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1● Earth
● Biosphere
● Biome
● Ecosystem
● Community
● Population
● Organism
● Systems
● Organs
● Tissues
● Cells
● Organelles
● Molecules
● Atoms
Cells in action
C H A P T E R
Chapter 1 Cells in action
3
Key knowledge● Cell structure: prokaryotic and eukaryotic cells at light and electron microscope levels; cellular
organisation
● Cell functioning; specialised parts of cells and their functions; biochemical processes including
photosynthesis and cellular respiration in terms of inputs and outputs; general role of enzymes in
biochemical activities of cells
Variety in living thingsAustralia is often described as a sunburnt country. This is indeed true when Victorian summer temperatures can reach as high as 47.2°C as it did in Mildura in January 1939. Did you know that temperatures can also drop as low as –11.7°C during winter as it did in Omeo in June 1965?
Humans can usually cope with these temperature ranges, as we are able to control our climate to a certain extent, but native plants and animals have to be able to survive in these extreme conditions and continue on with their daily activities of finding food and water, and reproducing. How do they manage to survive?
Cunningham skinks live in rocky areas in south-east Australia. Being lizards, they rely on the Sun to warm their body. They form family groupings that sleep together, eat together and bask together. In some places it is common to see a family of skinks basking on a warm rock, increasing their body temperature to the point where they are able to manage the morning defecation.
The tawny frogmouth is a species of nightjar that lives over most parts of the Australian continent. It has brown-grey feathers, roosts in trees and looks, for all intents and purposes, like a piece of bark. Being a bird, it is able to maintain a fairly constant body temperature at around 40°C. It is now known that frogmouths can survive the extreme cold of winter by going into a state of torpor, dropping their body temperature to as low as 29°C. A quick warm-up when the Sun comes out enables these birds to continue on with their daily routine.
Figure 1.1 Cunningham skinks.
Figure 1.2 Tawny frogmouth.
4 Unit 1
Figure 1.3 Grey saltbush.
R E V I E W1 Starting with the whole organism, zoom in on its structure. What are the different levels of structure?
Give an example of systems and organs from the above text.
2 Explain why the survival of the whole organism depends on the functioning of its individual cells.
bioTERMSsystemseveral organs that are interrelated and together perform a specific function
organsstructures of the body that perform one or more functions
tissuescells that work together to perform a similar function
Figure 1.4 Brush-tailed rock wallaby.
The grey saltbush is a common seaside plant. It grows in the shifting sands of coastal dunes and is able to endure the harsh winds, hot sun and salt spray that make up its everyday life. The floating seeds from this bush are now beginning to find their way across the ocean to New Zealand. These seeds have been found to germinate much better after a spell in seawater.
The brush-tailed rock wallaby lives amongst the rocky outcrops of the Grampians in western Victoria. Being a mammal, the rock wallaby maintains a constant body temperature and feeds its young on milk. The female can produce several different mixtures of milk at once, depending on the number and age of the young in its pouch. The adult rock wallaby is probably still growing and gains nutrition by feeding mainly on grass. It is constantly on the move.
In order to maintain this busy lifestyle the rock wallaby must have a constant supply of energy and raw materials to make new cells. It also needs a nervous system to coordinate all its varied activities, and much more. Later in this chapter we will explore how the activity of the rock wallaby’s cells helps its survival.
So how do these organisms survive in their environment? If it was possible to zoom in on each of these organisms, you would be able to see that they are made up of systems such as the digestive system, respiratory and reproductive systems. These systems are made up of organs that work together to perform specific functions; for example, the small intestine is an organ that digests and absorbs nutrients from the food that the organisms eat. As you get even closer you will see that each organ is made up of tissues, and that these tissues are made up of cells that work together to perform a similar function. If you could use a microscope to zoom in enough you would be able to see the individual cells within the tissue. Ultimately, the survival ability of each organism really depends upon the functioning of all its individual cells.
Chapter 1 Cells in action
5
Cells – the basic units of lifeThe four organisms discussed above are complex multicellular organisms; that is, they are made up of many cells. Let us investigate these smallest units of all living things – the cell.
The cell is the basic structural and functional unit of an organism. It is the smallest entity that still retains the properties of life. A cell can survive on its own or has the potential to do so. Its structure is highly organised and many chemical processes and reactions occur within it. A cell senses and responds to specific changes in its environment. It also has the potential to reproduce itself.
Cells differ enormously in size, shape and activities. Despite this, they have a remarkable similiarity in how they are structured, and what they do. These similarities and differences are used to group or classify cells, and the organisms that they make up, into smaller groupings.
The cell theoryIn 1838, Matthias Schleiden saw that each individual cell within a whole plant developed as an independent unit, and he thought that the nucleus probably had something to do with the development of each cell. In 1839, Theodore Schwann used his extensive knowledge of zoology and animal tissues to theorise that ‘Animals as well as plants consist of cells and cell products – and even though the cells are part of a whole organism, they have, to some extent, an individual life of their own’.
These observations, along with many microscopic examinations of a great variety of material, led Schleiden and Schwann to the belief that the great majority of organisms are composed of cells. This belief is embodied in the cell theory, which was proposed by these two scientists in 1839. The cell theory states that all living things are composed of one or more cells. The cell is the smallest entity that retains the properties of life.
Cell division was described for the first time in 1849 and this led to more information being added to the cell theory. In 1859, Rudolf Virchow proposed that all cells come from pre-existing cells. This had not been appreciated before as Schwann thought that new cells arose from tiny particles in the fluid between cells.
It is now known that organisms are made up of one of two different types of cells. The structure of these cells provides the groupings of organisms into kingdoms as seen in the Appendix (on the CD-ROM). The two different types of cells are prokaryotic cells and eukaryotic cells.
