Cell Structure and Function

Cell Structure and Function
This is a transmission electron micrograph of a neutrophil, a cell found in bone marrow Color has been added to highlight the various organelles (magnification: 27,500×)

Life Is Cellular Look closely at a part of a living thing, and what do you see? Hold a blade of grass up against the light, and you see tiny lines running the length of the blade Examine the tip of your finger, and you see the ridges and valleys that make up fingerprints Place an insect under a microscope, and you see the intricate structures of its wings and the spikes and bristles that protect its body As interesting as these close-up views may be, however, they're only the beginning of the story Look closer and deeper with a more powerful microscope, and you'll see that there is a common structure that makes up every living thing — the cell

The Discovery of the Cell
“Seeing is believing,” an old saying goes It would be hard to find a better example of this than the discovery of the cell Without the instruments to make them visible, cells remained out of sight and, therefore, out of mind for most of human history All of this changed with a dramatic advance in technology — the invention of the microscope

Early Microscopes It was not until the mid-1600s that scientists began to use microscopes to observe living things In 1665, Englishman Robert Hooke used an early compound microscope to look at a thin slice of cork, a plant material Under the microscope, cork seemed to be made of thousands of tiny, empty chambers Hooke called these chambers “cells” because they reminded him of a monastery's tiny rooms, which were called cells One of Hooke's illustrations of cells is shown in the figure to the right The term cell is used in biology to this day We now know, however, that cells are not empty but contain living matter

Cork Cells Using an early microscope, Hooke made this drawing of cork cells In Hooke's drawings, the cells look like empty chambers because he was looking at dead plant matter Today, we know that living cells are made up of many structures

Early Microscopes In Holland around the same time, Anton van Leeuwenhoek used a single-lens microscope to observe pond water and other things To his amazement, the microscope revealed a fantastic world of tiny living organisms that seemed to be everywhere, even in the very water he and his neighbors drank

The Cell Theory Soon, numerous observations made it clear that cells were the basic units of life In 1838, German botanist Matthias Schleiden concluded that all plants were made of cells The next year, German biologist Theodor Schwann stated that all animals were made of cells In 1855, the German physician Rudolf Virchow concluded that new cells could be produced only from the division of existing cells These discoveries, confirmed by other biologists, are summarized in the cell theory, a fundamental concept of biology

The Cell Theory The cell theory states:
All living things are composed of cells Cells are the basic units of structure and function in living things New cells are produced from existing cells

Exploring the Cell Following in the footsteps of Hooke, Virchow, and others, modern biologists still use microscopes to explore the cell However, today's researchers use microscopes and techniques more powerful than the pioneers of biology could have imagined Researchers can use fluorescent labels and light microscopy to follow molecules moving through the cell Confocal light microscopy, which scans cells with a laser beam, makes it possible to build three-dimensional images of cells and their parts High-resolution video technology makes it easy to produce movies of cells as they grow, divide, and develop

Exploring the Cell These new technologies make it possible for researchers to study the structure and movement of living cells in great detail Unfortunately, light itself limits the detail, or resolution, of images that can be made with the light microscope Like all forms of radiation, light waves are diffracted, or scattered, as they pass through matter, making it impossible to visualize tiny structures such as proteins and viruses with light microscopy

Exploring the Cell By contrast, as shown in the figure below, electron microscopes are capable of revealing details as much as 1000 times smaller than those visible in light microscopes because the wavelengths of electrons are much shorter than those of light Transmission electron microscopes (TEMs) make it possible to explore cell structures and large protein molecules Because beams of electrons can only pass through thin samples, cells and tissues must be cut first into ultrathin slices before they can be examined under a microscope

Exploring the Cell

Exploring the Cell With scanning electron microscopes (SEMs), a pencillike beam of electrons is scanned over the surface of a specimen For SEM images, specimens do not have to be cut into thin slices to be visualized The scanning electron microscope produces stunning three-dimensional images of cells Because electrons are easily scattered by molecules in the air, samples examined in both types of electron microscopes must be placed in a vacuum in order to be studied As a result, researchers chemically preserve their samples first and then carefully remove all of the water before placing them in the microscope This means that electron microscopy can be used to visualize only nonliving, preserved cells and tissues

Exploring the Cell In the 1990s, researchers perfected a new class of microscopes that produce images by tracing the surfaces of samples with a fine probe These scanning probe microscopes have revolutionized the study of surfaces and made it possible to observe single atoms Unlike electron microscopes, scanning probe microscopes can operate in ordinary air and can even show samples in solution Researchers have already used scanning probe microscopes to image DNA and protein molecules as well as a number of important biological structures

Prokaryotes and Eukaryotes
Cells come in a great variety of shapes and an amazing range of sizes Although typical cells range from 5 to 50 micrometers in diameter, the tiniest mycoplasma bacteria are only 0.2 micrometers across, so small that they are difficult to see under even the best light microscopes In contrast, the giant amoeba Chaos chaos may be 1000 micrometers in diameter, large enough to be seen with the unaided eye as a tiny speck in pond water Despite their differences, all cells have two characteristics in common They are surrounded by a barrier called a cell membrane; and, at some point in their lives, they contain the molecule that carries biological information—DNA

Prokaryotes and Eukaryotes
Cells fall into two broad categories, depending on whether they contain a nucleus The nucleus (plural: nuclei) is a large membrane-enclosed structure that contains the cell's genetic material in the form of DNA A membrane is a thin layer of material that serves as a covering or lining The nucleus controls many of the cell's activities Eukaryotes are cells that contain nuclei Prokaryotes are cells that do not contain nuclei Both words derive from the Greek words karyon, meaning “kernel,” or nucleus, and eu, meaning “true,” or pro, meaning “before” These words reflect the idea that prokaryotic cells evolved before nuclei developed

