Cell Biology: Structure, Function, and Processes | Summaries Biology

Cell: simplest structural and functional unit of life

Cell Shapes and Sizes

Squamous: thin,flat,scaly (sunny side up fried egg); line esophagus and

form epidermis

Cuboidal: square; liver cells

Columnar: taller, rectangular; lining of stomach and intestines

Polygonal: irregularly angular with 4+ sides

Sellate: “starlike”; nerve cells

Spheroidal to ovid: round oval; egg cells or WBC’s

Discoidal: disc-shaped; RBC’s

Fusiform: thick middle and tapered ends; smooth muscle cells

Fibrous: thread-like; skeletal muscles and nerve cell axons

~200 cells in human body with varied shapes

Cell size in micrometers, μm (10-6)

Basic Components of the Cell

Cytoplasm: contents of a cell b/w its plasma membrane and its nuclear

envelope, consisting of cytosol, organelles, inclusions and cytoskeleton

Plasma Membrane: made of proteins and lipids, surrounds cells & defines

boundaries(function and composition vary)

Cytoskeleton: supportive framework of protein filaments and tubules

Organelles: diverse structures that perform various metabolic tasks

Inclusions: foreign matter or stored cell products

Cytosol: Clear, featureless, gelatinous colloid in which organelles & other

structures of a cell are embedded

3.2 The Cell Surface

The plasma membrane defines the boundaries of the cell, governs its

interactions with other cells, and controls the passage of materials into and

out of the cell.

Hydrophilic phosphate-containing heads facing the water and hydrophobic

tails directed towards the center

Membrane Lipids

98% lipid (75% phospholipid, 20% cholesterol), 5%

glycolipid-phospholipids (contribute to the glycocalyx)

Cholesterol can stiffen the membrane in areas but higher concentrations of

cholesterol can increases membrane fluidity by preventing phospholipids

from packing closely together

Membrane Proteins

Proteins are only about 2% of molecules of the plasma membrane but

account for 50% of the membrane weight

Two classes of membrane proteins: transmembrane and peripheral

Transmembrane proteins: pass completely through the phospholipid

bilayer and contain hydrophobic and hydrophilic regions, Most are

glycoproteins bound to oligosaccharides on the extracellular side.

Peripheral proteins: do not protrude into the phospholipid layer but

adhere to either the inner or outer face of the membrane.

Functions of the membrane proteins include

1)Receptors are surface proteins that bind to molecules (chemical signals)

2) Second-messenger systems; when a messenger binds to a surface

receptor, it may trigger changes within the cell that produces a second

messenger in the cytoplasm.

3) Enzymes. Enzymes in the plasma membrane carry out the final stages

of starch and protein digestion in the small intestine, help produce second

messengers, and break down hormones.

4) Channel proteins. Channels are passages that allow water and

hydrophilic solutes to move through the membrane. Leak channels, are

always open and allow materials to pass through continually. Gated

channels open and close under different circumstances and allow solutes

through at some times but not others.

Gated channels respond to three types of stimuli 1)ligand gated channels

respond to chemical messengers 2)voltage gated channels to changes in

electrical potential (voltage) across the plasma membrane 3)

mechanically gated channels to physical stress on a cell

Carriers: transmembrane proteins that bind to glucose, electrolytes, and

other solutes, and transfer them to the other side of the membrane. Some

carriers, called pumps, consume ATP in the process.

Cell-identity markers: glycoproteins contribute to the glycocalyx acting as

an “identification tag” that enables our bodies to tell which cells belong to it

and which are foreign invaders.

Cell-adhesion molecules: Cells adhere to one another and to

extracellular material through membrane proteins called cell-adhesion

molecules.

Second Messengers

Epinephrine is the 1st messenger binds to a surface receptor, receptor

releases G protein, G protein binds to adenylate cyclase in the plasma

membrane, adenylate cyclase converts ATP to cyclicAMP (second

messenger, cAMP activates cytoplasmic enzyme kinase, kinases at

phosphate groups to other cytoplasmic enzymes.

