Cells

Q: What are the five main organelles found in an animal cell and their basic functions?

The five main organelles are the nucleus (contains DNA and controls cell activities), cytoplasm (site of chemical reactions), cell membrane (controls entry and exit of substances), mitochondria (site of aerobic respiration to release energy), and ribosomes (site of protein synthesis).

Q: Which three structures are found in plant cells but not in animal cells, and what are their functions?

Plant cells additionally have a cellulose cell wall (provides rigid support and shape), chloroplasts (contain chlorophyll to absorb light for photosynthesis), and a permanent vacuole (filled with cell sap to keep the cell turgid).

Q: What is the purpose of adding iodine solution and lowering the coverslip at an angle when preparing an onion slide?

Iodine solution stains the cell parts (such as the nucleus and cell walls) to make them clearly visible, and lowering the coverslip at an angle prevents trapping air bubbles under the glass.

Q: What is a stem cell and how do embryonic and adult stem cells differ in differentiation potential?

A stem cell is an unspecialised cell that can divide to produce more stem cells or differentiate into specialised cells; embryonic stem cells are pluripotent (can form almost any cell type), whereas adult stem cells are multipotent (can only form a limited range of related cell types).

Q: Where are stem cells located in plants and how long do they retain the ability to differentiate?

Plant stem cells are found in meristems (at the growing tips of roots and shoots) and retain the ability to divide and differentiate into any plant cell type throughout the entire life of the plant.

Q: What is diffusion and is it an active or passive process?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient; it is a passive process requiring no energy input.

Q: Which four factors increase the rate of diffusion across a membrane?

Diffusion rate increases with a steeper concentration gradient , a larger surface area , a shorter diffusion distance (thinner membrane), and a higher temperature .

Biology 1010 · CCEA GCSE · 8 min read · Updated: 31 Aug 2026

Unit 1: Cells, Living Processes and Biodiversity

Section 1.1: Cells — Comprehensive Study Notes

Welcome to your study guide for Cells! Whether you are aiming for top marks or just trying to get your head around the basics for your CCEA GCSE Biology exam, this guide breaks everything down into simple, step-by-step ideas. Cells are the fundamental building blocks of all living things—mastering this topic gives you the foundation for the entire course.

1. Cell Structure and Organelle Functions

All living organisms are made of cells. Some organelles (cell parts) are found in almost all cells, while others are unique to plants or bacteria.

A. Animal Cell Structure

Animal cells have five key structures that you need to know:

• Nucleus: Contains genetic material (DNA/chromosomes) and controls the activities of the cell.
• Cytoplasm: A jelly-like substance where most chemical reactions take place (controlled by enzymes).
• Cell Membrane: A selectively permeable (or semi-permeable) barrier that controls what enters and leaves the cell.
• Mitochondria: The sites of aerobic respiration, where energy is released for cellular work.
• Ribosomes: Tiny structures where protein synthesis occurs (where proteins are made).

B. Plant Cell Structure

Plant cells have all five structures found in animal cells, plus three extra structures:

• Cell Wall: Made of tough cellulose fibres. It provides rigid support, tensile strength, and structural protection to keep the cell shape.
• Chloroplasts: Contain the green pigment chlorophyll, which absorbs light energy needed for photosynthesis.
• Permanent Vacuole: A large central space filled with cell sap (a weak solution of sugars and salts). It keeps the cell firm (turgid) and helps support the plant.

Memory Trick: Remember the three Cs of plant-only parts: Cell wall, Chloroplasts, and Central permanent vacuole filled with Cell sap!

C. Bacterial Cells (Prokaryotes)

Bacteria are single-celled organisms that are much smaller than plant and animal cells. They do not have membrane-bound organelles (there is no true nucleus, no mitochondria, and no chloroplasts).

Key features of a bacterial cell include:

• Chromosomal DNA: A single circular loop of DNA floating freely in the cytoplasm (not inside a nucleus).
• Plasmids: Small, extra circular rings of DNA that can carry specific genes.
• Cell Wall: Provides protection and structure (note: this is not made of cellulose).
• Cell Membrane, Cytoplasm, and Ribosomes: Similar functions as in other cells.

Common Examiner Pitfalls to Avoid

• Never call the nucleus the "brain" of the cell. Always state that it contains genetic material and controls the cell's activities.
• Never say mitochondria "make", "create", or "produce" energy. Energy cannot be created! Mitochondria release energy from glucose through aerobic respiration.
• Do not confuse the cell wall and cell membrane. The cell wall is completely permeable and gives structural support. The cell membrane is selectively permeable and controls what moves in and out.

Section Key Takeaway: Animal cells have 5 main parts; plant cells have those 5 plus a cellulose cell wall, chloroplasts, and a permanent vacuole; bacterial cells lack a nucleus and have a free-floating loop of DNA and plasmids.