Prokaryotes Prokaryotic cells are generally smaller and simpler than eukaryotic cells, although there are many exceptions to this rule Prokaryotic cells have genetic material that is not contained in a nucleus Some prokaryotes contain internal membranes, but prokaryotes are generally less complicated than eukaryotes: NO MEMBRANE BOUND ORGANELLES Despite their simplicity, prokaryotes carry out every activity associated with living things They grow, reproduce, respond to the environment, and some can even move by gliding along surfaces or swimming through liquids The organisms we call bacteria are prokaryotes

Eukaryotes Eukaryotic cells are generally larger and more complex than prokaryotic cells Eukaryotic cells generally contain dozens of structures and internal membranes, and many are highly specialized: MEMBRANE BOUND ORGANELLES Eukaryotic cells contain a nucleus in which their genetic material is separated from the rest of the cell Eukaryotes display great variety Some eukaryotes live solitary lives as single-celled organisms Others form large, multicellular organisms. Plants, animals, fungi, and protists are eukaryotes

Cytoplasmic Organelles
Little organs Specialized cellular compartments, each performing its own job to maintain the life of the cell Membranous organelles: Bounded by a membrane similar in composition to the plasma membranre (minus the glycocalyx) This membrane enables them to maintain an internal environment different from that of the surrounding cytosol Examples: Mitochondria Peroxisomes Lysosomes Endoplasmic reticulum Golgi apparatus Nonmembranous organelles: Cytoskeleton Centrioles Ribosomes

Eukaryotic Cell Structure
In some respects, the eukaryotic cell is like a factory The first time you look at a microscope image of a cell, the cell seems impossibly complex Look closely at a eukaryotic cell, however, and patterns begin to emerge To see those patterns more clearly, we'll look at some structures that are common to eukaryotic cells, shown in the figure at right Because many of these structures act as if they are specialized organs, these structures are known as organelles, literally “little organs”

Characteristics of Cells
All cells have the same basic parts and some common functions A generalized human cell contains the plasma membrane, the cytoplasm, and the nucleus

Cell biologists divide the eukaryotic cell into two major parts: Nucleus Cytoplasm The cytoplasm is the portion of the cell outside the nucleus As you will see, the nucleus and cytoplasm work together in the business of life

Nucleus In the same way that the main office controls a large factory, the nucleus is the control center of the cell The nucleus contains nearly all the cell's DNA and with it the coded instructions for making proteins and other important molecules The structure of the nucleus is shown in the figure at right

THE NUCLEUS The nucleus is the control center of the cell and contains the cellular DNA Most cells have only one nucleus, but very large cells may be multinucleate Presence of more than one nucleus usually signifies that a larger-than-usual cytoplasmic mass must be regulated All body cells except mature red blood cells (anucleate) have nuclei The nucleus is larger than the cytoplasmic organelles It has three regions: Nuclear envelope (membrane) Nucleoli Chromatin

Nucleus

Nuclear Envelope The nucleus is surrounded by a nuclear envelope composed of two membranes The nuclear envelope is dotted with thousands of nuclear pores, which allow material to move into and out of the nucleus Like messages, instructions, and blueprints moving in and out of a main office, a steady stream of proteins, RNA, and other molecules move through the nuclear pores to and from the rest of the cell

Nuclear Envelope Is a double-membrane barrier (separated by a fluid-filled space) surrounding the nucleus Outer membrane is continuous with the rough ER of the cytoplasm and is studded with ribosomes on its external face Inner membrane is lined by a network of protein filaments ( the nuclear lamina) that maintains the shape of the nucleus At various points, nuclear pores penetrate areas where the membranes of the nuclear envelope fuse A complex of proteins, called a pore complex, lines each nuclear pore and regulates passage of large particles into and out of the nucleus Like other cell membranes, the nuclear envelope is selectively permeable, but here passage of substances is much freer than elsewhere Protein molecules imported from the cytoplasm and RNA molecules exported from the nucleus pass easily through the relatively large pores The nuclear envelope encloses the fluid and solutes of the nucleus, the nucleoplasm

Chromatin The granular material you can see in the nucleus is called chromatin Chromatin consists of DNA bound to protein Most of the time, chromatin is spread throughout the nucleus When a cell divides, however, chromatin condenses to form chromosomes These distinct, threadlike structures contain the genetic information that is passed from one generation of cells to the next

Chromatin (a): Appears as a fine, unevenly stained network, but special techniques reveal it as a system of bumpy threads weaving their way through the nucleoplasm Is roughly half DNA, the genetic material of the cell, and half globular histone proteins: Nucleosomes are the fundamental unit of chromatin, consisting of discus-shaped cores or clusters of eight histone proteins connected like beads on a string by a DNA molecule DNA winds around each nucleosome and continues on to the next cluster via linker DNA segments

Chromatin Histones provide physical means for packing the very long DNA molecules in a compact, orderly way, they also play an important role in gene regulation: In a nondividing cell, addition of phosphate or methyl groups to histone exposes different DNA segments, or genes, so that they can dictate the specifications for protein synthesis When a cell is preparing to divide, chromatin condenses into dense, rodlike chromosomes Chromosome compactness avoids entanglement and breakage of the delicate chromatin strands during the movements that occur during cell division

Nucleolus Most nuclei also contain a small, dense region known as the nucleolus The nucleolus is where the assembly of ribosomes begins

Nucleoli Dark-staining spherical bodies within the nucleus
NOT membrane bound There are typically one or two nucleoli per nucleus, but there may be more Site of the assembly of ribosomal subunits: Therefore, large in actively growing cells that are making large amounts of tissue proteins

Nucleus

NUCLEUS The nucleus is surrounded by a double membrane called the nuclear envelope. Inside the envelope is chromatin (combo of DNA and protein) which will become chromosomes. The nucleolus is the site where ribosomes are synthesized and partially assembled. The nucleus is porous and is the site where our genetic information is held.

