Barron's AP Biology - The Cell

Prokaryote

- Cells with no nuclear or internal membranes; i.e, lysosomes, vacuoles, and mitochondria
- Evolved 3.6 billion years ago
- Prokaryotes are classified in two domains: Archaea and Bacteria
- Contain small ribosomes
- Cells are small
- Contain naked, circular DNA

Eukaryotes

- Cells with internal membranes
- Evolved a billion years ago
- According to the theory of endosymbiosis, chloroplasts and mitochondria were formerly tiny prokaryotes that took up residence inside larger cells and formed permanent symbiotic relationship
- DNA is wrapped with histone proteins into chromosomes
- Metabolism is aerobic
- Cells are larger than prokaryotes

Form and Function Go Together

- All cells do not look alike. Their function dictates their form and vice versa

Why Cells Are So Small

- The surface area of the plasma membrane, which controls what enters and leaves a cell, limits the overall size of the cell. This is because while the surface area of a sphere increases at one rate, the volume increases at a much faster rate

Cell Organelles

Typical animal cell:
A. Endocytotic vacuole
B. Nucleolus
C. Smooth endoplasmic reticulum
D. Cell membrane
E. Cytoplasm (cytosol)
F. Rough endoplasmic reticulum
G. Ribosomes
H. Vacuole
I. Nuclear membrane
J. Mitochondria

Typical Plant Cell

Nucleolus

- Prominent region seem in the nucleus during interphase
- Where ribosome components are synthesized and assembled

Nucleus

- Contains chromosomes (DNA) wrapped into a chromatin network
- Surrounded by selectively permeable membrane that contains nuclear pores for the passage of large molecules like mRNA

Ribosomes

- Site of protein synthesis
- Found free in the cytoplasm or attached to the endoplasmic reticulum

Golgi Apparatus

- Packages and secretes substances produced in the ER
- Lies near nucleus; consists of flattened membranous sacs

Endoplasmic Reticulum (ER)

- Membranous system of channels and flattened sacs that traverses the cytoplasm
- Rough ER
- Site of protein synthesis
- Smooth ER
- Site of protein synthesis
- Connects rough ER to Golgi apparatus
- Carries out various detoxification processes

Lysosomes

- Sacs of hydrolytic enzymes surrounded by a single membrane
- Principal site of intracellular digestion of macromolecules
- Carry out programmed destruction of cells, apoptosis, using their hydrolytic enzyme
- Found in large numbers in phagocytic white blood cells
- Absent from plant cells

Peroxisomes

- Contain enzyme that converts hydrogen peroxide to harmless water
- In liver cells, detoxify alcohol

Mitochondria

- Site of aerobic respiration, the process that generates ATP
- Internal membranes are called cristae membranes
- Enclosed in a double membrane because in ancient times these were tiny free-living cells that took up residence inside larger organisms. This is the theory of endosymbiosis

Food Vacuoles

- Formed by phagocytosis
- Surrounded by a single membrane

Central Vacuoles

- Found in mature plant cells enclosed in a specialized membrane called a tonoplast

Contractile Vacuoles

- Found in freshwater protista like amoeba and paramecia
- Pump out excess water that diffuses inward because organisms live in a hypotonic environment

Chloroplasts

- Present in all living plant cells
- Site of photosynthesis
- Contains the green photosynthetic pigment chlorophyll that absorbs light energy for photosynthesis
- According to the Theory of Endosymbiosis, chloroplasts were once free-living prokaryotes

Cilia / Flagella

- Appendages that protrude from eukaryotic cells for locomotion
- Consist of special arrangement of microtubules: 9 pairs of microtubules + 2 singles (9 + 2)
- Cilia are short; flagella are long
- In paramecia, euglena, sperm, and human respiratory system

Cytoskeleton

- Complex network of protein filaments that extends through the cytoplasm and gives cell its shape and ability to move
- Ex: microtubules, microfilaments

Cell Wall

- Found in plant cells; not in animal cells
- In plants algae, it consists of cellulose
- In fungi, it consists of chitin
- Primary cell wall: immediately outside plasma membrane
- Secondary cell wall: where found, located outside primary wall

