Cell Biology

What you'll learn

Cell Biology forms a foundational component of the CXC CSEC Integrated Science syllabus. This revision guide covers the structure and function of plant and animal cells, distinguishes between prokaryotic and eukaryotic cells, and explains key cellular processes including diffusion, osmosis, and active transport. You will also learn about the organisation of cells into tissues, organs, and organ systems.

Key terms and definitions

Cell — the basic structural and functional unit of all living organisms

Organelle — a specialized structure within a cell that performs a specific function, such as the nucleus or mitochondrion

Cell membrane — a selectively permeable barrier that controls the movement of substances into and out of the cell

Diffusion — the net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient

Osmosis — the net movement of water molecules from a region of high water concentration (dilute solution) to a region of low water concentration (concentrated solution) through a selectively permeable membrane

Active transport — the movement of substances against a concentration gradient using energy from respiration

Mitosis — the process of cell division that produces two identical daughter cells for growth and repair

Tissue — a group of similar cells working together to perform a specific function

Core concepts

Cell structure and function

All living organisms are made up of cells. Cells contain specialized structures called organelles, each with specific functions.

Animal cell structures:

• Nucleus — contains genetic material (DNA) and controls all cell activities
• Cell membrane — controls what enters and leaves the cell; selectively permeable
• Cytoplasm — jelly-like substance where chemical reactions occur
• Mitochondria — site of aerobic respiration; releases energy from glucose
• Ribosomes — site of protein synthesis

Plant cell structures (in addition to animal cell structures):

• Cell wall — made of cellulose; provides structural support and protection
• Chloroplasts — contain chlorophyll; site of photosynthesis
• Permanent vacuole — filled with cell sap; maintains cell rigidity and stores substances
• Starch grains — storage form of glucose in plants

Caribbean applications: Sugar cane cells in Barbados, Jamaica, and Trinidad contain chloroplasts that convert sunlight into glucose through photosynthesis. The glucose is then converted to sucrose and stored in the stem cells.

Prokaryotic versus eukaryotic cells

Cells are classified into two main types based on their internal structure.

Eukaryotic cells:

• Contain a true nucleus enclosed by a nuclear membrane
• Have membrane-bound organelles (mitochondria, chloroplasts, endoplasmic reticulum)
• Include animal cells, plant cells, fungi, and protists
• Generally larger (10-100 μm)

Prokaryotic cells:

• Have no true nucleus; DNA floats freely in the cytoplasm
• No membrane-bound organelles
• Include bacteria
• Generally smaller (1-10 μm)
• Have a cell wall made of peptidoglycan (not cellulose)
• May have plasmids (small circular DNA molecules) and flagella for movement

Caribbean context: Bacteria that cause food spoilage in fish markets across Grenada and St. Lucia are prokaryotic cells. Understanding bacterial structure helps preserve Caribbean seafood products.

Movement of substances across membranes

The cell membrane is selectively permeable, allowing some substances to pass through while blocking others. Three main processes move substances across membranes:

Diffusion:

• Passive process requiring no energy
• Particles move down a concentration gradient
• Continues until equilibrium is reached
• Rate affected by: temperature, concentration gradient, surface area, distance

Examples in the body:

• Oxygen diffuses from alveoli into blood capillaries
• Carbon dioxide diffuses from blood into alveoli
• Nutrients diffuse from the small intestine into blood

Caribbean example: When seasoning meat with salt and lime juice (common in Jamaican jerk preparation), salt particles diffuse into the meat cells down a concentration gradient.

Osmosis:

• Special type of diffusion involving only water molecules
• Water moves through a selectively permeable membrane
• Moves from dilute solution (high water concentration) to concentrated solution (low water concentration)
• Continues until water concentration equalizes on both sides

Effects of osmosis on cells:

• Animal cells in pure water — water enters by osmosis; cell swells and may burst (lysis)
• Animal cells in concentrated solution — water leaves by osmosis; cell shrinks (crenation)
• Plant cells in pure water — water enters; cell becomes turgid (firm); cell wall prevents bursting
• Plant cells in concentrated solution — water leaves; cell becomes plasmolysed (membrane pulls away from cell wall)

Caribbean application: Salting fish (as done traditionally in coastal communities throughout the Caribbean) preserves it because the high salt concentration draws water out of bacterial cells by osmosis, preventing bacterial growth.

