11 Stage 1 Biology B | EXAM: Revision

Cells & Microorganisms, Biodiversity & Ecosystems

MRS GREN

The seven fundamental life processes carried out by living organisms: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.

Three tenets of Cell Theory

1. All living organisms are composed of one or more cells.

2. Cells are the smallest structural and functional units of life.

3. All cells arise from pre-existing cells.

Unicellular vs. multicellular organisms

Unicellular organisms consist of a single cell that carries out all life functions (e.g., bacteria, amoeba, paramecium, euglena, yeast, phytoplankton), whereas multicellular organisms consist of many coordinated cells that are often specialised (e.g., plants, animals, mushrooms).

Prokaryotic vs. eukaryotic cell structure

Prokaryotes lack a membrane-bound nucleus and organelles, possessing a single circular chromosome in the cytoplasm. Eukaryotes contain a membrane-bound nucleus with linear chromosomes and specialised membrane-bound organelles.

Organelles unique to plant cells compared to animal cells

A cellulose cell wall, chloroplasts for photosynthesis, and a large permanent vacuole.

Histones

Proteins around which DNA winds tightly to form chromosomes, allowing long strands of genetic material to fit compactly within the eukaryotic nucleus.

Fluid-mosaic model

A model of the cell membrane describing a flexible phospholipid bilayer ('fluid') embedded with laterally moving proteins, glycoproteins, and cholesterol ('mosaic').

Components of the phospholipid bilayer

Hydrophilic (water-attracting) phosphate heads facing the outward and inward aqueous environments, and hydrophobic (water-repelling) fatty acid tails facing the interior of the membrane.

Role of cholesterol in cell membranes

Maintains membrane fluidity and physical stability by preventing it from becoming too rigid in cold conditions or overly fluid in warm conditions.

Transmembrane vs. peripheral membrane proteins

Transmembrane proteins span the entire bilayer to transport substances or receive signals, while peripheral proteins attach temporarily to one surface to provide structural support or participate in signalling pathways.

Diffusion

The passive net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.

Osmosis

The net passive movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

Facilitated diffusion

Passive transport of substances (such as glucose or charged ions) down their concentration gradient using specific channel or carrier proteins.

Active transport

The movement of substances across a cell membrane against their concentration gradient requiring energy input in the form of $ATP$.

Surface area to volume ($SA:V$) ratio constraint

As cells grow larger, volume increases much faster than surface area, decreasing the $SA:V$ ratio and reducing material exchange efficiency; cells must divide to maintain a high ratio.

Endocytosis vs. exocytosis

Endocytosis engulfs large external substances into vesicles to bring them into the cell; exocytosis fuses internal vesicles with the membrane to release substances outside.

Mitosis vs. cytokinesis

Mitosis is the division of the cell nucleus to separate duplicated chromosomes, whereas cytokinesis is the physical division of the cytoplasm to yield two distinct daughter cells.

Binary fission

A form of asexual reproduction in prokaryotes where the circular chromosome replicates and the cell divides into two genetically identical cells without nuclear division.

Stages of mitosis in sequential order

1. Prophase (chromatin condenses, spindle forms, envelope breaks down)

2. Metaphase (chromosomes align at equator)

3. Anaphase (sister chromatids separate)

4. Telophase (nuclear envelopes reform, chromosomes decondense)

Diploid vs. haploid cells

Diploid cells ($2n$) carry two complete sets of homologous chromosomes (somatic cells), while haploid cells ($n$) contain only one set (gametes like sperm and ova) so fertilisation restores the diploid state.

Karyotype and Trisomy 21

A karyotype is an organised visual arrangement of homologous chromosome pairs used to detect abnormalities; Trisomy 21 is the presence of three copies of chromosome 21, resulting in Down syndrome.

Aerobic respiration word equation

$$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy (ATP)}$$

Fermentation (anaerobic respiration)

The incomplete breakdown of glucose without oxygen, yielding far less $ATP$; examples include lactic acid fermentation in animal muscle cells and alcohol fermentation in yeast producing ethanol and $\text{CO}_2$.

Photosynthesis word equation

$\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$ (powered by light energy captured by chlorophyll).

Autotrophs vs. heterotrophs vs. chemo-autotrophs

Autotrophs synthesise their own organic compounds using light energy; heterotrophs ingest other organisms for organic nutrients; chemo-autotrophs produce organic compounds using energy from inorganic chemical reactions.

