A Level Biology

Chromosomes are long sections of DNA, made up of genes, which are short lengths of DNA

They only become visible at the beginning of cell division, where they appear as sister chromatids (two tightly coiled genes) joined in the center by a centromere

Each species has a different number of chromosomes. Humans have 46 (23 pairs). The more similar the number of genes, the more closely related a species is. 

The number of chromosomes is halved in gametes (so humans have 23 in each gamete that, when combined into an embryo, make the full 46). Cells with the full number are diploid. Cells with half the usual amount are haploid

Mitosis

Produces two daughter cells that are genetically identical. 

This goes through the cell cycle once. What is the cell cycle you may ask?

This thing. Well, in mitosis we are just looking at the purple. The rest isn’t part of mitosis but it is very important leading up to mitosis.

The stages are 

Prophase - DNA condenses forming chromosomes. Centrioles move to opposite poles of the cell and start to develop spindle fibres. Towards the end, the nuclear envelope disappears

Metaphase - The chromosomes arrange themselves along the equator of the cell and become attached to spindle fibres

Anaphase - The centromeres split and the spindle fibres pull the now separated chromatids to opposite poles of the cell

Telophase - The chromatids form chromosomes again and uncoil. The spindle fibres break down and the nuclear membrane reforms. The cytoplasm begins to divide

After this comes cytokinesis where it splits in two. Mitosis is essential for growth and repair as it replaces tissue, and for organisms that reproduce asexually (i’d really rather use Agametic but i doubt they will accept it), allowing rapid reproduction, but lacking in genetic variation so these offspring are less able to adapt  

Interphase

Probably the most important part of the cell cycle. In this, the cell prepares to divide by doubling literally everything. It’s split into 3 sections

G1 - the first growth phase. Cell size increases a lil, copies some organelles, makes molecules it needs later. Sometimes the cell stays in this for an indefinite period (when it’s then known as G0)

Synthesis (S) - Synthesises copies of DNA 

G2 - cell size should now be doubled. Makes more proteins and organelles 

Finding out how much time is spent in a certain stage may be something they make you do as if also doing a level maths wasn’t enough. 

You may have a question like “If a root tip has 40 cells and 36 are in interphase calculate the proportion of time spent in interphase”

36/40 * 100 = 90%

If they give you a time frame like 24 hours, so to calculate how much time is spent in interphase if it is 90% of the cell cycle,

90/100 * 24 = 21.6 hours

Meiosis

Basically, mitosis twice, except the second time it produces 4 haploid daughter cells that are genetically different (aka gametes). When the chromatids are aligned in the center, genetic information is swapped/crossed over which leads to variation. Mitosis II follows immediately after mitosis I

Mitosis v Meiosis

2 daughter cells v 4 daughter cells

Diploid daughter cells v haploid daughter cells

No crossing over vs crossing over

Endocytosis

When a cell wants a large particle, it is absorbed by endocytosis. The cell membrane engulfs the particle, forming a vesicle, which enters the cytoplasm. There are two types of this:

Phagocytosis (literally means cell eating)

Pinocytosis (entry of liquid into the cell)

Exocytosis

Substances leave the cell after being transported through it in vesicles to the Golgi body, where it is then put into secretory vesicles and transported to the cell membrane, where the vesicle fuses with the cell membrane, emptying its contents outside the cell

In this unit, you need to know about Eukaryotes, Prokaryotes, and Viruses

Eukaryotic Cells

Animal and Plant cells

All have a nucleus and membrane bound organelles

Plant cells also have chloroplasts, cellulose cell walls, vacuoles

Fuctions in this post

They’re gonna give you an electron micrograph at some point and either ask you to identify organelles (that aren’t even there yes i’m looking at you unit 1 2016). This is a super in detail image of a cell.

Protein synthesis in terms of organelles shows how they work together

Ribosomes are made in the nucleolus and leave via nuclear pores

Nuclear pores allow mRNA through to the ribosomes

Protein synthesis occurs at the ribosome

The Rough ER transports polypeptides in transport vesicles to the Golgi Body

Polypeptide chains are modified, converted into their tertiary structure, and packaged in secretory vesicles by the Golgi Body

The secretory vesicles transport them to the cell membrane where they merge with it, releasing them by exocytosis

Mitochondria vs Chloroplasts

Similarities

Double membrane

Folded inner membrane

Circle of DNA for self replication

Ribosomes

Produce ATP

Differences

Mitochondria have cristae, chloroplasts have thylakoid membranes

Chloroplasts have photosynthetic pigments

Mitochondria have an inner matrix, chloroplasts have stroma

Prokaryotic Cells

No membrane bound organelles, no nucleus, no rough ER, no Golgi body, no mitochondria/chloroplasts

