Knowt
Cell Biology
1.1 Cell Structure
1.1.1 Eukaryotes and Prokaryotes
Eukaryotes: A type of cell found in plants and animals with a nucleus. Its components are:
Cell membrane
Cytoplasm
DNA enclosed in Nucleus
Prokaryotes: A type of cell found in bacteria that does not contain a nucleus. Its components are:
Cytoplasm
Cell Membrane
Cell Wall
A single DNA loop and plasmids (small rings of DNA)
Cell Size
Cells are microscopic, most animal and plant cells are 0.01 – 0.10 mm in size.
A microscope is needed to see them in detail.
Micrometre: used to express the size of microscopic structures such as cells, cellular organelles, and microorganisms. (symbol - μm)
Nanometre: used to express very small lengths, particularly at the nanoscale. (symbol - nm)
Best use for subcellular structures, for instance, ribosomes, or organisms such as viruses
Order of Magnitude
Refers to the approximate size or scale of a biological entity or process, typically expressed as a power of 10.
As cells are extremely small, orders of magnitude can be used to understand the size differences of one cell to another.
If an object is 10 times bigger than another, then we say it is 10¹ times bigger
If an object is 1000 times bigger than another, we say it is 10³ times bigger.
If an object is 10 times smaller than another, then we say it is 10-¹ times smaller.
Standard Form
Shows the size of numbers as powers of ten.
Standard form numbers are written as:
A × 10n
A is a number greater than one but less than 10
n is the index or power, always in powers of 10
Standard Form in Larger Numbers
A population of 120,000,000 microorganisms could be written as 1.2 × 10⁸.
This number can be written as 120,000,000.0.
If the decimal place is moved eight spaces to the left, we get 1.2.
So we put ‘x 108’ after 1.2 to show this.
Because the original number is greater than one metre, the minus sign before the 8 is not needed.
It makes a very large number easier to write down.
Standard Form in Smaller Numbers
A red blood cell's diameter of 7 μm or 0.000007 m could be written as 7 × 10-⁶ m.
This number can be written as 0.000007.
If we move the decimal place six spaces to the right, we get 7.0
So we put x 10-⁶ after 7 to show this.
Because the original number is less than one metre, we put a minus sign before 6.
It makes a very small number easier to write down.
Prefixes
They go before units of measurement (e.g.'metres’) to show the multiple of the unit.
Centi = 0.01
Milli = 0.001
Micro = 0.000,001
Nano = 0.000,000,001
1.1.2 Animal and Plant Cells
The following subcellular structures are essential in maintaining cellular functions, contributing to the overall well-being of the cell.
Components of Animal and Plant Cells
Nucleus: A large organelle in eukaryotic organisms that protects the majority of the DNA within each cell.
Cytoplasm: Responsible for holding the components of the cell and protecting them from damage.
Mitochondria: This is where the aerobic respiration reaction occurs and is considered the energy factory of the cell.
Cell Membrane: Regulates the transport of materials entering and exiting the cell.
Ribosome: It is where protein synthesis occurs and is found on a structure called “rough endoplasmic reticulum”.
Additional Components of Plant Cells (only)
Cell Wall: (also present in algal cells) is a wall made of cellulose to strengthen the cell.
Chloroplast: Produce energy through photosynthesis and oxygen-release processes, which sustain plant growth and crop yield.
Permanent Vacuole: It is found within the cytoplasm and functions as storage and transport, intracellular environmental stability, and response to injury.
To calculate the size or area of subcellular structures, you should find a shape (e.g. circle or rectangle) that resembles it.
The rules used to calculate the size/area of the shape (e.g. length x width for rectangle) should be applied to it.
1.1.3 Cell specialisation
Cell Specialisation
Cell is specialised by undergoing differentiation:A process where in “general” or “common” cells evolve to form specific cells that have specific functions.
Cells can either differentiate once early on or have the ability to differentiate their whole life (called stem cells).
Most cells only differentiate once in animals. On the other hand, cells retain their ability to differentiate in plants.
Example of Specialised Cell in Animals
Sperm Cells: Specialised to carry male’s DNA for fertilisation and reproduction.