Prokaryotic cellsIf you have ever eaten yoghurt then you would have eaten the bacteria Lactobacillus acidophilus. You would be eating prokaryotic cells. These types of cells are extremely small, have a simple internal structure with no membrane-bound organelles, and no membrane-bound nucleus. Prokaryotic cells are grouped within the kingdoms Bacteria and Archaea and include cyanobacteria and bacteria (see the Appendix, on the CD-ROM).
Figure 1.5 Lactobacillus acidophilus – a prokaryotic cell.
PRA
CTIC
AL ACTIVITY 1.1
bioTERMSorganellessmall structures within a cell that perform a specific function
Prokaryotic cells Eukaryotic cells
Cells of living things
6 Unit 1
capsule
bacterial flagellum
cell wall
pili
ribosomes
chromosome – DNA
plasma membrane
plasmid – DNA
cytoplasm
cell wallchromosome– DNA
ribosome
capsule cytoplasm
pili
plasma membrane
plasmid – DNA
Eukaryotic cellsSome single-celled organims and all multicellular organisms are made up of eukaryotic cells. Eukaryotic cells have a complex internal structure with many membrane-bound organelles and a membrane-bound nucleus as discussed below. Eukaryotic organisms are grouped within the kingdoms Protista, Fungi, Plantae and Animalia. Some single-celled organisms can have eukaryotic cells, for example those in the kingdom Protista (see the Appendix on the CD-ROM).
Figure 1.6 (a) Generalised diagram of a prokaryotic cell (b) A line drawing of the cell.
Figure 1.7 Summary of the characteristics of living cells.
a b
Kingdom Monera(bacteria)• single celled• lack distinct nucleus• one circular
DNA strand• lack membrane-bound
organelles• non-cellulose cell wall
Kingdom Plantae(plants)• multicellular• nucleus present• membrane-bound
organelles• chloroplasts present• cellulose cell wall• usually large vacuole
Kingdom Fungi• normally multicellular• nucleus present• membrane-bound
organelles• no chloroplasts• non-cellulose cell wall• vacuoles present
Kingdom Archaea• unicellular and prokaryotic• extremophiles – heat lovers and salt lovers• single circle of DNA• different cell wall structure to bacteria
Kingdom Protista• mainly single celled
or in colonies• nucleus present• membrane-bound
organelles• chloroplasts present in
many species but may be absent
• different types of cell walls
• vacuoles present
Kingdom Animalia(animals)• multicellular• nucleus present• membrane-bound
organelles• no chloroplasts• no cell wall• vacuoles small
or absent
7
Chapter 1 Cells in action
Table 1.1 Comparison of prokaryotic and eukaryotic cells.
Characteristic Prokaryotic cell Eukaryotic cell
Organelles Possess no internal membrane bound organelles
Possess internal membrane-bound organelles
Chromosomes Single and free Multiple and inside the nucleus
Nucleolus Absent Present
Ribosomes Present Present but different sizes to prokaryotic ribosomes
Endoplasmic reticulum Absent Present
Microtubules Absent Present
Enzymes used for cellular respiration
Attached to plasma membrane Attached to internal membrane of mitochondria
Chloroplasts Absent: enzymes of photosynthesis attached to lamellae
Present: enzymes of photosynthesis attached to internal membranes of chloroplasts
Cell wall Present but not made of cellulose Present in plants and fungi
Taking on different jobsMost cells are too small to see without the aid of a microscope. Red blood cells, for example, are about 8 millionths of a metre wide – about 2000 of them would fit across your thumbnail. Some eukaryotic cells, however, can be observed with the unaided human eye, such as the yolks of bird eggs, cells in the red part of watermelons and fish eggs, called caviar. Some nerve cells can be over 1 m in length.
Different types of cells share many similar features.What do the yolks in birds’ eggs,
caviar and human nerve cells have in common? There are many answers to this question:1 They all actively respire (the
chemical process that releases energy from glucose).
2 They are all cells that have become specialised to perform a specific function.
3 They are all single cells that can be seen without the aid of a microscope.Cells generally grow to the size they need to be in order to perform their specific
function. Nerve cells, for example, connect the spinal cord to muscles. If you have just put your bare foot onto a piece of jagged glass, you will need to lift it up quickly. Passing information between cells takes time. To ensure that there is a rapid transmission between the spinal cord and the muscles, there is only one long, thin cell connecting the two. No time is lost passing information from one cell to the next. Red blood cells on the other hand are very small as they need to be able to squeeze through capillaries, our smallest blood vessels.
Figure 1.8 Human nerve cells can be over 1 m in length.
8 Unit 1
R e v i e w3 Draw a time line to show the development of the cell theory.
4 State the complete cell theory.
5 a List two similarities in the structure and functioning of
prokaryotic and eukaryotic cells.
b List two differences in the structure and functioning of
prokaryotic and eukaryotic cells.
6 Refer to Figure 1.7 and list the kingdoms that contain organisms
with prokaryotic cells, and the kingdoms that contain organisms
with eukaryotic cells.
7 Explain why cells can differ so much in size.
8 Explain why cells are usually very small.
typical plant cell10–100 µm
Escherichia coli (bacterium)1–5 µm long
tobacco mosaic virus 300 nm long
polio virus30 nm
human red blood cell7–8 µm diameter
Chlamydomonas(green alga)5–6 µm
Trypanosoma(protozoan)25 µm long
HIV (AIDS virus)100 nm
T4 bacteriophage225 nm long
The real limiting factor on cell size is its ability to supply the interior of the cell with its requirements and to get rid of waste products. This all happens across the surface area (plasma membrane) of the cell. As a cell grows, it generally increases more quickly in volume than in surface area. As it grows, it reaches a point where the inward movement of requirements and the outward movement of wastes across the surface area are not fast enough to allow the cell to grow any more and still function efficiently.
