The Cell (Anatomy)

Cell

The basic structural and functional unit of all living organisms.

Cytology

The branch of biology that studies cells

Plasma Membrane

The outer surface of a cell that regulates the movement of substances in and out, maintaining homeostasis. It separates the cell's interior from its external environment.

Cytoplasm

The jelly-like substance within a cell that contains organelles and is the site for many metabolic processes. Two major components are the cytosol and organelles.

Cytosol

The fluid portion of cytoplasm that surrounds organelles, composed mainly of water, dissolved solutes, and organic molecules. Also called intracellular fluid.

Organelle

Specialized structures within a cell that perform distinct functions, such as energy production, protein synthesis, and waste processing. Examples: mitochondria, ribosomes, endoplasmic reticulum.

Nucleus

Large double membrane-bound organelle that contains the cell's genetic material (DNA) and regulates gene expression, control center of the cell. Functions are DNA replication and RNA transcription.

Chromosome

A thread-like structure made of DNA and proteins, contains thousands of genes. Found in nucleus of eukaryotic cells. Crucial for cell division and heredity.

Gene

A segment of DNA that contains the instructions for building proteins and determining traits in an organism. Control most aspects of cellular structure and functions.

Lipid Bilayer

Double layers made up of phospholipids, cholesterol, and glycolipids, which make up the plasma membrane.

Phospholipid

(75% of membrane lipids) a type of lipid that consists of a hydrophilic phosphate head (polar) and two hydrophobic fatty acid tails/ hydrocarbon chains (nonpolar).

How do phospholipids self-assemble in water?

They orient themselves in bilayer with hydrophilic heads facing outward to watery fluid on either side, hydrophobic tails facing towards each other, forming a nonpolar hydrophobic region in membrane interior.

Cholesterol

(20% of membrane lipids) type of lipid consisting of steroid rings and hydrocarbon tail (nonpolar) with an attached —OH (Hydroxyl) group (polar).

Glycolipid

(5% of membrane lipids) type of lipid consisting of carbohydrate groups (polar) and fatty acid tails (nonpolar)

Amphipathic Molecules

Have both polar and nonpolar parts

Integral Proteins

Extend into/through lipid bilayer and are firmly embedded. Most are transmembrane proteins, which span lipid bilayer, protrude into both ECF and cytosol. These are tightly attached to one side of bilayer by covalent bonding to fatty acids. Hydrophilic protrude into ECF or cytosol, hydrophobic extend among fatty acid tails.

Peripheral Proteins

Not as firmly embedded into the membrane. Attached to polar heads of membrane lipids or integral proteins in either surface of membrane.

Ion Channel

(Integral) Forms pore that specific ions can flow across membrane. Most membranes include specific channels for several common ions.

Carrier

(Integral) Transports specific substances across membrane by changing shape.

Receptor

(Integral) Recognizes specific ligand and alters cells functions in some way.

Enzyme

(Integral and Peripheral) Catalyzes reaction inside or outside the cell depending on where the active site faces.

Linker

(Integral and Peripheral) Anchors filaments inside and outside the plasma membrane, provides structural stability and shape to the cell. Also helps movement and connection of two cells.

Cell Identity Marker

(Glycoprotein) Distinguishes your own cells from anyone else’s. Important class of marker is the Major Histocompatibility (MHC) proteins.

Glycoproteins

Proteins with carbohydrate groups (oligosaccharides) attached to the ends which protrude into ECF. Many of these are integral proteins.

Glycocalyx

Extensive sugary coat formed by carbohydrate portions of glycolipids and glycoproteins. This allows cells to recognize each other, since the pattern of carbohydrates in these varies in cells.

Membrane Fluidity

This depends on the amount of cholesterol present and the number of double bonds in fatty acid tails of lipids which make up the bilayer, double bonds put “kinks” in the fatty acid tails which increase this, by preventing tight packing of lipid molecules into the membrane. Cholesterol makes lipid bilayer stronger. It allows interactions in the membrane like assembly of membrane proteins.