Has a streamlined shape and a long tail, efficient for swimming toward the egg.
Contains numerous mitochondria that supply the energy, allowing the cell to move.
The acrosome (top of the head) contains digestive enzymes that break down outer membrane layers of egg cells.
Contains numerous mitochondria that release energy from glucose to provide the energy needed for active transport.
Nerve Cells: Specialised to transmit the body's electrical impulses from one place to another.
Has long axons that allow it to carry signals over long distances.
Detritus (branch structures) receive signals, and axons transmit signals to other neurons, muscles, or glands.
Nerve endings contain numerous mitochondria that supply energy to make neurotransmitters, allowing the impulse to be passed from one cell to another.
Muscle Cells: Specialised for contraction and allowing movement.
Contains protein filaments (contractile proteins) that slide past each other when activated, shortening the cell.
Possess numerous mitochondria that provide the additional cellular energy necessary for contraction
Example of Specialised Cell in Plants
Root Hair Cells: Specialised to take up (absorb) water and mineral ions.
Have a large surface area (extended root hairs) that increases absorption.
Contains numerous mitochondria that release energy from glucose to provide the energy needed for active transport.
The large permanent vacuole impacts the movement speed of water from the soil to the cell.
Xylem Cells: Specialised to transport water up the stem of a plant and into the leaves.
Consists of dead cells
The cells that make up the xylem are adapted to their function:
They lose their end walls, so the xylem forms a continuous, hollow tube.
They become strengthened by a substance called “lignin” which gives strength and support to the plant).
Transport in the xylem is a physical process and therefore does not require energy.
Phloem Cells: Specialised to transport food products to parts of the plant where they are needed.
Moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as:
growing parts of the plant for immediate use
storage organs such as bulbs and tubers
developing seeds
It consists of living cells. The cells that make up the phloem are adapted to their function;
Sieve tubes: Specialised for transport and have no nuclei. Each sieve tube has a perforated end, so its cytoplasm connects one cell to the next.
Companion cells: One or more companion cells attached to each sieve tube provide this energy. Additionally, A sieve tube completely depends on its companion cell(s).
1.1.4 Cell differentiation
Cell Differentiation
A natural process through which a cell with less specificity develops and matures to become more distinct in terms of form and function.
It is a significant process for the development, growth, reproduction, and longevity of all multicellular organisms.
As an organism develops, cells transform into different types of cells.
Most types of animal cells differentiate at an early stage and lose this ability.
In mature animals, cell division is restricted to repair and replace damaged cells.
The cell acquires a different subcellular structure as it undergoes differentiation, enabling it to carry out specific functions, making it a specialised cell.
Many types of plant cells retain their ability to differentiate throughout life.
Plant cells only differentiate when they reach their final position in the plant, but they can still re-differentiate when it is moved to another position.
1.1.5 Microscopy
Microscopy: the technical field of using microscopes to view extremely small structures, samples, and/or objects that cannot be seen with the unaided eye.
Development of Microscopy Techniques throughout the Years
Light Microscopy
1590s - Janssen (Dutch spectacle makers), made an experiment where lenses were put in tubes, making the first compound of a microscope. However, their microscopes didn't last long, but their works are thought to have magnification from 3x to 9x.
1650 - Robert Hooke (British scientist), observed and drew cells using a compound microscope.
Late 1600s - Antoni van Leeuwenhoek (Dutch scientist) made a microscope with a single spherical lens that can be magnified up to 275x.
1800s - increased optical quality of lenses that are similar to the microscope used in today's generation.
Limitations
Resolving Power (cannot resolve beyond ~200 nm)
Electron Microscopy
A technique to acquire high-resolution images of biological and non-biological specimens.
Uses electron beams and their wave-like characteristics for higher magnification and resolution of an object’s image.
Has much higher magnification and resolving power than a light microscope.
Two Types of Electron Microscopes
Transmission Electron Microscope (TEM)
Uses a particle beam of electrons to visualize specimens and generate a highly-magnified image.
