Understanding Cell Structure & Key Biological Molecules for OCR A Level Biology

Biology Revision Overview

This revision guide covers two essential biology topics that form the foundation of your A-level studies. Cell structure helps you understand how life is organised at the microscopic level, whilst biological molecules reveals the chemistry behind living processes.

The cell structure section explores everything from organelles and their functions to the key differences between prokaryotic and eukaryotic cells. You'll also master microscopy skills, including magnification calculations and comparing different types of microscopes.

The biological molecules section starts with water's vital properties before moving on to macromolecules and polymers. Carbohydrates get special attention since they're fundamental energy sources and structural components in living organisms.

Exam Tip: These topics are heavily tested, so focus on understanding functions rather than just memorising structures.

Cell Types and Basic Structures

Prokaryotic cells are the simple ones - think bacteria. They're single-celled organisms that are much smaller and less complex than the cells in your body. These cells don't have a proper nucleus or membrane-bound organelles.

Eukaryotic cells are the complex ones found in animals, plants, and fungi. They're packed with organelles - specialised structures that each have specific jobs, like tiny factories within the cell. You can see their internal structures using electron microscopes, revealing what scientists call cell ultrastructure.

Animal cells contain organelles like the nucleus, mitochondria, ribosomes, and the rough endoplasmic reticulum (RER). Plant cells have all of these plus a few extras: a rigid cell wall made of cellulose, a large vacuole filled with cell sap, and chloroplasts for photosynthesis.

The plasma membrane surrounds all cells, controlling what enters and exits whilst responding to chemical signals like hormones.

Key Point: Remember that organelles are like specialised departments in a factory - each one has a specific function that keeps the cell running smoothly.

Organelle Functions

The nucleus is the cell's control centre, surrounded by a double membrane with pores. It contains chromatin (DNA plus proteins) and controls cell activities by managing protein synthesis. The nucleolus inside makes ribosomes.

Ribosomes are protein-making machines. Some float freely in the cytoplasm (making proteins that stay in the cell), whilst others attach to the rough endoplasmic reticulum (making proteins for export or membrane use).

The RER folds and processes proteins made by its attached ribosomes. The smooth endoplasmic reticulum (SER) lacks ribosomes and synthesises lipids instead. Vesicles transport materials around the cell like tiny delivery trucks.

Mitochondria are the cell's powerhouses, producing ATP through aerobic respiration. They have a double membrane with folded inner sections called cristae. Active cells need loads of mitochondria for energy. Lysosomes contain digestive enzymes to break down worn-out cell parts or digest invaders.

Memory Trick: Think of the cell as a busy factory where each organelle has a specialised job - nucleus as management, ribosomes as assembly lines, and mitochondria as the power plant.

Specialised Organelles and Structures

Chloroplasts are found only in plant cells and conduct photosynthesis. They contain thylakoid membranes stacked into grana, connected by lamellae. Some photosynthesis reactions happen in the grana, others in the stroma (the thick fluid surrounding the thylakoids).

The Golgi apparatus processes and packages proteins from the ER, creating vesicles for transport. It also makes lysosomes. Think of it as the cell's post office, preparing packages for delivery.

Centrioles are hollow cylinders made of microtubules (protein tubes). They're crucial during cell division, helping separate chromosomes. Most animal cells have them, but only some plant cells do.

Cilia and flagella are hair-like projections that help cells move. Cilia are short and numerous, moving substances along cell surfaces. Flagella are longer and act like propellers - sperm cells use flagella to swim towards eggs.

Plant vs Animal: Remember that plant cells have chloroplasts, a cell wall, and a large vacuole that animal cells lack, whilst animal cells typically have centrioles that most plant cells don't.

Protein Production and the Cytoskeleton

Protein production involves teamwork between organelles. Ribosomes make the proteins, the RER folds and processes them, vesicles transport them to the Golgi apparatus for final processing, then more vesicles deliver the finished products where they're needed.

Proteins destined for export (like glycoproteins in mucus) follow this complete pathway from ribosomes through the RER and Golgi to the cell surface.

The cytoskeleton is a network of protein threads running through the cytoplasm. It's made of microfilaments (very thin protein strands) and microtubules (tiny protein cylinders made of tubulin).

This protein network has four key functions: supporting and positioning organelles, maintaining cell shape, transporting materials within the cell, and enabling cell movement. Chromosome separation during cell division relies on microtubule contraction, and vesicle transport depends on cytoskeletal proteins.

Think of It: The cytoskeleton is like scaffolding in a building - it provides structure, support, and pathways for movement throughout the cell.

