Chapter 1.1 Living things consist of cells

TOPIC 1

CELLS AND MICROORGANISMS

Chapter 1.1 Living things consist of cells

Science Understanding

Living things are distinguishable from non-living things. The cell theory unifies all living things. Living things are made up of one or more cells. Cells:

? are the structural and functional units of life ? come from pre-existing cells ? contain hereditary material. The cell is the smallest independent unit of life. The cell membrane defines a cell; it separates the cell from its surroundings.

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Living things

The Earth is the only place known to sustain life. The term `life' refers to the vast diversity of living things that inhabit the planet. The scientific study of living things is called biology. Scientists who study living things are called biologists.

But what exactly is a living thing?

One way to define living things is to ask what makes living things alive. To most biologists, something is alive if it carries out fundamental processes associated with life called life processes. All living things carry out these same fundamental life processes. The main processes, together with examples are shown in the following table.

Life Process

Example

Maintaining a stable internal environment Humans sweating on a hot day

Controlled exchange of materials

Plants taking in carbon dioxide and giving out oxygen in daylight hours

Response to stimuli

Birds migrating for the winter

Obtaining energy and chemical elements Reptiles catching, killing and consuming prey

Transport of materials

Blood in fish carrying oxygen to muscle cells

Removal of waste

Human kidneys producing urine

Cell division

A fertilised ovum dividing many times to form an embryo

Growth and development

A caterpillar gradually changing into a butterfly

Independent movement

A koala climbing a tree

Reproduction

A bacterium dividing to produce two daughter cells

An organism is the name given to a living thing. Organisms can be unicellular (made up of one cell) or multicellular (made up of many cells), sometimes many billions, even trillions of cells. It has been estimated that there are about 35 trillion cells in an adult human being.

The cell is the fundamental or basic unit of life. It is often stated that the cell is the unit of structure and function of organisms. As such, the cell is the smallest independent unit of life that can carry out life's processes.

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LIVING THINGS CONSIST OF CELLS

CHAPTER 1.1

Cell types

Multicellular organisms consist of large numbers, sometimes billions, of cells of different types. The different cells are structured differently, some types of animal and plant cells are shown in Figures 111 and 112 below. Please note that the cells and structures are not drawn to the same scale.

1.1

red blood cells

sperm cell

Figure 111 Some animal cells

motor neuron cell

xylem cells

palisade cells

root epidermis cells

Figure 112 Some plant cells

The Cell Theory

The cell theory states that: ? all organisms consist of cells or the products of cells. ? all cells come from pre-existing cells by the process of cell division. ? cells are the building blocks of life.

cell membrane

cytoplasm

nucleus

10 ?m

membrane-enclosed organelles

Figure 113 A human cheek cell

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TOPIC 1

CELLS AND MICROORGANISMS

Cells

Cells consist of a volume of fluid enclosed by a membrane. The membrane is called the cell membrane and the enclosed fluid is called the cytoplasm. The cytoplasm is mostly made up of water in which substances are dissolved and some insoluble proteins. In animal and plant cells the cytoplasm also has structures in it called membrane-enclosed organelles that have specific functions; for example a nucleus that controls the overall function of the cell. An example of a cheek cell is shown in Figure 113, a scale bar is shown to help estimate the size of the cell. N.B. 1000 micrometres (?m) = 1mm

Unicellular organisms

Living things that consist of one cell only are called unicellular organisms; for example, bacteria (singular bacterium), fungi like yeasts, protists like amoeba, and algae (singular alga); for example, phytoplankton. Figures 114(a),(b),(c) and (d) show images of some common unicellular organisms, no uniform scale is used.

Figure 114 (a) Amoeba, (b) Bacteria, (c) Paramecium, (d) Euglena

Multicellular organisms

Living things that consist of large numbers of cells are called multicellular organisms; for example, mushrooms, animals and plants. Figure 115 (a)-(h) shows some images of a variety of multicellular organisms, no uniform scale is used.