Used to view thin specimens (tissue sections, molecules, etc.) through which electrons can pass, generating a projection image.
The resolving power is 0.2nm
Scanning Electron Microscope (SEM)
Uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens.
They are used to examine and analyse micro- and nanoparticle imaging characterization of solid objects.
The resolving power is 10nm
Magnification Differences
Light Microscope
Maximum Magnification: 1,500x
Resolution: Limited by the wavelength of visible light, resulting in challenges in visualizing subcellular structures in detail.
Low resolving power, with a poor surface view
Electron Microscope
Maximum Magnification: up to 1,000,000x
Resolution: High resolution helps to generate highly detailed images, providing an excellent insight into the internal structure of cells (but forms in greyscale).
High Resolving Power
Common Calculations
Magnification of Microscope: magnification of eyepiece lens x magnification of objective lens = magnification of microscope
Magnification of an Image: size of an image / real size of object = magnification
Standard Form: multiplying a certain number by a power of 10.
To compare the size of numbers, the ‘number’ multiplied by a power of 10 needs to be between 1 and 10.
Example:2.6 x 10-⁵ = 0.000026 and 5.5 x 10³ = 5500
1.1.6 Culturing Microorganisms (biology only)
Microorganisms
Refers to microscopic organisms.
They can be a bacteria, fungi, archaea, or protists.
To study such organisms, scientists need to grow them in the lab with the use of nutrients, or what we call “culturing”.
Culturing Microorganisms
A method of multiplying microbial organisms by letting them reproduce in a predetermined culture medium under controlled laboratory conditions.
It allows us to better understand them and their possible applications in our lives, along with what triggers them and how to destroy them if needed.
The culture medium contains carbohydrates (for energy), minerals, proteins, and vitamins.
2 Ways to Grow Microorganisms in the Lab
Nutrient Broth Solution: a liquid medium used for the cultivation of a wide variety of organisms from clinical specimens and other materials.
Involves mixing sterile nutrient broth to a suspension of bacteria (to be grown), stopping the flask with cotton wool to prevent air contamination, and shaking it regularly to provide oxygen to bacteria.
Agar Gel Plate: a thin layer of nutrient gel in a Petri dish that is used to grow bacteria and fungi in a microbiology laboratory.
Making Agar Plate
Choose a Recipe: Decide how many plates you will need. Our recipes will make 1 L (1000 mL) of media, enough to fill approximately forty 100 mm plates, but they can be scaled up or down as needed. Plan on using about 25 mL per 100 mm plate.
Gather Supplies
Prepare media: Use a glass container (ideally an Erlenmeyer flask) that will hold at least twice the volume of your media.
Sterilise: Make sure the agar dissolves completely. In media with 15% or more salt, the agar may be slow to dissolve. The media may look cloudy, or you may see small, translucent lens-like objects floating in it. Continue boiling until the media is completely clear; this may take longer than 15 minutes. Incompletely dissolved agar will leave your media squishy or fragile.
Pour into Plates: (1)Hot sterilised agar jelly is poured into a sterilised Petri dish, which is left to cool and set. (2) Wire loops called inoculating loops are dipped in a solution of the microorganism and spread over the agar evenly. (3) A lid is taped on, and the plate is incubated for a few days and stored upside down for microorganisms to grow.
Aseptic Technique
A set of procedures and techniques to prevent the introduction of unwanted organisms into a culture medium or the laboratory environment.
Steps and their reason to consider
Petri dishes and culture media must be sterilised before use: Sterilization prevents unwanted contamination from other microorganisms, also allowing scientists to control specific microorganisms to introduce in a culture medium.
Inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame: To prevent unwanted or cross-contamination.
The lid of the Petri dish should be secured with adhesive tape: A secure lid prevents air contamination and protects the plate from harmful external factors. There should still be enough openings for oxygen to come through.
The Petri Dish should be stored upside down: It prevents condensation from the lid to disrupt the growth process on the agar surface.
In school laboratories, cultures should generally be incubated at 25°C.: It is the most suitable temperature for microorganism growth. Also, it is for both the students' and teachers’ safety.