Metabolic wastes vs. faeces

Metabolic wastes (such as $\text{CO}_2$ from respiration and urea from protein deamination) are byproducts of cellular chemical reactions removed by excretion; faeces is unabsorbed, undigested food and gut microbes.

Plasmids and recombinant DNA technology

Plasmids are small, circular accessory loops of DNA in bacteria that can replicate independently and are used as vectors to insert foreign genes for gene cloning and biotechnology.

Ecological roles of phytoplankton and decomposers

Phytoplankton carry out aquatic photosynthesis and produce a major portion of Earth's oxygen; decomposers break down organic remains to recycle nutrients back into the abiotic environment.

Principles of food preservation

Techniques that inhibit microbial growth: chilling (lowers enzyme activity), heating (denatures enzymes and kills cells), drying or adding salt/sugar (removes available water via osmotic draw), and acidification (lowers pH).

Four key food safety practices

Clean, separate, cook, and chill.

Mechanism and targets of antibiotics

Substances that selectively inhibit bacterial cell walls, protein synthesis, or reproduction; they do not kill viruses because viruses lack cellular structures and metabolic pathways.

Spread of antibiotic resistance

Occurs via random mutations or horizontal plasmid exchange; resistant bacteria survive antibiotic exposure and reproduce rapidly, dominating the population.

Biotic vs. abiotic factors

Biotic factors are living components of an ecosystem (predators, competitors, prey); abiotic factors are non-living physical and chemical components (temperature, sunlight, moisture, salinity, pH).

Energy loss across trophic levels

Only a small fraction of energy transfers between levels because most is lost as metabolic heat during cellular respiration, unconsumed material, or undigested waste.

Zonation vs. stratification

Zonation is the horizontal spatial change of communities across an environmental gradient (e.g., intertidal rock platforms); stratification is the vertical layering of a habitat (e.g., rainforest canopy, understorey, and forest floor).

Three rock platform intertidal zones

1. Supra-littoral (splash zone: high desiccation, temperature swings)
2. Intertidal (tidal changes: wave action, alternating immersion/drying)
3. Sub-littoral (submerged: stable temperatures, reduced light at depth)

Nitrogen cycle key bacterial steps

1. Nitrogen fixation: converts inert atmospheric $\text{N}_2$ into ammonium or nitrates.

2. Ammonification: decomposes organic nitrogen into ammonium.

3. Denitrification: converts soil nitrates back into atmospheric $\text{N}_2$.

Phosphorus cycle characteristics

Recycles phosphorus essential for nucleic acids, $ATP$, and phospholipids; unlike other cycles, it lacks a significant atmospheric gaseous phase and relies on rock weathering and soil uptake.

Human disruptions to biogeochemical cycles

Fossil fuel burning and deforestation increase atmospheric $\text{CO}_2$; synthetic fertilisers and sewage runoffs cause nutrient overloading (eutrophication) in nitrogen and phosphorus cycles.

Primary vs. secondary ecological succession

Primary succession begins on newly exposed substrate lacking soil (colonised by pioneer species like lichens); secondary succession occurs after a disturbance where soil is already present.

Climax community

The stable, mature ecological community that emerges at the end stage of ecological succession in dynamic equilibrium with environmental conditions.

How do biologists decide whether something is living?

Biologists classify something as living if it carries out the fundamental life processes

Why is the cell theory considered a unifying idea in biology?

It explains that all living organisms are made of cells

What does it mean to say that cells are the structural and functional units of life?

Cells make up the bodies of living organisms and they also perform the basic processes needed for life.

Why is the cell membrane described as defining the cell?

It separates the cell's internal environment from the external environment and controls exchange across that boundary.

Why are unicellular organisms still considered fully living organisms?

Because a single cell can still carry out all life processes

What is the key difference between unicellular and multicellular organisms?

Unicellular organisms are made of one cell

Why are bacteria classified as prokaryotes?

They lack a nucleus and membrane-bound organelles

What features do all cells share

regardless of whether they are prokaryotic or eukaryotic?

How is DNA organised in a prokaryotic cell?

Usually as a single circular chromosome in the cytoplasm

How is DNA organised in a eukaryotic cell?

It is contained in a nucleus and arranged into multiple linear chromosomes.

Why are prokaryotic cells generally smaller than eukaryotic cells?

