Cellular Level of Organization

• The cellular level of organization concerns cells, the smallest living things.
• Cell biology is the study of cells, including their internal structures and chemical reactions.
• Cells are the structural and functional subunits of all body systems.

Cells and Parts

• Every cell has three principal parts: plasma membrane, cytoplasm, and nucleus.
• The plasma membrane is the flexible outer surface of the cell.
• The cytoplasm contains numerous organelles surrounded by cytosol.
• The nucleus is a large organelle that contains chromosomes.

Plasma Membrane

• The plasma membrane is a strong but flexible barrier between the interior of a cell and the outside world.
• The fluid mosaic model describes membrane structure.
  • A bilayer of phospholipids forms a structural foundation.
  • A variety of membrane proteins interact with the lipids.
  • Lipids and many proteins are able to move about freely.
• The plasma membrane is the major means that cells use to communicate with each other and the environment.
• Phospholipid bilayers create effective barriers for polar and charged molecules, and ions.
• Cholesterol and glycolipids are two types of lipids found in animal cell membranes.

Membrane Proteins

• Sub-classes of membrane proteins include integral, peripheral, and glycoproteins.
• Ion channels allow specific ions to move through water-filled pores.
• Carrier proteins transport specific substances across the membrane by changing shape.
  • Amino acids, needed to synthesize new proteins, enter body cells via carriers.
  • Carrier proteins are also known as transporters.
• Receptor proteins recognize specific ligands and alter cell function.
  • Antidiuretic hormone binds to receptors in the kidneys and changes the water permeability of certain plasma membranes.
• Enzymes catalyze reactions inside or outside the cell, depending on which direction the active site faces.
• Linker proteins anchor filaments inside and outside the plasma membrane, which provides structural stability and shape for the cell.
• Cell-identity markers such as glycoproteins distinguish cells from each other.

Membrane Fluidity and Permeability

• Various membrane functions require a balance between strength and flexibility of the membrane.
• Membrane fluidity refers to the viscosity of the lipid bilayer.
• Membrane fluidity depends mainly on the number of double bonds in the membranes' fatty acids.
• Small and non-polar molecules pass easily through phospholipid bilayers.
• The permeability of membranes to ions/polar molecules depends on the number of specific transport proteins.
• How quickly ions and molecules cross membranes relies on concentration and electrical gradients.

Active and Passive Processes

• Transport of molecules or ions across membranes can be classified as either passive or active.
• Passive processes are spontaneous with chemicals moving based on their kinetic energy.
  • Movement goes from higher to lower concentrations ("downhill").
  • Examples include simple diffusion, facilitated diffusion, and osmosis.
• Active processes require stored energy for chemicals to move.
  • Movement is from lower to higher concentrations ("uphill").
  • Examples include primary and secondary active transport, and endocytosis.

Diffusion

• The movement of solutes directly through the lipid bilayer is called simple diffusion.
• The movement of solutes with the help of membrane proteins is called facilitated diffusion.
• The rate of diffusion depends on the concentration gradient, temperature, mass of diffusing molecules, membrane surface area, and diffusion distance.
• Non-polar molecules move via simple diffusion.
• Many ions cross membranes through ion channels.
• Polar molecules are transported by carrier-mediated facilitated diffusion.

Osmosis

• Most membranes are selectively permeable, and allow water to move much more quickly than many solutes.
• Water moves in response to differences in solute concentrations, going toward the higher solute level.
• Osmotic gradients can have dramatic effects on cells.
• The three types of solutions are hypotonic, isotonic, and hypertonic.
• Most of our body fluids and cells are in osmotic equilibrium because water moves quickly.

Active Transport

• Energy is a requirement to move molecules, referred to as moving "uphill".
• Primary active transport uses ATP as the source of energy.
• Secondary active transport employs ion gradients as the source of energy.
• Vesicles can transport some large molecules into (endocytosis) and out of (exocytosis) cells without breaking them down.
• Primary active transport pumps ions "uphill" against their concentration gradients.
• ATP hydrolysis is used to power the process of primary active transport.
• The Na+/K+ pump and Ca2+/Mg2+ pump are examples of primary active transport.
• Secondary active transport also moves ions or molecules “uphill”, against their concentration gradients.
• Symporters and antiporters are two types of secondary active transport where many specific examples of each type exist in cells.
• Vesicles are small, spherical membrane sacs used to move large molecules in and out of cells, and between organelles.
  • Receptor-mediated endocytosis is an important example.
  • Phagocytosis allows some cells to "eat" large particles.
  • Bulk-phase endocytosis allows cells to take in fluid and small solutes together.
  • Transcytosis allows cells to transport large chemicals across an epithelium.

