KS3 Science Study Pack: Cells

Key Knowledge

Cells are the basic building blocks of living organisms. Every living thing is made of one or more cells. Most cells are too small to see clearly with the naked eye, so scientists use microscopes to view them.

In this topic you will learn how animal and plant cells are similar and different, how tiny cell structures carry out important jobs, how some cells are adapted for specialised functions, how diffusion moves particles into and out of cells, and how microscopes and magnification are used in biology.

By the end of this pack, you should be able to:

• describe cells as the smallest living units of organisms
• identify common structures in animal and plant cells
• explain the functions of organelles
• compare animal and plant cells using named structures
• explain how specialised cells are adapted to their jobs
• describe diffusion using particles and concentration
• use a light microscope safely and correctly
• calculate total magnification and actual size
• interpret diagrams, tables and practical results about cells and diffusion

What Are Cells?

A cell is the smallest living unit of an organism. Cells can carry out life processes such as respiration, growth, repair and responding to their surroundings.

Some organisms are unicellular, which means they are made of one cell. Examples include some simple organisms such as amoeba and yeast. Bacteria are also unicellular organisms, but their cells are simpler than plant and animal cells.

Other organisms are multicellular, which means they are made of many cells. Humans, oak trees, grass, cats, mushrooms and insects are multicellular. In multicellular organisms, cells often become specialised for particular jobs.

Examples:

• A human body contains many types of cells, including nerve cells, muscle cells, red blood cells and skin cells.
• A plant contains cells such as palisade cells, root hair cells and guard cells.
• Pondweed contains plant cells that may show chloroplasts clearly under a microscope.
• Onion epidermis cells are often used in school microscopy because they form a thin layer.

Most cells are microscopic. This means they can only be seen clearly using a microscope. A few cells are much larger, such as egg cells in some animals, but these are not typical examples.

From Cells To Organisms

In a multicellular organism, cells are organised in levels. Cells with similar jobs work together as tissues. Different tissues work together as organs. Organs work together in organ systems. Organ systems make up a whole organism.

cell -> tissue -> organ -> organ system -> organism

Human example:

muscle cell -> muscle tissue -> heart -> circulatory system -> human

Plant example:

palisade cell -> leaf tissue -> leaf -> shoot system -> plant

Level	Meaning	Human example	Plant example
Cell	Smallest living unit	Muscle cell	Palisade cell
Tissue	Group of similar cells working together	Muscle tissue	Palisade tissue
Organ	Different tissues working together	Heart	Leaf
Organ system	Organs working together	Circulatory system	Shoot system
Organism	Whole living thing	Human	Plant

Animal Cells

Animal cells contain several important structures. A small structure inside a cell with a particular function is called an organelle.

             Animal cell
        ______________________
       /                      \
      /   o  mitochondrion     \
     |                          |
     |        _________         |
     |       /         \        |
     |      | nucleus  |        |
     |       \_________/        |
     |                          |
     |  . . . ribosomes         |
     |                          |
     |   cytoplasm              |
      \                        /
       \______________________/
            cell membrane

Animal Cell Structures

Structure	Where found	Function	Useful memory clue
Nucleus	Plant and animal cells	Controls many cell activities and contains genetic information	Control centre
Cytoplasm	Plant and animal cells	Jelly-like substance where many chemical reactions happen	Cell jelly
Cell membrane	Plant and animal cells	Controls what enters and leaves the cell	Selective boundary
Mitochondria	Plant and animal cells	Where most aerobic respiration happens, releasing energy from food molecules	Respiration sites
Ribosomes	Plant and animal cells	Make proteins	Protein makers

The nucleus is not literally a brain. It controls many cell activities and contains genetic information, but it does not think.

The cell membrane is very important because cells need substances such as oxygen and glucose to enter, and waste substances such as carbon dioxide to leave.

Mitochondria do not make energy from nothing. They are where most aerobic respiration happens. Respiration releases energy from food molecules so the cell can use it for life processes.

Plant Cells

Plant cells contain the same basic structures as animal cells: nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Many plant cells also contain a cellulose cell wall, a permanent vacuole and chloroplasts.

                    Plant cell
        =================================
        | cell wall                     |
        |  ---------------------------  |
        |  | cell membrane           |  |
        |  |                         |  |
        |  |   chloroplasts  () ()   |  |
        |  |                         |  |
        |  |      ______________     |  |
        |  |     | permanent    |    |  |
        |  |     | vacuole      |    |  |
        |  |     |______________|    |  |
        |  |                         |  |
        |  | nucleus  (N)            |  |
        |  | mitochondria  o o       |  |
        |  | ribosomes . . .         |  |
        |  | cytoplasm fills space   |  |
        |  ---------------------------  |
        =================================