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Cell: simplest structural and functional unit of life Cell Shapes and Sizes Squamous: thin,flat,scaly (sunny side up fried egg); line esophagus and form epidermis Cuboidal: square; liver cells Columnar: taller, rectangular; lining of stomach and intestines Polygonal: irregularly angular with 4+ sides Sellate: “starlike”; nerve cells Spheroidal to ovid: round oval; egg cells or WBC’s Discoidal: disc-shaped; RBC’s Fusiform: thick middle and tapered ends; smooth muscle cells Fibrous: thread-like; skeletal muscles and nerve cell axons ~200 cells in human body with varied shapes Cell size in micrometers, μm (10-6) Basic Components of the Cell Cytoplasm: contents of a cell b/w its plasma membrane and its nuclear envelope, consisting of cytosol, organelles, inclusions and cytoskeleton Plasma Membrane: made of proteins and lipids, surrounds cells & defines boundaries(function and composition vary) Cytoskeleton: supportive framework of protein filaments and tubules Organelles: diverse structures that perform various metabolic tasks Inclusions: foreign matter or stored cell products Cytosol: Clear, featureless, gelatinous colloid in which organelles & other structures of a cell are embedded 3.2 The Cell Surface The plasma membrane defines the boundaries of the cell, governs its interactions with other cells, and controls the passage of materials into and out of the cell. Hydrophilic phosphate-containing heads facing the water and hydrophobic tails directed towards the center Membrane Lipids 98% lipid (75% phospholipid, 20% cholesterol), 5% glycolipid-phospholipids (contribute to the glycocalyx) Cholesterol can stiffen the membrane in areas but higher concentrations of cholesterol can increases membrane fluidity by preventing phospholipids from packing closely together Membrane Proteins Proteins are only about 2% of molecules of the plasma membrane but account for 50% of the membrane weight Two classes of membrane proteins: transmembrane and peripheral Transmembrane proteins: pass completely through the phospholipid bilayer and contain hydrophobic and hydrophilic regions, Most are glycoproteins bound to oligosaccharides on the extracellular side. Peripheral proteins: do not protrude into the phospholipid layer but adhere to either the inner or outer face of the membrane. Functions of the membrane proteins include 1)Receptors are surface proteins that bind to molecules (chemical signals)

Second-messenger systems; when a messenger binds to a surface receptor, it may trigger changes within the cell that produces a second messenger in the cytoplasm.
Enzymes. Enzymes in the plasma membrane carry out the final stages of starch and protein digestion in the small intestine, help produce second messengers, and break down hormones.
Channel proteins. Channels are passages that allow water and hydrophilic solutes to move through the membrane. Leak channels , are always open and allow materials to pass through continually. Gated channels open and close under different circumstances and allow solutes through at some times but not others. Gated channels respond to three types of stimuli 1)ligand gated channels respond to chemical messengers 2) voltage gated channels to changes in electrical potential (voltage) across the plasma membrane 3) mechanically gated channels to physical stress on a cell Carriers: transmembrane proteins that bind to glucose, electrolytes, and other solutes, and transfer them to the other side of the membrane. Some carriers, called pumps, consume ATP in the process. Cell-identity markers: glycoproteins contribute to the glycocalyx acting as an “identification tag” that enables our bodies to tell which cells belong to it and which are foreign invaders. Cell-adhesion molecules : Cells adhere to one another and to extracellular material through membrane proteins called cell-adhesion molecules. Second Messengers Epinephrine is the 1st messenger binds to a surface receptor, receptor releases G protein, G protein binds to adenylate cyclase in the plasma membrane, adenylate cyclase converts ATP to cyclicAMP (second messenger, cAMP activates cytoplasmic enzyme kinase, kinases at phosphate groups to other cytoplasmic enzymes.

