Cell Biology: Structure, Organelles and Function

Cells are the basic structural and functional units of life. Living cells contain structures called membranes, and they carry genetic material (DNA and RNA) capable of self-replication and of directing the synthesis of their proteins. Plant cells, animal cells and other eukaryotes are the result of the evolution of prokaryotic organisms similar to current bacteria. The precursors of eukaryotic cells are urcariots, which lost their capsule, increased in size, incorporated internal structures and developed a membrane system and a consistent cytoskeleton.

The Endosymbiotic Theory

The endosymbiotic theory (proposed by Lynn Margulis) holds that the urcariota could have engulfed bacteria, which became endosymbionts that constituted organelles within the cell.

Peroxisomes

Peroxisomes are membranous vesicles (0.2-1 micron) that contain enzymes using O2 to oxidize various substrates, producing H2O2. H2O2 is toxic to the cell, so the same organelle contains catalase, which breaks it down. These reactions are used in the detoxification of toxic substances in organisms (liver, kidney). They also participate in the synthesis of phospholipids and in beta-oxidation. They are heterogeneous in size, form and number, varying according to the type of cell and environmental conditions. In plants, glyoxysomes and peroxisomes are found.

Cilia and Flagella

Cilia and flagella are structures formed by microtubules and accessory proteins. They are covered by an extension of the plasma membrane. Their function is to move the cell or to move fluids over its surface. Cilia are numerous, short and effective for movement, while flagella are fewer, longer and anchored in the cell; they contain mitochondria and fibers that produce an undulating movement.

Chloroplasts

Chloroplasts are organelles exclusive to plant cells. They are lens-shaped (5-10 microns) and have a diameter of (2-4). They produce ATP and NADPH, which supply energy to the organism from solar energy.

Chloroplast Membranes

The membranes are: external, which contains porins; internal, which is selectively permeable; and the thylakoid membrane, arranged in flattened sacs called thylakoids, which are stacked in grana. The photosystems are responsible for capturing solar energy. The membrane contains unsaturated fatty acids.

The Three Spaces

The chloroplast has three spaces: the intermembrane space, similar to the cytosol; the stroma, which contains DNA, RNA, enzymes, ribosomes, starch granules and lipids; and the thylakoid space, enclosed by the thylakoid membrane.

Physiology of Photosynthesis

Chloroplasts convert light energy into chemical energy. Photosynthesis occurs in two stages: the light-dependent reactions, which take place in the thylakoid membranes, and the light-independent reactions (the Calvin cycle), which take place in the stroma. The molecules produced during the light reactions, ATP and NADPH, are used to synthesize organic molecules.

Mitochondria

Mitochondria are organelles present in the cells of nearly all aerobic organisms. They measure 1-4 microns in length and 0.2-1 micron in diameter. They have two membranes:

• External membrane: contains porins, channel proteins that allow the passage of small molecules.
• Internal membrane: impermeable to charged particles, contains enzymes and forms cristae.

There are two spaces: the intermembrane space, with a composition similar to the cytosol, and the matrix, which contains DNA, RNA, ribosomes and proteins.

Mitochondria originate from the division of existing mitochondria. Division can occur by binary fission (the cristae divide in two) or by segmentation (constriction). Growth implies the incorporation of molecules from the cytosol; proteins and lipids are synthesized inside the mitochondria and transported outward.

Physiology

Mitochondria convert energy. Aerobic respiration consumes O2 and produces CO2, and ATP is produced in the process.

• Matrix: contains the Krebs cycle enzymes, in which acetyl groups are oxidized, forming CO2.
• Internal membrane: contains the respiratory chain. The enzymes in the intermembrane space and the ATP synthase produce ATP from ADP inside the mitochondria.
• External membrane: contains enzymes involved in lipid synthesis.

History of Cell Discovery

• Robert Hooke: observed cells in cork.
• Antony van Leeuwenhoek: observed microorganisms.
• Robert Brown: described the cell nucleus.
• Theodor Schwann: proposed that organisms are made of cells, the structural unit of life.

Cell Theory

The cell theory states that the cell is the basic unit of structure and physiology of all living beings; each cell arises from a previous cell through division; and genetic information is transmitted from generation to generation.

Cellular Organization

There are two major types of cellular organization: prokaryotic and eukaryotic.

Viruses

Viruses are acellular, simple structures that lack their own metabolism. They are parasites, formed by a nucleic acid molecule surrounded by a protein coat, and they replicate only inside host cells.

Prokaryotes

Prokaryotes have no defined nucleus. Their DNA is a closed circular molecule located in the nucleoid region in the cytoplasm. They may contain a mesosome, plasmids, inclusions, a nucleoid, a capsule, a cell wall, a plasma membrane, flagella, pili, ribosomes and cytoplasm.

