Key Points

A cell is the fundamental structural and functional unit of all living organisms, capable of independent existence and performing the essential functions of life.

Formulated by Schleiden and Schwann, and later modified by Virchow, the cell theory states that all living organisms are composed of cells and products of cells, and all cells arise from pre-existing cells.

Prokaryotic cells lack a membrane-bound nucleus and organelles, with genetic material in a nucleoid. Eukaryotic cells possess a true nucleus and membrane-bound organelles like mitochondria and endoplasmic reticulum.

Prokaryotic cells are represented by bacteria and blue-green algae. They have a cell wall, 70S ribosomes, and may possess plasmids (small circular DNA) and mesosomes (infoldings of the plasma membrane).

Proposed by Singer and Nicolson, this model describes the plasma membrane as a quasi-fluid lipid bilayer with embedded proteins. This structure allows for lateral movement of proteins and is crucial for functions like cell growth and transport.

Transport across the membrane can be passive (diffusion, osmosis) without energy, or active, which requires energy (ATP) to move substances against a concentration gradient, such as the $\text{Na}^+/\text{K}^+$ pump.

A rigid outer layer found in plant cells, fungi, and algae that provides structural support and protection. In plants, it is mainly composed of cellulose, hemicellulose, and pectins.

This system includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles, which work in a coordinated manner for protein and lipid synthesis, modification, packaging, and transport.

A network of tubules divided into Rough ER (RER), with ribosomes for protein synthesis, and Smooth ER (SER), for lipid synthesis and detoxification.

Consists of flattened sacs called cisternae. It functions in modifying, sorting, and packaging proteins and lipids from the ER into vesicles for secretion or delivery to other organelles.

Lysosomes are single-membrane vesicles containing hydrolytic enzymes for intracellular digestion. Vacuoles are membrane-bound sacs for storage; in plants, a large central vacuole maintains turgor pressure.

A double-membraned organelle responsible for aerobic respiration and ATP synthesis. The inner membrane is folded into cristae to increase surface area for energy production.

Plastids include chloroplasts (for photosynthesis), chromoplasts (containing pigments like carotene), and leucoplasts (for nutrient storage like starch in amyloplasts).

A double-membraned organelle containing a fluid-filled stroma and stacks of membranous sacs called thylakoids, which form grana. Chlorophyll within the thylakoids traps light energy for photosynthesis.

Non-membranous organelles responsible for protein synthesis. Prokaryotes have 70S ribosomes (subunits 50S and 30S), while eukaryotes have 80S ribosomes (subunits 60S and 40S) in the cytoplasm.

A network of protein filaments (microtubules, microfilaments, intermediate filaments) in the cytoplasm. It provides mechanical support, maintains cell shape, and aids in motility and cell division.

Hair-like outgrowths responsible for movement. Their core, the axoneme, typically has a $9+2$ arrangement of microtubules, with nine doublets on the periphery and two central microtubules.

A double-membraned organelle containing the cell's genetic material (DNA) organized into chromatin. The nuclear envelope has pores for regulating the transport of molecules between the nucleus and cytoplasm.

During cell division, chromatin condenses to form chromosomes. Each chromosome has a primary constriction called the centromere, which holds two sister chromatids together.

Based on the centromere's position, chromosomes are classified as metacentric (middle), sub-metacentric (off-center), acrocentric (near one end), or telocentric (at the terminal end).