Part 1 of 3 · Cells — the Building Blocks of Life

TaskRead the passage, then answer the questions.

All living organisms are made up of cells. A cell is the smallest unit found in an organism, so cells are often called the building blocks of life. Cells are extremely small — about 100 animal cells would fit across the width of a full stop on this page.

Organisms differ enormously in how many cells they have. Some organisms, such as bacteria, are made of just one cell. A human being is made of many millions of cells. Around five million of your body cells die every second, and your body replaces them.

Cells were discovered in 1665 by Robert Hooke. Hooke built one of the first microscopes and used it to look at a thin slice of cork, which is a type of tree bark. He saw tiny boxes in the cork and named them cells. His microscope magnified about 30 times, which is very weak by modern standards, because the glass lenses were of poor quality.

Looking carefully and in detail at an object is called making an observation. Scientists record their observations by drawing what they see and writing down how much the object has been magnified.

Part 2 of 3 · Using a Light Microscope

TaskStudy the diagram, then read the passage below.

A light microscope shines a beam of light through the specimen and uses two lenses to magnify it. You look down the eyepiece lens. The objective lens is the lens closest to the specimen, and most microscopes have several objective lenses of different strengths on a turning nosepiece. The specimen sits on a glass slide held on the stage by clips.

The best modern light microscopes magnify up to about 2000 times and have a resolution of about 200 nanometres. That is a huge improvement on Hooke's microscope, because of better lenses, brighter light sources and stains that add contrast to the specimen.

Part 3 of 3 · Magnification and Resolution

TaskRead the passage, then work through the example.

Magnification tells you how much bigger something appears compared with its actual size. Both lenses in a microscope magnify the specimen, so you multiply them together to find the total magnification.

Resolution is different. It is the smallest distance between two points that can still be seen as two separate points rather than blurring into one. A microscope with poor resolution gives a large but fuzzy image, so high magnification is only useful if the resolution is good as well.

TaskComplete the table using the equation above.

Part 1 of 3 · Animal Cells and Their Organelles

TaskRead the passage and study the diagram.

When you look at cells through a microscope you can see smaller parts inside them. These parts are called organelles, and each one has an important function — a particular job it does for the cell. Animal cells have an irregular shape.

The nucleus controls everything that happens inside the cell, including its growth, and it contains the genetic information stored as DNA. The cytoplasm is a jelly-like substance that fills the cell and is where the chemical reactions of life take place. The cell membrane forms a barrier around the cell and controls which substances can enter and leave. The mitochondria are where respiration happens; respiration releases the energy the cell needs to carry out all its functions.

TaskComplete the table of organelles found in an animal cell.

Part 2 of 3 · Plant Cells and Their Extra Organelles

TaskRead the passage and study the diagram.

Plant cells contain all four of the organelles found in animal cells, plus three more. They also have a more regular, box-like structure, which allows them to fit together like bricks in a wall.

The cell wall is the protective outer layer of a plant cell. It is made of a tough fibre called cellulose, which makes the wall rigid and gives the cell strength and support. Do not confuse it with the cell membrane, which lies just inside it.

Chloroplasts are where photosynthesis happens. They contain a green pigment called chlorophyll, which absorbs energy transferred from the Sun. Chloroplasts are found in the green parts of a plant, so root cells do not contain them — there is no light underground.

The vacuole is a large space filled with a watery liquid called cell sap. It pushes outwards on the cell wall and keeps the cell firm, which helps the plant stand upright. Vacuoles can also store nutrients and waste products.

TaskComplete the table of the three extra plant organelles.

Part 3 of 3 · Comparing Plant and Animal Cells

TaskLook back at Fig 2.1 and Fig 2.2, then complete the table by writing plant, animal or both in each row.

Part 1 of 3 · What Is a Specialised Cell?

TaskRead the passage, then answer the questions.

An adaptation is a feature that makes an organism better suited to its environment or to its job. Polar bears have white fur so they are camouflaged in snow; camels have wide flat feet so they can walk on sand; giraffes have long necks so they can reach leaves high in a tree.

Cells can be adapted too. Most cells in your body contain a nucleus, cytoplasm, a cell membrane and mitochondria. However, many cells have changed their shape and structure so that they are suited to carry out one particular job. These changes are called structural adaptations, and cells with them are called specialised cells.

Your body contains over 200 different types of cell. If you look carefully at a specialised cell, its shape and its special features give you clues about what it does. A cell that needs to absorb a lot of something will usually have a large surface area; a cell that needs to move will usually have a streamlined shape and plenty of mitochondria.

Part 2 of 3 · Specialised Animal Cells

TaskStudy the diagrams, then read the passage.

Red blood cells transport oxygen around the body. They contain haemoglobin, a red pigment that joins to oxygen. Unlike almost every other animal cell, a red blood cell has no nucleus, which leaves more space for haemoglobin. Its disc-like shape, squashed in the centre, increases its surface area for taking up oxygen.

Nerve cells, also called neurones, carry electrical impulses around your body. They are very long and thin, so a single cell can reach from one part of the body to another, and they have branching connections at each end where they join to other nerve cells.

Sperm cells carry male genetic material to fertilise an egg cell. They have a streamlined head and a long tail, which lets the cell swim through liquid, and a large number of mitochondria just below the head to release the energy that powers the tail. The head also contains enzymes that let it digest its way into the egg cell.

