Welcome to Cells: The Building Blocks of Life!
Think of your favourite building or house. No matter how huge or complicated it looks from the outside, it is built from individual bricks placed side by side. In living things, those bricks are called cells!
Whether you are looking at a giant oak tree, a pet dog, or yourself in the mirror, every living organism is made of cells. In this chapter for CCEA GCSE Double Award Science (Unit B1), we will explore what is inside different types of cells, how we can view and measure them using microscopes, and how substances move into and out of them. Don't worry if biology feels tricky at times—we will take this step by step!
1. Cell Structures and Their Functions
Cells are tiny, but inside they have specialised parts called sub-cellular structures (or organelles). Each part has a specific job to keep the cell alive.
Animal Cells
Animal cells have four main parts that you must know:
• Nucleus: The control centre of the cell. It contains genetic material (DNA) that controls cell activities.
• Cytoplasm: A jelly-like substance where most chemical reactions take place.
• Cell Membrane: A flexible outer boundary that controls what enters and leaves the cell (like glucose and oxygen coming in, and waste products going out).
• Mitochondria (singular: mitochondrion): The powerhouses of the cell! This is where aerobic respiration happens, releasing energy 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:
• Cellulose Cell Wall: A rigid outer layer made of cellulose that gives the plant cell strength and support, preventing it from bursting.
• Chloroplasts: Green structures that contain the green pigment chlorophyll. Chlorophyll absorbs light energy for photosynthesis.
• Permanent Vacuole: A large space in the centre of the cell filled with cell sap (a watery solution of sugars and salts) that helps keep the cell firm and rigid (turgid).
Memory Trick: To remember the three parts found only in plant cells, remember C-C-V: Cell wall, Chloroplasts, and Vacuole!
Bacterial Cells
Bacteria are single-celled, microscopic organisms. They are much smaller and simpler than animal and plant cells. They are described as prokaryotic because they do not have a true nucleus.
Key features of a bacterial cell:
• No nucleus: Their genetic material is a circular loop of DNA floating freely in the cytoplasm.
• Plasmids: Small, circular rings of extra DNA.
• Non-cellulose Cell Wall: Provides protection and structure (note: it is not made of cellulose).
• Cell membrane and cytoplasm: Just like animal and plant cells.
• No mitochondria or chloroplasts: Bacterial cells lack membrane-bound organelles.
Common Mistake to Avoid: Many students write that the cell wall "controls what enters and leaves". This is incorrect! The cell membrane controls entry and exit; the cell wall provides structure and support.
Key Takeaway for Section 1: Animal cells have 4 main parts (nucleus, cytoplasm, membrane, mitochondria). Plant cells have those 4 plus 3 extras (cell wall, chloroplasts, permanent vacuole). Bacterial cells lack a nucleus and mitochondria, having a loop of DNA and plasmids instead.
2. Microscopes and Magnification Calculations
Because cells are microscopic, we need light microscopes to see them. Let's look at how we calculate how much bigger an image appears.
Total Magnification of a Light Microscope
A light microscope has two lenses: the eyepiece lens (where you look) and the objective lens (near the slide). To find the total magnification, multiply their powers together:
\(\text{Total Magnification} = \text{Eyepiece Lens Magnification} \times \text{Objective Lens Magnification}\)
Example: If the eyepiece lens is \(\times 10\) and the objective lens is \(\times 40\):
\(\text{Total Magnification} = 10 \times 40 = \times 400\)
The Magnification Equation (\(IAM\))
To calculate magnification from an image or diagram, use the classic formula triangle:
• \(\text{Image Size } (I) = \text{Actual Size } (A) \times \text{Magnification } (M)\)
• \(\text{Magnification } (M) = \frac{\text{Image Size } (I)}{\text{Actual Size } (A)}\)
• \(\text{Actual Size } (A) = \frac{\text{Image Size } (I)}{\text{Magnification } (M)}\)
Converting Units (Crucial Skill!)
Real cells are measured in micrometres (\(\mu\text{m}\)), while measurements on exam papers are made in millimetres (\(\text{mm}\)).
• To convert \(\text{mm}\) to \(\mu\text{m}\): multiply by \(1000\) (e.g. \(2\text{ mm} = 2 \times 1000 = 2000\ \mu\text{m}\))
• To convert \(\mu\text{m}\) to \(\text{mm}\): divide by \(1000\) (e.g. \(500\ \mu\text{m} = \frac{500}{1000} = 0.5\text{ mm}\))
Step-by-Step Worked Example
Question: An image of a plant cell measures \(30\text{ mm}\) across. The actual size of the cell is \(60\ \mu\text{m}\). Calculate the magnification.
