12 Chapter 1

Flagella and Cilia

Some animal cells have flagella (singular: flagellum), or whiplike tails. A flagellumhelps a cell move by using contractile proteins to spin it in a corkscrew motion,much like the propeller of a boat. Many cells have a single whiplike tail; however,some have multiple flagella. The human sperm cell is one example of a cell thatuses a flagellum for propulsion (Figure 4).

The cell membranes of other specialized cells are equipped with a number ofshorter, hairlike structures called cilia (singular: cilium). Like flagella, cilia havecontractile proteins that cause them to move. Most often, cilia appear to move ina coordinated fashion. In some cells, the main function of cilia is locomotion; inothers, it is to create fluid currents to move materials. The cells that line yourwindpipe are equipped with hundreds of thousands of cilia that move debristrapped in mucus away from your lungs.

Overview of Cell Structure

1. The cell theory states the following:• All living things are composed of one or more cells.• The cell is the smallest entity that retains the properties of life.• New cells arise only from cells that already exist.

2. Cells are composed of a nucleus and cytoplasm bound by a cell membrane.

3. The nucleus is the control centre of the cell.• It contains chromosomes, genes, DNA, and RNA.• It directs cell division and the formation of cell structures.

4. The cell membrane forms the boundary of the cell.• It is composed of proteins and lipids. It is selectively permeable.

5. The cytoplasm is where the work of the cell is done.

6. Flagella and cilia help cells move; cilia sometimes help move materials.

Practice

Understanding Concepts

1. Copy Table 1 into your notebook. Complete the third column.

Table 1: Functions of the Nucleus in Eukaryotic Cells

Area of cell Cell part Function

nucleus chromosomes

nucleolus

nuclear envelope

1.3 Looking at Cells and theCytoplasmic Organelles

A cell can be compared to a factory. Like a factory, the cell erects new cell structuresand repairs damaged structures. Nutrient molecules are taken in and fashioned

Figure 4

Sperm cells have flagella to help them swimto the egg.

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into molecules essential for life. The cytoplasm is the region of the cell in whichwork is accomplished, while the nucleus provides the directions. The cytoplasmhas specialized structures called organelles (small organs), which are visible whenviewed under an electron microscope (Figures 1 and 2).

The development of the transmission electron microscope opened a newwindow into the cell, allowing scientists to observe things they had previouslyonly imagined. However, the electron microscope has its limits. Because cellstructures must be embedded in plastic to be viewed, scientists are limited toobserving dead cells. A clearer understanding of a living cell requires differenttechnologies.

nucleus

nuclear pore

chromatin

nucleolus

nuclear envelope

rough ER

smooth ER

ribosomes

plastid

chloroplast

microtubules

plasma membrane

mitochondrion

microfilaments

vacuole

vesicle

Golgi apparatus

cell wall

nucleus

nuclear pore

chromatin

nucleolus

nuclear envelope

Golgi apparatus

lysosome

ribosomes

plasma membrane

rough ER

mitochondrion

vacuole being formed

microfilamentsflagellum(not always present)

smooth ER

centriole pair

microtubules

Figure 1

Plant cell and organelles

Figure 2

Animal cell and organelles

14 Chapter 1

Cell fractionation provides information about the thousands of chemicalreactions that occur simultaneously in a cell. In this technique, cells are brokenopen to release the organelles and other parts of the cytoplasm and then areplaced in a test tube that is spun in a centrifuge. The centrifuge is a machine thatrotates at high speeds to produce a force hundreds of thousands of times greaterthan normal gravity. Denser structures are driven toward the bottom of the testtube while less dense objects remain nearer the surface. Because each layer con-tains different cell parts, the layers can be separated and the chemical reactionsin each layer studied (Figure 3).

In another technique, living cells are treated with radioisotopes. The radio-activity emitted from the radioisotopes can be traced with special equipment. Theradioactive chemicals can be tracked through chemical reactions.

Looking at Cells

1. The transmission electron microscope limits observation to dead and non-living things.

2. Cell fractionation provides information about chemical reactions thatoccur in cells.• Cells are ground up, placed in a test tube, and spun in a centrifuge.• Denser structures fall to the bottom and less dense ones stay on top.• Layers with components of different density are separated and each layer

is studied.

3. Living cells can be treated with radioisotopes.• Radioactivity emitted can be traced.• Radioactive chemicals can be followed to determine the chemical reactions

in which they participate.

