Biology Exam Rev

Nucleus description and function

Nucleus: large, double-membrane-bound organelle containing the cell's DNA;

Nucleolus is a dense, non-membrane-bound region inside the nucleus.

• Function: The nucleus acts as the cell’s control centre, storing genetic info and directing protein synthesis. The nucleolus is specifically responsible for assembling ribosomes.

Mitochondrion description and function:

Description: A double-membrane organelle with a highly folded inner membrane (cristae).

Function: Known as the "powerhouse of the cell," it generates energy by converting glucose into adenosine triphosphate (ATP) through cellular respiration.

Chloroplast description and function:

Description: A double-membrane organelle found in plant cells and algae, containing green chlorophyll pigments arranged in stacks (thylakoids).

Function: Captures light energy to perform photosynthesis, converting solar energy, water, and carbon dioxide into glucose.

Cytoskeleton description and function:

Description: A dynamic network of protein filaments (microtubules, microfilaments, and intermediate filaments) extending throughout the cell’s cytoplasm

Function: Provides structural support, maintains cell shape, and enables intracellular transport and cell movement.

Difference to Cytoplasm: The cytoplasm is the entire gel-like fluid and material inside the cell membrane (excluding the nucleus). The cytoskeleton is the structural "scaffolding" made of proteins that sits within that fluid.

Cytoplasm description and function:

Description: The entire contents of a cell contained within the cell membrane, consisting of the cytosol (a thick, gel-like fluid), organelles, but excluding the nucleus in eukaryotic cells.

Function: Suspends and protects organelles, is a medium for biochemical reactions and helps transport of materials throughout the cell.

Vesicles description and function:

Description: Small membrane-bound sacs.

Function: Used for transporting and storing substances (like proteins and lipids) between organelles and in/out of the cell.

Rough ER description and function:

Description: A network of folded membranes studded with ribosomes on its outer surface.

Function: Folds, modifies, and transports proteins made by the attached ribosomes.

Smooth ER description and function:

Description: A tubular membrane network that lacks attached ribosomes.

Function: Responsible for lipid synthesis, carbohydrate metabolism, and the detoxification of drugs and poisons.

Golgi body/golgi apparatus description and function:

Description: A stack of flattened, unlinked membrane sacs (cisternae).

Function: Acts as the shipping and sorting centre; it modifies and packages proteins and lipids from the ER before directing them to their destinations.

Vacuole description and function:

Description: A membrane-bound storage sac. Plant cells typically have one large central vacuole, while animal cells have multiple smaller ones.

Function: Stores water, nutrients, and waste. In plants, the large central vacuole maintains turgor pressure to keep the plant upright.

Cell membrane description and function:

Description: A semi-permeable phospholipid bilayer embedded with proteins and cholesterol.

Function: Regulates what enters and exits the cell, providing protection and allowing communication with the external environment.

Cell wall description and function:

Description: A rigid outer layer surrounding the cell membrane, made of cellulose (in plants), chitin (in fungi), or peptidoglycan (in bacteria).

Function: Provides structural strength, rigidity, and protection, preventing the cell from bursting.

Ribosomes description and function:

Description: Tiny, non-membrane-bound structures made of ribosomal RNA (rRNA) and proteins, they can be found floating freely in the cytoplasm or attached to the Rough ER.

Function: Serve as the site of protein synthesis, where they translate genetic codes from messenger RNA (mRNA) into amino acid chains to build proteins.

Explain how cells are the basic unit of life

Cells are the basic unit of life because they are the smallest independent, self-sustaining units capable of performing life processes. In multicellular organisms, cells exhibit biological organisation: similar cells work together to form tissues, tissues group together to create organs, organs cooperate within body systems, and these systems ultimately form a complete individual.

Distinguish between prokaryotic and eukaryotic cells

• Prokaryotic cells are simple, single-celled organisms that lack a nucleus and membrane-bound organelles, with their DNA floating freely in the cytoplasm (e.g., bacteria and archaea).

