Collins Cambridge AS & A Level Biology

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Cell structure

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Scientists working for the Deep Carbon Observatory project have investigated life far below the Earth’s surface. They have shown that approximately 70% of the Earth’s bacteria and other microorganisms live underground. The scientists used microscopes to study samples from mines and boreholes, finding microorganisms living up to 5 km deep. Many of these organisms are very different from their surface-dwelling relatives, with a genetic diversity at least equal to, if not greater than, that found above the surface.

The ability of these organisms to survive in such extreme conditions will change the way scientists search for life elsewhere in the universe. Until now, attention has been focused on places where water may be found. Now scientists will need to consider the possibility of life surviving far below rocky surfaces too.

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Prior understanding

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In your previous studies you may have used microscopes or hand lenses to study biological specimens. You may have made drawings of specimens and calculated their magnification. You may be familiar with some different types of cell, such as animal and plant cells, as well as the functions of some of the structures inside cells.

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learning objectives

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In this chapter, you will learn how to use microscopy to study cells, including the function of an eyepiece graticule and stage micrometer scale, and distinguish between resolution and magnification. You will learn about two basic types of cell – prokaryotic and eukaryotic – as well as the key features of non-cellular viruses. You will also learn about the functions of cell structures, including the role of ATP in energy transfer.

1.1 The microscope in cell studies (syllabus 1.1.1–1.1.5) 1.2 Cells as the basic units of living organisms (syllabus 1.2.1–1.2.7)

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1 Cell structure

1.1 The microscope in cell studies

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Although lenses have been used as magnifying glasses for at least a thousand years, the first microscopes were built around the start of the 17th century. Some used a single lens but others were compound microscopes that used two lenses: an objective lens and an eyepiece lens, very similar to many microscopes today.

Later in the 17th century, scientists used microscopes to make the first detailed observations of biological structures. In Italy, Marcello Malpighi studied the structure of animal and plant tissues and was the first person to see blood capillaries. Cells were discovered and named by Robert Hooke in England, who thought that what he saw in a thin slice of cork resembled the small rooms, or cells, that monks lived in. Antonie Van Leeuwenhoek, a Dutch scientist, discovered bacteria, single-celled organisms called protists and sperm cells, as well as making many other detailed observations, for example of red blood cells and muscle fibres.

In the 20th century, electron microscopes were developed. These use a beam of electrons rather than light to produce an image, which shows much more detail than is possible with a light microscope. For example, the structure of viruses, which are much smaller than cells, was first seen using an electron microscope (although their existence had been known much earlier). Transmission electron microscopes (TEM) pass electrons through specimens to produce an image. Scanning electron microscopes (SEM) detect electrons that have been reflected from the surface of a specimen to produce a ‘three-dimensional’ image.

Images produced using a microscope are called micrographs: images produced by a light microscope are called photomicrographs (Figure 1.1a); images produced by an electron microscope are called electron micrographs (Figure 1.1b, c).

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Figure 1.1 Micrographs of: Amoeba, a single-celled organism, taken with a light microscope (a); a mitochondrion, taken with a TEM (b); a human egg cell covered with sperm cells, taken with an SEM (c).

1. Explain the difference between a photomicrograph and a photograph.

2. Look at Figure 1.1c. Explain how you can tell that the image has been taken with an SEM and not a TEM. 2

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1.1 The microscope in cell studies

Magnification and resolution

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Magnification Magnification is the number of times larger an image is than the actual object. For example, if the image of a cell is 50 mm long and the actual cell is 0.1 mm long, then the magnification is ×500.

Magnification can be calculated using the formula: size of image magnification = size of object The following units are used when measuring cells and cell structures: millimetres (mm), micrometres (μm) and nanometres (nm).

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Microscopes produce images larger than the original objects. This is called magnification.

Although in theory there is no limit to how many times you can magnify an image, at some point you will stop seeing any more detail. The ability to see detail is a measure of the microscope’s resolution (or resolving power). Think about a digital photograph: if you keep enlarging it, the magnification increases. However, when you get to the point where you can see the separate pixels, then greater magnification will not lead to greater resolution because you will not be able to see any more detail.

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Standard form is used to write very large or very small numbers. For example, 5 000 000 in standard form is 5 × 106 and 0.000005 is 5 × 10−6.

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• One millimetre is one-thousandth of a metre.

• One micrometre is one-thousandth of a millimetre and one-millionth of a metre.

• One nanometre is one-thousandth of a micrometre, one-millionth of a millimetre and one-billionth of a metre.

This is summarised in Table 1.1.

Symbol

Comparison with 1 metre

millimetre

mm

1 mm = 1 × 10−3 m

1 m = 1 × 103 mm

micrometre

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1 μm = 1 × 10−6 m

1 m = 1 × 106 μm

nanometre

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1 nm = 1 × 10−9 m

1 m = 1 × 109 nm

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Unit

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Worked example

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Table 1.1 Units used when making microscope observations of biological specimens.

a. Convert 5000 nanometres to micrometres.

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Answer 1000 nm = 1 μm, so 5000 nm = 5 μm.

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b. Convert 72 millimetres to nanometres. Give your answer in standard form.

Answer 1 mm = 1 × 106 nm, so 72 mm = 72 × 106 nm. In standard form this is 7.2 × 107 nm.

