B1: Cell Level Systems

What you'll learn

This module covers the fundamental organisation of cells and how they function as the basic units of life. You'll explore cell structures, enzyme function, movement of substances across membranes, and microscopy techniques. These concepts form the foundation for understanding all biological systems and appear frequently across OCR GCSE Biology papers.

Key terms and definitions

Eukaryotic cell — A cell containing a nucleus and membrane-bound organelles; found in animals, plants, fungi and protists.

Prokaryotic cell — A cell lacking a nucleus and membrane-bound organelles; genetic material is free in the cytoplasm; bacteria are prokaryotes.

Enzyme — A biological catalyst that speeds up the rate of chemical reactions without being used up; made of protein with a specific active site.

Diffusion — The net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient; a passive process requiring no energy.

Osmosis — The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Active transport — The movement of substances against a concentration gradient (from low to high concentration) using energy from respiration.

Magnification — How many times larger an image appears compared to the actual object size.

Resolution — The ability to distinguish between two separate points; the level of detail visible in an image.

Core concepts

Cell structures and functions

All living organisms are made of cells. Animal and plant cells are eukaryotic, containing distinct structures called organelles.

Animal cell structures:

• Nucleus — contains genetic material (DNA) organised into chromosomes; controls cell activities
• Cytoplasm — gel-like substance where most chemical reactions occur; contains enzymes
• Cell membrane — controls what enters and leaves the cell; partially permeable
• Mitochondria — site of aerobic respiration; releases energy from glucose
• Ribosomes — site of protein synthesis; found on rough endoplasmic reticulum or free in cytoplasm

Additional plant cell structures:

• Cell wall — made of cellulose; provides structural support; fully permeable
• Permanent vacuole — contains cell sap (solution of sugars and salts); maintains internal pressure
• Chloroplasts — contain chlorophyll; site of photosynthesis; absorb light energy

Bacterial cells are prokaryotic and much smaller than eukaryotic cells (typically 0.2-2.0 µm vs 10-100 µm).

Bacterial cell structures:

• Chromosomal DNA — single loop of DNA in cytoplasm; not enclosed in a nucleus
• Plasmids — small rings of extra DNA; can be transferred between bacteria
• Cell membrane — controls substance movement
• Cell wall — different composition to plant cell walls
• Flagella — long protein strands used for movement (not present in all bacteria)

Some bacteria have a slime capsule outside the cell wall for protection.

Specialised cells

Cells become specialised through a process called differentiation to carry out specific functions. The structure of each specialised cell relates directly to its function.

Examples of specialised animal cells:

Sperm cells — adapted for fertilisation

• Streamlined head with acrosome containing enzymes to penetrate egg
• Many mitochondria in middle section provide energy for tail movement
• Long tail (flagellum) for swimming to egg

Nerve cells (neurones) — adapted for transmitting electrical impulses

• Long axon carries impulses over long distances
• Branched dendrites increase connections to other nerve cells
• Myelin sheath insulates axon and speeds up transmission

Muscle cells — adapted for contraction

• Contain protein filaments that slide over each other to shorten cell length
• Many mitochondria provide energy for contraction
• Can store glycogen as energy reserve

Examples of specialised plant cells:

Root hair cells — adapted for water and mineral absorption

• Long extension increases surface area for absorption
• Thin cell wall reduces diffusion distance
• Many mitochondria for active transport of mineral ions

Xylem cells — adapted for water transport

• Cells die and form hollow tubes allowing continuous water columns
• Walls strengthened with lignin to withstand pressure
• No end walls between cells for uninterrupted flow

Phloem cells — adapted for transporting dissolved sugars

• Sieve tube elements have perforated end walls forming sieve plates
• Companion cells provide energy for active transport in sieve tubes
• Few organelles in sieve tubes to allow easier flow

Enzymes and digestion

Enzymes are biological catalysts that control the rate of metabolic reactions. Each enzyme has a specific active site with a complementary shape to its substrate (the molecule it acts upon).