Plant Cell Structures

Structure	Where found	Function	How it helps the plant
Nucleus	Plant and animal cells	Controls many cell activities and contains genetic information	Helps control cell functions
Cytoplasm	Plant and animal cells	Jelly-like substance where many chemical reactions happen	Provides a place for cell reactions
Cell membrane	Plant and animal cells	Controls what enters and leaves the cell	Allows useful substances in and wastes out
Mitochondria	Plant and animal cells	Where most aerobic respiration happens	Releases energy for cell processes
Ribosomes	Plant and animal cells	Make proteins	Helps the cell make needed proteins
Cell wall	Plant cells	Rigid cellulose layer that supports and strengthens the cell	Helps the plant stay upright
Permanent vacuole	Plant cells	Contains cell sap and helps keep the cell firm	Supports the cell by pressing outwards
Chloroplasts	Many green plant cells	Contain chlorophyll, which absorbs light for photosynthesis	Allows the plant to make glucose in photosynthesis

Not every plant cell contains chloroplasts. Root hair cells usually do not contain chloroplasts because they are underground and do not receive light. Chloroplasts are common in green parts of a plant, especially leaves.

Comparing Plant And Animal Cells

Plant and animal cells are not completely different. They share several important structures.

Structure	Animal cells	Plant cells
Nucleus	Usually present	Usually present
Cytoplasm	Present	Present
Cell membrane	Present	Present
Mitochondria	Present	Present
Ribosomes	Present	Present
Cell wall	Absent	Present
Chloroplasts	Absent	Present in many green plant cells
Permanent vacuole	Absent, although small temporary vacuoles may occur	Usually large and permanent

Similarities:

• Both plant and animal cells have cytoplasm.
• Both have a cell membrane.
• Both usually have a nucleus.
• Both have mitochondria.
• Both have ribosomes.

Differences:

• Plant cells have a cellulose cell wall, but animal cells do not.
• Many plant cells have chloroplasts, but animal cells do not.
• Plant cells usually have a large permanent vacuole, but animal cells do not.
• Plant cells often have a more regular shape because the cell wall supports them.

Worked Example: Identifying A Cell Type

A drawing shows a cell with a cell wall, a permanent vacuole and chloroplasts.

Question: Is it most likely a plant cell or an animal cell? Justify your answer.

Model answer: It is a plant cell. The evidence is that it has a cell wall, a permanent vacuole and chloroplasts. Animal cells do not have these structures.

Organelles And Their Jobs

An organelle is a small structure inside a cell that has a specific function. At KS3, you need to know the names and jobs of the main organelles in plant and animal cells.

Worked Example: Identifying Organelles From Clues

Clue: This structure controls what enters and leaves a cell.

Answer: Cell membrane.

How to work it out: Match the clue to the function. The word "controls" and the phrase "enters and leaves" tell you this is the boundary of the cell. That job belongs to the cell membrane.

More examples:

Clue	Structure	Reason
Contains genetic information	Nucleus	The nucleus controls many activities and contains genetic information
Jelly-like substance where reactions happen	Cytoplasm	Many chemical reactions occur in the cytoplasm
Absorbs light for photosynthesis	Chloroplast	Chloroplasts contain chlorophyll
Supports and strengthens a plant cell	Cell wall	The cellulose cell wall is rigid
Contains cell sap and keeps a plant cell firm	Permanent vacuole	The vacuole presses outwards and supports the cell
Where most aerobic respiration happens	Mitochondria	Respiration releases energy from food molecules
Makes proteins	Ribosomes	Ribosomes are protein-making structures

Specialised Cells

A specialised cell is a cell with adaptations that help it carry out a particular job. An adaptation is a feature that helps a cell perform its function.

Good structure-function explanations use cause and effect:

• "The cell has..."
• "This increases..."
• "Therefore it can..."

Specialised Animal And Plant Cells

Cell type	Where it is found	Main job	Adaptation	How the adaptation helps
Red blood cell	Blood	Carry oxygen	Biconcave disc shape	Gives a large surface area for oxygen exchange
Red blood cell	Blood	Carry oxygen	Contains haemoglobin	Haemoglobin carries oxygen
Red blood cell	Blood	Carry oxygen	No nucleus when mature	Gives more space for haemoglobin
Sperm cell	Male reproductive system	Fertilise an egg cell	Tail	Helps it swim towards the egg
Sperm cell	Male reproductive system	Fertilise an egg cell	Many mitochondria	Release energy for movement
Egg cell	Female reproductive system	Join with a sperm cell and provide materials for early development	Large cytoplasm containing nutrients	Provides resources for early cell divisions
Egg cell	Female reproductive system	Reproduction	Cell membrane changes after fertilisation	Helps stop more than one sperm entering
Nerve cell	Nervous system	Carry electrical messages	Long fibre	Carries messages over long distances
Nerve cell	Nervous system	Carry electrical messages	Branched endings	Connects with other nerve cells or effectors
Muscle cell	Muscles	Contract to cause movement	Contains protein fibres that can shorten	Allows the cell to contract
Muscle cell	Muscles	Contract	Many mitochondria	Release energy needed for contraction
Ciliated epithelial cell	Airways	Move mucus and trapped dust away from lungs	Tiny hairs called cilia	Cilia beat to move mucus along
Root hair cell	Plant roots	Absorb water and mineral ions	Long hair-like projection	Increases surface area for absorption
Palisade cell	Near the top of leaves	Absorb light for photosynthesis	Many chloroplasts	More light can be absorbed
Guard cell	Leaf surface	Open and close stomata	Changes shape	Controls gas exchange and water loss
Xylem cell	Plant stems, roots and leaves	Transport water and support plant	Hollow dead tubes with strengthened walls	Water can move through and the plant is supported

Red Blood Cell Diagram

     Red blood cell, side/top view

          _____________
       .-'             '-.
      /     shallow       \
     |      dip in         |
     |      centre         |
      \                   /
       '-._____________.-'

  Biconcave disc shape = large surface area
  Main role = transport oxygen in the blood

Worked Example: Red Blood Cell Adaptation

Question: Explain how a red blood cell is adapted to carry oxygen.

