Introduction to Cells

Cell Theory

• All living organisms are composed of one or more cells

• Cells are the smallest units of life

• All cells come from pre-existing cells

Atypical Cells

Striated Muscle Cell

• composed of sarcomeres

• each cell is multinucleated

• the average muscle fibre is approximately 30 mm long (larger than a typical cell)

• challenges the idea that a cell has one nucleus, as the striated muscle cell is comprised of multiple nuclei

Giant Algae: Acetabularia

• genus of single-celled green algae of gigantic size, ranging from 0.5 to 10 cm in length

• consists of the rhizoid (looks like small roots), the stalk and a top umbrella (made of branches that may fuse into a cap).

• challenges the idea that cells must be simple in structure and small in size

Aseptate Fungal Hyphae

• they are hyphae with many nuclei

• they have NO septa

• the result of this is shared cytoplasm and multiple nuclei

• challenges the idea that a cell is a single unit (the fungal hyphae have many nuclei, are very large and possess a continuous, shared cytoplasm

Investigating Cells and Tissues with a Microscope

• a typical cell ranges from 10 to 20 μm (micrometre) in diameter

Calculating Magnification

• Commonly used units when measuring cells or cellular parts are:

  • 1000 nm (nanometres) = 1 μm (micrometre)

  • 1000 μm (micrometres) = 1 mm (millimetre)

• magnification = size of drawing / actual size

• size of specimen = field of view (FOV) / number of specimens

  • ensure that all measurements are given to the same units

• total magnification = ocular lens magnification x objective lens magnification

Unicellular Organisms

• Unicellular organisms carry out seven life functions:

  • metabolism

  • growth

  • response (to a stimulus)

  • homeostasis

  • nutrition

  • reproduction

  • excretion

• homeostasis: the maintenance of a constant internal environment by regulating internal cell conditions.

• metabolism:

  • the regular set of life-supporting chemical reactions that takes place within the cells of living organisms

  • it is the sum of all chemical changes that take place in a cell (synthesis of new molecules + breakdown of the removal of others)

Surface Area to Volume Ratio

• the rate at which substances cross the cell membrane depends on their surface area

• as a cell grows, the volume of the cell increases by a power of 3 (cubed), whereas the surface area of the cell increases by the power of 2 (squared). As such, the surface area to volume ratio decreases as the cell gets bigger.

• as a cell increases in size, its surface area to volume ratio decreases

• a large surface area to volume ratio allows for:

  • heat loss to occur faster

  • a larger and faster exchange of waste materials

Specialized Structures

• villi and microvilli extend off the cell to increase surface area allowing for more efficient absorption

• folds in the plasma membrane are found in the:

  • immune system

  • lungs

  • gastrointestinal tract

• as a cell increases in size, its small surface area to volume ratio limits the rate of exchange of materials with its surroundings. A potential solution to this problem is developing projections from the cell membrane (microvilli) to increase its surface area

Multicellular Organisms

• The evolutionary steps of multicellular organisms are as follows:

  • organisms grew larger as they were no longer limited by the size of one cell

  • limited by the size of one cell in such an organism were able to specialize through differentiation

  • multicellular organisms displayed emergent properties

• genome: the complete set of genes, chromosomes or genetic material present in a cell or organism

• differentiation: when an unspecialized stem cell changes and carries out a specific function in the body. cells differentiate to form different cell types due to the expression of different genes

• according to emergent properties: a complex system possesses properties that its constituent parts do not have - the whole is more than the sum of its parts.

Cell Functions

• Cell Membrane

  • semi-permeable membrane that protects the cell

  • lets things move in and out of the cell

  • location: around the cell

• Nucleus

  • the control center of the cell

  • location: inside the cell, near the centre

• Nucleolus

  • makes ribosomes

  • location: inside the nucleus

• Nuclear Membrane

  • protects the nucleus

  • lets thing move in and out of the nucleus (pores)

  • location: around the nucleus

• Mitochondria

  • produces energy (powerhouse of the cell)

  • location: in the cytoplasm

• Golgi Bodies/Apparatus

  • packages and secretes waste

  • location: in cytoplasm

• Endoplasmic Reticulum (E.R.)

  • transports materials and sends messages to all parts of the cell

  • There are 2 types of ER: rough (has ribosomes) and smooth

    • both have the same function

  • location: attaches from cell membrane to nuclear membrane

• Ribosomes

  • makes proteins

  • location: in the cytoplasm or attached to the E.R.