2. Levels of Organisation

Complex living organisms are organised into a clear structural hierarchy:

\(\text{Organelle} \rightarrow \text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ System} \rightarrow \text{Organism}\)

Key Definitions

• Tissue: A group of similar specialised cells working together to perform a specific function (e.g., muscle tissue, epidermal tissue).
• Organ: A group of different tissues working together to carry out a particular function (e.g., heart, stomach, plant leaf).
• Organ System: A group of organs cooperating together to carry out a major life process (e.g., digestive system, circulatory system).
• Organism: A complete living entity made up of organ systems working together (e.g., a human, an oak tree).

Section Key Takeaway: Cells build tissues, tissues build organs, organs build systems, and systems make up the whole organism.

3. Microscopy and Calculations

A. Using a Light Microscope

When using a compound light microscope in the laboratory:

1. Always start by focusing with the lowest-power objective lens first. This gives the widest field of view and makes it easier to locate the specimen.
2. Turn the coarse focus knob to bring the stage close to the lens while looking from the side, then adjust to bring the image roughly into view.
3. Use the fine focus knob to make the image clear and sharp.

B. Preparing a Microscope Slide (Onion Epidermal Cells)

1. Peel a very thin layer (a single-cell monolayer) of epidermal tissue from the inner curve of an onion leaf using forceps.
2. Place the tissue flat onto a clean glass microscope slide with no folds.
3. Add a drop of iodine solution (this stain makes the nucleus and cell walls visible under the microscope).
4. Slowly lower a thin glass coverslip onto the slide at an angle using a mounted needle to avoid trapping air bubbles.

C. Calculating Total Magnification

\(\text{Total Magnification} = \text{Eyepiece Lens Magnification} \times \text{Objective Lens Magnification}\)

Example: If the eyepiece lens is \(\times 10\) and the objective lens is \(\times 40\):
\(\text{Total Magnification} = 10 \times 40 = \times 400\)

D. The Magnification Equation (\(I = A \times M\))

To calculate actual size, image size, or magnification, use the formula triangle:

\(\text{Image Size } (I) = \text{Actual Size } (A) \times \text{Magnification } (M)\)

\(\text{Magnification } (M) = \frac{\text{Image Size } (I)}{\text{Actual Size } (A)}\)

\(\text{Actual Size } (A) = \frac{\text{Image Size } (I)}{\text{Magnification } (M)}\)

E. Unit Conversions (Crucial Skill!)

In biology exams, image sizes are usually measured in millimetres (mm), but real cells are measured in micrometres (\(\mu\text{m}\)).

• \(1\text{ metre (m)} = 1\,000\text{ millimetres (mm)}\)
• \(1\text{ millimetre (mm)} = 1\,000\text{ micrometres (\mu m)}\)
• Standard form: \(1\text{ mm} = 1 \times 10^3\ \mu\text{m}\)

Rule: To convert \(\text{mm}\) to \(\mu\text{m}\), multiply by \(1\,000\). To convert \(\mu\text{m}\) to \(\text{mm}\), divide by \(1\,000\).

Step-by-Step Worked Example:
An image of a plant cell measures \(30\text{ mm}\) in length. The actual cell length is \(60\ \mu\text{m}\). Calculate the magnification.

Step 1: Convert units so they match.
\(30\text{ mm} \times 1\,000 = 30\,000\ \mu\text{m}\)

Step 2: Apply the formula.
\(\text{Magnification} = \frac{\text{Image Size}}{\text{Actual Size}} = \frac{30\,000\ \mu\text{m}}{60\ \mu\text{m}} = 500\)

Step 3: Write your answer with a multiplication symbol (\(\times\)).
\(\text{Answer} = \times 500\)

Examiner Warning: Never write units for magnification (e.g., do not write "\(500\text{ mm}\)"). Magnification is a multiplier, so write \(\times 500\).

Section Key Takeaway: Always convert both measurements to \(\mu\text{m}\) before using \(M = \frac{I}{A}\), and write magnification with a \(\times\) sign.

4. Stem Cells and Cell Differentiation

A. What is a Stem Cell?

A stem cell is an undifferentiated (unspecialised) cell that has the ability to divide by mitosis to produce more stem cells or differentiate into specialised cell types.

B. Types of Stem Cells

• Embryonic Stem Cells: Found in early human embryos. They are pluripotent, meaning they can differentiate into almost any type of specialised body cell (e.g., nerve cells, muscle cells, blood cells).
• Adult Stem Cells: Found in specific tissues such as bone marrow. They are multipotent, meaning they have a limited differentiation capacity and can only develop into related cell types (e.g., bone marrow stem cells can only become different types of blood cells).
• Plant Stem Cells (Meristems): Found in the growing tips of roots and shoots. Meristem cells retain the ability to divide and differentiate into any type of plant cell throughout the entire life of the plant.