Eg bacteria

Eukaryote v Prokaryote

Eukaryote

Larger

Large ribosomes attached to Rough ER

Membrane bound organelles

DNA in a nucleus

Double membrane nucleus

No plasmids

Cell wall of cellulose

Mitochondria

Prokaryote

Small

Small ribosomes free in cytoplasm

No membrane bound organelles

DNA free in cytoplasm

No nuclear envelope

Plasmids

Cell wall of peptidoglycan

No mitochondria. Uses mesosome

Viruses

The black sheep of the cell family. Like the weird neighbour you only occasionally see but probably summons satan in their spare time.

No membrane, no cytoplasm, no organelles, no chromosomes

Contain RNA

Made of a capsid (protein coat)

Can only reproduce in host cells

Outside the cell it exists as a viron

Your cells need to replicate their DNA in order to replicate themselves. This is done during interphase

Hydrogen bonds that were keeping the two strands together are broken

DNA unwinds

DNA polymerase catalyses the addition of free nucleotides to their complimentary base (the original DNA acts as a template)

This produces two identical DNA molecules.

Of course there is an experiment for this that you need to know

The Tales of Meselson and Stahl and the Semi-Conservative Hypothesis of DNA Replication

Their hypothesis is basically what you learnt above; that each DNA strand acts as a template for new DNA, and so each strand of DNA formed is a combination of the original and a new strand

They cultured bacterium Escherichia coli (E.coli) for several generations in a solution containing amino acids made with the heavy isotope 15N (which was absorbed into their nucleotides) and then switched to a 14N solution where they were allowed to replicate.

To extract the DNA, bacteria is centrifuged (they got centrifucked). They then observed at which points the DNA settled

The original 15N sample settled low down because it was heavy

The first generation culture in a 14N solution settled halfway down

The second generation culture settled both halfway down, and near the top (bc some had only light 14N wheras some had the heavier 15N)

There were other theories like conservative replication where the original strands stayed together, or the dispersive replication theory where they all mashed together like some Frankenstein monster DNA, but the experiment above proved them all wrong.

Organism that lives in or on a host, feeding off it at the expense of the host (it hurts the host) - they don't actually have eyes btw

Who can say no to that face

Parasites are nasty though

Adaptations

Hooks/suckers for attaching to gut wall

Thick cuticle for protection against enzymes and acid

Each segment has reproductive capabilities

Produces lots (and i mean LOTS) of offspring that can survive for ages outside a host

No sensory system as they don’t need to move

Very thin so large SA-V ratio which makes diffusion of predigested food easy

Species - A group of organisms that interbreed to produce fertile offspring

Species Richness - a measure of how many different species there are in a particular area

Species evenness - A comparison of the number of individuals of different dpecies in each area

Species diversity - the number of different species and individuals of those species in an area

Species diversity index - A quantitative measure of the species diversity of an area

Succession - Pioneer species develop in previously lifeless places. Secondary succession is where more species live in the area that was colonised before

Polymeric Loci - The position of an allele on a DNA strand. Shows difference in species (if they have same amounts in same places then same species)

How new species form:

Mutation - dofference in DNA

Variation - Difference in physical appearance/behavious

Competitive advantage - Some more suited to environment to live

Survival of the fittest - yeah you know this

Reproduction - better adapted reproduce

Pass advantageous alleles to offspring

There are currently 5 kingdoms; Prokaryote, Protoctista, Fungi, Plantae, and Animalia

They each have different characteristics

Plant 

Autotrophic - Make own sugars from simple molecules in photosynthesis

Cellulose cell wall

Multicellular 

Animal

Multicellular

Heterotrophic

Nervous and muscular systems

Fungi

Eukaryotic

Heterotroph

Chitin cell wall

Reproduction via spores

Mass of thread-like structures underground

Protoctista

Eukaryotic

Not much in common (the misfits)

Prokaryotic

Bacteria/Unicellular

Genetic material not membrane bound

When naming something, you go

Kingdom (eg Animal)

Phylum (eg Vertebrate)

Class (eg Mammal)

Order (eg Carnivore)

Family (eg Felicidae)

Genus (eg Panthera)

Species (eg Leo)

The above game me Lion. It is exactly the same as a Tiger, except for the species being Tigris.