• Vacuole

  • stores food and water

  • plants have one big vacuole; animals have multiple

  • location: in cytoplasm

• Lysosome

  • contains digestive enzymes

  • destroys bacteria, old cell parts

  • location: cytoplasm

Prokaryotic and Eukaryotic Cells

• Procaryotic cells existed before eukaryotic cells

• Pro=before

• eu=true

• eukaryotic cell: has a true nucleus

• prokaryotic cells: have a nucleoid region, no nucleus

Prokaryotic Cells

• one-celled organisms

• prokaryotic cells go through asexual reproduction, called binary fission

  • fast, simple, requires few resources and less energy

  • good for rapid survivals of conditions that are harsh

  • short-term gain as a species

• Process of binary fission

  • the chromosome is replicated semi-conservatively, beginning at the point of origin

  • beginning with the point of origin, the two copies of DNA move to opposite ends of the cell

  • the cell elongates (grows longer)

  • the plasma membrane grows inward and pinches off to form two separate but genetically identical cells

• parts:

  • plasma membrane

  • cytoplasm

  • pili

  • flagella

  • 70S ribosomes

  • no organelles

  • few internal structures

  • unicellular or colonial

  • cell wall

  • cell membrane

  • circular chromosomes

  • no nuclear membrane. Has nucleoid region

  • plasmids

Eukaryotic Cells

• contain organelles surrounded by membranes

• can be unicellular or multicellular

• eukaryotic cells have a compartmentalized cell structure, which refers to the formation of compartments within the cell by membrane-bound organelles.

• parts

  • linear chromosomes

  • plasma membrane

  • cytoplasm

  • mitochondria

  • 80S ribosomes

  • nucleus

  • nucleolus

  • smooth and rough ER (endoplasmic reticulum) Golgi apparatus

  • vesicle

  • lysosomes

  • centrioles

  • vacuole

  • cell wall (plants and fungi)

  • the cell membrane (all)

  • chloroplast (only in plant cells)

Cell Membrane

• the cell membrane is made of a phospholipid bilayer

• proteins, lipids and carbohydrates (CHO) are scattered throughout two rows of phospholipid (PPL) molecules

• proteins can be transmembrane/integral or peripheral

• amphipathic: has both hydrophilic and hydrophobic, allowing for a natural arrangement into the bilayer.

Structure

• hydrophilic heads face inside and outside of the membrane (the watery environments)

• hydrophobic fatty acids face each other in the middle of the membrane

Functions of Cell Membrane

• the function of the cell membrane is to control what enters and leaves the cell (the food goes in and waste goes out); acts as a barrier between the inside and outside of the cell

• the cell membrane is semi-permeable

  • small, soluble lipids can pass through

  • water can pass through

  • large molecules CANNOT pass through (due to how tightly packed the PPLs are)

  • some molecules require energy

• semi-permeable: only some molecules are allowed to pass freely

Fluid Mosaic Model

Phospholipid Bilayer

• lipid bilayer: 2 layers of PPLs

• phosphate head is polar (water-loving)

• fatty acid tails non-polar (water-fearing)

• proteins embedded in the membrane

Membrane Proteins

• functions of membrane proteins:

  • receptor sites for hormones (hormone binding sites)

  • immobilize enzymes

  • cell adhesion - makes tight junctions between cells

  • cell-to-cell communication

  • transport of materials

    • passive transport

    • active transport

Types of Proteins:

• Integral proteins

  • span across the membrane (transmembrane)

  • hydrophilic parts sticking out on either side of the membrane

• Peripheral proteins

  • found on the inner or outer surface of the membrane

  • can be attached to integral protein

• Glycoproteins

  • proteins with a carbohydrate (sugar) attached

  • usually involved in cell recognition, signalling or sites for hormone binding

• Cholesterol

  • found only in animal cells

  • type of lipid-steroid

  • many are hydrophobic but has -OH (hydroxyl) group which makes that part of hydrophilic (to interact with (PPL heads)

  • involved in controlling the fluidity of the cellular membrane

  • disrupts the PPL molecules - makes the cellular membrane more fluid (not to crystallize)/ but not too fluid - rigid ring structure

  • reduces the permeability of hydrophilic molecule

  

Stem Cells

• stem cell: an undifferentiated cell of a multicellular organism that can form more cells of the same type indefinitely, and from which certain other kinds of cells arise by differentiation

• properties of stem cells:

