Animal Cells

Human Anatomy

Animal Cells vs Other Eukaryotic Cells

• Plant Cells- contain rigid, mostly impermeable cell wall composed of cellulose, lignin, and pectin (polysaccharides)

• Fungi- contain rigid cell walls composed of chitin (N-acetylglucosamine, a glucose derivative)

• Bacteria- rigid peptidoglycan cell wall (complex sugar and amino acid mixture).

• Animal cells- fluid, cell membrane composed of a phospholipid bilayer, selectively permeable.

Animal cell membrane

Animal cell membrane

• The cell membrane: protect cells, allow cell to cell interaction, regulate flow (in and out) of material, provide large area for chemical reactions necessary for cellular survival (ex. uptake).

• Hydrophilic (water loving) heads face inward and outward, and is the attachment point for the negatively charged phosphate molecule. Its charge allows the head to be chemically attracted to water molecules.

• Hydrophobic (afraid of water) tails make up the inner part of the bilayer. These tails are composed of long chain hydrocarbons known as fatty acids. Cholesterol molecules are imbedded within the interior of the membrane. Their job is to stabilize the membrane, and deter entrance of water-soluble molecules into the cell.

Phospholipid Bilayer

Membrane proteins

• Protein-an organic macromolecule composed of amino acids

• Intrinsic- proteins embedded within cell membrane

• Used to allow essential ions to travel into/out of the cell (potassium, sodium, calcium), attachment to other cells, and communication with other cells.

• Glycocalyx- “cell coat”, outer layer composed of sugar molecules that aid in cell to cell interaction.

Selective Permeability

• Sounds complicated, Real simple• Intrinsic proteins allow for the entrance of certain

substances into the cell, but deter the entrance of others.

• Example: Ion channels in the membrane are activated or opened by the binding of a specific ion (known as a ligand). These ions can be Na+, K+, Ca2+, Cl- or ATP.

• These ions (ligands) bind the intrinsic membrane proteins causing them to open, allowing their entrance into the cell.

Homeostasis

• The maintenance of a constant internal cellular environment.

• The cell must constantly move substances into and out of itself to maintain homeostasis.

• It accomplishes homeostasis by diffusion and active transport.

Diffusion

• Simple diffusion- molecules move from areas of high concentration to low concentration without the use of ATP (energy). Substances diffuse until the cell reaches equilibrium (ex. oxygen or water).

• Osmosis- the simple diffusion of water down a gradient (from high to low).

• Facilitated diffusion- used to transport polar molecules into the cell. These molecules enter through protein channels (aka carrier proteins) in the membrane; movement is from high to low concentration.

Tonicity

• This refers to the concentration of Na+ in the cell relative to its environment.

• Isotonic-equal concentration inside and out.• Hypertonic- higher Na+ concentration outside

cell than inside: the result is cell shrinkage.• Hypotonic- lower Na+ concentration outside than

in. Not Good! The cell will burst and die.

Active Transport

• To combat hypotonic environments, the cell always attempts to maintain low intracellular levels of Na+.

• The cellular Na+/K+ pump uses ATP to pump sodium against the gradient.

• Using energy to move something from an already low concentration to a higher concentration is active transport.

• This means of movement is very energy expensive. For example, the Na+/K+ pump accounts for over 1/3 of the cell’s total ATP expenditure.

Endocytosis

• The bulk movement of material across the membrane into the cell.

• Types:• Pinocytosis- nonspecific uptake of extracellular fluid due

to the indentation of the membrane. A vesicle is formed from this indentation, and its contents are transported to cytoplasm.

• Phagocytosis- “cell eating”, nonspecific uptake of solid material. The material is then digested by enzymes from the lysosome. This is how macrophages kill unwanted intruders like viruses and harmful bacteria.

• Receptor mediated- specific receptors (intrinsic proteins) will recognize an extracellular molecule and binds it. The membrane indents, forming a vesicle.

Exocytosis

• Material being secreted from the cell into the extracellular environment.

• Golgi apparatus packages contents into vesicles (contents may be waste products or substances like histamine from mast cells). The contents are released into the extracellular environment, the vesicle fuses and becomes part of the membrane (this is how membrane replenishes); nothing is wasted.

The cytoplasm

• 2 parts: 1. cytomembrane and 2. cytosol• Cytomembrane- well defined, consists of smooth

and rough ER, golgi apparatus/bodies, vacuoles, and vesicles.

• Cytosol- aqueous layer in which the cytomembrane structures are suspended. Acts as a solvent in which molecules like simple sugars (solutes) are dissolved. Helps to maintain normal intracellular pressure.

