Cell Structure and Functions

Cell Structure and Functions

• Cell membrane: dynamic, fluid structure that encloses the cell and maintains differences between the cytosol and extracellular environment.
• Membrane structure: thin film of lipid and protein molecules held together by non-covalent interactions.
• Lipid molecules: arranged in a continuous double layer, about 5 nm thick, providing the basic fluid structure and impermeable barrier to water-soluble molecules.
• Lipid molecules constitute about 50% of the mass of animal cells, with approximately 10^9 lipid molecules in the plasma membrane of a small animal cell.

Membrane Proteins

• Perform most of the membrane's specific tasks, giving each type of cell membrane its characteristic functional properties.
• Amounts and types of membrane proteins vary highly, with some membranes having less than 25% protein (e.g., myelin membrane) and others having approximately 75% protein (e.g., internal membranes of mitochondria and chloroplast).
• There are always more lipid molecules than protein molecules in the cell membrane, with about 50 lipid molecules for each protein molecule.

Membrane Protein Functions

• Transport
• Enzymatic activity
• Signal transduction
• Intercellular joining
• Cell-cell recognition
• ECM attachment

Transmembrane Proteins

• Amphiphilic, with hydrophobic regions passing through the membrane and interacting with the hydrophobic tails of lipid molecules, and hydrophilic regions exposed to water on either side of the membrane.
• Covalent attachment of a fatty acid chain increases the hydrophobicity of some transmembrane proteins.

Peripheral Membrane Proteins

• Do not extend into the hydrophobic interior of the lipid bilayer.
• Bound to either face of the membrane by non-covalent interactions with other membrane proteins.
• Can be released from the membrane by gentle extraction procedures, exposure to solutions of very low or high ionic strength, or extreme pH.

Nucleus

• Spherical or oval structure containing almost all of the cell's hereditary information.
• Nuclear envelope: double membrane structure that surrounds the nucleus.
• Nuclear pores: control the movement of substances between the nucleus and cytoplasm.
• Nucleoli: spherical bodies where rRNA is synthesized.

DNA Packaging

• Nuclear DNA contains histones.
• Nucleosomes: 165 bp of DNA + 8 molecules of histones.
• Chromatin: threadlike appearance of DNA when not reproducing.
• Chromosomes: chromatin becomes short, thick bodies during cell division.

Endoplasmic Reticulum

• Extensive network of flattened membranous sacs or tubules called cisterns.
• Continuous with the nuclear envelope.
• Rough ER: studded with ribosomes.
• Smooth ER: no ribosomes on its surface, synthesizes phospholipids, fats, and steroids.

Ribosomes

• Sites of protein synthesis.
• Proteins synthesized to attached ribosomes enter cisterns within the ER for processing and sorting.
• In some cases, enzymes attach the proteins to carbohydrates to form glycoproteins or to phospholipids.

Golgi Complex

• An organelle consisting of 3-20 cisterns.
• Proteins synthesized by attached ribosomes are initially transported in this organelle.
• Proteins are modified and move from one cistern to another via transfer vesicles that bud from the cistern's edges.
• Enzymes in the cisterns modify the proteins to form glycoproteins, glycolipids, and lipoproteins.

Lysosomes

• Formed from Golgi complexes that look like membrane-enclosed spheres.
• Single-membraned and lack internal structure.
• Contain 40 kinds of digestive enzymes that break down various molecules, including bacteria.
• Abundant in human WBC.

Vacuoles

• Space or cavity in the cytoplasm that is enclosed by a membrane called the tonoplast.
• In plant cells, vacuoles occupy 5-90% of the cell volume, depending on the type of cell.
• Some serve as storage of proteins, sugars, organic acids, and inorganic ions; also, metabolic poisons and wastes (in plants).
• Help bring food to the cell.
• May take up water, enabling plant cells to increase in size and provide rigidity to leaves and stems.

Mitochondria

• Spherical or rod-shaped, appearing throughout the cytoplasm.
• Double-membrane: outer smooth, inner series of folds (cristae).
• Center part: matrix.
• No. in the cell varies, with liver having 1000-2000 mitochondria.
• Contains 70s ribosomes and some DNA of their own, as well as the machinery needed to replicate, transcribe, and translate the information encoded by their DNA.