Model answer: A red blood cell has a biconcave disc shape, which gives it a large surface area for oxygen to diffuse in and out. It contains haemoglobin, which carries oxygen. A mature red blood cell has no nucleus, so there is more space for haemoglobin.

Root Hair Cell Diagram

             Root hair cell

        ______________________
       |                      |
       |  nucleus             |----------------------.
       |  cytoplasm           |                      |
       |______________________|                      |
                                                      |
                         long hair-like extension     |
                         increases surface area       |
                         for absorbing water and      |
                         mineral ions                 |

Worked Example: Root Hair Cell Adaptation

Question: Explain how a root hair cell is adapted for absorption.

Model answer: A root hair cell has a long hair-like projection. This increases the surface area, so more water and mineral ions can be absorbed from the soil. Root hair cells usually do not contain chloroplasts because they are underground and do not receive light.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

Concentration means how much of a substance there is in a certain volume. A high concentration means there are many particles in a space. A low concentration means there are fewer particles in a space.

Particles in liquids and gases move randomly. Diffusion happens because of this random movement. Particles do not choose to move and they do not "want" to spread out. Over time, more particles move from the high concentration area to the low concentration area than in the opposite direction, so there is a net movement.

Everyday examples:

• A smell spreading through a room.
• Food colouring spreading through water.
• Oxygen moving from air in the lungs into the blood.
• Carbon dioxide moving from respiring cells into the blood.

Diffusion can happen in living and non-living systems. It is not only a process in living organisms.

Diffusion Across A Membrane

High oxygen concentration        Cell membrane        Low oxygen concentration

O O O O O O O O O O              |||||||||||||        O O
O O O O O O O O O O        --->  |||||||||||||  --->  O
O O O O O O O O O O              |||||||||||||        O

Net movement is from higher concentration to lower concentration.

Cell membranes control what enters and leaves cells. Small particles such as oxygen and carbon dioxide can diffuse across cell membranes.

Examples in organisms:

• Oxygen diffuses from the air sacs in the lungs into the blood because oxygen concentration is higher in the air sacs than in the blood entering the lungs.
• Carbon dioxide diffuses out of respiring cells into the blood because carbon dioxide concentration is higher in the cells.
• In leaves, carbon dioxide diffuses into leaf cells for photosynthesis, while oxygen can diffuse out.

Factors Affecting Diffusion Rate

Factor	Effect on diffusion	Example in cells or organisms
Concentration difference	A bigger difference usually makes diffusion faster	Oxygen diffuses quickly from air sacs into blood when the difference is large
Temperature	Higher temperature usually makes particles move faster, so diffusion is faster	Food colouring spreads faster in warm water
Surface area	Larger surface area allows more particles to cross at once	Root hair cells have a large surface area for absorption
Distance or thickness of barrier	Shorter distance or thinner barrier makes diffusion faster	Thin walls of air sacs help gas exchange

Worked Example: Diffusion Direction

Side of membrane	Oxygen concentration
Outside cell	80 arbitrary units
Inside cell	25 arbitrary units

Question: Which way will oxygen diffuse?

Model answer: Oxygen will diffuse from outside the cell to inside the cell. This is because oxygen moves by net movement from a higher concentration to a lower concentration across the cell membrane.

Microscopy

Microscopes are tools used to see objects that are too small to view clearly with the naked eye. In biology, light microscopes are used to view cells such as onion epidermis cells, cheek cells and pondweed cells.

Light Microscope Diagram

              eyepiece lens
                   ||
                   ||
                  /  \
                 /____\
                   ||
            objective lenses
                [x4 x10 x40]
                   ||
                  stage
             _____[slide]_____
            |      clips      |
            |_________________|
                   ||
              light source

        coarse focus knob  (large)
        fine focus knob    (small)

Microscope Parts

Part	Function	Correct use or safety point
Eyepiece lens	Lens you look through	Often x10 magnification
Objective lenses	Lenses near the specimen	Start with the lowest magnification objective
Stage	Platform that supports the slide	Keep the slide flat on the stage
Clips	Hold the slide in place	Use carefully so the slide does not crack
Light or mirror	Sends light through the specimen	Adjust light so the image is clear
Coarse focus knob	Moves stage or lens quickly for rough focusing	Use on low power first
Fine focus knob	Makes small focusing adjustments	Use to sharpen the image, especially on higher power

Safe And Correct Microscope Use

1. Carry the microscope carefully using two hands if instructed.
2. Place it on a stable bench.
3. Start with the lowest magnification objective lens.
4. Place the slide on the stage and secure it with clips.
5. Adjust the light source or mirror.
6. Use the coarse focus knob to bring the specimen into view.
7. Use the fine focus knob to sharpen the image.
8. Move to a higher magnification only after finding the specimen on low power.
9. Keep liquids away from electrical parts.
10. Follow your teacher's instructions for stains, slides and cleaning.