The Cytoplasmic Organelles

Mitochondria

Tiny oval-shaped organelles called mitochondria are often referred to as thepower plant of the cell (Figure 4). Each mitochondrion provides the cell withneeded energy in a series of chemical processes called cellular respiration. Duringthese processes, sugar molecules combine with oxygen to form carbon dioxideand water. Energy is also released and is temporarily stored in a compound calledadenosine triphosphate (ATP).

cell fractionation: the process by whichcell components are separated by centrifugation

radioisotopes: unstable chemicals thatemit bursts of energy as they break down

water and smallmolecules

proteins and lipids,ribosomes

lysosomes,mitochondria

DNA andnucleoplasm

Figure 3

inner matrix

intermembranespace

inner membrane

cristae

outer membraneFigure 4

An electron micrograph and a diagram of themitochondrion. In animal cells, mitochondriaare the largest of the cytoplasmic organelles.The one in the drawing is about a hundredthousand times larger than the real size.

mitochondria: organelles that providecells with a form of stored chemical energy

cellular respiration: the total of a seriesof chemical processes by which nutrients arebroken down to provide energy

adenosine triphosphate (ATP): acompound that temporarily stores chemicalenergy

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It is important to note that energy is not created in the mitochondria.Nutrient molecules such as glucose (a sugar) are transported into the cell. It is theprocess of breaking down chemical bonds in sugar molecules that releases energy.This energy is stored in ATP.

Mitochondria have two separate membranes: a smooth outer membraneand a folded inner membrane. The inner membrane consists of fingerlike pro-jections called cristae. Proteins called enzymes are located on the cristae. Theseenzymes speed up the reactions of cellular respiration.

The more active a particular cell is, the more mitochondria it will contain. Asa comparison, muscle cells have thousands of mitochondria, while adipose (fat-storing) cells have a much lower number.

The evolutionary origin of mitochondria presents one of the most baffling,but intriguing, questions for biologists. Mitochondria contain their own DNAunlike that found in the nucleus. Could this mean that the mitochondria wereonce separate organisms that invaded eukaryotic cells? The answer to this ques-tion is one of the many remaining mysteries of science.

Research in Canada: Killing Off Cancer Tumours

A difficulty with many methods of attacking tumour cells is that the tumourslearn to resist the agents we use against them. But as cancer researchers likeDr. Gurmit Singh (Figure 5) learn more about tumour biology, we can find newmethods of attack. Working at the Hamilton Regional Cancer Centre (andMcMaster University), Dr. Singh studies the way the mitochondria of a tumour’scells differ from those of normal cells. In healthy cells, any disruption of the mito-chondria causes cellular respiration to stop, and the cell ultimately dies. In tumourcells, the abnormal mitochondria allow the cells to continue to live and grow. Dr.Singh is trying to activate the normal cell death signals in tumour cells so that theywill die as normal cells do.

Ribosomes

Ribosomes are the organelles on which proteins are synthesized (Figure 6). Cellgrowth and reproduction require the constant synthesis of many different proteinmolecules. Proteins are composed of chains of smaller molecules called amino

acids. There are 20 different amino acids. The properties of a protein are deter-mined by the number and sequence of amino acids in the chain. Amino acids arechemically bonded together by enzymes at the ribosomes. The specific sequenceof amino acids is determined by instructions encoded in the DNA. A change inposition of a single amino acid can create a different protein.

Ribosomes are made of rRNA and proteins. Measuring just 20 nm in length,ribosomes are among the smallest organelles found in the cytoplasm. (There are

Although most pictures show mitochondriaas oval structures, scientists know that theshape can change quickly. Mitochondria swelland shrink in response to certain hormonesor drugs.

DID YOU KNOW ?

enzymes: protein molecules that increasethe rate at which biochemical reactions proceed

Figure 5

Dr. Gurmit Singh

Figure 6

A simpified illustration of a ribosome

amino acids: organic chemicals that canbe linked together to form proteins

16 Chapter 1

1 000 000 nm in 1 mm.) Yet, despite their minute size, ribosomes make up a greatportion of the cytoplasm. For example, it is estimated that in Escherichia coli, ribo-somes account for one quarter of the cell mass. The large number of ribosomespermits the simultaneous construction of many proteins within a single cell.

Endoplasmic Reticulum

vesicle

ribosomes

A network of interconnected canals carries materials throughout the cytoplasm.The canals, composed of parallel membranes, are referred to as endoplasmic

reticulum (ER). The membranes can appear either rough or smooth when viewedunder the electron microscope. The rough endoplasmic reticulum (RER) hasmany ribosomes attached to it (Figure 7). It is especially prevalent in cells thatspecialize in secreting proteins. For example, RER is highly developed in cells ofthe pancreas that secrete digestive enzymes. The smooth endoplasmic reticulum(SER) has no ribosomes attached and is the structure in which fats or lipids aresynthesized. SER is prevalent in cells of developing seeds and in animal cells thatsecrete steroid hormones.