Active transport:

• Requires energy from respiration (ATP)
• Moves substances against a concentration gradient (from low to high concentration)
• Uses carrier proteins in the cell membrane

Examples:

• Root hair cells absorb mineral ions from soil (even when soil concentration is lower than root cell concentration)
• Glucose absorption in the small intestine
• Kidney tubules reabsorb useful substances back into blood

Caribbean context: Dasheen, eddoes, and other root crops grown in St. Vincent and Dominica use active transport to absorb nutrients from volcanic soil, even when nutrient concentrations are low.

Levels of organisation

Cells are organized into increasingly complex structures:

Cell → Tissue → Organ → Organ system → Organism

Tissue — group of similar cells performing the same function

• Muscle tissue (contracts for movement)
• Epithelial tissue (covers and lines organs)
• Nerve tissue (transmits electrical impulses)
• Xylem and phloem tissue in plants (transport water and food)

Organ — structure made of different tissues working together

• Heart (muscle tissue, nerve tissue, blood vessels)
• Leaf (epidermis, palisade mesophyll, spongy mesophyll, xylem, phloem)
• Stomach (muscle tissue, glandular tissue, epithelial tissue)

Organ system — group of organs working together

• Digestive system (mouth, stomach, intestines, liver, pancreas)
• Circulatory system (heart, blood vessels)
• Respiratory system (nose, trachea, bronchi, lungs)

Caribbean example: The breadfruit tree (found throughout the Caribbean) has a root system (organ system) consisting of roots (organs) made of different tissues including xylem, phloem, and root hair cells.

Cell division: Mitosis

Mitosis is cell division that produces two genetically identical daughter cells. It is essential for growth, repair, and asexual reproduction.

Stages of the cell cycle:

1. Interphase — cell grows, DNA replicates, organelles duplicate; cell prepares for division
2. Prophase — chromosomes condense and become visible; nuclear membrane breaks down
3. Metaphase — chromosomes line up at the cell equator
4. Anaphase — chromosome copies separate and move to opposite poles of the cell
5. Telophase — nuclear membranes reform around each set of chromosomes; cytoplasm divides (cytokinesis)

Result: Two daughter cells, each with the same number of chromosomes as the parent cell

Importance:

• Growth (increasing cell number)
• Repair of damaged tissues
• Replacement of worn-out cells
• Asexual reproduction in organisms

Caribbean application: When a lizard in Trinidad loses its tail to escape a predator, mitosis in remaining tail cells allows tissue regeneration.

Specialized cells

Cells are adapted to perform specific functions. Their structure relates directly to their function.

Root hair cell:

• Long extension increases surface area for water and mineral absorption
• No chloroplasts (underground, no light)
• Many mitochondria provide energy for active transport of minerals

Palisade mesophyll cell:

• Packed with chloroplasts for maximum photosynthesis
• Located near upper leaf surface for maximum light absorption
• Cylindrical shape allows tight packing

Red blood cell:

• Biconcave disc shape increases surface area for oxygen absorption
• No nucleus provides more space for haemoglobin
• Flexible to squeeze through narrow capillaries

Sperm cell:

• Long tail (flagellum) for swimming to egg
• Many mitochondria in middle section provide energy for movement
• Acrosome contains enzymes to penetrate egg

Nerve cell (neuron):

• Long axon carries electrical impulses over long distances
• Branched dendrites connect to many other nerve cells
• Myelin sheath insulates and speeds up impulse transmission

Worked examples

Example 1: Osmosis in plant cells (4 marks)

A student placed potato chips in three different solutions for 30 minutes. The table shows the results:

Solution	Starting mass (g)	Final mass (g)
A	5.0	5.8
B	5.0	5.0
C	5.0	4.3

(a) Explain what happened to the potato chip in solution A. (2 marks) (b) What type of solution was B? (1 mark) (c) Describe the condition of the cells in the potato chip from solution C. (1 mark)

Mark scheme answers:

(a) Water moved into the potato cells by osmosis (1 mark) because solution A had a higher water concentration than the potato cells / solution A was more dilute than the cell sap (1 mark).