Endoplasmic Reticulum
Eukaryotic cells also contain an internal membrane system known as the endoplasmic reticulum, or ER, as shown in the figure at right The endoplasmic reticulum is the site where lipid components of the cell membrane are assembled, along with proteins and other materials that are exported from the cell

Ribosomes One of the most important jobs carried out in the cellular “factory” is making proteins Proteins are assembled on ribosomes Ribosomes are small particles of RNA and protein found throughout the cytoplasm They produce proteins by following coded instructions that come from the nucleus Each ribosome, in its own way, is like a small machine in a factory, turning out proteins on orders that come from its “boss”—the cell nucleus Cells that are active in protein synthesis are often packed with ribosomes

Ribosomes (a):Small staining granules consisting of protein and ribosomal RNA Each ribosome has two globular subunits that fit together Site of protein synthesis

Ribosome

Ribosomes Some float freely in the cytoplasm:
Make soluble proteins that function in the cytosol Some are attached to membranes, forming a complex called the rough endoplasmic reticulum: Synthesize proteins destined either for incorporation into cell membranes or for export from the cell Ribosomes can switch back-and-forth between the two types

The portion of the ER involved in the synthesis of proteins is called rough endoplasmic reticulum, or rough ER It is given this name because of the ribosomes found on its surface Newly made proteins leave these ribosomes and are inserted into the rough ER, where they may be chemically modified

Proteins that are released, or exported, from the cell are synthesized on the rough ER, as are many membrane proteins Rough ER is abundant in cells that produce large amounts of protein for export Other cellular proteins are made on “free” ribosomes, which are not attached to membranes

ER PROTEIN SYNTHESIS

The other portion of the ER is known as smooth endoplasmic reticulum (smooth ER) because ribosomes are not found on its surface In many cells, the smooth ER contains collections of enzymes that perform specialized tasks, including the synthesis of membrane lipids and the detoxification of drugs Liver cells, which play a key role in detoxifying drugs, often contain large amounts of smooth ER

Endoplasmic reticulum
Is an extensive system of interconnected tubes and parallel membranes enclosing fluid-filled cavities, called cisternae, that coils and twist throughout the cytosol Continuous with the nuclear membrane Two varieties: Rough ER Smooth ER

ENDOPLASMIC RETICULUM

Golgi Apparatus Proteins produced in the rough ER move next into an organelle called the Golgi apparatus, discovered by the Italian scientist Camillo Golgi As you can see in the figure at right, Golgi appears as a stack of closely apposed membranes The function of the Golgi apparatus is to modify, sort, and package proteins and other materials from the endoplasmic reticulum for storage in the cell or secretion outside the cell The Golgi apparatus is somewhat like a customization shop, where the finishing touches are put on proteins before they are ready to leave the “factory” From the Golgi apparatus, proteins are then “shipped” to their final destinations throughout the cell or outside of the cell

Golgi Apparatus

Golgi Apparatus Is a series of stacked, flattened, membranous sacs, shaped like hollow dinner plates, associated with swarms of tiny groups of membranous vesicles The main function of the Golgi apparatus is to modify, concentrate, and package the proteins and lipids made at the rough ER The transport vesicles that bud off from the rough ER move to and fuse with the membranes at its convex cis face (receiving side), of the Golgi apparatus

GOLGI ROLE

Lysosomes Even the neatest, cleanest factory needs a cleanup crew, and that's what lysosomes are Lysosomes are small organelles filled with enzymes One function of lysosomes is the digestion, or breakdown, of lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell

Lysosomes Lysosomes are also involved in breaking down organelles that have outlived their usefulness Lysosomes perform the vital function of removing “junk” that might otherwise accumulate and clutter up the cell A number of serious human diseases, including Tay-Sachs disease, can be traced to lysosomes that fail to function properly

Lysosomes Spherical membranous organelles that contain digestive enzymes Abundant in phagocytes, the cells that dispose of invading bacteria and cell debris Digest almost all kinds of biological molecules functioning best in acidic environments Thus called acid hydrolases The lysosomal membrane is adapted to serve lysosomal functions in two important ways: 1.Contains H+ (proton) pumps, ATPases that gather hydrogen ions from the surrounding cytosol to maintain the organelle’s acidic pH 2.It retains the dangerous acid hydrolases while permitting the final products of digestion to escape so that they can be used by the cell or excreted Hence, lysosomes provide sites where digestion can proceed safely within a cell

LYSOSOMES

LYSOSOMES Contains 40 different kinds of digestive enzymes
Digest food particles, bacteria, and worn-out or broken cell parts Small, spherical organelles surrounded by a single membrane Exist primarily in animal and fungal cells Role in early (embryonic) development Enzymes selectively destroy tissue

Lysosomes Function as a cell’s demolition crew:
Digesting particles taken in by endocytosis, particularly ingested bacteria, viruses, and toxins Degrading worn-out or nonfunctional organelles Performing metabolic functions, such as glycogen breakdown and release Breaking down nonuseful tissues, such as the webs between fingers and toes of a developing fetus and the uterine lining during menstruation Breaking down bone to release calcium ions into the blood