Keeping within boundariesEach cell is an independent unit, separated from other cells and its environment by the plasma membrane (cell membrane), the outermost barrier of a cell. The plasma membrane is composed of lipid molecules that are interspersed with tiny protein channels. It is through the protein channels that nutrients and water enter a cell and wastes are released. The structure and function of the plasma membrane is studied in detail in Chapter 3.
Figure 1.9 Cell size.
bioTERMSµm micron, 10–6 of a metrenm nanometre, 10–9 of a metre
biolinK
Cell size
9
Chapter 1 Cells in action
temporary vacuole – a temporary storage sac
plasma membrane – outermost barrier of the cell (selectively allows some substances to pass through it)
cytoplasm – fluid material where the activities of the cell occur
endoplasmic reticulum – intracellular and intercellular transport system
mitochondrion – site of cellular respiration
cell nucleus – coordinates all the activities of the cellnucleolus – involved
in the manufacture of proteins within the cell
ribosomes – site of protein synthesis
Golgi apparatus – system of membranes that package and store substances before their release
plasma membrane
temporary vacuole
nucleolus
cell nucleus
mitochondrion
cytoplasm
endoplasmic reticulum
ribosomes
Golgi apparatus
Figure 1.10 (a) A eukaryotic animal cell showing its plasma membrane and other organelles (b) A line drawing of the cell.
a
b
10 Unit 1
cytoplasm – fluid material where the activities of the cell occur
chloroplast – site of photosynthesis
nucleolus – involved in the manufacture of proteins within the cell
large permanent vacuole – a fluid-filled space that stores various materials
cell wall – provides extra support and protection to plant cells
mitochondrion – site of cellular respiration
plasma membrane – outermost barrier of the cell (selectively allows some substances to pass through it)
cell nucleus – coordinates all the activities of the cell
a
chloroplast
large permanent vacuole
mitochondrion
nucleolus
nucleus
cell wall
plasma membrane
cytoplasm
Figure 1.11 (a) A eukaryotic plant cell showing its plasma membrane, cell wall and other organelles (b) A line drawing of the cell.
b
R e v i e w
11
Chapter 1 Cells in action
9 a Name the structure that separates all cells from their outside environment.
b How do substances move through this structure?
10 What is the function of the cell wall?
11 In what organisms do you find a cell wall? What are their cell walls composed of?
12 Where do you find the cytoplasm in cells?
bioTERMSmitochondriaorganelles within the cytoplasm that are the site of aerobic respiration releasing energy for the cell
studEnt Cd
Plasma membrane
Animal cells have only a plasma membrane, whereas the cells found in plants, bacteria, fungi and most algae have the additional cell wall. The cell wall surrounds the plasma membrane and provides extra support and protection to these cells. The presence of a cell wall is one of the main differences between the major groupings of eukaryotic cells.
The cell wall in plant cells is composed of cellulose, a complex carbohydrate molecule. Some cells have a single cell wall, known as a primary cell wall. If extra support is needed, very rigid additional or secondary cell walls can be found. A tree trunk has the function of supporting the whole leaf canopy of the tree. The cells that make up the tree trunk therefore have to be very strong. These cells have a plasma membrane, a primary cell wall and a secondary cell wall. As these cells age, they die and lose their contents and plasma membrane, leaving only the cell walls intact. This creates long tube-like cells, ideal for carrying water from the roots to the leaves.
The cells that make up fungi have a cell wall that is composed of chitin, a polysaccharide. Bacterial cell walls are made up of proteins and polysaccharides (see Chapter 2).
Inside the boundaryExamine the light microscope images of cells in this section of the book. You should be able to see a granular or ‘spotty’ substance that is apparent between the plasma membrane and the nuclear membrane. This is the cytoplasm. This is where the activities of the cell are carried out. The cytoplasm is made up of a highly organised fluid material containing dissolved substances, called cytosol, and many membrane-bound organelles.
The power supplyThe rock wallaby carries out many functions in order to survive: sleeping, eating, drinking and fighting to name but a few. In order to be able to perform these tasks it must have a supply of energy. For its species to survive, it must also be able to reproduce. Inside the male rock wallaby’s reproductive system is an organ the function of which is to produce the male sex cell, sperm. The task of the sperm cell is to fertilise the egg in the reproductive tract of the female rock wallaby. In order to get there it must swim – very fast.
If you could zoom in on an individual sperm cell you would see that it is has a head that is nearly all nucleus. It contains the genetic material from the male parent. Behind the head is a section at the start of the tail that contains many tightly packed rod-shaped organelles called mitochondria. The rest of the tail thrashes rapidly to propel the sperm cell forward.
Figure 1.12 A plant cell of the Canadian Yew showing the primary and secondary cell wall.
12 Unit 1
cristae
outer membraneinner membrane
stalked particles on surface of crista
matrix
matrix
inner membraneouter membrane
cristae
Mitochondria and the cytoplasm are the sites of cellular respiration. Cellular respiration is a series of chemical reactions that involve a reaction between glucose and oxygen to produce carbon dioxide, water and heat energy:
glucose + oxygen carbon dioxide + water + heat energyC
6H
12O
6 + 6O
2 6CO
2 + 6H
2O
During certain stages of these chemical reactions energy is released and this is used to form ATP molecules. ATP is an energy storage molecule that is used by the cell to power cellular processes. In this example this energy is used to move the tail and make the sperm cell move forward.
Some of the reactions of cellular respiration occur in the cytoplasm, but the complete breakdown of glucose occurs on the internal membranes of the mitochondria. These membranes provide two important features to cellular respiration. First, their numerous foldings provide a large surface area for the reaction to occur and, second, the enzymes necessary for the chemical reactions are found there.
Figure 1.13 Human sperm cells.
bio B Y T EThe addition of extra
mitochondria to egg cells has been found to increase the chance of pregnancy in women who have so far failed to conceive.
