Anatomy and Physiology - Chapter 3: The Cellular Level of Organization Pt 1 Cell Structures

All human cells have three basic parts

1. Plasma membrane: flexible outer boundary

2. Cytoplasm: intracellular fluid (cytosol) containing organelles 

3. Nucleus: DNA containing control center 

cytoplasm

• All cellular material located between the plasma membrane and the nucleus 

• Composed of : 

  • Cytosol: gel like fluid component - water and soluble molecules 

  • Organelles: metabolic machinery structures of cell; each with specialized function; may/may not be membranous 

• May also have inclusions: insoluble molecules; vary with cell type

Cytoskeleton

• Elaborate network of rods that run throughout cytosol

• Provide cell shape and organization

  • Assist in movement and anchoring of cell components 

• Include microfilaments, microtubules, and intermediate filaments

Centrosome and Centrioles 

• Located near the nucleus; centrosome = cell centre 

• Microtubule organizing center 

• Centrioles = pair of barrel shaped microtubular organelles at right angles to each other 

  • 9 sets of microtubule triplets 

  • Also form the basis of cilia and flagella

cilia

whiplike, motile extensions on some cell surfaces, sweeping motion to move substances, move in one direction

flagella

longer extensions that move the whole cell

microvilli

• minute, fingerlike extensions of plasma membrane that project from surface of select cells

• Used to increase surface area for absorption

Ribosomes

• site of protein synthesis; consists of 2 subunits

• made of protein and ribosomal RNA (rRNA)

• Two forms

  • free ribosomes: free floating in cytosol

  • membrane-bound ribosomes: attached to membrane of ER

Rough Endoplasmic Reticulum

• surface appears rough - covered with attached ribosomes

  • used to synthesize proteins that are secreted from the cell

• proteins enter cisterns and are modified as they move through fluid-filled tubes

• final protein enclosed in vesicle and sent to Golgi complex for further processing

Smooth Endoplasmic Recticulum

• Network of looped tubules continuous with Rough ER

• contains enzymes in its membranes that function in…

  • lipid metabolism: cholesterol and steroid-based hormone synthesis, making lipids for lipoproteins, absorption, synthesis, and transport of fats

  • detoxification of certain chemicals

  • conversion of glycogen to glucose

  • storage and release of calcium

    • sarcoplasmic reticulum is specialized smooth ER found in skeletal and cardiac muscle cells

Golgi Complex

• Stacked and flattened membranous cistern sacs

• modifies, concentrates, and packages proteins and lipids received from ER

• Depending on its contents, final transport vesicle can go to

  • Pathway A: proteins to be used outside of the cell

  • Pathway B: contents (lipids, proteins) directly inserted into plasma membrane

  • Pathway C: Remain in cell holding contents in vesicle until needed

Lysosomes

• Spherical membranous bags containing digestive enzymes made by golgi complex

  • digest ingested bacteria, viruses, and toxins

  • degrade nonfunctional organelles

  • metabolic functions: break down and release glycogen, break down and release Ca2+ from bones

  • intracellular release if injured causes cells to self digest (autoanalysis)

Peroxisomes

• Membranous sacs containing powerful detoxifying substances that neutralize toxins (like free radicals) and break down organic substances like fatty acids

• two main detoxifiers: oxidase uses oxygen to convert toxins to hydrogen peroxide and then to H2O

Proteasomes

• Barrel-shaped structures that destroy unneeded, damaged, or faulty proteins by cutting long proteins into smaller peptides

Mitochondria

• Produces most of the cell’s ATP via aerobic cell respiration

• enclosed by double membranes, inner membrane has many folds where cell respiration reactions occur using membrane proteins

• can divide - contain their own DNA, RNA, and ribosomes

• play a role in apoptosis (programmed cell death)

Nucleus

• Largest organelle, contains the genetic library of blueprints for synthesis of nearly all 100,000+ cellular proteins

• most cells are uninucleate, but skeletal muscle, certain bone cells, and some liver cells are multinucleate, red blood cells are anucleate

Nuclear Envelope

• double membrane barrier enclosing the nucleoplasm

  • outer layer is continuous with rough ER and is covered with ribosomes

• Nuclear pores allow substances in and out of nucleus

Nucleoli

• dark staining spherical bodies within nucleus that are involved in ribosomal RNA (rRNA) synthesis + other types of RNA and ribosome subunit assembly

• usually one or two per cell

Chromatin

• Consists of threadlike (non-dividing cell) strands of DNA, histone proteins, and RNA

• arranged in nucleosomes which consist of DNA wrapped around histones

  • chemical alterations of histones affect DNA → help regulate gene expression

• Image shows sister chromatin

• Chromosomes are condensed chromatin

  • helps protect fragile chromatin threats during cell division

Plasma Membrane

flexible yet sturdy barrier that surrounds cell and contains the cytoplasm

fluid mosaic model

functions of the plasma membrane

1. physical barrier

2. selective permeability

3. communication

4. cell recognition

Membrane lipids

75% phospholipids - polar, hydrophilic heads with nonpolar, hydrophobic tails

5% glycolipids - lipids with sugar groups; found on outer membrane surface

20% cholesterol - increases membrane stability

Membrane Proteins

• allow cell communication with environment

• most have specialized membrane functions

• float freely or are attached to intracellular structures

• integral (transmembrane) proteins are firmly inserted into membrane transport enzymes or receptors

• peripheral proteins are loosely attached - enzymes, motor proteins for shape changes during cell division and muscle concentration, cell-to-cell connections

Membrane fluidity

lipids and proteins move easily in the bilayer

cholesterol stabilizes membrane and reduces fluidity

membrane permeability

• selectively permeable

• lipid bilayer is ALWAYS permeable to small, nonpolar, uncharged molecules

• Transmembrane proteins increase permeability

• macromolecules are only able to pass by vesicular transport

Membrane gradients

electrochemical gradient

concentration gradient: different in chemical concentration between one side of the plasma membrane and the other

electrical gradient: difference in ion concentration between one side of the plasma membrane and the other - creates electrical charges on the membrane