3. The length of a sperm cell is measured as 50 μm. Give the length, in standard form, in (a) mm, (b) nm.

4. The diameter of the nucleus in a human cell is approximately 6 μm. An electron micrograph is made of the nucleus at magnification ×4000. a. Give the actual diameter of the nucleus in nanometres. b. Calculate the diameter of the image of the nucleus. Give your answer in millimetres.

Human cells range in diameter from about 7 μm (red blood cell) to about 100 μm (egg cell). Bacteria cells, which are usually smaller than human cells, have diameters between 0.2 μm and 10 μm.

Tip Always try to show your working in calculations, even if you are not specifically asked for it.

Tip When writing in standard form, the first part of the number must be at least 1.0 but smaller than 10.

Tip You may need to give an answer in different units from those in a question.

For magnification calculations, make sure both the image and object measurements are in the same unit.

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1 Cell structure You can work out the magnification of a compound light microscope (see Figure 1.2) by multiplying together the magnifications of the eyepiece lens and the objective lens.

(The magnification is usually shown on the side of a lens.) For example, if an eyepiece lens has a magnification of ×10 and the objective lens has a magnification of ×40, then the total magnification = 10 × 40 = ×400.

Eyepiece lenses

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Coarse focusing dial

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Fine focusing dial

Stage

Specimen

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Condenser lens

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Objective lenses

Light source

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Figure 1.2 A compound light microscope. Note that often the eyepiece and objective lenses are each composed of more than one lens.

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Resolution Resolution is the ability to distinguish between two objects that are close together – to be able to see them as separate objects. The reason that electron microscopes (see Figure 1.3) produce images with a greater resolution than light microscopes is to do with wavelength.

Electrons have a much smaller wavelength than the wavelength of light, which allows for greater resolution of the image. Light microscopes have a resolution limit between 200 nm and 250 nm. The limit of resolution of an SEM is about 1 nm and that of a TEM about 0.5 nm. This means that the maximum useful magnification of a light microscope is approximately ×1500, whereas for electron microscopes, magnification can be up to ×500 000 for a TEM (and about half that for a SEM). As a comparison, Figure 1.4 shows chloroplasts viewed with both a light microscope and a TEM. Only the latter clearly shows the internal structure of a chloroplast.

Comparison of light microscopes and electron microscopes Although electron microscopes allow much greater magnification and resolution than light microscopes, they do have some disadvantages. The images produced by electron microscopes do not show colour (because they are formed by the action of electrons, not light). If you see a coloured electron micrograph this is because the colour has been added afterwards, for example by a computer. These images are known as false-colour images.

The other main disadvantage is that electron microscopes cannot be used to view living tissue or living organisms. This is because the prepared specimens have to be placed in a vacuum inside the microscope.

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Cathode (electron gun) Anode Electron beam

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Condenser lens (electromagnetic ‘lens’) Vacuum

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Figure 1.4 (a) Micrographs of plant cells, taken with a light microscope – the dark green structures are chloroplasts; (b) micrograph of a single chloroplast, taken with a transmission electron microscope.

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Specimen

Projector lens

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Black-and-white image

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Objective lens

Fluorescent screen Camera chamber

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Figure 1.3 A transmission electron microscope.

Electron microscopes are also much larger, more expensive, and more complicated to maintain than light microscopes.

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5. Explain the difference between resolution and magnification.

6. Give one advantage and one disadvantage of an image produced by an electron microscope compared with one produced by a light microscope.

Experimental skills 1.1: Preparing and observing slides

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Preparing slides of biological material for viewing under a light microscope is usually quite straightforward.

For looking at individual cells, the specimen should be no more than one cell thick. This can be obtained by peeling off a suitable layer of tissue, for example from the inside of a piece of onion. Another way is to take a sample of tissue and squash it so the cells separate from each other. Stains are often added to the specimen to make the cell structures more visible.

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1.1 The microscope in cell studies

Preparing an onion cell slide A student carried out the following steps to prepare and then observe a slide of onion cells: 1. Peel a layer of tissue from the inside of a piece of onion.

2. Place the tissue on a microscope slide, making sure the tissue is flat and not folded over.

3. Add a drop of yellow/brown iodine solution (iodine in potassium iodide solution) as a stain.

4. Carefully lower a coverslip; place one side down first to avoid trapping air bubbles.

5. Gently press down on the coverslip with tissue paper to absorb any excess liquid.

Questions P1. Suggest reasons for the following steps in preparing a microscope slide: a. making sure that the specimen tissue is not folded over b. using a stain c. using a coverslip.

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1 Cell structure

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1. Turn the eyepiece or the slide to line up the scale on the graticule with the scale on the stage micrometer, as shown in Figure 1.6.

2. Count the number of divisions on the graticule that match up with a set distance on the stage micrometer scale.

3. Calculate the distance between each division on the eyepiece graticule.

Eyepiece graticule scale (arbitrary units)

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1. Place the slide on the microscope stage, securing it with the clips.

2. Move the objective lens with the lowest power (magnification), so it is above the specimen.

3. Adjust the mirror (or light source) to ensure the specimen is suitably bright when observed.

4. Move the objective lens as close as it goes to the slide without touching it. Looking through the eyepiece, use the coarse focusing dial to move the objective lens away from the slide until the specimen comes into focus. Use the fine focusing dial to make the image as sharp as possible. Move the slide around on the stage to observe different parts of the specimen.

5. Use the objective lenses with higher powers, in the same way, to observe the cells in more detail, before making biological drawings of several cells.

the distance between each division on the scale then you can use the graticule to make measurements of the specimen you observe. You can work out this distance using a stage micrometer scale – this is a microscope slide with another scale etched on to it. Working out the distance between the divisions on the graticule scale is an example of calibration.

