Cell

Cells

The basic structural unit of all living organisms and the smallest unit that is considered alive

Cell theory

States that all living things are composed of cells, cells are the smallest unit of life, and all cells arise only from pre-existing cells

Magnification

The number of times larger an image is than the specimen, while resolution is the ability to separate small objects that are very close together

Magnification

Number of times larger an image is than the specimen, while resolution is the ability to separate small objects that are very close together

Electron microscopes

Have much higher resolution than light microscopes, allowing for the study of ultrastructure (fine structures of cells like organelles)

Eukaryote Structure

Have a true nucleus surrounded by a double-layered nuclear envelope containing chromatin (DNA and histones). Contain membrane-bound organelles such as mitochondria (aerobic respiration), Rough Endoplasmic Reticulum (protein synthesis with attached 80S ribosomes), Golgi apparatus (processing and packaging), and lysosomes (digestive enzymes)

Eukaryote Structure

Have a true nucleus surrounded by a double-layered nuclear envelope containing chromatin (DNA and histones). Contain membrane-bound organelles such as mitochondria (aerobic respiration), Rough Endoplasmic Reticulum (protein synthesis with attached 80S ribosomes), Golgi apparatus (processing and packaging), and lysosomes (digestive enzymes)

Animal cells

Lack a cell wall and chloroplasts, often having temporary vacuoles and centrioles

Plant cells

Have a rigid cellulose cell wall, chloroplasts for photosynthesis, and a large permanent vacuole

Fungal cells

Have cell walls typically containing chitin

Atypical Eukaryotic Cells

Some structures challenge traditional cell theory, such as

striated muscle fibres - extremely long and multinucleated

aseptate fungal hyphae - no cell partitions

mature red blood cells - no nucleus

sieve tube elements in phloem - lack a nucleus and many organelles

Organelles Defined

Discrete subunits of cells adapted to perform specific tasks, such as the nucleus, mitochondria, ribosomes, and vesicles. Cell walls, the cytoskeleton, and cytoplasm are not considered organelles

Advantages of Compartmentalization

By surrounding areas with membranes, cells can concentrate enzymes and substrates for specific reactions, separate damaging processes (like the hydrolytic enzymes in lysosomes), and maintain specific conditions like pH that optimize enzyme activity.

Nucleus and Cytoplasm Separation

The separation of the nucleus from the cytoplasm by the nuclear envelope allows for the regulation of gene expression. For instance, it provides a compartment for mRNA splicing and modification to occur before the mRNA moves to the cytoplasm for translation, ensuring that only "mature" messages are read by ribosomes

Phagocytic Vacuoles

Specialized compartments (vesicles) formed when a cell captures and ingests foreign particles like bacteria. These vacuoles then fuse with lysosomes to destroy the pathogen, demonstrating how compartmentalization protects the rest of the cell's contents from digestive enzymes

Differentiation

Multicellular life begins as a single zygote, which divides to produce unspecialized cells. These cells then undergo differentiation, where specific genes are activated or repressed to allow the cell to develop a specialized form and function

Morphogens

Signalling molecules that exist in a concentration gradient across a developing embryo. The specific concentration of morphogens in a cell's environment triggers different patterns of gene expression, directing the cell's eventual specialization

Stem Cells

Undifferentiated cells that have two key properties: they can undergo unlimited division (self-renewal) and can differentiate into various cell types.

Potency Levels

Totipotent, Pluripotent and Multipotent

Totipotent

Can differentiate into any body cell type and the placenta; these are only found in the first few divisions after fertilization

Pluripotent

Can develop into any body cell type but cannot form a placenta

Multipotent

Can only develop into a limited range of closely related cell types (e.g., adult blood stem cells can become red or white blood cells)

Stem Cell Niches

Specific microenvironments in the adult body (such as bone marrow or hair follicles) that maintain stem cells in a dormant, undifferentiated state until they receive stimuli to self-renew or differentiate

Cell Size and Specialization

Cell size is adapted to function. For example, the human egg is large (100 µm) to store nutrients, while the sperm is small (5 µm head) for mobility; neurons can be extremely long (up to 1 m) to transmit signals over distances

Surface Area-to-Volume Ratio (SA:V)

A primary constraint on cell size. As a cell increases in size, its volume increases faster than its surface area. A small SA:V ratio in large cells means they cannot transport enough oxygen and nutrients in, or waste out, to maintain metabolism. Large cells often adapt by becoming flattened or developing membrane extensions like microvilli to increase their surface area.