C. Medical Applications and Ethical Debates

• Current Therapies: Bone marrow transplants are used successfully to treat blood disorders such as leukaemia by replacing faulty blood-producing stem cells.
• Potential Therapies: Scientists are researching stem cells to replace damaged cells in conditions like paralysis (repairing damaged spinal nerve tissue) and diabetes (producing insulin-secreting pancreas cells).
• Ethical Issues: The use of embryonic stem cells causes debate because harvesting them involves destroying a human embryo, which some people consider to be the destruction of potential human life.

Section Key Takeaway: Embryonic stem cells can form almost any cell type; adult stem cells have limited potential; plant meristems can differentiate throughout the plant's life.

5. Diffusion

A. What is Diffusion?

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient.

It is a passive process, meaning it does not require energy input from the cell.

B. Factors Affecting the Rate of Diffusion

There are four main factors that affect how fast diffusion happens:

1. Concentration Gradient: A steeper concentration gradient (a greater difference in concentration between two areas) results in a faster rate of diffusion.
2. Surface Area: A larger surface area of the membrane allows more particles to cross at the same time, leading to a faster rate of diffusion.
3. Diffusion Distance (Membrane Thickness): A shorter distance (thinner barrier or membrane) means particles have less distance to travel, resulting in a faster rate of diffusion.
4. Temperature: At higher temperatures, particles have more kinetic energy and move faster, leading to a faster rate of diffusion.

Section Key Takeaway: Diffusion is the movement of particles from high to low concentration. It is speeded up by a steeper gradient, larger surface area, shorter distance, and higher temperature.

Quick Revision Checklist

Before sitting your Unit 1 exam, make sure you can:

• Label all 5 parts of an animal cell and all 8 parts of a plant cell.
• Describe the difference between bacterial DNA and eukaryotic DNA.
• State the order of biological organisation from organelle to organism.
• Use \(I = A \times M\) accurately, converting \(\text{mm}\) to \(\mu\text{m}\) by multiplying by \(1\,000\).
• Explain the role of iodine stain and coverslips in slide preparation.
• Compare embryonic, adult, and plant meristem stem cells.
• State the definition of diffusion and explain all 4 factors that affect its rate.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the five main organelles found in an animal cell and their basic functions?

The five main organelles are the nucleus (contains DNA and controls cell activities), cytoplasm (site of chemical reactions), cell membrane (controls entry and exit of substances), mitochondria (site of aerobic respiration to release energy), and ribosomes (site of protein synthesis).

Which three structures are found in plant cells but not in animal cells, and what are their functions?

Plant cells additionally have a cellulose cell wall (provides rigid support and shape), chloroplasts (contain chlorophyll to absorb light for photosynthesis), and a permanent vacuole (filled with cell sap to keep the cell turgid).

How does the genetic material in a bacterial cell differ from that in plant and animal cells?

Bacteria lack a true nucleus; their genetic material consists of a single circular loop of chromosomal DNA floating freely in the cytoplasm, along with small extra rings of DNA called plasmids.

What is the hierarchy of organisation in multicellular organisms from simplest to most complex?

\(\text{Organelle} \rightarrow \text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ System} \rightarrow \text{Organism}\)

What is the purpose of adding iodine solution and lowering the coverslip at an angle when preparing an onion slide?

Iodine solution stains the cell parts (such as the nucleus and cell walls) to make them clearly visible, and lowering the coverslip at an angle prevents trapping air bubbles under the glass.

How is magnification calculated using the magnification formula triangle?

\(\text{Magnification } (M) = \frac{\text{Image Size } (I)}{\text{Actual Size } (A)}\), ensuring that both image size and actual size are converted into the same units first.

What is a stem cell and how do embryonic and adult stem cells differ in differentiation potential?

A stem cell is an unspecialised cell that can divide to produce more stem cells or differentiate into specialised cells; embryonic stem cells are pluripotent (can form almost any cell type), whereas adult stem cells are multipotent (can only form a limited range of related cell types).

Where are stem cells located in plants and how long do they retain the ability to differentiate?

Plant stem cells are found in meristems (at the growing tips of roots and shoots) and retain the ability to divide and differentiate into any plant cell type throughout the entire life of the plant.

What is diffusion and is it an active or passive process?

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient; it is a passive process requiring no energy input.

Which four factors increase the rate of diffusion across a membrane?

Diffusion rate increases with a steeper concentration gradient, a larger surface area, a shorter diffusion distance (thinner membrane), and a higher temperature.

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