Comprehensive AS Level Biology Notes: Prokaryotic and Eukaryotic Cells

Prokaryotic Cells and Viruses

Prokaryotic cells are the simplest cellular structures you'll encounter, found in bacteria and blue-green algae. Unlike the cells in your body, they don't have a membrane-bound nucleus - instead, their DNA floats freely in the cytoplasm within a region called the nucleoid.

These cells are surprisingly efficient with their basic setup. They've got 70S ribosomes for protein synthesis, and their cell walls are made from peptidoglycan (also called murein), not cellulose like plants. The mesosome - formed by the cell membrane folding inwards - serves as the site for aerobic respiration.

Viruses are even simpler and technically aren't even alive. They're incredibly tiny (electron microscope required!) and consist of just a protein coat surrounding DNA or RNA. Some have spikes for attaching to specific cells, and bacteriophages are viruses that specifically target bacteria.

Key Point: Remember that prokaryotic cells are complete living organisms, whilst viruses need to hijack other cells to reproduce!

Eukaryotic Cell Structure

Eukaryotic cells are the sophisticated type found in plants, animals, and fungi - they're packed with membrane-bound organelles that prokaryotes lack. Think of organelles as specialized departments in a factory, each with specific jobs that keep the cell running smoothly.

The nucleus is your cell's command centre, housing DNA and controlling protein synthesis. It's surrounded by a double membrane with pores that allow mRNA to exit. Inside, you'll find the nucleolus, which produces ribosomes and rRNA.

Mitochondria are the cell's powerhouses, producing ATP through aerobic respiration. Their inner membrane folds into cristae, creating massive surface areas for enzyme attachment. Metabolically active cells like muscle and liver cells are packed with these energy factories.

Exam Tip: When describing organelles, always mention their membrane structure and specific function - examiners love these details!

Protein Synthesis and Transport Systems

The endoplasmic reticulum (ER) is your cell's highway system, transporting molecules throughout the cytoplasm. Rough ER has ribosomes attached, specialising in protein transport, whilst smooth ER handles lipid synthesis and transport.

Ribosomes are protein-making machines found freely in the cytoplasm or attached to rough ER. Eukaryotic cells use 80S ribosomes, whilst prokaryotes stick with 70S versions. They're made from RNA and protein, with specific sites for mRNA and tRNA attachment.

The Golgi body acts like a post office, modifying and packaging proteins received from the rough ER. It's brilliant at producing enzymes, forming lysosomes, and creating glycoproteins. Everything gets transported in vesicles that bud off from its surfaces.

Memory Trick: Think "Rough ER = Ribosomes = pRotein transport" - the alliteration helps you remember!

Specialized Organelles

Lysosomes are the cell's recycling centres, found mainly in animal cells. These vesicles contain digestive enzymes that break down worn-out organelles and cellular waste. They're formed by budding off from the Golgi body.

Centrioles are crucial for cell division, found in animal cells near the nucleus. These hollow cylinders work in pairs, moving to opposite poles during division to help pull chromosomes apart via spindle fibres.

Chloroplasts are plant cell specialists conducting photosynthesis. They capture sunlight energy, convert it to ATP, and release oxygen from water. Like mitochondria, they contain their own DNA and 70S ribosomes, making them self-replicating.

Vacuoles serve different purposes - small ones in animal cells for storage, but massive ones in plant cells that provide structural support and maintain turgor pressure.

Plant vs Animal: Remember that chloroplasts and large vacuoles are plant-exclusive, whilst centrioles are typically animal-only!

Cell Walls and Levels of Organisation

Plant cell walls are made from cellulose (beta-glucose chains) and provide structural strength whilst resisting vacuole expansion. Plasmodesmata are microscopic channels connecting plant cells, allowing communication and transport between them.

The key differences between cell types are straightforward. Prokaryotes (bacteria) have no membrane-bound organelles and 70S ribosomes, with cell walls made of murein. Eukaryotes (plants, animals, fungi) have membrane-bound organelles and 80S ribosomes.

Levels of organisation follow a logical hierarchy in multicellular organisms. Similar cells group into tissues (like muscle tissue), different tissues combine into organs (like your heart), organs work together as systems (like your circulatory system), and systems combine to form complete organisms.

Exam Focus: Understanding this hierarchy is essential for questions about how structure relates to function at different biological levels!

Tissue Types and Functions

Epithelial tissue forms protective barriers, covering internal and external body surfaces. It lacks blood vessels but contains nerve endings, with cells sitting on a basement membrane made of collagen and proteins.

Cuboidal epithelial tissue appears in kidney tubules and ovary surfaces, providing protection from abrasion and water loss. Ciliated epithelial tissue lines your respiratory tract, with hair-like cilia that wave to move mucus upwards - essential for keeping your lungs clean.

Muscle tissue comes in two main types. Smooth muscle found in blood vessels and intestines allows fine control without conscious effort. Striated (skeletal) muscle attaches to bones for voluntary movement and shows distinctive striped patterns under a microscope.

Real-World Connection: Every time you cough to clear mucus, you're relying on ciliated epithelial tissue doing its job perfectly!

Connective Tissue

Connective tissue is your body's structural framework, found in tendons, ligaments, and cartilage. It fills spaces between organs and tissues whilst providing essential structural support throughout your body.

This tissue type is incredibly versatile, adapting to provide exactly the right amount of support where needed. Whether it's the tough tendons connecting muscle to bone or the flexible cartilage cushioning your joints, connective tissue keeps everything properly positioned and functioning.

Remember: Connective tissue literally connects and supports - the name tells you exactly what it does!