Concentration Gradient

Difference in concentration of chemicals from one place to another, like from the inside to the outside of the membrane. Oxygen molecules and sodium ions (Na+) are more concentrated in ECF than in cytosol; carbon dioxide molecules and potassium ions (K+) are more concentrated in the cytosol than in ECF.

Electrical Gradient

Difference in electrical charges between two regions. The charge difference is called membrane potential because it happens across the membrane.

Electrochemical Gradient

Combined influence of the concentration and electrical gradients on movement of a particular ion.

Passive Processes

Substance moves down its concentration or electrical gradient to cross the membrane using only its own kinetic energy, no input of energy from cell.

Active Processes

Cellular energy is used to drive the substance “uphill” against the concentration or electrical gradient. The energy is used in the form of ATP.

Diffusion

(Passive) Random mixing of particles in a solution occurs because of particles’ kinetic energy. Both solutes, dissolved substances, and solvent undergo this process.

Simple Diffusion

(Passive) Substances move freely through the bilayer of membrane of cells without the help of transport proteins. Nonpolar, hydrophobic molecules such as oxygen, carbon dioxide, and nitrogen gases, fatty acids, steroids, and fat-soluble vitamins (A, D, E, K), as well as uncharged polar molecules like water, urea, and small alcohols may pass through this.

Carrier-mediated Facilitated Diffusion

(Passive) Carrier moves a solute down its concentration gradient across plasma membrane. Solute binds to carrier on one side of membrane then released on the other side after changing shape. Solute binds more often to the side with higher concentration. After concentration is the same on both sides, solute molecules bind to the carrier on cytosolic side and move to ECF, same as from ECF to cytosol. Rate of diffusion is determined by steepness of concentration gradient across membrane.

Osmosis

(Passive) Form of Diffusion in which a solvent moves through selectively permeable membrane. When the solvent is water, it moves from areas of higher water concentration to lower concentration.

Water molecules pass through plasma membrane in two ways:

1. By moving between neighboring phospholipid molecules in lipid bilayer via simple diffusion

2. By moving through aquaporins (integral membrane proteins that function as water channels)

Primary Active Transport

(Active) Energy derived from hydrolysis of ATP changes shape of carrier protein, which pumps substance across membrane against concentration gradient. Typical body cell expends 40% of ATP it generates.

Secondary Active Transport

(Active) Energy stored in a Na+ or H+ concentration gradient is used to drive other substances across membrane against their concentration gradients. It indirectly uses energy obtained from hydrolysis of ATP.

Vesicular Transport

(Active) Transport in vesicles (endocytosis and exocytosis); variety of substances are transported in vesicles between structures within cells. Vesicles import materials from and release materials into ECF.

Endocytosis

(Active) Form of vesicular transport in which materials move into cell in a vesicle formed from plasma membrane.

Exocytosis

(Active) Form of vesicular transport in which materials move out of cell by fusion with the plasma membrane of vesicles formed inside the cell.

Receptor-mediated Endocytosis

(Active) Highly selective type of endocytosis where cells take up specific ligands. Vesicle forms after a receptor protein in the membrane recognizes and binds to a specific particle in ECF. Cells take up cholesterol-containing low-density lipoproteins (LDL’s), transferrin, some vitamins, antibodies, and certain hormones.

Steps in Receptor-mediated Endocytosis

1. Binding

2. Vesicle formation

3. Uncoating

4. Fusion with endosome

5. Recycling receptors to plasma membrane

6. Degradation in lysosomes

Phagocytosis

(Active) “Cell eating” Form of endocytosis where the cell engulfs large solid particles, such as worn-out cells, bacteria, or viruses. Two specific kinds of body cells (phagocytes) are able to carry this out, macrophages (located in body tissues) and neutrophils (type of white blood cell).

Pinocytosis (Bulk Phase)

Form of endocytosis where tiny droplets of ECF are taken up. Receptor proteins are not involved; solutes dissolved in ECF are brought into cell.

