Cells: The Working Units of Life

Cells: The Working Units of Life
4 Cells: The Working Units of Life

Chapter 4 Cells: The Working Units of Life
Key Concepts 4.1 Cells Provide Compartments for Biochemical Reactions 4.2 Prokaryotic Cells Do Not Have a Nucleus 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments 4.4 The Cytoskeleton Provides Strength and Movement 4.5 Extracellular Structures Provide Support and Protection For Cells and Tissues

Chapter 4 Opening Question
What do the characteristics of modern cells indicate about how the first cells originated?

Concept 4.1 Cells Provide Compartments for Biochemical Reactions
Cell theory was the first unifying theory of biology: Cells are the fundamental units of life. All organisms are composed of cells. All cells come from preexisting cells.

Important implications of cell theory: Studying cell biology is the same as studying life. Life is continuous—all the way back to the evolution of the first living cells.

Most cells are tiny, in order to maintain a good surface area-to-volume ratio. The volume of a cell determines its metabolic activity per unit of time. The surface area of a cell determines the amount of substances that can enter or leave the cell.

Figure 4.1 The Scale of Life
Figure 4.1 The Scale of Life This logarithmic scale shows the relative sizes of molecules, cells, and multicellular organisms.

Figure 4.2 Why Cells Are Small
Figure 4.2 Why Cells Are Small As an object grows larger, its volume increases more rapidly than its surface area. Cells must maintain a large surface area-to-volume ratio in order to function. This explains why multicellular organisms must be composed of many small cells rather than a few large ones.

As cells grow larger, metabolic activity and need for resources and rate of waste production increases faster than surface area. Some large cells increase surface area by folds in the cell membrane.

To see small cells, there are two types of microscopes: Light microscopes—use glass lenses and light Resolution = 0.2 μm Electron microscopes—electromagnets focus an electron beam Resolution = 2 nm

Figure 4.3 Microscopy Figure 4.3 Microscopy Light and electron microscopes are used to examine cell structures.

Chemical analysis of cells involves breaking them open to make a cell-free extract. The composition and chemical reactions of the extract can be examined. Properties of the cell-free extract are the same as those inside the cell. Cell structures and macromolecules can be separated according to size in a centrifuge.

Figure 4.4 Centrifugation
Figure 4.4 Centrifugation Structures within cells can be separated from one another on the basis of size and density, and the isolated structures can then be analyzed chemically.

The cell membrane: A selectively permeable barrier that allows cells to maintain a stable internal environment (homeostasis) Important in communication and receiving signals Often has proteins for binding and adhering to adjacent cells

Two types of cells: Prokaryotes have no membrane-enclosed compartments. Eukaryotes have membrane-enclosed compartments called organelles, such as the nucleus.

Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus
Are enclosed by a cell membrane Have DNA located in the nucleoid region The rest of the cytoplasm consists of: Cytosol (water and dissolved material) and suspended particles Ribosomes—sites of protein synthesis

Figure 4.5 A Prokaryotic Cell
Figure 4.5 A Prokaryotic Cell The bacterium Pseudomonas aeruginosa illustrates the typical structures shared by all prokaryotic cells. This bacterium also has a protective outer membrane and a capsule, which are not present in all prokaryotes.

Most prokaryotes have a rigid cell wall outside the cell membrane. Bacterial cell walls contain peptidoglycans. Some bacteria have an additional outer membrane that is very permeable. Other bacteria have a slimy layer of polysaccharides, called the capsule.

Some bacteria, including cyanobacteria, have an internal membrane system that contains molecules needed for photosynthesis.

Some prokaryotes swim by means of flagella, made of the protein flagellin. A motor protein anchored to the cell membrane or outer membrane spins each flagellum and drives the cell.

Figure 4.6 Prokaryotic Flagella (Part 1)
Figure 4.6 Prokaryotic Flagella (A) Flagella contribute to the movement and adhesion of prokaryotic cells. (B) Complex protein ring structures anchored in the cell membrane form a motor unit that rotates the flagellum and propels the cell.

