Biology unit 1 - area of study 1 outcome 1

What is the role of enzymes in biological reactions?

Enzymes act as biological catalysts, speeding up chemical reactions without being consumed in the process.

How do enzymes interact with substrates?

Enzymes bind to substrates at their active site, forming an enzyme–substrate complex that lowers activation energy and allows the reaction to occur faster.

What factors can affect enzyme activity?

Temperature, pH, substrate concentration, and enzyme concentration can influence enzyme activity. Extreme conditions can cause denaturation and loss of function.

What happens when an enzyme is denatured?

The shape of its active site changes, so the substrate no longer fits, and the enzyme can no longer catalyse reactions efficiently.

Why are enzymes specific to particular reactions?

The active site’s shape is complementary to only one specific substrate, ensuring that enzymes catalyse only their intended reactions.

What are the two main types of cells?

Prokaryotic and eukaryotic cells.

How do prokaryotic and eukaryotic cells differ structurally?

• Prokaryotes: No nucleus or membrane-bound organelles (e.g., bacteria).

• Eukaryotes: Have a nucleus and membrane-bound organelles (e.g., animal and plant cells).

What are the key differences between bacterial, archaeal, and eukaryotic cells?

• Bacteria: Peptidoglycan cell walls, circular DNA.

• Archaea: Unique membrane lipids, survive extreme environments.

• Eukaryotes: Linear DNA within a nucleus, complex internal structure.

Why are cells considered the basic units of life?

All living organisms are composed of one or more cells, and all cellular functions — metabolism, reproduction, and growth — occur within cells.

What distinguishes unicellular from multicellular organisms?

• Unicellular organisms consist of one cell that performs all life functions.

• Multicellular organisms have many specialised cells that work together.

Why are cells described as the structural and functional units of life?

Because every living organism’s structure and biological processes depend on the activity and cooperation of its cells.

How do cellular processes demonstrate the unity of life?

All cells rely on similar molecular processes — such as DNA replication, protein synthesis, and energy conversion — showing a common origin of life.

What is the function of chloroplasts in plant cells?

Chloroplasts are the site of photosynthesis. They contain chlorophyll, which captures light energy to convert carbon dioxide and water into glucose and oxygen.

What are the structural features of chloroplasts that help in photosynthesis?

They have thylakoid membranes arranged in grana for light-dependent reactions, and a stroma where the light-independent (Calvin cycle) reactions occur.

What is the function of mitochondria?

Mitochondria are the site of cellular respiration, where glucose is broken down to produce ATP, the energy currency of the cell.

How are chloroplasts and mitochondria similar and different?

Both have double membranes, their own DNA, and produce energy.

• Mitochondria: Use glucose to make ATP (respiration).

• Chloroplasts: Use light energy to make glucose (photosynthesis).

What is the function of the nucleus?

The nucleus controls cellular activities by storing DNA, which contains the genetic instructions for protein synthesis.

What is the function of the endoplasmic reticulum (ER)?

• Rough ER: Synthesises and transports proteins.

• Smooth ER: Synthesises lipids and detoxifies substances.

What is the function of the Golgi apparatus?

It modifies, packages, and distributes proteins and lipids for secretion or use within the cell.

What are some unique features of plant cells compared to animal cells?

• Plant cells have a cell wall, chloroplasts, and a large central vacuole.

• Animal cells have centrioles and lysosomes but no cell wall or chloroplasts.

What is the structure of the plasma membrane?

It is a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates, forming a fluid mosaic model that regulates entry and exit of substances.

What is the role of phospholipids in the plasma membrane?

Phospholipids have hydrophilic heads and hydrophobic tails, creating a barrier that allows only certain molecules to pass through.

What role does cholesterol play in the plasma membrane?

Cholesterol helps maintain membrane stability and fluidity, especially during temperature changes.

What is osmosis?

Osmosis is the passive movement of water molecules across a semi-permeable membrane from a region of low solute concentration to high solute concentration.

What is facilitated diffusion?

The passive movement of molecules across the membrane via channel or carrier proteins, down their concentration gradient.

What is active transport?

The movement of molecules against their concentration gradient using energy (ATP) and specific transport proteins.

How does the plasma membrane selectively allow substances to pass?

It is semi-permeable, allowing small nonpolar molecules (e.g. O₂, CO₂) to diffuse easily, while hydrophilic substances require proteins or channels to pass through.

What is the difference between passive and active transport?

• Passive transport (osmosis, diffusion, facilitated diffusion): No energy required; moves down concentration gradient.