Vacuoles Every factory needs a place to store things, and cells contain places for storage as well Some kinds of cells contain saclike structures called vacuoles that store materials such as water, salts, proteins, and carbohydrates In many plant cells there is a single, large central vacuole filled with liquid The pressure of the central vacuole in these cells makes it possible for plants to support heavy structures such as leaves and flowers

Vacuoles Vacuoles are also found in some single-celled organisms and in some animals The paramecium contains a vacuole called a contractile vacuole By contracting rhythmically, this specialized vacuole pumps excess water out of the cell The control of water content within the cell is just one example of an important process known as homeostasis Homeostasis is the maintenance of a controlled internal environment

VACUOLES Fluid-filled cavities or sacs
Organelles often found in plants Store enzymes and waste products (like lysosomes in animal cells) Some of the waste products are toxic and need to be kept away from the rest of the cell In a mature cell, makes up about 90% of the volume

VACUOLES Vacuoles are a second common characteristic of plant cells. These are fluid-filled organelles that store enzymes and wastes.

Mitochondria and Chloroplasts
All living things require a source of energy Factories are hooked up to the local power company, but what about cells? Most cells get energy in one of two ways—from food molecules or from the sun

Mitochondria Nearly all eukaryotic cells, including plants, contain mitochondria (singular: mitochondrion) Mitochondria are organelles that convert the chemical energy stored in food into compounds that are more convenient for the cell to use Mitochondria are enclosed by two membranes—an outer membrane and an inner membrane The inner membrane is folded up inside the organelle

Mitochondria One of the most interesting aspects of mitochondria is the way in which they are inherited In humans, all or nearly all of our mitochondria come from the cytoplasm of the ovum, or egg cell This means that when your relatives are discussing which side of the family should take credit for your best characteristics, you can tell them that you got your mitchondria from Mom!

Mitochondria Sausage-shaped membranous organelle
In living cells they squirm, elongate, and change shape almost continuously Power plants of the cell, providing most of its ATP supply Enclosed by two membranes, each with the general structure of the plasma membrane Outer membrane is smooth and featureless Inner membrane folds inward, forming shelflike cristae that protrude into the matrix, the gel-like substance within the mitochondrion Intermediate products of food fuels are broken down to water and carbon dioxide by teams of enzymes, some dissolved in the mitochondrial matrix and others forming part of the crista membrane

MITOCHONDRION

Mitochondria Site of aerobic respiration (requires oxygen)
Contain their own DNA and RNA and are able to reproduce themselves Capable of fission Contain approximately 37 genes that direct the synthesis of some proteins required for mitochondrial functions Believed that mitochondria arose from bacteria that invaded the ancestors of plant and animal cells

PLASTIDS Large organelles in plants Store food or pigments
Organelle where solar energy is converted into chemical energy and stored Types: Chloroplast: Contains a green pigment (chlorophyll) that absorbs sunlight as the first step in photosynthesis Chromoplasts: Synthesis and store pigments such as orange carotenes, yellow xanthophylls, and various red pigments, some of which function in trapping sunlight for energy Give certain plants their distinctive colors Leucoplasts: Store food such as starches, proteins, and lipids

Chloroplasts Plants and some other organisms contain chloroplasts
Chloroplasts are organelles that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis Chloroplasts are the biological equivalents of solar power plants Like mitochondria, chloroplasts are surrounded by two membranes Inside the organelle are large stacks of other membranes, which contain the green pigment chlorophyll

CHLOROPLAST

Organelle DNA Unlike other organelles that contain no DNA, chloroplasts and mitochondria contain their own genetic information in the form of small DNA molecules Lynn Margulis, an American biologist, has suggested that mitochondria and chloroplasts are actually the descendants of ancient prokaryotes Margulis suggests that the prokaryotic ancestors of these organelles evolved a symbiotic relationship with early eukaryotes, taking up residence within the eukaryotic cell One group of prokaryotes had the ability to use oxygen to generate ATP: These prokaryotes evolved into mitochondria Other prokaryotes that carried out photosynthesis evolved into: Chloroplasts This idea is called the endosymbiotic theory

Cytoskeleton A supporting structure and a transportation system complete our picture of the cell as a factory As you know, a factory building is supported by steel or cement beams and by columns that support its walls and roof Eukaryotic cells have a structure—the cytoskeleton—that helps support the cell The cytoskeleton is a network of protein filaments that helps the cell to maintain its shape The cytoskeleton is also involved in movement Microfilaments and microtubules are two of the principal protein filaments that make up the cytoskeleton

Cytoskeleton Series of rods running through the cytosol, supporting cellular structures and aiding in cell movement There are three types of rods in the cytoskeleton:not covered by membranes Microtubules Microfilaments Intermediate filaments

Cytoskeleton The cytoskeleton is a network of protein filaments that helps the cell to maintain its shape and is involved in many forms of cell movement

Microfilaments Microfilaments are threadlike structures made of a protein called actin They form extensive networks in some cells and produce a tough, flexible framework that supports the cell Microfilaments also help cells move Microfilament assembly and disassembly is responsible for the cytoplasmic movements that allow cells, such as amoebas, to crawl along surfaces

Microfilaments Thinnest elements of the cytoskeleton
Strands of the protein actin (ray) Each cell has its own unique arrangements (NO TWO CELLS ARE ALIKE) Nearly all cells have a fairly dense cross-linked network of microfilaments attached to the cytoplasmic side of their plasma membrane that strengthens the cell surface