Lock and key model:

1. Substrate fits into active site like a key into a lock
2. Enzyme-substrate complex forms
3. Reaction occurs and products are released
4. Enzyme remains unchanged and can be reused

Factors affecting enzyme activity:

Temperature:

• Increasing temperature increases kinetic energy and collision rate between enzyme and substrate
• Rate of reaction increases up to the optimum temperature (typically 37°C for human enzymes)
• Above optimum temperature, enzyme denatures — active site shape changes permanently
• Substrate no longer fits; enzyme cannot function

pH:

• Each enzyme has an optimum pH where it works most effectively
• Different enzymes have different optimum pH values (e.g., pepsin works best at pH 2; amylase at pH 7)
• pH values too high or too low cause denaturation
• Hydrogen ions disrupt bonds maintaining the active site shape

Substrate concentration:

• More substrate molecules increase collision frequency with enzyme active sites
• Rate increases until all active sites are occupied
• Further increases in substrate concentration have no effect — enzyme concentration becomes limiting factor

Digestive enzymes:

• Carbohydrases break down carbohydrates to simple sugars

  • Amylase (made in salivary glands and pancreas) breaks down starch to maltose

• Proteases break down proteins to amino acids

  • Pepsin (made in stomach) works in acidic conditions
  • Trypsin (made in pancreas) works in alkaline conditions

• Lipases break down lipids (fats) to glycerol and fatty acids

  • Made in pancreas; work in small intestine

Bile (produced in liver, stored in gall bladder) neutralises stomach acid and emulsifies fats, increasing surface area for lipase action.

Transport across membranes

Substances move into and out of cells through the cell membrane by three main processes.

Diffusion is a passive process requiring no energy input from the cell. The rate of diffusion depends on:

• Concentration gradient — greater difference = faster diffusion
• Temperature — higher temperature = faster particle movement
• Surface area — larger area = more diffusion
• Diffusion distance — shorter distance = faster diffusion

Examples: oxygen diffuses into cells for respiration; carbon dioxide diffuses out as a waste product; urea diffuses from cells into blood plasma.

Osmosis is the diffusion of water across a partially permeable membrane. Water moves from dilute solutions (high water potential) to concentrated solutions (low water potential).

Effects on plant cells:

• In pure water or dilute solution, water enters by osmosis; cell becomes turgid (swollen and firm) — cell wall prevents bursting
• In concentrated solution, water leaves by osmosis; cell becomes plasmolysed (cytoplasm shrinks away from cell wall)
• Turgidity provides support in non-woody plant tissues

Effects on animal cells:

• In pure water, cells absorb water and may burst (lyse) — no cell wall for protection
• In concentrated solution, cells lose water and shrivel (crenation)
• Cells function best in isotonic solutions

Active transport moves substances against their concentration gradient using energy from respiration. This allows cells to absorb substances even when concentrations inside are higher than outside.

Examples:

• Root hair cells absorb mineral ions from dilute soil solutions
• Small intestine absorbs glucose from gut when gut concentration is lower than blood concentration
• Nerve cells pump ions to maintain electrical potential

Microscopy and cell observation

Light microscopes use light and lenses to magnify specimens up to approximately ×1500. Maximum resolution is about 200 nm (0.2 µm).

Using a light microscope:

1. Place slide on stage and secure with clips
2. Select lowest power objective lens
3. Use coarse focus to move stage close to objective lens
4. Look through eyepiece and use coarse focus to move stage away until image appears
5. Adjust fine focus for clear image
6. Increase magnification by rotating to higher power objective lens
7. Refocus using fine focus only

Electron microscopes use beams of electrons instead of light, achieving much higher magnification (up to ×2,000,000) and resolution (0.2 nm).

Two types:

• Transmission electron microscope (TEM) — electrons pass through very thin specimens; produces flat 2D images
• Scanning electron microscope (SEM) — electrons bounce off specimen surface; produces 3D images

Electron microscopes revealed internal cell structures including mitochondria and ribosomes.

Magnification calculations:

Magnification = Image size ÷ Actual size

Rearranged: Image size = Magnification × Actual size

Rearranged: Actual size = Image size ÷ Magnification

Remember the triangle: I over A M (Image over Actual × Magnification)

Always convert units to the same measurement before calculating (typically micrometres, µm).