The following steps explain how to calibrate an eyepiece graticule:

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Observing onion cells The student observed the onion cell slide under a light microscope, carrying out the following steps:

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Biological drawings should identify clearly and simply the main features of what is being observed. There should be no shading. Drawings and labels should be in pencil, and a ruler should be used for label lines. Drawings should be given a title and show the magnification where possible.

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P2. Suggest reasons for the following steps in observing a microscope slide: a. focusing by starting with the objective lens close to the slide and then moving it away b. focusing first with the coarse focusing dial and then with the fine focusing dial c. making your first observations with a low power objective lens before moving to a higher power lens.

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Figure 1.5 shows an onion cell slide observed using a light microscope.

Produce an accurate biological drawing of one of the onion cells Figure 1.5 Onion cells observed with a light microscope. shown in Figure 1.5.

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Using an eyepiece graticule and stage micrometer scale An eyepiece graticule fits inside the eyepiece and shows a scale when you look through it. If you know

Stage micrometer scale (marked in 0.01 mm and 0.1 mm divisions) Figure 1.6 Calibrating an eyepiece graticule.

P4. Look at Figure 1.6. a. State how many eyepiece graticule divisions correspond with 0.1 mm on the stage micrometer scale. b. Calculate the distance between the divisions on the eyepiece graticule. Give your answer in millimetres in standard form. c. A student used the eyepiece graticule to observe a cell. The diameter of the cell measured 12 divisions on the graticule scale. Calculate the actual diameter of the cell. Give your answer in micrometres.

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1.1 The microscope in cell studies

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a. The total area of the blood sample on the slide is 4 cm2. Use information from the shaded squares in Figure 1.7 to estimate the total number of red blood cells on the slide. b. Suggest how the accuracy of this estimate could be improved.

Link In Chapter 18, quadrats are used to estimate population sizes during ecological fieldwork. Using a microscope grid follows the same principle of using samples to estimate a total number.

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P5. Figure 1.7 shows red blood cells observed using a light microscope and a grid. Each square on the grid measures 1 mm by 1 mm.

Figure 1.7 Red blood cells viewed through a light microscope.

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Using a grid to estimate the number of cells The number of cells on a slide, or in the field of view, can be estimated, for example by using a grid, which can be fitted inside the eyepiece or be on a slide itself. By counting the number of cells in several sections of the grid, an estimate can be made of the total number of cells.

The following steps explain how to do this: 1. Count the number of cells in several sections of the grid.

2. Calculate the mean number of cells per section.

3. Multiply the mean number by how many times bigger the whole sample is than a single section.

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d. The student then used a different objective lens with a higher magnification to observe the cell nucleus in more detail. The student measured the diameter of the nucleus using the same eyepiece graticule. Explain what else the student needs to do to make the measurement accurate.

Assignment 1.1: Observing human cheek cells

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A student used a cotton bud to wipe the inside of their cheek. They wiped the cotton bud across a clean microscope slide. They put one drop of methylene blue stain (a dark blue stain) on the smear and placed a coverslip over it. They observed the slide with a light microscope. Figure 1.8 shows one of the cheek cells as seen under the microscope.

Questions A1. Look at Figure 1.8. a. The cell diameter shown measures 68 mm on the photomicrograph. The magnification is Ă—12 000.

Calculate the actual diameter of the cell in micrometres. Give your answer to two significant figures. b. Describe how you could calculate a more accurate value for the average diameter of the cell.

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Figure 1.8 Photomicrograph of a human cheek cell.

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1 Cell structure

A2. Three students made biological drawings of the cheek cell, as shown in Figure 1.9.

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Cell membrane Cytoplasm

Cell membrane Nucleus Cytoplasm

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Nucleus

Figure 1.9 Biological drawings of a human cheek cell.

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a. Explain which diagram shows the best biological drawing of the cheek cell. b. State how the diagram you have chosen in part (a) could be improved further.

Key ideas

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➜➜Light microscopes produce magnified images, allowing the study of cells and their components. ➜➜Electron microscopes produce images of greater resolution and magnification than light microscopes, allowing the detailed study of cell organelles. ➜➜A transmission electron microscope (TEM) produces images by passing electrons through an object, whereas a scanning electron microscope (SEM) produces images from electrons reflected from the surface of an object. ➜➜Magnification is how many times bigger an image is than the original object. ➜➜Resolution is the shortest distance between two separate objects that can still be seen as separate. ➜➜Eyepiece graticules and stage micrometer scales can be used to make measurements of objects seen through microscopes.

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1.2 Cells as the basic units of living organisms

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All living organisms are made up of cells. Some organisms consist of a single cell, for example Amoeba, whereas an adult human may be made up of over 37 trillion cells (one trillion = 1012), consisting of many different types (see Figure 1.10). Current estimates are that there are around 1.3 million different species on the planet, but there are only two basic types of cell: eukaryotic cells and prokaryotic cells. Organisms with eukaryotic cells include all animals, plants and fungi. Organisms with prokaryotic cells include bacteria. Prokaryotic cells are smaller and lack many of the features of eukaryotic cells, such as a nucleus. Even smaller than prokaryotic cells are viruses, which do not have a cellular structure and are not considered as living because they do not carry out all the functions of living organisms: for example they do not respire.

Scientists used to classify living things into two groups: those with eukaryotic cells and those with prokaryotic cells.