Microtubules Microtubules, as shown in the figure to the right, are hollow structures made up of proteins known as tubulins In many cells, they play critical roles in maintaining cell shape Microtubules are also important in cell division, where they form a structure known as the mitotic spindle, which helps to separate chromosomes In animal cells, tubulin is also used to form a pair of structures known as centrioles Centrioles are located near the nucleus and help to organize cell division Centrioles are not found in plant cells

Microtubules Microtubules also help to build projections from the cell surface, which are known as cilia (singular: cilium) and flagella (singular: flagellum), that enable cells to swim rapidly through liquids Cilia and flagella can produce considerable force; and in some cells they move almost like the oars of a boat, pulling or pushing cells through the water

Microtubules Largest diameter
Hollow tubes made of spherical protein subunits called tubulins Most radiate from a small region of cytoplasm near the nucleus called the centrosome Constantly growing from the centrosome, disassembling, and then reassembling

Cytoskeleton

Cell Boundaries When you first study a country, you may begin by examining a map of the country's borders Before you can learn anything about a nation, it's important to understand where it begins and where it ends The same principle applies to cells Among the most important parts of a cell are its borders, which separate the cell from its surroundings All cells are surrounded by a thin, flexible barrier known as the cell membrane Many cells also produce a strong supporting layer around the membrane known as a cell wall

Cell Membrane The cell membrane regulates what enters and leaves the cell and also provides protection and support The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer As you can see in the figure at right, there are two layers of lipids, hence the name bilayer The lipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings

Cell Membrane In addition to lipids, most cell membranes contain protein molecules that are embedded in the lipid bilayer Carbohydrate molecules are attached to many of these proteins There are so many kinds of molecules in cell membranes that scientists describe the membrane as a “mosaic” of different molecules A mosaic is a work of art made of individual tiles or other pieces assembled to form a picture or design Some of the proteins form channels and pumps that help to move material across the cell membrane Many of the carbohydrates act like chemical identification cards, allowing individual cells to identify one another

The Fluid Mosaic Model The inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds and amounts of lipids they contain: The majority of membrane phospholipids are unsaturated (like phosphatidyl choline), a condition which kinks their tails (increasing the space between them) and increases fluidity Glycolipids, phospholipids with attached sugar groups, are found only in the outer membrane (5% of membrane) Sugar group makes that end of the glycolipid molecule polar, whereas the fatty acid tails are nonpolar Cholesterol (20% of membrane) stabilizes the lipid membrane by wedging its platelike hydrocarbon rings between the phospholipid tails and restraining movement of the phospholipids Lipid rafts (20%), dynamic assemblies of saturated phospholipids (which pack together tightly) associated with unique lipids called sphinolipids and lots of cholesterol are also found only in the outer membrane More stable and orderly and less fluid than the rest of the membrane Include or exclude specific proteins to various extents Assumed to function in cell signaling

The Fluid Mosaic Model Two distinct populations of membrane proteins:
Integral Peripheral

Functions of Membrane Proteins
Proteins make up about 50% of the plasma membrane by mass and are responsible for most of the specialized membrane functions Transport Enzymatic activity Receptors for signal transduction Intercellular joining Cell-cell recognition Attachment to the cytoskeleton and extracellular matrix (ECM)

Plasma Membrane: Structure
Plasma membrane (cell membrane) defines the extent of the cell, separating two of the body’s major fluid compartments: Intracellular fluid within cells Extracellular fluid outside cells

Cell Membrane

The Fluid Mosaic Model Plasma membrane is composed of a double layer of phospholipids embedded with small amounts of cholesterol and proteins dispersed in it The phospolipid bilayer is composed of two layers of phospholipids lying tail to tail: Polar head is charged and hydrophilic (hydro=water, philic=loving) Exposed to water inside (intracellular) and outside (extracellular) the cell Attracted to water Nonpolar tail is made of two fatty acid chains and is hydrophobic (phobia=hating) Avoid water Line up in the center of the membrane

Cell Walls Cell walls are present in many organisms, including plants, algae, fungi, and many prokaryotes Cell walls lie outside the cell membrane Most cell walls are porous enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through easily The main function of the cell wall is to provide support and protection for the cell

Cell Walls Most cell walls are made from fibers of carbohydrate and protein These substances are produced within the cell and then released at the surface of the cell membrane where they are assembled to form the wall Plant cell walls are composed mostly of cellulose, a tough carbohydrate fiber Cellulose is the principal component of both wood and paper, so every time you pick up a sheet of paper, you are holding the stuff of cell walls in your hand

CELL WALL Plant cells Surround the cell membrane and helps support and protect the cell Rigid covering Made primarily of long chains of cellulose embedded in hardening compounds such as pectin and lignin Pores allow ions and molecules to pass to and from the cell membrane

CELL WALL Two types: both give strength to the cell Primary:
Formed during cell growth Secondary: Formed after growth has ceased

PLANT CELL DIVISION CELL WALL FORMATION
Middle lamella forms between the new cells Intercellular glue of pectin (substance that makes jelly gel) Primary wall is formed between the middle lamella and the cell membrane Cellulose fibers are laid down in layers (all of the fibers in one direction) Growing cell is thus able to expand sequentially at a right angle to the most recent fiber layer Functions both to protect and allow the cell to grow Secondary wall forms after growth of the primary wall ceases Made of cellulose and lignin (makes wall woody) fibers interwoven so that no further expansion of the cell is possible Functions to strengthen the mature cell When you hold a piece of wood, you are holding secondary cell walls

Diffusion Through Cell Boundaries
Every living cell exists in a liquid environment that it needs to survive It may not always seem that way; yet even in the dust and heat of a desert, the cells of cactus plants, scorpions, and vultures are bathed in liquid One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side