Figure 1.15 An electron micrograph of a mitochondrion in longitudinal section (magnification 80 000).
Figure 1.14 Internal structure of a mitochondrion.
BIOLINK
Cell structure and function
13
Chapter 1 Cells in action
microtubule
mitochondrion
microfilaments
plasma membrane
endoplasmic reticulum
EnzymesEnzymes are organic catalysts that speed up chemical reactions such as cellular respiration. Without enzymes the reactions that occur in living things and their cells would be so slow as to hardly proceed at all. Enzymes are involved in building up reactions, such as photosynthesis, and breaking down reactions, such as cellular respiration. You will study enzymes in more detail in Chapter 2.
Building cell structuresProteins are needed for organisms to be able to grow, repair damage and make new cells. All types of cells therefore should have organelles that manufacture proteins. Sure enough, whether we look at eukaryotic or prokaryotic cells, small organelles called ribosomes are found. Furthermore, cells that produce proteins that will be used in other tissues, such as hormone-producing cells, have an abundance of ribosomes.
Ribosomes are small structures visible only with an electron microscope. They are found scattered freely throughout the cytoplasm and build simple amino acids into proteins that are used within the cell. They can also be found attached to the endoplasmic reticulum (see below) and these ribosomes produce proteins (e.g. enzymes and hormones) that are exported for use in other cells.
Supporting cell structureIf a cell is essentially a fluid-filled sac, what stops it from being flattened, ruptured and squashed? This doesn’t happen because eukaryotic cells have an internal skeletal structure called a cytoskeleton. The cytoskeleton is a three-dimensional structure that occurs in the cytoplasm and provides shape to the cell. It is made up of structures called microtubules and microfilaments.
Microtubules are hollow, cylindrical tubes approximately 20 nm in diameter. They act as a scaffold to determine cell shape. They also provide a set of ‘rails’ for the cell organelles to travel around the cytoplasm. This allows the constant mixing and movement of the cytoplasm known as cytoplasmic streaming. This can be seen in a living Elodea leaf under the light microscope. The chloroplasts can be seen to be slowly moving around the vacuole and other cell structures.
One of their more remarkable properties is the apparent ease with which microtubules come apart and reassemble. They can be assembled in one part of the cell where they are needed, then taken apart and reassembled later in another part of the cell.
Animal cells contain two rod-like structures that are located at right angles to each other. These structures are called centrioles. One of the roles of centrioles is to organise and produce microtubules.
Centrioles are very prominent in animal cells that are about to divide. They replicate themselves just before cell division begins to produce two pairs. They give rise to the spindle fibres onto which chromosomes attach themselves. When the spindle fibres contract the attached chromosomes can be moved around the cell (see Chapter 6).
Microfilaments are contractile proteins about a quarter of the diameter of microtubules. They are solid and not tubular. Like microtubules, they can be readily assembled and disassembled. They occur in bundles in the cytoplasm and when they contract they can cause the cell to change shape; this is especially apparent in the contraction of muscle cells.
bioTERMSribosomessmall structures in cells that build amino acids into complex proteins
cytoskeletonthe system of microtubules and microfilaments within a cell that supports and gives shape to it, helps movement and reproduction
microtubuleshollow, cylindrical tubes in cells that act as scaffolding to determine cell shape and aid movement
spindle fibresmicrotubules, produced during cell division, that move chromosomes in precise directions
chromosomesstructures made of a DNA molecule with associated proteins
Figure 1.16 The cytoskeleton of a cell.
14 Unit 1
R e v i e w13 Which organelle allows cells to access energy so they can carry out
activities? Explain how it does this.
14 Explain why you would expect human muscle cells to contain
more mitochondria than a cell in your big toe.
15 What is the function of ribosomes? Why would you expect to find
more ribosomes in a hormone-producing cell than a skin cell?
16 What structure assists cells in keeping their three-
dimensional shape?
17 a State one difference and one similarity between microtubules
and microfilaments.
b What other roles do these structures have?
Transport within the cellProteins, such as those that make up microfilaments, are produced within the ribosomes. But how does a cell manage if a certain protein is produced in one part of the cell, and required for use in another part of the cell?
The endoplasmic reticulum is an interconnecting system of thin membrane sheets dividing the cytoplasm into compartments and channels. Substances are able to move around the cell within these channels. The membrane of the endoplasmic reticulum is able to pinch off into small sacs called vesicles and deliver proteins to all parts within
the cell. The endoplasmic reticulum is therefore an intracellular transport system.Most of the endoplasmic reticulum in cells is studded with ribosomes and
thus is known as rough endoplasmic reticulum. In this way the proteins produced by the ribosomes can move directly into the endoplasmic reticulum and move about the cell. Many of these proteins, however, are not required by the cell in which they are made but are to be exported or secreted into other cells. Such proteins include enzymes and hormones. Therefore, the endoplasmic reticulum is also an intercellular transport system helping to move proteins from one cell to another.
In certain parts of some cells, the endoplasmic reticulum has no ribosomes attached to it and is known as smooth endoplasmic
reticulum. The amount and function of this smooth endoplasmic reticulum depends on the type of cell it is located in. Its main role is to
transport proteins, synthesise lipids, and to assist in the manufacture of plasma membranes. In liver cells it also detoxifies drugs, and in adrenal cortical
cells it produces the steroid hormone.
Packaging and distributionThe main diet of the rock wallaby is grass. The cells in grass, a plant, have a cell wall. In order to be able to digest and absorb the nutrients from inside the grass, the cell wall must be broken down by enzymes. Cells in the digestive glands of the rock wallaby produce such enzymes. Being protein, the digestive enzyme is produced initially by the ribosomes on the rough endoplasmic reticulum. It moves through the channels within the endoplasmic reticulum where it is secreted within the cytoplasm of the cell. From there it moves into the Golgi apparatus where different enzymes put the final touches to it, and it is packaged and stored before being secreted from the cell to move into the intestines of the wallaby. This is where it can begin its work digesting the cellulose in the cell wall of the grass.