Figure 115 (a)-(h) A variety of multicellular organisms

Helpful Online RESOURCES to find the location

These organisms, photographed by an associate one of the authors, all live together somewhere in the world, can you work out where? If you wish, scan this QR code to learn more.

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LIVING THINGS CONSIST OF CELLS

CHAPTER 1.1

Cells and DNA

All cells contain the chemical DNA (deoxyribonucleic acid) found in special structures called chromosomes. This is a very special chemical that enable cells to:

? make important molecules like proteins for cellular function. ? divide and produce new cells.

Heredity is defined as the passing on of characteristics genetically from one generation to the next. It is the chemical DNA (found in the nucleus of cells in multicellular organisms) that is responsible for passing on this genetic material from parent to offspring.

Chromosomes in cells of multicellular organisms

In the nucleus of cells of multicellular organisms exists a nucleic acid called DNA, or deoxyribonucleic acid. DNA is a very long macromolecule; molecules called histones are usually associated with DNA. A human cell may contain about 2 metres of DNA and the total length of DNA in a human body is hundreds of thousands of kilometres. Such a long molecule can fit into a space as small as a nucleus by being packed into structures called chromosomes. Figure 116 illustrates how these various components are organised. This matter will be more thoroughly discussed in Topic 3.

histones packed together

cytoplasm

nucleus

1.1

chromosome AN ANIMAL CELL

nucleotide bases

a histone

DNA

one strand of DNA

Figure 116 Chromosomes and DNA

The Cell Membrane

The cell membrane is a structure found in all cells; it defines a cell and separates the cell from its surroundings.

The cell membrane is found in all living cells and controls the entry and exit of materials in and out of the cell. The membrane is a thin layer (0.01?m) consisting of two layers of phospholipid molecules with embedded proteins. Figure 117 shows the structure of a `typical' membrane. (N.B. 1000?m = 1mm)

Some history about the development of the understanding of cell membrane structure and function is given in Chapter 1.5.

bilayer phospholipid membrane protein

phospholipid

Figure 117 The structure of a cell membrane

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TOPIC 1

CELLS AND MICROORGANISMS

The model used to describe the structure and function of the cell membrane is now known as the `Fluid-mosaic' model.

Fluid: This part of the terminology refers to the ability of both the lipids and proteins to move sideways within the membrane structure. The membrane relies on the fluid nature to carry out its functions.

Mosaic: The term mosaic refers to the pattern of protein molecules that are either embedded into or attached to the phospholipid bi-layer. It is these proteins that mainly determine the function of the cell membrane.

The light microscope

An instrument that is used to produce a much bigger (or magnified) image of what is viewed is called a microscope. The type of microscope students use in school laboratories is called a light microscope. What is viewed using a light microscope is called the specimen. Light microscopes are able to magnify up to about 1500?. A photograph (Figure 118(a)) and a diagram (Figure 118(b)) of a light microscope are shown below and you will probably have the opportunity to use one of these in the laboratory to examine cells and other structures.

lens

focussing mechanism arm

base

body tube revolving nosepiece

objective lens mechanical stage stage rack and pinion substage mirror

Figure 118(a) A photograph of a microscope Figure 118(b) A labelled drawing of a microscope

Figure 119 An electron microscope

The electron microscope

This is a very specialised microscope that uses a beam of electrons to provide images with a high degree of resolution enabling the user to see inside the workings of a cell.

Using an electron microscope it is possible to see images of structures magnified tens or even hundreds of thousands of times. A photograph of an electron microscope is shown in Figure 119

Key Concepts

1. All life forms have a number of distinguishing processes that identify them from nonliving things. 2. Cell theory states that all living organisms are made up of cells and the products of cells and that

cells come from pre-existing cells. 3. Cells are the unit of structure and function in all living organisms. 4. Cells contain DNA which is the molecule of heredity. 5. All organisms are either unicellular or multicellular. 6. All cells have a cell membrane which defines them.

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