Higher magnification does not always mean a better image. It can be harder to focus and may need more light. Low power is best for first finding the specimen.

Preparing An Onion Epidermis Slide

1. Peel a very thin layer of onion epidermis.
2. Place it flat on a clean microscope slide.
3. Add a drop of water or iodine stain if your teacher tells you to use it.
4. Lower a coverslip carefully at an angle to reduce air bubbles.
5. Place the slide on the stage.
6. Observe using low power first, then higher power.
7. Draw and label visible structures such as the cell wall, cytoplasm, nucleus and vacuole if visible.

Safety notes:

• Handle slides and coverslips carefully because glass can break.
• Use iodine stain only as instructed by a teacher.
• Wash hands after practical work.

Viewing Cheek Cells

Cheek cells are animal cells, so they do not have a cell wall or chloroplasts.

Safety and hygiene:

• Use sterile cotton buds only.
• Do not share swabs.
• Dispose of swabs safely according to school rules.
• Wash hands and clean benches.
• Use stain only as instructed by a teacher.

Biological Drawings

A good biological drawing should include:

• a clear title
• clear single lines
• no shading
• labels with straight lines
• labels that do not cross each other
• correct scientific names
• magnification if known
• only what is actually observed

Worked Example: Drawing Cells From A Microscope

Student A draws onion cells with clear outlines, a title, straight label lines and no shading.

Student B draws onion cells with heavy shading, sketchy lines, no title and labels that cross.

Student A's drawing is better because it follows biological drawing rules. It is clear, labelled, titled and not shaded.

Magnification

Magnification means how many times larger an image is than the actual object.

Resolution means how much detail can be distinguished. Magnification and resolution are not the same thing. A blurry image can be highly magnified but still have poor resolution.

Total Microscope Magnification

To calculate total magnification:

total magnification = eyepiece magnification x objective magnification

Worked example:

eyepiece lens = x10
objective lens = x40

total magnification = eyepiece x objective
total magnification = 10 x 40
total magnification = x400

Magnification Equation

magnification = image size / actual size

Rearranged:

actual size = image size / magnification
image size = magnification x actual size

Worked example:

A cell image is 30 mm long. The actual cell is 0.06 mm long.

magnification = image size / actual size
magnification = 30 / 0.06
magnification = x500

Unit Conversion

Microscopy often uses millimetres and micrometres.

1 mm = 1000 micrometres

Worked example:

Convert 2 mm to micrometres.

2 mm = 2 x 1000
2 mm = 2000 micrometres

Real-World Examples

Cells and microscopes are used in many real situations:

• Doctors study blood samples to help diagnose disease.
• Scientists use microscopes to investigate microorganisms.
• Plant scientists check crops for disease.
• Medical laboratories study cheek cells, blood cells and other samples.
• Red blood cells transport oxygen around the body.
• Nerve cells carry electrical messages between body parts.
• Ciliated epithelial cells move mucus and trapped dust away from the lungs.
• Root hair cells absorb water and mineral ions from soil.
• Palisade cells absorb light for photosynthesis in leaves.
• Diffusion allows oxygen and carbon dioxide to move into and out of cells.

Working Scientifically: Investigating Cells And Diffusion

Scientists plan investigations carefully so results are valid and reliable.

Important terms:

• Independent variable: the variable changed by the scientist.
• Dependent variable: the variable measured.
• Control variables: variables kept the same to make the test fair.
• Fair test: an investigation where only the independent variable is changed.
• Repeatability: whether similar results are obtained when the same person repeats the method using the same equipment.
• Reliability: how trustworthy results are, often improved by repeats and consistent methods.
• Accuracy: how close a measurement is to the true value.
• Precision: how close repeated measurements are to each other.
• Evaluation: judging the quality of a method and results, including limitations and improvements.

Diffusion Investigation With Agar Cubes

Agar jelly cubes can be used as model cells. If the cubes contain indicator, they may change colour when acid or alkali diffuses into them. Smaller cubes often change colour throughout more quickly.

Important limitation: agar cubes are models. They are not living cells, do not have working organelles, and do not have living cell membranes.

Scientific idea	Example for agar cube diffusion investigation
Independent variable	Size of agar cube
Dependent variable	Time taken for the cube to change colour throughout
Control variables	Same agar type, same solution, same temperature, same volume of solution
Repeatability	Repeat each cube size several times
Accuracy	Use a stopwatch carefully and cut cubes to measured sizes
Possible improvements	Use more repeats, calculate a mean, use a ruler to cut cubes more accurately

Fair method summary:

1. Cut agar cubes of different sizes, such as 1 cm, 2 cm and 3 cm.
2. Measure each cube carefully.
3. Place each cube in the same volume and concentration of solution.
4. Keep the temperature the same.
5. Time how long each cube takes to change colour throughout.
6. Repeat each size at least three times.
7. Calculate a mean time for each cube size.
8. Compare results and look for anomalies.

