Cell Structure
All cells are the basic theory of the structure and functional units of life formed by
pre-existing cells by cell division
1. All organisms are made up of one or more cells
2. Cells are the fundamental unit of life
3. All cells come from preexisting cells
There are two types of cells:
Prokaryotes
Less complex with no true nucleus or other membrane-bound
organelles. Organisms within the domain bacteria and archaea
have a single prokaryotic cell.
Eukaryotes
Contains a nucleus and membrane-bound organelles and is found in organisms such as animals,
plants, fungi and protozoa.
Animal Cell
Plant Cell
Comparison
● Eukaryotes can be unicellular or multicellular organisms
● Prokaryotes are always unicellular
Characteristic
Prokaryote
Eukaryote
DNA storage
Free-floating in the cytoplasm.
Long circular nucleoids and
smaller plasmids
Located in the nucleus.
Composed of multiple
chromosomes
Membrane-bound organelles
None
Nucleus, Golgi apparatus,
endoplasmic, reticulum, etc
Cell wall
Present in all bacteria and
archaea
Found only in plants, fungi and
some protists
Size
0.1 - 0.5 μm
10 - 100 μm
Observing Cells
Light Microscopes
● Uses light to visualise cells ranging from 1mm to 100 nm
● Magnification → the ratio of enlargement (or reduction) of an image
● Resolution → ability of the lens to show the fine details
● Field of View (FOV) → area visible when looking through the microscope and can be
used to calculate the size of the cell
Electron Microscope
● Resolve objects less than 1 nm
● Uses electron beams instead of light and can come in 2 types:
○ Scanning electron microscope (SEM): Scans the surface of the specimen
○ Transmission electron microscope (TEM): Produces images of internal organelles
Cell Function
Fluid Membrane mosaic model
● Identify that there is movement of molecules into and out of cells
● Helps the cell membrane maintain its role as a barrier between the inside and outside of
the cell environments
Substances that enter and leave cells
● These substances are called wastes
○ Urea, uric acid and excess carbon dioxide
● Products secreted by cells that may be needed to coat the outside of the cell (mucus) or
may pass to other cells (hormones)
Boundaries
● Cell membrane are selectively permeable because they allowed only certain molecules
to pass through them
● Microscopic pores that exist in the cell membrane determine what molecules may not
enter a cell
● These pores work in a manner similar to the gates in the boundary or fence surrounding
the school. People who are associated with the school are allowed in and strangers are
refused entry
Cell Membranes and Nutrition
● Cells must obtain nutrients to survive while expelling waste materials that can become
toxic is accumulated in large amounts
○ This is facilitated by the cell membrane
○ Aids in exchanging these materials within their internal/ external environment
● The membrane is composed of phospholipid bilayer
○ Containing millions of phospholipid molecules
○ Hydrophilic heads facing outwards towards water
○ Hydrophobic tails facing inwards away from water
● Embedded in the membrane are structures of lipids, carbohydrates and proteins:
Diffusion
The habit of liquids and gasses moving down the concentration gradient from high to low. It is
a passive process, meaning it needs no input of energy.
Osmosis
Osmosis is a type of diffusion, that refers to the movement of water molecules down the
concentration gradient through a cell's semipermeable membrane without requiring energy
input.
● As water is charged the mvt is not directly through the bilayer of lipids
● Mvt occurs through special tiny protein channels - AQUAPORINS
● This makes the mvt much more rapid than expected
● Mvt rate is affected by
○ Solute concentration
○ Opposing physical pressure or tension exerted on the water
Diffusion and osmosis help explain how nutrients & wastes are transported in and out of cells
Passive Transport (high → low)
● Some small, non-polar and neutral substances (water, oxygen, carbon dioxide) can
easily move through the membrane via passive transport
○ Moves water molecules through osmosis and ions through facilitated diffusion
using protein channels embedded within cell membrane
○ Moves substances down their concentration gradient
● It is a result from the random movement of particles (Brownian motion)
○ This allows the particles to move from an area of high concentration to an area of
low concentration = equilibrium
Active Transport (low → high)
● Larger, polar and charged molecules (lipids, carbohydrates) cannot easily cross the cell
membrane and therefore move via active transport
○ Active transport requires both the protein channels and ATP
○ Moves substances up their concentration gradient
Simple Diffusion
The movement (mvt) of any molecules from a region of high concentration to low concentration
until equilibrium is reached. It does not require any energy input.