Egg cells are large and bulky and are found in the ovaries. Their cytoplasm contains a large store of nutrients to supply energy and growth after fertilisation, and a jelly coat that protects the egg and hardens after fertilisation to stop other sperm entering.

Ciliated cells line the air passages of your lungs and throat. They are covered in tiny hairs called cilia which beat together to sweep mucus, trapped dust and bacteria back up the throat, protecting the lungs from damage.

Part 3 of 3 · Specialised Plant Cells

TaskRead the passage, then complete the table.

Palisade cells are found near the top of a leaf and carry out photosynthesis. They are tall and thin and packed with chloroplasts, which gives them a large surface area for absorbing energy transferred from the Sun. Being near the top of the leaf means they receive light first.

Root hair cells are found in plant roots and absorb water and minerals from the soil. Each cell has a long hair-like extension, which greatly increases its surface area for absorbing. The cell wall is thin, so minerals pass through easily, and the large vacuole stores the water taken in. Root hair cells contain no chloroplasts, because no light reaches them underground.

TaskComplete the table of specialised cells.

Part 1 of 3 · Diffusion

TaskRead the passage and study the diagram.

Imagine your favourite food being cooked in the kitchen. You will usually smell the food long before you see it. The particles that carry the smell spread out from the kitchen, where there are lots of them, into the rest of the house, where there are very few. This spreading out is called diffusion.

Diffusion is the random movement of particles spreading out from an area of high concentration to an area of low concentration. Concentration means how many particles of a substance there are in a given space. Nothing pushes the particles — they simply move about randomly, and because there are more of them in the crowded area, more move out of it than into it.

Diffusion continues until the concentration of the particles is the same everywhere. That is why, some time after the toast burns, you can smell it just as strongly in the living room as in the kitchen.

Part 2 of 3 · Substances Moving In and Out of Cells

TaskRead the passage, then complete the practical.

All the cells in your body need glucose, a substance gained from food, and oxygen so they can carry out respiration. During respiration, energy is transferred for the cell to use. Glucose and oxygen are carried around your body in the blood, and they diffuse into the cells that need them.

Some reactions inside cells make waste products. Carbon dioxide is produced during respiration. It diffuses out of the cell into the blood, and the blood transports it to the lungs where you breathe it out.

Diffusion does not always happen at the same speed. Raising the temperature makes particles move faster, because they have more energy, so they diffuse at a faster rate.

TaskRecord your observations in the table.

Part 3 of 3 · Diffusion in Plant Cells

TaskRead the passage and study the diagram.

Plants need a constant supply of water for photosynthesis. Water diffuses into a plant through the root hair cells. There is a high concentration of water in the soil and a lower concentration inside the root hair cell, so water moves into the cell by diffusion. It then travels up the stem in the veins and into the leaves.

Inside a leaf cell, water diffuses through the cell wall and the cell membrane into the vacuole. As the vacuole fills, it pushes outwards on the cell wall and makes the cell rigid. Millions of firm cells together hold the plant upright.

If a plant is not watered, the vacuoles shrink. The cells become floppy, they no longer push out on their cell walls, and the plant wilts. If the plant is watered again quickly the vacuoles refill and the plant recovers; if not, it eventually dies.

Part 1 of 3 · What Is a Unicellular Organism?

TaskRead the passage, then answer the questions.

Not all living organisms are as complicated as you are. The first organisms on Earth were made up of a single cell, and many organisms alive today still consist of only one cell. These are called unicellular organisms: uni means one and cellular means made from cells.

A unicellular organism must do everything with just one cell. That single cell has to feed, grow, move, remove its waste and reproduce. Plants and animals are multicellular — they are made of many cells, and different specialised cells share the work between them.

Two unicellular organisms you will study are the amoeba and the euglena. Both live in water, both have a nucleus and a cell membrane like an animal cell, and both reproduce by binary fission. In binary fission the nucleus divides first, then the cytoplasm divides, producing two identical cells.

Part 2 of 3 · The Amoeba

TaskStudy the diagram, then read the passage.

An amoeba is a unicellular organism with no fixed shape. Amoebae are found in fresh water, salt water, wet soil and even inside animals. The nucleus controls the cell's growth and reproduction and the cell membrane forms its boundary.

An amoeba moves by changing the shape of its body. It pushes out an extension called a pseudopod in the direction it wants to travel, and the rest of the cell slowly flows after it.

Amoebae eat algae, bacteria and plant cells. The amoeba stretches a pseudopod around the piece of food until it has surrounded it completely, forming a food vacuole. This is called engulfing. The food vacuole then digests the food.

Living in fresh water means water keeps entering the cell. The contractile vacuole collects this excess water and expels it, which stops the amoeba bursting.

Part 3 of 3 · The Euglena

TaskStudy the diagram, then read the passage.

Like amoebae, euglenas contain cytoplasm, a nucleus, a cell membrane and a contractile vacuole. Unlike amoebae, they have a fixed shape and they contain chloroplasts, which make them look green. The chloroplasts trap energy transferred from the Sun so a euglena can make its own food by photosynthesis.

A euglena also has an eye spot, which detects light, and a flagellum. The flagellum is a long tail-like structure that spins like a propeller, making the euglena swim towards the light. Swimming into the light lets it maximise the amount of food it makes.

When there is not enough light to photosynthesise, a euglena feeds like an amoeba. It surrounds and engulfs other microorganisms, such as bacteria and algae. Being able to feed in two ways means a euglena has features of both a plant and an animal.

TaskComplete the comparison table.