Figure 1.17 Endoplasmic reticulum studded with ribosomes (rough).
bioTERMSintracellularoccurring within a cell
intercellularoccurring between cells
15
Chapter 1 Cells in action
R E V I E W13 Which organelle allows cells to access energy so they can carry out
activities? Explain how it does this.
14 Explain why you would expect human muscle cells to contain
more mitochondria than a cell in your big toe.
15 What is the function of ribosomes? Why would you expect to find
more ribosomes in a hormone-producing cell than a skin cell?
16 What structure assists cells in keeping their three-
dimensional shape?
17 a State one difference and one similarity between microtubules
and microfilaments.
b What other roles do these structures have?
The Golgi apparatus (or Golgi body) was first discovered in the brain cells of owls by the Italian physician Camillo Golgi at the end of the 19th century. It consists of a system of membranes within the cytoplasm. Parts of the Golgi apparatus membrane are able to pinch off into small sacs called vesicles. It is these vesicles that move to the plasma membrane, where they join the membrane and discharge their contents to the outside of the cell (Figure 1.19).
Figure 1.19 How the Golgi apparatus removes a secretion from a cell.
Recycling and reuseInevitably, organelles within the cytoplasm of cells reach their ‘use by’ date and wear out. Instead of wasting the raw materials that make up these organelles, the cell has an ingenious method of recycling and reuse. This is the job carried out by lysosomes (‘lysis’ means to break apart), one of the special organelles found within the cytoplasm of animal cells. Lysosomes are formed by the Golgi apparatus. They contain digestive enzymes that are responsible for splitting complex chemical compounds into simpler ones. These simpler ones can then be used as building blocks for new compounds and organelles.
Sometimes lysosomes may destroy the entire cell. This happens when the lysosome membrane ruptures, liberating the enzymes, which then digest the contents of the cell, killing it in the process. This is known as apoptosis or cell suicide. Lysosomes are therefore sometimes referred to as ‘suicide sacs’.
Figure 1.18 Electron micrograph of the Golgi apparatus (magnification 80 000).
bioTERMSapoptosisprogrammed cell death
vesicles pinched off roughendoplasmic reticulum fuseto form flattened cavity
vesicle fuses with plasmamembrane and releasessecretion
vesicle containingsecretion pinched offflattened cavity
Golgi apparatusstack of flattened cavitieslined with a smoothendoplasmic reticulum
rough endoplasmic reticulum
BIOLINK
Diversity of cells
R E V I E W
16 Unit 1
plasma membrane
cytoplasm cytoplasm
a b
transport into cell transport out of cell
endocytosis exocytosis
transport of liquid
pinocytosis
transport of particles
phagocytosis
Moving in and outOther molecules must also be able to move in and out of cells. Many are too large to move passively across the plasma membrane, so there must be another way. The plasma membrane has an ingenious solution to this problem. It can sink in and engulf large particles and liquids within its environment in a process known as endocytosis. It encloses the material within it to form an endocytic vesicle, which then stores or transports the material within the cytoplasm.
Conversely, exocytic vesicles are associated with transporting large molecules and particles across the plasma membrane and out of the cell. During exocytosis, a small membrane-bound vesicle moves through the cytoplasm to the plasma membrane, where it joins with it and then releases its contents to the exterior of the cell.
You will study endocytosis and exocytosis in more detail in Chapter 3.
Figure 1.20 The process of (a) exocytosis and (b) endocytosis.
18 What are the two main roles of the endoplasmic reticulum?
19 State the difference between smooth endoplasmic reticulum and rough endoplasmic reticulum.
20 Explain the role of the Golgi apparatus in the transport of materials out of the cell. What kinds of materials does it package?
21 Why are lysosomes like cell recycling stations?
22 Explain one similarity and one difference between endocytosis and exocytosis.
Figure 1.21 Transport of substances into and out of cells.
17
Chapter 1 Cells in action
a
b
nucleolus
nucleus
Coordinating cell activitiesAs you can see cells carry out many and varied tasks, usually at the same time. To be an efficient system a cell needs to have some way of coordinating all of these activities. This is a main function of the nucleus. The nucleus is one of the most noticeable features you can observe in a eukaryotic cell. (see Figure 1.10).
DNA (deoxyribonucleic acid) is the main molecule found within the nucleus. DNA codes for the production of proteins that carry out a variety of activities within the cell.
The nucleus is separated from the rest of the cell by the porous nuclear membrane (or envelope). It is composed of a fatty substance (lipid) with small holes or pores within it, as shown in Figure 1.23. This allows charged particles (ions) and small water-soluble molecules to move freely across it.
A dark-staining structure within the nucleus is called the nucleolus. One or more of these can be seen in cells when they are not dividing. The nucleolus is made of protein and a type of nucleic acid called ribosomal RNA (ribonucleic acid). Nucleoli are involved in the manufacture of proteins in the cell.
Figure 1.22 (above) Transmission electron micrograph of the nucleus of a pancreas acinar cell (magnification 17 000).
Figure 1.23 Electron micrograph of a porous nuclear membrane (a) seen in section (arrowed) and (b) surface view (magnification 75 000).
b
18 Unit 1
two outer membranes
granum
stromathylakoid membrane
inner membrane system
a
Different ways of doing thingsThe Australian eucalypt is found in various forms across the continent. The majestic river red gum with its green-grey leaves lines the banks of the Murray River. The smaller, shiny green-leafed snow gum graces our Alps. The stunted red mallee gum, so called because of its newly grown red leaves, grows in scrub regions. Many eucalypts have spectacular flowers of red, pink, yellow or cream.