● Can involve the mvt of solid, liquid or gases through any medium
● Mvt is along a concentration gradient
● Rate is determined by the concentration gradient
○ The greater the difference the faster the diffusion
● Can speed up or slow down depending on temperature
● Biological eg: lipid molecules and small simple molecules diffuse across cell membranes
Facilitated diffusion
Similar to simple diffusion but the mvt is assisted by carrier proteins in the membrane
● Each protein acts as a membrane-transporter is specific to one solute or several similar
solutes
● Similar to simple diffusion but the movement is assisted by Carrier proteins in the
membrane
● Allows the mvt of large or charged molecules to occur more rapidly
● Can act as channel protein or carrier protein
○ Channel is faster form
■ Drinking straw
○ Carrier protein binds with the solute
■ Uptake of mineral salts from the soil (plants)
Cell membrane permeability
The permeability of a cell membrane refers to its ability to allow the cells to exchange liquids
and materials between the cells internal and external environment.
● The movement of materials in and out of a cell allows essential materials to enter and
keep wastes out
● It also allows cells to communicate with each other
● Semipermeable membrane are those which only lets solvents, such as water, pass
through them
Molecule/ion
Examples
Permeability of membrane to
the molecule/ion
Small, uncharged molecule
Oxygen, carbon dioxide
Permeable
lipid-soluble, non-polar
molecule
Alcohol, chloroform, steroids
Permeable
Smal, polar molecule
Water, urea
Permeable or semipermeable
Small ion
Potassium, sodium, chloride
Impermeable (ion passes
through protein channels)
Large, polar, water-soluble
molecule
Amino acid, glucose
Impermeable
Endocytosis
Endo = inside; cyto = cell
Endocytosis is the process by which large particles are moved into the cell using the help of the
cell membrane. There are three types of endocytosis:
● Phagocytosis → particle is engulfed by the membrane and mineral is digested
● Pinocytosis → cell membrane engulfs liquid containing dissolved molecules
● Receptor-mediated endocytosis → pinocytosis that is initiated by protein receptors in
the cell membrane
Exocytosis
Exo = outside; cyto = cell
Exocytosis is the process by which substances such as antibodies and enzymes that have
important functions elsewhere and waste products in the organism need to be removed.
Factors Affecting Diffusion
There are several factors that affect the date and style of transportation of molecules across the
membrane.
● Size of molecules
○ Glucose and amino acids are large and require facilitated diffusion
● Concentration gradient
○ Relative concentration of solutes on either side of the membrane affects the
speed at which diffusion occurs.
● Surface Area to Volume ratio (SA:V)
○ The smaller the cell, the larger the SA:V which means substances can reach the
centre of the cell quicker than a large cell with a small SA:V
Surface area to volume (SA:V)
● Larger cells have greater metabolic needs so they need to exchange more nutrients and
wastes with their environment
● As the cell of cell increases, the SA:V of the cell decreases
● Smaller cells can exchange matter with their environment more efficiently
Increasing SA:V
Compartmentalisation
Cell compartmentalisation also allows eukaryotic cells to be much bigger than prokaryotic cells.
This is because it:
● Reduces the amount of exchange that needs to occur across the cell membrane to
maintain a stable environment
● Creates more space for membrane-bound enzymes, allowing increased activity in the
cell
Flattened shape
● As cell increases in volume, the distance from the centre of the cell to the cell
membrane also increases
● Flattening a cube while keeping the volume constant results in a larger surface area, and
therefore a larger SA:V
Membrane extensions
● Cells involved in absorbing nutrients or secreting wastes counteract the SA:V ratio
problem by extending the SA of the cell membranes
● Come animal cells have dick-like extensions of the cell membrane called microvilli
● Root hairs in plants are lateral extensions of root cells allowing plants to absorb water
and nutrients
Cell Requirements
Substances needed by cells are used in 2 main ways
1. As essential building blocks cells & living tissue are made
2. As a source of stored energy for the cell
Inorganic nutrients
Inorganic nutrients
Positions in cells
Water
- Oxygen
- Hydrogen
90% of the protoplasm
Uses in cell activities
● Transport medium in cells &
organisms
● Important solvent for many
molecules inside cells
● Medium in which all chemical
reactions in cell take place
Mineral salts:
Dissolves as ions in the
Chlorides, nitrites, phosphates and cytoplasm and in vacuoles in
carbonates of sodium, magnesium, plant cells
calcium, potassium and ammonium
(e.g. NaCl)
● Assist all chemical reactions
● Used in synthesis of many
macromolecules & body
tissues
● Sodium ions & chloride ions
assist in water balance in cells
& are essential for cell
membrane functioning &
function of nerve & muscle
cells
Gases
- CO2
- Oxygen
● Carbon Dioxide:
○ Used during
photosynthesis
○ Released as a product
of aerobic cellular
respiration
● Oxygen:
○ Used during aerobic
cellular respiration to
release energy
○ Released as product of
photosynthesis
Dissolves in protoplasm, used
and/or produced in
chloroplasts & mitochondria
Organic Compounds
● Biomacromolecules → large organic molecules required for structure & to maintain
biochemical processes involved in effective functioning of all living cells
● Four main types
○ Carbohydrates
○ Lipids
○ Proteins
○ Nucleic acids