Structure of a cell

Cell theory

1. all living things are composed of one or more cells
2. the cell is the basic unit of life
3. new cells arise from pre-existing cells

Prokaryotic and Eukaryotic cells

• 4 key components of cell: plasma membrane, cytoplasm, DNA, ribosomes
• prokaryotic cells are one single open space(without membrane walls inside)
• prokaryotic DNA is in nucleoid
• the surface-area-to-volume ratio(exchange capacity of the cell) regulates cell size

Eukaryotic cells

• membrane bound nucleus
• a number of membrane-bound organelles
• multiple linear chromosomes

Plasma membrane and cytoplasm

• plasma membrane: a double layer consists of phospholipids, divides in&out of the cell
• its two-layer structure called a phospholipid bilayer
• cytoplasm: everything found inside the plasma membrane(in prokaryotes), between the plasma membrane and the nuclear envelope(nucleus)(in eukaryotes)
• cytosol=goo

Nucleus and ribosomes

• nuclear envelope: made up of two layers of membrane
• in the space between the two layers: endoplasmic reticulum
• nucleolus: new ribosomes are assembled(by ribosomal RNA)

• in prokaryotes DNA is organized into a loop structure chromosome
• in eukaryotes DNA is organized into a string structure chromosome

• endoplasmic reticulum: rough one(with ribosomes)/ smooth one(without ribosomes)

The endomembrane system

• a group of membranes and organelles in eukaryotic cells
• nuclear envelope/ lysosomes/ endoplasmic reticulum/ Golgi apparatus and the plasma membrane

The endoplasmic reticulum

• the rough endoplasmic reticulum(rough ER): ribosomes(attached to its cytoplasmic surface) make proteins>packaged into vesicles>shipped to the Golgi apparatus
• the smooth endoplasmic reticulum(smooth ER): no ribosomes. synthesis of carbohydrate/lipids/steroid hormones, detoxification, storage(of calcium ions)

The Golgi apparatus

• cis face(the receiving side)>modify proteins and lipids>tagged and sorted, packaged into vesicles(again)>trans face(exit)>bud from the Golgi

• lysosomes: an organelle contains digestive enzymes(in plants cells, called vacuole)
• phagocytosis(process): pathogen(like a virus) engulfed by macrophages(white blood cell)>a phagosome contain pathogen>lysosome involve, destroy the pathogen

Mitochondria and chloroplasts

• Mitochondria act as the powerhouse of the cell, through cellular respiration
• Chloroplasts act the same for plants and algae, through photosynthesis
• photosynthesis
• thylakoid: contains light-harvesting complexes that include chlorophyll 
• Mitochondria make adenosine triphosphate(ATP)
• ATP making process=cellular respiration

The cytoskeleton

• 3 types of protein fibers in the cytoskeleton(in eukaryotes)
• microfilaments: the narrowest, made of actin, highways inside the cell 
• intermediate filaments: specialized to bear tension, maintain the shape of the cell
• microtubules: help the cell resist compression forces
• microtubules are components of eukaryotic cell structure: flagella, cilia, centrosomes
• flagella: like a moving tale of a sperm
• cilia: like a short hair in nostrils(nose holes)
• both have 9+2 array

• centrosomes consists of 2 centrioles(=modified basal body)
• centrosomes: microtubule organizing center, its exact function is at open research

Extracellular structures and cell-cell junctions

• extracellular matrix(ECM): a complex meshwork of proteins and carbohydrates, a major component is the protein collagen

• proteoglycan complex: interwoven with collagen fibers
• integrins: embedded in the plasma membrane. anchor the cell, sense its environment(ex. blood clotting)
• fibronectin: act as bridges between integrins and other ECM proteins(collagen)
• cell wall: plant's supportive extracellular structure(collagen for animals), a major organic molacule is cellulose

Cell-cell junctions

• plasmodesmata: have a hole to allow direct cytoplasmic exchange between two cells
• gap junctions: channels between neighboring cells, 6 connexins=a connexon 

• tight junctions: create a watertight seal between two adjacent animal cells(ex. our bladder) 
• claudins: tight junction proteins
• desmosomes: junctions of animal cell. pin adjacent cells together, ensuring stretching skin or muscle remain connected in an unbroken sheet
• cadherins: adhesion(sticking) proteins, hold the membranes together