Chloroplasts

• Found in green algae and plants.
• Membrane-enclosed structure containing pigment chlorophyll and enzymes required for photosynthesis.
• Thylakoids: flattened membrane sacs that contain chlorophyll.
• Grana: stacks of thylakoids.

Peroxisomes

• Single-membrane-bound organelles.
• Contain one or more enzymes that use molecular oxygen to remove hydrogen atoms from specific organic substrates.
• Do not contain DNA or ribosomes.
• Usually contain 50 different enzymes involved in different biochemical pathways.

Centrosome

• Located near the nucleus.
• Two components: pericentriolar area and centriole.
• Pericentriolar material: region of the cytosol composed of a dense network of small protein fibers; organizing center for the mitotic spindle; microtubule formation in nondividing cells.
• Centrioles: composed of nine clusters of three microtubules (triplets) arranged in a circular pattern.

Cellular Compartmentalization

• Advantages of compartmentalization:
  1. Allows the cell to carry out different metabolic activities due to the established physical boundaries.
  2. Generate a specific micro-environment to spatially or temporally regulate a biological process.
  3. Establish specific locations or cellular addresses for which processes should occur.

Tissues

• Groups of cells with a common structure and function.
• Four types:
  1. Epithelial: outside of the body and lines organs and cavities.
  2. Connective: binds and supports other tissues.
  3. Nervous: senses stimuli and transmits signals.
  4. Muscle: capable of contracting when stimulated by nerve impulses.

Peptide Bond Formation

• Peptide bond formation is a highly endergonic process, requiring the hydrolysis of high-energy phosphate bonds.
• Water is removed during the process, resulting in a bond between the α-carboxyl group of one amino acid and the α-amino group of another.

Characteristics of the Peptide Bond

• The peptide bond is planar, with adjacent α-carbons, carbonyl oxygen, α-amino nitrogen, and associated hydrogen atoms lying in the same plane.
• The peptide bond has a partial double-bond character due to resonance, preventing rotation around the bond axis.
• Peptide bonds are resistant to denaturation by heat or high concentrations of urea, but can be hydrolyzed by strong acid or base at elevated temperatures.

Conformation of Proteins

• A protein in its native state has a unique three-dimensional structure (conformation).
• Primary structure refers to the linear sequence of amino acids joined by peptide bonds, forming a covalent "backbone" of the polypeptide.
• The amino acid sequence is coded for by DNA and determines the final three-dimensional form of the protein.

Amino Acid Characteristics

• Basic amino acids have side chains with net positive charges at neutral pH, including histidine, arginine, and lysine.
• Proline is an imino acid with an aliphatic side chain, bonded to both the nitrogen and α-carbon atoms, and is more conformationally restricted due to its ring structure.

Post-Translational Modifications

• More than 100 different kinds of amino acids can be formed through post-translational modifications, including:
  • Hydroxylation of prolines and lysines in collagen
  • Methylation of lysines and histidines in muscle myosin
  • Carboxylation of glutamates in blood clotting and bone proteins
  • Phosphorylation of serine, threonine, and tyrosine molecules

Peptides and Polypeptides

• Peptide chains refer to the linking of amino acids.
• Polypeptides are composed of many amino acids linked together.
• Amino acids in polypeptide chains are referred to as residues.

Secondary Structure

• α-helices have 3.6 amino acid residues per turn, with a right-handed (clockwise) orientation.
• β-pleated sheets form through hydrogen bonds between peptide bonds in different chains.
• β-pleated sheets can be parallel or antiparallel, depending on the direction of the chains.

Tertiary Structure

• Tertiary structure refers to the spatial relations of more distant residues.
• Folding of polypeptide chains occurs due to associations between secondary structures, resulting in a state of lowest energy (greatest stability) for the protein.
• Hydrophobic side chains are located in the interior of the structure, while hydrophilic side chains are on the outside, in contact with water.