• Eukaryotic cells can be unicellular or multicellular and contain a distinct nucleus and membrane-bound organelles like mitochondria and chloroplasts (e.g., plants, animals, fungi, and protists).

• Eukaryotic cells are usually bigger and more complex than prokaryotic.

Describe the difference between organelles in animal and plant cells

Plant Cells Only:

• Rigid cell wall (for structural support)

• Chloroplasts (to make food via photosynthesis).

• Singular large central vacuole to maintain water pressure and cell shape. Their shape is fixed and rectangular.

Animal Cells Only:

• Centrioles (for cell division)

• Lysosomes (for digesting waste).

• Multiple small, temporary vacuoles instead of a central one.

• Their shape is flexible and irregular/round.

Justify the type and relative number of organelles based on the function of different cell types

• Muscle Cells: Have plentiful mitochondria to generate the massive amounts of energy (ATP) needed for constant contraction.

• Pancreas Cells: Have extensive Rough ER and Golgi bodies to continuously synthesise, package, and secrete digestive proteins (enzymes).

• White Blood Cells: Have high numbers of lysosomes packed with digestive enzymes to destroy engulfed bacteria and viruses.

• Red Blood Cells: Have zero organelles (no nucleus/mitochondria) to maximise internal space for carrying oxygen payload.

• Leaf vs. Root Cells: Leaf cells have abundant chloroplasts to capture sunlight for photosynthesis, while underground root cells have none because they live in total darkness.

What is the step-by-step pathway of protein exocytosis in cells and key idea behind this

Key idea: Organelles don’t work by themselves, they form interconnected systems to achieve complex cellular process such as protein exocytosis.

Nucleus: sends DNA instructions (as mRNA) out into the cell

Ribosomes read instructions and build protein in the rough ER

Vesicles pinch off the ER and carry the unfinished protein to the Golgi

Golgi modifies, sorts and tags the protein with a destination

Another vesicle pinches off the Golgi, moves to the cell membrane, fuses with it and releases protein outside of cell.

Define DNA:

Deoxyribonucleic acid: The double-helix (stranded) nucleic acid that stores genetic information, codes for proteins, and serves as the hereditary material passed across generations.

What is DNA made of (in terms of subunits)

• Repeating subunits called nucleotides.

• Each nucleotide has 3 parts:

  1. A sugar molecule (deoxyribose)

  2. A phosphate molecule

  3. A nitrogenous base (A, T, C, or G)

How do nucleotides link together

• They connect in a 5’ (five-prime) to 3’ (three-prime) direction.

• The phosphate sits at the 5' end.

• The free OH group sits at the 3' end, acting as the "landing pad" where the next nucleotide must attach.

Name the four types of nitrogenous bases and which ones they bind to:

Adenine (A) and Guanine (G) are types of Purines

Thymine (T) and Cytosine (C) are types of pyramidines

A - T and C - G

Define gene:

• A gene is the basic physical and functional unit of heredity.

• Structure: it’s a specific sequence of nucleotides along a segment of DNA.

• Function: It contains the coded instructions needed to build a specific molecule, usually a protein, which determines an organism's traits.

Define allele:

Versions of the same gene. e.g. if a gene was eye colour, the alleles would be green, brown etc.

How does variation in nucleotide sequences lead to variation in genes?

• A gene is a specific sequence of nucleotide bases (A, T, C, G).

• Changing this sequence creates alternative versions of that gene - alleles.

• This alter the instructions for building proteins by swapping amino acids, changing the structure, function and type of protein

Define chromosome:

• A chromosome is a tightly coiled thread of DNA (2m) wrapped around packaging proteins (histones)

• Acts as a structure that houses and organises an organism's genes.

• Humans have 46 chromosomes (23 pairs) inside the nucleus of almost every cell.

• A single DNA molecule forms a single chromosome.

Largest to smallest: nucleus, cells, gene, chromosome, DNA

Largest —> smallest: cell, nucleus, chromosome, DNA, gene