Step 1: Make sure both values are in the same units (convert \(\text{mm}\) to \(\mu\text{m}\)):
\(\text{Image size} = 30\text{ mm} \times 1000 = 30000\ \mu\text{m}\)
Step 2: Write the formula:
\(\text{Magnification} = \frac{\text{Image Size}}{\text{Actual Size}}\)
Step 3: Put the numbers in and calculate:
\(\text{Magnification} = \frac{30000}{60} = \times 500\)
Key Takeaway for Section 2: Always convert all measurements into the same units (\(\mu\text{m}\)) before using the formula \(M = \frac{I}{A}\).
3. Levels of Organisation
In multicellular organisms, cells do not work in isolation. They are organised into complex biological systems in a specific hierarchy:
1. Organelle: A tiny structure inside a cell with a specific job (e.g. nucleus, mitochondrion).
2. Cell: The basic functional unit of a living organism (e.g. muscle cell, root hair cell).
3. Tissue: A group of similar specialised cells working together to carry out a specific function (e.g. muscle tissue, xylem tissue).
4. Organ: A structure made of different tissues working together to perform a function (e.g. heart, stomach, leaf).
5. Organ System: A group of organs working together to carry out a major bodily process (e.g. circulatory system, digestive system).
6. Organism: A complete living individual (e.g. a human, an oak tree).
Order to remember: Organelle \(\rightarrow\) Cell \(\rightarrow\) Tissue \(\rightarrow\) Organ \(\rightarrow\) Organ System \(\rightarrow\) Organism
Key Takeaway for Section 3: Life builds upwards from small organelles inside single cells all the way up to whole living organisms.
4. Stem Cells
Most cells in an adult body are differentiated (specialised) to do one job (for example, red blood cells carry oxygen, nerve cells transmit electrical impulses). Once a cell specialises, it cannot change into another type.
However, stem cells are special!
What is a Stem Cell?
A stem cell is an undifferentiated cell that has the ability to divide by mitosis and develop into different specialised cell types.
Types of Stem Cells
• Embryonic Stem Cells: Found in early embryos. They are exciting to scientists because they can differentiate into any type of cell in the body.
• Adult Stem Cells: Found in specific tissues like bone marrow. They are more limited and can only develop into certain cell types (e.g. bone marrow stem cells can only become different types of blood cells).
• Plant Meristems: Found in the growing tips of roots and shoots in plants. Unlike animal stem cells, plant meristem cells retain the ability to divide and differentiate into any plant tissue throughout the plant's entire life.
Medical Uses and Ethical Issues
Stem cells offer huge potential in medicine:
• They can be used to replace damaged or diseased tissues, such as in treatments for leukaemia (blood cancer), paralysis, or diabetes.
• Ethical Issues: Using embryonic stem cells is controversial because obtaining them results in the destruction of an early human embryo. Some people believe an embryo has the status of a human life from conception, while others argue the potential to cure severe diseases outweighs this concern.
Key Takeaway for Section 4: Stem cells are unspecialised. Embryonic stem cells can form any cell type; adult stem cells are limited. Plant meristems allow plants to grow and differentiate throughout their life.
5. Movement in and out of Cells: Diffusion
Cells need to take in useful substances (like oxygen and glucose) and remove waste products (like carbon dioxide). One of the main ways substances move 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 until they are evenly distributed.
Everyday Analogy: If someone sprays perfume in one corner of a room, the scent particles are at a high concentration there. Over time, they spread out into areas of lower concentration until the whole room smells of perfume!
Diffusion is a passive process, meaning it does not require energy from respiration.
Factors Affecting the Rate of Diffusion
Three main factors speed up or slow down diffusion:
1. Concentration Gradient: A bigger difference in concentration between two areas makes particles move faster.
2. Temperature: At higher temperatures, particles have more kinetic energy, so they move and spread out faster.
3. Surface Area: A larger surface area allows more particles to pass through a membrane at the same time, increasing the rate of diffusion.
Surface Area to Volume Ratio (\(\text{SA:V}\))
• Single-celled organisms (like bacteria) are very small and have a large surface area to volume ratio. Substances can diffuse directly into the cell across its surface fast enough to supply all its needs.
• Multicellular organisms (like humans and plants) are large and have a small surface area to volume ratio. Diffusion across their outer surface alone is far too slow to reach cells deep inside. Therefore, multicellular organisms require specialised exchange surfaces (like lungs or gills) and transport systems (like blood vessels) to survive.
Key Takeaway for Section 5: Diffusion is the passive movement of particles from high to low concentration. It is sped up by a higher temperature, steeper concentration gradient, and larger surface area.
Quick Chapter Review Checklist
Can you do the following before your exam?
• Identify and label the parts of animal, plant, and bacterial cells.
• State the function of the nucleus, cytoplasm, cell membrane, mitochondria, cell wall, chloroplasts, and vacuole.
• Calculate magnification and convert between \(\text{mm}\) and \(\mu\text{m}\) using \(\text{Image} = \text{Actual} \times \text{Magnification}\).
• Order the levels of organisation from organelle to organism.
• Explain what a stem cell is and distinguish between embryonic and adult stem cells.
• Define diffusion and explain how temperature, concentration gradient, and surface area affect its rate.