Unit conversions:

• 1 mm = 1000 µm
• 1 µm = 1000 nm
• 1 m = 1,000,000 µm

Cell division and organisation

Cells in multicellular organisms are organised into hierarchical levels:

Cell → Tissue → Organ → Organ system → Organism

• Tissue — group of similar cells working together (e.g., muscle tissue, epithelial tissue)
• Organ — group of different tissues working together (e.g., stomach, leaf)
• Organ system — group of organs working together (e.g., digestive system, respiratory system)

The cell cycle is the series of stages cells go through as they grow and divide.

Interphase (growth and DNA replication):

• Cell grows and increases subcellular structures (mitochondria, ribosomes)
• DNA replicates to form two copies of each chromosome
• Cells spend most of their time in this phase

Mitosis (nuclear division):

• Chromosomes line up along cell centre
• Cell fibres pull each chromosome copy to opposite poles
• Nucleus divides into two identical nuclei

Cytokinesis (cell division):

• Cytoplasm and cell membrane divide
• Two identical daughter cells form

Mitosis is important for growth, repair, and asexual reproduction. Daughter cells are genetically identical to parent cell and to each other.

Worked examples

Example 1: A student observes a bacterial cell under an electron microscope. The image measures 45 mm in length. The actual length of the bacterial cell is 1.5 µm. Calculate the magnification. (3 marks)

Answer:

• Convert image size to same units: 45 mm = 45,000 µm (1 mark)
• Magnification = Image size ÷ Actual size (1 mark)
• Magnification = 45,000 ÷ 1.5 = ×30,000 (1 mark)

Example 2: Explain why muscle cells contain many mitochondria. (2 marks)

Answer:

• Mitochondria are the site of aerobic respiration (1 mark)
• Muscle cells require lots of energy for contraction/movement (1 mark)

Examiner note: Must link structure to function for full marks

Example 3: A student investigates the effect of temperature on the enzyme amylase breaking down starch. Describe how the student could use iodine solution to determine when all the starch has been broken down. (3 marks)

Answer:

• Take samples of the starch-amylase mixture at regular time intervals (1 mark)
• Add each sample to iodine solution (1 mark)
• Starch is broken down when iodine remains orange/brown instead of turning blue-black (1 mark)

Common mistakes and how to avoid them

• Confusing diffusion and osmosis — Remember osmosis is specifically the movement of water molecules through a partially permeable membrane; diffusion refers to any particles moving down a concentration gradient

• Stating enzymes are "killed" at high temperatures — Enzymes denature (change shape permanently) but they're not alive, so cannot be killed; use correct terminology

• Mixing up magnification formula — Always write the formula out: Magnification = Image ÷ Actual. Check your answer makes sense (magnification should be greater than ×1 for enlarged images)

• Forgetting unit conversions — Convert all measurements to the same units before calculating; show your conversion in working for method marks

• Describing active transport as "using ATP" — State that active transport uses energy from respiration; ATP is A-level content not required at GCSE

• Labelling plant cells with "mitochondria" in chloroplasts — Plant cells have both mitochondria AND chloroplasts as separate organelles; chloroplasts photosynthesise, mitochondria respire

Exam technique for "B1: Cell Level Systems"

• Command words matter — "Describe" requires you to state what happens; "Explain" requires you to say why/how it happens with reasoning. For example, "Describe the effect of temperature on enzymes" needs observations; "Explain..." needs mechanism (increased collisions, denaturation)

• Link structure to function — When asked about specialised cells, always connect the structural adaptation to its specific purpose. Don't just list features without explaining their advantage

• Show your working — In calculations, write out the formula, show substitution of numbers, and include units in your final answer. Partial marks are available for method even if final answer is incorrect

• Use correct magnification units — Express magnification as "×200" not "200 times" in answers. Always include the multiplication symbol

Quick revision summary

Cells are the basic units of life. Eukaryotic cells (animals and plants) contain a nucleus and organelles; prokaryotic cells (bacteria) lack these. Specialised cells are adapted to their functions through differentiation. Enzymes are biological catalysts with specific active sites affected by temperature and pH. Substances cross membranes by diffusion, osmosis (water only), or active transport (requiring energy). Magnification = Image ÷ Actual. Cells are organised into tissues, organs, and organ systems. Mitosis produces two identical daughter cells for growth and repair.