Figure 4.6 Prokaryotic Flagella (Part 2)
Figure 4.6 Prokaryotic Flagella (A) Flagella contribute to the movement and adhesion of prokaryotic cells. (B) Complex protein ring structures anchored in the cell membrane form a motor unit that rotates the flagellum and propels the cell.

Cytoskeleton: Some rod-shaped bacteria have a network of helical actin-like protein structures to help maintain their shape.

Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments
Eukaryotic cells have a cell membrane, cytoplasm, and ribosomes, as well as membrane-enclosed compartments called organelles. Each organelle plays a specific role in the cell.

Figure 4.7 Eukaryotic Cells (Part 1)
Figure 4.7 Eukaryotic Cells Animal and plant cells share many structures and organelles. Structures present in the cells of plants but not animals include the cell wall and the chloroplasts. Plants do not have centrioles. Note that the electron micrographs are two-dimensional “slices,” whereas cells are three-dimensional.

Ribosomes translate the nucleotide sequence of a messenger RNA molecule into a polypeptide. They occur in both prokaryotic and eukaryotic cells and consist of one large and one small subunit. Each subunit consists of ribosomal RNA (rRNA) bound to smaller protein molecules.

Ribosomes are not membrane-bound organelles. In eukaryotes, they are free in the cytoplasm, attached to the endoplasmic reticulum, or inside mitochondria and chloroplasts. In prokaryotes, ribosomes float freely in the cytoplasm.

The nucleus is usually the largest organelle:
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments The nucleus is usually the largest organelle: Location of DNA and DNA replication Site where DNA is transcribed to RNA Contains the nucleolus, where assembly of ribosomes from RNA and proteins begins

The nucleus is surrounded by two membranes that form the nuclear envelope. Nuclear pores in the envelope control movement of molecules between nucleus and cytoplasm. In the nucleus, DNA combines with proteins to form chromatin in long, thin threads called chromosomes. The outer membrane of the envelope is continuous with the endoplasmic reticulum.

The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes. Tiny, membrane-surrounded vesicles shuttle substances between the various components, as well as to the cell membrane.

Figure 4.8 The Endomembrane System
Figure 4.8 The Endomembrane System Membranes of the nucleus, endoplasmic reticulum (ER), and Golgi apparatus form a network that is connected by vesicles. Parts of the membrane move between these organelles. Membrane synthesized in the smooth ER becomes sequentially part of the rough ER, then the Golgi apparatus, then vesicles formed from the Golgi apparatus. These vesicles may eventually fuse with, and become part of, the cell membrane.

Rough endoplasmic reticulum (RER) Smooth endoplasmic reticulum (SER)
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Endoplasmic reticulum (ER)—network of interconnected membranes in the cytoplasm, with a large surface area Two types of ER: Rough endoplasmic reticulum (RER) Smooth endoplasmic reticulum (SER)

Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface. Newly made proteins enter the RER lumen where they are chemically modified and tagged for delivery to specific locations. The proteins are transported in vesicles that pinch off from the ER. All secreted proteins and most membrane proteins pass through the RER.

Polypeptides are transported into the RER lumen as they are being synthesized. In the lumen they are folded into their tertiary structures. Many are linked to carbohydrate groups, becoming glycoproteins. Many glycoproteins are important in recognition and interactions between cells.

