A2.2 Cell Structure

Cells as the basic structural unit of life

All living organisms are composed of one or more cells, and the cell is the smallest unit capable of carrying out all vital functions such as metabolism, growth, and reproduction

Cell theory

States that all living organisms are made of cells, cells are the basic unit of structure and function, and all cells arise from pre-existing cells

Deductive reasoning in cell theory

Using cell theory to predict that any newly discovered organism must be composed of one or more cells

Quantitative observation

Observations involving numerical measurements, such as measuring cell size using microscopes and graticules

Microscopy

The use of microscopes to observe structures too small to be seen with the naked eye

Temporary mount

A microscope slide prepared by placing a specimen in liquid under a coverslip for short-term observation

Staining in microscopy

The use of dyes to increase contrast by binding to specific cell structures so they become more visible under a microscope

Coarse adjustment knob

Used for rapid, large movements to bring the specimen roughly into focus

Fine adjustment knob

Used for small, precise movements to sharpen the image once roughly focused

Eyepiece graticule

A scale inside the eyepiece used to measure cell size when calibrated

Stage micrometer

A slide with a known scale used to calibrate an eyepiece graticule

Magnification

The number of times larger an image is compared to the actual specimen, calculated as image size ÷ actual size

Scale bar

A line on a micrograph that represents a specific actual length, allowing size estimation

Light microscopy advantages

Allows observation of living cells, colour images, and relatively simple preparation

Electron microscopy advantages

Much higher resolution and magnification, allowing detailed ultrastructure of cells to be observed

Freeze fracture microscopy

Technique where membranes are split to reveal internal membrane structure such as phospholipid bilayers

Cryogenic electron microscopy

Samples are rapidly frozen to preserve structure without chemical fixation, reducing distortion

Immunofluorescence

Uses fluorescently labelled antibodies to locate specific proteins in cells under a light microscope

Structures common to all cells

DNA as genetic material, cytoplasm mainly composed of water, ribosomes, and a plasma membrane

Plasma membrane

A phospholipid bilayer with proteins that controls the movement of substances in and out of the cell

Cytoplasm

A water-based matrix containing enzymes and metabolites where many metabolic reactions occur

DNA as genetic material

Stores information needed for cell structure, function, and inheritance

Prokaryote

A cell lacking a nucleus and membrane-bound organelles, typically smaller and simpler than eukaryotic cells

Prokaryotic cell wall

Made of peptidoglycan, providing protection and preventing osmotic lysis

Nucleoid

Region in prokaryotic cells containing naked circular DNA not enclosed by a membrane

70S ribosomes

Smaller ribosomes found in prokaryotes and in mitochondria and chloroplasts

Eukaryote

A cell with a nucleus and membrane-bound organelles

Compartmentalization

Separation of cell functions into membrane-bound organelles to increase efficiency

Nucleus

Contains chromosomes made of DNA bound to histones and controls gene expression

Nuclear envelope

Double membrane surrounding the nucleus with nuclear pores for transport

Nuclear pores

Protein complexes that regulate movement of RNA and proteins between nucleus and cytoplasm

Rough endoplasmic reticulum

Membrane system with ribosomes attached, involved in protein synthesis and transport

Smooth endoplasmic reticulum

Involved in lipid synthesis, detoxification, and calcium storage

Golgi apparatus

Modifies, sorts, and packages proteins into vesicles for transport or secretion

Mitochondrion

Site of aerobic respiration and ATP production, surrounded by a double membrane

Lysosome

Vesicle containing hydrolytic enzymes for intracellular digestion

Vacuole

Large vesicle in plant cells involved in storage, support, and maintaining turgor pressure

Cytoskeleton

Network of microtubules and microfilaments that maintain cell shape and enable movement

Unicellular organism life processes

A single cell carries out homeostasis, metabolism, nutrition, movement, growth, response, excretion, and reproduction

Differences between plant and animal cells

Plant cells have cell walls, chloroplasts, and large vacuoles, while animal cells do not

Atypical eukaryotic cells

Cells with unusual structures such as multinucleate skeletal muscle cells or anucleate red blood cells

(HL) Endosymbiotic theory

Explains the origin of mitochondria and chloroplasts as free-living prokaryotes engulfed by ancestral eukaryotic cells

(HL) Evidence for endosymbiosis

Presence of 70S ribosomes, naked circular DNA, double membranes, and independent replication in mitochondria and chloroplasts

(HL) Cell differentiation

Process by which cells become specialized due to differential gene expression triggered by environmental and developmental signals

(HL) Evolution of multicellularity

Multicellularity evolved repeatedly, allowing increased body size, division of labour, and specialized tissue

Compare prokaryotic and eukaryotic cells in terms of structure and organization

Prokaryotic cells lack a nucleus and membrane-bound organelles, have naked circular DNA in a nucleoid region, 70S ribosomes, and are generally smaller, whereas eukaryotic cells have a nucleus with linear DNA bound to histones, membrane-bound organelles such as mitochondria and ER, 80S ribosomes, and a compartmentalized cytoplasm that increases efficiency

Explain why compartmentalization is an advantage in eukaryotic cells

Compartmentalization allows incompatible reactions to occur simultaneously, concentrates enzymes and substrates to increase reaction rates, provides specialized conditions such as low pH in lysosomes, and allows greater regulation and efficiency of metabolism

Describe how you would calculate the actual size of a cell from an electron micrograph

Measure the image size of the cell using a ruler, divide by the magnification to obtain actual size, ensure units are consistent, or alternatively use the scale bar by comparing the measured cell length to the scale bar length

Explain why electron microscopes are required to observe ribosomes but not whole cells

ibosomes are approximately 20–30 nm in diameter, which is below the resolution limit of light microscopes (~200 nm), while whole cells are much larger and can be resolved using light microscopy

Explain how evidence from mitochondria supports the endosymbiotic theory (HL)

Mitochondria contain 70S ribosomes similar to prokaryotes, naked circular DNA, replicate independently of the nucleus, and are surrounded by a double membrane, all of which support the idea that they originated as free-living bacteria engulfed by ancestral eukaryotic cells

Predict whether a newly discovered organism with ribosomes, DNA, and no nucleus is prokaryotic or eukaryotic and justify your answer

The organism is prokaryotic because the absence of a nucleus and presence of ribosomes and DNA in the cytoplasm match prokaryotic cell structure

Explain how a unicellular organism can carry out all life processes within a single cell

A unicellular organism maintains homeostasis through membrane transport, carries out metabolism using cytoplasmic enzymes, obtains nutrients by absorption or ingestion, removes wastes by diffusion or exocytosis, responds to stimuli via receptors, grows, and reproduces by cell division

Explain why red blood cells are described as atypical eukaryotic cells

Red blood cells are anucleate and lack most organelles, increasing space for haemoglobin and improving oxygen transport, which makes them structurally different from typical eukaryotic cell