Measuring Concentration
The cytoplasm of a cell contains a solution of many different substances in water Recall that a solution is a mixture of two or more substances The substances dissolved in the solution are called solutes The concentration of a solution is the mass of solute in a given volume of solution, or mass/volume Example: If you dissolved 12 grams of salt in 3 liters of water, the concentration of the solution would be 12 g/3 L, or 4 g/L (grams per liter) If you had 12 grams of salt in 6 liters of water, the concentration would be 12 g/6 L, or 2 g/L The first solution is twice as concentrated as the second solution

Diffusion In a solution, particles move constantly
They collide with one another and tend to spread out randomly As a result, the particles tend to move from an area where they are more concentrated to an area where they are less concentrated, a process known as diffusion When the concentration of the solute is the same throughout a system, the system has reached equilibrium

Passive Process Diffusion
Diffusion is a process in which substances scatter evenly throughout the environment from an area of higher concentration to an area of lower concentration Molecules move randomly, collide and ricochet off one another, changing direction with each collision Overall effect of this erratic movement is that molecules or ions move away from areas where they are in higher concentration to areas where their concentration is lower, so we say that molecule diffuse along, or down, their concentration gradient until equilibrium The greater the difference in concentration between the two areas, the faster the net diffusion of the particles

Passive Process Diffusion
Driving force is the kinetic energy of the molecules themselves: The speed is influenced by: Molecular size: Smaller the faster Temperature: Warmer, the faster Example: Peeling an onion, releases volatile substances that diffuse through the air, dissolving in the fluid film covering your eyes forming irritating sulfuric acid

Diffusion What do diffusion and equilibrium have to do with cell membranes? Suppose a substance is present in unequal concentrations on either side of a cell membrane, as shown in the figure at right If the substance can cross the cell membrane, its particles will tend to move toward the area where it is less concentrated until equilibrium is reached At that point, the concentration of the substance on both sides of the cell membrane will be the same

Diffusion Because diffusion depends upon random particle movements, substances diffuse across membranes without requiring the cell to use energy Even when equilibrium is reached, particles of a solution will continue to move across the membrane in both directions However, because almost equal numbers of particles move in each direction, there is no further change in concentration

Diffusion

Passive Process Simple Diffusion
Nonpolar and lipid-soluble substances diffuse directly through the lipid bilayer Examples: oxygen, carbon dioxide, fat-soluble vitamins, and alcohol

Osmosis Although many substances can diffuse across biological membranes, some are too large or too strongly charged to cross the lipid bilayer If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it A membrane is impermeable to substances that cannot pass across it Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot

Osmosis Water passes quite easily across most membranes, even though many solute molecules cannot An important process known as osmosis is the result Osmosis is the diffusion of water through a selectively permeable membrane

How Osmosis Works Look at the beaker on the left in the figure at right There are more sugar molecules on the left side of the selectively permeable membrane than on the right side That means that the concentration of water is lower on the left than it is on the right The membrane is permeable to water but not to sugar This means that water can cross the membrane in both directions, but sugar cannot As a result, there is a net movement of water from the area of high concentration to the area of low concentration

How Osmosis Works

Osmolarity When equal volumes of aqueous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in the system, net diffusion of both solute and water occurs, each moving down its own concentration gradient Eventually, equilibrium is reached when the water concentration on the left equals that on the right, and the solute concentration on both sides is the same

How Osmosis Works Water will tend to move across the membrane to the left until equilibrium is reached At that point, the concentrations of water and sugar will be the same on both sides of the membrane When this happens, the two solutions will be isotonic, which means “same strength” When the experiment began, the more concentrated sugar solution was hypertonic, which means “above strength,” as compared to the dilute sugar solution The dilute sugar solution was hypotonic, or “below strength”

Osmolarity If we consider the same system, but make the membrane impermeable to solute molecules, we see quite a different result: Water quickly diffuses from the left to the right compartment and continues to do so until its concentration is the same on the two sides of the membrane Notice that in this case equilibrium results from the movement of WATER ALONE (the solutes are prevented from moving) Also the movement of water leads to dramatic changes in the volumes of the two compartments

Osmotic Pressure For organisms to survive, they must have a way to balance the intake and loss of water Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane Osmotic pressure can cause serious problems for a cell Because the cell is filled with salts, sugars, proteins, and other molecules, it will almost always be hypertonic to fresh water This means that osmotic pressure should produce a net movement of water into a typical cell that is surrounded by fresh water If that happens, the volume of a cell will increase until the cell becomes swollen Eventually, the cell may burst like an overinflated balloon The effects of osmosis are shown in the figure at right

Osmotic Pressure

Osmotic Pressure Fortunately, cells in large organisms are not in danger of bursting Most cells in such organisms do not come in contact with fresh water Instead, the cells are bathed in fluids, such as blood, that are isotonic These isotonic fluids have concentrations of dissolved materials roughly equal to those in the cells themselves

Osmotic Pressure Other cells, such as plant cells and bacteria, which do come into contact with fresh water, are surrounded by tough cell walls The cell walls prevent the cells from expanding, even under tremendous osmotic pressure However, the increased osmotic pressure makes the cells extremely vulnerable to injuries to their cell walls

Facilitated Diffusion
A few molecules, such as the sugar glucose, seem to pass through the cell membrane much more quickly than they should One might think that these molecules are too large or too strongly charged to cross the membrane, and yet they diffuse across quite easily

Passive Process Facilitated Diffusion
In facilitated diffusion substances are moved through, even though they are unable to pass through the lipid bilayer of the plasma membrane, by either: Binding to protein carriers in the membrane Moving through channels Examples: glucose and other sugars, amino acids, and ions