Expected result: smaller cubes change colour throughout more quickly because the diffusion distance to the centre is shorter. Smaller cubes also have a larger surface area compared with their volume than larger cubes.

Data And Skills Tasks

Task 1: Microscope Magnification Table

Eyepiece lens	Objective lens	Total magnification
x10	x4	?
x10	x10	?
x10	x40	?
x15	x20	?

Questions:

1. Calculate each total magnification.
2. Which objective lens is best for first finding the specimen: x4, x10 or x40? Explain why.

Model answers:

1. x10 x x4 = x40; x10 x x10 = x100; x10 x x40 = x400; x15 x x20 = x300.
2. The x4 objective is best for first finding the specimen because it gives the lowest magnification and a wider view.

Task 2: Cell Size And Image Size

Image size	Magnification	Actual size
40 mm	x200	?
25 mm	x500	?
18 mm	x300	?

Use:

actual size = image size / magnification

Questions:

1. Calculate the actual size for each cell in mm.
2. Convert the first actual size into micrometres.

Model answers:

1. 40 / 200 = 0.20 mm; 25 / 500 = 0.05 mm; 18 / 300 = 0.06 mm.
2. 0.20 mm = 0.20 x 1000 = 200 micrometres.

Task 3: Plant Versus Animal Cell Identification

Unknown cell	Description
A	Has nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. No cell wall or chloroplasts are visible.
B	Has a cell wall, permanent vacuole, nucleus and chloroplasts.
C	Has a cell wall and a large vacuole, but no chloroplasts. It comes from a root.

Questions:

1. Identify each cell as plant or animal.
2. Justify each answer using named structures.

Model answers:

1. A is an animal cell. B is a plant cell. C is a plant cell.
2. A has only the shared structures and lacks a cell wall and chloroplasts, so it is likely to be animal. B has a cell wall, permanent vacuole and chloroplasts, so it is plant. C has a cell wall and large vacuole, so it is plant, even though it has no chloroplasts because root cells are underground.

Task 4: Diffusion Practical Results

A student investigated how temperature affects the time for a coloured substance to spread through water.

Temperature of water in degrees Celsius	Time for colour to spread in seconds
10	180
20	130
30	95
40	60
50	65

Questions:

1. Describe the overall pattern.
2. Identify the independent variable.
3. Identify the dependent variable.
4. Which result may be anomalous?
5. Suggest one improvement.

Model answers:

1. As temperature increases, the time for the colour to spread usually decreases, so diffusion is faster at higher temperatures.
2. Temperature of the water.
3. Time for the colour to spread.
4. The 50 degrees Celsius result may be anomalous because it takes longer than at 40 degrees Celsius, even though the pattern suggests it should be faster.
5. Repeat each temperature three times and calculate a mean, or control the volume of water and amount of colouring more carefully.

Task 5: Surface Area And Diffusion Model

Agar cube side length	Time to change colour throughout
1 cm	4 min
2 cm	9 min
3 cm	18 min

Questions:

1. Which cube changed colour throughout fastest?
2. Describe the pattern.
3. Explain the pattern using diffusion distance.
4. Give one limitation of using agar cubes as model cells.

Model answers:

1. The 1 cm cube changed colour fastest.
2. Larger cubes took longer to change colour throughout.
3. In larger cubes, particles have a longer distance to diffuse to reach the centre. Smaller cubes have a shorter diffusion distance and a larger surface area compared with their volume.
4. Agar cubes are not living cells and do not have real cell membranes or organelles.

Task 6: Specialised Cell Matching

Match each adaptation to the correct cell type and function.

Adaptation	Cell type	Function
Many chloroplasts	?	?
Tail and many mitochondria	?	?
Long hair-like projection	?	?
Cilia on the surface	?	?
Long fibre with branched endings	?	?

Model answers:

• Many chloroplasts: palisade cell, absorbs light for photosynthesis.
• Tail and many mitochondria: sperm cell, swims to fertilise an egg cell.
• Long hair-like projection: root hair cell, absorbs water and mineral ions.
• Cilia on the surface: ciliated epithelial cell, moves mucus and trapped dust.
• Long fibre with branched endings: nerve cell, carries electrical messages.

Full sentence example: A root hair cell has a long hair-like projection, which increases surface area so it can absorb more water and mineral ions from the soil.

Task 7: Microscope Method Ordering

Jumbled steps:

A. Use the fine focus knob to sharpen the image. B. Place the slide on the stage and secure it with clips. C. Start with the lowest power objective lens. D. Use the coarse focus knob to bring the specimen into view. E. Increase magnification if more detail is needed. F. Adjust the light source.