• Active transport: Requires energy (ATP); moves against concentration gradient.

What is binary fission?

Binary fission is a simple form of asexual reproduction in prokaryotic cells, where one cell divides into two genetically identical daughter cells.

What are the main steps of binary fission?

1. DNA replication – circular DNA is copied.

2. Chromosome segregation – DNA attaches to the plasma membrane and separates.

3. Elongation – cell grows and DNA moves to opposite poles.

4. Cytokinesis – new cell wall and membrane form, producing two identical cells.

Why does binary fission enable rapid population growth?

It is fast and simple, as it doesn’t involve mitotic stages or sexual reproduction — ideal for prokaryotes in stable environments.

How is binary fission different from mitosis?

• Occurs in: Prokaryotes (binary fission) vs. Eukaryotes (mitosis).

• DNA type: Single circular DNA vs. linear chromosomes.

• Organelles: Absent in prokaryotes.

• Speed: Binary fission is faster.

Why is binary fission advantageous in certain environments?

It allows rapid adaptation and colonisation when conditions are favourable

What are the main stages of the eukaryotic cell cycle?

1. Interphase – cell growth and DNA replication (G₁, S, G₂).

2. Mitotic (M) phase – mitosis (nuclear division) and cytokinesis (cytoplasmic division).

What happens in each sub-phase of mitosis?

• Prophase: Chromosomes condense; spindle forms.

• Metaphase: Chromosomes line up at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear membranes reform; chromosomes uncoil.

What happens during cytokinesis?

• Animal cells: The membrane pinches inward to form a cleavage furrow.

• Plant cells: A cell plate forms between the two nuclei, which becomes the new cell wall.

What is the purpose of mitosis and cytokinesis?

To produce two genetically identical diploid cells for growth, repair, and maintenance in multicellular organisms.

How do checkpoints regulate the cell cycle?

They monitor DNA damage, replication accuracy, and spindle attachment to ensure cells only divide when conditions are correct.

What is apoptosis?

Apoptosis is a controlled, programmed process of cell death that removes old, damaged, or unnecessary cells to maintain tissue health.

What are the key stages of apoptosis?

• Cell shrinkage and DNA fragmentation.

• Blebbing of the cell membrane.

• Formation of apoptotic bodies.

• Phagocytosis by neighbouring cells or immune cells.

What enzymes are involved in apoptosis?

Caspases – a family of protease enzymes that trigger the breakdown of cellular components during apoptosis.

Why is apoptosis important?

• Removes damaged or infected cells.

• Shapes tissues during embryonic development (e.g., removal of webbing between fingers).

• Prevents cancerous growth by eliminating abnormal cells.

What happens if apoptosis is disrupted?

• Too little apoptosis: Uncontrolled cell growth → cancer.

• Too much apoptosis: Tissue damage or degenerative diseases.

What are stem cells?

Stem cells are unspecialised cells that can differentiate into different cell types and have the ability to self-renew through continuous division.

What are the two main properties of stem cells?

1. Differentiation – ability to become specialised cells (e.g., nerve, blood, muscle).

2. Self-renewal – ability to divide and produce more stem cells.

What is totipotency?

Totipotent stem cells can develop into any cell type, including embryonic and extraembryonic (placental) cells.

Example: the zygote and cells of the early embryo (first few divisions).

What is pluripotency?

Pluripotent stem cells can develop into any cell type within the body (but not placental tissues).

Example: embryonic stem cells from the blastocyst.

What is the difference between totipotent and pluripotent stem cells?

• Totipotent → can form all body and placental cells.

• Pluripotent → can form all body cells but not placental cells.

What are multipotent stem cells?

Multipotent stem cells can differentiate into a limited range of related cell types.

Adult (somatic) stem cells like bone marrow stem cells that form red and white blood cells.

Where are stem cells found?

• Embryonic stem cells: in the early embryo (pluripotent).

• Adult stem cells: in tissues like bone marrow, skin, and the intestine (multipotent).

How are stem cells used in medicine?

• Regenerative medicine: replacing or repairing damaged tissue (e.g., spinal cord injuries, burns).

• Treatment of disease: e.g., bone marrow transplants for leukaemia.

Why is stem cell research important?

It helps scientists understand development, cell differentiation, and genetic diseases, and offers potential for organ regeneration and drug testing.

What are the ethical concerns around stem cell use?

The use of embryonic stem cells raises ethical debates about the destruction of embryos versus the potential for life-saving medical treatments.