Glossary: Cell the Unit of Life | Biology A-Level - A Level PDF Download

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What is a Cell?

• Cell: Cell is the fundamental structural and functional unit of all living organisms. 

An Overview of Cell

Organelles: The eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria, microbodies and vacuoles.

Eukaryotic cells & Prokaryotic cells: Eukaryotic cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic.

Centrosome: Animal cells contain another non-membrane bound organelle called centrosome which helps in cell division.

Prokaryotic Cells

• Plasmids: Many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
• Gram positive & Gram negative bacteria: Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram viz., those that take up the gram stain are Gram positive and the others that do not are called Gram negative bacteria.
• Slime layer & Capsule: Glycocalyx differs in composition and thickness among different bacteria. It could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the capsule.
• Mesosome: A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae.
• Pili & Fimbriae: Besides flagella, Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell.
• Inclusion bodies: Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.

Eukaryotic Cells

• Polysome: Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
• Passive transport: The membrane is selectively permeable to some molecules present on either side of it. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.
• Osmosis: Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.
• Active transport: A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. Such a transport is an energy dependent process, in which ATP is utilised and is called active transport, e.g., Na+/K+ Pump.
• Stroma: The inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma.
• Thylakoids: A number of organised flattened membranous sacs called the thylakoids, are present in the stroma
• Cytoskeleton: An elaborate network of filamentous proteinaceous structures consisting of microtubules, microfilaments and intermediate filaments present in the cytoplasm is collectively referred to as the cytoskeleton. 
• Hub & Spokes: The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
• Perinuclear Space: A nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.