The plasma membrane (also called plasmalemma) is a thin, flexible outer membrane that separates intracellular fluid from extracellular fluid.
Structure of the Plasma Membrane
Composition:
Made of a double layer of lipids including:
Phospholipids
Cholesterol
Glycolipids
Membrane Proteins: Embedded within the lipid bilayer.
Phospholipids
Description:
Most abundant lipids in the plasma membrane.
Structure:
Hydrophilic heads: Attracted to water, facing inward and outward.
Hydrophobic tails: Avoid water, resting in the center of the membrane.
Membrane Proteins
Types:
Integral Proteins:
Most abundant; extend across the membrane.
May act as receptors.
Peripheral Proteins:
Exist mainly on the cytoplasmic side; provide network support via filamentous structures.
Glycocalyx:
A carbohydrate covering involved in cell-to-cell recognition, composed of sugars from glycoproteins or glycolipids.
Functions of the Plasma Membrane
Barrier Function:
Serves as an external barrier against substances.
Receptor Function:
Proteins act as receptors for hormones and neurotransmitters; facilitate cell recognition.
Transport Function:
The membrane is a selectively permeable barrier, allowing certain substances to pass while restricting others.
Movements of Substances Across the Plasma Membrane
Passive Processes:
Substances move freely through the lipid bilayer down their concentration gradient (high to low concentration) without energy (ATP).
Diffusion: Movement of small, uncharged molecules (e.g., O₂, CO₂, fat-soluble molecules).
Active Processes:
Substances move against their concentration gradient (low to high concentration), requiring ATP.
Active Transport: Involves integral proteins to transport larger water-soluble or charged molecules (e.g., glucose, amino acids, ions).
Vesicular Transport:
Larger particles and macromolecules are transported via vesicles.
Two types:
Exocytosis
Endocytosis
Exocytosis
Definition:
Membrane-lined cytoplasmic vesicles fuse with the plasma membrane, releasing contents to the outside of the cell.
Example: Mucus and protein secretions from glands.
Mechanism:
vSNAREs from the vesicle bind with tSNAREs from the plasma membrane, facilitating fusion.
Endocytosis
Definition:
Brings large molecules into the cell by infolding the plasma membrane, forming cytoplasmic vesicles.
Key Protein: Clathrin, which helps in membrane deformation.
Types of Endocytosis:
Phagocytosis (Cell Eating):
Plasma membrane forms pseudopodia around large molecules (e.g., bacteria, debris) and engulfs them, forming a phagosome that fuses with lysosomes for digestion.
Pinocytosis (Cell Drinking):
Involves small infoldings of the membrane surrounding extracellular fluid and dissolved molecules, primarily for nutrient absorption in intestinal cells.
Receptor-Mediated Endocytosis:
Selective uptake of specific molecules (e.g., insulin, LDL) via receptor binding. Molecules are internalized in a protein-coated vesicle, lysosomes release vesicle contents, and receptors are recycled.
Familial Hypercholesterolemia
Definition:
An inherited condition where cells lack receptors for binding cholesterol-carrying LDLs (Low-Density Lipoproteins).
Consequences:
Cholesterol cannot enter the cells, leading to accumulation in blood, causing:
Hypercholesterolemia
Atherosclerosis
Increased risk of stroke or myocardial infarction.
Cytoplasm
Definition:
Region between the nucleus and plasma membrane; consists of:
Cytosol: Viscous fluid with water, ions, enzymes.
Inclusions: Stored nutrients, pigments.
Organelles: Specific cellular structures.
Ribosomes
Description:
Dark-staining granules, non-membrane bound.
Function: Site of protein production.
Structure: Composed of two subunits: proteins and ribosomal RNA (rRNA).
Types:
Free Ribosomes: Produce proteins used in the cytosol.
Ribosomes on rER: Produce proteins for cell membrane or export.
Process: Protein synthesis occurs through Translation, directed by DNA, with the messenger RNA (mRNA) carrying instructions.
Rough Endoplasmic Reticulum (rER)
Definition:
A network of membrane-walled envelopes (cisternae) studded with ribosomes.
Function: Ribosomes synthesize proteins that enter the cisternae for secretion or incorporation into the cell membrane.
Smooth Endoplasmic Reticulum (sER)
Definition:
A membranous system of sacs and tubules without ribosomes.
Functions:
Lipid and steroid synthesis
Lipid metabolism
Drug detoxification.
Golgi Apparatus
Structure:
Stack of 3-10 disc-shaped membrane-bound cisternae.
Faces:
Cis Face: Convex side for receiving materials.
Trans Face: Concave side for shipping materials.
Function: Sorts products from rER, packing them into vesicles for delivery.
Products: Includes secretory granules and lysosomes.
Mitochondria
Description:
Rod-like organelles surrounded by two membranes; inner membrane is folded into projections called cristae.
Function: Main site of ATP synthesis, generating cellular energy.
Lysosomes
Structure:
Spherical membrane-bound sacs containing digestive enzymes known as Acid hydrolases.
Function: Site of intracellular digestion; break down organelles and substances ingested by the cell.
Mechanism: Fuse with phagosomes to release enzymes for breakdown.
Diseases:
Tay-Sachs Disease: Inherited condition where specific lysosomal enzymes are lacking, resulting in glycolipid accumulation leading to severe neurological impairment.
Gaucher's Disease: Caused by lack of the glucocerebrosidase enzyme, leading to harmful accumulation of substances; three types exist with differing symptoms:
Type 1: Bone disease, anemia, enlarged spleen.
Type 2: Infantile onset with severe neurological involvement.
Type 3: Symptoms may appear later in life, with varied organ involvement.
Peroxisomes
Definition:
Membrane-walled sacs containing enzymes.
Enzymes: Oxidase (neutralizes free radicals) and catalase (converts hydrogen peroxide to harmless substances).
Functions: Breakdown of toxins such as alcohol and formaldehyde.
Cytoskeleton
Definition:
Network of rods throughout the cytosol that supports cell structure and movement.
Types:
Microtubules: Cylindrical structures made of tubulin proteins, involved in cell shape and organelle transport.
Microfilaments (Actin Filaments): Fine filaments of actin involved in cell movement and muscle contraction.
Intermediate Filaments: Tough and stable fibers that provide tensile strength.
Centrosome and Centrioles
Centrosome:
Spherical structure near the nucleus, composed of an outer protein matrix and inner pair of centrioles.
Function: Organizes microtubules, crucial during cell division.
Centrioles:
Paired cylindrical structures composed of triplets of microtubules, forming the base of cilia and flagella, and organizing the spindle apparatus during mitosis.