Their structure is simpler

Why are membrane-bound organelles important in eukaryotic cells?

They compartmentalise different functions

What is the main structural difference between plant and animal cells?

Plant cells have a cell wall

What is the role of histones in cells?

Histones are proteins that help DNA pack tightly into chromosomes so it fits inside the nucleus.

Why is DNA packaging necessary in multicellular organisms?

Because the DNA molecule is extremely long

What is heredity?

Heredity is the passing of characteristics from parents to offspring through DNA.

Why is DNA essential for cell function?

It contains the instructions for making proteins and for cell division.

What is the main role of the cell membrane in maintaining homeostasis?

It controls the movement of substances in and out of the cell

What does selectively permeable mean in relation to the cell membrane?

It means the membrane allows some substances to cross more easily than others.

How does the phospholipid bilayer contribute to selective permeability?

The hydrophilic heads and hydrophobic tails create a barrier that limits which substances can move freely through the membrane.

What is the role of transmembrane proteins?

They span the membrane and help transport substances or act as receptors for signals.

What is the role of peripheral proteins?

They are attached to one side of the membrane and help with support

Why is cholesterol important in the cell membrane?

It helps maintain membrane stability and controls how fluid or rigid the membrane is.

What does the fluid-mosaic model explain?

It explains that the membrane is fluid because components can move sideways

Why can oxygen and carbon dioxide pass directly through the membrane?

They are small

Why do ions usually need membrane proteins to cross the cell membrane?

Because ions are charged and cannot easily pass through the hydrophobic centre of the membrane.

What is the difference between passive transport and active transport?

Passive transport does not require energy and moves substances down a concentration gradient

What is diffusion?

The net movement of particles from a region of high concentration to a region of low concentration.

What is osmosis?

The movement of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

What is facilitated diffusion?

The passive movement of substances across a membrane using a transport protein.

Why do glucose and some ions need facilitated diffusion?

They are too large

What determines how fast materials cross the cell membrane?

Surface area to volume ratio

Why do larger cells struggle to exchange materials efficiently?

As cells grow

Why is a high surface area to volume ratio important for cell survival?

It allows materials to enter and leave the cell fast enough to meet metabolic demands.

Why do cells divide rather than just keep growing?

They need to maintain a high surface area to volume ratio so exchange of materials stays efficient.

What is the function of endocytosis?

It brings large substances into the cell by forming a vesicle from the membrane.

What is the function of exocytosis?

It moves large substances out of the cell when vesicles fuse with the cell membrane.

Why are endocytosis and exocytosis important for cells?

They allow movement of large particles that cannot cross the membrane by diffusion or transport proteins.

What is the purpose of cell division in multicellular organisms?

To allow growth

Why is cell division essential for unicellular organisms?

It is the method by which they reproduce asexually.

What must happen before a cell can divide successfully?

Its DNA must be replicated so each new cell receives a complete copy.

What is the role of mitosis?

It separates duplicated chromosomes in the nucleus to produce genetically identical nuclei.

What is the role of cytokinesis?

It divides the cytoplasm to form two separate daughter cells.

What is the key result of mitosis and cytokinesis together?

They produce two genetically identical daughter cells.

How is binary fission different from mitosis?

Binary fission occurs in prokaryotes and is simpler

Why can binary fission happen so quickly in bacteria?

Bacteria have simple structures

What is one major disadvantage of binary fission?

It produces little genetic variation

What is the advantage of genetically identical daughter cells in mitosis?

They maintain the same genetic information needed for body growth and repair.

What happens during prophase of mitosis?

Chromosomes condense

What happens during metaphase of mitosis?

Chromosomes line up at the equator of the spindle.

What happens during anaphase of mitosis?

Sister chromatids separate and move to opposite poles of the cell.

What happens during telophase of mitosis?

New nuclear membranes form and the chromosomes decondense.

Why is mitosis described as a form of asexual reproduction in some eukaryotes?

It can produce offspring that are genetically identical to the parent cell without gamete fusion.

What is a diploid cell?

A cell that contains a full set of chromosomes arranged in homologous pairs.

What is a haploid cell?

A cell that contains one chromosome from each homologous pair.

Why are sperm and ova haploid?

So that when they fuse during fertilisation

What is a karyotype used for?

It is used to observe chromosome number and structure

What is trisomy 21?

A condition where a person has three copies of chromosome 21

Why do living cells require energy?

They need energy for movement