Chapter 18 explains why organisms are now classified into three main groups.

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1.2 Cells as the basic units of living organisms

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Structure of eukaryotic cells

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7. Look at Figure 1.10a. Explain whether Stentor is a prokaryotic or a eukaryotic cell.

b.

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Animals, plants, fungi and many single-celled organisms are composed of eukaryotic cells. Although there are many different types of eukaryotic cell, they have many features in common.

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Cell surface membrane Figure 1.11 shows the structure of a typical animal cell. It has many of the features found in other types of eukaryotic cells too. Figure 1.12 shows part of an animal cell, as seen using an electron microscope.

Centrioles – associated with cell division

Nuclear envelope

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Endoplasmic reticulum (rough)

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Cytosol

Cell surface membrane Lysosome Intercellular space Golgi body – secretes substances into vesicles

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Nucleolus Nucleus

Chromatin Ribosome (attached)

Ribosome (free)

Figure 1.10 A micrograph of Stentor, a single-celled organism (a). A human is composed of trillions of cells – a multicellular organism (b).

Nuclear pore Endoplasmic reticulum (smooth) Mitochondrion

Microtubules Microfilaments Provide support and are involved in movement within cell

Figure 1.11 The structure of a typical animal cell, as seen using an electron microscope.

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1 Cell structure

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In Chapter 4, you will see how the structure of the cell surface membrane controls the movement of substances into and out of cells.

Figure 1.12 An electron micrograph of part of an animal cell.

The animal cell is surrounded by a cell surface membrane (often just known as a cell membrane). As well as containing all the other cell structures, and separating the cell from its surroundings, the membrane also controls the substances that enter and leave the cell. Inside the cell are many smaller structures called organelles within a semi-liquid substance called cytosol. The cytosol and the organelles (apart from the nucleus) make up the cytoplasm.

Nucleus In many cells, the nucleus (see Figures 1.13 and 1.14) is the largest organelle, with a typical diameter of about 10 Οm. It contains most of the cell’s deoxyribonucleic acid (DNA),

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1.2 Cells as the basic units of living organisms

Endoplasmic reticulum

Ribosomes

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Nucleolus The nucleolus produces the RNA that is used to make ribosomes

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which carries the coded information needed to control cell division and other cell processes. During cell division, the DNA condenses, forming short thread-like structures called chromosomes. The material that chromosomes are made from, a mixture of DNA and proteins called histones, is called chromatin. At other times, the chromosomes ‘unravel’ and the DNA spreads out within the nucleus; the chromatin in the nucleus then has a grainy appearance, with one or more darker regions visible, known as nucleoli. A nucleolus is the site of ribonucleic acid (RNA) synthesis. RNA is used to make ribosomes in the cytoplasm. The nucleus is surrounded by a double membrane known as the nuclear envelope (or nuclear membrane), which has pores (gaps) that allow RNA molecules to move out of the nucleus into the cytoplasm.

Nuclear pore Inner membrane

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Nuclear envelope Outer membrane The nuclear envelope is made up of two membranes. The outer membrane is continuous with the membranes of the endoplasmic reticulum

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Chromatin When a cell is not dividing, the DNA is in the form of chromatin, some of which stains darkly

Figure 1.13 A nucleus.

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8. Suggest why it is important to control what enters and leaves a cell.

9. During cell division, the DNA in the nucleus condenses, forming compact chromosomes. In between divisions, much of the DNA ‘unravels’. Suggest an explanation for these changes.

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Endoplasmic reticulum and ribosomes Endoplasmic reticulum (ER; see Figures 1.15 and 1.16) is a network of membranes that form channels throughout the cytoplasm. Some of these channels join up with the nuclear envelope and some may join with the cell surface membrane. There are two types of ER:

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• Rough endoplasmic reticulum (rough ER, or RER) has ribosomes attached to its outer surface, giving it a ‘bumpy’ appearance – hence its name. Ribosomes are very small organelles formed of RNA and are where proteins are synthesised. The proteins move into the channels formed by the membranes, through which they travel to other parts of the cell.

• Smooth endoplasmic reticulum (smooth ER, or SER) has no ribosomes associated with it. It is involved in several functions, including the synthesis of hormones and lipids.

Although most ribosomes are attached to the rough ER, some are found free in the cytoplasm. Some ribosomes are also located inside mitochondria and chloroplasts.

Those found in the cytoplasm, including those on rough ER, are sometimes described as 80S ribosomes. (The S refers to how quickly they form sediment in a process called centrifugation.) Those found in mitochondria and chloroplasts are smaller 70S ribosomes.

Figure 1.14 False-colour electron micrograph of nucleus. The nucleus is shown in light orange and the nucleolus as a darker brown area. You can see the doublelayered nuclear envelope as two black lines. The green organelles are mitochondria.

Link There is more about the structure of DNA, chromosomes and RNA, and how they are involved in cell division and protein synthesis, in Chapters 5, 6 and 16.

Link In Chapter 6, you will see how ribosomes, and other types of RNA, are involved in producing proteins.

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1 Cell structure Ribosomes Ribosomes are the smallest organelles in the cell. They are made of protein and RNA. The RNA in ribosomes is produced in the nucleolus. Ribosomes build proteins. Many ribosomes are attached to the rough endoplasmic reticulum

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Cytosol The cytosol (the solution outside the endoplamsic reticulum) is a solution containing enzymes, food-storage compounds and waste products about to be expelled

Rough endoplasmic reticulum The rough endoplasmic reticulum consists of sheets of membrane to which ribosomes are attached

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Smooth endoplasmic reticulum The smooth endoplasmic reticulum is a tubular phospholipid membrane that does not have ribosomes attached

Intracisternal (lumenal) space

Figure 1.15 Endoplasmic reticulum.