Calculating Bacterial Population Growth
Bacteria can multiply by binary fission as fast as every 20 minutes if they have a nutrient supply and a suitable temperature.
Mean Division Time: the average time it takes for one bacterial cell to divide once.
Formula: bacteria at beginning x 2 ⁿᵘᵐᵇᵉʳ ᵒᶠ ᵈⁱᵛⁱˢⁱᵒⁿˢ = bacteria at end
To calculate the number of divisions (time the population is left for / the mean division time for that bacteria)
The number of bacteria at the end of the growth period can be very large, so it is common for it to be left in standard form.
Example: The mean division time for bacteria population A is 20 minutes. If the observation begins with one bacterium, calculate how many bacteria will be present after six hours.
Calculate the division of bacteria in 6 hours
In this example, the bacteria divide every 20 minutes, and will therefore divide three times every hour, 60/20 = 3
If the bacteria grow for six hours, each bacterium will divide 3 times per hour. 3 times × 6 hours = 18 times.
Calculate the number of bacteria in the population
bacteria at beginning x 2 ⁿᵘᵐᵇᵉʳ ᵒᶠ ᵈⁱᵛⁱˢⁱᵒⁿˢ = bacteria at end
Number of bacteria at the beginning = 1
Number of divisions = 18
1 × 2¹⁸ = 1 × 262,144 = 262,144 bacteria
In standard form, 262,144 bacteria can also be written as 2.62 × 10⁵ bacteria
1.2 Cell Division
1.2.1 Chromosomes
The nucleus contains genetic information.
Chromosomes: Threadlike structures in the nucleus made of protein and a single molecule of DNA that carries the genomic information from cell to cell.
Each chromosome carries a large number of genes.
DNA Molecule: known as deoxyribonucleic acid (DNA), is the molecule that carries genetic information for the development and functioning of an organism.
Genes: the basic physical and functional unit of heredity and the segments of DNA that carry instructions for building and maintaining the organism
In body cells, the chromosomes are normally found in pairs.
23 pairs of chromosomes are in each cell of the body.
Formation of Pairs
Inheritance: Inheriting one from your parents results in acquiring 46 chromosomes total in each cell.
Mitosis and Cell Division: During cell division, each daughter cell receives a complete set of chromosomes, maintaining the diploid nature of body cells.
Sex Chromosomes: A type of chromosome that identifies the sex of an individual.
1.2.2 Mitosis and the Cell Cycle
Cell Cycle
The series of steps that a cell must undergo to divide.
Involves many repetitions of cellular growth and reproduction.
During the cycle, the genetic material is doubled and then divided into two identical cells.
Stages of Cell Cycle
1 - Interphase
The longest part of the cell cycle.
The stage where the cells and organelles grow and increase in number. As the synthesis of proteins occurs, DNA is replicated and energy stores are increased before moving into mitosis.
2 - Mitosis
The stage where the cell divides its previously copied DNA and cytoplasm to make two new, identical daughter cells.
The chromosomes line up at the equator of the cell, and cell fibres pull each chromosome of the ‘X’ to either side of the cell.
3 - Cytokinesis
The stage where two identical daughter cells form when the cytoplasm and cell membranes divide.
Mitosis
A step or stage of the cycle where the cell divides.
A way of making more cells that are genetically the same as the parent cell.
It is crucial in the development of embryos, along with the growth and development of our bodies.
A vital part of asexual reproduction, as this type of reproduction only involves one organism, so to produce offspring it simply replicates its cells.
1.2.3 Stem cells
Stem Cells
These are cells with the potential to develop into numerous types of cells in the body.
It is significant in repairing damaged tissues and is essential in blood cancer and blood disorder treatments.
Types of Stem Cells
Embryonic Stem Cells
It can be obtained from early-stage embryos.
It represents natural units of embryonic development and tissue regeneration.
Functions to regenerate or repair diseased tissue and organs.
Scientists can clone or culture these cells, directing them to differentiate into almost any cell in the body.
Could potentially be used to replace insulin-producing cells in those suffering from diabetes, new neural cells for diseases such as Alzheimer’s, or nerve cells for those paralysed with spinal cord injuries.
