the globular protein actin– Make up the cell cortex

• Intermediate filaments– Maintain cell and tissue structures

Fig. 4-26 (a-c), p. 72

Fig. 4-26d, p. 72

Motor Proteins

• Motor proteins– Accessory proteins that move molecules

through cells on tracks of microtubules and microfilaments

– Energized by ATP– Example: kinesins

Motor Proteins: Kinesin

Cilia, Flagella, and False Feet

• Eukaryotic flagella and cilia– Whiplike structures formed from microtubules

organized into 9 + 2 arrays– Grow from a centriole which remains in the

cytoplasm as a basal body

• Psueudopods– “False feet” used by amoebas and other

eukaryotic cells to move or engulf prey

Moving Cells• Flagellum of the human sperm, and pseudopods of a

predatory amoeba

Fig. 4-29a, p. 73

Eukaryotic Flagella and Cilia

Fig. 4-29b, p. 73

Fig. 4-29c, p. 73

4.13 Key Concepts:

A Look at the Cytoskeleton

• Diverse protein filaments reinforce a cell’s shape and keep its parts organized

• As some filaments lengthen and shorten, they move cell structures or the whole cell

Summary: Components of Prokaryotic and Eukaryotic

Cells