Tonicity

(Osmosis) Measure of solutions ability to change volume of cells by altering the water content.

Isotonic Solution

Solution in which the cell maintains its normal shape and volume.

Hypotonic Solution

Solution that has lower concentration of solutes than cytosol in the RBC’s. (Water molecules enter the cell faster than they leave)

Hypertonic Solution

Solution that has higher concentration of solutes than cytosol in RBC’s.

Sodium-Potassium Pump

(Primary active transport) Mechanism that expels sodium ions (Na+) from cells and brings potassium ions (K+) in.

Symporters

(Secondary active transport) Carriers move two substances in the same direction.

Example: Glucose and amino acids are symporters of Na+

Antiporters

(Secondary active transport) Carriers move two substances in opposite directions.

Example: Ca2+ and H+ are antiporters of Na+

Cytoskeleton

Network of protein filaments that extends throughout cytosol. This and other organelles are composed of microfilaments, intermediate filaments and microtubules.

Microfilaments

Thinnest element of cytoskeleton, composed of the proteins, actin and myosin. Most prevalent at the edge of cell. It helps generate movement and provide mechanical support to microvilli.

Intermediate Filaments

Thicker than microfilaments; thinner than microtubules. Several different strong proteins can form these. Found in parts of cell subject to mechanical stress. They stabilize organelles and attach cells to each other.

Example: Keratin

Microtubules

Largest of cytoskeletal components, long, unbranched hollow tubes composed mainly of the protein tubulin. Assembly begins in the centrosome. They help determine cell shape and are dominant components to flagella and cilia.

Example: Tubulin

Ribosomes

Sites of protein synthesis, high content of ribonucleic acid (RNA). Each ribosome contains 50 proteins. Consist of 2 subunits, 1 large, 1 small, which are made separately in the nucleolus (spherical body in nucleus). Once produced, they are released separately then come together in cytoplasm. Attached to outer surface of nuclear membrane and ER. Also found in mitochondria, to synthesize mitochondrial proteins.

Rough Endoplasmic Reticulum

Continuous with nuclear membrane and usually folded into a series of flattened sacs (cisternae). Outer surface is studded with ribosomes (sites of protein synthesis). Synthesizes glycoproteins and phospholipids transferred into organelles, inserted in membrane or secreted during exocytosis.

Smooth Endoplasmic Reticulum

Extends from rough ER to form network of membrane tubules. Synthesizes fatty acids and steroids like testosterone and estrogen. Inactivates/detoxifies drugs and removes the phosphate group from glucose-6-phosphate. Stores and releases calcium ions that trigger contraction in muscle fibers. Outer surface is smooth (No ribosomes). Functions are lipid and drug metabolism and glycogen breakdown.

Golgi Complex

(First step in transport) Consists of 3-20 saccules, small, flattened, curved membranous sacs. More extensive in cells that secrete proteins. It modifies, sorts, packs and transports proteins received from rough ER, forms lysosomes and secretory vesicles that discharge processed proteins via exocytosis into ECF.

Lysosomes

Spherical membranous vesicles that form from Golgi complex. Contain up to 60 kinds of digestive and hydrolytic enzymes that break down molecules after endocytosis. Digests phagosome, autophagy, autolysis (cell death). This organelles membrane has transporters that move products of digestion like glucose, fatty acids, and amino acids into cytosol.

Mitochondria

Powerhouse of cell. Consists of internal and external membranes that have small fluid filled space in between. Internal membrane contains series of folds called cristae, and the fluid filled cavity is the mitochondrial matrix.

Nuclear Envelope

Double membrane that separates the nucleus from cytoplasm (Lipid bilayers similar to plasma membrane). Outer layer is continuous with rough ER.

Nuclear Pore

Openings in nuclear envelope that consist of circular arrangement of proteins surrounding central opening.

Chromatin

Threadlike mass of genetic material, consisting of DNA and histone proteins, present in nucleus of a nondividing or interphase cell.

Transcription

(Occurs in nucleus) Genetic information represented by sequence of base triplets in DNA serves as a template for copying the information into complementary sequence of codons.