Adult Stem Cells
It is found in most parts of the body, including the brain, bone marrow, blood vessels, skin, teeth, and heart.
If found in bone marrow, it can form many types of cells including blood cells.
Meristems of Plants
Is the centre of active mitotic cell division where plant growth occurs.
It is found in the roots and shoot tips.
Capable of differentiating into any type of plant (and have this ability throughout the life of the plant).
Can be used to make clones of the plant (necessary if the parent plant has certain desirable features), either for research, large production or to save a rare plant from extinction.
Therapeutic Cloning
Refers to producing an embryo with the same genes as the patient.
Obtaining embryonic stem cells from the produced embryo to grow into cells that a patient needs, such as new tissues or organs.
An advantage of this is that stem cells from an embryo would not be rejected by a patient's body, so it is used for medical treatment.
Benefits and Risks of Using Stem Cells in Medical Research and Treatments
Benefits
It can be used to replace damaged or diseased body parts.
Helps to enhance the growth of new healthy skin tissue, enhance collagen production, stimulate hair development after incisions or loss, and help substitute scar tissue with newly developed healthy tissue.
Unwanted embryos in fertility clinics can be purposeful, as they would otherwise be discarded.
Risks
People may have religious or ethical objections, as it is seen as interference with the natural process of reproduction.
Difficulty in finding stem cell donors.
Mutations have been observed in stem cells cultured for several generations, and some mutated stem cells have been observed to behave like cancer cells.
Removal of stem cells destroys the embryo.
Cultured stem cells could be contaminated with viruses that can infect the patient it will be transferred to.
1.3 Transport in cells
1.3.1 Diffusion
Substances may move into and out of cells across the cell membranes by diffusion.
Diffusion
The spreading out of the particles of any substance in solution, or particles of a gas, results in a net movement from an area of higher concentration to an area of lower concentration.
Some substances that are transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange and the waste product urea from cells into the blood plasma for excretion in the kidney.
Factors that affect diffusion rate
Temperature: High temperature increases energy and the movement of the molecules, therefore, increases the rate of diffusion. Lower temperature decreases the molecule’s energy, which decreases the rate of diffusion.
Concentration Gradient: The diffusion rate is directly proportional to the concentration gradient. A higher concentration gradient equates to a faster rate of diffusion.
Size of Molecule: The diffusion rate is inversely proportional to the size of a molecule, which means, the smaller the size of the molecule, the faster the rate of diffusion.
Presence of Membrane: The surface area of a membrane affects the rate of diffusion. As the surface area of the membrane increases, the rate of diffusion also increases, as there is more space for molecules to diffuse across the membrane.
Surface Area to Volume Ratio (SA:V): refers to how much surface area an object or collection of objects has per unit volume.
Formula: Find the Volume (length x width x height) and the Surface Area (length x width), and write the ratio in the smallest whole numbers.
If this is large, the organism is less likely to require specialized exchange surfaces and a transport system because the rate of diffusion is sufficient in supplying and removing the necessary gases.
To write the ratio in the smallest whole numbers, divide SA by V and V by V.
Example: The Surface Area is 54 and the Volume is 27.
54/27 : 27/27 = 2:1
In multicellular organisms, the need for exchange surfaces and transport systems is related to the concept of surface area to volume ratio, as their SA:V is small and cannot rely on diffusion alone. Instead, surfaces and organ systems have several adaptations that allow molecules to be transported in and out of cells.
The surfaces and organ systems are specialized in multicellular organisms for exchanging materials, allowing sufficient molecules to be transported into and out of cells for the organism’s needs. The effectiveness of an exchange surface is increased by:
having a large surface area
a thin membrane that provides a short diffusion path
Having an efficient blood supply (in animals)
Being ventilated (for gaseous exchange in animals).
Examples adapted for exchange materials
Lungs in Mammals: Lungs are specifically adapted for efficient respiratory gas exchange.
As the oxygen is transferred to the blood while carbon dioxide is transferred to the lungs, the exchange takes place in the millions of alveoli in the lungs that are covered in tiny capillaries that supply the blood.
