MCAT Biology - The Cell

History of Cell Theory - before 1600

History of Cell Theory - before 1600

organisms were assumed to be complete and inseparable

Robert Hooke (1665)

used a compound microscope on cork; called the spaces he saw “cells” as in rooms in a monastery

Anton van Leeuwenhoek (1674)

looked at living cells under microscope

History of Cell Theory - past 1700

Later researchers could separate cells, concluded tissues were made of cells, and function of tissue is dependent on cells

Richard Virchow (1850)

observed diseased cells come from normal cells

First Tenet of Cell Theory

All living things are composed of cells

Second Tenet of Cell Theory

The cell is the basic functional unit of life.

Third Tenet of Cell Theory

Cells arise only from pre-existing cells.

Fourth Tenet of Cell Theory (later addition)

Cells carry genetic information in the form of DNA which is passed from mother to daughter cell.

Are viruses living cells?

Dilemma; violate third and fourth

Eukaryote

Organism made of cell(s) that contain a true nucleus enclosed in a membrane and other membrane-bound organelles; larger ribosomes - 40S&60S

Cell/Plasma membrane

semi-fluid semipermeable barrier around a cell; regulates what enters and leaves the cell; surrounds cytoplasm; consists of a phospholipid bilayer with embedded lipids and proteins

Cytosol

The soluble portion of the cytoplasm, which includes molecules and small particles, such as ribosomes, but not the organelles covered with membranes; allows for diffusion of molecules through the cells

Mitosis

Eukaryotic cell division in which the parent cell divides into two identical daughter cells containing the same number of chromosomes

Organelle

specialised structure within a cell; allows compartmentalisation of functions

Nucleus

contains genetic material in the form of DNA

Gene

A segment of DNA on a chromosome that codes for a specific trait

Histone

protein molecule around which DNA is tightly coiled in chromatin

Chromatin

Substance found in eukaryotic chromosomes that consists of DNA tightly coiled around histones

Chromosomes

threadlike structures made of chromatin; only in this form when dividing

Nuclear membrane/envelope

double membrane that surrounds the nucleus

nuclear pore

a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm

nucleolus

subsection of nucleus where ribosomal RNA (rRNA) is synthesised; appears darker in images

Mitochondrion

“powerhouse of the cell”, produces ATP, a store of energy for the cell

Outer mitochondrial membrane

barrier btwn cytosol and inner environment; maintains proper concentration gradients

Inner mitochondrial membrane

contains molecules and enzymes of the electron transport chain (ETC)

Cristae

foldings/invaginations called cristae maximize surface area for ETC

Mitochondrial matrix

space inside of the inner membrane of the mitochondrion

Intermembrane space

the region between the inner membrane and the outer membrane of a mitochondrion

Mitochondrial replication

semi-autonomous, containing their own genes and replicating independently via binary fission

cytoplasmic/extranuclear inheritance

transmission of genetic material independent of the nucleus

endosymbiotic theory

mitochondria (& chloroplasts) originated from symbiotic bacteria that were engulfed by ancestral eukaryotic cells

apoptosis

programmed cell death

Mitochondrial apoptosis

induce cell death by releasing enzymes from the electron transport chain

Lysosome

Membrane-bound structure containing hydrolytic enzymes capable of breaking down many substrates ingested by endocytosis or cellular waste products

Endosomes

transport, package, and sort cell material traveling to/from the cell membrane, trans-golgi, or lysosomal pathway

Lysosomal apoptosis

autolyse and release enzymes into cytoplasm

Endoplasmic Reticulum (ER)

series of interconnected double membranes contiguous w/ nuclear envelope, folded into numerous invaginations, with a central lumen; two types: smooth & rough

Rough ER (RER)

studded with ribosomes for translation of proteins destined for secretion into the lumen

Smooth ER (SER)

lacking ribosomes, involved in lipid synthesis (phospholipids), detoxification of drugs/poisons, and transportation of proteins from RER to Golgi apparatus

Golgi apparatus

Stacked membrane-bound sacs that transport, label, and send materials around and out of the cell

vesicles

small membrane sacs that specialize in moving products into, out of, and within a cell

Gologi apparatus modificationt

Add carbohydrate, phosphate, sulfate groups to cellular products

Signal sequences

added by Golgi apparatus for direct delivery of product to a particular cellular location

secretion

secondary vesicles merges with the cell membrane through exocytosis, releasing products out of the cell

Peroxisome

Contains hydrogen peroxide; breaks down of long chain fatty acids via beta-oxidation; participates in synthesis of phospholipids; contains enzymes involved in pentose phosphate pathway

Cytoskeleton

Provides structure to cell, maintains shape, acts as a conduit for transport, made of microfilaments

microfilaments

Solid polymerized rods of actin, organized into bundles and networks, resistant to compression and fracture

cytokinesis

division of materials btwn daughter cells; microfilaments form cleavage furrow, organize as a ring at division site, contract to pinch off connection between daughter cells

microtubules

Hollow polymers of tubulin proteins that radiate through the cell, serve as primary pathways for movement by kinesin and dynein, form cilia and flagella

cilia

projections from cell involved in movement of material along surface

flagella

long, whip-like structures used for propulsion toward food or away from toxins/immune cells

structure of cilia/flagella (eukaryote)

