Cell Membrane Overview

Cell Membrane

• The plasma membrane (cell membrane), plasma lemma, or limiting membrane is extremely thin (7.5 to 10 nm).
• It's not visible under Light Microscopy (LM) without special stains (e.g., silver).
• Electron Microscopy (EM) reveals a trilaminar appearance: two electron-dense layers separated by a translucent layer.
• The membrane is primarily composed of a phospholipid bilayer with glycolipids and cholesterol.
• Phospholipid molecules have a hydrophilic head and hydrophobic tails.
• The hydrophilic heads face the intracellular and extracellular environments.
• The hydrophobic tails position themselves within the membrane.
• The trilaminar appearance results from the staining of the hydrophilic heads on either side of the membrane.
• Proteins are embedded in the membrane.
• Proteins can be peripheral (located on the membrane surface) or integral (integrated within the bilayer), and some span both the inner and outer surfaces (transmembrane proteins).
• Carbohydrates are also present predominantly on the outer membrane surface, linked to lipids (glycolipids) or proteins (glycoproteins) forming a glycocalyx.
• The cell membrane acts as a barrier and regulates the passage of substances into and out of the cell, maintaining a constant intracellular environment.
• It also plays a role in cell signaling.

Vesicular Transport

• Endocytosis and exocytosis are two types of vesicular transport.
• Endocytosis involves the pinching off of the cell membrane to form vesicles that engulf materials from outside the cell.
• Pinocytosis ("cell drinking") is one form; it engulfs fluids.
• Phagocytosis ("cell eating") involves engulfing larger particles or cells.
• Receptor-mediated endocytosis is a specialized form where specific receptors bind foreign molecules, initiating the process.
• Exocytosis is the release of materials from intracellular vesicles to the extracellular environment.
• The process is affected by calcium levels in the cytoplasm; higher calcium leads to more exocytosis.
• Exocytosis pathways include constitutive (continuous) and regulated (stimulated).

Mitochondria

• Mitochondria have a distinctive shape, typically spherical or rod-like.
• They are not visible in light microscopy (LM) without special vital stains like Janus green B.
• Mitochondria are localized in cells according to their energy utilization, e.g., more prominent in active cells like heart muscle (cardiac cells) as well as in ion-transferring cells and spermatozoa.
• They have their own DNA and can replicate independently from the cell.
• Mitochondrial DNA and RNA are contained within the organelle.
• The interior of the mitochondria is partitioned by inner membranes; folds are called cristae.
• The inner membrane encloses the matrix, containing enzymes necessary for the citric acid/Krebs cycle, and the electron transport chain is present there.
• The process of oxidative phosphorylation occurs on the cristae, generating ATP (energy).

Endoplasmic Reticulum

• Endoplasmic reticulum (ER) is a network of channels and sacs formed by continuous membrane.
• It is classified as either rough (RER), having ribosomes attached to its surface, or smooth (SER), lacking ribosomes.
• RER appears as a basophilic area in the cytoplasm, while SER is not visible in light microscopy.
• RER is associated with protein synthesis and modification, targeting proteins for secretion or insertion into the cell membrane.
• SER is involved in steroid hormone synthesis, lipid metabolism, drug detoxification, calcium storage, and glycogen metabolism.

Golgi Apparatus

• The Golgi apparatus is composed of flattened sacs (cisternae).
• The Golgi apparatus is usually organized in stacks with dilated peripheries.
• The Golgi apparatus has two faces: the cis (receiving) face and the trans (shipping) face.
• The cis face is associated with small vesicles coming from the ER, carrying immature proteins.
• The trans face is associated with large secretory vesicles containing mature proteins.
• Functions include protein modification (such as glycosylation), concentration, packaging of proteins into secretory vesicles, lysosome formation, and membrane recycling.

Lysosomes

• Lysosomes are sites for intracellular digestion.
• Their enzymes optimally function in acidic pH.
• The organelle can be seen in electron microscopy (EM).
• Lysosomes are types: primary and secondary, containing enzymes that degrade cellular substances.

Peroxisomes

• Peroxisomes are membrane-enclosed organelles.
• Function in removing hydrogen peroxide and breaking down fatty acids into smaller molecules.
• Peroxisomes utilize oxidative enzymes and generate hydrogen peroxide as a by-product.

Ribosomes

• Ribosomes are non-membrane bound organelles.
• Ribosomes are composed of rRNA and proteins.
• Two forms are free in the cytoplasm and bound to RER.
• Ribosomes function in protein synthesis by translating mRNA into amino acid chains.

Proteasomes

• Proteasomes are non-membrane bound.
• They are responsible for protein degradation.
• They target improperly folded or damaged proteins for destruction.

Cytoskeleton

• Cytoskeleton is vital for maintaining cell shape and facilitating internal movement.
• Composed of microtubules, intermediate filaments, and microfilaments, or actin filaments.

Centrioles

• Centrioles organize microtubules during cell division, playing a critical role in forming the mitotic spindle, determining the basis of cilia, and forming flagella.
• Centrioles are composed of microtubules arranged in a specific configuration.

Cilia

• Cilia are hair-like projections on the cell surface.
• Cilia are responsible for moving substances across the cell surface.
• Cilia have a characteristic structure of microtubules, organized in a 9+2 arrangement

Filaments

• Filaments are protein fibers supporting cell structure.
• Three types are thinner actin, intermediate filaments, and thick myosin filaments.

Inclusions

• Inclusions are stored materials within the cell.
• Includes stored food, pigments, and crystals. Lipofuscin is a type of aging pigment. Melanin is a natural skin pigment. Hemoglobin is an oxygen-carrying protein pigment found in red blood cells. Glycogen is a type of stored carbohydrate.

Nucleus

• The nucleus is the largest and most prominent organelle in the cell.
• It has a rounded or oval shape but can be irregular, and a central location within a cell body.
• The nucleus can be observed by light microscopy (LM) and contains chromatin, nucleolus, nuclear envelope, and nuclear matrix.
• Chromatin comprises DNA and associated proteins.
• The nucleolus is a region within the nucleus where ribosome components are synthesized.
• The nuclear envelope ensures the separation of the nucleus from the cytoplasm. It has a double membrane. Nuclear pores allow passage into and out of the nucleus.
• The nuclear matrix provides structural support to the nucleus.