Unit 1: Biology – Cells Study Notes
Welcome to your study guide for Cells! Cells are the microscopic building blocks of every living thing on Earth, from the tiniest bacterium to giant oak trees and human beings. Understanding cells is the foundation of GCSE Biology. Don't worry if some of the terminology looks unfamiliar right now — we will break down each concept step by step.
---1. Cell Structure and Organelles
An organelle is a tiny structure inside a cell that carries out a specific job. Think of a cell like a bustling miniature factory where each organelle has its own workstation.
Animal Cells
Animal cells contain four main structures that you need to know:
• Nucleus: Contains genetic material (chromosomes made of DNA) and controls the activities of the cell. Analogy: The main office or control centre of the factory.
• Cytoplasm: A jelly-like substance where most chemical reactions take place.
• Cell Membrane: A selectively permeable (semi-permeable) barrier that controls which substances enter and leave the cell. Analogy: The security gates at the factory entrance.
• Mitochondria: The site of aerobic respiration, where energy is released from glucose for the cell to use.
Plant Cells
Plant cells contain all four structures found in animal cells (nucleus, cytoplasm, cell membrane, and mitochondria), plus three extra structures:
• Cell Wall: A rigid outer layer made of cellulose. It provides shape, strength, and structural support to the cell.
• Chloroplasts: Structures containing the green pigment chlorophyll, which absorbs light energy needed for photosynthesis.
• Large Permanent Vacuole: A fluid-filled space containing cell sap (a watery solution of sugars and salts). It helps keep the cell firm and turgid, supporting the plant.
Bacterial Cells
Bacterial cells are much smaller and simpler than plant and animal cells. They are prokaryotic organisms:
• They do not have a membrane-bound nucleus. Instead, their genetic material (DNA) floats freely in the cytoplasm or in small circular rings called plasmids.
• They are surrounded by a cell membrane and a cell wall (which has a different structure from plant cell walls).
Examiner Warnings & Common Pitfalls
• Mitochondria Misconception: Never say mitochondria "create" or "produce" energy. Energy cannot be created; it is released during respiration. Also, remember that respiration is a chemical reaction, not the same thing as breathing!
• Plant Cells Have Mitochondria Too: A very common mistake is thinking plants only have chloroplasts. Plant cells need to respire day and night, so they contain both chloroplasts and mitochondria.
• Cell Wall vs. Cell Membrane: The cell wall does not control what enters and leaves (it is fully permeable). The cell membrane controls entry and exit.
Key Takeaway: Animal cells have 4 main parts (nucleus, cytoplasm, membrane, mitochondria). Plant cells have all 4 of these PLUS 3 extras (cell wall, chloroplasts, permanent vacuole). Bacteria have no nucleus.
---2. Levels of Biological Organisation
Multicellular organisms (like humans and plants) are organized into hierarchical levels. Each level increases in size and complexity:
\(\text{Organelle} \rightarrow \text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ System} \rightarrow \text{Organism}\)
• Organelle: A specialised structure within a cell (e.g., nucleus, mitochondrion).
• Cell: The basic structural and functional unit of all living organisms (e.g., muscle cell, skin cell).
• Tissue: A group of similar cells working together to perform a shared function (e.g., muscle tissue).
• Organ: A collection of different tissues working together to carry out a specific job (e.g., the heart, stomach, or a plant leaf).
• Organ System: A group of related organs working together to perform a major bodily function (e.g., the circulatory system, digestive system).
• Organism: A complete, independent living individual made up of cooperating organ systems (e.g., a human, a dog, an oak tree).
Key Takeaway: Cells join to form tissues, tissues combine into organs, organs make up organ systems, and organ systems create a whole organism.
---3. Microscopy and Magnification
Because cells are microscopic, biologists use light microscopes to view them.
Key Parts of a Light Microscope
• Eyepiece Lens: The lens you look through at the top (usually has a magnification of \(10\times\)).
• Objective Lenses: A set of lenses on a rotating nosepiece with different magnification powers (e.g., \(4\times\), \(10\times\), \(40\times\)).
• Stage & Stage Clips: The flat platform where the glass slide is placed and held securely.
• Light Source / Mirror: Directs light upward through the specimen.
• Coarse Focus Knob: Moves the stage up and down rapidly to bring the image into general focus.
• Fine Focus Knob: Moves the stage slightly to sharpen the image clarity.
Calculating Total Magnification
To find out how much larger a microscope is making an object appear:
\(\text{Total Magnification} = \text{Magnification of Eyepiece Lens} \times \text{Magnification of Objective Lens}\)
Example: If the eyepiece is \(10\times\) and the objective lens is \(40\times\):
\(\text{Total Magnification} = 10 \times 40 = 400\times\)
The Magnification Formula: The \(I / A M\) Triangle
In written exam questions, you will often need to calculate magnification, image size, or actual size using this core formula:
\(\text{Magnification} = \frac{\text{Image Size}}{\text{Actual Size}} \quad \left(M = \frac{I}{A}\right)\)
You can rearrange this formula:
• To find Actual Size: \(\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}} \quad \left(A = \frac{I}{M}\right)\)
• To find Image Size: \(\text{Image Size} = \text{Magnification} \times \text{Actual Size} \quad \left(I = M \times A\right)\)
Essential Unit Conversion
Cell measurements are usually given in micrometres (\(\mu\text{m}\)), while measurements taken with a ruler on exam papers are in millimetres (\(\text{mm}\)).