Correct order:

1. C
2. B
3. F
4. D
5. A
6. E

Safety reason: Starting on low power reduces the chance of crashing a high-power objective lens into the slide and makes the specimen easier to find.

Task 8: Biological Drawing Evaluation

Student A: uses clear single lines, no shading, straight label lines, a title and labels for cell wall and nucleus.

Student B: uses sketchy lines, dark shading, no title and labels that cross over each other.

Question: Which drawing is better and why?

Model answer: Student A's drawing is better because it uses clear single lines, has no shading, includes a title and uses straight label lines. These features make it a more accurate biological drawing.

Common Misconceptions

Misconception	Correct idea
All cells contain chloroplasts.	Chloroplasts are found in many green plant cells, but not in animal cells and not in all plant cells.
All cells are visible without microscopes.	Most cells are microscopic. Some cells can be larger, but this is not typical.
Plant cells and animal cells are completely different.	They share nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
The cell wall and cell membrane are the same thing.	The cell wall is a rigid support outside the membrane in plant cells. The cell membrane controls entry and exit.
The vacuole is empty.	The permanent vacuole contains cell sap and helps keep the plant cell firm.
The nucleus is literally a brain.	The nucleus controls many cell activities and contains genetic information, but it is not a brain.
Mitochondria make energy from nothing.	Mitochondria are where most aerobic respiration happens, releasing energy from food molecules.
Diffusion happens because particles choose to move.	Particles move randomly. Net movement is from higher concentration to lower concentration.
Diffusion only happens in living things.	Diffusion happens in gases and liquids in living and non-living systems.
Larger cells always exchange substances faster.	Smaller cells often exchange substances more effectively because diffusion distances are shorter and surface area compared with volume is greater.
Higher magnification always gives a better image.	Low power is best for finding the specimen first. High power only helps if the image is focused and well lit.
Microscope drawings should look artistic.	Biological drawings should be clear, simple, labelled and accurate, with no shading.
Magnification and resolution mean the same thing.	Magnification is how many times larger the image is. Resolution is how much detail can be distinguished.

Key Vocabulary

Term	Definition
Cell	Smallest living unit of an organism
Unicellular	Made of one cell
Multicellular	Made of many cells
Tissue	Group of similar cells working together
Organ	Structure made of different tissues working together
Organ system	Group of organs working together
Organism	A living thing
Organelle	Small cell structure with a specific function
Nucleus	Organelle that controls many cell activities and contains genetic information
Cytoplasm	Jelly-like substance where many chemical reactions happen
Cell membrane	Boundary that controls what enters and leaves the cell
Mitochondria	Organelles where most aerobic respiration happens
Ribosomes	Organelles that make proteins
Cell wall	Rigid cellulose layer that supports and strengthens plant cells
Permanent vacuole	Plant cell structure containing cell sap that helps keep the cell firm
Cell sap	Liquid inside the permanent vacuole
Chloroplast	Organelle containing chlorophyll that absorbs light for photosynthesis
Chlorophyll	Green pigment that absorbs light
Photosynthesis	Process by which plants make glucose using light
Specialised cell	Cell adapted for a particular function
Adaptation	Feature that helps a cell or organism carry out a function
Diffusion	Net movement of particles from higher concentration to lower concentration
Concentration	Amount of a substance in a certain volume
Microscopic	Too small to see clearly with the naked eye
Light microscope	Instrument using lenses and light to magnify small objects
Magnification	How many times larger an image is than the actual object
Resolution	Ability to distinguish detail in an image
Eyepiece lens	Lens you look through on a microscope
Objective lens	Lens near the specimen on a microscope
Stage	Platform that holds the microscope slide
Coarse focus	Microscope adjustment for rough focusing
Fine focus	Microscope adjustment for small focusing changes
Independent variable	Variable changed in an investigation
Dependent variable	Variable measured in an investigation
Control variable	Variable kept the same in a fair test
Reliability	How trustworthy results are, often improved by repeats
Repeatability	Whether repeating the same method gives similar results
Accuracy	How close a measurement is to the true value
Precision	How close repeated measurements are to each other
Evaluation	Judging the quality of a method and results

Practice Questions

Multiple-Choice Questions

1. Which structure controls what enters and leaves a cell? A. Nucleus
   B. Cell membrane
   C. Cell wall
   D. Ribosome

2. Which structure is found in plant cells but not animal cells? A. Cytoplasm
   B. Mitochondrion
   C. Cell wall
   D. Cell membrane

3. What is diffusion? A. Particles choosing to move where they are needed
   B. Net movement from low concentration to high concentration
   C. Net movement from high concentration to low concentration
   D. Movement only in living cells

4. Which microscope lens should usually be used first? A. Highest power objective
   B. Lowest power objective
   C. Eyepiece removed
   D. Any lens, because it makes no difference

5. Which cell is adapted with many chloroplasts? A. Red blood cell
   B. Palisade cell
   C. Sperm cell
   D. Ciliated epithelial cell

6. Which statement about root hair cells is correct? A. They usually contain many chloroplasts because they are underground.
   B. Their long projection increases surface area for absorption.
   C. They carry electrical messages.
   D. They transport oxygen in blood.

