Cell Biology Fundamentals

• Chapter 5 is about cells being the working units of life.
• Cells are the fundamental units of life

The Cell Theory

• It is an important unifying principle of biology.
• First stated in 1838.
• The three tenets are:
• Cells are the fundamental units of life.
• All organisms are composed of one or more cells.
• All cells come from preexisting cells.
• Modern cells evolved from a common ancestor.

Common Characteristics of All Cells

• Plasma cell membrane is the outer boundary that surrounds the cell, and it consists of a phospholipid bilayer with embedded proteins.
• It acts as a selectively permeable barrier.
• Allows for a relatively constant internal environment.
• Plays an important role in communicating with adjacent cells and receiving signals from the extracellular environment.
• Contains proteins responsible for binding and adhering to adjacent cells.
• Plays an important structural role and contributes to cell shape.
• Cytoplasm is everything inside the cell except for the nucleus.
• Cytosol is the fluid cytoplasm not contained inside another compartment.
• Region of DNA is the nucleoid in prokaryotes and the nucleus in eukaryotes.
• Ribosomes are sites of protein synthesis.
• Cell size is limited by the surface area-to-volume ratio.
• The surface area of a cell determines the amount of substance that can enter it and the amount of waste products that can exit it.
• As a living cell grows larger, its chemical activities increase, thus increasing the need for resources and the rate of waste production increases.
• Cells are small in order to maintain a large surface area-to-volume ratio, so they can exchange adequate quantities of materials with their environment.
• Substances must move from one site to another within the cell; the smaller the cell, the more easily this is accomplished.
• There are two types of cells:
• Prokaryotic
• Eukaryotic
• Prokaryotic cells are bacteria and archaea.
• They have no membrane-enclosed internal compartments.
• Eukaryotic cells are eukarya.
• They have membrane-enclosed organelles in which different functions occur.
• Prokaryotic Cells Are the Simplest Cells

Prokaryotic Cells Structures

• Capsule is outside the cell wall, made of polysaccharides, and is slimy.
• It protects cell from white blood cell attacks, retains moisture, and helps in attaching to other cells.
• Outer membrane encloses the peptidoglycan layer of the cell wall.
• Cell wall (most prokaryotes have this structure) is outside the cell membrane and is rigid.
• It contains peptidoglycan and supports the cell and gives it shape.
• Flagella helps in swimming.
• Pili project from the cell surface and are hair-like.
• They help in adhering to other cells and in the movement of genetic material from one cell to another.
• Cytoskeleton is a system of protein filaments.
• It maintains cell shape and plays roles in cell movement and cell division.
• Eukaryotic Cells Contain Organelles