The colourful presentation of these trees’ leaves and flowers is due to a group of organelles called plastids. Plastids are organelles that contain coloured pigments. The three general types of plastids are chloroplasts, chromoplasts and leucoplasts.
Eucalypts, like all other plants, produce their own simple sugars through the chemical reactions that make up photosynthesis. The energy that they need to power photosynthesis comes from the Sun. How do plants utilise this energy source when it is so far away? Plants have leaves and sometimes stems whose cells contain chloroplasts. These are oval-shaped organelles containing green pigment called chlorophyll.
Figure 1.24 Snow gums.
Figure 1.25 (a) Generalised sketch showing the grana and stroma of a chloroplast (b) False colour transmission electron micrograph of a chloroplast from Coleus blumei (magnification 5000).
BIOLINK
Cells
19
Chapter 1 Cells in action
Chlorophyll is able to absorb light energy and make it available for use in photosynthesis. Photosynthesis is a series of reactions that occur in the stroma and thylakoid membrane system of the chloroplast (see Figure 1.25). During these reactions carbon dioxide and water are combined to produce glucose, oxygen and water:
carbon dioxide + water glucose + oxygen + water6CO
2 + 12H
2O C
6H
12O
6 + 6O
2 + 6H
2O
The internal membranes of the chloroplast are folded many times to provide more surface area for chemical reactions of photosynthesis to occur. They are also associated with the enzymes necessary to speed up the chemical reactions involved.
The red mallee gum has juvenile leaves that are red. It is the chromoplasts that contain coloured pigments (other than green), which give the red mallee gum leaves their characteristic colouration. They have carotenoid but no chlorophyll pigments. They turn green as they mature and produce chlorophyll. Carotenoid results in the red-yellow colouration seen in some leaves, flowers, fruits and roots such as carrots.
Leucoplasts do not contain any pigments; they are colourless plastids. They include amyloplasts, which often store starch grains (see Figure 1.26).
Moving things aboutIn addition to the magnificent colours of the Australian eucalypt, another stunning feature is the height to which some species are able to grow. Eucalyptus globulus, the southern blue gum, has been known to grow as tall as 70 m. All the photosynthetic tissue is in the leaves, so how do the actively respiring root cells obtain the sugars they need for cellular respiration? How do the leaves obtain the water they need for photosynthesis?
The answer lies in cell specialisation, where cells have taken on special features to enable them to carry out their task. Complex, multicellular plants have two transport systems that make up their vascular system – the xylem and the phloem.
The xylem is concerned with the transport of water and mineral ions from the roots to the leaves. Xylem tissue is made up of two main types of cells, the tracheids and the vessels. Both of these types of cells are dead and hollow, which makes them very suitable for conducting water and mineral ions. Tracheids are long cells with pointed ends and many holes in the walls. Since their sloping ends lie next to each other water can flow from one cell to the next. Vessels are formed from cylindrical cells standing end-to-end. Their end walls initially have little holes in them, but the ends gradually break down, creating long continuous tubes. Their side walls are thickened with lignin spirals and rings, making them very strong.
Figure 1.26 Leucoplasts (the darker, smaller circles) in onion root cells.
Figure 1.27 Scanning electron micrograph of ash tree showing xylem with pits.
bioTERMSstromathe jelly-like semifluid interior of a chloroplast
thylakoidinterconnecting folded membranes within chloroplasts
lignina complex carbohydrate found in thickened cell walls of xylem vessels
20 Unit 1
Companion cells Sieve cells
plasmodesmata connecting sieve cell with companion cell
nucleus
cytoplasm contains numerous organelles
sieve plate
sieve pore
cytoplasmic filaments
endoplasmic reticulum fragment
organelles
cellulose wall
The phloem transports sugars in solution through the plant. Phloem tissue consists mainly of sieve cells and associated companion cells (see Figure 1.28). Sieve cells are long and tubular in shape and have sieve-like plates at their end walls. Sieve cells are joined end to end to form sieve tubes and it is through the sieve tubes that the sugar solution flows. They have no nucleus but each sieve cell has a companion cell next to it. The companion cell has a nucleus and it controls the functioning not only of itself but also of its neighbouring sieve cell.
VacuolesA large part of the cytoplasm in mature plant cells is composed of a fluid-filled space called a vacuole. The fluid or cell sap in the vacuole serves as a storage space for sugars, minerals, proteins and water. The vacuole is able to expand, often taking up 50 to 90% of the volume of the cell. As the size of the vacuole increases, more and more pressure is exerted on the cell wall. This forces the flexible cell wall to bulge, thus increasing the size of the whole cell. As the size of the vacuole increases, the remaining cytoplasm becomes a narrow band between the plasma membrane and vacuole.
Moving from place to placeMany cells need to be able to move around in order to meet their requirements for survival, or to perform a specific function. Some cells actively move, such as sperm cells with their thrashing flagella. Others are moved about because the medium in which they are located moves them, such as red blood cells moving in the plasma that makes up blood.
Figure 1.28 Sieve and companion cells. Sieve cells have no nucleus and few organelles, which are pushed up against the cell wall. Companion cells have a prominent nucleus and their cytoplasm contains numerous organelles.
Figure 1.29 Plant cell showing vacuole. Notice how the cell wall is extended.
R E V I E W23 Which organelle controls the functioning of the whole cell?
24 How is the nuclear membrane similar to the plasma membrane?
25 Where is DNA found? Where is RNA found?
26 Name the three general types of plastids. What is the function of
each type?
27 Explain how cell specialisation provides an answer to a survival
problem, using xylem or phloem as your example.
28 When plants do not have enough water, they wilt. Once watered,
they will stand up straight again. Explain the role of the vacuole in
this process.
21
Chapter 1 Cells in action
R E V I E W29 Why do cells have to be able to move from place to place?