  • self-renewal - the ability to divide and form new stem cells

  • potency - the ability to differentiate into different cells

Types of Stem Cells

Totipotent

• each cell is able to develop into a new cell including extra-embryonic tissue

• forms to make an entire organism

• can be taken from early 1-3 days embryos

  • this is called the morula, which is the first cells that are formed following the fertilization of an egg cell

• can differentiate into placental cells

Pluripotent

• cells that can form any type of cell (there are 220 cell types)

• can be taken from the blastocyst (5 to 14 days)

• can differentiate into all body cells; not able to make an entire organism

Multipotent

• also called adult stem cells

• appear in 14-day-old embryos and beyond

• the stem cells will continue down certain lineages and cannot naturally turn back into pluripotent cells or switch lineages

• can be taken from fetal tissue, cord blood and adult stem cells.

• can only differentiate into some closely related types of body cells.

• located in the "stem cell niche"

Characteristics of Stem Cells

• unspecialized

• divide rapidly

• differentiate into several types of cells

• have a large nucleus relative to the volume of cytoplasm

• useful because they provide therapies for diseases and other health problems

Embryonic Stem Cells

• nearly unlimited growth potential

• is able to differentiate into any type of cell in the body

• higher risk of becoming tumour cells than adult stem cells

• lower chance of genetic damage due to the accumulation of mutations than with adult stem cells

• often genetically different from an adult patent that is receiving the tissue

• removing cells from an embryo kills it unless only one or two cells are taken.

Cord Blood Stem Cells

• obtained and stored easily

• commercial collection and storage services are already available

• fully compatible with the tissues of the adult that grows from the baby, so no rejection problems occur

• limited capacity to differentiate into different cell types - only naturally develop into blood cells, but research may lead to the production of other types

• limited quantities of stem cells from one baby's cord

• the umbilical cord is discarded whether or not stem cells are taken from it

Adult Stem Cells

• difficult to obtain as there are very few of them and they are buried deep in tissues

• less growth potential than embryonic stem cells

• less chance of malignant tumours developing than from embryonic stem cells

• limited capacity to differentiate into different cell types

• fully compatible with the adult's tissues, so rejection problems do not occur

• removal of stem cells does not kill the adult from which the cells are taken.

The Controversy of Stem Cells

• embryonic stem cells are derived from

  • extra blastocysts that would otherwise be discarded after IVF

  • the blood from the umbilical cord

  • adult tissues such as bone marrow

• extracting stem cells destroys the developing blastocyst (embryo)

• can also be derived in a lab (induced pluripotent stem cells)

• Therapeutic Uses of Stem Cells

  • Can be used in the treatment of leukemia and Stargardt's disease

Stages of Embryogenesis

• cleavage

• totipotent, 8-cell stage

• pluripotent, turns into a blastocyst

Cellular Transport

Types of Cell Transport

Passive Transport

• the cell does NOT use energy

• simple diffusion

• facilitated diffusion

• osmosis

• it is the movement of ions or molecules across a membrane from a region of a higher concentration to a region with a lower concentration

• Simple Diffusion:

  • the random movement of particles from an area of high concentration to an area of low concentration

  • continues until all molecules are even spaces (equilibrium is reached)

  • note: molecules will still move around but stay spread out

• Facilitated Diffusion

  • involves the transport of ions or molecules across a membrane through a membrane protein along the concentration gradient

  • from highly specific channels through the membrane

  • structure determines which particles can travel through

  • can be open or closed in response to signals from hormones, electric charge, pressure, light

  • requires channel proteins

• Osmosis

  • diffusion of water through a selectively permeable membrane

  • water moves from high to low concentration

  • some cells have water channels called aquaporins

    • this increases the permeability of water in that cell

  • Hypotonic Solutions:

    • fluid surrounding the cell has a LOWER solute concentration than the cell's cytoplasm

    • water diffuses INTO the cell by osmosis (cell grows)

  • Isotonic Solutions:

    • fluid surrounding the cell has an EQUAL solute concentration as the cell's cytoplasm

    • water diffuses into and out of the cell by osmosis in equal amounts (the cell does not change size)

  • Hypertonic Solution:

    • fluid surrounding the cell has a HIGHER solute concentration than the cell's cytoplasm

    • water diffuses OUT of the cell through osmosis.

Active Transport

• the cell uses ATP directly to move molecules or ions from one side of the membrane to the other

• primary

• secondary

• involves concentration gradient

• the cell uses energy (ATP)

• actively moves molecules to where they are needed

• movement from an area of low concentration to an area of high concentration

• against the concentration gradient

Secondary Active Transport:

• uses an electrochemical gradient as a source of energy to transport ions or molecules across a cell membrane

• eg. ion pumps are carrier proteins that use ATP to pump ions across the membrane.