Eukaryotic Cells

• They are about 10 times larger than prokaryotes.
• They have membrane-enclosed organelles that carry out specific functions and are similar in all of them.
• Ribosomes are sites of protein synthesis.
• They have similar structures in prokaryotes and eukaryotes.
• Consist of ribosomal RNA (rRNA) and more than 50 different protein molecules.
• Nucleus is usually the largest organelle.
• It contains most of the cell's DNA.
• The assembly of ribosomes begins in a region called the nucleolus.
• It is surrounded by the nuclear envelope, which is a double membrane.
• Nuclear pores in the envelope control movement of molecules across the envelope.
• Endomembrane system is an interconnected system of membrane-enclosed compartments.
• Tiny, membrane-surrounded vesicles shuttle substances between the various components.
• The membranes and the materials they contain are in constant motion within alive cells.
• Endoplasmic reticulum (ER) is a network of membranes in the cytoplasm with a large surface area.
• Rough endoplasmic reticulum (RER) have ribosomes attached.
• Newly made proteins enter the RER lumen and are modified, folded, and transported to other regions.
• Smooth endoplasmic reticulum (SER) has no ribosomes.
• Chemically modifiers small molecules such as drugs and pesticides.
• Is the site of glycogen degradation in animal cells.
• Synthesizes lipids and steroids.
• Golgi apparatus consists of flattened sacs (cisternae) and small vesicles.
• Receives proteins from the RER and modifies them.
• Concentrates, packages, and sorts proteins.
• In plant cells, it is where polysaccharides for cell walls are synthesized.
• Lysosomes contain digestive enzymes that hydrolyze macromolecules into monomers.
• Primary lysosomes originate from the Golgi apparatus.
• Food molecules enter the cell by phagocytosis to form a phagosome.
• Phagosomes fuse with primary lysosomes to form secondary lysosomes.
• Enzymes hydrolyze the food molecules.
• Wastes are ejected by exocytosis.
• Mitochondria transform energy in fuel molecules such as glucose to the bonds of energy-rich ATP (cellular respiration).
• Cells that require a lot of energy have many mitochondria.
• They can reproduce and divide independently of the nucleus.
• They are made up of two membranes with an outer membrane and an inner membrane folds inward to form cristae, creating a surface area for embedded proteins involved in cellular respiration.
• Mitochondrial matrix contains enzymes, DNA, and ribosomes to make the proteins needed for cellular respiration.
• Plastids occur only in plants, some protists, come in different types, and have different functions.
• Chloroplasts converts light energy into chemical energy during photosynthesis.
• Have a double membrane and can divide independently of the nucleus.
• The inner membrane forms thylakoids which contain chlorophyll and other pigments that harvest light energy.
• Chromoplasts contain red, orange, and yellow pigments, and give color to flowers.
• Leucoplasts store starches and fats.
• Plant and protist cells have vacuoles.
• Provide structure for plant cells-water enters the vacuole by osmosis, creating turgor pressure.
• Store pigments in flowers and fruits to attract pollinators.
• May also store digestive enzymes, waste products and toxic compounds to deter herbivores.
• Cytoskeleton supports and maintains cell shape.
• Holds organelles in position.
• Moves organelles.
• Involved in cytoplasmic streaming.
• Interacts with extracellular structures to hold cell in place.
• Composed of three types of filaments.
• Three components of the cytoskeleton:
• Microfilaments help a cell or parts of a cell move, determine cell shape, and is how pseudopodia is formed.
• Intermediate filaments anchor cell structures in place and have many kinds.
• Microtubules are long, hollow cylinders and form a rigid internal skeleton in some cells.
• Cilia and eukaryotic flagella are made of microtubules in “9 + 2” array.
• Cilia are short, hundreds on one cell, move stiffly to propel the cell or move fluid over a cell
• Flagella are longer, usually one or two present, and movement is snakelike
• Extracellular Materials Provide Support
• Extracellular structures are important for protection, support, and interacting with other cells.
• They are secreted to the outside of the cell membrane.
• Consist of macromolecules embedded in a gel-like medium (eukaryotes).
• Plant cell wall has cellulose fibers that are embedded in other complex polysaccharides and proteins.
• Provides rigid yet flexible support for the plant.
• Are a barrier to disease organisms.
• Adjacent plant cells are connected by cell membrane-lined channels called plasmodesmata, which permit movement of water, ions, small molecules, RNA and proteins.
• Many animal cells are surrounded by an extracellular matrix, which is composed of fibrous proteins such as collagen, gel-like proteoglycans and other proteins.
• Holds cells together in tissues.
• Contributes to properties of bone, cartilage, and skin
• Filters materials passing between different tissues.
• Orients cell movements in development and tissue repair.
• Plays a role in chemical signaling.
• Eukaryotic Cells Evolved in Several Steps
• Evidence of the first eukaryotes appears in the fossil record 2.7 billion years ago.
• The advent of compartmentalization and evolution of eukaryotic cells was a major event in the history of life.
• The nucleus and endomembrane system may have originated from the inward folds of the cell membrane in prokaryotes.
• Enclosed compartments would be advantageous, where chemicals could be concentrated and chemical reactions would proceed more efficiently.
• Some organelles arose by symbiosis.
• The theory of endosymbiosis proposes that mitochondria and plastids (chloroplasts) arose when one cell engulfed another cell.
• Many of the ingested cell's genes were transferred to the host's DNA but the symbionts retained specialized functions.