Cell Structure and Function

• All organisms consist of cells.
• Cells are often recognized as the simplest collections of matter that can live.
• All cells are related through descent from earlier cells.
• Cellular structure is correlated with cellular function

Types of Cells

• All organisms are comprised of prokaryotic or eukaryotic cells.
• Basic cell features are a plasma membrane, semifluid substance called cytosol, chromosomes, and ribosomes.
• Prokaryotic cells lack a nucleus, and have DNA in an unbound region called the nucleoid.
• Prokaryotic cells lack membrane-bound organelles, and have have cytoplasm bound by the plasma membrane
• Eukaryotic cells have DNA in a nucleus bounded by a membranous nuclear envelope.
• Eukaryotic cells have many membrane-bound organelles.
• The cytoplasm in eukaryotic cells is the region between the plasma membrane and nucleus.
• Eukaryotic cells are generally much larger than prokaryotic cells

Plasma Membrane

• The plasma membrane is a selective barrier.
• The plasma membrane allows passage of oxygen, nutrients, and waste.

Eukaryotic Cell

• A eukaryotic cell has internal membranes that partition the cell into organelles.
• Plants and animals share most of the same organelles.

Nucleus

• The nucleus contains most of the cell’s genes and is very conspicuous.
• The nuclear envelope encloses the nucleus, separating it from the cytoplasm.
• The nuclear envelope is a double membrane, with each membrane consisting of a lipid bilayer.
• The nuclear envelope regulates the entry and exit of molecules.
• The nuclear envelope regulates nuclear shape.

Chromatin, Nucleolus & Ribosomes

Endomembrane System

• The endomembrane system regulates protein traffic and performs metabolic functions in the cell.
• Components of the endomembrane system include the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells.
• The ER is continuous with the nuclear envelope.
• There are two distinct regions of the ER: Smooth ER and Rough ER.
• Smooth ER synthesizes lipids, metabolizes carbohydrates, detoxifies poisons, and stores calcium.
• Rough ER has ribosomes that secrete glycoproteins, distribute transport vesicles, and act as a membrane "factory."

Golgi Apparatus

• Functions of the Golgi apparatus include modifying products from the ER, manufacturing certain macromolecules, and sorting and packaging materials into transport vesicles.

Lysosomes

• A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules, including fats, polysaccharides, proteins, and nucleic acids.
• Some cells engulf others through phagocytosis, forming a food vacuole.
• Lysosomes break down a cell’s own organelles via autophagy.

Vacuoles

• Vacuoles are diverse maintenance compartments.
• Food vacuoles are formed via phagocytosis.
• Contractile vacuoles pump excess water out of cells.
• Central vacuoles hold organic compounds and water

Mitochondria and Chloroplasts

• Mitochondria and chloroplasts convert energy from one form to another.
• Mitochondria are sites of cellular respiration, a process that generates ATP.
• Chloroplasts are sites of photosynthesis.
• Peroxisomes are oxidative organelles.
• Mitochondria and chloroplasts are not part of the endomembrane system; they have a double membrane, have proteins made by free ribosomes, and contain their own DNA.
• Mitochondria are in nearly all eukaryotic cells.
• Cristae present a large surface area for enzymes that synthesize ATP.

Chloroplasts

• Chloroplasts belong to a family of organelles called plastids.
• Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis.
• Chloroplasts are found in leaves and other green organs of plants and algae.

Peroxisomes

• Peroxisomes are specialized compartments bounded by a single membrane that produce H2O2.

Cytoskeleton

• The cytoskeleton is a network of fibers extending throughout the cytoplasm.
• The cytoskeleton provides cellular organization and mechanical support, and is composed of microtubules, microfilaments, and intermediate filaments.
• Vesicles can travel along the cytoskeleton via "monorails."

Microtubules

• In many cells, microtubules develop from a centrosome near the nucleus.
• Microtubules control the beating of cilia and flagella, which are locomotor structures of some cells.

Cilia and Flagella

• Cilia and flagella share a common ultrastructure.
• They have a core of microtubules sheathed by the plasma membrane.
• They have a basal body that anchors the cilium or flagellum.
• They have a motor protein called dynein, which drives the bending movements of a cilium or flagellum.
• Dynein arms alternately grab, move, and release the other microtubules.
• Protein cross-links limit sliding.
• Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum.

Microfilaments

• Microfilaments that function in cellular motility contain the protein myosin in addition to actin.
• Localized contraction via actin and myosin also drives ameboid movement.
• Pseudopodia extend and contract through the reversible assembly and contraction of actin subunits into microfilaments.
• Cytoplasmic streaming is a circular flow of cytoplasm within cells.
• In plant cells, actin-myosin interactions partially drive cytoplasmic streaming.
• Microvillus bear tension; resist pulling forces in the cell , form a supportive network called the cortex

Intermediate Filaments

• The intermediate filaments are more permanent cytoskeleton fixtures than other two classes.

Cell Walls

• Most cells synthesize and secrete materials that are external to the plasma membrane.
• This includes cell walls of plants, the extracellular matrix (ECM) of animal cells and intercellular junctions.
• The cell wall is an extracellular structure that distinguishes plant cells from animal cells.
• Prokaryotes, fungi, and some protists also have cell walls.
• The cell wall protects the plant cell, maintains its shape, and prevents excess water uptake.
• Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein.
• Channels present btw adjacent cells

Extracellular Matrix

• Animal cells lack cell walls yet are covered by an ECM
• Function of ECM: support, adhesion, movement, regulation

Intercellular Junctions

• Neighboring cells interact; intercellular junctions facilitate this contact.
• Water and small solutes pass through intercellular junctions