Golgi Apparatus

Figure 7

Electron micrograph and diagram of roughendoplasmic reticulum. The micrograph showsthe endoplasmic reticulum as parallel yellow-and-green linear structures.

endoplasmic reticulum: a series ofcanals that transport materials throughoutthe cytoplasm

The Golgi apparatus (Figure 8) was first described by the Italian physicianCamillo Golgi in 1898. Golgi had stained cells from a barn owl and found a newcytoplasmic structure. Half a century later, electron microscopy confirmed Golgi’sobservations. The Golgi apparatus stores, modifies, and packages proteins fromthe rough endoplasmic reticulum. The Golgi apparatus looks like as a stack of flat-tened balloons, which are actually membranous sacs piled on top of each other.

Figure 8

Electron micrograph and diagram of the Golgiapparatus. The Golgi apparatus appears asred-and-green structures in the micrograph.

Golgi apparatus: a protein-packagingorganelle composed of membranous sacs

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These membrane-bound structures pinch off at the ends to produce smaller pro-tein-filled sacs called vesicles. The vesicles move toward the plasma membrane,fuse with it, and empty their contents outside the cell in a process called exocy-

tosis. Through exocytosis, large molecules, such as hormones and enzymes, arereleased from cells.

Vesicles can also be formed when the plasma membrane brings materialsinto the cell by a process called endocytosis (Figure 9). Many different vesiclesare produced within the cell and their main function is transport.

vesicles: small sacs or packets that arereleased by the Golgi apparatus. Vesicles areimportant in the processes of exocytosis andendocytosis.

exocytosis: a process by which particlesare released from a cell by fusing a particle-filled vesicle with the cell membrane

endocytosis: a process by which the cellmembrane wraps around a particle andpinches off a vesicle inside the cell

endocytosisexocytosis

Lysosomes

Lysosomes are organelles bound by a single membrane and formed by the Golgiapparatus. They contain a variety of enzymes that break down large moleculesand cell parts within the cytoplasm. Food particles that are brought into the cellare broken down into smaller molecules, which can then be used by the cell.Lysosomes are found only in animal cells.

Lysosomes also play an important role in the human body’s defence mecha-nism by destroying harmful substances that find their way into the cell. Whenwhite blood cells encounter and engulf invading bacteria, the lysosomes releasetheir digestive enzymes, destroying the bacteria and the white blood cell. Thefluid and protein fragments that remain after the cells have been destroyed makeup a substance called pus. The enzymes released from the lysosomes also destroydamaged or worn-out cells (Figure 10).

More than 30 different hereditary diseases have been linked to defective diges-tive enzymes in the lysosomes. Tay-Sachs disease, for example, is caused by an

Figure 9

Exocytosis and endocytosis

White blood cell digested material isreleased from cellenzymes digest

bacterium

lysosomesbeing formed

Golgi apparatus

lysosome fuses withvesicle containingbacterium

Figure 10

Acting as “suicide sacs,” lysosomes releaseenzymes that destroy damaged or worn-outcells.

lysosomes: vesicles that contain a variety ofenzymes able to break down large molecules

18 Chapter 1

enzyme deficiency that results in the accumulation of waste materials. A buildupof waste products in the cells of the body can cause brain damage.

Microfilaments and Microtubules

Microfilaments are pipelike structures found in the cytoplasm that help provideshape and movement for the cells (Figure 11). Muscle cells have many microfil-aments. Microtubules are tiny tubelike fibres that transport materials throughoutthe cytoplasm. Composed of proteins, the microtubules are found in cilia andflagella.

(a) (b) (c)

Figure 11

The structural organization of (a) micro-filaments and (b) microtubules. Diagram(c) shows how both structures form anetwork throughout a cell, attaching tothe cell membrane and organelles.

Research in Canada: Artificial Cells

Dr. Thomas Chang (Figure 12), a scientist at McGill University, invented artifi-cial cells in 1957 when he was still an undergraduate student. Because artificialcells function in much the same way as natural cells, they have the capacity toreplace biological cells.

Continuing research by Dr. Chang and other scientists around the world hasresulted in artificial blood cells and artificial cells being used in the treatment ofdiabetes and liver failure. Artificial blood cells are not true blood cells but rathermembrane-bound sacs containing modified hemoglobin molecules that can beused for short-term oxygen transport.

For patients suffering from blood poisoning, charcoal-filled artificial cellscan be used to filter toxins from the blood. In a procedure called hemoperfusion,a patient’s contaminated blood is circulated through a column filled with artifi-cial cells containing charcoal. The charcoal absorbs the toxins and remains in theartificial cells due to the artificial cell’s membrane, which acts as a barrier. Othertypes of artificial cells are being tested that would treat hereditary diseases andmetabolic disorders.

Practice

Understanding Concepts

1. Copy Table 1 into your notebook. Complete column 3.

Figure 12

Dr. Thomas Chang