Smooth endoplasmic reticulum (SER)— more tubular, no ribosomes
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Smooth endoplasmic reticulum (SER)— more tubular, no ribosomes Chemically modifies small molecules such as drugs and pesticides Site of glycogen degradation in animal cells Site of synthesis of lipids and steroids Stores calcium ions, which trigger many cell responses

Receives proteins from the RER and can further modify them
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Golgi apparatus: flattened sacs (cisternae) and small membrane-enclosed vesicles. Receives proteins from the RER and can further modify them Concentrates, packages, and sorts proteins Adds carbohydrates to proteins Site of polysaccharide synthesis for plant cell walls

Golgi apparatus has three regions:
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Golgi apparatus has three regions: cis region: receives vesicles containing proteins from the ER trans region: vesicles bud off from the Golgi apparatus and travel to the cell membrane or to lysosomes medial region: in between trans and cis regions

Figure 4.8 The Endomembrane System
Figure 4.8 The Endomembrane System Membranes of the nucleus, endoplasmic reticulum (ER), and Golgi apparatus form a network that is connected by vesicles. Parts of the membrane move between these organelles. Membrane synthesized in the smooth ER becomes sequentially part of the rough ER, then the Golgi apparatus, then vesicles formed from the Golgi apparatus. These vesicles may eventually fuse with, and become part of, the cell membrane.

Primary lysosomes originate from the Golgi apparatus. They contain hydrolases (digestive enzymes), and are the site where macromolecules are hydrolyzed into monomers.

Macromolecules may enter the cell by phagocytosis—part of the cell membrane encloses the material and a phagosome is formed. Phagosomes then fuse with primary lysosomes to form secondary lysosomes. Enzymes in the secondary lysosome hydrolyze the food molecules.

Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm
Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm Lysosomes are sites for the hydrolysis of material taken into the cell by phagocytosis.

Phagocytes are cells specialized to take in materials and break them down. Autophagy is the programmed destruction of cell components. Cells break down their own materials, and even entire organelles, within lysosomes. Lysosomal storage diseases occur when lysosomes fail to digest cell components.

In eukaryotes, breakdown of energy-rich molecules begins in the cytosol. The partially digested molecules enter the mitochondria, where chemical energy is converted to energy-rich ATP. Cells that require a lot of energy often have more mitochondria.

Mitochondria have two membranes: Outer membrane—very porous
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Mitochondria have two membranes: Outer membrane—very porous Inner membrane—extensive folds called cristae increase surface area The fluid-filled matrix contains enzymes, DNA, and ribosomes.

Plant and algae cells contain plastids that can differentiate into organelles—some are used for storage. Chloroplast: contains chlorophyll; site of photosynthesis Photosynthesis converts light energy into chemical energy (anabolic process).

Chloroplasts have two membranes, plus internal membranes called thylakoids. Granum—a stack of thylakoids; light energy is converted to chemical energy on these membranes. Stroma—aqueous matrix around grana; contains ribosomes and DNA; carbohydrates are synthesized here.

Other plastids: Chromoplasts make and store red, yellow, and orange pigments, especially in flowers and fruits.

Leucoplasts store macromolecules such as starch.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Leucoplasts store macromolecules such as starch.

Other organelles perform specialized functions. Peroxisomes collect and break down toxic by-products of metabolism, such as H2O2, using specialized enzymes. Glyoxysomes (only in plants)—lipids are converted to carbohydrates for growth.

Vacuoles (mainly in plants and fungi) have several functions:
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Vacuoles (mainly in plants and fungi) have several functions: Storage of waste products and toxic compounds; some may deter herbivores. Structure for plant cells—water enters the vacuole by osmosis, creating turgor pressure.

Reproduction—vacuoles in flowers and fruits contain pigments whose colors attract pollinators and aid seed dispersal. Catabolism—digestive enzymes in seed vacuoles hydrolyze stored food for early growth.

Contractile vacuoles in freshwater protists get rid of excess water entering the cell due to solute imbalance. The contractile vacuole enlarges as water enters, then quickly contracts to force water out through special pores.

