Topic 2: Cells

2.1.1 Outline the cell theory + 2.1.2 Discuss the evidence for the cell theory.

1. Cells are derived from other cells.

Evidence:

• Observation of mitosis (production of two cells from one cell) and binary fission in prokaryotes)

2. Cells are the smallest unit of life. 

Evidence:

• Organelles isolated from rest of cell cannot survive in the long-term.
• Single-celled unicellular organism CAN survive in isolation in the long-term.

3. Living organisms are composed of cells. 

• Development of light microscope allowing magnification required to observe cells.

Exceptions:

• Living organisms contain extracellular material such as the following:
• Vitreous humour of eye
• Mineral deposits of bones
• Cellulose fibres in plant cell walls

2.1.3 State that unicellular organisms carry out all the functions of life.

G - Growth

H - Homeostasis

R - Response

R - Reproduction

M - Metabolism

N – Nutrition

2.1.4 Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the appropriate SI unit.

Molecules: 1nm                                                       DNA Double Helix: 2nm              

Membrane thickness: 10 nm                              Large virus: 100nm

Bacteria: 1 micrometre                                    Organelles: up to 10 micrometres

Nucleus: 20 micrometres                               Eukaryotic cell: up to 100 micrometres

molecules (1 nm), thickness of membranes (10 nm), viruses (100 nm), bacteria (1 µm),organelles (up

to 10 µm), and most cells (up to 100 µm).

2.1.6 Explain the importance of the surface area to volume ratio as a factor limiting cell size.

• Gas, nutrient, waste exchange is a function of cell surface area
• Heat, waste and gas production is a function of cell volume
• Surface area increases by a square whereas volume increases by a cube.
• Thus, cells cannot exchange substances fast enough for metabolism to occur, and cell divides.

2.1.7 State that multicellular organisms show emergent properties.

• Emergent properties – Whole greater than the sum of the individual parts. 
• Individual cells of a multicellular organisms are ineffective when separated, e.g. if nerve cells are separated, brain does not function properly. 

2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others.

• All cells in a living organism contain the same genome -
• the same genes that could be either expressed or not expressed. Differentiation is when certain genes in the genome of multicellular organisms are expressed, 
• causing cells to become specialized in structure and function. 
• Examples of differentiated cells are nerve cells that have the function of carrying nerve impulses through the central nervous system. 
• What determines what type of cells that cells will differentiate into is the position of that cell relative to other cells, hormones, cell-to-cell signals and chemicals that all have the potential to switch genes in that cell on or off.

2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways.

2.1.10 Outline one therapeutic use of stem cells.

• Stem cells are undifferentiated cells that retain the capacity to divide and the ability to specialize in different ways to produce a large number of identical cells. 
• These stem cells can be used in medical research or treatment of diseases such as leukemia, 
• because the stem cells can be used to form a variety of different tissues to replace damaged cells. 
• For example, the stem cells can be differentiated into bone marrow cells for leukemia patients, 
• so that the cancerous white blood cells can be replaced. 

• Stem cells have/retain capacity to divide;
• can be used to produce cell cultures/large number of identical cells;
• can be used to repair/replace damaged/lost cells/tissue;
• stem cells are undifferentiated/have not yet differentiated/specialized;
• can differentiate/specialize in different ways/ are pluripotent/totipotent;
• can be used to form a variety of different tissues / form organs;
• used in medical research;
• used in treatment of named disease, e.g., leukemia;

2.2.1 Draw and label a diagram of the ultrastructure of E.coli as an example of a prokaryote.

2.3.6 Outline two roles of extracellular components.

• Plant cell wall maintains shape of plant
• Prevents excessive water uptake
• Holds plant upright against force of gravity
• Animal cells secrete glycoproteins that form the extracellular matrix
• extracellular maxtric functions in SAM. 
• S - Support 
• A -Adhesion
• M - Movement

2.4.1 Draw and label a diagram to show the structure of membranes. 

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of the cell membranes.

• Fatty acid tails are hydrophobic and repel water
• Phosphate heads are hydrophilic and attract water
• Phosphate heads immersed in water-based solution due to hydrogen bonding occurring between phosphate and water molecules.
• Fatty acid tails point inwards due to their hydrophobic properties.
• Hydrophobic interactions cause fatty acid tails to remain in contact with each other.
• Results in lipid bilayer.

2.4.3 List the functions of membrane proteins.

• Hormone binding sites
• Immobilized enzymes
• Cellular recognition
• Cell adhesion
• Pumps for active transport
• Channels for passive transport

2.4.4 Define diffusion and osmosis.

• Osmosis is a type of diffusion involving water. 
• Osmosis is the passive movement of water particles across a partially permeable membrane from a lower solute concentration (high concentration of water) to a higher solute concentration (low concentration of water). 
• Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration.
• Passive simply means that no chemical energy such as ATP is required to move substances across the membrane;
• as apposed to active transport which does require chemical energy in the form of ATP to move substances across the membrane.

