Cell Structure and Membranes

Cell Structures

• A cell consists of three basic structures: plasma membrane, cytoplasm, and organelles

Membranes

• There are two types of membranes: epithelial and connective tissue membranes
• Epithelial membranes include:
  • Cutaneous membranes
  • Serous membranes
  • Mucous membranes
• Serous membranes:
  • Line the inner face of body cavities not connected to the external environment
  • Consist of a simple, squamous epithelium (mesothelium) lined up on a thin connective tissue
  • Secrete a serous fluid that wets the surface
• Mucous membranes:
  • Line body cavities directly or indirectly connected to the external environment
  • Special cells secrete mucus
• Synovial membranes:
  • Line the joint space inside movable joints
  • Do not contain a continuous epithelium
  • Made up of flat connective tissue cells and adipose tissue elements

Cell Membrane

• The lipid bilayer forms the main structure of biological membranes
• Proteins in the membrane determine its function
• Proteins can be classified into two classes: peripheral proteins and integral proteins
• Integral proteins:
  • Can be completely embedded in the lipid bilayer
  • Can have parts that protrude towards the surface
  • Can be embedded in the outer or inner half of the lipid bilayer
  • Can extend across the lipid bilayer and meet the aqueous microenvironment on both sides of the membrane (transmembrane proteins)
• Peripheral proteins:
  • Completely outside the lipid layer
  • Do not constitute a continuous layer
• Functions of proteins in the membrane:
  • Transport: form hydrophilic channels and transport substances across the membrane
  • Enzyme activity: act as enzymes in metabolic pathways
  • Signal transduction: allow signal molecules to bind and transmit messages to the cell
  • Binding between cells: bind neighboring cells together
  • Cell-cell recognition: enable cell recognition by other cells
  • Attachment to the extracellular matrix: attach the cytoskeleton to the extracellular matrix

Carbohydrates of Membrane

• Proteoglycans are carbohydrates attached to the outer surface of the cell with small proteins
• Functions:
  • Form recognition regions of the cell
  • Create a negatively charged cell surface
  • Make cells stick together
  • Act as receptors

Functions of Cell Membrane

• Separate the cell from the external environment and control the entry and exit of matter
• Selective permeability: allow certain substances to enter the cell while keeping others out
• Ensure cell and organelle integrity
• Transport molecules across the membrane
• Initiate metabolic reactions
• Regulate metabolic reactions
• Bind cells together

Endoplasmic Reticulum (ER)

• Functions:
  • Participate in protein synthesis
  • Perform protein glycosylation
  • Make quality control of proteins
  • Selectively send synthesized proteins to other organelles to form their structure
• RER (Rough Endoplasmic Reticulum):
  • Involved in protein synthesis, glycosylation, and quality control
  • Sends proteins to other organelles to form their structure
• SER (Smooth Endoplasmic Reticulum):
  • Involved in steroid hormone synthesis, glycogen production, cholesterol and bile production, detoxification, and storing calcium ions

Mitochondria

• A double-membrane organelle with variable numbers depending on the cell's function
• Contains unique DNA and ribosomes
• Functions:
  • Acts as the energy (ATP) production center of the cell
  • Stores excess calcium in the cytosol and returns it to the cell when necessary
  • Plays a role in hormone synthesis
  • Initiates apoptosis (programmed cell death)

Ribosomes

• Found individually or in groups on the surface of the cytosol-facing membrane of the RER, on the surface of the outer nucleus, and in the cytoplasm
• Made up of protein and ribonucleic acid (RNA) molecules
• Functions:
  • Carry out protein synthesis

Golgi Apparatus

• Not found in prokaryotes and red blood cells
• Exchanges products synthesized in the cell and ensures they are transported to their destination by packaging
• Consists of three parts: flat sacs, secretory granules, and vacuoles
• Functions:
  • Makes secretion
  • Synthesizes glycoproteins
  • Packages and concentrates secretions
  • Forms lysosomes in the RER region
  • Preserves membrane integrity by adding vesicles to the cell membrane by exocytosis

Lysosomes

• Defense and digestive organelle
• Contains degrading enzymes called hydrolase produced in the ER and Golgi complex
• Functions:
  • Digests large molecule nutrients
  • Digests bacteria, viruses, and toxins
  • Releases calcium in bone