Concept 4.4 The Cytoskeleton Provides Strength and Movement
Supports and maintains cell shape Holds organelles in position Moves organelles Involved in cytoplasmic streaming Interacts with extracellular structures to anchor cell in place

The cytoskeleton has three components with very different functions: Microfilaments Intermediate filaments Microtubules

Microfilaments: Help a cell or parts of a cell to move Determine cell shape Made from actin monomers that attach to the “plus end” and detach at the “minus end” of the filament

Figure 4.10 The Cytoskeleton (Part 1)
Figure The Cytoskeleton Three highly visible and important structural components of the cytoskeleton are shown here in detail. Specific stains were used to visualize them in a single cell. These structures maintain and reinforce cell shape and contribute to cell movement.

Dynamic instability: the filaments can shorten (more detachment) or lengthen (more assembly) This allows for quick assembly or breakdown of the cytoskeleton. In muscle cells, actin filaments are associated with the motor protein myosin; their interactions result in muscle contraction. Motor protein (molecular motor): any protein that causes movement within a cell

Intermediate filaments: At least 50 different kinds in six molecular classes Tough, ropelike protein assemblages; more permanent than other filaments and do not show dynamic instability Anchor cell structures in place Resist tension, maintain rigidity

Microtubules: Thickest cytoskeleton elements. Form a rigid internal skeleton for some cells or regions Act as a framework for motor proteins to move structures in the cell

Microtubules are made from dimers of the protein tubulin—chains of dimers surround a hollow core. They have (+) and (–) ends and show dynamic instability. Polymerization results in a rigid structure; depolymerization leads to its collapse.

Microtubules form an internal skeleton for moveable cellular appendages: Cilia—short, usually many present; move stiffly to propel a cell, or move fluid over a stationary cell Flagella—longer, usually one or two present; push or pull cell through water

Figure Cilia Figure Cilia (A) This unicellular eukaryotic organism (a ciliated protist) can coordinate the beating of its cilia, allowing rapid movement. (B) A cross section of a single cilium shows the arrangement of the microtubules and proteins.

Cilia and flagella microtubules are arranged in a “9 + 2” pattern: Doublets—nine fused pairs of microtubules form a cylinder One unfused pair in center Motion occurs as doublets slide past each other.

Dynein—a motor protein that can change shape and drives the sliding of doublets Nexin—protein that crosslinks doublets and prevents sliding, so cilia bend Kinesin—motor protein that binds to vesicles in the cell and “walks” them along the microtubule

Figure 4.12 A Motor Protein Moves Microtubules in Cilia and Flagella
Figure A Motor Protein Moves Microtubules in Cilia and Flagella The motor protein dynein causes microtubule doublets to slide past one another. If the protein nexin is present to anchor the microtubule doublets together, the flagellum or cilium bends.

Figure 4.13 A Motor Protein Drives Vesicles along Microtubules
Figure A Motor Protein Drives Vesicles along Microtubules (A) Kinesin delivers vesicles or organelles to various locations in the cell by moving along microtubule “railroad tracks.” (B) The process is seen by time-lapse photography at half-second intervals in the protist Dictyostelium.

Cytoskeletal structure may be observed under the microscope, and function can be observed in a cell with that structure. Observations may suggest that a structure has a function, but correlation does not establish cause and effect.

Two methods are used to determine a link between a structure (A) and its function (B): Inhibition: use a drug to inhibit A, if B still occurs, then A does not cause B. Mutation: if genes for A are missing and B does not occur, A probably causes B.

Figure The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology (Part 1) Figure The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology In test tubes, the drug cytochalasin B prevents microfilament formation from monomeric precursors. This led to the question: Will the drug work like this in living cells and inhibit the movement of Amoebaa? [a T. D. Pollard and R. R Weihing CRC Critical Reviews of Biochemistry 2: 1–65.]

Figure The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology (Part 2) Figure The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology In test tubes, the drug cytochalasin B prevents microfilament formation from monomeric precursors. This led to the question: Will the drug work like this in living cells and inhibit the movement of Amoebaa? [a T. D. Pollard and R. R Weihing CRC Critical Reviews of Biochemistry 2: 1–65.]