Taxonomy - The science of defining groups or biological organisms based on shared characteristics and giving names to groups

Alpha taxonomy - finsing and naming species

Beta taxonomy - classification of ranks higher than species

Microtaxonomy - deciding how to define species

Macrotaxonomy - Study of groups at ranks higher than species

The father of all this is Carl Linnaeus, who implemented a binomial naming system

Phylogeny

Another way of classification that tries to bypass the confusion and sort things into their true houses families

A measure of how varied an ecosystem is. Can be measured in terms of genes, species, or habitat

Genetic Diversity - A measure of how many variations there are in the genetic code between individuals of a particular species,  or between different species

Species Diversity - A measure of how many different species are present in an area, and how many of each species there is

Habitat Diversity - A measure of how many different habitats are present in an area

Diversity is strongly affected by its habitat (eg desert = small, forest - big)

Habitats usually contain several species

Animals that are endemic are native to one region only (Opposite is cosmopolitan distribution)

Biodiversity hotspots have high levels of biodiversity, but are threatened (34 of them)

Ecosystems rely on interdependence of organisms to maintain stability (basically ~we’re all in this togeetherrr~)

Hedgerows! Everyone loves hedgerows! They’re full of habitats and species! People are always complaining about the loss of hedgerows bc even though it gives us more farmland, it allows food webs to fall apart, disease to spread, and crops to die :A:

Spearman’s Rank can be used to calculate diversity

Looking at an animal’s teeth, you can tell a lot about what it eats

You’re looking for 4 types of teeth: Incisor, Canines, Premolars, or Molars

This is the skull of a herbivore (Sheep to be specific). It has incisors in the front, Premolars in the middle, and molars at the back. There is a gap between the front and back teeth (Diastema)

This is of a carnivore (doggie). Its teeth are way sharper for tearing at meat. It has canines and incisor at the front, as well as carnassial teeth towards the back

Cow Gut

Remember what i said about the hole being cut in the side of the cow? Yeah this is what goes on with cows

Grass is cut by teeth and swallowed down oesophagus to rumen

Rumen mixes food with microbes that secrete enzymes to digest cellulose

This is fermented

Fermended grass passes to reticulum and reforms as cud

Cud can be regurgitated (lovely) and swallowed several times

Cud passes to omasum where water/organic acids are absorbed)

The abomasum (true stomach) digests protein at PH2

Digested food passes to small intestine

For this I was shown a video where a cow literally had a hole in the side of its stomach just so scientists could see it digest food. Creepy.

We all know how we eat, more or less. Food goes in mouth. Food sits in stomach for a bit. Digested food then passes out as poop. It does get a lil more scientific than that though

Some key words

Ingestion - intake of food into mouth Digestion - breaking down large, insoluble food molecules into smaller, soluble ones using enzymes Egestion -Passing out of undigested food from the anus (pooping) Peristalsis - A series of wave like contractions in the smooth muscle that moves food along digestive tract your digestive system doing a giant macarena and no one can convince me otherwise

I should be drawing these but i’m getting rlly lazy okay

The Gut

Mouth

Ingestion

Digestion of starch

Slightly alkaline

Oesophagus

muscular tube carrying food from mouth to stomach (NOT involved in digestion)

Stomach

Elastic and muscular organs which can expand

Highly folded mucosa

Gastric pits secrete gastric juce

Contraction and relaxation of muscular wall churns food

Digestion of protein

Gastric Juice

Acidit PH2 (HCL - kills bacteria)

Duodenum

Digestion of carbohydrates/fats/proteins

Ileum

Many tiny folds in walls (vili)

Vili have microvili (like knife knife)

Smooth muscle makes them contract and sway for more contact

Network of capillaries transport/absorb nutrients

Digestion of carbohydrate/fat/protein

Colon

Absorption of water

Rectum

Storage of faeces

Anus

Egestion

Gut Wall

Its 3 main layers are the Serosa(circular,longitudinal layers of smooth muscle), Submucosa (connective tissue), and Mucosa (Inner layer)

Digestion

When protein is broken down, two enzymes are needed. Endopeptidase hydrolyse peptide bonds inside the protein, while Exopeptidase hydrolyses bonds on the outside (Endo = in, Exo = out)

Pepsin is used for digestion. It could digest the gut wall itself, so it stays in dormant form and is only activated by H ions

The small intestine has no enzymes, but has bile and pancreatic juice which do have enzymes. It’s slightly alkaline

Gastric Pits have a goblet cell at the top that secretes mucus, the chief cells/zymogen secrete enzymes, and the oxyntic cell secretes HCL

Bile

Made in liver

Stored in gall bladder

Passes through bile duct into duodenum

No enzymes

Bile salts emulsify lipids by lowering surface tension and breaking into small globules

Alkaline

Pancreatic Juice

Secreted by Islet cells

Enters duodenum through pancreatic duct

Enzymes are: Endopeptidases, Trypsinogen, Enterokinase(turns Trypsin into Trypsinogen), Amylase, Lipase

You remember this diagram from GCSE?