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Golgi body The Golgi body (also known as the Golgi apparatus or Golgi complex; see Figures 1.17 and 1.18) is an organelle that stores substances such as proteins before they are transported elsewhere in the cell. The substances may also be modified, for example by adding short carbohydrate chains to proteins to form glycoproteins. The Golgi body is formed from a group of flattened spaces or vesicles. The processed substances are packaged into their own vesicles, which leave the Golgi body and either move to other parts of the cell or fuse with the cell surface membrane to release their contents outside the cell (a process known as exocytosis).

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In Chapter 4, you will find out more about how exocytosis moves substances out of cells.

10. Look at Figure 1.16. What type of ER does it show? Explain your answer.

11. Suggest why some substances are transported through the ER rather than just diffusing throughout a cell.

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Figure 1.16 Electron micrograph of endoplasmic reticulum.

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Many of the stages of respiration occur in the mitochondria: in the matrix and on the inner membrane. You will learn about this in Chapter 12.

12. Which process occurs in both the Golgi body and the endoplasmic reticulum?

A. Movement of substances between cells B. Production of new DNA C. Production of new substances D. Protein synthesis 13. Suggest one type of cell that will contain a large number of Golgi bodies.

Mitochondria Mitochondria (singular: mitochondrion; see Figures 1.19 and 1.20) are spherical or elongated (‘sausage-shaped’) organelles found in most eukaryotic cells. They can be relatively large compared with other organelles, being around 0.5–1.5 μm wide and 3–10 μm long. They are the site of aerobic respiration. Respiration is a process that

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1.2 Cells as the basic units of living organisms breaks down substances such as glucose to release adenosine triphosphate (ATP). Aerobic respiration uses oxygen to release more ATP per glucose molecule than can be released in anaerobic respiration (respiration in the absence of oxygen). It is ATP that directly provides the energy that cells transfer for energy-requiring processes, such as active transport (see Chapter 4) and muscle contraction (see Chapter 15).

Material passes out of cell

Cell surface membrane

Lysosome 5

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Secretory vesicle

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Figure 1.18 Electron micrograph of the Golgi body.

Golgi body

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Rough endoplasmic reticulum

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1 Vesicles containing proteins ‘bud off ’ from the RER and fuse with the Golgi body.

2 Inside the Golgi, the proteins are modified ready for use elsewhere in the cell, or for export.

3 Vesicles containing the modified proteins ‘bud off ’ from the Golgi body.

4 Some of these vesicles move to the cell surface membrane, where they fuse with it and empty their contents outside of the cell.

5 Some of the vesicles are lysosomes, containing hydrolase enzymes that can hydrolyse material inside the cell.

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Figure 1.17 The Golgi body.

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Mitochondria have two membranes: an outer membrane and a highly folded inner membrane (the folds are known as cristae). The cristae hold many of the enzymes involved in respiration, although there are other enzymes involved found in the fluidfilled internal space (known as the matrix).

Mitochondria contain 70S ribosomes and their own small circular molecules of DNA. Lysosomes Lysosomes (look back at Figure 1.11) are vesicles produced by the Golgi body, containing digestive enzymes called lysozymes. Lysosomes are used within cells to break down old or unneeded organelles, and they are used by white blood cells called phagocytes to digest any engulfed pathogens. Lysosomes are also used to destroy unwanted cells and tissues, for example to reduce the muscle of the uterus after birth, and to reduce milkproducing tissue after weaning.

Link In Chapter 11, you will learn about the essential role that phagocytes, and other types of white blood cell, have in the body’s responses to infection.

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1 Cell structure DNA The DNA in mitochondria is made up of circular molecules. It contains the information required for the mitochondrion to replicate, and also for making some of the enzymes used in the reactions of respiration

Matrix The matrix contains many of the enzymes that control some stages of respiration

Ribosome (70S) Outer membrane

Figure 1.20 Electron micrograph of a mitochondrion.

Membrane-bound ATP synthase molecules

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Mitochondrial envelope

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Inner membrane

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Outer membrane The outer membrane controls the passage of materials into and out of the mitochondrion

Cristae The inner membrane is highly folded to form cristae. Folding gives more surface area for enzymes/pathways used to make ATP. Most of the enzymes that control energy transfer reactions in the later stages of respiration are attached to the cristae

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Intermembrane space

Figure 1.19 A mitochondrion.

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14. Suggest one type of cell that will contain large numbers of mitochondria. Explain your choice.

15. Suggest why it is important that lysozymes inside a cell remain inside lysosomes.

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Figure 1.21 The cilia (the tiny ‘hairs’ around the outside) of Paramecium move, propelling it through water.

Centrioles, microtubules and cilia Centrioles are small organelles found in all animal cells but absent in many plants.

They are the site of formation of fine protein filaments known as microtubules, or microfilaments. Microtubules help form a supportive network inside a cell, supporting its shape or helping it to move. They also organise and move organelles within a cell, and they form the spindle (spindle fibres) during cell division. Some microtubules form external structures called cilia that are involved with movement: some singlecelled organisms use cilia to move (see Figure 1.21); they are also found inside multicellular animals, for example on ciliated epithelial cells, which move to expel mucus from the lungs.

