A2.2 Cell Structure

A2.2.1: Cells as the basic structural unit of all living organisms 

Cells are the basic structural unit of all living things 

• All living things are composed of cells.

• Cells are the basic units of structure and function.

• Cells come from preexisting cells.

A2.2.2: Microscopy skills 

• Microscopes magnify the size of images.

• Resolution is the ability of a microscope to distinguish the details of a specimen or sample. 

• Magnification = measured length / actual length 

• Actual length = measured length/magnification 

• µm in a mm = 1000 

• nm in a um = 1000

• nm in a mm = 1000000

A2.2.3: Developments in microscopy

Cryogenic electron microscopy 

• Cryogenic electron microscopy allows scientists to view proteins and other biomolecules which do not readily crystalise. 

Freeze Fracture Electron Microscopy 

• Freeze-fracture electron microscopy is a technique used to examine the ultrastructure of rapidly frozen biological samples, such as plasma membranes.

• Membranes are rapidly frozen, and then fractured in area of weakness

• separating the phospholipid bilayer and through integral proteins.

• The procedure has allowed scientists to analyse the structure of plasma membranes, and allowed the identification of integral proteins, leading to the development of the Singer-Nicolson model of membrane structure

Fluorescent Stains and Immunofluorescence in Light Microscopy.

• Immunofluorescence is a technique used  to visualize a specific protein or antigen in cells or tissue by binding a specific antibody chemically attached to a fluorescent dye.

• The specific antibodies attach to specific proteins within biological tissue.

• The sample can then be analyzed using a fluorescence microscope.

• The advantages of immunofluorescence is that the stains are specific so scientists are able to study the location, distribution and quantity of specific biomolecules. Fluorescent stains can be used with living tissue, allowing scientists to study dynamic processes such as cell division.

• Fluorescent stains can be used to detect molecules at low concentrations

• Different coloured fluorescent stains can be used to label different molecules allowing the study of the interactions between molecules.

A2.2.4: Structures common to cells in all living organisms

• Cells can be classified as prokaryotic cells or eukaryotic cells 

• All cells share following features 

  • A phospholipid plasma membrane which controls what enters and exits the cell.

  • Cytoplasm is composed of mainly water, which is where most metabolism occurs.

  • DNA as the genetic material.

  • Ribosomes for protein synthesis.

A2.2.5: Prokaryotic cell structure 

- Prokaryotes have a simple cell structure without compartmentalism.

- Prokaryotes are a diverse group of organisms with a wide variety of structures.

Cell structures 

A2.2.6: Eukaryotic cell structure 

• Eukaryotic cells have chromosomes located in a nucleus, as well as a variety of membrane bound organelles.

• All animals, fungi and plants are eukaryotes.

•  Organelles are structures with a specialized function found within cells.

Similarities between prokaryotes and eukaryotes 

• A phospholipid plasma membrane which controls what enters and exits the cell.

• Cytoplasm where most metabolism occurs.

• DNA as the genetic material.

• Ribosomes for protein synthesis.

A2.2.7: Processes of life in unicellular organisms 

Processes of life 

• Homeostasis is the maintenance of internal conditions within a narrow range 

• Metabolism is the complex network of interdependent and interacting chemical reactions occurring in living organisms.

• Nutrition is the processes that organisms use to obtain and use food (nutrients) for growth and development.

• Movement is the changing of the position of the organism.

• Excretion is the removal of metabolic waste

• Growth is the increase in mass or size of an organism.

• Response to stimuli is the ability of organisms to respond to internal or external stimuli.

• Reproduction is the production of offspring.

A2.2.8: Differences in eukaryotic cell structure between animals, fungi and plants

A2.2.9: Atypical cell structure in eukaryotes

• Eukaryotes have single nucleus except for 

  • Aseptate fungal hyphae: Fungi have a filamentous structure called hyphae, which may be separated by internal cell walls called septa. 

  • Some fungal hyphae are not separated by septa, forming one long multinucleate cell.

  • Skeletal muscle cells, also known as muscle fibers, are multinucleate.

  • Red blood cells, also known as erythrocytes, do not have a nucleus.

  • Phloem sieve tube elements do not have a nucleus.

A2.2.10: Cell types and cell structures viewed in light and electron micrographs 

Nucleoid Region - Prokaryotes

• The chromosome of prokaryotes is located in the nucleoid region.

• The prokaryotic chromosome is naked (not associated with proteins), and contains the genetic information for the growth and development of the cell.

• The nucleoid region is visible on micrographs as a lighter irregularly-shaped region within the cytoplasm of the cell.

• The peptidoglycan cell wall of prokaryotes surrounds the cell.

• The cell wall is seen as a dark line around the outside of the cell.

• The plasma membrane is pushed against the cell wall.

Nucleus - Eukaryotes

• The nucleus is a large structure found in the centre of animal cells and pushed up against the cell wall in plant cells.

• The nucleus has a double membrane (envelope) with pores.

• The pores are highlighted by the short arrows in the micrograph

Mitochondria - Eukaryotes

• A mitochondrion has a double membrane.

• The outer membrane is smooth.

• The inner membrane, cristae, is highly folded, allowing mitochondria to be identified in micrographs.

Chloroplast - Plants (Eukaryotes)

• The chloroplast has a double outer membrane with many membranes within the chloroplast.

• Chloroplasts will only be found in plant cells capable of carrying out photosynthesis.  