The moist, thin walls of alveoli allow much efficient gas exchange through diffusion.
Small Intestine in Mammals: Small intestines are part of the digestive system, responsible for nutrient absorption.
In this organ, cells have villi and microvilli projections that help absorb the nutrients from digested foods that pass through, into the bloodstream.
Gills in Fish: Gills are where gas exchange takes place in a fish, which also serves as a way to extract oxygen from water.
Each gill has gill filaments and gill lamellae, which is where the diffusion of oxygen into the blood and diffusion of carbon dioxide into the water takes place.
Blood flows in one direction while the water flows in the other. Greater temperature equates to greater movement of particles that results in more collisions, expediting the rate of diffusion.
Roots: Roots of plants are adapted to absorb water and mineral ions.
This part has root hair cells, resulting in a large surface area that is projected into the soil while increasing the rate of water and nutrient absorption.
Leaves: Leaves are specifically adapted for gas exchange in plants.
Carbon dioxide diffuses through stomata for photosynthesis, whilst oxygen and water vapour move out through them.
The stomata are the tiny pores that facilitate the exchange of gases during photosynthesis.
1.3.2 Osmosis
Osmosis
Through osmosis, water may move across cell membranes.
It is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
It influences the transport of nutrients and the release of metabolic waste products.
Higher osmotic pressure protects the plants against drought injury.
Types of Osmosis
Endosmosis: It refers to when a substance is placed in a hypotonic solution, the solvent molecules move inside the cell and the cell becomes turgid or undergoes deplasmolysis.
Exosmosis: It refers to when a substance is placed in a hypertonic solution, the solvent molecules move outside the cell and the cell becomes flaccid or undergoes plasmolysis.
Examples of Osmosis
In plants, the absorption of water from the soil is due to osmosis. The plant roots have a higher concentration than the soil, therefore, the water flows into the roots.
In humans, people who suffer from cholera are also affected by osmosis. The bacteria that overpopulate the intestines reverse the flow of absorption and do not allow water to be absorbed by the intestines, resulting in dehydration.
In animals, if a saltwater fish is placed in water with different salt concentrations, the fish will eventually die due to the entry or exit of water in the cells of the fish.
The Rate of Water Uptake
Example: 0.59 grams of water were taken up by the potato cylinder. This took place over 40 minutes, so the water uptake in an hour, assuming that the rate was constant, would be:
Formula: change in mass x 60 minutes/period of time measured in minutes
Water uptake in 1 hour = 0.59 x 60/40 = 0.89 g/hr
Therefore, the rate of water uptake is 0.89 g/hr
Percentage of Change in Mass
In comparing the changes in mass of different potato cylinders, calculate the percentage change in mass.
Example: The mass of the potato cylinder at the start is 2.22 grams and 2.81 grams at the end. Change in mass is +0.59 grams
Formula: change in mass = mass at the end - mass at start/mass at start x 100
change in mass = 2.81-2.22 / 2.22 x 100 or 0.59/2.22 x 100
0.59/2.22 x 100 = 26.6 %
Therefore, the percentage of change in mass is 26.6%.
1.3.3 Active transport
Active Transport: A process that is required to move molecules or substances against a concentration gradient that requires energy from respiration.
In plants, active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil, as ions are essential for a plant’s healthy growth.
In animals, it allows glucose molecules to be moved across the gut wall into the blood. The glucose molecules in the intestine might be in a higher concentration than in the intestinal cells and blood, but there will also be times when glucose concentration in the intestine might be lower.
Comparing diffusion, osmosis, and active transport
Diffusion: refers to the movement of particles from a high to a lower concentration.
Moved Substances: Carbon dioxide, oxygen, water, food substances, wastes, and most lipids
Required Energy: None
Osmosis: refers to the diffusion of water across a membrane.
Moved Substances: Water
Required Energy: None
Active Transport: refers to how substances move against a concentration gradient.
Moved Substances: Mineral ions, glucose and amino acids
Required Energy: Adenosine Triphosphate or ATP