Endoplasmic Reticulum

• Membrane bound structure, maze-like, found in the cytoplasm (cytomembrane). Allows for circulation of molecules throughout the cytoplasm. Also stores enzymes and non-enzymatic proteins.

• Biology dogma- all enzymes are proteins, but all proteins are not enzymes.

• Enzyme- acts as a catalyst to make biochemical reaction more efficient. It does this by lowering the activation energy. Ex. Na+/K+ ATPase makes the active transport of these ions more energy efficient.

• Some enzymes can speed reactions up by as much as 1000% or more.

Endoplasmic Reticulum

• Rough ER Functions: Ribosomal attachment, site for protein synthesis, serves in transport of proteins throughout the cell (cell’s highway system).

• Smooth ER function: Site for lipid synthesis, metabolizes carbohydrates and steroids, helps regulate intracellular calcium levels.

Ribosomes• Found in rough AND smooth ER,

ribosomal numbers in the smooth ER are much lower than in rough ER. Some are found floating in the cytosol.

• Ribosomes are sites for protein synthesis (translation) and contain ribosomal ribonucleic acid (rRNA).

• Polysomes- clusters of ribosomes held together by strands of messenger RNA.

• In humans, our type of ribosomes are 80S, and consist of 40S and 60S subunits.

Golgi Complex

• Found in the cytoplasm (cytomembrane), it is a collection of membranes whose function is to sort, package, and secrete proteins and lipids.

• Example: Ribosomes synthesize protein which are transferred to the Golgi. The vesicles fuse and enter the Golgi. The contents are modified in the Golgi and packaged into secretory vesicle. These vesicles travel to the cell membrane where they fuse and release contents. The Golgi is also the production site for lysosomes.

• Golgi complexes are found in abundance cells that secrete substances into the blood stream. Ex. Pancreatic (insulin), nerve cells (neurotransmitters), mast cells (histamine).

Lysosomes

• Membrane bound organelles that serve to digest organic compounds (ex. proteins) in the cell.

• They accomplish this task by binding to vesicles that have engulfed material from outside the cell (endocytosis). Then, secrete an enzyme called hydrolase. This enzyme is very powerful and quickly degrades whatever it contacts.

• Sometimes the lysosome may rupture, spilling hydrolase into the cell. Hydrolase will then begin digesting the cell from the inside out.

Endosymbiotic Theory

• Billions of years ago, a eukaryotic cell ancestor engulfed a smaller organism that could conduct aerobic respiration. The smaller cell was incorporated into the eukaryotic cell, along with its DNA, and a mutualistic relationship formed.

• The smaller organism is the mitochondria we find today in animal cells.

• The mitochondria is protected inside of eukaryotic animal cells. In return, it allows animal cells to undergo respiration (using O2 and other components to produce ATP).

Endosymbiotic Theory

• Symbiosis- close relationship between two organisms.

• Parasitism- one organism benefits, the other is harmed

• Mutualism- both organism are benefited

• Commensalism- one organism derives a benefit, the other organism is neither benefited or harmed.

Endosymbiotic Theory

• Evidence:• Mitochondria cell membrane contains

peptidoglycan, which is found in bacteria cells.• Mitochondrial DNA is circular (like bacteria)

instead of a double helix, which differs from the cell nucleus.

• Mitochondrial ribosomes are 70S type, which is also consistent with those found in bacteria. All other animal cell ribosomes are mostly 80S.

Mitochondria

• Functions:• To produce ATP by aerobic (oxygen involved)

respiration. Glucose, pyruvate, and NADH (nicotinamide) are oxidized and converted to ATP. Aerobic respiration can produce as many as 30 moles of ATP for every 1 mol of glucose consumed. In comparison, anaerobic (oxygen deprived) produces only 2 moles to 1 mol of glucose.

• Calcium storage • Sends signal to begin Apoptosis

Apoptosis

• Programmed cell death regulated by the mitochondria.• When a cell has outlived its usefulness, it will commit

suicide in the form of apoptosis.• Apoptotic cells are tagged with a protein called ubiquitin

(most abundant protein in body), which serves as a label indicating the cell must be destroyed and recycled.

• After a series of complex biochemical processes, the cell is sent to the proteosome; which is basically a garbage disposal. The cell is degraded and its remains recycled and distributed to other cells.

Nucleus

• Control center of the cell: Site of Replication and Transcription, also storage center for chromosomes.

• Nuclear pore complex- the nuclear membrane/envelope is porous. What goes in and out is controlled only by pore size. Pore sizes are setup in such a way as to prevent DNA departure, but permit the exit of RNA (goes to ER to be translated into protein).