The Glycocalyx External to the plasma membrane all animal cells have a coat called the glycocalyx, composed of the carbohydrate moieties of membrane glycolipids and glycoproteins. It is chemically unique to everyone. The glycocalyx provides 1)Protection; cushions the plasma membrane and protects it from physical/chemical injury

Immunity-Infection; enables the immune system to recognize and attack 3)Defense against cancer; enable the immune system to recognize and attack cancer cells 4)Transplant compatibility; forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants.
Cell adhesion; binds cells together so that tissues don’t fall apart Extensions of the Cell Surface Many cells have surface extensions called microvilli, cilia, flagella, and pseudopods. Microvilli : extensions of the plasma membrane that serve primarily to increase a cell’s surface area. They are best developed in cells specialized for absorption&sectretion, such as the epithelial cells of the intestines and kidneys. Cilia : hair-like processes with various functions. Some serve as sensory, some monitor the flow of fluid, some open calcium gates. Motile cilia are less widespread, occurring in the respiratory tract, uterine, internal cavities of the brain. They beat in waves that sweep across the surface of an epithelium, always in the same direction propelling (power strokes) such materials as mucus, and egg cell, or cerebrospinal fluid. Flagellum: whip-like tail beats in a more snakelike/corkscrew fashion. (sperm in humans) Pseudopods: A temporary cytoplasmic extension of a cell used for locomotion and phagocytosis (WBC neutrophils crawl by means of fingerlike pseudopods). Cellular membranes are important in that they control the passage of materials into and out of the organelles and the cell as a whole. It is selectively permeable: allowing some things through and preventing others. Passive mechanisms require no energy (ATP) expenditure by the cell. The random molecular motion of the particles themselves provides the necessary energy. Passive mechanisms include filtration, diffusion, and osmosis. Active Mechanisms consume ATP, these include active transport and vesicular transport. Carrier-mediated mechanisms use a membrane protein to transport substances from one side of the membrane to the other. Filtration (example of passive transport) Filtration is a process in which a physical pressure forces fluid through a selectively permeable membrane. (like water moving through a filter leaving behind the coffee grounds) The most important case of filtration in physiology is seen in the blood capillaries, where blood pressure forces fluid through gaps in the capillary wall. This is how water, salts, nutrients, and other solutes are transferred from the bloodstream to the tissue fluid and how the kidneys filter wastes from the blood. Hydrostatic pressure Capillaries hold back larger particles such as blood cells and protein. In most cases water and solutes filter through narrow gaps b/w the capillary cells.

