Cell Structure: Prokaryotes and Eukaryotes

Module 1: Cells as the Basis of Life

• Cell Structure distinguishes one cell from another.

Cell Structure

• Study different cellular structures.
• Examine prokaryotic and eukaryotic cells.
• Describe technologies used to determine cell structure and function.
• Investigate prokaryotic and eukaryotic cell structures.
• Draw scaled diagrams of a variety of cells.
• Compare and contrast different cell organelles and arrangements.
• Model the structure and function of the fluid mosaic model of the cell membrane.

Prokaryotes vs. Eukaryotes

• Cells exists as unicellular (single celled) or multicellular (multi celled) organisms.

Prokaryotes

• 'Pro' means before, 'karyon' means 'nucleus.'
• Prokaryotes are unicellular organisms including Archaea and bacteria.
• Prokaryotes are very small and can be found everywhere.

Eukaryotes

• 'Eu' means true, 'karyon' means 'nucleus.'
• Eukaryotes are larger and more complex.
• They are multicelluar organisms including plants and animals.
• Eukaryotes can be divided into protists, fungi, plants and animals.

Prokaryotes vs. Eukaryotes comparison

• Prokaryotes have no membrane bound organelles.
• Eukaryotes have membrane bound organelles.
• Prokaryotes have no nucleus, and Eukaryotes have a nucleus
• Prokaryote DNA is circular in shape and contained in plasmids.
• Eukaryote DNA is linear and found in the nucleus
• Prokaryotes are smaller, typically 0.1-5µm e.g. Cyanobacteria.
• Eukaryotes are larger (up to 10 times), typically 10-100µm e.g. Humans.

Endosymbiosis Theory

• This theory explains how eukaryotic cells could have evolved from prokaryotic cells.
• The theory states that some large prokaryotic cells engulfed some small bacteria and they lived as symbionts.
• Mitochondria and chloroplasts both contain their own DNA that is arranged in a similar way to prokaryotes.

Parts of a Cell

• Nucleus is found in both animal and plant cells.
• Vacuole is found in both animal and plant cells, but is larger in plants.
• Chloroplast only exits in plant cells.
• Mitochondria exits in both plant and animal cells.
• Cell wall exist only in plants cells.
• The cell membrane is found in animal and plant cells.
• Plants may have lytic vacuoles, which act like lysosomes in animal cells.
• Plant cells have microtubules and secretory vesicles.
• Cell membrane and plasma membrane are the same structure.

Nucleus

• A large organelle surrounded by a double-layered membrane.
• It contains most of the genetic material (DNA and proteins).
• The nucleus is the control centre of the cell and coordinates all cells activities
• It contains the nucleolus which makes ribosomes.

Ribosome

• Ribosomes are composed of two units.
• They are very tiny and therefore can only can be seen with an electron microscope.
• They are made of proteins and ribosomal RNA (rRNA) and are the sites of protein synthesis.
• They are not membrane-bound organelles.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a network of intracellular membranous sacs and tubules.
• It links the cell membrane and other membranous organelles.
• Rough ER has ribosomes attached and actively produces and exports proteins.
• Smooth ER does not have ribosomes attached, but contains enzymes involved in the synthesis of phospholipids.

Golgi Apparatus

• The Golgi Body is a stack of flattened, smooth membrane sacs.
• It is responsible for bundling and packaging macromolecules (proteins and lipids) as they are synthesised within the cell.

Lysosome

• Lysosomes are sphere shaped organelles for digestion and waste removal.
• They ingest bacteria, broken cell parts and other unwanted materials.

Mitochondria

• Mitochondria are membrane bound organelles.
• They are responsible for cellular respiration.
• The highly folded inner membrane increases the surface area over for which chemical reactions take place.

Chloroplasts

• Chloroplasts are membrane bound organelles involved in photosynthesis.
• They contain chlorophyll (green pigment) and thylakoids (disc-shaped).
• Chloroplasts trap light energy in the process of photosynthesis.

Vacuole

• Vacuoles are membrane bound storage compartments of a cell.
• Animal cells have many small, temporary vacuoles and plant cells contain a single, large, permanent vacuole.
• Vacuoles in plants provide structural support.

Cell Wall

• The cell wall is a rigid structure that surrounds the cell membrane of plant cells, fungal cells and some prokaryotic cells.
• The cell wall provides support for the cell.

Cell Membrane

• The cell membrane is a semi-permeable barrier that controls the exchange of materials into and out of the cell.

Biological Drawings

• Drawings should be made with a pencil and ruler;
• Drawings should be large, 2D only, and labelled clearly with a title, magnification and scale.
• There should be no sketching or shading. For example;
  • Length of a red blood cell = 8µm; the length of your drawing = 4cm
  • Actual length = 8μm; Length of drawing = 4cm ; Scale = 2µm/1cm
  • Therefore, 1cm in a drawing represents 2µm.