2.4.5 Explain passive transport across membranes by simple diffusion and facilitated diffusion.

• Diffusion is the movement of particles from a region of high concentration to a region of low concentration
• Passive transport involves kinetic molecular energy from random movements of particles
• This energy used to travel across membrane unassisted, known as simple diffusion
• Passive transport means that kinetic molecular energy from random movements of particles is used for diffusion down a concentration gradient;
• from a region of high concentration to a region of low concentration, 
• through tiny gaps in the membrane. 
• This is possible for small non-polar substances such as steroid hormones, urea, lipids, oxygen and carbon dioxide.
• Water is able to diffuse through the phospholipid bilayer via simple diffusion, despite the hydrophobic fatty acid tails. 
• Facilitated diffusion occurs when molecules are too large or polar to travel through the phospholipid bilayer. 
• Thus, protein channels embedded in the plasma membrane allow large and non-polar substances to diffuse through the membrane via facilitate diffusion. 
• These protein channels are specific and so have binding sites for certain substances. 
• These protein channels can be gated to control the flow of a substance in and out of a cell.

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes.

• Energy from ATP is required to transport substances against the concentration gradient from a region of low concentration to a region of high concentration. This process is termed active transport. Active transport occurs when a molecule or ion attaches to a specific binding site on the carrier protein. This carrier protein then changes shape after reacting with ATP, letting the substance pass through the hydrophilic core, to the other side of the membrane.
• Examples include:
• Glucose absorption across the epithelial cells in the ileum of the small intestine.
• Potassium and sodium pumps that restore the resting potential of a nerve fibre.
• Absorption of potassium and other ions.

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum , Golgi apparatus and plasma membrane.

• Vesicles formed from rER transport proteins to Golgi apparatus;
• these vesicles fuse with membranes of Golgi apparatus;
• proteins are processed as they move through Golgi apparatus;
• (transport) vesicles bud off/leave Golgi apparatus;
• vesicles move through cytoplasm;
• (vesicles) fuse with plasma membrane;
• contents released to outside of cell / exocytosis;
• cells use vesicles to secrete substances such as hormones/digestive
• enzymes/other appropriate example;
• vesicles may contain cell products other than proteins;
• Credit drawings which fully explain the points above. 6 max

2.4.8 Describe how the fluidity of the membrane allows it to change shape,break and re-form during endocytosis and exocytosis.

• endocytosis occurs when a membrane encloses a target particle;
• fluidity of membrane permits movement of membrane;
• membrane sinks inwardly/forms pit/invaginates to enclose particle;
• membrane seals back on itself / edges fuse;
• one membrane layer / two phospholipid layers enclose particle making
• vesicle;
• inner phospholipid layer of (original) membrane becomes outer
• phospholipid layer of vesicle membrane;
• outer phospholipid layer of (original) membrane becomes inner
• phospholipid layer of vesicle membrane;
• vesicle breaks away from membrane/moves into cytoplasm;
• changes in membrane shape require energy;
• specific example of endocytosis (e.g. pinocytosis, phagocytosis);
• Accept any of the above points in an annotated diagram.

2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G 2), mitosis and cytokinesis.

• Interphase is an active period in the life of a cell when many metabolic reactions occur. 
• It consists of three phases: G1, S & G2. During the G1 (Gap 1) phase protein is synthesized and organelles are replicated. Thereafter in the S (Synthesis) phase, DNA replication occurs. 
• Lastly, during G2 (Gap 2), centrioles in animal cells are replicated and mitochondria are replicated in both animal and plant cells. 
• Chloroplasts are replicated in plant cells (animal cells do not contain chloroplasts).

2.5.3 State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase.

• Mitosis is the division of a eukaryotic nucleus into two genetically identical nuclei. 
• Mitosis consists of a sequence of stages: prophase → metaphase → anaphase → telophase. (To gain full marks, three statements have to be given for each phase)

Prophase

1. DNA supercoils and becomes visible under light microscope.

2. Spindle microtubules grow from the poles of the cell from the microtubule organising centre (MTOC) to the chromosomes. (Spindle microtubules separate and pull chromosomes to opposite sides of cell).

3. Nuclear membrane breaks down.

Metaphase

1. Mitotic spindle is formed completely. 

2. Microtubules attach to centromeres.

3. Microtubules move the chromosomes to the equator of the cell.

Anaphase

1. Pairs of sister chromatids separate at the centromere and 

2. are pulled to opposite poles by the microtubules.

3. Microtubules contract

Telophase

1. Nuclear membrane reforms around chromosomes at both poles. 

2. DNA uncoils

3. Mitotic spindle breaks down.

2.5.5 Explain how mitosis produces two genetically identical nuclei.

Diagram coming soon.

2.5.6 State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.