Eukaryotic Cells - OCR A Level Biology Revision Notes 2023

Cell Surface Membrane

• Surrounds all cells and controls material exchange between internal and external environments.
• Partially permeable; composed of a phospholipid bilayer (~10 nm).
• Dynamic structure with moving phospholipids and proteins.

Cell Wall

• Present in plant cells, absent in animal cells.
• Provides structural support via cellulose in plants and peptidoglycan in bacteria.
• Features plasmodesmata for cytoplasmic connections.

Nucleus

• Present in all eukaryotic cells (except red blood cells).
• Separated from cytoplasm by nuclear envelope with pores for mRNA, ribosomes, enzymes, and signaling molecules exchange.
• Contains chromatin (DNA-histone complex) forming chromosomes.
• Includes nucleolus for ribosome production.

Mitochondria

• Site of aerobic respiration, visible under light microscope.
• Double membrane structure with inner membranes folded into cristae.
• Matrix contains enzymes for ATP production, mitochondrial DNA, and ribosomes.

Chloroplasts

• Found in plant cells, larger than mitochondria.
• Double-membrane with thylakoids stacking into grana, connected by lamellae.
• Site of photosynthesis: light-dependent reactions in thylakoids, light-independent in stroma.
• Contains chlorophyll, DNA, and ribosomes.

Ribosomes

• Found in all cells, free in cytoplasm or on rough ER.
• Composed of rRNA and proteins; 80S in eukaryotes, 70S in prokaryotes, mitochondria, and chloroplasts.
• Site of protein synthesis.

Endoplasmic Reticulum (ER)

Rough ER

• Has ribosomes on surface, involved in protein processing.

Smooth ER

• Lacks ribosomes, involved in lipid, carbohydrate, and steroid processing.

Golgi Apparatus

• Flattened membrane sacs modifying and packaging proteins/lipids into vesicles.
• Involved in protein export, lysosome formation, and organelle delivery.

Vacuoles

• Large permanent vacuoles in plants, surrounded by tonoplast.
• Temporary and small in animal cells.

Vesicles

• Membrane-bound sacs for transport and storage within cells.

Lysosomes

• Specialized vesicles with hydrolytic enzymes for waste breakdown.
• Important in immune response and apoptosis.

Centrioles

• Made of microtubules; form centrosomes for spindle fiber organization in cell division.
• Absent in flowering plants and fungi.

Microtubules

• Component of cytoskeleton, providing support and movement.
• Composed of alpha and beta tubulin dimers forming protofilaments, making up microtubules.

Microvilli

• Cell membrane projections increasing surface area for exchange processes.

Cilia

• Hair-like structures made from microtubules; move substances over cell surfaces.

Flagella

• Similar to cilia but longer, used for cell movement, e.g., in sperm cells.