How does this happen? The answer is that cell membranes have protein channels that make it easy for certain molecules to cross the membrane Red blood cells, for example, have a cell membrane protein with an internal channel that allows glucose to pass through it Only glucose can pass through this channel, and it can move through in either direction This cell membrane protein is said to facilitate, or help, the diffusion of glucose across the membrane The process, shown to the right, is known as facilitated diffusion Hundreds of different protein channels have been found that allow particular substances to cross different membranes

During facilitated diffusion, molecules, such as glucose, that cannot diffuse across the cell membrane's lipid bilayer on their own move through protein channels instead

Although facilitated diffusion is fast and specific, it is still diffusion Therefore, a net movement of molecules across a cell membrane will occur only if there is a higher concentration of the particular molecules on one side than on the other side This movement does not require the use of the cell's energy

Facilitated Diffusion Carriers
Is a transmembrane integral protein (sometimes called a permease) that shows specificity for molecules of a certain polar substance or class of substances that are too large to pass through membrane channels Examples: sugars and amino acids Mechanism: Carrier-Mediated Facilitated Diffusion Changes in shape of the carrier allow it to first envelop and then release the transported substance, shielding it en route from the nonpolar regions of the membrane Essentially, the binding site is moved from one face of the membrane to the other by changes in the conformation of the carrier protein

Facilitated Diffusion Channels
Transmembrane proteins that serve to transport substances, usually ions or water, through aqueous channels from one side of the membrane to the other Types of Channels are: Open Channels: Are always open (leakage channels) and simply allow ion or water fluxes according to concentration gradients Gated and Controlled Channels: Gated: Binding or association sites exist within the channel and the channel is selective due to pore size and the charges of the amino acids lining the channel Controlled: open or close by various chemical or electrical signals

Integral Membrane Proteins
Firmly inserted into the plasma membrane Some protrude from one membrane face only, BUT most are transmembrane proteins that span the entire width of the membrane and protrude on BOTH sides All have BOTH hydrophobic and hydrophilic regions: This allows them to interact BOTH with the nonpolar lipid tails buried in the membrane and with water inside and outside the cell

Integral Membrane Proteins
Mainly involved in transport: Some cluster together to form channels, or pores, through which small, water-soluble molecules or ions can move, thus bypassing the lipid part of the membrane Some act as carriers that bind to a substance and then move it through the membrane Some are receptors for hormones or other chemical messengers and relay messages to the cell interior (process called signal transduction)

Active Transport As powerful as diffusion is, cells sometimes must move materials in the opposite direction—against a concentration difference This is accomplished by a process known as active transport As its name implies, active transport requires energy The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins or “pumps” that are found in the membrane itself Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis The transport of these larger materials sometimes involves changes in the shape of the cell membrane

Active Transport Similar to facilitated diffusion in that both require carrier proteins that combine specifically and reversibly with the transported substances HOWEVER, facilitated diffusion always honors concentration gradients because its driving force is kinetic energy IN CONTRAST, the active transporters or solute pumps move solutes, most importantly ions (such as Na+, K+, and Ca2+), uphill against a concentration gradient To do this work, cells must expend the energy of ATP Very selective involving chemicals that cannot pass by diffusion Classified according to their energy source

Primary Active Transport
Because Na+ and K+ leak slowly but continuously through channels in the plasma membrane along their concentration gradient (and cross rapidly in stimulated muscles and nerve cells), the Na+-K+ pump operates more or less continuously as an antiport to simultaneously drive Na+ out of the cell against a steep concentration gradient and pump K+ back in

Molecular Transport Active Transport
Small molecules and ions are carried across membranes by proteins in the membrane that act like energy-requiring pumps Many cells use such proteins to move calcium, potassium, and sodium ions across cell membranes Changes in protein shape, as shown in the figure at right, seem to play an important role in the pumping process A considerable portion of the energy used by cells in their daily activities is devoted to providing the energy to keep this form of active transport working. The use of energy in these systems enables cells to concentrate substances in a particular location, even when the forces of diffusion might tend to move these substances in the opposite direction

Molecular Transport Active Transport

Endocytosis Larger molecules and even solid clumps of material may be transported by movements of the cell membrane One of these movements is called endocytosis Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane The pocket that results breaks loose from the outer portion of the cell membrane and forms a vacuole within the cytoplasm Large molecules, clumps of food, and even whole cells can be taken up in this way Two examples of endocytosis are phagocytosis and pinocytosis

ENDOCYTOSIS

Endocytosis Phagocytosis
Phagocytosis means “cell eating” In phagocytosis, extensions of cytoplasm surround a particle and package it within a food vacuole The cell then engulfs it Amoebas use this method of taking in food Engulfing material in this way requires a considerable amount of energy and, therefore, is correctly considered a form of active transport

Phagocytosis Type of endocytosis in which some relatively large or solid material, such as a clump of bacteria or cell debris, is engulfed by the cell Cytoplasmic extensions called pseudopods form and flow around the particle and engulf it Vesicle formed is called a phagosome In most cases, the phagosome then fuses with a lysosome and its contents are digested In humans, certain WBC and macrophages

Endocytosis Pinocytosis
In a process similar to endocytosis, many cells take up liquid from the surrounding environment Tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell This process is known as pinocytosis

Pinocytosis Also called fluid-phase endocytosis (cell drinking)
Bit of infolding plasma membrane surrounds a very small volume of extracellular fluid containing dissolved molecules This droplet enters the cell in an endosome Unlike phagocytosis, pinocytosis is a routine activity of most cells, affording them a nonselective way of sampling the extracellular fluid Important in cells that absorb nutrients, such as cells that line the intestines