Plasma Membrane

- Selectively-permeable - fluid mosaic model
- Consists of a lipid bilayer with proteins dispersed throughout
- In vertebrates, cholesterol molecules are embedded in the interior of membrane for stability
- External surface has glycoproteins that functions in cell-to-cell communications
- Contains protein channels, pumps, and enzymes

Transport

- Movement of a substance into or out of a cell
- Can be active or passive
- Passive: requires no expenditure of energy
- Active: requires energy

Passive Transport

- Movement of molecules down a gradient from a region of high concentration to a region of low concentration; no energy (ATP) required
- Simple diffusion: no membrane required
- Facilitated diffusion: molecules diffuses through membrane channels
- Osmosis: type of diffusion where water diffuses across a membrane
- Countercurrent exchange: special case of simple diffusion - flow of adjacent fluids in opposite direction to maximize rate of diffusion

Water Potential

- Symbol is Greek letter psi = Ѱ
- Water moves across a membrane from a solution with higher water potential to a solution with lower water potential
- Water potential of pure water = 0
- Addition of solute to water lowers Ѱ
- Water flows from hypotonic solutions to hypertonic solutions

What Happens to a Cell in a Hypertonic Solution?

- Cell shrinks because water flows from higher water potential to lower water potential
- Hypertonic means higher concentration of solute
- Cell shrinking is called plasmolysis

What Happens to a Cell in a Hypotonic Solution?

- Animal cells swell or burst because water moves from higher water potential to lower water potential
- Hypotonic means lower concentration of solute
- Plant cells do not burst; they swell and become turgid

What Happens to a Cell in an Isotonic Solution?

- Nothing happens to the cell because the concentrations of the solutions outside and inside the cell are equal

Aquaporins

- Special water channel proteins in certain cells that facilitate the rapid diffusion of massive amounts of water across a cell membrane
- They do not alter the water potential gradient; they only speed the rate of diffusion

Active Transport

- Movement of molecules against a gradient from a region of low concentration to high concentration
- Requires energy (ATP)

Sodium-Potassium Pump

- Pumps Na+ and K+ ions across axon membranes against a gradient
- Returns the neuron to its resting state - polarized
- While the pump is working, no impulse can pass along the axon - an interval called the refractory period

Pinocytosis

- Cell drinking
- The uptake of large, dissolved particles

Phagocytosis

- Engulfing of large particles or small cells by pseudopods

Ion-channel receptors

- Allosteric receptors that opens and shuts a gate in a membrane allowing an influx of ions

Cytoplasmic Receptors

- Reception: small, non-polar ligands diffuse directly through the plasma membrane and into the cytoplasm where they bind with an intracellular receptor
- Transduction: Once activated, the receptor converts a molecular signal to a cellular response
- Response: Can be a single step of a more complex signal transduction pathway

Signal Transduction Pathway

- A multistep process in which a small number of extracellular signaling molecules produce a major cellular response via a cascade effect
- The similarity in these pathways in bacteria, plants, and animals suggests that the pathways evolved hundreds of millions of years ago in a common ancestor
- Advantage of a multistep pathway: it provides many opportunities for coordination and regulation

Gap Junctions

- Another example of cell-to-cell communication
- Provide cytoplasmic channels between adjacent cells in animals
- Large molecules can pass from cell to cell
- In the heart, they enable cells to coordinate contractions of muscle tissue
- Plasmodesmata - channels between cells in plants

Apoptosis

- Programmed cell death, brought about by signals that activate a series of suicide proteins
- Essential to the development of the nervous system in embryos and to normal operation of the immune system
- Different pathways involving about 15 different caspases carry out apoptosis
- Triggering signals can come from the nucleus of a cell when the DNA is irreparably damaged or from the endoplasmic reticulum when excessive protein misfolding occurs
- Signals can originate outside the cell and bind with all cell-surface receptor that triggers a signal transduction pathway to initiate apoptosis
- Mitochondria can initiate and apoptotic pathway by leaking relay protein into the cytoplasm
- An important example of cell communication