Well, here we start talking about Partial pressure and other cool stuff

Partial Pressure - pressure exerted by 1 gas in a mixture of gases Atmospheric pressure at sea level is 100kPa (kilo Pascals - no not the chameleon) In Alveoli, this is 13kPa In tissue this is only 5kPa

The higher the partial pressure, the more blood is saturated

Oxygen is carried around by Haemoglobin (up to 4 oxygen can be carried by 1 haemoglobin)

Loading is when red blood cell has oxygen put into it. Unloading is the opposite.

Haemoglobin has a high affinity for oxygen. This means it is hard to loose the first oxygen, but then the rest follow more easily. When gaining oxygen, the first one is easily gained, but it’s harder for the rest.

I find it easy to picture a jar full of ping pong balls. If you turn the jar upside down, the balls stay in place. It’s always hard to pull the first ball out of the jar, but once that goes, they all fall out. It’s the same with putting balls into the jar. The first one is easy, but it gets harder as more go in.

Factors that affect it

CO2 levels are higher if you do exercise. CO2 dissolves to form acid (which makes it more acidic). More respiration = higher temperature. These all cause Haemoglobin to give up more, or less oxygen

Blood passing through the tissue never looses all its oxygen

Bohr Effect

This is why and how the heart beats

You also need to be able to identify the valves (Just remember Tri means 3 and Bi means 2). Also, the pulmonary and aortic valves can just be called Semilunar Valves

Atrial Distole - Atria fill with blood which begins to flow in ventricles as soon as their walls relax

Atrial Systole - Contraction of Atria pump blood into Ventricles

Ventricular Systole - Contraction of ventricles pump blood into Aorta and pulmonary arteries

You know that blip thing you always get on tv when a person is in hospital? It’s a thing and signals when each of these phases occur: the ECG

A more complicated way of drawing it is this

A - Atrioventricular valves close B - Semilunar valves open C - Semilunar valves close D - Atrioventricular valves open

The heart is made up of cardiac muscle that is myogenic (beats by itself). It doesn’t need to receive impulses from nerves to make it contract, but it does.

Sinoatrial Node initiates 

This signal travels across Atria (cause it contractions) to the AV Node

A slight delay (to allow valves to close), then AV node then causes ventricles to contract

Or to put it in a science paragraph

Blood is passively filling the Atria. A nerve impulse travels to he Sinoatrial Node. During Atrial Systole, this impulse travels across the Atria to the AV Node, causing contractions in the Atria ans pushing blood into the Ventricles. The impulse travels down the Bundle of His and up the Purkinje Network, where it then, after a small delay, caused the ventricles to contract. This forces blood out of the heart, and the process repeats.

There are 2 types; Open and closed

Open - in direct contact with cells Closed - not in direct contact

Insects

Open system

Hemolymph (insect blood) bathes cells

Circulatory system only carries nutrient and waste (which is why bug blood is icky green as it has no haemoglobin)

Single blood vessel runs along dorsal side of insect

Long tube shaped heart pumps blood at low pressure

Materials exchanged with body cells

Closed systems can be single, or double

Single, it only needs one push to go around the body. Double, it needs one push to go to the lungs, and another push to go around the body.

Earthworms have 

closed systems

Blood has oxygen carrying pigment

Major blood vessels are: Ventral blood vessel (distributes blood), Branches of Ventral (takes blood to structures), Dorsal blood vessel (collects blood)

Have pseudo hearts (lots of em)

Fish

Blood pressure reduced

Limits rate of delivery

Efficient for their level of activity and being cold blooded

Humans

High blood pressure (with all these exams I don’t blame us)

Higher activity rate & have to maintain body temperature

Need rapid delivery

When looking at a heart diagram, it’s the opposite way around. Right becomes left, and left becomes right. (it’s just a confusing thing sigh)

You need to know about these. The Tunica Adventitia is made up of collagen and elastic fibres. The Tunica Media is made of elastic lamellae and muscle. The Tunica Intima is made of endothelium and connective tissue. The lumen is the space in which blood flows through

Capillaries are 1 cell thick which makes them ideal for diffusion.