Receptor-mediated endocytosis
Very selective Main mechanism for the specific endocytosis and transcytosis of most macromolecules by body cells Receptors for this process are plasma membrane proteins that bind only with certain substances Both the receptors and attached molecules are internalized in a clathrin-coated pit Substances taken up include: Enzymes Insulin Some hormones Low-density lipoproteins (such as cholesterol attached to a transport protein) Iron UNFORTUNATELY: flu viruses and diphtheria toxin use this route to enter and attack our cells

Exocytosis Many cells also release large amounts of material from the cell, a process known as exocytosis During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell The removal of water by means of a contractile vacuole is one example of this kind of active transport

Exocytosis The substance to be removed from the cell is first enclosed in a membranous sac called a vesicle Vesicle migrates to the plasma membrane, fuses with it, and then ruptures, spilling the sac contents out of the cell Special transmembrane proteins on the vesicle called v-SNAREs (v for vesicles) recognize certain plasma membrane proteins, called t-SNAREs (t for target), and bind Lipid layers corkscrew without mixing

The Diversity of Cellular Life
Earth is sometimes called a living planet, and for good reason From its simple beginnings, life has spread to every corner of the globe, penetrating deep into the earth and far beneath the surface of the seas The diversity of life is so great that you might have to remind yourself that all living things are composed of cells, use the same basic chemistry, follow the same genetic code, and even contain the same kinds of organelles This does not mean that all living things are the same It does mean that their differences arise from the ways in which cells are specialized to perform certain tasks and the ways in which cells associate with each other to form multicellular organisms

Unicellular Organisms
Cells are the basic living units of all organisms, but sometimes a single cell is a little more than that Sometimes, a cell is the organism A single-celled organism is also called a unicellular organism Unicellular organisms do everything that you would expect a living thing to do They grow, respond to the environment, transform energy, and reproduce In terms of their numbers, unicellular organisms dominate life on Earth One example of a unicellular organism is the bacterium shown to the right

Unicellular Organisms

Multicellular Organisms
Organisms that are made up of many cells are called multicellular There is a great variety among multicellular organisms However, all multicellular organisms depend on communication and cooperation among specialized cells Cells throughout an organism can develop in different ways to perform different tasks This process is called cell specialization

Specialized Animal Cells
Animal cells are specialized in many ways Red blood cells are specialized to transport oxygen Red blood cells contain a protein that binds to oxygen in the lungs and transports the oxygen throughout the body where it is released Cells specialized to produce proteins, for example, are found in the pancreas The pancreas is a gland that produces enzymes that make it possible to digest food. As you might expect, pancreatic cells are packed with ribosomes and rough ER, which are where proteins are produced Pancreatic cells also possess large amounts of other organelles needed for protein export, including a well-developed Golgi apparatus and clusters of storage vacuoles loaded with enzymes

The human ability to move is result of the specialized structures of muscle cells These cells generate force by using a dramatically overdeveloped cytoskeleton Skeletal muscle cells are packed with fibers arranged in a tight, regular pattern Those fibers are actin microfilaments and a cytoskeletal protein called myosin When they contract, muscle cells use chemical energy to pull these fibers past each other, generating force Whether your muscles are large or small, your muscle cells themselves are “bulked up” with these specialized cytoskeletal proteins to a degree that makes them the body's undisputed heavy-lifting champions

Specialized Plant Cells
A plant basking in the sunlight may seem quiet and passive, but it is actually interacting with the environment at every moment It rapidly exchanges carbon dioxide, oxygen, water vapor, and other gases through tiny openings called stomata on the undersides of leaves Highly specialized cells, known as guard cells, regulate this exchange Guard cells monitor the plant's internal conditions, changing their shape according to those conditions For example, when the plant can benefit from gas exchange, the stomata open The stomata close tightly when the plant's internal conditions change

Cells vary greatly in their size, shape, and function

CELL DIVERSITY

Levels of Organization
Biologists have identified levels of organization that make it easier to describe the cells within a multicellular organism The levels of organization in a multicellular organism are individual cells, tissues, organs, and organ systems These levels of organization are shown at right

MULTICELLULAR ORGANIZATION
Tissue: group of similar cells that carry out a common function (epithelial, muscle, nerves, etc.) Organ: several types of tissues that interact to perform a specific function (stomach, skin, etc.) Organ system: group of organs that work together to perform a set of related tasks (digestive system: mouth, esophagus, stomach, small intestine, large intestine, and several other organs)

Tissues In multicellular organisms, cells are the first level of organization Similar cells are grouped into units called tissues A tissue is a group of similar cells that perform a particular function The collection of cells that produce digestive enzymes in the pancreas makes up one kind of tissue Most animals have four main types of tissue: Muscle Epithelial Nervous Connective tissue

Organs Many tasks within the body are too complicated to be carried out by just one type of tissue In these cases, many groups of tissues work together as an organ For example, each muscle in your body is an individual organ. Within a muscle, however, there is much more than muscle tissue There are nerve tissues and connective tissues Each type of tissue performs an essential task to help the organ function.

Organ Systems In most cases, an organ completes a series of specialized tasks A group of organs that work together to perform a specific function is called an organ system

Organ Systems The organization of the body's cells into tissues, organs, and organ systems creates a division of labor among those cells that makes multicellular life possible Specialized cells such as nerve and muscle cells are able to function precisely because other cells are specialized to obtain the food and oxygen needed by those cells This overall specialization and interdependence is one of the remarkable attributes of living things. Appreciating this characteristic is an important step in understanding the nature of living things

Multicellular Organisms

Cell Structure and Function

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Cell Structure and Function

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