30 State two different ways in which cells are able to move.
31 What function do cilia have, other than moving a cell?
32 Give one reason why red blood cells do not have flagella to help them move.
Figure 1.30 Paramecium is able to move around using cilia.
PRA
CTIC
AL ACTIVITY 1.2Active movementThe single-celled eukaryotic organism, Paramecium, lives in freshwater ponds. It cannot photosynthesise. It obtains its nutrients from its watery surroundings. In order for it to gain sufficient nutrients, Paramecium must move around within its environment. If you look carefully at Figure 1.30 you will see fine hair-like structures covering the outer surface of the Paramecium. These are called cilia. The rhythmic waving motion of the cilia propels it through its surroundings.
The human sperm cell has to be able to propel itself through a watery environment to reach the ovum and achieve fertilisation. It does this by the means of a long whip-like tail called a flagellum (see Figure 1.13).
Cilia are usually shorter and more numerous than flagella but are otherwise similar. Both have the ability to wave up and down or lash back and forth and their functions depend on this.
Flagella are nearly always associated with locomotion but cilia, which are more common, perform other functions as well. For example, they are often found lining ducts, tubules, and other specialised surfaces (e.g. lung tissue) along which materials are moved by means of their rapid and rhythmical beating.
Flagella and cilia contain microtubules (see page 13) arranged in organised patterns.
Passive movementRed blood cells move through the human circulatory system. As they move, they release oxygen to be used by actively respiring cells. They return to the lungs to load up with more oxygen before they begin their journey again. Red blood cells do not have cilia or flagella; rather they are able to move because the medium which surrounds them – the blood – is being actively pumped through the blood vessels by the heart.
Some single-celled organisms move around in lakes and rivers because the water currents carry them. In this way they are removed from areas which they have depleted of food stocks and filled with waste material, to new areas full of new resources.
The discovery of cellsWe are indebted to the early microscopists for our current knowledge about cell structure and function. Although their techniques were crude by today’s standards, they paved the way for developments in microscopes that they would never have dreamt of.
In the early 1600s, Galileo Galilei put together some glass lenses in a cylinder and found that they magnified objects. Although he was not a biologist, he was the first person to observe and record a biological specimen through a microscope. He observed and was fascinated by the incredible geometric pattern of the tiny eyes of an insect.
22 Unit 1
This early work by Galileo was the beginning of the study of cells as the basis of life. Across Europe, from Italy, then in France and England, scientists began
to explore the microscopic world, the existence of which had never been suspected.
By the middle of the 17th century, a curator of instruments for the Royal Society of England, Robert Hooke, used a
microscope to observe thinly sliced cork from a mature tree. When you look at Figure 1.31 you might be able to see a
resemblance to what Hooke described as small rooms. He used the Latin name cella (meaning ‘small room’) for these structures, having no idea that they were actually living components of the tree.
By the late 17th century a Dutch shopkeeper, Anton van Leeuwenhoek, used his skills in making lenses to construct microscopes which, although they were still very simple, improved the magnification and clarity of the many specimens that were being observed.
In 1831, Robert Brown, using improved lenses, was able to focus sharply on plant cells. He noticed that most
cells contained an opaque spot, which he called a nucleus.
Different kinds of microscopes and microscopy
The type of microscope you use in your school laboratory is a light (or optical) microscope. This is the same type of microscope used by scientists when
cells were first discovered. If you look at material with a simple light microscope, fine structures within the cell will not be visible. Technology that is more sophisticated is needed to view the smallest parts of cells. Our knowledge of cell structure and function has been greatly advanced in recent years because of advances in the types of microscopes.
Cells are generally colourless and it is difficult to see detail using a light microscope. Stains and dyes can be added to cells to make some features more prominent. The disadvantage of this is that the stain kills cells, so is no good for looking at living cells.
Light rays from a light source beneath the stage are transmitted through two glass lenses in series, the objective and ocular (eyepiece) lenses. Depending on their strength, these two lenses together provide magnifications of up to 400 times.
In theory it might seem possible to magnify an object indefinitely by adding more glass lenses in series, but in practice this produces only a larger, fuzzier picture; the resolution is not improved and no more detail is visible.
Therefore, it seems that even though the invention of the light microscope has had a profound influence on biology, there is a limit to the amount of detail (or resolving power) that it can show.
Since the 1950s, microscopic studies have been revolutionised by the development of the electron microscope. This instrument uses an electron beam instead of light, and electromagnets instead of glass lenses. The electrons are recorded on a photographic plate, which then forms a viewable image on a screen.
Figure 1.31 Robert Hooke’s drawing of cork cells.
bioTERMSlight microscopea microscope that uses light rays to enlarge an image of a specimen through glass lenses
BIOLINK
Microscopes and magnification
b
23
Chapter 1 Cells in action
a
Figure 1.32 (a) A high-resolution electron microscope currently used in biological research. (b) A scanning electron micrograph of a tick. Note the pittings on the surface of its cuticle (magnification 80).
BIOLINK
SEM
BIOBOX 1.1DIFFERENT KINDS OF MICROSCOPES AND MICROSCOPY
The electron microscope can give clear pictures that are magnified 500 000 times. It is important to appreciate what this
means in practice: with the electron microcope an object the size of a pinhead can be enlarged to the point at which it
has a diameter of well over a kilometre; a cell with a diameter of 10 µm finishes up with a diameter of 5 m.
The electron microscope has had a profound effect on biology. Materials that we formerly described as
structureless have been shown to have an elaborate internal organisation, and so-called homogeneous fluids are now
known to contain a variety of complex structures. The electron microscope has opened up a new world whose existence
was barely realised 50 years ago.
But there are problems. The material for examination has to be mounted in a vacuum, and is therefore dead
before it can be viewed. This, coupled with the preliminary treatment to which the material has to be subjected, may
distort the delicate structures inside cells and create images that are not real.