Cytoskeleton Functions and Prokaryotic Cells

The cytoskeleton drives cell movement in several ways. During cell division, microtubules contract to separate chromosomes through the phases: prophase, metaphase, and anaphase. Vesicle transport around the cell relies entirely on cytoskeletal protein tracks.

Cilia and flagella movement happens because cytoskeletal protein filaments run through them and contract. In single cells like sperm, this cytoskeletal action propels the entire cell forward.

Prokaryotic cells are much simpler than eukaryotic ones. Bacterial cells are roughly one-tenth the size of eukaryotic cells, making them difficult to study with normal microscopes.

Key bacterial structures include the bacterial chromosome (circular DNA), plasmids (small rings of DNA), cell wall, plasma membrane, and often a flagellum for movement. Unlike eukaryotic cells, bacteria lack membrane-bound organelles and a proper nucleus.

Size Matters: Prokaryotic cells are so small that you need powerful electron microscopes to see their internal details clearly.

Microscopy and Magnification

Magnification tells you how much bigger an image appears compared to the actual specimen. The formula is straightforward: magnification = image height ÷ object height.

You can calculate magnification using a scale bar - that line you see on microscope photos with a measurement label. Measure the scale bar in millimetres, convert to micrometres (multiply by 1000), then divide by the actual length written on the scale bar.

Resolution is completely different from magnification - it's how much detail you can see. Think of it as the microscope's ability to distinguish between two points that are close together. If the resolution isn't good enough, just increasing magnification won't help you see more detail.

Light microscopes have a maximum resolution of about 0.2 micrometres and useful magnification up to ×1500. They're perfect for viewing whole cells and tissues. Laser scanning confocal microscopes use fluorescent dyes and can create 3D images by combining multiple scans at different depths.

Exam Focus: Practice magnification calculations - they're common exam questions and easy marks once you've got the method down.

Electron Microscopes

Electron microscopes use electron beams instead of light, giving much higher resolution than light microscopes. This means incredibly detailed images of tiny structures like ribosomes and organelle interiors.

Transmission Electron Microscopes (TEMs) fire electrons through very thin specimen slices. Denser parts absorb more electrons, appearing darker in the final image. TEMs produce high-resolution 2D images perfect for studying internal organelle structures, but specimens must be sliced extremely thinly.

Scanning Electron Microscopes (SEMs) work differently - they scan electron beams across the specimen surface. Electrons get knocked off the specimen and collected to form 3D images showing surface details. However, SEM images have lower resolution than TEM images.

Both electron microscope types require specimens to be treated with heavy metals like lead. This is like staining samples for light microscopy - the metal ions scatter electrons to create contrast between different structures.

Remember: TEM = internal detail in 2D, SEM = surface detail in 3D, both give much better resolution than light microscopes.

Microscope Comparison and Sample Preparation

Here's what you need to know about microscope capabilities. Light microscopes max out at 0.2 micrometre resolution and ×1500 magnification. TEMs achieve 0.002 micrometre resolution with magnification over ×1,000,000. SEMs match TEM resolution but usually stay under ×500,000 magnification.

Electron micrographs are always produced in black and white, though colours can be added later to make interpretation easier. The heavy metal treatment creates the contrast you see in these images.

For light microscopy, staining is crucial because many specimens are transparent. Methylene blue and eosin are common stains that get absorbed differently by various cell parts, creating the contrast needed to distinguish structures.

You'll use an eyepiece graticule with a stage micrometer to calibrate measurements. The stage micrometer has known measurements that help you work out the actual size of what you're viewing through the eyepiece graticule.

Practical Tip: Different stains highlight different cell components - choose your stain based on what you want to observe.

Using Light Microscopes

Staining makes transparent specimens visible by creating contrast. Some cell parts absorb more stain than others, so you get heavily stained and lightly stained areas that you can distinguish easily.

Dry mounts are the simplest slide preparation method. Just place your specimen (like hair, insect parts, or pollen) directly on the slide. Perfect for non-living samples that don't need to stay moist.

Wet mounts involve placing specimens in liquid (usually water) on the slide. This technique works brilliantly for living samples like tiny aquatic organisms that need to stay hydrated to remain active.

When using a light microscope, always start with the lowest power objective lens to locate your specimen, then switch to higher powers for detail. The condenser and diaphragm control light intensity reaching your specimen. Use the coarse focus for low power and fine focus for medium and high power lenses.

The turret rotates to bring different objective lenses into position. Remember that higher magnification gives a smaller field of view, so find your specimen on low power first.

Top Tip: Always start on low power and work your way up - it's much easier to find your specimen this way than starting on high magnification.