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You will find out how the spindle is involved in moving chromosomes during cell division in Chapters 5 and 16.

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In Chapter 9, you will see how different cells, including ciliated epithelial cells, help maintain the health of the human gas exchange system.

Microvilli Microvilli are small finger-like projections found on the surface of some cells to increase surface area. For example, they are found on the surface of the epithelium cells covering the villi in the small intestine, where they increase surface area for the absorption of digested food (see Figure 1.22).

16. Smoking tobacco damages the ciliated epithelial cells in our lungs. Suggest why this leads to the heavy cough of many smokers.

17. Microvillus inclusion disease is a health condition in which microvilli in the small intestine do not form properly. The condition can often be fatal. Suggest why.

Cell walls and plasmodesmata Figure 1.23 shows the structure of a typical plant cell. It has many of the features found in animal cells, but in addition, plant cells also contain a cell wall, chloroplasts, plasmodesmata and a large permanent vacuole. Figure 1.24 shows a plant cell, as seen using an electron microscope.

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1.2 Cells as the basic units of living organisms

Microvilli

Mitochondrion

Smooth endoplasmic reticulum

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Figure 1.22 An epithelial cell on the surface of a villus.

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Rough endoplasmic reticulum Pinocytotic vesicles

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Nucleus

Rough endoplasmic reticulum

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Smooth endoplasmic reticulum

Golgi body Chloroplast

Middle lamella

Ribosome

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Cell wall

Cell surface membrane

Nuclear envelope Nucleolus

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Membrane around vacuole (tonoplast)

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Chromatin Large vacuole

Figure 1.24 A scanning electron micrograph of a plant cell.

Mitochondrion

Link Plasmodesma – fine strand that connects adjacent cells

Figure 1.23 The structure of a typical plant cell, as seen using an electron microscope.

Outside the cell surface membrane, the plant cell is surrounded by a cell wall. Cell walls are found on the outside of the cells of plants, algae and fungi, but never on animal cells. Plant and algal cell walls are made of a substance called cellulose, which is a polysaccharide (see Chapter 2). Cellulose is a long molecule, and in the cell wall, parallel cellulose molecules are packed together in bundles called microfibrils. The cell wall is made of many layers of these microfibrils embedded in a substance called pectin. In different layers, the microfibrils run in different directions, adding strength to the structure (see Figure 1.25). Unlike the cell surface membrane, the cell wall is completely permeable and does not control in any way the substances that enter or leave

You will learn about the structure and function of cellulose and other polysaccharides in Chapter 2.

Link How cells absorb water is explained in Chapter 4. In Chapter 7, you will see how water and other substances are transported from one part of a plant to another.

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1 Cell structure

18. Suggest why dehydration causes a plant to wilt (droop).

19. Suggest how large animals support their body if they do not have cell walls.

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Figure 1.25 Electron micrograph of cellulose microfibrils in the cell wall of an alga.

a cell. Its function is to provide the cell with support. When a plant cell absorbs water, its volume increases and the cell surface membrane presses against the cell wall, making the whole structure rigid. This is why even non-woody plants can support themselves to stay upright.

Joining adjacent plant cells together is a layer of pectin called the middle lamella.

Plasmodesmata (see Figure 1.23) are tiny holes through the middle lamella between adjacent cells through which cytoplasmic connections allow substances to move from one cell to another.

The cell walls of fungi are made of a different polymer called chitin.

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Stroma The stroma is a fluid containing enzymes that uses ATP generated in the early stages of photosynthesis to fix carbon dioxide into sugar

Lipid droplet

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Thylakoid The thylakoid membrane has many chlorophyll molecules embedded in it. These absorb light energy and then transfer it to other molecules. This stage of photosynthesis generates ATP

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In Chapter 13, you will see how the different parts of a chloroplast are involved in the different stages of photosynthesis.

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Chloroplasts Chloroplasts (see Figures 1.26 and 1.27) are organelles found in plant and algal cells and are the site of photosynthesis. The green pigment chlorophyll in chloroplasts absorbs light, and the subsequent energy transfer is used to build up glucose and other complex organic molecules.

Chloroplasts have two outer membranes (forming the chloroplast envelope) as well as a series of internal membranes forming thylakoids that stack to form grana (singular: granum). The thylakoid membranes have chlorophyll and other photosynthetic pigments embedded within them. The liquid within the chloroplasts is called stroma.

Like mitochondria, chloroplasts contain 70S ribosomes and their own small circular molecules of DNA. They also contain starch grains, and sometimes lipid droplets.

Enlarged model of part of the chloroplast

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Chloroplast envelope

Inner membrane Starch grains Excess carbohydrate made during photosynthesis is temporarily stored as starch grains Granum The thylakoids are arranged in stacks, each called a granum. This greatly increases the efficiency of light capture by the chloroplast

Ribosomes (70S)

Thylakoids

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Outer membrane The outer membrane controls movement of molecules in and out of the chloroplast

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DNA Chloroplast DNA is circular and contains information necessary for the formation and function of the chloroplast

Stroma

Figure 1.26 The structure of a chloroplast.

Figure 1.27 Electron micrograph of a chloroplast.

Vacuole All cells contain some liquid-filled spaces called vacuoles. Many are small and short-lived – for example vesicles. However, plant cells usually have a large permanent vacuole, which can take up much of the space inside the cell (see Figure 1.23). The vacuole is surrounded by a single membrane called the tonoplast and contains a fluid called cell sap. The main

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1.2 Cells as the basic units of living organisms functions of plant vacuoles are to help maintain cell rigidity by absorbing water so the cell contents press against the cell wall, and to store substances such as sucrose and amino acids, as well as waste products that may be harmful to the rest of the cell.