7. What is the function of mitochondria? A. To absorb light
   B. To support the cell wall
   C. To make proteins
   D. To release energy during aerobic respiration

8. If an eyepiece is x10 and the objective is x20, what is the total magnification? A. x30
   B. x100
   C. x200
   D. x2000

Fill-In-The-Blank Questions

Use these words: nucleus, cytoplasm, membrane, diffusion, chloroplasts, ribosomes, vacuole, tissue.

1. The jelly-like substance where many chemical reactions happen is the ________.
2. The ________ controls many cell activities and contains genetic information.
3. The cell ________ controls what enters and leaves the cell.
4. ________ make proteins.
5. ________ contain chlorophyll and absorb light for photosynthesis.
6. The permanent ________ contains cell sap.
7. A group of similar cells working together is a ________.
8. The net movement of particles from higher concentration to lower concentration is called ________.

Short-Answer Questions

1. State two structures found in both plant and animal cells.
2. State two structures found in plant cells but not animal cells.
3. Explain why a root hair cell usually does not contain chloroplasts.
4. Describe the job of the nucleus.
5. Explain why a cell membrane is important.
6. Give one example of diffusion in the human body.
7. State one safety rule when viewing cheek cells.
8. Why should microscope drawings not be shaded?

Diagram Labelling Question: Animal Cell

Label A-E using the words: nucleus, cytoplasm, cell membrane, mitochondrion, ribosomes.

        ______________________
       /                      \
      /     B                 \
     |          ______        |
     |         /  A   \       |
     |         \______/       |
     |                      C |
     |     D       . . . E    |
      \                      /
       \____________________/

Diagram Labelling Question: Plant Cell

Label A-H using the words: cell wall, cell membrane, cytoplasm, nucleus, chloroplast, permanent vacuole, mitochondrion, ribosomes.

        =========================
        | A                     |
        | --------------------  |
        | | B                |  |
        | |   C       () D   |  |
        | |                  |  |
        | |     _________    |  |
        | |    |    E    |   |  |
        | |    |_________|   |  |
        | |   F      o G     |  |
        | |       . . . H    |  |
        | --------------------  |
        =========================

Compare-And-Contrast Question

Compare plant and animal cells. Include at least three similarities and three differences.

Data Interpretation Question

A student measured carbon dioxide concentration on each side of a cell membrane.

Region	Carbon dioxide concentration
Inside respiring cell	70 units
Outside cell in blood	25 units

1. Which way will carbon dioxide diffuse?
2. Explain your answer using concentration language.
3. Name the cell structure it crosses.

Practical Method Question

A student wants to investigate whether temperature affects diffusion using food colouring and water.

1. Identify the independent variable.
2. Identify the dependent variable.
3. Give two control variables.
4. Explain why repeats would improve the investigation.
5. Suggest one possible source of error.

Calculation Questions

1. Eyepiece lens = x10. Objective lens = x40. Calculate total magnification.
2. Eyepiece lens = x15. Objective lens = x10. Calculate total magnification.
3. A cell image is 24 mm long. The actual cell is 0.08 mm long. Calculate magnification.
4. A cell image is 50 mm long at x250 magnification. Calculate the actual size.
5. Convert 0.15 mm into micrometres.

Longer 8-Mark Question

A student investigates diffusion using agar jelly cubes of different sizes.

Describe a fair method for the investigation. In your answer, include:

• the independent variable
• the dependent variable
• at least three control variables
• how repeats would be used
• the expected pattern of results
• why the pattern happens
• one limitation of using agar cubes as model cells

Model Answers

Multiple-Choice Answers

1. B. Cell membrane.
2. C. Cell wall.
3. C. Net movement from high concentration to low concentration.
4. B. Lowest power objective.
5. B. Palisade cell.
6. B. Their long projection increases surface area for absorption.
7. D. To release energy during aerobic respiration.
8. C. x200.

Fill-In-The-Blank Answers

1. cytoplasm
2. nucleus
3. membrane
4. ribosomes
5. chloroplasts
6. vacuole
7. tissue
8. diffusion

Short-Answer Model Answers

1. Any two from nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
2. Any two from cell wall, chloroplasts and permanent vacuole.
3. A root hair cell is underground and does not receive light, so it does not need chloroplasts for photosynthesis.
4. The nucleus controls many cell activities and contains genetic information.
5. The cell membrane controls what enters and leaves the cell, allowing useful substances in and waste substances out.
6. Oxygen diffuses from air sacs in the lungs into the blood, or carbon dioxide diffuses from cells into the blood.
7. Use sterile cotton buds, do not share swabs, dispose of swabs safely, wash hands, or use stain only as instructed.
8. Biological drawings should be clear and accurate. Shading can hide detail and make structures harder to see.

Animal Cell Diagram Answers

A = nucleus
B = cytoplasm
C = cell membrane
D = mitochondrion
E = ribosomes

Plant Cell Diagram Answers

A = cell wall
B = cell membrane
C = cytoplasm
D = chloroplast
E = permanent vacuole
F = nucleus
G = mitochondrion
H = ribosomes

Compare-And-Contrast Model Answer

Plant and animal cells are similar because both have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. The nucleus controls many cell activities, the cytoplasm is where many chemical reactions happen, and the cell membrane controls what enters and leaves the cell.