In eukaryotes, these structures have two components:
Concept 4.5 Extracellular Structures Provide Support and Protection for Cells and Tissues Extracellular structures are secreted to the outside of the cell membrane. In eukaryotes, these structures have two components: A fibrous macromolecule A gel-like medium in which fibers are embedded

Plant cell wall: semi-rigid structure outside the cell membrane
Concept 4.5 Extracellular Structures Provide Support and Protection for Cells and Tissues Plant cell wall: semi-rigid structure outside the cell membrane The fibrous component is the polysaccharide cellulose. The gel-like matrix contains cross-linked polysaccharides and proteins.

Figure 4.15 The Plant Cell Wall
Figure The Plant Cell Wall The semirigid cell wall provides support for plant cells. It is composed of cellulose fibers embedded in a matrix of polysaccharides and proteins.

The plant cell wall has three major roles:
Concept 4.5 Extracellular Structures Provide Support and Protection for Cells and Tissues The plant cell wall has three major roles: Provides support for the cell and limits its volume by remaining rigid Acts as a barrier to infection Contributes to form during growth and development

Concept 4.5 Extracellular Structures Provide Support and Protection for Cells and Tissues
Adjacent plant cells are connected by cell membrane-lined channels called plasmodesmata. These channels allow movement of water, ions, small molecules, hormones, and some RNA and proteins.

Many animal cells are surrounded by an extracellular matrix. The fibrous component is the protein collagen. The gel-like matrix consists of proteoglycans. A third group of proteins links the collagen and the matrix together.

Figure 4.16 An Extracellular Matrix
Figure An Extracellular Matrix Cells in the kidney secrete an extracellular matrix called the basal lamina that separates them from nearby blood vessels. The basal lamina filters materials that pass between the kidney and the blood.

Extracellular matrices in animal cells: Hold cells together in tissues
Concept 4.5 Extracellular Structures Provide Support and Protection for Cells and Tissues Extracellular matrices in animal cells: Hold cells together in tissues Contribute to physical properties of cartilage, skin, bone, and other tissues Help filter materials (e.g., in kidneys) Orient cell movement during development and tissue repair

Proteins such as integrin connect the extracellular matrix to the cell membrane. These proteins bind to microfilaments in the cytoplasm and to collagen fibers in the extracellular matrix. For cell movement, the protein changes shape and detaches from the collagen.

Figure 4.17 Cell Membrane Proteins Interact with the Extracellular Matrix
Figure Cell Membrane Proteins Interact with the Extracellular Matrix In this example, integrin mediates the attachment of animal cells to the extracellular matrix.

Cell junctions are specialized structures that protrude from adjacent cells and “glue” them together: • Tight junctions • Desmosomes • Gap junctions

Tight junctions prevent substances from moving through spaces between cells. Desmosomes hold cells together but allow materials to move in the matrix. Gap junctions are channels that run between membrane pores in adjacent cells, allowing substances to pass between the cells.

Figure 4.18 Junctions Link Animal Cells (Part 1)
Figure Junctions Link Animal Cells Although all three types of junctions are shown in the cell at right, they don’t necessarily all occur in the same cell.

Answer to Opening Question
Synthetic cell models, or protocells, can demonstrate how cell properties may have originated. Combinations of molecules can produce a cell-like structure, with a lipid “membrane” and water-filled interior. As in modern cells, the membrane allows only certain things to pass, while RNA inside the cell can replicate itself.

Figure A Protocell Figure A Protocell A protocell can be made in the lab and can carry out some functions of modern cells—in particular, it provides a compartment for biochemical reactions.

Answer to Opening Question
Life probably began with single-celled organisms resembling modern bacteria. These cells lack structures that are typically preserved as fossils, but recent advances in microscopy have shown 800 million-year-old fossil cells that resemble the protocells.

Cells: The Working Units of Life

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