These are called artefacts (‘of artificial making’). The electron microscopist is always on the look-out for such
artefacts and uses every means to prevent them occurring.
The electron microscope shown in Figure 1.32(a) is called a transmission electron microscope because the
electrons pass through the specimen. In the more recently developed scanning electron microscope (SEM), solid
specimens are bombarded with a beam of electrons, which causes secondary electrons to be emitted from the surface
layers of the specimen. These electrons are recorded on a photographic plate and the image is viewed on a screen, as
with the transmission electron microscope. The scanning electron microscope enables details of the surface to be seen
very clearly. However, it can only magnify up to about 80 000 times.
24 Unit 1
Visual summary
lysosomes
Golgi apparatus
endoplasmic reticulum
ribosomes
mitochondria
nucleus – DNA
Eukaryotic
Plantae
Animalae
Protista
Fungi
cytoskeleton cell shape
chloroplasts
endocytosis exocytosis
cells
Plasma Membrane tissues
organsProkaryotic Cell
single cell
no membrane-bound organelles Cytoplasm
control centre
Archaea power supply
construct proteins
support and protection
intra- /inter- cell transport
transports water and minerals
package and export
transports sugar phloem
recycle and reuseleucoplasts chromoplasts
plastids
Plants Only
bacteria
cell wall
xylem
membrane-bound organelles
multicellular systems
25
Chapter 1 Cells in action
Key termsapoptosis cytosol microfilaments ribosomes
cell DNA microtubules sieve cells
cell specialisation electron microscope mitochondria smooth endoplasmic reticulum
cell theory endocytosis multicellular spindle fibres
cellular respiration endoplasmic reticulum nm stroma
cellulose enzymes nuclear membrane system
centrioles eukaryotic nucleolus thylakoid
chlorophyll exocytosis nucleus tissue
chloroplasts flagellum organ tracheids
chromoplasts Golgi apparatus organelles vacuole
chromosomes intercellular phloem vesicles
cilia intracellular photosynthesis vessels
companion cells leucoplasts plasma membrane xylem
cytoplasm light microscope plastids µm
cytoplasmic streaming lignin prokaryotic
cytoskeleton lysosomes rough endoplasmic reticulum
Apply understandings If you were given an unknown cell, how
would you be able to tell if it were:a eukaryotic or prokaryotic?b from a plant or animal?
Match each structure with its function.
Organelle/structure
Function
i nucleus a collecting and packaging centre of the cell
ii endoplasmic reticulum
b photosynthesis and storage
iii lysosome c transport of substances around the cell
iv mitochondria d control centre of the cell
v Golgi apparatus
e aerobic respiration, which releases energy to the cell
vi chloroplast f breakdown of materials
Distinguish between:a rough and smooth endoplasmic reticulumb plasma membrane and cell wallc leucoplast, chloroplast and chromoplastd chloroplast and chlorophyll.
Match the type of cell tissue that would have an abundance of a certain type of organelle.
Tissue type Organelle in abundance in cell
i Muscle a Chloroplasts
ii White blood cells b Mitochondria
iii Leaf c Ribosomes and Golgi apparatus
iv Pancreas d Lysosomes
iii
26 Unit 1
a Is the cell shown in Figure 1.33 from a plant or animal? Give reasons for your answer.
b Would this photograph be taken from a cell viewed with an electron or light microscope? Give your reasons.
c Name each of the arrowed organelles.d Which features of this cell indicate that it
is a eukaryotic cell?e Some organelles may be present in this
cell but are not shown in the photograph. Suggest why this might be the case.
Why do you think it is an advantage to a eukaryotic cell to possess different types of organelles?
Certain cells have densely packed mitochondria and the cristae (infolded projections of a mitochondrion) are very close together. What would you predict about the function of such cells? Explain your reasoning.
Radioactively labelled amino acids were supplied to a pancreatic cell, which produces digestive enzymes to be released into the digestive system.a In which organelles of the cell would they
be subsequently detected? List them in order.
b In what form would they appear in these organelles? You might like to present your answer as a diagram.
Below is a diagram of a typical plant cell.
Figure 1.34
Investigate and inquire
Figure 1.33
A
B
C
D
XE
27
Chapter 1 Cells in action
The Golgi bodies in plant cells are smaller than those in animal cells and are thought to be involved in the formation of the components of the cell wall as well as other complex molecules.a From Figure 1.34 write the letters that correspond to the: cell wall Golgi apparatusb What is the main chemical component of the plant cell wall?c What is the function of the cell wall?d In the diagram above, name the structure labelled X.
Phytophthera cinnamomi is a microscopic organism that belongs to group known as the water moulds. It needs moist conditions to survive. It invades the root system of susceptible plants, forming a mass of threads – the mycelium – that interferes with the plant’s ability to take in and circulate water and nutrients by causing lesions (areas that appear rotten.) It is blamed for ‘forest dieback’ in native forests of Victoria as well as Western Australia and Tasmania, causing millions of dollars’ worth of damage annually to forest industries, as well as placing many plant populations under serious threat.a Is Phytophthera a prokaryotic or eukaryotic organism?b Explain your answer to (a) above.c Name the part of the vascular tissue of infected plants most likely to be infected
by Phytophthera. It is possible to isolate individual organelles by differential centrifugation. In this
technique, cells are broken open and their contents released. The solution is kept ice cold and is spun in a centrifuge at a speed of rotation that causes the heaviest organelles to be thrown to the bottom, forming a sediment. The other lighter organelles remain floating in the clear fluid above the sediment.a Why is the solution kept ice cold?b Considering the relative sizes of organelles, which organelles would you expect
to find in the sediment and which in the clear fluid?c Techniques that are used to analyse the clear fluid include electrophoresis and
paper chromatography. Research how these techniques are carried out. Make a list of five organ systems of the human body and say which organs belong
to each system.