The material in the nucleus is called nucleoplasm this includes chromatin - fine threadlike matter composed of DNA and protein and one+ dark staining masses called nucleoli where ribosomes are produced. The Endoplasmic Reticulum (ER) is a system of interconnected channels called cisterns. In areas called rough endoplasmic reticulum , the cisterns are parallel flattened sacs covered with granules called ribosomes. In areas called smooth endoplasmic reticulum , the cisterns are more tubular, branch more extensively, and lack ribosomes. The cisterns of the smooth ER are continuous with those of the rough, so the two are different parts of the same network. The ER synthesizes steroids and other lipids, detoxifies alcohol and other drugs, and manufactures nearly all membranes of the cell. The rough ER produces phospholipids and proteins of the plasma membrane and synthesizes proteins that are either secreted from the cell or packaged in organelles such as lysosomes. Smooth ER is abundant in cells that engage extensively in detoxification (liver&kidney), abundant in hormone producing organs testes & ovaries, and stores calcium for muscle contraction in skeletal and cardiac muscle. Ribosomes Ribosomes: A granule found free in the cytoplasm or attached to the rough endoplasmic reticulum or nuclear envelope composed of ribosomal RNA and enzymes; specialized to read the nucleotide sequence of messenger RNA and assemble a corresponding sequence of amino acids to make protein. Ribosomes in the cytoplasm make enzymes and proteins for use in those organelles. Ribosomes attach to the rough ER when they make proteins destined to me packaged in the lysosome or secreted. Golgi Complex Golgi Complex is a small system of cisterns that synthesize carbohydrates and put the finishing touches on protein and glycoprotein synthesis. The Golgi receives newly synthesized proteins from the rough ER. It sorts them, cuts and splices some, and adds carbohydrate moieties to some. Finished cells product breaks up into membrane bound Golgi vesicles. Some Golgi vesicles become lysosomes, some migrate to the plasma membrane and fuse with it, and some become secretory vesicles that store cell product for later release (breast milk, digestive enzymes). Lysosomes Lysosome: A membrane bound organelle containing a mixture of enzymes with a variety of intracellular and extracellular roles in digesting foreign matter, pathogens, and expired organelles. Lysosomes hydrolyze proteins, nucleic acids, complex carbohydrates, phospholipids, and others. WBC’s use lysosomes to digest phagocytized bacteria. Lysosomes also digest and dispose of surplus or nonvital organelles and other cell components in order to recycle their nutrients to more important cell needs, autophagy. Also assists in “cell suicide”. This shrinkage is due to autolysis, the digestion of surplus cells by their own lysosomal enzymes. Peroxisomes Resemble lysosomes but contain different enzymes. Their general function is to use molecular oxygen O 2 to oxidize organic molecules. These reactions produce hydrogen peroxide H 2 O 2 is then used to oxidize other molecules, and excess is broken down to water and oxygen by an enzyme called catalase. THey neutralize free radicals and detoxify. They also decompose fatty acids into two carbon fragments that the mitochondria use as an energy source for ATP synthesis. Abundant in liver and kidney cells. Proteaosomes Main function is protein disposal. Proteasomes are hollow, cylindrical complexes of proteins located in both the cytoplasm and the nucleus. A cell tags undesirable proteins for destruction and transports them to a proteasome. As the unwanted protein passes through this organelle, the proteasome’s enzymes unfold it and break it down into short peptides and free amino acids. Proteosomes degrade more than 80% of a cell's proteins. Mitochondria Mitochondria are organelles specialized for synthesizing ATP. The mitochondrion is surrounded by a double membrane. The inner membrane usually has folds called cristae projecting like shelves across the organelle. The space between the cristae, called the matrix, contains ribosomes, enzymes used in ATP synthesis, and many small circular DNA molecules called mitochondrial DNA. Mitochondria are the powerhouses of the cell. (Energy is not made here but is extracted from organic compounds and transferred to ATP, primarily by enzymes located on the cristae) Centrioles Centriole: an organelle composed of a short cylinder of nine triplets of microtubules, usually paired with another centriole perpendicular to it; origin of the mitotic spindle; identical to the basal body of a cilium or flagellum. Near the nucleus most cells have a small clear patch of cytoplasm called the centrosome containing a pair of mutually

perpendicular centrioles. Vesicular Transport Vesicular transport: the movement of particles or fluid droplets through the plasma membrane by the process of endocytosis or exocytosis. The three forms of endocytosis (bringing into the cell) are phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis: “cell eating” is the process of engulfing particles such as bacteria, dust, and cellular debris. Ex. when a neutrophil encounters a bacterium, it surrounds it with pseudopods and traps it in a vesicle called a phagosome. Pinocytosis: the process by which a cell can take in large droplets of ECF (water) Receptor-mediated endocytosis: is a more selective form of either phagocytosis or pinocytosis. It enables a cell to take in more specific molecules from the ECF with a minimum of unnecessary matter. Receptors bind to a ligand then cluster into pits that pinch off into vesicles. Exocytosis: vesicular transport that releases material from the inside of the cell to the outside of the cell The function of the nucleolus is ribosome production The site of ribosome production in a cell is the Nucleolus Basal bodies and the axonemes of flagella and cilia originate from nonmembranous Centrioles

Cell Biology: Structure, Function, and Processes, Summaries of Biology

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