Technologies - Microscopes

• Magnification explains how many times larger the object is made to look.
• Resolution is the degree of detail achieved when magnifying.
• A light microscope can see the cell wall, cell membrane, cytoplasm and nucleus of cells.
• Electron microscopes allow a view of structures like the chloroplast, mitochondria, rough endoplasmic reticulum, nucleus, Golgi body etc.

Cell Membrane - The Fluid Mosaic Model

• Cell membrane/plasma membrane is a phospholipid bilayer.
• It is selectively permeable, only allowing certain molecules or ions into or out of the cell.
• It allows the concentration of substances inside cells to remain constant compared to outside.
• The proteins floating on and in it create a 'lipid sea' making in a fluid mosiac model.
• The cell membrane has the ability to flow and change shape. Protein molecules are embedded in it like a mosaic, and this can move or be fixed.
• Each phospholipid has a head (hydrophilic) and a tail (hydrophobic).

Cell Function

• Cells coordinate their activities within their internal and external environment.
• Study the way in which materials move into and out of cells.
• Conduct a practical investigation modelling diffusion and osmosis.
• Examine the roles of active transport, endocytosis and exocytosis.
• Relate the exchange of materials across membranes to the surface-area-to-volume ratio, concentration gradients and characteristics of the materials being exchanged.
• Investigate cell requirements.
• Investigate the biochemical processes of photosynthesis, cell respiration and the removal of cellular products and wastes in eukaryotic cells.
• Model the action of enzymes in cells and investigate the effects of the environment on enzyme activity through the collection of primary or secondary data

Movement in and out of cells

• Substances must pass through the semi-permeable cell membrane to move in and out of cells.
• Exchanging materials is important for cell functioning and communication between cells.
• The molecules size, shape affect permeability.
  • small, uncharged molecules (oxygen, carbon dioxide) are permeable.
  • lipid-soluble, non-polar molecule (alcohol, chloroform, steroids) are permeable.
  • small, polar molecule (water, urea) is permeable or semipermeable.
  • small ions such as potassium, sodium and chloride are impermeable without protein channels.
  • large, polar, water-soluble molecule (amino acid, glucose) are impermeable without protein channels.

Active and Passive Transport

• Active Transport requires energy and usually goes against a concentration gradient such as the sodium and potassium pump.
• Passive Transport requires no energy and follows with the concentration gradient.

Diffusion

• Diffusion happens via passive transport where particles move from an area of high concentration to an area of low concentration until equilibrium is reached.

Facilitated Diffusion

• Facilitated diffusion is required for large particles (glucose and amino acids) to pass the cell membrane.
• Facilitated diffusion requires carrier proteins and channel proteins to assist in movement.

Osmosis

• Osmosis happens via passive transport where water molecules move from an area of high concentration to an area of low concentration until equilibrium is reached.
• Hypertonic solutions move water out of the cell leading it to become flaccid.
• Hypotonic solutions move water into the cell leading it to becomes turgid.
• Isotonic solutions lead to equilibrium.

Endocytosis

• 'Endo' = inside & 'cyto' = cell.
• Endocytosis happens when large particles move into the cell with the help of the cell membrane (active transport)
• Three types of endocytosis:
  • Phagocytosis happens when a particle is engulfed.
  • Pinocytosis happens when liquid is engulfed.
  • Receptor-mediated endocytosis, showing the engulfment of different materials by the cell membrane and the release of the vesicle within the cell.

Exocytosis

• 'Έχο' = outside & 'cyto' = cell
• Exocytosis happens when materials are transported from the inside to the outside of the cell in membrane bound vesicles that fuse with the plasma membrane.

Surface Area to Volume Ratio

• The smaller the cell, the larger the SA:V, improving the speed at which substances reach the middle of the cell.
• As cell size increases, the SA:V decrease.
• Smaller cells can exchange matter with their environment more efficiently. For example; a 5cm cube:
  1. Calculate the surface area.
  2. Calculate volume.
  3. Calculate surface area divided by volume.
  4. This will give the ratio.
  5. SA = (5x5)x6 = 150cm²
  6. V = 5x5x5 = 125cm³
  7. SA:V = 150/125 = 1.2

Materials Being Exchanged

-Chemical factors impacting exchange:

• Uncharged molecules can easily pass through the membrane.
• Water, sodium and potassium cannot pass through, but water passes through the membrane through aquaporins.
• Physical factors impacting exchange:
  • The size and shape affect the ability of molecules to move through the membrane.
  • Small molecules can diffuse easily.
  • Large molecules, like glucose, use carrier proteins to assist their movement (endocytosis and exocytosis).

Cell Requirements

• Cells require organic and inorganic molecules to perform their functions.
  • Organic containing carbon attached to hydrogen.
  • Inorganic carbon (if present) is not attached to hydrogen’s