(b) Isotonic solution / same concentration as cell sap (1 mark)

(c) Cells became plasmolysed / shriveled / lost water / became flaccid (1 mark)

Example 2: Cell structure and function (6 marks)

Compare the structure of a typical plant cell and a typical animal cell.

Mark scheme answer:

Similarities:

• Both have a nucleus that contains DNA / controls cell activities (1 mark)
• Both have cell membrane that controls entry and exit of substances (1 mark)
• Both have cytoplasm where reactions occur (1 mark)
• Both have mitochondria for respiration (1 mark)

Differences:

• Plant cells have cell wall (made of cellulose) but animal cells do not (1 mark)
• Plant cells have chloroplasts for photosynthesis but animal cells do not (1 mark)
• Plant cells have permanent vacuole but animal cells have small temporary vacuoles (1 mark)

(Any 6 marks from above points)

Example 3: Active transport (3 marks)

Explain why root hair cells contain many mitochondria. (3 marks)

Mark scheme answer:

Mitochondria are the site of aerobic respiration (1 mark), which releases energy (1 mark). Root hair cells need energy for active transport of mineral ions from the soil into the cell against a concentration gradient (1 mark).

Common mistakes and how to avoid them

• Confusing diffusion and osmosis — Remember that osmosis specifically refers to water movement through a selectively permeable membrane; diffusion can involve any particles in any medium (solid, liquid, or gas).

• Stating that osmosis requires energy — Both osmosis and diffusion are passive processes requiring no energy input. Only active transport requires energy from respiration.

• Mixing up plant and animal cell structures — Create a clear comparison table. Remember the three structures unique to plant cells: cell wall, chloroplasts, and permanent vacuole.

• Describing concentration gradient incorrectly — Always specify direction: particles move FROM high concentration TO low concentration in diffusion. Water moves FROM high water concentration (dilute) TO low water concentration (concentrated) in osmosis.

• Forgetting that cell walls prevent plant cells from bursting — When plant cells are in pure water, they become turgid but do not burst because the strong cellulose cell wall resists expansion.

• Using vague terminology — Avoid phrases like "things move in and out." Instead, specify: "water molecules move by osmosis" or "glucose molecules move by diffusion."

Exam technique for Cell Biology

• Command word awareness — "State" requires a simple answer without explanation (1 mark). "Explain" requires a reason using "because" or "so that" (2-3 marks). "Compare" requires both similarities AND differences. "Describe" requires characteristics or changes observed without explanation.

• Label diagrams accurately — Use a ruler for label lines. Ensure lines touch the structure being identified but do not cross. Write labels horizontally outside the diagram. CXC examiners deduct marks for poor presentation.

• Use comparative language — When comparing cells, use "whereas," "but," "while," or "however" to show clear differences. Example: "Plant cells have chloroplasts, whereas animal cells do not."

• Show working in calculations — For percentage change questions (common with osmosis practicals), write the formula: (change ÷ original) × 100. Show each calculation step for partial credit even if final answer is wrong.

Quick revision summary

Cells are the basic units of life, containing specialized organelles including the nucleus, mitochondria, and cell membrane. Plant cells additionally have cell walls, chloroplasts, and permanent vacuoles. Substances cross membranes by diffusion (particles moving down concentration gradients), osmosis (water moving through selectively permeable membranes), and active transport (requiring energy from respiration). Cells organize into tissues, organs, and organ systems. Mitosis produces identical daughter cells for growth and repair. Understanding cell structure and function is essential for explaining life processes tested in CSEC examinations.