Messenger RNA (mRNA)

Directs synthesis of proteins

Ribosomal RNA (rRNA)

Joins with ribosomal proteins to make ribosomes

Transfer RNA (tRNA)

Binds to amino acid and holds it in place on ribosomes until incorporated into protein during translation. One end of tRNA carries specific amino acids, opposite end consists of triplet nucleotides (anticodon). By pairing bases, tRNA anticodon attaches to mRNA codon. Each tRNA binds to one amino acid.

Translation

Peptidyl binds tRNA carrying polypeptide chain. Aminoacyl binds tRNA carrying the next amino acid to be added. Exit binds tRNA before its released from ribosome.

Somatic Cell Division

Cell undergoes a nuclear division called mitosis.

Reproductive Cell Division

Mechanism that produces gametes (cells needed to form next generation of sexually reproducing organisms).

Cell Cycle

Orderly sequence of events in which a somatic cell duplicates and divides in two. Some cells divide more than others. One member of each pair is inherited from each parent. Two chromosomes making pairs are homologous chromosomes.

Interphase

Cell replicates its DNA and produces additional organelles and cytosolic components in anticipation of cell division.

Mitotic Phase

Consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division) which forms two identical cells.

G1 Phase

(8-10 hours) Cell metabolically active; duplicates organelles and cytosolic components; centrosome replication begins

S Phase

(8 hours) DNA replication

G2 Phase

(4-6 hours) Cell growth continues; enzymes and other proteins are synthesized; centrosome replication completed

G0 Phase

(Nondividing cell) Exits from cell cycle

Prophase

• Chromatin condenses into chromosomes

• Nuclear membrane begins to break down

• Mitotic spindle forms from centrosomes (moving to opposite sides of cell)

• Nucleolus disappears

Metaphase

• Microtubules align centromeres at the middle of mitotic spindle (metaphase plate)

Anaphase

• centromeres separate making each pair of chromatids and move to opposite sides of cell (Chromatids now called chromosomes)

Telophase

• Chromosomes uncoil and revert to chromatin form

• Nuclear envelope forms

• Nucleolus reappears

Examples of Membranous Organelles

Rough ER, smooth ER, Golgi apparatus, mitochondria, plastids, nucleus, vacuoles and lysosomes

Examples of Non-membranous Organelles

Ribosomes, cytoskeletal structures, centrioles, cilia and flagella

Examples of Surface Structures

Microvilli, cilia and flagella

Centrosome

(Near the nucleus) microtubule organizing center, contains pair of centrioles and pericentriolar matrix

Centrioles

Cylindrical structures composed of 9 clusters of microtubular triplets arranged in a circular pattern

Structure of Mitochondria

Double membrane, with central cavity. DNA within mitochondria, mitochondria matrix.

Structure of Golgi Complex

Flattened curved membranous sacs, convex side faces ER, concave side faces cell membrane

Protein Packaging and Transport

Rough ER→ Golgi Complex→ Secretory Vesicle→ Exterior

pH Level of Lysosomes

The interior pH is 5; cytosol pH is 7.

Nucleolus

Spherical, dark bodies within the nucleus

Genetic Processing

DNA replication→ DNA transcription→ RNA translation = protein

• Reverse transcription (in some viruses) is when RNA transcripts into DNA

Diffusion of Solutes

Happens through lipid bilayer, permeable to nonpolar, uncharged molecules, slight permeability to water and urea, impermeable to polar, charged molecules. Also happens through membrane channels (ion channels), small, hydrophilic inorganic molecules (K+, Na+, Cl-) as well as gated channels.

Passive Membrane Ion Channels

always open, ion specific

Voltage Gated Membrane Ion Channels

Regulated by changes in membrane potential, ion specific

Ligand Gated Membrane Ion Channels

Regulated by the binding of chemicals, ion specific

Facilitated Transport

Utilizes transporters, transport maximum depends on saturation. Concentration gradient is important for this.

Energy Source for Primary Active

ATP