They are different because plant cells have a cellulose cell wall, a permanent vacuole and often chloroplasts, but animal cells do not. The cell wall supports and strengthens the plant cell. The permanent vacuole contains cell sap and helps keep the cell firm. Chloroplasts contain chlorophyll and absorb light for photosynthesis. Animal cells usually have a less regular shape because they do not have a rigid cell wall.

Data Interpretation Model Answer

1. Carbon dioxide will diffuse from inside the respiring cell to outside the cell into the blood.
2. This is because the concentration is higher inside the cell, at 70 units, than outside in the blood, at 25 units. Diffusion is the net movement from higher concentration to lower concentration.
3. It crosses the cell membrane.

Practical Method Model Answer

1. The independent variable is the temperature of the water.
2. The dependent variable is the time taken for the food colouring to spread, or a measured distance spread in a set time.
3. Control variables could include the volume of water, amount of food colouring, size of container, starting position of the colouring and method of timing.
4. Repeats improve reliability because anomalous results can be spotted and a mean can be calculated.
5. A source of error could be adding different amounts of food colouring each time or starting the stopwatch at slightly different times.

Calculation Model Answers

1. Total magnification = 10 x 40 = x400.
2. Total magnification = 15 x 10 = x150.
3. Magnification = image size / actual size = 24 / 0.08 = x300.
4. Actual size = image size / magnification = 50 / 250 = 0.20 mm.
5. 0.15 mm = 0.15 x 1000 = 150 micrometres.

Longer 8-Mark Model Answer

The student could investigate diffusion using agar jelly cubes of different sizes, such as 1 cm, 2 cm and 3 cm cubes. The independent variable is the size of the agar cube. The dependent variable is the time taken for the cube to change colour throughout. Control variables should include the concentration of the solution, the volume of solution, the temperature, the type of agar and the timing method.

The student should cut the cubes accurately using a ruler, place each cube into the same volume and concentration of solution, and start timing. They should record how long each cube takes to change colour throughout. Each cube size should be repeated at least three times and a mean should be calculated. This improves reliability and helps identify anomalies.

The expected result is that smaller cubes change colour throughout faster than larger cubes. This is because particles have a shorter distance to diffuse to reach the centre of a smaller cube. Smaller cubes also have a larger surface area compared with their volume. A limitation is that agar cubes are only models. They are not living cells and do not have real cell membranes, cytoplasm or organelles.

Revision Checklist

Use this checklist to test your revision.

Cells And Organisation

• I can define a cell as the smallest living unit of an organism.
• I can explain the difference between unicellular and multicellular organisms.
• I can put cell, tissue, organ, organ system and organism in the correct order.
• I can give a human example of cell organisation.
• I can give a plant example of cell organisation.

Animal And Plant Cells

• I can label an animal cell diagram.
• I can label a plant cell diagram.
• I can state the functions of the nucleus, cytoplasm and cell membrane.
• I can state the functions of mitochondria and ribosomes.
• I can state the functions of the cell wall, chloroplasts and permanent vacuole.
• I can compare plant and animal cells using similarities and differences.
• I can explain why not every plant cell contains chloroplasts.

Specialised Cells

• I can define specialised cell and adaptation.
• I can explain how a red blood cell is adapted to carry oxygen.
• I can explain how a root hair cell is adapted for absorption.
• I can describe adaptations of sperm cells, egg cells, nerve cells, muscle cells and ciliated epithelial cells.
• I can describe adaptations of palisade cells and guard cells or xylem cells.
• I can write structure-function explanations using clear cause and effect.

Diffusion

• I can define diffusion as net movement from higher concentration to lower concentration.
• I can explain diffusion using random particle movement.
• I can predict the direction of diffusion from concentration data.
• I can describe diffusion across a cell membrane.
• I can explain why diffusion is important for gas exchange.
• I can describe how concentration difference, temperature, surface area and diffusion distance affect diffusion rate.

Microscopy

• I can label the eyepiece lens, objective lenses, stage, clips, light source, coarse focus and fine focus.
• I can describe how to use a microscope safely.
• I can explain why low power is used first.
• I can describe how to prepare an onion epidermis slide.
• I can state safety rules for viewing cheek cells.
• I can describe the features of a good biological drawing.

Magnification And Data

• I can calculate total magnification using eyepiece x objective.
• I can use magnification = image size / actual size.
• I can rearrange the magnification equation to calculate actual size.
• I can convert between mm and micrometres using 1 mm = 1000 micrometres.
• I can interpret tables about cell size, diffusion and magnification.
• I can identify patterns, anomalies, limitations and improvements in practical results.

Working Scientifically

• I can identify independent, dependent and control variables.
• I can describe a fair test for diffusion.
• I can explain why repeats improve reliability.
• I can comment on accuracy and possible sources of error.
• I can evaluate a model, such as agar cubes representing cells.