Veins have a thick Tunica Adventitia, a poorly developed Tunica intima, and their lumen is generally larger than arteries. They also have valves

Vascular Bundles

Roots 

Provides drill structure

Enables plant to push into roots

Xylem is strongest so at centre

Phloem in 4 separate sections

Stem

Xylem on inside (Provides flexible support)

Cambium layer has meristem cells

Leaf

Xylem on top of phloem (only in dioctyledonous plants)

Xylem

Made of 2 types of cell; vessel and tracheids (both dead and have lignin in cell walls - the plant literally kills its own cells to make these)

Transports water/minerals

Provides strength and support

Lignin (waterproofing)is laid out in spiral patterns

Transpiration

Flow of water through the plant

Rate is dependent on

Temperature

Humidity

Air movement

Light

After water enters the root via osmosis, there are 3 ways in which it can move

Apoplast: Between well walls of neighhbouring cells

Symplast: Through plasma membrane/plasmodesmata to cytoplasm

Vacuolar: Through plasma membrane/plasmodesmata to cytoplasm and through vacuoles

As water approaches the xylem, its apoplast pathway is blocked by the epidermis. The suberin forms a band (Casparian strip) around the cells which forces water into the symplast way

There are 3 ways in which people think water moves through the xylem

Cohesion-Tension Theory: Water pulled up due to transpiration flow (Water attracts)

Root pressure: Water is pushed upwards(lower potential in root = higher hydrostatic force in base of xylem pushing it up

Capillarity: Water molecules climb up narrow tubes because they are attracted to the polar molecules of the tube

When it reaches the leaf, It travels through cells to the spongy mesophyll, then into the air spaces and out through the stomata

Uptake of minerals

Plants obtain minerals from the soil

Nitrogen enters as nitrates

Ions move into roots via diffusion/active transport

Ions move across roots in water solution

At endodermis, they are actively transported to bypass Casparian Strip

Translocation

Phloem - transports product of photosynthesis from source (leaf) to sink (root)

Sieve tubes 

Laid end to end

sieve plates holy (end walls)

Mature ones have no nucleus

Cytoplasm pushed to side but kept alive by companion tube

Companion cell

Lots of mitochondria

Very thin cell walls

Nucleus

Connected to sieve tube through plasmodesmata

Evidence  (for something, idk)

They used bugs called Aphids. Well, more specifically they used their stylets (feeding tubes). They let the bug stick it in the plant and then cut it off, leaving the stylet in the plant. Poor bug

Ringing (removing a strip of bark)

Radioactive labeling - CO2 isotope added which was used in photosynthesis, then sap is monitored to see where it goes

Autoradiography - ‘foggy’ film has radioactive isotope (i think they took photos?)

Mass flow Hypothesis - Sucrose is actively transported out and pushed down by hydrostatic pressure where it is acctively transported into the sink. Osmosis moves water

Water is lost through the stomata mainly

Opening

Hydrogen ions leave guard cells

Potassium ions enter

Chloride ions enter

Osmotic pressure increases

Water potential decreases

Water enters

Guard cells become turgid

Closing

Potassium ions leave guard cell

Chloride ions leave

Osmotic pressure decreases

Water potential increases

Water leaves

Guard cells become flaccid

Xerophytes (hot climates)

Thick cuticle

Small leaf (spines)

Curved leaf

Less stomata

Epidermal hairs

Curved, hairy leaves raises humidity so water is taken away more slowly.

Hydrophytes (wet climates)

No cuticle

No stomata

Reduced vascular tissue

Small leaves

Air spaces

You thought that was it? Ahahahahaha-no. I’m just spreading this out over 2 posts

Insects

Inefficient

Open circulatory system

Lungs don’t meed demands

oxygen travels as a gas

Simple tracheal system

At rest, it isn’t in need of oxygen as much. When active, the fluid reduces to increase the surface area to allow for more diffusion.

Humans

Have a small SA-V ratio so need a specialised system (lungs)

Alveoli

Single layer of flattened epithelial cells

Capillaries close

1 cell thick

large SA-V ratio (circular and folded)

Secrete sufactant to prevent collapse

How we breathe

Inspiration -  Intercostal muscles contract, diaphram lowers, rips pull out. This increases volume and lowers pressure, forcing air in.

Expiration -  Intercostal muscles relax, diaphram raises, ribs go back to normal. This increases pressure and lowers volume, forcing air out.

Plants

My notes on this are confused with water transport so i’ll try my best

From what i can tell, the leaf

is thin and flat (large SA)

Spongy mesophyll allows for gases to circulate

Permeated by air spaces

Stomata allows gas exchange

Same adaptations for photosynthesis apply to this?