Eukaryotes

Eukaryotic animal cells have very complex metabolic activity. The plasma membrane delimits the cell, and the nucleus is separated from the cytoplasm by a double membrane. The cytoplasm contains the cytosol, an aqueous phase with components of the cytoskeleton, along with organelles.

Plant cells are adapted to a hypotonic environment. Differences from animal cells include plastids (chloroplasts for photosynthesis), large vacuoles, and a rigid cell wall for mechanical support and protection. Plant cells lack centrioles.

Cell Differentiation

Cell differentiation produces different cell types, and tissue organization determines the characteristics of multicellular organisms.

Plant Cells

Plant tissues are characterized by a thick cell wall. Their main role is structural support, provided by dead cells.

Animal Cells

Animals have more than 200 types of cells. Their tissues include:

• Epithelial: exchange of nutrients and secretion of substances.
• Connective: joins tissues and organs, providing support and protection.
• Blood: leukocytes (defense) and erythrocytes (oxygen transport).
• Muscle: elongated cells that contract.
• Nervous: generation and transmission of nerve impulses.

Stem Cells

Stem cells are undifferentiated cells that give rise to adult cell types. They are used to repair damaged organs and tissues. Cell renewal occurs through division and differentiation of cells that replace those lost in adult tissues.

Cell Sizes

Approximate dimensions: virus 100 nm, bacteria 2 microns, animal cell 25 microns, plant cell 40 microns.

Cell Functions

Cell functions are related to structure and level of organization. For example, the plasma membrane receives stimuli and generates responses, such as neurotransmitters or hormones.

Cell Division and Evolution

Cell division and renewal are key to the evolution of species, from simple prokaryotes to complex eukaryotes (mitosis and meiosis). DNA replication occurs in every case.

Nutrition and Metabolism

Autotrophic cells produce their own organic molecules through photosynthesis or chemosynthesis, using energy from oxidation of inorganic molecules. Heterotrophic cells obtain energy by oxidizing organic molecules.

Metabolism includes catabolism, which degrades organic molecules to obtain energy, and anabolism, which builds structures such as carbohydrates, lipids and proteins. Metabolic reactions are linked through intermediary metabolites, forming metabolic pathways in which each reaction is catalyzed by a specific enzyme.

Cellular Respiration

Cellular respiration is a catabolic process that converts glucose energy into ATP:

• Glycolysis: glucose is converted to pyruvic acid.
• Acetyl-CoA formation: pyruvate reacts with coenzyme A.
• Krebs cycle: produces CO2 and reduced coenzymes.
• Respiratory chain: NADH and FADH2 are oxidized and O2 is reduced to H2O.

Fatty acid beta-oxidation also feeds into the Krebs cycle and the respiratory chain.

Fermentation

Fermentation is a partial oxidation that occurs without oxygen, producing different molecules.

Anabolism (Synthesis)

Anabolism builds new bonds to create complex molecules, using ATP and reducing power as energy and precursors. Autotrophs use simple molecules such as CO2, H2O and NH3, while heterotrophs obtain nutrients from the diet, and catabolic reactions provide precursors.

Catabolism and anabolism are not isolated; they are interdependent processes. Energy obtained is used for biosynthesis of molecules and for cellular work.

Extracellular Matrix

The extracellular matrix fills the spaces between cells and tissues. It has several functions: it facilitates diffusion of nutrients, provides suitable microenvironments, and allows cells to move.

Composition

• Fibrous structural proteins: collagen provides strength; elastin provides elasticity.
• Glycosaminoglycans: gelatinous polysaccharides such as hyaluronic acid, which promotes water retention.
• Adhesion proteins: fibronectin binds to cell surface receptors; laminin is found beneath epithelial tissues.

Techniques for Studying Cells

Separation and Fragmentation

Cells and tissues are separated by mechanical or chemical methods. Cells are broken by hypotonic solutions or by blending, and organelles are separated by centrifugation.

Cell Culture

Cell culture allows observation and study of cell behavior. Cultivation is limited to a finite number of divisions, except for cancer cells. Culture depends on the nature of the substrate, the concentration of nutrients, and pH.

Microscopy

• Light field: the objective lens collects light directly from the specimen, and the eyepiece magnifies the image.
• Darkfield: only scattered light rays reach the eye.
• Phase contrast: allows observation of living cells without staining.
• Polarized light: reveals structures with optical activity.
• Fluorescence: uses fluorescent dyes to highlight specific structures.
• Confocal: produces three-dimensional images through electronic reconstruction.
• Electron microscopy: uses an electron beam generated by a cathode and accelerated toward an anode in a vacuum. Transmission (TEM) passes electrons through the sample, while scanning (SEM) does not pass through the sample but scans its surface.