• e.g. sodium/potassium pumps are important in nerve responses

• concentration gradient→a difference in concentration between one side of the membrane and other

• electrochemical gradient→the combination of a concentration gradient and an electric potential (voltage) across a membrane; stores potential energy that can be used by the cell

• Sodium-potassium pump

  • maintaining an electrochemical gradient between the inside and outside of the cell; the inside is negatively charged

  • for every 3 sodium (Na+) ions that leaves, 2 potassium (K+) come in

  • the electrochemical gradient is used by nerve cells to relay signals

Membrane-Assisted Transport

• the cell uses energy (from ATP)

• endocytosis (phagocytosis, pinocytosis), exocytosis

  • this is done by vesicles

• endocytosis

  • materials coming INTO the membrane

  • a cell engulfs material by folding the cell membrane around it, pinching it off to form a vesicle

  • e.g. antibodies from mom's milk, bacteria and viruses (immune system cells)

• phagocytosis

  • cell-eating

  • endocytosis involving solid particles

• pinocytosis

  • cell-drinking

  • endocytosis involving liquid particles

• exocytosis

  • materials coming OUT of the membrane

  • vesicles fuse with the cell membrane and empty their contents into the extracellular environment

  • macromolecules and other large particles use this method to leave the cell

• vesicles

  • a small sac of membrane

  • formed by "pinching" off of the plasma membrane or membrane-bound organelles (Golgi or ER)

  • moves things around the cell

Factors affecting diffusion

• molecule size

• molecule polarity

• molecule or ion charge

• temperature

• pressure

Types of membrane proteins

• channel proteins

• carrier proteins

Carrier Proteins:

• bind to specific molecules

• transport them across the membrane and release them on the other side

• changes shape

• lower rates of diffusion than channel proteins

• e.g. potassium channel in the axon (neurons)

  • K+ channels are voltage-gated in axon

  • when charges are more positive outside the axon, the channel is closed

  • when charges are more negative outside the axon, the channel opens temporarily allowing the k+ to leave through diffusion

• in a hypotonic solution, water moves INTO the cell and the cell swells and bursts - the cell becomes turgid for plants and lysis for animals.

• in a hypertonic solution, water moves OUT of the cell, and the cell shrinks and shrivels, this is called plasmolysis in plants and crenation in animal

Origin of Cells

Miller-Urey Experiment

• simulated the conditions of early Earth's atmosphere - methane, hydrogen, ammonia gas

• used electricity to simulate lightning and storms

• Results:

  • a variety of small organic molecules were produced

  • e.g. amino acids (the building blocks of protein) and other carbon compounds

Conditions for Emergence of Life

• Production of carbon compounds like sugars and amino acids (simple organic molecules)

• Assembly of carbon compounds

  • inorganic chemicals, such as iron sulphide, supply energy which can be used to assemble carbon compounds into polymers

• Formation of membranes

  • If amphipathic carbon compounds (like PPLs) were made - they can readily form vesicles that resemble the cell membrane. this creates a separate environment (allowing chemical reactions to occur) on the inside compared to the outside

• Development of a mechanism for inheritance

  • early genetic material was not DNA but RNA, which has the ability to replicate by itself and can also be a catalyst.

  • since inheritance requires copying and passing genes onward and enzymatic reactions (catalyst), RNA was the best-suited molecule

Endosymbiosis

• prokaryotes most likely gave rise to archaebacteria and eubacteria

Origin of Eukaryotic Cells

• some prokaryotes lose their cell walls which allowed them to consume material through their cell membrane more easily.

• this allowed cell membranes to fold inwards and evolved into membrane-bound organelles (which explains the appearance of mitochondria and chloroplasts)

Endosymbiotic Theory

• early eukaryotic cells engulf aerobic heterotopic bacteria

• bacteria were surrounded by a cell membrane and were not digested

• they then entered a symbiotic relationship with the host cell and evolved into mitochondria

• early eukaryotic cells engulf photosynthetic bacteria and then evolved into chloroplasts

• Support for the endosymbiotic theory

  • mitochondria and chloroplasts are different from other organelles as:

  • they are surrounded by 2 membranes

  • have their own circular DNA

  • have 70s ribosomes

  • replicate their own DNA and undergo division independently (binary fission) from a host cell