Sap Vacuole = Plants (Eukaryotes)

• The sap vacuole in plants is a large vacuole with a single membrane.

• It is located in the centre of the cell, pushing all other organelles against the cell wall

Rough endoplasmic reticulum - eukaryotes

• Rough endoplasmic reticulum is a network of tubules within eukaryotic cells.

• The rough appearance is as a result of many ribosomes attached to the surface of the rough endoplasmic reticulum.

Smooth endoplasmic reticulum - eukaryotes

• Smooth endoplasmic reticulum is a network of tubules within eukaryotic cells.

• There are no ribosomes present on the tubules, so they have a smooth appearance.

Golgi Apparatus - Eukaryotes 

• The Golgi apparatus is a series of stacked, flattened membranes. 

• Vesicles (small vacuoles) are seen around the Golgi apparatus.

Chromosome - EUkaryotes 

• Chromosomes become visible during mitosis and meiosis.

• Chromosomes consist of two elongated DNA molecules (chromatids) held together until anaphase.

Ribosomes - Eukaryotes

• Ribosomes are found floating in the cytoplasm of prokaryotic and eukaryotic cells.

• Ribosomes are also found attached to the rough endoplasmic reticulum in eukaryotic cells.

• Ribosomes appear as spherical dots with a dark centre on micrographs

Cell wall - plants (eukaryotes)

• Plant cell walls surround the cell.

• The plasma membrane is located on the inside of the cell wall.

Plasma membrane - eukaryotes

•  In plant cells the plasma membrane is pushed against the inside of the cell.

• In animal cells, the plasma membrane is the outer boundary of the cell

Microvilli - Animals (Eukaryotes)

• Microvilli are found on epithelial cells of the small intestine, and in the proximal convoluted tubule of nephrons in the kidney.

• Microvilli appear as long finger-like extensions of a cell, significantly increasing the surface area.

A2.2.11: Drawing and annotation based on electron micrographs

• Function of nucleus

  • Contains chromosomes

  • Contain the genetic information for the growth and development of the cells

  • The RNA required for translation is produced within the nucleus

Function of mitochondria:

• Produce ATP by aerobic respiration

 Function of the sap vacuole 

• Sap vacuoles are found in plant cells, and store nutrients and wastes, and maintain turgor pressure.

• Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. 

Rough endoplasmic reticulum

• Is a site fro protein synthesis 

• The proteins are then transported to the Golgi apparatus 

Smooth endoplasmic reticulum

• Production of lipids adn detoxification of harmful substances 

Golgi apparatus 

• Receives proteins from teh rough endoplasmic reticulum 

• Modifies the proteins and packages them n vesicles fro secretion 

Chroloplasts

• Contain chlyropyll and carry out photosynthesis for the plant 

Chromosome

• Located in teh nucleus, with DNA wrapped around histone proteins 

• Contain teh genetic information for growth adn development of the cell 

Plasma memberan - animals                                            Plants 

Controls what renters adn exists the cell 

Secretory vescieles drawing g

• Transport proteins from teh Golgi apparatus to teh plasma membrane, where the proteins are secreted by exocytosis 

Microvilli

• Increase the surface area of a cells, increasing the available surface area for transport of materials  

A2.2.12: Origin of eukayotic cells by endosymbiosis 

Endosymbiotic theory 

• Evidence suggests that all eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually.

• Mitochondria and chloroplasts later evolved by endosymbiosis.

• The endosymbiotic theory proposes that a large cell engulfed a small aerobic prokaryotic cell. 

• The small cell was not digested, but developed a mutualistic relationship with the host cell. 

• The host cell provided protection, and received ATP energy from the aerobic cell. 

• The aerobic cell evolved into mitochondria.

• Later the same process occurred with a photosynthetic prokaryote, which evolved into chloroplasts.

Evidence for theory 

• Chloroplasts and mitochondria are a similar size to modern prokaryotes and share many characteristics which support the endosymbiotic theory.

• These characteristics include:

• A single circular chromosome with naked DNA.

• 70S ribosomes (larger 80S ribosomes are present in eukaryotic cells cytoplasm) for synthesising proteins.

• Reproducing in the same manner through a process called binary fission.

• Chloroplasts and mitochondria also have double membranes with the outer membrane resulting from the endocytosis of the original, small prokaryotic cell. 

A2.2.13: Cell differentiation as the process fro developing specialized tissues in multicellular organisms 

Stem cells are 

• Undifferentiated cells.

• Capable of differentiating into specialized cells.

• Capable of endlessly reproducing.

Stem cells differentiate into specialized cells 

• The basis for stem cell differentiation is different patterns of gene expression, often triggered by changes in the internal or external environment.

• The genes that are expressed (turned on) determine the structure and function of the specialized cell.

Meristemtic Tissue iinPlants 

• Plant cells contain meristematic tissue. 

• Meristematic tissue contains cells that are:

• Undifferentiated cells.

• Capable of differentiating into specialized cells.

• Capable of endlessly reproducing.

A2.2.14: Evolution of multicellularity 

• Multicellularity has evolved independently at least 25 times.

• Many fungi, eukaryotic algae and all animals and plants are multicellular.

Advantages

• Multicellular organisms evolved specialized tissues to carry out a range of functions

• resulting in more efficient organisms, leading to longer life spans. 

• The specialized tissue allows more efficient use of resources.

• Multicellular organisms are larger than unicellular organisms

• so are better protected from predators, and are capable of consuming smaller organisms.