Comparison of the structure of typical plant and animal cells Plant and animal cells have many structural features in common, although there are differences. These similarities and differences can be seen in Figures 1.11 and 1.23.

Worked example

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20. Not all plant cells contain chloroplasts. Suggest why and give an example of a cell without chloroplasts.

21. Some of the waste products stored in plant cell vacuoles have an unpleasant taste to animals. Suggest how storing these products may benefit a plant.

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a. Compare the structures of a typical plant cell and animal cell.

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Answer Plant and animal cells have many structures in common: cell surface membrane; nucleus, including the nuclear envelope and nucleolus; rough and smooth endoplasmic reticulum; Golgi bodies; mitochondria; ribosomes; lysosomes; centrioles and microtubules. There are some features found in plant cells that are not found in animal cells: cell wall; chloroplasts; plasmodesmata; large permanent vacuole with tonoplast.

In Chapter 4, you will learn about how plant cells, and their vacuoles, absorb water by osmosis.

b. Suggest reasons for the similarities and differences between a typical plant and animal cell.

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Answer Plant and animal cells have many features in common because they need them for the same reasons: for example, a cell surface membrane to control movement of substances in and out of the cell, a nucleus containing genetic information in DNA, and mitochondria for respiration. However, whereas animals take in the organic substances they need as food from their environment, plants build up their organic substances by photosynthesis, and so need chloroplasts for this. Plants also need cell walls and large vacuoles to make their cells rigid, which is necessary for the support of the plant; animals gain support either through having a firm skeleton, by living in water, or by being very small and not needing to support their body weight against gravity.

Structure of prokaryotic cells

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Prokaryotic cells are much smaller than eukaryotic cells, and many of the structures commonly present in eukaryotic cells are absent. Prokaryotic cells are thought to have evolved much earlier and to have later given rise to eukaryotic cells. Many scientists think that eukaryotic cells formed when prokaryotic cells joined together to form larger cells. This is called the ‘endosymbiotic’ theory. The theory was first suggested at the start of the 20th century by Russian botanist Konstantin Mereschkowski, and later developed by other scientists, most notably, in the second half of the century, by US evolutionary biologist Lynn Margulis. Some of the evidence for the theory comes from the similarities in structure found between prokaryotic cells and mitochondria and chloroplasts, which, it is suggested, are descended from once separate, free-living prokaryotic cells.

Figure 1.28 shows the structure of a typical bacterium as an example of a prokaryotic cell.

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1 Cell structure Slime capsule

Cytosol

Cell wall

Flagellum

Cell surface membrane Main loop of DNA

Ribosomes Plasmid

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Figure 1.28 The structure of a typical bacterium, a single-celled prokaryotic cell.

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Key features of typical bacterial cells include the following:

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In Chapter 10 you will see examples of bacteria that cause infectious disease (pathogens).

Chapter 19 explains how bacterial plasmids are used in genetic technology.

• They are unicellular. Although some do form strands joined end to end, they do not form complex multicellular organisms like many eukaryotic cells.

• They are small, generally 1–5 μm in diameter, compared with eukaryotic cells, which are typically 10–100 μm in diameter.

• They have cell walls composed of peptidoglycan, a polymer made of polysaccharide and peptide chains (see Chapter 2). By contrast, eukaryotic cells have cell walls made of cellulose (plants and algae) or chitin (fungi), or have no cell wall at all (animals).

• Their DNA is circular and ‘naked’ (not associated with histone proteins). Eukaryotic DNA is associated with histones to form chromatin and chromosomes (although mitochondria and chloroplasts do have circular DNA). In addition to a main DNA loop, bacteria also contain much smaller loops of DNA called plasmids, although, in total, a prokaryotic cell contains far less DNA than a eukaryotic cell.

• They contain 70S ribosomes, unlike the 80S ribosomes found in eukaryotic cells (although mitochondria and chloroplasts have 70S ribosomes).

• They do not contain organelles surrounded by a double membrane, such as nuclei, mitochondria and chloroplasts.

• They do not have endoplasmic reticulum, Golgi bodies, lysosomes and microtubules.

• Some bacterial cells have flagella (singular: flagellum), which help them to ‘swim’.

• Many bacterial cells have an extra layer outside the cell wall called a capsule (or slime capsule) made of polysaccharide (see Chapter 2). Capsules provide protection against attack from other cells, as well as from physical harm such as desiccation (drying out).

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a. Compare the outer structures of a typical bacterial cell and animal cell.

In questions like this, do not just refer to ‘it’ in your answer. Make sure you are clear which type of cell you are referring to.

Answer Both types of cell are surrounded by a cell membrane; however, a bacterial cell also has a peptidoglycan cell wall. Many bacterial cells also have a capsule layer outside the cell wall. b. Bacterial cells, like all prokaryotic cells, do not have mitochondria. However, they still need to respire to produce ATP. Suggest where respiration occurs in a bacterial cell.

Answer In bacteria, all stages of respiration occur in the cytoplasm.

22. Describe how the DNA in prokaryotic cells is different from that in eukaryotic cells.

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1.2 Cells as the basic units of living organisms

Viruses Viruses are tiny particles that are much smaller than prokaryotic cells, being around 20–400 nm in diameter. They are non-cellular structures composed of a nucleic acid core (either DNA or RNA; see Chapter 6) surrounded by a protein coat called a capsid.