Nine pairs of microtubules with 2 in the center, known as the 9 + 2 structure, found only in eukaryotes

centrioles

Organizing centers for microtubules, found in the centrosome, migrate to opposite poles during mitosis to organize the mitotic spindle; nine triplets of tubules with hollow centre

intermediate filaments

involved in cell-cell adhesion, maintenance of cytoskeleton, and anchoring organelles

ex. keratin, desmin, vimentin, lamins

Tissue

A group of similar cells that perform the same function

Types of tissues

epithelial, connective, muscle, nervous

epithelial tissue

covers the body and lines organs; provides protection against pathogen invasion and desiccation, absorption, secretion, sensation; Tightly joined as one cohesive unit to underlying connective tissue via the basement membrane

parenchyma

re the functional parts of the organ; often epithelial tissue

polarised epithetial cells

One side faces a lumen, while the other interacts with underlying blood vessels and structural cells

lumen

hollow space

simple epithelial tissue

one layer of cells

stratified epithelial tissue

multiple layers of cells

pseudostratified epithelial tissue

single layer, but appear as multiple, due to varying size of cells

cuboidal epithelial tissue

cells are cube shaped

columnar epithelial tissue

cells are long and thin

squamous epithelial tissue

cells are flat and scale-like

Connective tissue

support body and provides framework; produce and secrete collagen and elastin to form extracellular matrix

ex. bone, cartilage, tendons, ligaments, adipose, blood

stroma

support structure of individual organs

Prokaryote

An organism with cells that lack a nucleus and membrane bound organelles

Phylogenetic Domains

Three categories of known life: Archaea, Bacteria (eubacteria), and Eukarya (eukaryotes)

Archaea

Domain of unicellular prokaryotes that have cell walls that do not contain peptidoglycan; often extremophiles with ability to use alternative sources of energy; resistant to many antibiotics

Ways archaea are similar to eukaryotes

start translation with methionine, contain similar RNA polymerases, associate DNA w/ histones

Ways archaea are similar to bacteria

single circular chromosome, divide via binary fission or budding, similar bacterial structure

Bacteria

Domain of unicellular prokaryotes that have cell walls containing peptidoglycan; contain cell membrane and cytoplasm; some have flagella/fimbriae; primitive cytoskeleton; smaller ribosomes - 30S&50S

Mutualistic symbiotes

both organisms benefit

ex. vit. K and B7 (biotin) is created by bacteria in the human body

Pathogens

disease causing agents, inc. bacteria and viruses

parasites

An organism that lives in or on another organism, deriving nourishment at the expense of its host, usually without killing it

intracellular pathogen

A pathogen that lives within a host cell.

ex. Chlamydia trachomatis

extracellular pathogen

pathogens that don't invade a cell; live in environment around cells and travel via the bloodstream and lymphatic system

ex. Clostridium tetani

cocci

spherical bacteria

ex. Streptococcus pyogenes

bacilli

rod-shaped bacteria

ex. Escherichia coli

spirilli

spiral-shaped bacteria

ex. Treponema pallidum, Borrelia burgdorferi, Leptospira interrogans

obligate aerobes

require oxygen for metabolism

anaerobes

does not require oxygen, metabolises via fermentation

obligate anaerobe

presence of oxygen produces reactive radicals leading to cell death

facultative anaerobe

can toggle btwn aerobic and anaerobic metabolism

aerotolerant anaerobe

can’t use oxygen, but aren’t harmed

nucleoid region

where singular circular ring of DNA is stored

bacterial colonies

individual bacteria living in a group to help each other; often covered in a substance to unify them; improve their chance of survival by being in this group

Cell Wall

outermost barrier that protects the cell/plasma membrane, provides structure, and controls the movement of solutes into and out of the bacterium; layer of peptidoglycan

peptidoglycan

a polymer made of amino acids and sugars; may protect against the host's immune system

Gram staining

crystal violet stain, followed by counterstain with safranin; thicker layer of peptidoglycan; contains lipoteichoic acid that may activate the human immune system.

Gram-positive bacteria

envelope absorbs the crystal violet stain, appearing purple in color; thinner layer of peptidoglycan; separated from membrane by periplasmic space; lipopolysaccharides trigger immune response in humans which lead to a stronger inflammatory response than lipoteichoic acid

Gram-negative bacteria

envelope absorbs safranin counterstain, appearing pink- red

Bacterial flagella

may have one, two, many; composed of filament, basal body, hook

chemotaxis

movement by a cell or organism in reaction to a chemical stimulus

Penicillin

targets the enzyme that crosslinks peptidoglycan in the cell wall, leading to cell death; Streptococcus pyogenes and Treponema pallidum are still effectively targeted by penicillin

filament (bacterial flagella)

hollow helical structure composed of flagellin

basal body (bacterial flagella)

complex structure anchors flagellum to cytoplasmic membrane; motor that rotates at rates to 300 Hz; basal body rotation provides torque on filament: spins, causing forward propulsion