\(1\text{ mm} = 1000\text{ }\mu\text{m}\)
• To convert \(\text{mm}\) to \(\mu\text{m}\): Multiply by \(1000\). (e.g., \(5\text{ mm} \times 1000 = 5000\text{ }\mu\text{m}\))
• To convert \(\mu\text{m}\) to \(\text{mm}\): Divide by \(1000\). (e.g., \(250\text{ }\mu\text{m} \div 1000 = 0.25\text{ mm}\))
Step-by-Step Worked Example
Question: A cell image has a measured width of \(12\text{ mm}\). The magnification is \(400\times\). Calculate the actual size of the cell in micrometres (\(\mu\text{m}\)).
Step 1: Convert image size from \(\text{mm}\) to \(\mu\text{m}\):
\(12\text{ mm} \times 1000 = 12000\text{ }\mu\text{m}\)
Step 2: Use the formula \(A = \frac{I}{M}\):
\(\text{Actual Size} = \frac{12000\text{ }\mu\text{m}}{400} = 30\text{ }\mu\text{m}\)
Key Takeaway: Always check your units before calculating magnification. Remember that \(1\text{ mm} = 1000\text{ }\mu\text{m}\).
---4. Movement of Substances: Diffusion
Cells need to take in useful substances (like oxygen and glucose) and remove waste products (like carbon dioxide). One primary way substances move across the cell membrane is by diffusion.
What is Diffusion?
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.
• Passive Process: Diffusion does not require energy (ATP) from the cell.
• Everyday Analogy: If someone sprays perfume in one corner of a classroom, the scent particles gradually spread out across the room from where they are highly concentrated to where there are fewer particles.
Factors Affecting the Rate of Diffusion
Three main factors speed up or slow down diffusion:
• Concentration Gradient: A bigger difference in concentration between two areas makes particles move and spread out faster.
• Temperature: At higher temperatures, particles have more kinetic energy, so they move and diffuse more rapidly.
• Surface Area: A larger surface area allows more particles to move across a membrane at the same time, increasing the rate of diffusion.
Key Takeaway: Diffusion is the passive movement of particles from high to low concentration. It is speeded up by a steeper gradient, higher temperature, and larger surface area.
---5. Stem Cells and Differentiation
What is a Stem Cell?
Most cells in an adult body are specialised (e.g., red blood cells, nerve cells) — they have a specific shape and features to perform one role and cannot turn into other cell types.
A stem cell is an undifferentiated cell that retains the ability to divide and develop into specialised cell types.
Sources of Stem Cells
• Embryonic Stem Cells: Taken from early-stage embryos. They have the ability to differentiate into almost any type of cell in the body.
• Adult Stem Cells: Found in certain adult tissues, such as bone marrow. They have a limited ability to differentiate (e.g., bone marrow stem cells can only form different types of blood cells).
• Plant Meristems: Found in the growing tips of plant roots and shoots. Meristem cells can divide and differentiate into any plant cell type throughout the entire life of the plant.
Medical Applications of Stem Cells
Because stem cells can replace damaged or diseased tissues, doctors use them (or are researching their use) to treat conditions such as:
• Leukaemia: A cancer of the blood, treated using bone marrow stem cell transplants.
• Paralysis: Replacing damaged nerve cells in the spinal cord.
• Diabetes: Replacing insulin-producing cells in the pancreas.
The Ethical Debate
While stem cell research offers enormous medical hope, the use of embryonic stem cells raises ethical concerns:
• Arguments in favour: Can treat previously incurable diseases, improve quality of life, and use spare embryos from fertility clinics that would otherwise be destroyed.
• Ethical objections: An embryo has the potential to become a human life; some people believe destroying an embryo to harvest stem cells is ethically wrong.
Key Takeaway: Embryonic stem cells can become almost any cell type, while adult stem cells are more limited. Plant meristems produce new cells throughout a plant's life.
---Quick Chapter Review: Checklist for Success
Check if you can confidently answer each of these questions before your exam:
1. Can you name the 4 parts in an animal cell and explain what each one does?
2. Can you name the 3 extra parts in a plant cell and state their functions?
3. Can you describe the structure of a bacterial cell?
4. Can you write down the levels of organisation from organelle to organism in order?
5. Can you use \(M = \frac{I}{A}\) and convert \(\text{mm}\) to \(\mu\text{m}\)?
6. Can you give the exact definition of diffusion and list three factors that speed it up?
7. Can you compare embryonic and adult stem cells and state two medical conditions they can treat?