Cell Structure & Ultrastructure

Prokaryotic cells are simple, single-celled organisms that lack a true nucleus and other membrane-bound organelles. Their genetic material (a single, circular chromosome) is located in a region called the nucleoid. They possess ribosomes (70S, smaller than eukaryotic), a cell wall (often peptidoglycan), a cell membrane, and sometimes a capsule, flagella, or pili. Examples include bacteria and archaea.

Eukaryotic cells are more complex, possessing a true nucleus that encloses their linear genetic material (chromosomes), and a variety of membrane-bound organelles. They are typically larger than prokaryotic cells and include animal cells, plant cells, fungi, and protists. Eukaryotic ribosomes are larger (80S).

The nucleus is a large, membrane-bound organelle that contains the cell's genetic material (DNA) organised into chromosomes. It is enclosed by a double membrane called the nuclear envelope, which has nuclear pores to regulate the passage of molecules. Inside the nucleus is the nucleolus, responsible for synthesising ribosomal RNA (rRNA) and assembling ribosomes. The nucleus controls all cell activities by regulating gene expression.

Mitochondria are often called the 'powerhouses' of the cell. They are the primary sites of aerobic respiration, where glucose is broken down to produce ATP (adenosine triphosphate), the cell's main energy currency. Each mitochondrion has a double membrane: a smooth outer membrane and a highly folded inner membrane called the cristae. The folds increase the surface area for enzyme reactions. The inner compartment, the matrix, contains enzymes, mitochondrial DNA, and ribosomes.

Chloroplasts are organelles found in plant cells and some protists, responsible for photosynthesis. They also have a double membrane. Inside, a fluid-filled space called the stroma contains enzymes, chloroplast DNA, and ribosomes. Within the stroma are stacks of flattened sacs called thylakoids, which are arranged into grana (singular: granum). Chlorophyll, the pigment that captures light energy, is located on the thylakoid membranes.

The ER is an extensive network of interconnected membranes forming sacs (cisternae) and tubules throughout the cytoplasm. There are two types: 1. Rough Endoplasmic Reticulum (RER): Studded with ribosomes, it is involved in the synthesis, folding, modification, and transport of proteins destined for secretion, insertion into membranes, or delivery to other organelles. 2. Smooth Endoplasmic Reticulum (SER): Lacks ribosomes. It is involved in lipid synthesis (e.g., steroids, phospholipids), detoxification of drugs and poisons, and storage of calcium ions.

Ribosomes are small, non-membrane-bound organelles responsible for protein synthesis (translation). They are composed of ribosomal RNA (rRNA) and proteins. Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S). They can be found free in the cytoplasm, synthesising proteins for use within the cell, or attached to the RER, synthesising proteins for secretion or insertion into membranes.

The Golgi apparatus consists of a stack of flattened membrane-bound sacs called cisternae. It receives proteins and lipids from the ER, further modifies them, sorts them, and packages them into vesicles for transport to other cellular destinations or for secretion outside the cell. It also plays a role in the formation of lysosomes.

Lysosomes are membrane-bound vesicles containing powerful hydrolytic enzymes. They are involved in intracellular digestion, breaking down waste materials, cellular debris, and foreign particles (e.g., bacteria). They also play a role in programmed cell death (apoptosis). Lysosomes are more common and prominent in animal cells.

Vacuoles are membrane-bound sacs used for storage, waste removal, and maintaining turgor pressure. * Plant cells typically have a large, central vacuole that stores water, nutrients, and waste products, and helps maintain cell turgor against the cell wall. * Animal cells may have small, temporary vacuoles for storage or transport, but they are not as prominent as in plant cells.

The cell wall is a rigid outer layer found in plant cells, fungi, bacteria, and some protists, located outside the cell membrane. It provides structural support, protection, and prevents excessive water uptake. In plants, it is primarily composed of cellulose; in fungi, chitin; and in bacteria, peptidoglycan.

The cell membrane is a selectively permeable barrier that encloses the cytoplasm of all cells. It regulates the passage of substances into and out of the cell, maintains cell integrity, and is involved in cell signalling and recognition. It is primarily composed of a phospholipid bilayer with embedded proteins.

The fluid mosaic model describes the structure of the cell membrane. It proposes that the membrane is a dynamic, fluid structure composed of a 'mosaic' of components: a phospholipid bilayer, with various proteins (integral and peripheral) embedded within or associated with it. The phospholipids and many proteins can move laterally within the membrane, giving it fluidity. Cholesterol (in animal cells) helps regulate membrane fluidity. Carbohydrate chains (glycoproteins and glycolipids) are found on the outer surface, involved in cell recognition and adhesion.