Cytoplasm

• This is everything between the nucleus and the cell membrane.
• Cytosol is the intracellular fluid, mostly water with many dissolved chemicals.
• Organelles are structures that each perform specific functions for the cell.
• The cytoskeleton is a network of protein filaments that stretches throughout the cytosol.
  • Major functions include determining cell shape, organizing the contents of the cell, moving organelles, moving chromosomes during cell division, and creating/moving membrane vesicles (in phagocytosis, etc).

Centrosome

• The centrosome makes new microtubules in nondividing cells.
• The centrosome forms the mitotic spindle during cell division.

Cilia and Flagella

• Cilia move fluid along the cell surface.
• Flagella move cells through the medium.

Ribosomes

• Ribosomes have two major subunits made of both RNA and proteins.free
• Free ribosomes make proteins used in the cytosol.
• Attached ribosomes make proteins used in membranes and for export.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (or just “ER”) is an extensive membrane network within the cytosol.
• Rough ER contains bound ribosomes.
• Smooth ER has enzymes involved in metabolism of lipids and drugs.

Golgi Complex

• Modifies and sorts proteins produced by the rough ER
• Generates vesicles for moving molecules around inside

Lysosomes, Peroxisomes, Proteasomes

• Lysosomes contain digestive enzymes used to break down ingested material, worn-out parts of cells, and the entire cell.
• Peroxisomes contain oxidative enzymes important in metabolism.
• Proteasomes break down worn out or unneeded proteins.

Mitochondria

• Mitochondria contain enzymes that help cells produce large amounts of ATP in a process called cellular respiration.
• Mitochondria contain both an inner and outer membrane.
• Mitochondria self-replicate, using their own ribosomes, also containing some of their genes on their own DNA.

Nucleus

• The nucleus acts as the central control center of the cell.
• A double-walled nuclear envelope separates the nucleus from the cytoplasm.
• The nucleolus is a site within the nucleus that produces new ribosomes.
• Chromosomes contain genes which store the information to build and run cells.
• The human genome consists of 23 pairs of chromosomes.
• Each chromosome contains DNA combined with histone proteins to form chromatin.
• Histones allow DNA to be tightly packed.
• Sister chromatids are connected at the centromere after DNA replication.

Genetic Information

• Transcription is the process where a DNA sequence is copied (or transcribed) into an RNA sequence.
• Messenger RNA (mRNA) contains a sequence to encode proteins.
• The mRNA sequence is used to make new proteins in a process called translation.
• Transcription transfers genetic information from DNA into one of three major classes of RNA (messenger, ribosomal, and transfer RNA).
• RNA polymerase, an enzyme, creates RNA molecules.
• Regulation of transcription is a way that cells control which proteins get made.

Translation

• Ribosomes have two subunits and combine when mRNA is available.
• Ribosomes are made of proteins and rRNA, and have multiple binding sites for mRNA and tRNA.
• tRNA's help line up the correct amino acids to make a new protein.
• All non-gamete producing cells of the body are produced by mitosis.

Cell Division

• The cell cycle is an orderly sequence of events by which somatic cells replicate.
• Cells grow and duplicate their DNA during interphase.
• Cells divide their chromosomes and nuclei during mitosis.
• After mitosis, cells finish dividing during cytokinesis.
• DNA molecules are copied during the S phase.
• Chromosomes are divided into stages including interphase, prophase, metaphase, anaphase, telophase.
• The structure of the double helix allows both copies of the new DNA to have the same sequence.
• At the end of the S phase, sister chromatids have been created even though they are not visible until mitosis.
• Meiosis = Reproductive Cell Division.
• Gametes are unique cells formed in sexual reproduction.
• Gametes contain half as many chromosomes as all other cells and are referred to as haploid cells.
• Chromosome pairs exchange DNA early in the first division in a process called crossing over, a form of genetic recombination.
• At fertilization, male and female gametes combine to create a new diploid individual, which will then undergo mitosis.