Some viruses also have an outer envelope made of phospholipids (see Chapter 2).

Figure 1.29 shows the structure of a typical virus.

Nucleic acid core (DNA or RNA)

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Capsid

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Figure 1.29 The structure of a virus particle.

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Although viruses are associated with living organisms, they are not considered to be alive themselves. They are not made of cells, and they do not demonstrate the seven characteristics of living organisms (movement, respiration, sensitivity, growth, reproduction, excretion, nutrition). The only characteristic they do demonstrate is reproduction, but even then, they need living cells to be able to reproduce. When a virus invades a cell, the cell replicates the viral DNA or RNA, makes new protein coats and assembles new viruses. The viruses then burst out of the cell, killing it.

23. How many times greater in size is the diameter of a prokaryotic cell of 5 μm compared with that of a virus with a diameter of 25 nm?

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Chapter 6 explains how DNA is replicated and how proteins are synthesised. In Chapter 10, you will see the factors involved in the control of one type of virus: HIV.

In calculations like this, convert all the values to the same unit. Usually it is easier to convert to the smaller unit.

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➜➜Eukaryotic cells are larger and more complex than prokaryotic cells, containing many structures not present in prokaryotic cells: nucleus, endoplasmic reticulum, Golgi bodies, mitochondria, chloroplasts, lysosomes, centrioles, microtubules. ➜➜Although eukaryotic and prokaryotic cells may have some features in common, there are differences: in the size of the cells, in the size of ribosomes, in the composition of cell walls. ➜➜Prokaryotic cells have some features not found in eukaryotic cells, such as plasmids. ➜➜Animal and plant cells share many cell structures, but only plant cells have a cell wall, chloroplasts and a large permanent vacuole. ➜➜Cells use ATP from respiration for energy-requiring processes. ➜➜Viruses are non-living, non-cellular particles that can only reproduce by attacking living cells.

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1 Cell structure

Chapter overview Review the mini mind map to link new learning with your prior learning, and to highlight which of the key concepts underpin the chapter.

Micrographs

Viruses

Using microscopes Observation and experiment

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Cell as the units of life

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Non-cellular particles

Prokaryotic cells

Eukaryotic cells

Other structures

Plant cells

Organelles

Biochemical processes

Respiration

DNA, the molecule of heredity

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Animal cells

Cell structures

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Types of cell

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Try copying this mini mind map and expanding upon it. Use your notes from other chapters to help you explore how the essential ideas, theories and principles can be linked further together.

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Learning outcomes Now that you have finished this chapter you should be able to:

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• make temporary preparations of cellular material suitable for viewing with a light microscope • draw cells from microscope slides and photomicrographs • calculate magnifications of images and actual sizes of specimens using drawings, photomicrographs and electron micrographs (scanning and transmission) • use an eyepiece graticule and stage micrometer scale to make measurements and use the appropriate units, millimetre (mm), micrometre (µm) and nanometre (nm) • define ‘resolution’ and ‘magnification’ and explain the differences between these terms, with reference to light microscopy and electron microscopy • recognise organelles and other cell structures found in eukaryotic cells and outline their structures and functions, limited to: cell surface membrane nucleus, nuclear envelope and nucleolus rough endoplasmic reticulum smooth endoplasmic reticulum Golgi body (Golgi apparatus or Golgi complex) ¦ ¦ ¦ ¦ ¦

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Chapter review mitochondria (including small circular DNA) ribosomes (80S in the cytoplasm and 70S in chloroplasts and mitochondria) lysosomes centrioles and microtubules cilia microvilli chloroplasts (including small circular DNA) cell wall plasmodesmata large permanent vacuole and tonoplast of plant cells describe and interpret photomicrographs, electron micrographs and drawings of typical animal and plant cells compare the structure of typical plant and animal cells state that cells use ATP from respiration for energy-requiring processes outline key structural features of a prokaryotic cell as found in a typical bacterium, including: unicellular generally 1–5 μm diameter peptidoglycan cell wall naked circular DNA 70S ribosomes absence of organelles surrounded by double membranes compare the structure of a prokaryotic cell as found in a typical bacterium with a typical eukaryotic cell state that viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some have an outer envelope made of phospholipids. ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦ ¦

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Chapter review

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1. Which cell structure is not found in prokaryotic cells?

A. 70S ribosome B. 80S ribosome C. Cell wall D. Plasmid 2. Where is RNA made?

A. Centriole B. Golgi body C. Lysosome D. Nucleolus 3. A photomicrograph image of a cell has a diameter of 15 mm. The actual cell has a diameter of 75 μm. Calculate the magnification of the image.

4. A student uses a light microscope with an eyepiece lens of magnification ×10 and an objective lens of magnification ×25. Calculate the total magnification of the image produced by the microscope.

5. Figure 1.30 shows an electron micrograph of a leaf.

State precisely what type of microscope was used to produce the micrograph in Figure 1.30. Give a reason for your answer.

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1 Cell structure

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Figure 1.30 Electron micrograph of a section through a leaf.

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6. Give three structures found in plant cells that are not found in animal cells.

7. Explain where most ATP is produced in a eukaryotic cell.

8. Describe the function of a lysosome.

9. Give three features of a prokaryotic cell that distinguish it from a eukaryotic cell.

10. Viruses are not regarded as living organisms. Give two reasons why.

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